Evaluation of Coronary Stents and Stenoses at Different Heart Rates With Dual Source Spiral CT (DSCT)

OBJECTIVES

Evaluation of coronary arteries at higher heart rates and in the presence of coronary stents remains problematic. The utilization of dual source computed tomography (DSCT) might improve the visualization of the coronary arteries under these conditions by imaging at a temporal resolution of 83 milliseconds, independent of heart rate.

MATERIALS AND METHODS

Vessel phantoms (diameter 2-4 mm) were attached to a robotic device to simulate cardiac motion and scanned with a DSCT system. The phantoms had either inserts leading to 50% stenosis or carried stents. Images were evaluated for motion artifacts and measurements of the normal, stenotic, and in-stent lumen at different heart rates (50-120 bpm) were performed. Quantile regression analysis was performed to investigate heart rate dependence of the measurement errors.

RESULTS

Visualization of the stenoses and stents was possible without motion artifacts at heart rates of up to 120 bpm. Image quality was similar for the static (0 bpm) and the dynamic (50-120 bpm) scans. Errors for diameter measurements of the vessel lumen and the stenotic lumen were low (3-mm vessel: 1-2%), but considerable for in-stent diameter measurements (3-mm stent: 27-32%). A window/level setting of 1500/300 Hounsfield units was more favorable for stent evaluation. No heart rate dependence was found.

CONCLUSIONS

Depiction of coronary stents with DSCT is possible across a large range of simulated heart rates without motion artifacts and with image quality superior to that of previous generations of CT scanners.

An assessment of the use of skin flashes in helical tomotherapy using phantom and in-vivo dosimetry

BACKGROUND AND PURPOSE

In helical tomotherapy the nature of the optimizing and planning systems allows the delivery of dose on the skin using a build-up compensating technique (skin flash). However, positioning errors or changes in the patient's contour can influence the correct dosage in these regions. This work studies the behavior of skin-flash regions using phantom and in-vivo dosimetry.

MATERIALS AND METHODS

The dosimetric accuracy of the tomotherapy planning system in skin-flash regions is checked using film and TLD on phantom. Positioning errors are induced and the effect on the skin dose is investigated. Further a volume decrease is simulated using bolus material and the results are compared.

RESULTS

Results show that the tomotherapy planning system calculates dose on skin regions within 2 SD using TLD measurements. Film measurements show drops of dose of 2.8% and 26% for, respectively, a 5mm and 10mm mispositioning of the phantom towards air and a dose increase of 9% for a 5mm shift towards tissue. These measurements are confirmed by TLD measurements. A simulated volume reduction shows a similar behavior with a 2.6% and 19.4% drop in dose, measured with TLDs.

CONCLUSION

The tomotherapy system allows adequate planning and delivery of dose using skin flashes. However, exact positioning is crucial to deliver the dose at the exact location.

Hyperfast parallel-beam and cone-beam backprojection using the cell general purpose hardware

Tomographic image reconstruction, such as the reconstruction of computed tomography projection values, of tomosynthesis data, positron emission tomography or SPECT events, and of magnetic resonance imaging data is computationally very demanding. One of the most time-consuming steps is the backprojection. Recently, a novel general purpose architecture optimized for distributed computing became available: the cell broadband engine (CBE). To maximize image reconstruction speed we modified our parallel-beam backprojection algorithm [two dimensional (2D)] and our perspective backprojection algorithm [three dimensional (3D), cone beam for flat-panel detectors] and optimized the code for the CBE. The algorithms are pixel or voxel driven, run with floating point accuracy and use linear (LI) or nearest neighbor (NN) interpolation between detector elements. For the parallel-beam case, 512 projections per half rotation, 1024 detector channels, and an image of size 512(2) was used. The cone-beam backprojection performance was assessed by backprojecting a full circle scan of 512 projections of size 1024(2) into a volume of size 512(3) voxels. The field of view was chosen to completely lie within the field of measurement and the pixel or voxel size was set to correspond to the detector element size projected to the center of rotation divided by square root of 2. Both the PC and the CBE were clocked at 3 GHz. For the parallel backprojection of 512 projections into a 512(2) image, a throughput of 11 fps (LI) and 15 fps (NN) was measured on the PC, whereas the CBE achieved 126 fps (LI) and 165 fps (NN), respectively. The cone-beam backprojection of 512 projections into the 512(3) volume took 3.2 min on the PC and is as fast as 13.6 s on the cell. Thereby, the cell greatly outperforms today's top-notch backprojections based on graphical processing units. Using both CBEs of our dual cell-based blade (Mercury Computer Systems) allows to 2D backproject 330 images/s and one can complete the 3D cone-beam backprojection in 6.8 s.

Superiority of synchrony of 256-slice cone beam computed tomography for acquiring pulsating objects. Comparison with conventional multislice computed tomography

PURPOSE

A prototype 256-slice cone beam computed tomography (CT) provides complete volumetric data within a single gantry rotation (1 s/rotation) with 0.5 mm slice-thickness.

MATERIALS AND METHODS

Calcified phantoms (200-400 HU) were attached to the balloon of a pulsating phantom and moved at a rate of 5-90/min. Acquisition was performed during one to-and-fro motion at each pulsation rate without electrocardiogram (ECG)-gating. Each period was divided into 10 phases, and compared to conventional multislice CT scanning without ECG-gating.

RESULTS

At 5-20/min, the configuration of calcified phantoms continued to the through-plane without gaps. At 60/min, duplicated calcified phantoms at end-systole and end-diastole were observed without motion. At 90/min, motion could be observed without gaps but was more blurred, and total calcified volume, Agatston scores, mean and max CT values of three phantoms were almost equal compared with those at static state. However, at 60/min, total calcified volume, scores, mean and max CT values of three phantoms were decreased to 64%, 37%, 80% and 56%, respectively, compared with those at static state. In multislice CT, even at lower rates, there were gaps in the through-plane. At 60/min, total calcified volume, scores, mean and max CT values of three phantoms were decreased to only 8%, 3%, 79% and 53%, respectively, compared with static state.

CONCLUSION

This new prototype's unique character (synchrony) enables the acquisition of pulsating objects. These can be acquired without gaps in the through-plane even in the absence of ECG-gating. However, its present temporal resolution only permits accurate quantitative evaluation of calcium up to 20/min.

Pulsatility Imaging of Saccular Aneurysm Model by 64-Slice CT with Dynamic Multiscan Technique

The feasibility of imaging pulsatility in an aneurysm model with the high-resolution dynamic multiscan technique of 64-slice computed tomography (CT) was studied. A pulsatile aneurysm phantom was constructed and imaged with dynamic multiscan technique. The aneurysm model was filled with iodinated contrast material (250 Hounsfield Units) and was scanned with use of a gantry rotation time of 0.33 seconds, slice thickness of 1.2 mm, effective coverage of 24 mm, and total imaging time of 4 seconds. Images were reconstructed at 50-msec intervals. The visualization of wall motion was qualitatively evaluated by direct comparison of four-dimensional images versus phantom motion. Pulsatility imaging without perceptible artifact or need for cardiac gating was achieved with the use of this technique.

Automated volumetry of solid pulmonary nodules in a phantom: accuracy across different CT scanner technologies

OBJECTIVES

The accuracy of automated volumetry for pulmonary nodules in a phantom using different CT scanner technologies from single-slice spiral CT (SSCT) to 64-slice multidetector-row CT (MDCT) was compared.

MATERIALS AND METHODS

A lung phantom with 5 different categories of pulmonary nodules was scanned using a single-slice spiral CT, a 4-slice MDCT, a 16-slice MDCT and a 64-slice MDCT. Each category comprised of 7-9 nodules each (total n = 40) with different known volumes. Standard dose and low dose protocols were performed using thin and thick collimation. Image data were reconstructed at the thinnest slice thickness. Data sets were analyzed with a dedicated volumetry software. Volumes of all nodules were calculated and compared.

RESULTS

Mean absolute percentage error (APE) for all nodules was 8.65% (+/-7.29%) for the SSCT, 10.26% (+/-8.25%) for the 4-slice MDCT, 8.19% (+/-7.57%) for the 16-slice MDCT and 7.89% (+/-7.39%) for the 64-slice MDCT. There was statistically significant influence of the scanner type, protocol, anatomic location, and nodule volume on APE, but overall, APEs were comparable.

CONCLUSION

Computer-aided volumetry showed accurate measurements in all tested scanner types. This finding has important implications for nodule assessment and follow-up.

Contrast-Enhanced Abdominal Angiographic CT for Intra-abdominal Tumor Embolization: A New Tool for Vessel and Soft Tissue Visualization

C-Arm cone-beam computed tomography (CACT), is a relatively new technique that uses data acquired with a flat-panel detector C-arm angiography system during an interventional procedure to reconstruct CT-like images. The purpose of this Technical Note is to present the technique, feasibility, and added value of CACT in five patients who underwent abdominal transarterial chemoembolization procedures. Target organs for the chemoembolizations were kidney, liver, and pancreas and a liposarcoma infiltrating the duodenum. The time for patient positioning, C-arm and system preparation, CACT raw data acquisition, and data reconstruction for a single CACT study ranged from 6 to 12 min. The volume data set produced by the workstation was interactively reformatted using maximum intensity projections and multiplanar reconstructions. As part of an angiography system CACT provided essential information on vascular anatomy, therapy endpoints, and immediate follow-up during and immediately after the abdominal interventions without patient transfer. The quality of CACT images was sufficient to influence the course of treatment. This technology has the potential to expedite any interventional procedure that requires three-dimensional information and navigation.

Comparison of gated blood pool SPECT and spiral multidetector computed tomography in the assessment of right ventricular functional parameters: validation with first-pass radionuclide angiography

OBJECTIVE

The aim of this study was to compare gated blood pool single photon emission computed tomography (SPECT) (GBPS) and multidetector row computed tomography (MDCT) for the determination of right ventricular ejection fraction (RVEF) and right ventricular volumes (RVV) and to compare first-pass radionuclide angiography (FP-RNA) as the gold standard.

METHODS

Twenty consecutive patients (11 men, 9 women) referred for MDCT for the evaluation of the presence of coronary artery disease underwent FP-RNA and GBPS.

RESULTS

The mean right ventricular end-diastolic volume (EDV) calculated with GBPS revealed a statistically significant lower value than that of MDCT. The mean right ventricular end-systolic volume (ESV) calculated with GBPS was also lower than that of MDCT. A comparison of right ventricular EDV from GBPS and MDCT yielded a correlation coefficient of 0.5972. Right ventricular ESV between GBPS and MDCT showed a correlation coefficient of 0.5650. The mean RVEFs calculated with FP-RNA (39.8% +/- 4.0%), GBPS (43.7% +/-6.9%0), and MDCT (40.4% + 7.7%) showed no statistical differences (Kruskal-Wallis statistics 4.538, P = 0.1034). A comparison of RVEFs from FP-RNA and GBPS yielded a correlation coefficient of 0.7251; RVEFs between FP-RNA and MDCT showed a correlation coefficient of 0.6166 and between GBPS and MDCT showed a correlation coefficient of 0.6367.

CONCLUSION

The RVEF, EDV, and ESV calculated by GBPS had good correlation with those obtained with MDCT. In addition, there were no statistical differences of RVEF calculated from FP-RNA, GBPS, and MDCT. However, with regard to RVV, EDV and ESV from GBPS revealed statistically significantly lower values than those of MDCT. Although reasonable correlations among these modalities were obtained, the agreement among these three modalities was not good enough for interchangeable use in the clinical setting. Also, these results should be confirmed in patients with cardiac diseases in future larger population-based studies.

Handling data redundancy in helical cone beam reconstruction with a cone-angle-based window function and its asymptotic approximation

A cone-angle-based window function is defined in this manuscript for image reconstruction using helical cone beam filtered backprojection (CB-FBP) algorithms. Rather than defining the window boundaries in a two-dimensional detector acquiring projection data for computed tomographic imaging, the cone-angle-based window function deals with data redundancy by selecting rays with the smallest cone angle relative to the reconstruction plane. To be computationally efficient, an asymptotic approximation of the cone-angle-based window function is also given and analyzed in this paper. The benefit of using such an asymptotic approximation also includes the avoidance of functional discontinuities that cause artifacts in reconstructed tomographic images. The cone-angle-based window function and its asymptotic approximation provide a way, equivalent to the Tam-Danielsson-window, for helical CB-FBP reconstruction algorithms to deal with data redundancy, regardless of where the helical pitch is constant or dynamically variable during a scan. By taking the cone-parallel geometry as an example, a computer simulation study is conducted to evaluate the proposed window function and its asymptotic approximation for helical CB-FBP reconstruction algorithm to handle data redundancy. The computer simulated Forbild head and thorax phantoms are utilized in the performance evaluation, showing that the proposed cone-angle-based window function and its asymptotic approximation can deal with data redundancy very well in cone beam image reconstruction from projection data acquired along helical source trajectories. Moreover, a numerical study carried out in this paper reveals that the proposed cone-angle-based window function is actually equivalent to the Tam-Danielsson-window, and rigorous mathematical proofs are being investigated.

Flat Panel Detector-Based Volumetric Computed Tomography (fpVCT)

OBJECTIVES

Evaluation of a silicon-based flat panel volumetric computed tomography (fpVCT) and multislice CT in terms of volumetry of phantoms with different algorithms. Furthermore, to compare the different volumetric analysis methods themselves.

MATERIALS AND METHODS

Four phantoms of different materials have been scanned with fpVCT (GE prototype with circular gantry with 2 aSi/CsI flat panel detector) and a 64-slice spiral CT (MSCT: LightSpeed VCT). Three spherical phantoms of different materials and 1 phantom with an irregular shape were evaluated. True volumes were calculated in dependence from the diameter or by water displacement method. Imaging parameters (80 kVp, 100 mA) and the position of the phantoms were identical in both techniques. After reconstruction of the images different algorithms have been used 4 times for each phantom. These analysis methods have been performed: Region growing, threshold method, planimetry, 3-dimensional volumetry measurement by using the equation of an ellipsoid (ellipse) and an advanced lung analysis modus [single advanced lung analysis (ALA)]. The mean values and the standard deviations have been evaluated and compared with the true volumes.

RESULTS

In all phantoms fpVCT showed better results with lower deviations from the true values than in MSCT, especially for small volumes of the phantoms. However, the results of the ALA single method demonstrated no significant difference between the fpVCT and MSCT. The comparison of the different analysis methods revealed that 3-dimensional measurement with the ellipse method was the worst method for volume estimation, especially for the irregularly formed phantom.

CONCLUSION

fpVCT was superior to MSCT in the volumetry of small objects. The ellipse method has been shown to be the worst for volumetry with the highest relative deviations from the true volume value. The single ALA method shows the lowest standard deviation thereby revealing a reproducible volumetric method for small nodules. However, further future developments of volumetric analysis methods are necessary to use them accurately in daily routine. Due to the truly isotropic volume data set with high spatial resolution fpVCT is a powerful tool for the volumetry of small nodules.

Multichannel CT: evaluating the spine in postoperative patients with orthopedic hardware

Evaluating the spine in patients with metal orthopedic hardware is challenging. Although the effectiveness of conventional computed tomography (CT) can be limited by severe beam-hardening artifacts, the evolution of multichannel CT in recent years has made available new techniques that can help minimize these artifacts. Multichannel CT allows faster scanning times, resulting in reduced motion artifacts; thinner sections, with which it is possible to create a scanned volume of isotropic voxels with equivalent image resolution in all planes; and the generation of a higher x-ray tube current, which may result in better penetration of metal hardware and reduction of artifacts. Although 140 kVp and high milliamperage-second exposure are recommended for imaging patients with hardware, caution should always be exercised, particularly in children, young adults, and patients undergoing multiple examinations. The acquisition of multiplanar reformatted images in the axial, sagittal, coronal, and oblique planes and of three-dimensional volume-rendered images optimizes image interpretation. Wide window settings are best for reviewing images when hardware is present. The integrity of hardware is best assessed with multiplanar average intensity projection. Soft-tissue structures are best visualized by interactively varying the window width and level settings. Implementation of these techniques can yield diagnostic-quality images and aid in patient treatment.

Optimizing 4D cone-beam CT acquisition protocol for external beam radiotherapy

PURPOSE

Four-dimensional cone-beam computed tomography (4D-CBCT) imaging is sensitive to parameters such as gantry rotation speed, number of gantry rotations, X-ray pulse rate, and tube current, as well as a patient's breathing pattern. The aim of this study is to optimize the image acquisition on a patient-specific basis while minimizing the scan time and the radiation dose.

METHODS AND MATERIALS

More than 60 sets of 4D-CBCT images, each with a temporal resolution of 10 phases, were acquired using multiple-gantry rotation and slow-gantry rotation techniques. The image quality was quantified with a relative root mean-square error (RE) and correlated with various acquisition settings; specifically, varying gantry rotation speed, varying both the rotation speed and the number of rotations, and varying both the rotation speed and tube current to keep the radiation exposure constant. These experiments were repeated for three different respiratory periods.

RESULTS

With similar radiation dose, 4D-CBCT images acquired with low current and low rotation speed have better quality over images obtained with high current and high rotation speed. In general, a one-rotation low-speed scan is superior to a two-rotation double-speed scan, even though they provide the same number of projections. Furthermore, it is found that the image quality behaves monotonically with the relative speed as defined by the gantry rotation speed and the patient respiratory period.

CONCLUSIONS

The RE curves established in this work can be used to predict the 4D-CBCT image quality before a scan. This allows the acquisition protocol to be optimized individually to balance the desired quality with the associated scanning time and patient radiation dose.

Diagnostic accuracy of multislice computed tomography for the detection of coronary artery disease in diabetic patients

BACKGROUND

Diabetes mellitus is an important risk factor for coronary artery disease. Cardiac multislice computed tomography (MSCT) permits visualization of the coronary arteries with good sensitivity and specificity. However, at present, there are no data whether MSCT allows an accurate assessment of coronary arteries of diabetic patients, in comparison to nondiabetic patients. Thus, we compared the catheter-controlled MSCT results from diabetic and nondiabetic patients in a cohort of 116 patients with regard to sensitivity, specificity, positive predictive value, and negative predictive value, as well as image quality.

METHODS AND MATERIALS

Twenty-two diabetic patients (age, 64.6+/-8.5 years; number of risk factors, 3.4+/-1.1) and 94 nondiabetic patients (age, 64.2+/-9.2 years; number of risk factors, 2.4+/-1.0) were examined by MSCT (Sensation 16 Speed 4 D, Siemens, Forchheim, Germany; gantry rotation time, 375 ms) and invasive coronary angiography. MSCT results were compared, blinded to the results of the coronary angiography with regard to the presence or absence of a significant stenosis (>50%) in a modified American Heart Association 13-segment model. Image quality was assessed on a qualitative scale between 1 (very good) and 5 (invisible) for each segment.

RESULTS

Sensitivity, specificity, positive predictive value, and negative predictive value were statistically not different in diabetic and nondiabetic patients (0.85/0.98/0.92/0.96 vs. 0.84/0.97/0.91/0.95). One diabetic and three nondiabetic patients had to be excluded from analysis. Diabetic patients had relevantly more risk factors (P < .05), but calcium scoring was not different in both groups (Agatston score 1090+/-1278 vs. 798+/-1033). The image quality in both cohorts was comparable.

CONCLUSIONS

MSCT allows the assessment of the coronary arteries noninvasively in diabetic patients with a good sensitivity and specificity, and diabetes does not have an impact on the number of evaluable segments. Thus, MSCT is a noninvasive tool in the care of these patients.

[Evaluation of the accuracy of 3-D spiral CT in diagnosis of maxillofacial fracture]

PURPOSE

To investigate the value of three dimensional (3-D) CT in diagnosis of maxillofacial fractures.

METHODS

72 patients with maxillofacial fractures were scanned by three dimensional (3-D) spiral CT, The three dimensional images of maxillofacial fractures were detected by using various rotation axes. All images confirmed clinically were analyzed, in terms of types and locations. The accuracy of X-ray film, 2-D CT and 3-D CT in fractures were compared.

RESULTS

3-D spiral CT provided a clear stereoscopic view of the complex maxillofacial fractures and their peripheral relations. Compared with radiography and 2-D CT, 3D CT was superior in revealing the location, displacement, accompanying fractures and deformities with high resolution and vivid visualization.

CONCLUSIONS

3-D CT is of important clinical value in diagnosis of complex maxillofacial fractures. It helps display clearly the location and displacement of complex maxillofacial fractures and design the surgical plan.

Accuracy of automated volumetry of pulmonary nodules across different multislice CT scanners

The purpose of this study was to compare the accuracy of an automated volumetry software for phantom pulmonary nodules across various 16-slice multislice spiral CT (MSCT) scanners from different vendors. A lung phantom containing five different nodule categories (intraparenchymal, around a vessel, vessel attached, pleural, and attached to the pleura), with each category comprised of 7-9 nodules (total, n = 40) of varying sizes (diameter 3-10 mm; volume 6.62 mm(3)-525 mm(3)), was scanned with four different 16-slice MSCT scanners (Siemens, GE, Philips, Toshiba). Routine and low-dose chest protocols with thin and thick collimations were applied. The data from all scanners were used for further analysis using a dedicated prototype volumetry software. Absolute percentage volume errors (APE) were calculated and compared. The mean APE for all nodules was 8.4% (+/-7.7%) for data acquired with the 16-slice Siemens scanner, 14.3% (+/-11.1%) for the GE scanner, 9.7% (+/-9.6%) for the Philips scanner and 7.5% (+/-7.2%) for the Toshiba scanner, respectively. The lowest APEs were found within the diameter size range of 5-10 mm and volumes >66 mm(3). Nodule volumetry is accurate with a reasonable volume error in data from different scanner vendors. This may have an important impact for intraindividual follow-up studies.

SPECT/CT imaging using a spiral CT scanner for anatomical localization: Impact on diagnostic accuracy and reporter confidence in clinical practice

PURPOSE

To evaluate the incremental benefit in routine clinical practice of computed tomography (CT) scans acquired for anatomical localization on an integrated SPECT/CT which incorporates a spiral CT scanner, in comparison with conventional planar and SPECT scanning.

METHODS

The first 50 studies acquired on the integrated system were evaluated by two experienced nuclear medicine physicians who were aware of the patient's clinical history. These included bone scans, gallium scans, octreotide scans, sestamibi parathyroid scans and MIBG scans. For each patient study, abnormalities were assessed on planar and SPECT images for location and provisional diagnosis and a quantitative scale was used to assess reporter confidence. The fused SPECT/CT images were then reviewed and the location and provisional diagnosis noted and reporter confidence was assessed using the same quantitative scale.

RESULTS

There were 129 abnormalities detected in 50 patient studies. For localization of abnormalities, the inclusion of the CT resulted in a minor change in 16% of cases and a significant change in 11% over planar/SPECT imaging alone. The confidence of localization was improved moderately in 19% and improved significantly in 6%. For diagnosis, SPECT/CT resulted in a minor change in 10% and a significant change in 9% over planar/SPECT imaging. The confidence of diagnosis was improved moderately in 10% and improved significantly in a further 10% of cases. For the final scan interpretation, there would have been no change in 44% patients, a minor change in 30% and a significant change in 26% with the use of SPECT/CT.

CONCLUSION

Use of integrated SPECT/CT with a high spatial resolution, spiral CT used for anatomical localization improves accuracy and reporter confidence in clinical practice. As a result, final reports were different in 56% of the cases, including being significantly different in 26% patients compared to reporting with planar/SPECT alone.

[New devices in radiation oncology]

The development of sophisticated conformal radiation techniques, such as intensity-modulated radiation therapy, image-guided radiation therapy, adaptative radiation therapy, and radiosurgery, implies precise and accurate targeting. To achieve this goal, a lot of new devices and techniques have been designed and are now available in radiation therapy departments : modern 3D-imaging systems, sophisticated treatment planning systems, breathing-adapted radiotherapy equipments (for gating and tracking techniques), in-room 3D-imaging systems, tomotherapy, etc. Purpose of this review is to briefly present the new equipments which are now used in radiation therapy departments in conformal therapy.

Evaluation of the spatial resolution characteristics of a cone-beam breast CT scanner

The purpose of this study was to examine the spatial resolution of a prototype pendant-geometry cone-beam breast computed tomography (CT) system. Modulation transfer functions (MTFs) of the reconstructed image in the coronal (x and y) plane were computed as a function of the cone angle, the radial distance from the axis of rotation, the size of the reconstruction matrix, the back-projection filter used, and the number of projections acquired for the reconstruction. The results show that the cone angle and size of the reconstruction matrix have minimal impact on the MTF, while the MTF degraded radially from the axis of rotation (from 0.76 at 2.6 mm from axis of rotation down to 0.37 at 76.9 mm from axis of rotation at f=0.5 mm(-1)). The Ramp reconstruction filter increases the MTF near the axis of rotation relative to the Shepp-Logan filter, while an increase in the number of projections from 500 to 1000 increased the MTF near the periphery of the reconstructed image. The MTF in the z direction (anterior-posterior direction) was also evaluated. The z-direction MTF values tend to be higher when compared to the coronal MTF (0.85 at f =0.5 mm(-1)), and tend to be very constant throughout the coronal plane direction. The results suggest that an increase in the MTF for the prototype breast CT system is possible by optimizing various scanning and reconstruction parameters.

Four-dimensional cone-beam computed tomography using an on-board imager

On-board cone-beam computed tomography (CBCT) has recently become available to provide volumetric information of a patient in the treatment position, and holds promises for improved target localization and irradiation dose verification. The design of currently available on-board CBCT, however, is far from optimal. Its quality is adversely influenced by many factors, such as scatter, beam hardening, and intra-scanning organ motion. In this work we quantitatively study the influence of organ motion on CBCT imaging and investigate a strategy to acquire high quality phase-resolved [four-dimensional (4D)] CBCT images based on phase binning of the CBCT projection data. An efficient and robust method for binning CBCT data according to the patient's respiratory phase derived in the projection space was developed. The phase-binned projections were reconstructed using the conventional Feldkamp algorithm to yield 4D CBCT images. Both phantom and patient studies were carried out to validate the technique and to optimize the 4D CBCT data acquisition protocol. Several factors that are important to the clinical implementation of the technique, such as the image quality, scanning time, number of projections, and radiation dose, were analyzed for various scanning schemes. The general references drawn from this study are: (i) reliable phase binning of CBCT projections is accomplishable with the aid of external or internal marker and simple analysis of its trace in the projection space, and (ii) artifact-free 4D CBCT images can be obtained without increasing the patient radiation dose as compared to the current 3D CBCT scan.

Chord-based image reconstruction in cone-beam CT with a curved detector

Modern computed tomography (CT) scanners use cone-beam configurations for increasing volume coverage, improving x-ray-tube utilization, and yielding isotropic spatial resolution. Recently, there have been significant developments in theory and algorithms for exact image reconstruction from cone-beam projections. In particular, algorithms have been proposed for image reconstruction on chords; and advantages over the existing algorithms offered by the chord-based algorithms include the high flexibility of exact image reconstruction for general scanning trajectories and the capability of exact reconstruction of images within a region of interest from truncated data. These chord-based algorithms have been developed only for flat-panel detectors. Many cone-beam CT scanners employ curved detectors for important practical considerations. Therefore, in this work, we have derived chord-based algorithms for a curved detector so that they can be applied to reconstructing images directly from data acquired by use of a CT scanner with a curved detector. We have also conducted preliminary numerical studies to demonstrate and evaluate the reconstruction properties of the derived chord-based algorithms for curved detectors.

Experimental 16-Row CT Evaluation of In-Stent Restenosis using New Stationary and Moving Cardiac Stent Phantoms: Experimental Examination

PURPOSE

The aim of this study was to evaluate in-stent restenosiss using a newly developed stationary and moving cardiac stent phantom with three built-in artificial stenoses and a 16-row MDCT.

MATERIALS AND METHODS

A newly developed coronary stent phantom with three artificial stenoses--low (approx. 30 %), medium (approx. 50 %) and high (approx. 70 %)--was attached to a moving heart phantom and used to evaluate the ability of 16-row MDCT to visualize in-stent restenosis. High resolution scans (16 x 0.75 mm, 250 mm FOV) were made to identify the baseline for image quality. The non-moving phantom was scanned (16 x 0.75 mm, routine cardiac scan protocol) first without and then with implementation of an ECG signal at various simulated heart rates (HR 40 to 120 bpm) and pitches (0.15 to 0.3). The moving cardiac phantom was scanned at the same simulated heart rates but at a pitch of 0.15. Images were reconstructed at every 10 % of the RR interval using a multi-cycle real cone-beam reconstruction algorithm. Multi-planar reformations (MPR) were made for the image evaluation. The image quality was assessed using a three-point scale, and stent patency and stenoses detection were evaluated using a four-point scale. To evaluate the image quality and to grade the stent stenoses, the median values were calculated while considering the reconstruction interval.

RESULTS

The image quality for the static phantom was adequate in 97 % of the measurements. In this phantom, every stenosis was detected independent of the pitch and heart rate used. The dynamic stent phantom yielded the best results at 0 %, 40 %, and 50 % of the RR interval at a pitch of 0.15. The low stenosis was visible at a simulated heart rate of up to 80 bpm. Patency can be detected at heart rates greater than 80 bpm.

CONCLUSION

The newly developed moving stent phantom allowed a nearly in-vivo condition for detecting re-stenoses within a stent. In this phantom study the use of a 16-row MDCT allowed the detection of re-stenosis within a coronary stent at a heart rate of up to 80 bpm. This phantom can then be used for future studies, e. g. with a 64-row MDCT.

Reproducible Simulation of Respiratory Motion in Porcine Lung Explants

PURPOSE

To develop a model for exactly reproducible respiration motion simulations of animal lung explants inside an MR-compatible chest phantom.

MATERIALS AND METHODS

The materials included a piston pump and a flexible silicone reconstruction of a porcine diaphragm and were used in combination with an established MR-compatible chest phantom for porcine heart-lung preparations. The rhythmic inflation and deflation of the diaphragm at the bottom of the artificial thorax with water (1 - 1.5 L) induced lung tissue displacement resembling diaphragmatic breathing. This system was tested on five porcine heart-lung preparations using 1.5T MRI with transverse and coronal 3D-GRE (TR/TE = 3.63/1.58, 256 x 256 matrix, 350 mm FOV, 4 mm slices) and half Fourier T2-FSE (TR/TE = 545/29, 256 x 192, 350 mm, 6 mm) as well as multiple row detector CT (16 x 1 mm collimation, pitch 1.5, FOV 400 mm, 120 mAs) acquired at five fixed inspiration levels. Dynamic CT scans and coronal MRI with dynamic 2D-GRE and 2D-SS-GRE sequences (image frequencies of 10/sec and 3/sec, respectively) were acquired during continuous "breathing" (7/minute). The position of the piston pump was visually correlated with the respiratory motion visible through the transparent wall of the phantom and with dynamic displays of CT and MR images. An elastic body splines analysis of the respiratory motion was performed using CT data.

RESULTS

Visual evaluation of MRI and CT showed three-dimensional movement of the lung tissue throughout the respiration cycle. Local tissue displacement inside the lung explants was documented with motion maps calculated from CT. The maximum displacement at the top of the diaphragm (mean 26.26 [SD 1.9] mm on CT and 27.16 [SD 1.5] mm on MRI, respectively [p = 0.25; Wilcoxon test]) was in the range of tidal breathing in human patients.

CONCLUSION

The chest phantom with a diaphragmatic pump is a promising platform for multi-modality imaging studies of the effects of respiratory lung motion.

[Simulation and experimental study of cone artifact in spiral CT]

OBJECTIVE

To study regular patterns of cone artifact resulted from interpolation algorithm of spiral CT.

METHODS

Based on the principle of interpolation algorithm and back-projection reconstruction, a mathematical model of the reconstructed image was established to clarify the relation of the scanning parameters and the characteristics of the scanned object with the cone artifact. Experiments were carried out by a set of acrylic phantoms on siemens plus 4 CT scanners.

RESULTS

The artifact in the image was directly proportional to the table increment per gantry rotation of the scanner, and was positively correlated to the tangent of half cone-angle and inversely to the radii in the reconstruction plane of the phantom. The theoretical analysis was validated by experimental results.

CONCLUSION

The cone artifact is related to the scanning parameters and the characteristics of the scanned object.