Initial In Vivo Quantification of Tc-99m Sestamibi Uptake as a Function of Tissue Type in Healthy Breasts Using Dedicated Breast SPECT-CT

Evaluation of image quality at the detector's edge of dedicated breast positron emission tomography

BACKGROUND

Using phantoms and clinical studies in prone hanging breast imaging, we assessed the image quality of a commercially available dedicated breast PET (dbPET) at the detector's edge, where mammary glands near the chest wall are located. These are compared to supine PET/CT breast images of the same clinical subjects.

METHODS

A breast phantom with four spheres (16-, 10-, 7.5-, and 5-mm diameter) was filled with ¹⁸F-fluorodeoxyglucose solution (sphere-to-background activity concentration ratio, 8:1). The spheres occupied five different positions from the top edge to the centre of the detector and were scanned for 5 min in each position. Reconstructed images were visually evaluated, and the contrast-to-noise ratio (CNR), contrast recovery coefficient (CRC) for all spheres, and coefficient of variation of the background (CV_B) were calculated. Subsequently, clinical images obtained with standard supine PET/CT and prone dbPET were retrospectively analysed. Tumour-to-background ratios (TBRs) between breast cancer near the chest wall (close to the detector's edge; peripheral group) and at other locations (non-peripheral group) were compared. The TBR of each lesion was compared between dbPET and PET/CT.

RESULTS

Closer to the detector's edge, the CNR and CRC of all spheres decreased while the CV_B increased in the phantom study. The disadvantages of this placement were visually confirmed. Regarding clinical images, TBR of dbPET was significantly higher than that of PET/CT in both the peripheral (12.38 ± 6.41 vs 6.73 ± 3.5, p = 0.0006) and non-peripheral (12.44 ± 5.94 vs 7.71 ± 7.1, p = 0.0183) groups. There was no significant difference in TBR of dbPET between the peripheral and non-peripheral groups.

CONCLUSION

The phantom study revealed poorer image quality at < 2-cm distance from the detector's edge than at other more central parts. In clinical studies, however, the visibility of breast lesions with dbPET was the same regardless of the lesion position, and it was higher than that in PET/CT. dbPET has a great potential for detecting breast lesions near the chest wall if they are at least 2 cm from the edge of the FOV, even in young women with small breasts.

Applications of Computational Methods in Biomedical Breast Cancer Imaging Diagnostics: A Review

With the exponential increase in new cases coupled with an increased mortality rate, cancer has ranked as the second most prevalent cause of death in the world. Early detection is paramount for suitable diagnosis and effective treatment of different kinds of cancers, but this is limited to the accuracy and sensitivity of available diagnostic imaging methods. Breast cancer is the most widely diagnosed cancer among women across the globe with a high percentage of total cancer deaths requiring an intensive, accurate, and sensitive imaging approach. Indeed, it is treatable when detected at an early stage. Hence, the use of state of the art computational approaches has been proposed as a potential alternative approach for the design and development of novel diagnostic imaging methods for breast cancer. Thus, this review provides a concise overview of past and present conventional diagnostics approaches in breast cancer detection. Further, we gave an account of several computational models (machine learning, deep learning, and robotics), which have been developed and can serve as alternative techniques for breast cancer diagnostics imaging. This review will be helpful to academia, medical practitioners, and others for further study in this area to improve the biomedical breast cancer imaging diagnosis.

Characterization of a multi-pinhole molecular breast tomosynthesis scanner

In recent years, breast imaging using radiolabelled molecules has attracted significant interest. Our group has proposed a multi-pinhole molecular breast tomosynthesis (MP-MBT) scanner to obtain 3D functional molecular breast images at high resolutions. After conducting extensive optimisation studies using simulations, we here present a first prototype of MP-MBT and evaluate its performance using physical phantoms. The MP-MBT design is based on two opposing gamma cameras that can image a lightly compressed pendant breast. Each gamma camera consists of a 250 × 150 mm² detector equipped with a collimator with multiple pinholes focusing on a line. The NaI(Tl) gamma detector is a customised design with 3.5 mm intrinsic spatial resolution and high spatial linearity near the edges due to a novel light-guide geometry and the use of square PMTs. A volume-of-interest is scanned by translating the collimator and gamma detector together in a sequence that optimises count yield from the scan region. Derenzo phantom images showed that the system can reach 3.5 mm resolution for a clinically realistic ^99mTc activity concentration in an 11-minute scan, while in breast phantoms the smallest spheres visible were 6 mm in diameter for the same scan time. To conclude, the experimental results of the novel MP-MBT scanner showed that the setup had sub-centimetre breast tumour detection capability which might facilitate 3D molecular breast cancer imaging in the future.

An On-Board Spectral-CT/CBCT/SPECT Imaging Configuration for Small-Animal Radiation Therapy Platform: A Monte Carlo Study

This study investigated the feasibility of a highly specific multiplexed image-guided small animal radiation therapy (SART) platform based on triple imaging from on-board single-photon emission computed tomography (SPECT), spectral-CT, and cone-beam CT (CBCT) guidance in radiotherapy treatment. As a proof-of-concept, the SART system was built with the capability of triple on-board image guidance by utilizing an x-ray tube and a single cadmium zinc telluride (CZT) semiconductor photon-counting imager via a Monte Carlo simulation study. The x-ray tube can be set at a low tube current for imaging mode and a high tube current for radiation therapy mode, respectively. In the imaging mode, both x-ray and gamma-ray projection data were collected by the imager to reconstruct CBCT, SPECT and spectral CT images of small animals being treated. The modulation transfer function (MTF) of the pixelated CZT imager measured was 8.6 lp/mm. The overall performances of the CBCT and SPECT imaging of the system were evaluated with sufficient spatial resolution and imaging quality to be fitted into the SART platform. The material differentiation and decomposition capacities of spectral CT within the system were verified using K-edge imaging, image-based optimal energy weighted imaging, and image-based linear material decomposition methods. The triple imaging capability of the system was demonstrated using a PMMA phantom containing gadolinium, iodine and radioisotope ^99mTc inserts. All the probes were clearly identified in the registered image. The results demonstrated that a novel SART platform with high-quality on-board CBCT, spectral-CT, SPECT image guidance is technically feasible by using a single semiconductor imager, thus affording comprehensive image guidance from anatomical, functional, and molecular levels for radiation treatment beam delivery.

Comparison of fan beam, slit-slat and multi-pinhole collimators for molecular breast tomosynthesis

Recently, we proposed and optimized dedicated multi-pinhole molecular breast tomosynthesis (MBT) that images a lightly compressed breast. As MBT may also be performed with other types of collimators, the aim of this paper is to optimize MBT with fan beam and slit-slat collimators and to compare its performance to that of multi-pinhole MBT to arrive at a truly optimized design. Using analytical expressions, we first optimized fan beam and slit-slat collimator parameters to reach maximum sensitivity at a series of given system resolutions. Additionally, we performed full system simulations of a breast phantom containing several tumours for the optimized designs. We found that at equal system resolution the maximum achievable sensitivity increases from pinhole to slit-slat to fan beam collimation with fan beam and slit-slat MBT having on average a 48% and 20% higher sensitivity than multi-pinhole MBT. Furthermore, by inspecting simulated images and applying a tumour-to-background contrast-to-noise (TB-CNR) analysis, we found that slit-slat collimators underperform with respect to the other collimator types. The fan beam collimators obtained a similar TB-CNR as the pinhole collimators, but the optimum was reached at different system resolutions. For fan beam collimators, a 6-8 mm system resolution was optimal in terms of TB-CNR, while with pinhole collimation highest TB-CNR was reached in the 7-10 mm range.

Experimental validation of absolute SPECT/CT quantification for response monitoring in breast cancer

PURPOSE

Recent developments in iterative image reconstruction enable absolute quantification of SPECT/CT studies by incorporating compensation for collimator-detector response, attenuation, and scatter as well as resolution recovery into the reconstruction process (Evolution; Q.Metrix package; GE Healthcare, Little Chalfont, UK). The aim of this experimental study is to assess its quantitative accuracy for potential clinical ^99m Tc-sestamibi (MIBI)-related SPECT/CT application in neoadjuvant chemotherapy response studies in breast cancer.

METHODS

Two phantoms were filled with MIBI and acquired on a SPECT/CT gamma camera (Discovery 670 Pro; GE Healthcare), that is, a water cylinder and a NEMA body phantom containing six spheres that were filled with an activity concentration reflecting clinical MIBI uptake. Subsequently, volumes-of-interest (VOI) of each sphere were drawn (semi)automatically on SPECT using various isocontour methods or manually on CT. Finally, prone MIBI SPECT/CT scans were acquired 5 and 90 min p.i. in a locally advanced breast cancer patient.

RESULTS

Activity concentration in the four largest spheres converged after nine iterations of evolution. Depending on the count statistics, the accuracy of the reconstructed activity concentration varied between -4.7 and -0.16% (VOI covering the entire phantom) and from 6.9% to 10% (8.8 cm ⌀ cylinder VOI placed in the center of the phantom). Recovery coefficients of SUV_max were 1.89 ± 0.18, 1.76 ± 0.17, 2.00 ± 0.38, 1.89 ± 0.35, and 0.90 ± 0.26 for spheres with 37, 28, 22, 17, and 13 mm ⌀, respectively. Recovery coefficients of SUV_mean were 1.07 ± 0.06, 1.03 ± 0.09, 1.17 ± 0.21, 1.10 ± 0.20, and 0.52 ± 0.14 (42% isocontour); 1.10 ± 0.07, 1.02 ± 0.09, 1.13 ± 0.19, 1.06 ± 0.19, and 0.51 ± 0.13 (36% isocontour with local background correction); and 0.96, 1.09, 1.03, 1.03, and 0.29 (CT). Patient study results were concordant with the phantom validation.

CONCLUSIONS

Absolute SPECT/CT quantification of breast studies using MIBI seems feasible (<17% deviation) when a 42% isocontour is used for delineation for tumors of at least 17 mm diameter. However, with tumor shrinkage, response evaluation should be handled with caution, especially when using SUV_max .

System geometry optimization for molecular breast tomosynthesis with focusing multi-pinhole collimators

Imaging of ^99mTc-labelled tracers is gaining popularity for detecting breast tumours. Recently, we proposed a novel design for molecular breast tomosynthesis (MBT) based on two sliding focusing multi-pinhole collimators that scan a modestly compressed breast. Simulation studies indicate that MBT has the potential to improve the tumour-to-background contrast-to-noise ratio significantly over state-of-the-art planar molecular breast imaging. The aim of the present paper is to optimize the collimator-detector geometry of MBT. Using analytical models, we first optimized sensitivity at different fixed system resolutions (ranging from 5 to 12 mm) by tuning the pinhole diameters and the distance between breast and detector for a whole series of automatically generated multi-pinhole designs. We evaluated both MBT with a conventional continuous crystal detector with 3.2 mm intrinsic resolution and with a pixelated detector with 1.6 mm pixels. Subsequently, full system simulations of a breast phantom containing several lesions were performed for the optimized geometry at each system resolution for both types of detector. From these simulations, we found that tumour-to-background contrast-to-noise ratio was highest for systems in the 7 mm-10 mm system resolution range over which it hardly varied. No significant differences between the two detector types were found.

Implementation and CT sampling characterization of a third-generation SPECT-CT system for dedicated breast imaging

Stand-alone cone beam computed tomography (CT) and single-photon emission computed tomography (SPECT) systems capable of complex acquisition trajectories have previously been developed for breast imaging. Fully three-dimensional (3-D) motions of SPECT systems provide views into the chest wall and throughout the entire volume. The polar tilting capability of the CBCT system has shown improvement in sampling close to the chest wall, while eliminating cone beam artifacts. Here, a single hybrid SPECT-CT system, with each individual modality capable of independently traversing complex trajectories around a common pendant breast volume, was developed. We present the practical implementation of this design and preliminary results of the CT system. The fully 3-D SPECT was nested inside the suspended CT gantry and oriented perpendicular to the CT source-detector pair. Both subsystems were positioned on a rotation stage, with the combined polar and azimuthal motions enabling spherical trajectories. Six trajectories were used for initial evaluation of the tilt capable CT system. The developed system can achieve polar tilt angles with a [Formula: see text] positioning error and no hysteresis. Initial imaging results demonstrate that additional off-axis projection views of various geometric resolution phantoms facilitate more complete sampling, more consistent attenuation value recovery, and markedly improved reconstructions. This system could have various applications in diagnostic or therapeutic breast imaging.

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.

Molecular breast tomosynthesis with scanning focus multi-pinhole cameras

Planar molecular breast imaging (MBI) is rapidly gaining in popularity in diagnostic oncology. To add 3D capabilities, we introduce a novel molecular breast tomosynthesis (MBT) scanner concept based on multi-pinhole collimation. In our design, the patient lies prone with the pendant breast lightly compressed between transparent plates. Integrated webcams view the breast through these plates and allow the operator to designate the scan volume (e.g. a whole breast or a suspected region). The breast is then scanned by translating focusing multi-pinhole plates and NaI(Tl) gamma detectors together in a sequence that optimizes count yield from the volume-of-interest. With simulations, we compared MBT with existing planar MBI. In a breast phantom containing different lesions, MBT improved tumour-to-background contrast-to-noise ratio (CNR) over planar MBI by 12% and 111% for 4.0 and 6.0 mm lesions respectively in case of whole breast scanning. For the same lesions, much larger CNR improvements of 92% and 241% over planar MBI were found in a scan that focused on a breast region containing several lesions. MBT resolved 3.0 mm rods in a Derenzo resolution phantom in the transverse plane compared to 2.5 mm rods distinguished by planar MBI. While planar MBI cannot provide depth information, MBT offered 4.0 mm depth resolution. Our simulations indicate that besides offering 3D localization of increased tracer uptake, multi-pinhole MBT can significantly increase tumour-to-background CNR compared to planar MBI. These properties could be promising for better estimating the position, extend and shape of lesions and distinguishing between single and multiple lesions.

Three dimensional dose distribution comparison of simple and complex acquisition trajectories in dedicated breast CT

PURPOSE

A novel breast CT system capable of arbitrary 3D trajectories has been developed to address cone beam sampling insufficiency as well as to image further into the patient's chest wall. The purpose of this study was to characterize any trajectory-related differences in 3D x-ray dose distribution in a pendant target when imaged with different orbits.

METHODS

Two acquisition trajectories were evaluated: circular azimuthal (no-tilt) and sinusoidal (saddle) orbit with ±15° tilts around a pendant breast, using Monte Carlo simulations as well as physical measurements. Simulations were performed with tungsten (W) filtration of a W-anode source; the simulated source flux was normalized to the measured exposure of a W-anode source. A water-filled cylindrical phantom was divided into 1 cm(3) voxels, and the cumulative energy deposited was tracked in each voxel. Energy deposited per voxel was converted to dose, yielding the 3D distributed dose volumes. Additionally, three cylindrical phantoms of different diameters (10, 12.5, and 15 cm) and an anthropomorphic breast phantom, initially filled with water (mimicking pure fibroglandular tissue) and then with a 75% methanol-25% water mixture (mimicking 50-50 fibroglandular-adipose tissues), were used to simulate the pendant breast geometry and scanned on the physical system. Ionization chamber calibrated radiochromic film was used to determine the dose delivered in a 2D plane through the center of the volume for a fully 3D CT scan using the different orbits.

RESULTS

Measured experimental results for the same exposure indicated that the mean dose measured throughout the central slice for different diameters ranged from 3.93 to 5.28 mGy, with the lowest average dose measured on the largest cylinder with water mimicking a homogeneously fibroglandular breast. These results align well with the cylinder phantom Monte Carlo studies which also showed a marginal difference in dose delivered by a saddle trajectory in the central slice. Regardless of phantom material or filled fluid density, dose delivered by the saddle scan was negligibly different than the simple circular, no-tilt scans. The average dose measured in the breast phantom was marginally higher for saddle than the circular no tilt scan at 3.82 and 3.87 mGy, respectively.

CONCLUSIONS

Not only does nontraditional 3D-trajectory CT scanning yield more complete sampling of the breast volume but also has comparable dose deposition throughout the breast and anterior chest volume, as verified by Monte Carlo simulation and physical measurements.

Characterization of simulated incident scatter and the impact on quantification in dedicated breast single-photon emission computed tomography

The objective was to characterize the changes seen from incident Monte Carlo-based scatter distributions in dedicated three-dimensional (3-D) breast single-photon emission computed tomography, with emphasis on the impact of scatter correction using the dual-energy window (DEW) method. Changes in scatter distributions with 3-D detector position were investigated for prone breast imaging with an ideal detector. Energy spectra within a high-energy scatter window measured from simulations were linearly fit, and the slope was used to characterize scatter distributions. The impact of detector position on the measured scatter fraction within various photopeak windows and the [Formula: see text] value (ratio of scatter within the photopeak and scatter energy windows) useful for scatter correction was determined. Results indicate that application of a single [Formula: see text] value with the DEW method in the presence of anisotropic object scatter distribution is not appropriate for trajectories including the heart and liver. The scatter spectra's slope demonstrates a strong correlation to measured [Formula: see text] values. Reconstructions of fixed-tilt 3-D acquisition trajectories with a single [Formula: see text] value show quantification errors up to 5% compared to primary-only reconstructions. However, a variable-tilt trajectory provides improved sampling and minimizes quantification errors, and thus allows for a single [Formula: see text] value to be used with the DEW method leading to more accurate quantification.

Non-rigid registration of serial dedicated breast CT, longitudinal dedicated breast CT and PET/CT images using the diffeomorphic demons method

RATIONALE AND OBJECTIVES

Dedicated breast CT and PET/CT scanners provide detailed 3D anatomical and functional imaging data sets and are currently being investigated for applications in breast cancer management such as diagnosis, monitoring response to therapy and radiation therapy planning. Our objective was to evaluate the performance of the diffeomorphic demons (DD) non-rigid image registration method to spatially align 3D serial (pre- and post-contrast) dedicated breast computed tomography (CT), and longitudinally-acquired dedicated 3D breast CT and positron emission tomography (PET)/CT images.

METHODS

The algorithmic parameters of the DD method were optimized for the alignment of dedicated breast CT images using training data and fixed. The performance of the method for image alignment was quantitatively evaluated using three separate data sets; (1) serial breast CT pre- and post-contrast images of 20 women, (2) breast CT images of 20 women acquired before and after repositioning the subject on the scanner, and (3) dedicated breast PET/CT images of 7 women undergoing neo-adjuvant chemotherapy acquired pre-treatment and after 1 cycle of therapy.

RESULTS

The DD registration method outperformed no registration (p < 0.001) and conventional affine registration (p ≤ 0.002) for serial and longitudinal breast CT and PET/CT image alignment. In spite of the large size of the imaging data, the computational cost of the DD method was found to be reasonable (3-5 min).

CONCLUSIONS

Co-registration of dedicated breast CT and PET/CT images can be performed rapidly and reliably using the DD method. This is the first study evaluating the DD registration method for the alignment of dedicated breast CT and PET/CT images.