Evaluation of x-ray scatter properties in a dedicated cone-beam breast CT scanner

A simulation study to evaluate the effect of 2D antiscatter grid primary transmission on flat panel detector based CBCT image quality

Purpose. Two-dimensional antiscatter grids' (2D-ASGs) septal shadows and their impact on primary transmission play a critical role in cone-beam computed tomography (CBCT) image noise and artifact characteristics. Therefore, a numerical simulation platform was developed to evaluate the effect of 2D-ASG's primary transmission on image quality, as a function of grid geometry and CBCT system properties.Methods. To study the effect of 2D-ASG's septal shadows on primary transmission and CBCT image quality, two new methods were introduced; one to simulate projection signal gradients in septal shadows, and the other to simulate septal shadow variations due to gantry flex. Signal gradients in septal shadows were simulated by generating a system point spread function that was directly extracted from projection images of 2D-ASG prototypes in experiments. Variations in septal shadows due to gantry flex were simulated by generating oversampled shadow profiles extracted from experiments. Subsequently, the effect of 2D-ASG's septal shadows on primary transmission and image quality was evaluated.Results.For an apparent septal thickness of 0.15 mm, the primary transmission of 2D-ASG varied between 72%-90% for grid pitches 1-3 mm. In low-contrast phantoms, the effect of 2D-ASG's radiopaque footprint on information loss was subtle. At high spatial frequencies, information loss manifested itself as undersampling artifacts, however, its impact on image quality is subtle when compared to quantum noise. Effects of additive electronic noise and gantry flex induced ring artifacts on image quality varied as a function of grid pitch and septal thickness. Such artifacts were substantially less in lower resolution images.Conclusion. The proposed simulation platform allowed successful evaluation of CBCT image quality variations as a function of 2D-ASG primary transmission properties and CBCT system characteristics. This platform can be potentially used for optimizing 2D-ASG design properties based on the imaging task and properties of the CBCT system.

Deep learning for x-ray scatter correction in dedicated breast CT

BACKGROUND

Accurate correction of x-ray scatter in dedicated breast computed tomography (bCT) imaging may result in improved visual interpretation and is crucial to achieve quantitative accuracy during image reconstruction and analysis.

PURPOSE

To develop a deep learning (DL) model to correct for x-ray scatter in bCT projection images.

METHODS

A total of 115 patient scans acquired with a bCT clinical system were segmented into the major breast tissue types (skin, adipose, and fibroglandular tissue). The resulting breast phantoms were divided into training (n = 110) and internal validation cohort (n = 5). Training phantoms were augmented by a factor of four by random translation of the breast in the image field of view. Using a previously validated Monte Carlo (MC) simulation algorithm, 12 primary and scatter bCT projection images with a 30-degree step were generated from each phantom. For each projection, the thickness map and breast location in the field of view were also calculated. A U-Net based DL model was developed to estimate the scatter signal based on the total input simulated image and trained single-projection-wise, with the thickness map and breast location provided as additional inputs. The model was internally validated using MC-simulated projections and tested using an external data set of 10 phantoms derived from images acquired with a different bCT system. For this purpose, the mean relative difference (MRD) and mean absolute error (MAE) were calculated. To test for accuracy in reconstructed images, a full bCT acquisition was mimicked with MC-simulations and then assessed by calculating the MAE and the structural similarity (SSIM). Subsequently, scatter was estimated and subtracted from the bCT scans of three patients to obtain the scatter-corrected image. The scatter-corrected projections were reconstructed and compared with the uncorrected reconstructions by evaluating the correction of the cupping artifact, increase in image contrast, and contrast-to-noise ratio (CNR).

RESULTS

The mean MRD and MAE across all cases (min, max) for the internal validation set were 0.04% (-1.1%, 1.3%) and 2.94% (2.7%, 3.2%), while for the external test set they were -0.64% (-1.6%, 0.2%) and 2.84% (2.3%, 3.5%), respectively. For MC-simulated reconstruction slices, the computed SSIM was 0.99 and the MAE was 0.11% (range: 0%, 0.35%) with a single outlier slice of 2.06%. For the three patient bCT reconstructed images, the correction increased the contrast by a mean of 25% (range: 20%, 30%), and reduced the cupping artifact. The mean CNR increased by 0.32 after scatter correction, which was not found to be significant (95% confidence interval: [-0.01, 0.65], p = 0.059). The time required to correct the scatter in a single bCT projection was 0.2 s on an NVIDIA GeForce GTX 1080 GPU.

CONCLUSION

The developed DL model could accurately estimate scatter in bCT projection images and could enhance contrast and correct for cupping artifact in reconstructed patient images without significantly affecting the CNR. The time required for correction would allow its use in daily clinical practice, and the reported accuracy will potentially allow quantitative reconstructions.

A novel hardware duo of beam modulation and shielding to reduce scatter acquisition and dose in cone-beam breast CT

PURPOSE

In cone-beam breast CT, scattered photons form a large portion of the acquired signal, adversely impacting image quality throughout the frequency response of the imaging system. Prior simulation studies provided proof of concept for utilization of a hardware solution to prevent scatter acquisition. Here, we report the design, implementation, and characterization of an auxiliary apparatus of fluence modulation and scatter shielding that does indeed lead to projections with a reduced level of scatter.

METHODS

An apparatus was designed for permanent installation within an existing cone-beam CT system. The apparatus is composed of two primary assemblies: a "Fluence Modulator" (FM) and a "Scatter Shield" (SS). The design of the assemblies enables them to operate in synchrony during image acquisition, converting the sourced x-rays into a moving narrow beam. During a projection, this narrow beam sweeps the entire fan angle coverage of the imaging system. As the two assemblies are contingent on one another, their joint implementation is described in the singular as apparatus FM-SS. The FM and the SS assemblies are each comprised a metal housing, a sensory system, and a robotic system. A controller unit handles their relative movements. A series of comparative studies were conducted to evaluate the performance of a cone-beam CT system in two "modes" of operation: with and without FM-SS installed, and to compare the results of physical implementation with those previously simulated. The dynamic range requirements of the utilized detector in the cone-beam CT imaging system were first characterized, independent of the mode of operation. We then characterized and compared the spatial resolution of the imaging system with, and without, FM-SS. A physical breast phantom, representative of an average size breast, was developed and imaged. Actual differences in signal level obtained with, versus without, FM-SS were then compared to the expected level gains based on previously reported simulations. Following these initial assessments, the scatter acquisition in each projection in both modes of operation was investigated. Finally, as an initial study of the impact of FM-SS on radiation dose in an average size breast, a series of Monte Carlo simulations were coupled with physical measurements of air kerma, with and without FM-SS.

RESULTS

With implementation of FM-SS, the detector's required dynamic range was reduced by a factor of 5.5. Substantial reduction in the acquisition of the scattered rays, by a factor of 5.1 was achieved. With the implementation of FM-SS, deposited dose was reduced by 27% in the studied breast.

CONCLUSIONS

The disclosed implementation of FM-SS, within a cone-beam breast CT system, results in reduction of scatter-components in acquired projections, reduction of dose deposit to the breast, and relaxation of requirements for the detector's dynamic range. Controlling or correcting for patient motion occurring during image acquisition remains an open problem to be solved prior to practical clinical usage of FM-SS cone-beam breast CT.

Intraoperative cone-beam and slot-beam CT: 3D image quality and dose with a slot collimator on the O-arm imaging system

PURPOSE

To characterize the 3D imaging performance and radiation dose for a prototype slot-beam configuration on an intraoperative O-arm™ Surgical Imaging System (Medtronic Inc., Littleton, MA) and identify potential improvements in soft-tissue image quality for surgical interventions.

METHODS

A slot collimator was integrated with the O-arm^™ system for slot-beam axial CT. The collimator can be automatically actuated to provide 1.2° slot-beam longitudinal collimation. Cone-beam and slot-beam configurations were investigated with and without an antiscatter grid (12:1 grid ratio, 60 lines/cm). Dose, scatter, image noise, and soft-tissue contrast resolution were evaluated in quantitative phantoms for head and body configurations over a range of exposure levels (beam energy and mAs), with reconstruction performed via filtered-backprojection. Qualitative imaging performance across various anatomical sites and imaging tasks was assessed with anthropomorphic head, abdomen, and pelvis phantoms.

RESULTS

The dose for a slot-beam scan varied from 0.02-0.06 mGy/mAs for head protocols to 0.01-0.03 mGy/mAs for body protocols, yielding dose reduction by ∼1/5 to 1/3 compared to cone-beam, owing to beam collimation and reduced x-ray scatter. The slot-beam provided an ∼6-7× reduction in scatter-to-primary ratio (SPR) compared to the cone-beam, yielding SPR ∼20-80% for head and body without the grid and ∼7-30% with the grid. Compared to cone-beam scans at equivalent dose, slot-beam images exhibited an ∼2.5× increase in soft-tissue contrast-to-noise ratio (CNR) for both grid and gridless configurations. For slot-beam scans, a further ∼10-30% improvement in CNR was achieved when the grid was removed. Slot-beam imaging could benefit certain interventional scenarios in which improved visualization of soft tissues is required within a fairly narrow longitudinal region of interest ( ± 7 mm in z )--for example, checking the completeness of tumor resection, preservation of adjacent anatomy, or detection of complications (e.g., hemorrhage). While preserving existing capabilities for fluoroscopy and cone-beam CT, slot-beam scanning could enhance the utility of intraoperative imaging and provide a useful mode for safety and validation checks in image-guided surgery.

CONCLUSIONS

The 3D imaging performance and dose of a prototype slot-beam CT configuration on the O-arm^™ system was investigated. Substantial improvements in soft-tissue image quality and reduction in radiation dose are evident with the slot-beam configuration due to reduced x-ray scatter.

Incorporation of residual attention modules into two neural networks for low-dose CT denoising

PURPOSE

The low-dose computed tomography (CT) imaging can reduce the damage caused by x-ray radiation to the human body. However, low-dose CT images have a different degree of artifacts than conventional CT images, and their resolution is lower than that of conventional CT images, which can affect disease diagnosis by clinicians. Therefore, methods for noise-level reduction and resolution improvement in low-dose CT images have inevitably become a research hotspot in the field of low-dose CT imaging.

METHODS

In this paper, residual attention modules (RAMs) are incorporated into the residual encoder-decoder convolutional neural network (RED-CNN) and generative adversarial network with Wasserstein distance (WGAN) to learn features that are beneficial to improving the performances of denoising networks, and developed models are denoted as RED-CNN-RAM and WGAN-RAM, respectively. In detail, RAM is composed of a multi-scale convolution module and an attention module built on the residual network architecture, where the attention module consists of a channel attention module and a spatial attention module. The residual network architecture solves the problem of network degradation with increased network depth. The function of the attention module is to learn which features are beneficial to reduce the noise level of low-dose CT images to reduce the loss of detail in the final denoising images, which is also the key point of the proposed algorithms.

RESULTS

To develop a robust network for low-dose CT image denoising, multidose-level torso phantom images provided by a cooperating equipment vendor are used to train the network, which can improve the network's adaptability to clinical application. In addition, a clinical dataset is used to test the network's migration capabilities and clinical applicability. The experimental results demonstrate that these proposed networks can effectively remove noise and artifacts from multidose CT scans. Subjective and objective analyses of multiple groups of comparison experiments show that the proposed networks achieve good noise suppression performance while preserving the image texture details.

CONCLUSION

In this study, two deep learning network models are developed using multidose-level CT images acquired from a commercial spiral CT scanner. The two network models can reduce and even remove streaking artifacts, and noise from low-dose CT images confirms the effectiveness of the proposed algorithms.

Evaluation of scatter rejection and correction performance of 2D antiscatter grids in cone beam computed tomography

PURPOSE

We have been investigating two-dimensional (2D) antiscatter grids (2D ASGs) to reduce scatter fluence and improve image quality in cone beam computed tomography (CBCT). In this work, two different aspects of 2D ASGs, their scatter rejection and correction capability, were investigated in CBCT experiments. To correct residual scatter transmitted through the 2D ASG, it was used as a scatter measurement device with a novel method: grid-based scatter sampling.

METHODS

Three focused 2D ASG prototypes with grid ratios of 8, 12, and 16 were developed for linac-mounted offset detector CBCT geometry. In the first phase, 2D ASGs were used as a scatter rejection device, and the effect of grid ratio on CT number accuracy and contrast-to-noise ratio (CNR) evaluated in CBCT images. In the second phase, a grid-based scatter sampling method which exploits the signal modulation characteristics of the 2D ASG's septal shadows to measure and correct residual scatter transmitted through the grid was implemented. To evaluate CT number accuracy, the percent change in CT numbers was measured by changing the phantom from head to pelvis size and configuration.

RESULTS

When 2D ASG was used as a scatter rejection device, CT number accuracy increased and the CT number variation due to change in phantom dimensions was reduced from 23% to 2-6%. A grid ratio of 16 yielded the lowest CT number variation. All three 2D ASGs yielded improvement in CNR, up to a factor of two in pelvis-sized phantoms. When 2D ASG prototypes were used for both scatter rejection and correction, CT number variations were reduced further, to 1.3-2.6%. In comparisons with a clinical CBCT system and a high-performance radiographic ASG, 2D ASG provided higher CT number accuracy under the same imaging conditions.

CONCLUSIONS

When 2D ASG is used solely as a scatter rejection device, substantial improvement in CT number accuracy can be achieved by increasing the grid ratio. Two-dimensional ASGs also provided significant CNR improvement even at lower grid ratios. Two-dimensional ASGs used in conjunction with the grid-based scatter sampling method provided further improvement in CT number accuracy, irrespective of the grid ratio, while preserving 2D ASGs' capacity to improve CNR. The combined effect of scatter rejection and residual scatter correction by 2D ASG may accelerate implementation of new techniques in CBCT that require high quantitative accuracy, such as radiotherapy dose calculation and dual energy CBCT.

An analysis of scatter characteristics in x-ray CT spectral correction

X-ray scatter remains a major physics challenge in volumetric computed tomography (CT), whose physical and statistical behaviors have been commonly leveraged in order to eliminate its impact on CT image quality. In this work, we conduct an in-depth derivation of how the scatter distribution and scatter to primary ratio (SPR) will change during the spectral correction, leading to an interesting finding on the property of scatter. Such a characterization of scatter's behavior provides an analytic approach of compensating for the SPR as well as approximating the change of scatter distribution after spectral correction, even though both of them might be significantly distorted as the linearization mapping function in spectral correction could vary a lot from one detector pixel to another. We conduct an evaluation of SPR compensations on a Catphan phantom and an anthropomorphic chest phantom to validate the characteristics of scatter. In addition, this scatter property is also directly adopted into CT imaging using a spectral modulator with flying focal spot technology (SMFFS) as an example to demonstrate its potential in practical applications. For cone-beam CT scans at both 80 and 120 kVp, CT images with accurate CT numbers can be achieved after spectral correction followed by the appropriate SPR compensation based on our presented scatter property. In the case of the SMFFS based cone-beam CT scan of the Catphan phantom at 120 kVp, after a scatter correction using an analytic algorithm derived from the scatter property, CT image quality was significantly improved, with the averaged root mean square error reduced from 297.9 to 6.5 Hounsfield units (HU).

A feasibility study of time of flight cone beam computed tomography imaging

BACKGROUND

The time of flight (TOF) cone beam computed tomography (CBCT) was recently shown to reduce the X-ray scattering effects by 95% and improve the image CNR by 110% for large volume objects. The advancements in X-ray sources like in compact Free Electron Lasers (FEL) and advancements in detector technology show potential for the TOF method to be feasible in CBCT when imaging large objects.

OBJECTIVE

To investigate the feasibility and efficacy of TOF CBCT in imaging smaller objects with different targets such as bones and tumors embedded inside the background.

METHODS

The TOF method used in this work was verified using a 24 cm phantom. Then, the GATE software was used to simulate the CBCT imaging of an 8 cm diameter cylindrical water phantom with two bone targets using a modeled 20 keV quasi-energetic FEL source and various TOF resolutions ranging from 1 to 1000 ps. An inhomogeneous breast phantom of similar size with tumor targets was also imaged using the same system setup.

RESULTS

The same results were obtained in the 24 cm phantom, which validated the applied CBCT simulation approach. For the case of 8 cm cylindrical phantom and bone target, a TOF resolution of 10 ps improved the image contrast-to-noise ratio (CNR) by 57% and reduced the scatter-to-primary ratio (SPR) by 8.63. For the case of breast phantom and tumor target, image CNR was enhanced by 12% and SPR was reduced by 1.35 at 5 ps temporal resolution.

CONCLUSIONS

This study indicates that a TOF resolution below 10 ps is required to observe notable enhancements in the image quality and scatter reduction for small objects around 8 cm in diameter. The strong scattering targets such as bone can result in substantial improvements by using TOF CBCT.

A fluence modulation and scatter shielding apparatus for dedicated breast CT: Theory of operation

PURPOSE

To introduce an auxiliary apparatus of fluence modulation and scatter shielding for dedicated breast computed tomography (bCT) and the corresponding patient-specific method of image acquisition.

METHODS

The apparatus is composed of two assemblies, referred to herein as the "Dynamic Fluence Gate" (FG) and "Scatter Shield" (SS), that work in synchrony to form a narrow beam sweeping the entire fan angle coverage of the imaging system during a projection. The apparatus, as a whole, is referred to as FG-SS. FG and SS are pre-patient and post-patient units, respectively. Each is composed of a rotating drum, on top of which are installed two sheets of high x-ray attenuating material, a sensory system, and the constituent robotics. The sheets of each unit are positioned such that an opening - a window Fluence Modulation and Scatter Shielding is formed between them. The rotations of the drums and positioning of the sheets are synchronized and adjusted such that a line of sight is created between the source, FG window, the breast, and the SS window. With line of sight achieved, the narrow beam transitions from the source to the detector. The fluence of the narrow beam during a projection depends on the size, shape, and positioning of the breast. The FG-SS method of imaging is discussed mathematically and demonstrated using computer simulations. A series of Monte Carlo simulations are conducted to evaluate the performance of the system as relates to its impact on the imager's dynamic range, dose distribution to the breast, noise inhomogeneity in reconstructed images, and scatter buildup in projections within small, medium, and large breasts composed of homogeneous medium breast tissue.

RESULTS

Implementation of FG-SS results in near scatter-free projections, reduction in both dose and the required dynamic range of the imager, and equalization of the quantum noise distribution in the reconstructed image. Using the disclosed design, the dynamic range was reduced by factors ranging from 1.6 to 5.5 across the range of breast sizes studied. A reduction in the acquisition of the scattered rays, varying between the factors of 6.1 (in the small breast) and 9.8 (in that large breast) was achieved and consequently, shading artifacts were suppressed. Noise in the CT image was equalized by reducing the overall spatial variation from 29% to <5% in small breast and from 45% to 14% in the large breast. An overall reduction in deposited dose to the breast was achieved - between 26% and 39% depending on the breast size.

CONCLUSIONS

Utilization of the FG-SS apparatus and technique was demonstrated via simulations to result in: significant reductions in dose to the breast, reductions in scatter uptake in projections, reduced required dynamic range of the imager, and homogenizing of quantum noise throughout the reconstructed image.

Framework for Photon Counting Quantitative Material Decomposition

In this paper, the accuracy of material decomposition (MD) using an energy discriminating photon counting detector was studied. An MD framework was established and validated using calcium hydroxyapatite (CaHA) inserts of known densities (50 mg/cm³, 100 mg/cm³, 250 mg/cm³, 400 mg/cm³), and diameters (1.2, 3.0, and 5.0 mm). These inserts were placed in a cardiac rod phantom that mimics a tissue equivalent heart and measured using an experimental photon counting detector cone beam computed tomography (PCD-CBCT) setup. The quantitative coronary calcium scores (density, mass, and volume) obtained from the MD framework were compared with the nominal values. In addition, three different calibration techniques, signal-to-equivalent thickness calibration (STC), polynomial correction (PC), and projected equivalent thickness calibration (PETC) were compared to investigate the effect of the calibration method on the quantitative values. The obtained MD estimates agreed well with the nominal values for density (mass) with mean absolute percent errors (MAPEs) 8 ± 11% (9 ± 15%) and 4 ± 6% (9 ± 14%) for STC and PETC calibration methods, respectively. PC displayed large MAPEs for density (27 ± 9%), and mass (25 ± 12%). Volume estimation resulted in large deviations between true and measured values with notable MAPEs for STC (40 ± 90%), PC (40 ± 80%), and PETC (40 ± 90%). The framework demonstrated the feasibility of quantitative CaHA mass and density scoring using PCD-CBCT.

Spectral Signatures of X-ray Scatter Using Energy-Resolving Photon-Counting Detectors

Energy-resolving photon-counting detectors (PCDs) separate photons from a polychromatic X-ray source into a number of separate energy bins. This spectral information from PCDs would allow advancements in X-ray imaging, such as improving image contrast, quantitative imaging, and material identification and characterization. However, aspects like detector spectral distortions and scattered photons from the object can impede these advantages if left unaccounted for. Scattered X-ray photons act as noise in an image and reduce image contrast, thereby significantly hindering PCD utility. In this paper, we explore and outline several important characteristics of spectral X-ray scatter with examples of soft-material imaging (such as cancer imaging in mammography or explosives detection in airport security). Our results showed critical spectral signatures of scattered photons that depend on a few adjustable experimental factors. Additionally, energy bins over a large portion of the spectrum exhibit lower scatter-to-primary ratio in comparison to what would be expected when using a conventional energy-integrating detector. These important findings allow flexible choice of scatter-correction methods and energy-bin utilization when using PCDs. Our findings also propel the development of efficient spectral X-ray scatter correction methods for a wide range of PCD-based applications.

Shading artifact correction in breast CT using an interleaved deep learning segmentation and maximum-likelihood polynomial fitting approach

PURPOSE

The purpose of this work was twofold: (a) To provide a robust and accurate method for image segmentation of dedicated breast CT (bCT) volume data sets, and (b) to improve Hounsfield unit (HU) accuracy in bCT by means of a postprocessing method that uses the segmented images to correct for the low-frequency shading artifacts in reconstructed images.

METHODS

A sequential and iterative application of image segmentation and low-order polynomial fitting to bCT volume data sets was used in the interleaved correction (IC) method. Image segmentation was performed through a deep convolutional neural network (CNN) with a modified U-Net architecture. A total of 45 621 coronal bCT images from 111 patient volume data sets were segmented (using a previously published segmentation algorithm) and used for neural network training, validation, and testing. All patient data sets were selected from scans performed on four different prototype breast CT systems. The adipose voxels for each patient volume data set, segmented using the proposed CNN, were then fit to a three-dimensional low-order polynomial. The polynomial fit was subsequently used to correct for the shading artifacts introduced by scatter and beam hardening in a method termed "flat fielding." An interleaved utilization of image segmentation and flat fielding was repeated until a convergence criterion was satisfied. Mathematical and physical phantom studies were conducted to evaluate the dependence of the proposed algorithm on breast size and the distribution of fibroglandular tissue. In addition, a subset of patient scans (not used in the CNN training, testing or validation) were used to investigate the accuracy of the IC method across different scanner designs and beam qualities.

RESULTS

The IC method resulted in an accurate classification of different tissue types with an average Dice similarity coefficient > 95%, precision > 97%, recall > 95%, and F1-score > 96% across all tissue types. The flat fielding correction of bCT images resulted in a significant reduction in either cupping or capping artifacts in both mathematical and physical phantom studies as measured by the integral nonuniformity metric with an average reduction of 71% for cupping and 30% for capping across different phantom sizes, and the Uniformity Index with an average reduction of 53% for cupping and 34% for capping.

CONCLUSION

The validation studies demonstrated that the IC method improves Hounsfield Units (HU) accuracy and effectively corrects for shading artifacts caused by scatter contamination and beam hardening. The postprocessing approach described herein is relevant to the broad scope of bCT devices and does not require any modification in hardware or existing scan protocols. The trained CNN parameters and network architecture are available for interested users.

A dedicated breast-PET/CT scanner: Evaluation of basic performance characteristics

PURPOSE

Application of advanced imaging techniques, such as PET and x ray CT, can potentially improve detection of breast cancer. Unfortunately, both modalities have challenges in the detection of some lesions. The combination of the two techniques, however, could potentially lead to an overall improvement in diagnostic breast imaging. The purpose of this investigation is to test the basic performance of a new dedicated breast-PET/CT.

METHODS

The PET component consists of a rotating pair of detectors. Its performance was evaluated using the NEMA NU4-2008 protocols. The CT component utilizes a pulsed x ray source and flat panel detector mounted on the same gantry as the PET scanner. Its performance was assessed using specialized phantoms. The radiation dose to a breast during CT imaging was explored by the measurement of free-in-air kerma and air kerma measured at the center of a 16 cm-diameter PMMA cylinder. Finally, the combined capabilities of the system were demonstrated by imaging of a micro-hot-rod phantom.

RESULTS

Overall, performance of the PET component is comparable to many pre-clinical and other dedicated breast-PET scanners. Its spatial resolution is 2.2 mm, 5 mm from the center of the scanner using images created with the single-sliced-filtered-backprojection algorithm. Peak NECR is 24.6 kcps; peak sensitivity is 1.36%; the scatter fraction is 27%. Spatial resolution of the CT scanner is 1.1 lp/mm at 10% MTF. The free-in-air kerma is 2.33 mGy, while the PMMA-air kerma is 1.24 mGy. Finally, combined imaging of a micro-hot-rod phantom illustrated the potential utility of the dual-modality images produced by the system.

CONCLUSION

The basic performance characteristics of a new dedicated breast-PET/CT scanner are good, demonstrating that its performance is similar to current dedicated PET and CT scanners. The potential value of this system is the capability to produce combined duality-modality images that could improve detection of breast disease. The next stage in development of this system is testing with more advanced phantoms and human subjects.

The role of off-focus radiation in scatter correction for dedicated cone beam breast CT

PURPOSE

Dedicated cone beam breast CT (CBBCT) suffers from x-ray scatter contamination. We aim to identify the source of the significant difference between the scatter distributions estimated by two recent methods proposed by our group and to investigate its effect on CBBCT image quality.

METHOD

We recently proposed two novel methods of scatter correction for CBBCT, using a library based (LB) technique and a forward projection (FP) model. Despite similar enhancement on CBBCT image qualities, these two methods obtain very different scatter distributions. We hypothesize that the off-focus radiation (OFR) is the contributor and results in nontrivial signals in x-ray projections, which is ignored in the scatter estimation via the LB method. Experiments using a thin wire test tool are designed to study the effect of OFR on CBBCT spatial resolution by measuring the point spread function (PSF) and the modulation transfer function (MTF). A narrow collimator setting is used to suppress the OFR-induced signals. In addition, "PSFs" and "MTFs" are measured on clinical CBBCT images obtained by the LB and FP methods using small calcifications as point sources. The improvement of spatial resolution achieved by suppressing OFR in the wire experiment as well as in the clinical study is quantified by the improvement ratios of PSFs and spatial frequencies at different MTF values. Our hypothesis that OFR causes the imaging difference between the FP and LB methods is verified if these ratios obtained from experimental and clinical data are consistent.

RESULTS

In the wire experiment, the results show that suppression of OFR increases the maximum signal of the PSF by about 14% and reduces the full-width-at-half-maximum (FWHM) by about 12.0%. Similar improvement on spatial resolution is achieved by the FP method compared with the LB method in the patient study. The improvement ratios of spatial frequencies at different MTF values without OFR match very well in both studies at a level of around 16%, with an average root-mean-square difference of 0.47%.

CONCLUSION

The results of the wire experiment and the clinical study indicate that the main difference between the LB and FP methods is whether the OFR-induced signals are included after scatter correction. Our study further shows that OFR significantly affects the image spatial resolution of CBBCT, indicating that the visualization of micro-calcifications is susceptible to OFR contamination. Our finding is therefore important in further improvement of diagnostic performance of CBBCT.

Transmission characteristics of a two dimensional antiscatter grid prototype for CBCT

AIM

High fraction of scattered radiation in cone-beam CT (CBCT) imaging degrades CT number accuracy and visualization of low contrast objects. To suppress scatter in CBCT projections, we developed a focused, two-dimensional antiscatter grid (2DASG) prototype. In this work, we report on the primary and scatter transmission characteristics of the 2DASG prototype aimed for linac mounted, offset detector geometry CBCT systems in radiation therapy, and compared its performance to a conventional one-dimensional ASG (1DASG).

METHODS

The 2DASG is an array of through-holes separated by 0.1 mm septa that was fabricated from tungsten using additive manufacturing techniques. Through-holes' focusing geometry was designed for offset detector CBCT in Varian TrueBeam system. Two types of ASGs were evaluated: (a) a conventional 1DASG with a grid ratio of 10, (b) the 2DASG prototype with a grid ratio of 8.2. To assess the scatter suppression performance of both ASGs, Scatter-to-primary ratio (SPR) and scatter transmission fraction (Ts) were measured using the beam stop method. Scatter and primary intensities were modulated by varying the phantom thickness between 10 and 40 cm. Additionally, the effect of air gap and bow tie (BT) filter on SPR and Ts were evaluated. Average primary transmission fraction (T_P ) and pixel specific primary transmission were also measured for both ASGs. To assess the effect of transmission characteristics on projection image signal-to-noise ratio (SNR), SNR improvement factor was calculated. Improvement in contrast to noise ratio (CNR) was demonstrated using a low contrast object.

RESULTS

In comparison to 1DASG, 2DASG reduced SPRs by a factor of 3 to 6 across the range of phantom setups investigated. Ts values for 1D and 2DASGs were in the range of 21 to 29%, and 5 to 14% respectively. 2DASG continued to provide lower SPR and Ts at increased air gap and with BT filter. Tp of 1D and 2DASGs were 70.6% and 84.7% respectively. Due to the septal shadow of the 2DASG, its pixel specific primary transmission values varied between 32.5% and 99.1%. With respect to 1DASG, 2DASG provided up to factor of 1.7 more improvement in SNR across the SPR range investigated. Moreover, 2DASG provided improved visualization of low contrast objects with respect to 1DASG and NOASG setups.

CONCLUSIONS

When compared to a conventional 1DASG, 2DASG prototype provided noticeably lower SPR and Ts values, indicating its superior scatter suppression performance. 2DASG also provided 19% higher average primary transmission that was attributed to the absence of interseptal spacers and optimized grid geometry. Our results indicate that the combined effect of lower scatter and higher primary transmission provided by 2DASG may potentially translate into more accurate CT numbers and improved contrast resolution in CBCT images.

X-ray scatter correction for dedicated cone beam breast CT using a forward-projection model

PURPOSE

The quality of dedicated cone-beam breast CT (CBBCT) imaging is fundamentally limited by x-ray scatter contamination due to the large irradiation volume. In this paper, we propose a scatter correction method for CBBCT using a novel forward-projection model with high correction efficacy and reliability.

METHOD

We first coarsely segment the uncorrected, first-pass, reconstructed CBBCT images into binary-object maps and assign the segmented fibroglandular and adipose tissue with the correct attenuation coefficients based on the mean x-ray energy. The modified CBBCT are treated as the prior images toward scatter correction. Primary signals are first estimated via forward projection on the modified CBBCT. To avoid errors caused by inaccurate segmentation, only sparse samples of estimated primary are selected for scatter estimation. A Fourier-Transform based algorithm, herein referred to as local filtration hereafter, is developed to efficiently estimate the global scatter distribution on the detector. The scatter-corrected images are obtained by removing the estimated scatter distribution from measured projection data.

RESULTS

We evaluate the method performance on six patients with different breast sizes and shapes representing the general population. The results show that the proposed method effectively reduces the image spatial non-uniformity from 8.27 to 1.91% for coronal views and from 6.50 to 3.00% for sagittal views. The contrast-to-deviation ratio is improved by an average factor of 1.41. Comparisons on the image details reveal that the proposed scatter correction successfully preserves fine structures of fibroglandular tissues that are lost in the segmentation process.

CONCLUSION

We propose a highly practical and efficient scatter correction algorithm for CBBCT via a forward-projection model. The method is attractive in clinical CBBCT imaging as it is readily implementable on a clinical system without modifications in current imaging protocols or system hardware.

Characterization of X-ray scattering for various phantoms and clinical breast geometries using breast CT on a dedicated hybrid system

OBJECTIVE

The purpose of this study was to utilize a dedicated breast CT system using a 2D beam stop array to physically evaluate the scatter to primary ratios (SPRs) of different geometric phantoms and prospectively acquired clinical patient data.

METHODS

Including clinically unrealizable compositions of 100% glandular and 100% fat, projection images were acquired using three geometrically different phantoms filled with fluids simulating breast tissue. The beam stop array method was used for measuring scatter in projection space, and creating the scatter corrected primary images. 2D SPRs were calculated. Additionally, a new figure of merit, the 3D normalized scatter contribution (NSC) volumes were calculated.

RESULTS

The 2D SPR values (0.52-1.10) were primarily dependent on phantom geometry; a secondary dependence was due to their uniform density; 2D SPRs were low frequency and smoothly varying in the uniformly filled phantoms. SPRs of clinical patient data followed similar trends as phantoms, but with noticeable deviations and high frequency components due to the heterogeneous distribution of glandular tissue. The maximum measured patient 2D SPRs were all <0.6, even for the largest diameter breast. These results demonstrate modest scatter components with changing object geometries and densities; the 3D NSC volumes with higher frequency components help visualize scatter distribution throughout the reconstructed image volumes. Furthermore, the SPRs in the heterogeneous clinical breast cases were underestimated by the equivalent density, uniformly filled phantoms.

CONCLUSIONS

These results provide guidance on the use of uniformly distributed density and differently shaped phantoms when considering simulations. They also clearly demonstrate that results from patients can vary considerably from 2D SPRs of uniformly simulated phantoms.

Library based x-ray scatter correction for dedicated cone beam breast CT

PURPOSE

The image quality of dedicated cone beam breast CT (CBBCT) is limited by substantial scatter contamination, resulting in cupping artifacts and contrast-loss in reconstructed images. Such effects obscure the visibility of soft-tissue lesions and calcifications, which hinders breast cancer detection and diagnosis. In this work, we propose a library-based software approach to suppress scatter on CBBCT images with high efficiency, accuracy, and reliability.

METHODS

The authors precompute a scatter library on simplified breast models with different sizes using the geant4-based Monte Carlo (MC) toolkit. The breast is approximated as a semiellipsoid with homogeneous glandular/adipose tissue mixture. For scatter correction on real clinical data, the authors estimate the breast size from a first-pass breast CT reconstruction and then select the corresponding scatter distribution from the library. The selected scatter distribution from simplified breast models is spatially translated to match the projection data from the clinical scan and is subtracted from the measured projection for effective scatter correction. The method performance was evaluated using 15 sets of patient data, with a wide range of breast sizes representing about 95% of general population. Spatial nonuniformity (SNU) and contrast to signal deviation ratio (CDR) were used as metrics for evaluation.

RESULTS

Since the time-consuming MC simulation for library generation is precomputed, the authors' method efficiently corrects for scatter with minimal processing time. Furthermore, the authors find that a scatter library on a simple breast model with only one input parameter, i.e., the breast diameter, sufficiently guarantees improvements in SNU and CDR. For the 15 clinical datasets, the authors' method reduces the average SNU from 7.14% to 2.47% in coronal views and from 10.14% to 3.02% in sagittal views. On average, the CDR is improved by a factor of 1.49 in coronal views and 2.12 in sagittal views.

CONCLUSIONS

The library-based scatter correction does not require increase in radiation dose or hardware modifications, and it improves over the existing methods on implementation simplicity and computational efficiency. As demonstrated through patient studies, the authors' approach is effective and stable, and is therefore clinically attractive for CBBCT imaging.

Bowtie filters for dedicated breast CT: theory and computational implementation

PURPOSE

To design bowtie filters with improved properties for dedicated breast CT to improve image quality and reduce dose to the patient.

METHODS

The authors present three different bowtie filters designed for a cylindrical 14-cm diameter phantom with a uniform composition of 40/60 breast tissue, which vary in their design objectives and performance improvements. Bowtie design #1 is based on single material spectral matching and produces nearly uniform spectral shape for radiation incident upon the detector. Bowtie design #2 uses the idea of basis material decomposition to produce the same spectral shape and intensity at the detector, using two different materials. Bowtie design #3 eliminates the beam hardening effect in the reconstructed image by adjusting the bowtie filter thickness so that the effective attenuation coefficient for every ray is the same. All three designs are obtained using analytical computational methods and linear attenuation coefficients. Thus, the designs do not take into account the effects of scatter. The authors considered this to be a reasonable approach to the filter design problem since the use of Monte Carlo methods would have been computationally intensive. The filter profiles for a cone-angle of 0° were used for the entire length of each filter because the differences between those profiles and the correct cone-beam profiles for the cone angles in our system are very small, and the constant profiles allowed construction of the filters with the facilities available to us. For evaluation of the filters, we used Monte Carlo simulation techniques and the full cone-beam geometry. Images were generated with and without each bowtie filter to analyze the effect on dose distribution, noise uniformity, and contrast-to-noise ratio (CNR) homogeneity. Line profiles through the reconstructed images generated from the simulated projection images were also used as validation for the filter designs.

RESULTS

Examples of the three designs are presented. Initial verification of performance of the designs was done using analytical computations of HVL, intensity, and effective attenuation coefficient behind the phantom as a function of fan-angle with a cone-angle of 0°. The performance of the designs depends only weakly on incident spectrum and tissue composition. For all designs, the dynamic range requirement on the detector was reduced compared to the no-bowtie-filter case. Further verification of the filter designs was achieved through analysis of reconstructed images from simulations. Simulation data also showed that the use of our bowtie filters can reduce peripheral dose to the breast by 61% and provide uniform noise and CNR distributions. The bowtie filter design concepts validated in this work were then used to create a computational realization of a 3D anthropomorphic bowtie filter capable of achieving a constant effective attenuation coefficient behind the entire field-of-view of an anthropomorphic breast phantom.

CONCLUSIONS

Three different bowtie filter designs that vary in performance improvements were described and evaluated using computational and simulation techniques. Results indicate that the designs are robust against variations in breast diameter, breast composition, and tube voltage, and that the use of these filters can reduce patient dose and improve image quality compared to the no-bowtie-filter case.

A post-reconstruction method to correct cupping artifacts in cone beam breast computed tomography

In cone beam breast computed tomography (CT), scattered radiation leads to nonuniform biasing of CT numbers known as a cupping artifact. Besides being visual distractions, cupping artifacts appear as background nonuniformities, which impair efficient gray scale windowing and pose a problem in threshold based volume visualization/segmentation. To overcome this problem, we have developed a background nonuniformity correction method specifically designed for cone beam breast CT. With this technique, the cupping artifact is modeled as an additive background signal profile in the reconstructed breast images. Due to the largely circularly symmetric shape of a typical breast, the additive background signal profile was also assumed to be circularly symmetric. The radial variation of the background signals was estimated by measuring the spatial variation of adipose tissue signals in front view breast images. To extract adipose tissue signals in an automated manner, a signal sampling scheme in polar coordinates and a background trend fitting algorithm were implemented. The background fits compared with targeted adipose tissue signal value (constant throughout the breast volume) to get an additive correction value for each tissue voxel. To test the accuracy, we applied the technique to cone beam CT images of mastectomy specimens. After correction, the images demonstrated significantly improved signal uniformity in both front and side view slices. The reduction of both intraslice and interslice variations in adipose tissue CT numbers supported our observations.

Effects on image quality of a 2D antiscatter grid in x-ray digital breast tomosynthesis: Initial experience using the dual modality (x-ray and molecular) breast tomosynthesis scanner

PURPOSE

Radiation scattered from the breast in digital breast tomosynthesis (DBT) causes image degradation, including loss of contrast between cancerous and background tissue. Unlike in 2-dimensional (2D) mammography, an antiscatter grid cannot readily be used in DBT because changing alignment between the tube and detector during the scan would result in unacceptable loss of primary radiation. However, in the dual modality breast tomosynthesis (DMT) scanner, which combines DBT and molecular breast tomosynthesis, the tube and detector rotate around a common axis, thereby maintaining a fixed tube-detector alignment. This C-arm geometry raises the possibility of using a 2D (cellular) focused antiscatter grid. The purpose of this study is to assess change in image quality when using an antiscatter grid in the DBT portion of a DMT scan under conditions of fixed radiation dose.

METHODS

Two 2D focused prototype grids with 80 cm focal length were tested, one stack-laminated from copper (Cu) and one cast from a tungsten-polymer (W-poly). They were reciprocated using a motion scheme designed to maximize transmission of primary x-ray photons. Grid-in and grid-out scatter-to-primary ratios (SPRs) were measured for rectangular blocks of material simulating 30%, 50%, and 70% glandular tissue compositions. For assessment of changes in image quality through the addition of a grid, the Computerized Imaging Reference Systems, Inc., phantom Model 011A containing a set of 1 cm thick blocks simulating a range of glandular/adipose ratios from 0/100 to 100/0 was used. To simulate 6.5 and 8.5 cm thick compressed breasts, 1 cm thick slices of PMMA were added to the Model 011A phantom. DBT images were obtained with and without the grid, with exposure parameters fixed for a given compressed thickness. Signal-difference-to-noise ratios (SDNRs), contrast, and voxel value-based attenuation coefficients (μ) were measured for all blocks from reconstructed phantom images.

RESULTS

For 4, 6, and 8 cm tissue-equivalent block phantom thicknesses, the inclusion of the W-poly grid reduced the SPR by factors of 5, 6, and 5.8, respectively. For the same thicknesses, the copper grid reduced the SPR by factors of 3.9, 4.5, and 4.9. For the 011A phantom, the W-poly grid raised the SDNR of the 70/30 block from 0.8, -0.32, and -0.72 to 0.9, 0.76, and 0.062 for the 4.5, 6.5, and 8.5 cm phantoms, respectively. It raised the SDNR of the 100/0 block from 3.78, 1.95, and 1.0 to 3.79, 3.67, and 3.25 for the 4.5, 6.5, and 8.5 cm phantoms, respectively. Inclusion of the W-poly grid improved the accuracy of image-based μ values for all block compositions. However, smearing of attenuation across slices due to limited angular sampling decreases the sensitivity of voxel values to changing composition compared to theoretical μ values.

CONCLUSIONS

Under conditions of fixed radiation dose to the breast, use of a 2D focused grid increased contrast, SDNR, and accuracy of estimated attenuation for mass-simulating block compositions in all phantom thicknesses tested, with the degree of improvement depending upon material composition. A 2D antiscatter grid can be usefully incorporated in DBT systems that employ fully isocentric tube-detector rotation.

A Novel Scatter Correction Method for Cone-Beam Computed Tomography

In this article, a novel scatter correction approach was proposed based on the Klein-Nishina formulation. Through a series of deductions from this formulation, a principle was proposed that the photon intensity distribution was determined by the attenuation coefficient μ and the path length l. This means if 2 pencil beams pass through 2 objects with the same μl, even if the attenuation coefficient μ and the path length l of the objects are different, they will still achieve the same photon intensity distribution, that is, the same point spread function. Subsequently, a novel scatter correction approach was established after a series of deductions based on this principle. The simulations and experiments demonstrated the correctness of our principle and the comparable correction effect of our scatter correction approach compared with the beam stop array method. Furthermore, because of the character of our method, the program has very high parallel computing features, which can dramatically increase the computation speed.

X-ray scatter correction method for dedicated breast computed tomography: improvements and initial patient testing

A previously proposed x-ray scatter correction method for dedicated breast computed tomography was further developed and implemented so as to allow for initial patient testing. The method involves the acquisition of a complete second set of breast CT projections covering 360° with a perforated tungsten plate in the path of the x-ray beam. To make patient testing feasible, a wirelessly controlled electronic positioner for the tungsten plate was designed and added to a breast CT system. Other improvements to the algorithm were implemented, including automated exclusion of non-valid primary estimate points and the use of a different approximation method to estimate the full scatter signal. To evaluate the effectiveness of the algorithm, evaluation of the resulting image quality was performed with a breast phantom and with nine patient images. The improvements in the algorithm resulted in the avoidance of introduction of artifacts, especially at the object borders, which was an issue in the previous implementation in some cases. Both contrast, in terms of signal difference and signal difference-to-noise ratio were improved with the proposed method, as opposed to with the correction algorithm incorporated in the system, which does not recover contrast. Patient image evaluation also showed enhanced contrast, better cupping correction, and more consistent voxel values for the different tissues. The algorithm also reduces artifacts present in reconstructions of non-regularly shaped breasts. With the implemented hardware and software improvements, the proposed method can be reliably used during patient breast CT imaging, resulting in improvement of image quality, no introduction of artifacts, and in some cases reduction of artifacts already present. The impact of the algorithm on actual clinical performance for detection, diagnosis and other clinical tasks in breast imaging remains to be evaluated.

A general method for cupping artifact correction of cone-beam breast computed tomography images

PURPOSE

Cone-beam breast computed tomography (CBBCT), a promising breast cancer diagnostic technique, has been under investigation for the past decade. However, owing to scattered radiation and beam hardening, CT numbers are not uniform on CBBCT images. This is known as cupping artifact, and it presents an obstacle for threshold-based volume segmentation. In this study, we proposed a general post-reconstruction method for cupping artifact correction.

METHODS

There were four steps in the proposed method. First, three types of local region histogram peaks were calculated: adipose peaks with low CT numbers, glandular peaks with high CT numbers, and unidentified peaks. Second, a linear discriminant analysis classifier, which was trained by identified adipose and glandular peaks, was employed to identify the unidentified peaks as adipose or glandular peaks. Third, adipose background signal profile was fitted according to the adipose peaks using the least squares method. Finally, the adipose background signal profile was subtracted from original image to obtain cupping corrected image

RESULTS

In experimental study, standard deviation of adipose tissue CT numbers was obviously reduced and the CT numbers were more uniform after cupping correction by proposed method; in simulation study, root-mean-square errors were significantly reduced for both symmetric and asymmetric cupping artifacts, indicating that the proposed method was effective to both artifacts.

CONCLUSIONS

A general method without a circularly symmetric assumption was proposed to correct cupping artifacts in CBBCT images for breast. It may be properly applied to images of real patient breasts with natural pendent geometry.

Bowtie filters for dedicated breast CT: Analysis of bowtie filter material selection

PURPOSE

For a given bowtie filter design, both the selection of material and the physical design control the energy fluence, and consequently the dose distribution, in the object. Using three previously described bowtie filter designs, the goal of this work is to demonstrate the effect that different materials have on the bowtie filter performance measures.

METHODS

Three bowtie filter designs that compensate for one or more aspects of the beam-modifying effects due to the differences in path length in a projection have been designed. The nature of the designs allows for their realization using a variety of materials. The designs were based on a phantom, 14 cm in diameter, composed of 40% fibroglandular and 60% adipose tissue. Bowtie design #1 is based on single material spectral matching and produces nearly uniform spectral shape for radiation incident upon the detector. Bowtie design #2 uses the idea of basis-material decomposition to produce the same spectral shape and intensity at the detector, using two different materials. With bowtie design #3, it is possible to eliminate the beam hardening effect in the reconstructed image by adjusting the bowtie filter thickness so that the effective attenuation coefficient for every ray is the same. Seven different materials were chosen to represent a range of chemical compositions and densities. After calculation of construction parameters for each bowtie filter design, a bowtie filter was created using each of these materials (assuming reasonable construction parameters were obtained), resulting in a total of 26 bowtie filters modeled analytically and in the penelope Monte Carlo simulation environment. Using the analytical model of each bowtie filter, design profiles were obtained and energy fluence as a function of fan-angle was calculated. Projection images with and without each bowtie filter design were also generated using penelope and reconstructed using FBP. Parameters such as dose distribution, noise uniformity, and scatter were investigated.

RESULTS

Analytical calculations with and without each bowtie filter show that some materials for a given design produce bowtie filters that are too large for implementation in breast CT scanners or too small to accurately manufacture. Results also demonstrate the ability to manipulate the energy fluence distribution (dynamic range) by using different materials, or different combinations of materials, for a given bowtie filter design. This feature is especially advantageous when using photon counting detector technology. Monte Carlo simulation results from penelope show that all studied material choices for bowtie design #2 achieve nearly uniform dose distribution, noise uniformity index less than 5%, and nearly uniform scatter-to-primary ratio. These same features can also be obtained using certain materials with bowtie designs #1 and #3.

CONCLUSIONS

With the three bowtie filter designs used in this work, the selection of material is an important design consideration. An appropriate material choice can improve image quality, dose uniformity, and dynamic range.

Radiation doses in volume-of-interest breast computed tomography--A Monte Carlo simulation study

PURPOSE

Cone beam breast computed tomography (breast CT) with true three-dimensional, nearly isotropic spatial resolution has been developed and investigated over the past decade to overcome the problem of lesions overlapping with breast anatomical structures on two-dimensional mammographic images. However, the ability of breast CT to detect small objects, such as tissue structure edges and small calcifications, is limited. To resolve this problem, the authors proposed and developed a volume-of-interest (VOI) breast CT technique to image a small VOI using a higher radiation dose to improve that region's visibility. In this study, the authors performed Monte Carlo simulations to estimate average breast dose and average glandular dose (AGD) for the VOI breast CT technique.

METHODS

Electron-Gamma-Shower system code-based Monte Carlo codes were used to simulate breast CT. The Monte Carlo codes estimated were validated using physical measurements of air kerma ratios and point doses in phantoms with an ion chamber and optically stimulated luminescence dosimeters. The validated full cone x-ray source was then collimated to simulate half cone beam x-rays to image digital pendant-geometry, hemi-ellipsoidal, homogeneous breast phantoms and to estimate breast doses with full field scans. 13-cm in diameter, 10-cm long hemi-ellipsoidal homogeneous phantoms were used to simulate median breasts. Breast compositions of 25% and 50% volumetric glandular fractions (VGFs) were used to investigate the influence on breast dose. The simulated half cone beam x-rays were then collimated to a narrow x-ray beam with an area of 2.5 × 2.5 cm(2) field of view at the isocenter plane and to perform VOI field scans. The Monte Carlo results for the full field scans and the VOI field scans were then used to estimate the AGD for the VOI breast CT technique.

RESULTS

The ratios of air kerma ratios and dose measurement results from the Monte Carlo simulation to those from the physical measurements were 0.97 ± 0.03 and 1.10 ± 0.13, respectively, indicating that the accuracy of the Monte Carlo simulation was adequate. The normalized AGD with VOI field scans was substantially reduced by a factor of about 2 over the VOI region and by a factor of 18 over the entire breast for both 25% and 50% VGF simulated breasts compared with the normalized AGD with full field scans. The normalized AGD for the VOI breast CT technique can be kept the same as or lower than that for a full field scan with the exposure level for the VOI field scan increased by a factor of as much as 12.

CONCLUSIONS

The authors' Monte Carlo estimates of normalized AGDs for the VOI breast CT technique show that this technique can be used to markedly increase the dose to the breast and thus the visibility of the VOI region without increasing the dose to the breast. The results of this investigation should be helpful for those interested in using VOI breast CT technique to image small calcifications with dose concern.

High fidelity system modeling for high quality image reconstruction in clinical CT

Today, while many researchers focus on the improvement of the regularization term in IR algorithms, they pay less concern to the improvement of the fidelity term. In this paper, we hypothesize that improving the fidelity term will further improve IR image quality in low-dose scanning, which typically causes more noise. The purpose of this paper is to systematically test and examine the role of high-fidelity system models using raw data in the performance of iterative image reconstruction approach minimizing energy functional. We first isolated the fidelity term and analyzed the importance of using focal spot area modeling, flying focal spot location modeling, and active detector area modeling as opposed to just flying focal spot motion. We then compared images using different permutations of all three factors. Next, we tested the ability of the fidelity terms to retain signals upon application of the regularization term with all three factors. We then compared the differences between images generated by the proposed method and Filtered-Back-Projection. Lastly, we compared images of low-dose in vivo data using Filtered-Back-Projection, Iterative Reconstruction in Image Space, and the proposed method using raw data. The initial comparison of difference maps of images constructed showed that the focal spot area model and the active detector area model also have significant impacts on the quality of images produced. Upon application of the regularization term, images generated using all three factors were able to substantially decrease model mismatch error, artifacts, and noise. When the images generated by the proposed method were tested, conspicuity greatly increased, noise standard deviation decreased by 90% in homogeneous regions, and resolution also greatly improved. In conclusion, the improvement of the fidelity term to model clinical scanners is essential to generating higher quality images in low-dose imaging.

A breast-specific, negligible-dose scatter correction technique for dedicated cone-beam breast CT: a physics-based approach to improve Hounsfield Unit accuracy

The purpose of this research was to develop a method to correct the cupping artifact caused from x-ray scattering and to achieve consistent Hounsfield Unit (HU) values of breast tissues for a dedicated breast CT (bCT) system. The use of a beam passing array (BPA) composed of parallel-holes has been previously proposed for scatter correction in various imaging applications. In this study, we first verified the efficacy and accuracy using BPA to measure the scatter signal on a cone-beam bCT system. A systematic scatter correction approach was then developed by modeling the scatter-to-primary ratio (SPR) in projection images acquired with and without BPA. To quantitatively evaluate the improved accuracy of HU values, different breast tissue-equivalent phantoms were scanned and radially averaged HU profiles through reconstructed planes were evaluated. The dependency of the correction method on object size and number of projections was studied. A simplified application of the proposed method on five clinical patient scans was performed to demonstrate efficacy. For the typical 10-18 cm breast diameters seen in the bCT application, the proposed method can effectively correct for the cupping artifact and reduce the variation of HU values of breast equivalent material from 150 to 40 HU. The measured HU values of 100% glandular tissue, 50/50 glandular/adipose tissue, and 100% adipose tissue were approximately 46, -35, and -94, respectively. It was found that only six BPA projections were necessary to accurately implement this method, and the additional dose requirement is less than 1% of the exam dose. The proposed method can effectively correct for the cupping artifact caused from x-ray scattering and retain consistent HU values of breast tissues.

Experimental validation of a single shaped filter approach for CT using variable source-to-filter distance for examination of arbitrary object diameters

The purpose of this study was to validate the use of a single shaped filter (SF) for computed tomography (CT) using variable source-to-filter distance (SFD) for the examination of different object diameters.A SF was designed by performing simulations with the purpose of achieving noise homogeneity in the reconstructed volume and dose reduction for arbitrary phantom diameters. This was accomplished by using a filter design method thats target is to achieve a homogeneous detector noise, but also uses a correction factor for the filtered back projection process. According to simulation results, a single SF designed for one of the largest phantom diameters meets the requirements for all diameters when SFD can be adjusted. To validate these results, a SF made of aluminium alloy was manufactured. Measurements were performed on a CT scanner with polymethyl methacrylate (PMMA) phantoms of diameters from 40-100 mm. The filter was positioned at SFDs ranging from 97-168 mm depending on the phantom diameter. Image quality was evaluated for the reconstructed volume by assessing CT value accuracy, noise homogeneity, contrast-to-noise ratio weighted by dose (CNRD) and spatial resolution. Furthermore, scatter distribution was determined with the use of a beam-stop phantom. Dose was measured for a PMMA phantom with a diameter of 100 mm using a calibrated ionization chamber.The application of a single SF at variable SFD led to improved noise uniformity and dose reduction: noise homogeneity was improved from 15% down to about 0%, and dose was reduced by about 37%. Furthermore, scatter dropped by about 32%, which led to reduced cupping artifacts and improved CT value accuracy. Spatial resolution and CNRD was not affected by the SF.By means of a single SF with variable SFD designed for CT, significant dose reduction can be achieved and image quality can be improved by reducing noise inhomogeneity as well as scatter-induced artifacts.

Characterization of scatter magnitude and distribution in dedicated breast computed tomography with bowtie filters

Scatter contamination of projection images in cone-beam computed tomography (CT) degrades the image quality. The use of bowtie filters in dedicated breast CT can decrease this scatter contribution. Three bowtie filter designs that compensate for one or more aspects of the beam-modifying effects due to differences in path length in a projection were studied. These designs have been investigated in terms of their ability to reduce the scatter contamination in projection images acquired in a dedicated breast CT geometry. The scatter magnitude was measured as the scatter-to-primary ratio (SPR) using experimental and Monte Carlo techniques for various breast phantom diameters and tube voltages. The results show that a 55% reduction in the center SPR value could be obtained with the bowtie filter designs. On average, the bowtie filters reduced the center SPR by approximately 18% over all breast diameters. The distribution of the scatter was calculated at a range of different locations to produce scatter distribution maps for all three bowtie filter designs. With the inclusion of the bowtie filters, the scatter distribution was more uniform for all breast diameters. The results of this study will be useful in designing scatter correction methods and understanding the benefits of bowtie filters in dedicated breast CT.

Investigation of iterative image reconstruction in low-dose breast CT

There is interest in developing computed tomography (CT) dedicated to breast-cancer imaging. Because breast tissues are radiation-sensitive, the total radiation exposure in a breast-CT scan is kept low, often comparable to a typical two-view mammography exam, thus resulting in a challenging low-dose-data-reconstruction problem. In recent years, evidence has been found that suggests that iterative reconstruction may yield images of improved quality from low-dose data. In this work, based upon the constrained image total-variation minimization program and its numerical solver, i.e., the adaptive steepest descent-projection onto the convex set (ASD-POCS), we investigate and evaluate iterative image reconstructions from low-dose breast-CT data of patients, with a focus on identifying and determining key reconstruction parameters, devising surrogate utility metrics for characterizing reconstruction quality, and tailoring the program and ASD-POCS to the specific reconstruction task under consideration. The ASD-POCS reconstructions appear to outperform the corresponding clinical FDK reconstructions, in terms of subjective visualization and surrogate utility metrics.

Power spectrum analysis of the x-ray scatter signal in mammography and breast tomosynthesis projections

PURPOSE

To analyze the frequency domain characteristics of the signal in mammography images and breast tomosynthesis projections with patient tissue texture due to detected scattered x-rays.

METHODS

Acquisitions of x-ray projection images of 19 different patient breasts were simulated using previously acquired volumetric patient images. Acquisition of these images was performed with a dedicated breast CT prototype system, and the images were classified into voxels representing skin, adipose, and glandular tissue with a previously validated automated algorithm. The classified three dimensional images then underwent simulated mechanical compression representing that which is performed during acquisition of mammography and breast tomosynthesis images. The acquisition of projection images of each patient breast was simulated using Monte Carlo methods with each simulation resulting in two images: one of the primary (non-scattered) signal and one of the scatter signal. To analyze the scatter signal for both mammography and breast tomosynthesis, two projections images of each patient breast were simulated, one with the x-ray source positioned at 0° (mammography and central tomosynthesis projection) and at 30° (wide tomosynthesis projection). The noise power spectra (NPS) for both the scatter signal alone and the total signal (primary + scatter) for all images were obtained and the combined results of all patients analyzed. The total NPS was fit to the expected power-law relationship NPS(f) = k/f β and the results were compared with those previously published on the power spectrum characteristics of mammographic texture. The scatter signal alone was analyzed qualitatively and a power-law fit was also performed.

RESULTS

The mammography and tomosynthesis projections of three patient breasts were too small to analyze, so a total of 16 patient breasts were analyzed. The values of β for the total signal of the 0° projections agreed well with previously published results. As expected, the scatter power spectrum reflected a fast drop-off with increasing spatial frequency, with a reduction of four orders of magnitude by 0.1 lp/mm. The β values for the scatter signal were 6.14 and 6.39 for the 0° and 30° projections, respectively.

CONCLUSIONS

Although the low-frequency characteristics of scatter in mammography and breast tomosynthesis were known, a quantitative analysis of the frequency domain characteristics of this signal was needed in order to optimize previously proposed software-based x-ray scatter reduction algorithms for these imaging modalities.

Photon counting spectral breast CT: effect of adaptive filtration on CT numbers, noise, and contrast to noise ratio

PURPOSE

Photon counting spectral (PCS) computed tomography (CT) shows promise for breast imaging. An issue with current photon-counting detectors is low count rate capabilities, artifacts resulting from nonuniform count rate across the field of view, and suboptimal spectral information. These issues are addressed in part by using tissue-equivalent adaptive filtration of the x-ray beam. The purpose of the study was to investigate the effect of adaptive filtration on different aspects of PCS breast CT.

METHODS

The theoretical formulation for the filter shape was derived for different filter materials and evaluated by simulation and an experimental prototype of the filter was fabricated from a tissue-like material (acrylic). The PCS CT images of a glandular breast phantom with adipose and iodine contrast elements were simulated at 40, 60, 90, and 120 kVp tube voltages, with and without adaptive filter. The CT numbers, CT noise, and contrast-to-noise ratio (CNR) were compared for spectral CT images acquired with and without adaptive filters. Similar comparison was made for material-decomposed PCS CT images.

RESULTS

The adaptive filter improved the uniformity of CT numbers, CT noise, and CNR in both ordinary and material decomposed PCS CT images. At the same tube output the average CT noise with adaptive filter, although uniform, was higher than the average noise without adaptive filter due to x-ray absorption by the filter. Increasing tube output, so that average skin exposure with the adaptive filter was same as without filter, made the noise with adaptive filter comparable to or lower than that without adaptive filter. Similar effects were observed when energy weighting was applied, and when material decompositions were performed using energy selective CT data.

CONCLUSIONS

An adaptive filter decreases count rate requirements to the photon counting detectors which enables PCS breast CT based on commercially available detector technologies. Adaptive filter also improves image quality in PCS breast CT by decreasing beam hardening artifacts and by eliminating spatial nonuniformities of CT numbers, noise, and CNR.

Modelling the physics in the iterative reconstruction for transmission computed tomography

There is an increasing interest in iterative reconstruction (IR) as a key tool to improve quality and increase applicability of x-ray CT imaging. IR has the ability to significantly reduce patient dose; it provides the flexibility to reconstruct images from arbitrary x-ray system geometries and allows one to include detailed models of photon transport and detection physics to accurately correct for a wide variety of image degrading effects. This paper reviews discretization issues and modelling of finite spatial resolution, Compton scatter in the scanned object, data noise and the energy spectrum. The widespread implementation of IR with a highly accurate model-based correction, however, still requires significant effort. In addition, new hardware will provide new opportunities and challenges to improve CT with new modelling.

Modelling the physics in the iterative reconstruction for transmission computed tomography

There is an increasing interest in iterative reconstruction (IR) as a key tool to improve quality and increase applicability of x-ray CT imaging. IR has the ability to significantly reduce patient dose; it provides the flexibility to reconstruct images from arbitrary x-ray system geometries and allows one to include detailed models of photon transport and detection physics to accurately correct for a wide variety of image degrading effects. This paper reviews discretization issues and modelling of finite spatial resolution, Compton scatter in the scanned object, data noise and the energy spectrum. The widespread implementation of IR with a highly accurate model-based correction, however, still requires significant effort. In addition, new hardware will provide new opportunities and challenges to improve CT with new modelling.

Monte Carlo study of the effects of system geometry and antiscatter grids on cone-beam CT scatter distributions

PURPOSE

The proliferation of cone-beam CT (CBCT) has created interest in performance optimization, with x-ray scatter identified among the main limitations to image quality. CBCT often contends with elevated scatter, but the wide variety of imaging geometry in different CBCT configurations suggests that not all configurations are affected to the same extent. Graphics processing unit (GPU) accelerated Monte Carlo (MC) simulations are employed over a range of imaging geometries to elucidate the factors governing scatter characteristics, efficacy of antiscatter grids, guide system design, and augment development of scatter correction.

METHODS

A MC x-ray simulator implemented on GPU was accelerated by inclusion of variance reduction techniques (interaction splitting, forced scattering, and forced detection) and extended to include x-ray spectra and analytical models of antiscatter grids and flat-panel detectors. The simulator was applied to small animal (SA), musculoskeletal (MSK) extremity, otolaryngology (Head), breast, interventional C-arm, and on-board (kilovoltage) linear accelerator (Linac) imaging, with an axis-to-detector distance (ADD) of 5, 12, 22, 32, 60, and 50 cm, respectively. Each configuration was modeled with and without an antiscatter grid and with (i) an elliptical cylinder varying 70-280 mm in major axis; and (ii) digital murine and anthropomorphic models. The effects of scatter were evaluated in terms of the angular distribution of scatter incident upon the detector, scatter-to-primary ratio (SPR), artifact magnitude, contrast, contrast-to-noise ratio (CNR), and visual assessment.

RESULTS

Variance reduction yielded improvements in MC simulation efficiency ranging from ∼17-fold (for SA CBCT) to ∼35-fold (for Head and C-arm), with the most significant acceleration due to interaction splitting (∼6 to ∼10-fold increase in efficiency). The benefit of a more extended geometry was evident by virtue of a larger air gap-e.g., for a 16 cm diameter object, the SPR reduced from 1.5 for ADD = 12 cm (MSK geometry) to 1.1 for ADD = 22 cm (Head) and to 0.5 for ADD = 60 cm (C-arm). Grid efficiency was higher for configurations with shorter air gap due to a broader angular distribution of scattered photons-e.g., scatter rejection factor ∼0.8 for MSK geometry versus ∼0.65 for C-arm. Grids reduced cupping for all configurations but had limited improvement on scatter-induced streaks and resulted in a loss of CNR for the SA, Breast, and C-arm. Relative contribution of forward-directed scatter increased with a grid (e.g., Rayleigh scatter fraction increasing from ∼0.15 without a grid to ∼0.25 with a grid for the MSK configuration), resulting in scatter distributions with greater spatial variation (the form of which depended on grid orientation).

CONCLUSIONS

A fast MC simulator combining GPU acceleration with variance reduction provided a systematic examination of a range of CBCT configurations in relation to scatter, highlighting the magnitude and spatial uniformity of individual scatter components, illustrating tradeoffs in CNR and artifacts and identifying the system geometries for which grids are more beneficial (e.g., MSK) from those in which an extended geometry is the better defense (e.g., C-arm head imaging). Compact geometries with an antiscatter grid challenge assumptions of slowly varying scatter distributions due to increased contribution of Rayleigh scatter.

Cupping artifact correction and automated classification for high-resolution dedicated breast CT images

PURPOSE

To develop and test an automated algorithm to classify the different tissues present in dedicated breast CT images.

METHODS

The original CT images are first corrected to overcome cupping artifacts, and then a multiscale bilateral filter is used to reduce noise while keeping edge information on the images. As skin and glandular tissues have similar CT values on breast CT images, morphologic processing is used to identify the skin mask based on its position information. A modified fuzzy C-means (FCM) classification method is then used to classify breast tissue as fat and glandular tissue. By combining the results of the skin mask with the FCM, the breast tissue is classified as skin, fat, and glandular tissue. To evaluate the authors' classification method, the authors use Dice overlap ratios to compare the results of the automated classification to those obtained by manual segmentation on eight patient images.

RESULTS

The correction method was able to correct the cupping artifacts and improve the quality of the breast CT images. For glandular tissue, the overlap ratios between the authors' automatic classification and manual segmentation were 91.6% ± 2.0%.

CONCLUSIONS

A cupping artifact correction method and an automatic classification method were applied and evaluated for high-resolution dedicated breast CT images. Breast tissue classification can provide quantitative measurements regarding breast composition, density, and tissue distribution.

Characterization of the homogeneous tissue mixture approximation in breast imaging dosimetry

PURPOSE

To compare the estimate of normalized glandular dose in mammography and breast CT imaging obtained using the actual glandular tissue distribution in the breast to that obtained using the homogeneous tissue mixture approximation.

METHODS

Twenty volumetric images of patient breasts were acquired with a dedicated breast CT prototype system and the voxels in the breast CT images were automatically classified into skin, adipose, and glandular tissue. The breasts in the classified images underwent simulated mechanical compression to mimic the conditions present during mammographic acquisition. The compressed thickness for each breast was set to that achieved during each patient's last screening cranio-caudal (CC) acquisition. The volumetric glandular density of each breast was computed using both the compressed and uncompressed classified images, and additional images were created in which all voxels representing adipose and glandular tissue were replaced by a homogeneous mixture of these two tissues in a proportion corresponding to each breast's volumetric glandular density. All four breast images (compressed and uncompressed; heterogeneous and homogeneous tissue) were input into Monte Carlo simulations to estimate the normalized glandular dose during mammography (compressed breasts) and dedicated breast CT (uncompressed breasts). For the mammography simulations the x-ray spectra used was that used during each patient's last screening CC acquisition. For the breast CT simulations, two x-ray spectra were used, corresponding to the x-ray spectra with the lowest and highest energies currently being used in dedicated breast CT prototype systems under clinical investigation. The resulting normalized glandular dose for the heterogeneous and homogeneous versions of each breast for each modality was compared.

RESULTS

For mammography, the normalized glandular dose based on the homogeneous tissue approximation was, on average, 27% higher than that estimated using the true heterogeneous glandular tissue distribution (Wilcoxon Signed Rank Test p = 0.00046). For dedicated breast CT, the overestimation of normalized glandular dose was, on average, 8% (49 kVp spectrum, p = 0.00045) and 4% (80 kVp spectrum, p = 0.000089). Only two cases in mammography and two cases in dedicated breast CT with a tube voltage of 49 kVp resulted in lower dose estimates for the homogeneous tissue approximation compared to the heterogeneous tissue distribution.

CONCLUSIONS

The normalized glandular dose based on the homogeneous tissue mixture approximation results in a significant overestimation of dose to the imaged breast. This overestimation impacts the use of dose estimates in absolute terms, such as for risk estimates, and may impact some comparative studies, such as when modalities or techniques with different x-ray energies are used. The error introduced by the homogeneous tissue mixture approximation in higher energy x-ray modalities, such as dedicated breast CT, although statistically significant, may not be of clinical concern. Further work is required to better characterize this overestimation and potentially develop new metrics or correction factors to better estimate the true glandular dose to breasts undergoing imaging with ionizing radiation.

Effect of slice thickness on detectability in breast CT using a prewhitened matched filter and simulated mass lesions

PURPOSE

Dedicated breast CT (bCT) is an emerging technology with the potential to improve the detection of breast cancer in screening and diagnostic capacities. Typically, the 3D volume reconstructed from the scanner is displayed as sectional images. The purpose of this study was to evaluate the effect of section thickness on the detectability of simulated masses using a prewhitened matched filter (PWMF) as a model observer.

METHODS

A breast CT scanner has been designed and fabricated in the authors' laboratory with more than 200 women imaged in IRB-approved phase I and phase II trials to date. Of these, 151 bilateral data sets were selected on the basis of low artifact content, sufficient breast coverage, and excluding cases with breast implants. BIRADS breast density ratings were available for 144 of these patients. Spherical mass lesions of diameter 1, 2, 3, 5, 11, and 15 mm were mathematically generated and embedded at random locations within the parenchymal region of each bCT volume. Microcalcifications were not simulated in this study. For each viewing plane (sagittal, axial, and coronal) and section thickness (ranging from 0.3 to 44 mm), section images of the breast parenchyma containing the lesion were generated from the reconstructed bCT data sets by averaging voxels over the length of the section. Using signal known exactly (SKE) model observer methodology, receiver operating characteristic (ROC) curve analysis was performed on each generated projected image using a PWMF based model observer. ROC curves were generated for each breast data set, and the area under the ROC curve (AUC) was evaluated as well as the sensitivity at 95% specificity.

RESULTS

For all lesion sizes, performance rises modestly to a peak before falling off substantially as section thickness increases over the range of the study. We find that the optimal section thickness tracks the size of the lesion to be detected linearly with a small positive offset and slopes ranging from 0.27 to 0.44. No significant differences were observed between left and right breasts. Performance measures are negatively correlated with measures of breast density, with an average correlation coefficient of -0.48 for the BIRADS breast density score and -0.81 for the proportion of glandular tissue in the breast interior.

CONCLUSIONS

This study shows quantitatively how PWMF detection performance of a known lesion size is influenced by section thickness in dedicated breast CT. While the optimal section thickness is tuned to the size of the lesion being detected, overall performance is more robust for thin section images compared to thicker images.

X-ray scatter correction method for dedicated breast computed tomography

PURPOSE

To improve image quality and accuracy in dedicated breast computed tomography (BCT) by removing the x-ray scatter signal included in the BCT projections.

METHODS

The previously characterized magnitude and distribution of x-ray scatter in BCT results in both cupping artifacts and reduction of contrast and accuracy in the reconstructions. In this study, an image processing method is proposed that estimates and subtracts the low-frequency x-ray scatter signal included in each BCT projection postacquisition and prereconstruction. The estimation of this signal is performed using simple additional hardware, one additional BCT projection acquisition with negligible radiation dose, and simple image processing software algorithms. The high frequency quantum noise due to the scatter signal is reduced using a noise filter postreconstruction. The dosimetric consequences and validity of the assumptions of this algorithm were determined using Monte Carlo simulations. The feasibility of this method was determined by imaging a breast phantom on a BCT clinical prototype and comparing the corrected reconstructions to the unprocessed reconstructions and to reconstructions obtained from fan-beam acquisitions as a reference standard. One-dimensional profiles of the reconstructions and objective image quality metrics were used to determine the impact of the algorithm.

RESULTS

The proposed additional acquisition results in negligible additional radiation dose to the imaged breast (∼0.4% of the standard BCT acquisition). The processed phantom reconstruction showed substantially reduced cupping artifacts, increased contrast between adipose and glandular tissue equivalents, higher voxel value accuracy, and no discernible blurring of high frequency features.

CONCLUSIONS

The proposed scatter correction method for dedicated breast CT is feasible and can result in highly improved image quality. Further optimization and testing, especially with patient images, is necessary to characterize its impact on clinical performance.

Fast patient-specific Monte Carlo brachytherapy dose calculations via the correlated sampling variance reduction technique

PURPOSE

To demonstrate potential of correlated sampling Monte Carlo (CMC) simulation to improve the calculation efficiency for permanent seed brachytherapy (PSB) implants without loss of accuracy.

METHODS

CMC was implemented within an in-house MC code family (PTRAN) and used to compute 3D dose distributions for two patient cases: a clinical PSB postimplant prostate CT imaging study and a simulated post lumpectomy breast PSB implant planned on a screening dedicated breast cone-beam CT patient exam. CMC tallies the dose difference, ΔD, between highly correlated histories in homogeneous and heterogeneous geometries. The heterogeneous geometry histories were derived from photon collisions sampled in a geometrically identical but purely homogeneous medium geometry, by altering their particle weights to correct for bias. The prostate case consisted of 78 Model-6711 (125)I seeds. The breast case consisted of 87 Model-200 (103)Pd seeds embedded around a simulated lumpectomy cavity. Systematic and random errors in CMC were unfolded using low-uncertainty uncorrelated MC (UMC) as the benchmark. CMC efficiency gains, relative to UMC, were computed for all voxels, and the mean was classified in regions that received minimum doses greater than 20%, 50%, and 90% of D(90), as well as for various anatomical regions.

RESULTS

Systematic errors in CMC relative to UMC were less than 0.6% for 99% of the voxels and 0.04% for 100% of the voxels for the prostate and breast cases, respectively. For a 1 × 1 × 1 mm(3) dose grid, efficiency gains were realized in all structures with 38.1- and 59.8-fold average gains within the prostate and breast clinical target volumes (CTVs), respectively. Greater than 99% of the voxels within the prostate and breast CTVs experienced an efficiency gain. Additionally, it was shown that efficiency losses were confined to low dose regions while the largest gains were located where little difference exists between the homogeneous and heterogeneous doses. On an AMD 1090T processor, computing times of 38 and 21 sec were required to achieve an average statistical uncertainty of 2% within the prostate (1 × 1 × 1 mm(3)) and breast (0.67 × 0.67 × 0.8 mm(3)) CTVs, respectively.

CONCLUSIONS

CMC supports an additional average 38-60 fold improvement in average efficiency relative to conventional uncorrelated MC techniques, although some voxels experience no gain or even efficiency losses. However, for the two investigated case studies, the maximum variance within clinically significant structures was always reduced (on average by a factor of 6) in the therapeutic dose range generally. CMC takes only seconds to produce an accurate, high-resolution, low-uncertainly dose distribution for the low-energy PSB implants investigated in this study.

Scatter correction for full-fan volumetric CT using a stationary beam blocker in a single full scan

PURPOSE

Applications of volumetric CT (VCT) are hampered by shading and streaking artifacts in the reconstructed images. These artifacts are mainly due to strong x-ray scatter signals accompanied with the large illumination area within one projection, which lead to CT number inaccuracy, image contrast loss and spatial nonuniformity. Although different scatter correction algorithms have been proposed in literature, a standard solution still remains unclear. Measurement-based methods use a beam blocker to acquire scatter samples. These techniques have unrivaled advantages over other existing algorithms in that they are simple and efficient, and achieve high scatter estimation accuracy without prior knowledge of the imaged object. Nevertheless, primary signal loss is inevitable in the scatter measurement, and multiple scans or moving the beam blocker during data acquisition are typically employed to compensate for the missing primary data. In this paper, we propose a new measurement-based scatter correction algorithm without primary compensation for full-fan VCT. An accurate reconstruction is obtained with one single-scan and a stationary x-ray beam blocker, two seemingly incompatible features which enable simple and efficient scatter correction without increase of scan time or patient dose.

METHODS

Based on the CT reconstruction theory, we distribute the blocked data over the projection area where primary signals are considered approximately redundant in a full scan, such that the CT image quality is not degraded even with primary loss. Scatter is then accurately estimated by interpolation and scatter-corrected CT images are obtained using an FDK-based reconstruction algorithm.

RESULTS

The proposed method is evaluated using two phantom studies on a tabletop CBCT system. On the Catphan©600 phantom, our approach reduces the reconstruction error from 207 Hounsfield unit (HU) to 9 HU in the selected region of interest, and improves the image contrast by a factor of 2.0 in the high-contrast regions. On an anthropomorphic head phantom, the reconstruction error is reduced from 97 HU to 6 HU in the soft tissue region and image spatial nonuniformity decreases from 27% to 5% after correction.

CONCLUSIONS

Our method inherits the main advantages of measurement-based methods while avoiding their shortcomings. It has the potential to become a practical scatter correction solution widely implementable on different VCT systems.

A general framework and review of scatter correction methods in x-ray cone-beam computerized tomography. Part 1: Scatter compensation approaches

Since scattered radiation in cone-beam volume CT implies severe degradation of CT images by quantification errors, artifacts, and noise increase, scatter suppression is one of the main issues related to image quality in CBCT imaging. The aim of this review is to structurize the variety of scatter suppression methods, to analyze the common structure, and to develop a general framework for scatter correction procedures. In general, scatter suppression combines hardware techniques of scatter rejection and software methods of scatter correction. The authors emphasize that scatter correction procedures consist of the main components scatter estimation (by measurement or mathematical modeling) and scatter compensation (deterministic or statistical methods). The framework comprises most scatter correction approaches and its validity also goes beyond transmission CT. Before the advent of cone-beam CT, a lot of papers on scatter correction approaches in x-ray radiography, mammography, emission tomography, and in Megavolt CT had been published. The opportunity to avail from research in those other fields of medical imaging has not yet been sufficiently exploited. Therefore additional references are included when ever it seems pertinent. Scatter estimation and scatter compensation are typically intertwined in iterative procedures. It makes sense to recognize iterative approaches in the light of the concept of self-consistency. The importance of incorporating scatter compensation approaches into a statistical framework for noise minimization has to be underscored. Signal and noise propagation analysis is presented. A main result is the preservation of differential-signal-to-noise-ratio (dSNR) in CT projection data by ideal scatter correction. The objective of scatter compensation methods is the restoration of quantitative accuracy and a balance between low-contrast restoration and noise reduction. In a synopsis section, the different deterministic and statistical methods are discussed with respect to their properties and applications. The current paper is focused on scatter compensation algorithms. The multitude of scatter estimation models will be dealt with in a separate paper.

Evaluation of the absorbed dose to the breast using radiochromic film in a dedicated CT mammotomography system employing a quasi-monochromatic x-ray beam

PURPOSE

A dual modality SPECT-CT prototype system dedicated to uncompressed breast imaging (mammotomography) has been developed. The computed tomography subsystem incorporates an ultrathick K-edge filtration technique producing a quasi-monochromatic x-ray cone beam that optimizes the dose efficiency of the system for lesion imaging in an uncompressed breast. Here, the absorbed dose in various geometric phantoms and in an uncompressed and pendant cadaveric breast using a normal tomographic cone beam imaging protocol is characterized using both thermoluminescent dosimeter (TLD) measurements and ionization chamber-calibrated radiochromic film.

METHODS

Initially, two geometric phantoms and an anthropomorphic breast phantom are filled in turn with oil and water to simulate the dose to objects that mimic various breast shapes having effective density bounds of 100% fatty and glandular breast compositions, respectively. Ultimately, an excised human cadaver breast is tomographically scanned using the normal tomographic imaging protocol, and the dose to the breast tissue is evaluated and compared to the earlier phantom-based measurements.

RESULTS

Measured trends in dose distribution across all breast geometric and anthropomorphic phantom volumes indicate lower doses in the medial breast and more proximal to the chest wall, with consequently higher doses near the lateral peripheries and nipple regions. Measured doses to the oil-filled phantoms are consistently lower across all volume shapes due to the reduced mass energy-absorption coefficient of oil relative to water. The mean measured dose to the breast cadaver, composed of adipose and glandular tissues, was measured to be 4.2 mGy compared to a mean whole-breast dose of 3.8 and 4.5 mGy for the oil- and water-filled anthropomorphic breast phantoms, respectively.

CONCLUSIONS

Assuming rotational symmetry due to the tomographic acquisition exposures, these results characterize the 3D dose distributions in an uncompressed human breast tissue volume for this dedicated breast imaging device and illustrate advantages of using the novel ultrathick K-edge filtered beam to minimize the dose to the breast during fully-3D imaging.

Experimental validation of a method characterizing bow tie filters in CT scanners using a real-time dose probe

PURPOSE

Beam-shaping or "bow tie" (BT) filters are used to spatially modulate the x-ray beam in a CT scanner, but the conventional method of step-and-shoot measurement to characterize a beam's profile is tedious and time-consuming. The theory for characterization of bow tie relative attenuation (COBRA) method, which relies on a real-time dosimeter to address the issues of conventional measurement techniques, was previously demonstrated using computer simulations. In this study the feasibility of the COBRA theory is further validated experimentally through the employment of a prototype real-time radiation meter and a known BT filter.

METHODS

The COBRA method consisted of four basic steps: (1) The probe was placed at the edge of a scanner's field of view; (2) a real-time signal train was collected as the scanner's gantry rotated with the x-ray beam on; (3) the signal train, without a BT filter, was modeled using peak values measured in the signal train of step 2; and (4) the relative attenuation of the BT filter was estimated from filtered and unfiltered data sets. The prototype probe was first verified to have an isotropic and linear response to incident x-rays. The COBRA method was then tested on a dedicated breast CT scanner with a custom-designed BT filter and compared to the conventional step-and-shoot characterization of the BT filter. Using basis decomposition of dual energy signal data, the thickness of the filter was estimated and compared to the BT filter's manufacturing specifications. The COBRA method was also demonstrated with a clinical whole body CT scanner using the body BT filter. The relative attenuation was calculated at four discrete x-ray tube potentials and used to estimate the thickness of the BT filter.

RESULTS

The prototype probe was found to have a linear and isotropic response to x-rays. The relative attenuation produced from the COBRA method fell within the error of the relative attenuation measured with the step-and-shoot method. The BT filter thickness estimates resulting from the dual energy scans on the breast CT system were equivalent to the manufacturing specifications. The clinical CT evaluation produced data conceptually similar to previous computer simulations and plausible relative attenuation profiles were observed.

CONCLUSIONS

The COBRA method is a fast and accurate method for BT filter characterization, which requires a simple experimental setup in a clinical environment. Because of the ease of data acquisition, multienergy scans can be acquired which allow characterization of the BT filter thickness.

A statistical, task-based evaluation method for three-dimensional x-ray breast imaging systems using variable-background phantoms

PURPOSE

For the last few years, development and optimization of three-dimensional (3D) x-ray breast imaging systems, such as digital breast tomosynthesis (DBT) and computed tomography, have drawn much attention from the medical imaging community, either academia or industry. However, there is still much room for understanding how to best optimize and evaluate the devices over a large space of many different system parameters and geometries. Current evaluation methods, which work well for 2D systems, do not incorporate the depth information from the 3D imaging systems. Therefore, it is critical to develop a statistically sound evaluation method to investigate the usefulness of inclusion of depth and background-variability information into the assessment and optimization of the 3D systems.

METHODS

In this paper, we present a mathematical framework for a statistical assessment of planar and 3D x-ray breast imaging systems. Our method is based on statistical decision theory, in particular, making use of the ideal linear observer called the Hotelling observer. We also present a physical phantom that consists of spheres of different sizes and materials for producing an ensemble of randomly varying backgrounds to be imaged for a given patient class. Lastly, we demonstrate our evaluation method in comparing laboratory mammography and three-angle DBT systems for signal detection tasks using the phantom's projection data. We compare the variable phantom case to that of a phantom of the same dimensions filled with water, which we call the uniform phantom, based on the performance of the Hotelling observer as a function of signal size and intensity.

RESULTS

Detectability trends calculated using the variable and uniform phantom methods are different from each other for both mammography and DBT systems.

CONCLUSIONS

Our results indicate that measuring the system's detection performance with consideration of background variability may lead to differences in system performance estimates and comparisons. For the assessment of 3D systems, to accurately determine trade offs between image quality and radiation dose, it is critical to incorporate randomness arising from the imaging chain including background variability into system performance calculations.