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Purpose: The purpose of this study was to verify the dosimetric performance of Acuros XB (AXB), a grid-based Boltzmann solver, in intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT). Methods: The Radiological Physics Center (RPC) head and neck (H&N) phantom was used for all calculations and measurements in this study. Clinically equivalent IMRT and VMAT plans were created on the RPC H&N phantom in the Eclipse treatment planning system (version 10.0) by using RPC dose prescription specifications. The dose distributions were calculated with two different algorithms, AXB 11.0.03 and anisotropic analytical algorithm (AAA) 10.0.24. Two dose report modes of AXB were recorded: dose-to-medium in medium (Dm,m) and dose-to-water in medium (Dw,m). Each treatment plan was delivered to the RPC phantom three times for reproducibility by using a Varian Clinac iX linear accelerator. Absolute point dose and planar dose were measured with thermoluminescent dosimeters (TLDs) and GafChromic® EBT2 film, respectively. Profile comparison and 2D gamma analysis were used to quantify the agreement between the film measurements and the calculated dose distributions from both AXB and AAA. The computation times for AAA and AXB were also evaluated. Results: Good agreement was observed between measured doses and those calculated with AAA or AXB. Both AAA and AXB calculated doses within 5% of TLD measurements in both the IMRT and VMAT plans. Results of AXB_Dm,m (0.1% to 3.6%) were slightly better than AAA (0.2% to 4.6%) or AXB_Dw,m (0.3% to 5.1%). The gamma analysis for both AAA and AXB met the RPC 7%/4 mm criteria (over 90% passed), whereas AXB_Dm,m met 5%/3 mm criteria in most cases. AAA was 2 to 3 times faster than AXB for IMRT, whereas AXB was 4–6 times faster than AAA for VMAT. Conclusions: AXB was found to be satisfactorily accurate when compared to measurements in the RPC H&N phantom. Compared with AAA, AXB results were equal to or better than those obtained with film measurements for IMRT and VMAT plans. The AXB_Dm,m reporting mode was found to be closer to TLD and film measurements than was the AXB_Dw,m mode. AXB calculation time was found to be significantly shorter (× 4) than AAA for VMAT.

Purpose: The deterministic Acuros XB (AXB) algorithm was recently implemented in the Eclipse treatment planning system. The goal of this study was to compare AXB performance to Monte Carlo (MC) and two standard clinical convolution methods: the anisotropic analytical algorithm (AAA) and the collapsed-cone convolution (CCC) method. Methods: Homogeneous water and multilayer slab virtual phantoms were used for this study. The multilayer slab phantom had three different materials, representing soft tissue, bone, and lung. Depth dose and lateral dose profiles from AXB v10 in Eclipse were compared to AAA v10 in Eclipse, CCC in Pinnacle 3 , and EGSnrc MC simulations for 6 and 18 MV photon beams with open fields for both phantoms. In order to further reveal the dosimetric differences between AXB and AAA or CCC, three-dimensional (3D) gamma index analyses were conducted in slab regions and subregions defined by AAPM Task Group 53. Results: The AXB calculations were found to be closer to MC than both AAA and CCC for all the investigated plans, especially in bone and lung regions. The average differences of depth dose profiles between MC and AXB, AAA, or CCC was within 1.1, 4.4, and 2.2%, respectively, for all fields and energies. More specifically, those differences in bone region were up to 1.1, 6.4, and 1.6%; in lung region were up to 0.9, 11.6, and 4.5% for AXB, AAA, and CCC, respectively. AXB was also found to have better dose predictions than AAA and CCC at the tissue interfaces where backscatter occurs. 3D gamma index analyses (percent of dose voxels passing a 2%/2 mm criterion) showed that the dose differences between AAA and AXB are significant (under 60% passed) in the bone region for all field sizes of 6 MV and in the lung region for most of field sizes of both energies. The difference between AXB and CCC was generally small (over 90% passed) except in the lung region for 18 MV 10 � 10 cm 2 fields (over 26% passed) and in the bone region for 5 � 5 and 10 � 10 cm 2 fields (over 64% passed). With the criterion relaxed to 5%/2 mm, the pass rates were over 90% for both AAA and CCC relative to AXB for all energies and fields, with the exception of AAA 18 MV 2.5 � 2.5 cm 2 field, which still did not pass. Conclusions: In heterogeneous media, AXB dose prediction ability appears to be comparable to MC and superior to current clinical convolution methods. The dose differences between AXB and AAA or CCC are mainly in the bone, lung, and interface regions. The spatial distributions of these differences depend on the field sizes and energies. V C 2011 American Association of Physicists in Medicine. [DOI: 10.1118/1.3582690]

Purpose: In this article, we describe a method to estimate the spatial dose variation, average dose and mean glandular dose (MGD) for a real breast using Monte Carlo simulation based on cone beam breast computed tomography (CBBCT) images. We present and discuss the dose estimation results for 19 mastectomy breast specimens, 4 homogeneous breast models, 6 ellipsoidal phantoms, and 6 cylindrical phantoms. Methods: To validate the Monte Carlo method for dose estimation in CBBCT, we compared the Monte Carlo dose estimates with the thermoluminescent dosimeter measurements at various radial positions in two polycarbonate cylinders (11- and 15-cm in diameter). Cone-beam computed tomography (CBCT) images of 19 mastectomy breast specimens, obtained with a bench-top experimental scanner, were segmented and used to construct 19 structured breast models. Monte Carlo simulation of CBBCT with these models was performed and used to estimate the point doses, average doses, and mean glandular doses for unit open air exposure at the iso-center. Mass based glandularity values were computed and used to investigate their effects on the average doses as well as the mean glandular doses. Average doses for 4 homogeneous breast models were estimated and compared to those of the corresponding structured breast models to investigate the effect of tissue structures. Average doses for ellipsoidal and cylindrical digital phantoms of identical diameter and height were also estimated for various glandularity values and compared with those for the structured breast models. Results: The absorbed dose maps for structured breast models show that doses in the glandular tissue were higher than those in the nearby adipose tissue. Estimated average doses for the homogeneous breast models were almost identical to those for the structured breast models(p=1). Normalized average doses estimated for the ellipsoidal phantoms were similar to those for the structured breast models (root mean square (rms) percentage difference=1.7%;p=0.01), whereas those for the cylindrical phantoms were significantly lower (rms percentage difference=7.7%;p

Purpose: In this study, the authors investigated the feasibility of a dual resolution volume-ofinterest VOI cone beam breast CT technique and compared two implementation approaches in terms of dose saving and scatter reduction. Methods: With this technique, a lead VOI mask with an opening is inserted between the x-ray source and the breast to deliver x-ray exposure to the VOI while blocking x rays outside the VOI. A CCD detector is used to collect the high resolution projection data of the VOI. Low resolution cone beam CT CBCT images of the entire breast, acquired with a flat panel FP detector, were used to calculate the projection data outside the VOI with the ray-tracing reprojection method. The Feldkamp‐Davis‐Kress filtered backprojection algorithm was used to reconstruct the dual resolution 3D images. Breast phantoms with 180 m and smaller microcalcifications MCs were imaged with both FP and FP-CCD dual resolution CBCT systems, respectively. Two approaches of implementing the dual resolution technique, breast-centered approach and VOI-centered approach, were investigated and evaluated for dose saving and scatter reduction with Monte Carlo simulation using a GEANT4 package. Results: The results showed that the breast-centered approach saved more breast absorbed dose than did VOI-centered approach with similar scatter reduction. The MCs in fatty breast phantom, which were invisible with FP CBCT scan, became visible with the FP-CCD dual resolution CBCT scan. Conclusions: These results indicate potential improvement of the image quality inside the VOI with reduced breast dose both inside and outside the VOI. © 2009 American Association of Physicists in Medicine. DOI: 10.1118/1.3187225

This work is to demonstrate that high quality cone beam CT images can be generated for a volume of interest (VOI) and to investigate the exposure reduction effect, dose saving, and scatter reduction with the VOI scanning technique. The VOI scanning technique involves inserting a filtering mask between the x-ray source and the breast during image acquisition. The mask has an opening to allow full x-ray exposure to be delivered to a preselected VOI and a lower, filtered exposure to the region outside the VOI. To investigate the effects of increased noise due to reduced exposure outside the VOI on the reconstructed VOI image, we directly extracted the projection data inside the VOI from the full-field projection data and added additional data to the projection outside the VOI to simulate the relative noise increase due to reduced exposure. The nonuniform reference images were simulated in an identical manner to normalize the projection images and measure the x-ray attenuation factor for the object. Regular Feldkamp–Davis–Kress filtered backprojection algorithm was used to reconstruct the 3D images. The noise level inside the VOI was evaluated and compared with that of the full-field higher exposure image. Calcifications phantom and low contrast phantom were imaged. Dose reduction was investigated by estimating the dose distribution in a cylindrical water phantom using Monte Carlo simulation based Geant4 package. Scatter reduction at the detector input was also studied. Our results show that with the exposure level reduced by the VOI mask, the dose levels were significantly reduced both inside and outside the VOI without compromising the accuracy of image reconstruction, allowing for the VOI to be imaged with more clarity and helping to reduce the breast dose. The contrast-to-noise ratio inside the VOI was improved. The VOI images were not adversely affected by noisier projection data outside the VOI. Scatter intensities at the detector input were also shown to decrease significantly both inside and outside the VOI in the projection images, indicating potential improvement of image quality inside the VOI and contribution to dose reduction both inside and outside the VOI.

Anti-scatter grids have been widely used to reject scatter and increase the perceptibility of low-contrast object in chest radiography; however they also attenuate the primary x-rays, resulting in a substantial degradation of primary information. Compensation for this degradation requires the use of higher exposure technique hence higher dose to the patient. A more efficient approach to reject scatter is the slot-scan imaging technique which employs a narrow scanning x-ray fan beam in conjunction with a slit or slot shaped solid state detector or an area detector used with an aft-collimator. With this approach, scatter can be rejected effectively without the need to attenuate primary x-rays. This paper demonstrates an electronic aft-collimation method, referred to as the alternate line erasure and readout (ALER) technique, for implementing the slot-scan digital radiography with a modern flat-panel detector. With this technique, instead of first exposing the detector and then reading the image line by line, the image line on the leading edge of the scanning fan beam is reset to erase the scatter accumulated prior to the arrival of the fan beam x-rays, while the image line on the trailing edge of the scanning fan beam is read out to acquire the image signals following the fan-beam exposure. These reset and readout processes are alternated and repeated as the x-ray fan beam scans across the detector. An anthropomorphic chest phantom was imaged to evaluate the scatter rejection ability and the low-contrast performance for the ALER technique and compare them with those for the anti-scatter grid method in full-field chest imaging. With a projected beam width of 16 mm , the slot-scan/ALER technique resulted in an average reduction of the scatter-to-primary ratios by 81%, 84%, 82%, and 86% versus 65%, 73%, 74%, and 73% with the anti-scatter grid method in the lungs, mediastinum, retrocardium, and subdiaphragm, respectively. The average CNR for the slot-scan/ALER technique was found to improve by 135%, 133%, 176%, and 87% versus 15%, 15%, 38%, and − 11 % with the anti-scatter grid method in the mediastinum, retrocardium, subdiaphragm, and lungs, respectively. These results demonstrated that the slot-scan/ALER technique can be used to achieve equally effective scatter rejection but substantially higher low-contrast performance than the anti-scatter grid method.

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 were 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 intra-slice and inter-slice variations in adipose tissue CT numbers supported our observations.

This work is to demonstrate that high quality cone beam CT images can be generated for a volume of interest (VOI) and to investigate the exposure reduction effect, dose saving, and scatter reduction with the VOI scanning technique. The VOI scanning technique involves inserting a filtering mask between the x-ray source and the breast during image acquisition. The mask has an opening to allow full x-ray exposure to be delivered to a preselected VOI and a lower, filtered exposure to the region outside the VOI. To investigate the effects of increased noise due to reduced exposure outside the VOI on the reconstructed VOI image, we directly extracted the projection data inside the VOI from the full-field projection data and added additional data to the projection outside the VOI to simulate the relative noise increase due to reduced exposure. The nonuniform reference images were simulated in an identical manner to normalize the projection images and measure the x-ray attenuation factor for the object. Regular Feldkamp–Davis–Kress filtered backprojection algorithm was used to reconstruct the 3D images. The noise level inside the VOI was evaluated and compared with that of the full-field higher exposure image. Calcifications phantom and low contrast phantom were imaged. Dose reduction was investigated by estimating the dose distribution in a cylindrical water phantom using Monte Carlo simulation based Geant4 package. Scatter reduction at the detector input was also studied. Our results show that with the exposure level reduced by the VOI mask, the dose levels were significantly reduced both inside and outside the VOI without compromising the accuracy of image reconstruction, allowing for the VOI to be imaged with more clarity and helping to reduce the breast dose. The contrast-to-noise ratio inside the VOI was improved. The VOI images were not adversely affected by noisier projection data outside the VOI. Scatter intensities at the detector input were also shown to decrease significantly both inside and outside the VOI in the projection images, indicating potential improvement of image quality inside the VOI and contribution to dose reduction both inside and outside the VOI.

Mammography is the only technique currently used for detecting microcalcification (MC) clusters, an early indicator of breast cancer. However, mammographic images superimpose a three-dimensional compressed breast image onto two-dimensional projection views, resulting in overlapped anatomical breast structures that may obscure the detection and visualization of MCs. One possible solution to this problem is the use of cone beam computed tomography (CBCT) with a flat-panel (FP) digital detector. Although feasibility studies of CBCT techniques for breast imaging have yielded promising results, they have not shown how radiation dose and x-ray tube voltage affect the accuracy with which MCs are detected by CBCT experimentally. We therefore conducted a phantom study using a FP-based CBCT system with various mean glandular doses and kVp values. An experimental CBCT scanner was constructed with a data acquisition rate of 7.5 frames/s. 10.5 and 14.5 cm diameter breast phantoms made of gelatin were used to simulate uncompressed breasts consisting of 100% glandular tissue. Eight different MC sizes of calcium carbonate grains, ranging from 180-200 microm to 355-425 microm, were used to simulate MCs. MCs of the same size were arranged to form a 5 x 5 MC cluster and embedded in the breast phantoms. These MC clusters were positioned at 2.8 cm away from the center of the breast phantoms. The phantoms were imaged at 60, 80, and 100 kVp. With a single scan (360 degrees), 300 projection images were acquired with 0.5 x, 1x, and 2x mean glandular dose limit for 10.5 cm phantom and with 1x, 2x, and 4x for 14.5 cm phantom. A Feldkamp algorithm with a pure ramp filter was used for image reconstruction. The normalized noise level was calculated for each x-ray tube voltage and dose level. The image quality of the CBCT images was evaluated by counting the number of visible MCs for each MC cluster for various conditions. The average percentage of the visible MCs was computed and plotted as a function of the MGD, the kVp, and the average MC size. The results showed that the MC visibility increased with the MGD significantly but decreased with the breast size. The results also showed that the x-ray tube voltage affects the detection of MCs under different circumstances. With a 50% threshold, the minimum detectable MC sizes for the 10.5 cm phantom were 348(+/-2), 288(+/-7), 257(+/-2) microm at 3, 6, and 12 mGy, respectively. Those for the 14.5 cm phantom were 355 (+/-1), 307 (+/-7), 275 (+/-5) microm at 6, 12, and 24 mGy, respectively. With a 75% threshold, the minimum detectable MC sizes for the 10.5 cm phantom were 367 (+/-1), 316 (+/-7), 265 (+/-3) microm at 3, 6, and 12 mGy, respectively. Those for the 14.5 cm phantom were 377 (+/-3), 334 (+/-5), 300 (+/-2) microm at 6, 12, and 24 mGy, respectively.

Purpose: To validate a Monte Carlo model and evaluate the dosimetric capabilities of a novel commercial CT/MR compatible Fletcher applicator for cervical cancer brachytherapy. Methods: MCNPX 2.7.0 was used to model the Fletcher CT/MR shielded applicator (FA) and 192Ir HDR source. Energy deposition was calculated with a track length estimator modified by an energy-dependent heating function. A high density polystyrene phantom was constructed with three film pockets for validation of the MCNPX model. Three planes of data were calculated with the MCNPX model corresponding to the three film planes in phantom. The planes were located 1 cm from the most anterior, posterior, and medial extents of the FA right ovoid. Unshielded distributions were calculated by modeling the shielded cells as air instead of the tungsten alloy. A third order polynomial fit to the OD to dose curve was used to convert OD of the three film planes to dose. Each film and MCNPX plane dose distribution was normalized to a point 2 cm from the center of the film plane and in a region of low dose gradient. MCNPX and film were overlaid and compared with a distance-to-agreement criterion of (±2%/±2mm). Shielded and unshielded distributions were overlaid and a percent shielded plot was created. Results: 85.2%, 97.1%, and 96.6% of the MCNPX points passed the (±2%/±2mm) criterion respectively for the anterior, lateral, and posterior film comparison planes. A majority of the points in the anterior plane that exceeded the DTA criterion were either along edges of where the film was cut or near the terminal edges of the film. The percent shielded matrices indicated that the maximum % shielding was 50%. Conclusion: These data confirm the validity of the FA Monte Carlo model. The FA ovoid can shield up to 50% of the dose in the anteroposterior direction.

Purpose: To quantify the difference in dose to PTV_EVAL and OARs (skin and rib) as calculated by (TG43) and heterogeneous calculations (CCC). Methods: 25 patient plans (5 Contura and 20 SAVI) were selected for analysis. Clinical dose distributions were computed with a commercially available treatment planning algorithm (TG43-D_(w,w)) and then recomputed with a pre-clinical collapsed cone convolution algorithm (CCCD_( m,m)). PTV_EVAL coverage (V90%, V95%), and rib and skin maximum dose were compared via percent difference. Differences in dose to normal tissue (V150cc, V200cc of PTV_EVAL) were also compared. Changes in coverage and maximum dose to organs at risk are reported in percent change, (100*(TG43 − CCC) / TG43)), and changes in maximum dose to normal tissue are absolute change in cc (TG43 − CCC). Results: Mean differences in V90, V95, V150, and V200 for the SAVI cases were −0.2%, −0.4%, −0.03cc, and −0.14cc, respectively, with maximum differences of −0.78%, −1.7%, 1.28cc, and 1.01cc, respectively. Mean differences in the 0.1cc dose to the rib and skin were −1.4% and −0.22%, respectively, with maximum differences of −4.5% and 16%, respectively. Mean differences in V90, V95, V150, and V200 for the Contura cases were −1.2%, −2.1%, −1.8cc, and −0.59cc, respectively, with maximum differences of −2.0%, −3.16%, −2.9cc, and −0.76cc, respectively. Mean differences in the 0.1cc dose to the rib and skin were −2.6% and −3.9%, respectively, with maximum differences of −3.2% and −5.7%, respectively. Conclusion: The effects of translating clinical knowledge based on D_(w,w) to plans reported in D_(m,m) are minimal (2% or less) on average, but vary based on the type and placement of the device, source, and heterogeneity information.

Purpose: Stereography can provide a visualization of the skin surface for radiation therapy patients. The aim of this study was to verify the registration algorithm in a commercial image analysis software, 3dMDVultus, for the fusion of stereograms and CT images. Methods: CT and stereographic scans were acquired of a head phantom and a deformable phantom. CT images were imported in 3dMDVultus and the surface contours were generated by threshold segmentation. Stereograms were reconstructed in 3dMDVultus. The resulting surfaces were registered with Vultus algorithm and then exported to in-house registration software and compared with four algorithms: rigid, affine, non-rigid iterative closest point (ICP) and b-spline algorithm. RMS (root-mean-square residuals of the surface point distances) error between the registered CT and stereogram surfaces was calculated and analyzed. Results: For the head phantom, the maximum RMS error between registered CT surfaces to stereogram was 6.6 mm for Vultus algorithm, whereas the mean RMS error was 0.7 mm. For the deformable phantom, the maximum RMS error was 16.2 mm for Vultus algorithm, whereas the mean RMS error was 4.4 mm. Non-rigid ICP demonstrated the best registration accuracy, as the mean of RMS errors were both within 1 mm. Conclusion: The accuracy of registration algorithm in 3dMDVultus was verified and exceeded RMS of 2 mm for deformable cases. Non-rigid ICP and b-spline algorithms improve the registration accuracy for both phantoms, especially in deformable one. For those patients whose body habitus deforms during radiation therapy, more advanced nonrigid algorithms need to be used.

Purpose: To implement and investigate a total variation based reconstruction algorithm for cone beam breast CT. The algorithm is focused on reconstruction using under‐sampled projection views. Method and Materials: The implementation of the algorithm includes two major steps. In the reconstruction step, an algebraic reconstruction method is adopted. In the next step, the total variation is calculated and minimized. The initial image is updated and the iteration repeats. A segmented breast model was used to test the algorithm in this study. The cone beam CTimages of a mastectomy breast specimen were corrected, filtered and segmented to form a breast model which consists of a 3D map of dense tissue structures. Re‐projection of the model was conducted with 9 degree angular increment to generate 40 projection images over 360°. The total variation based algorithm was then used to reconstruct the breast model with the under‐sampled projection views. The reconstructed images were compared with the original breast model and the percentage deviation was computed for the evaluation. Results: Iterative reconstruction with total variation minimization was performed on under‐ sampled re‐projections of the breast model. The structures of the breast were satisfactorily reconstructed, compared to the breast model. The percentage deviation was found to be 7.0%. Further improvement and optimization of the algorithm are under progress. Conclusion: The preliminary reconstruction result of a breast model demonstrates that under‐sampled breast CT projections can be reconstructed using the total variation iterative method, which provides novel opportunity for breast CT study.

Accurate and fast projector/back‐projector (PBP) is the key to successful simulation and iterative reconstruction of tomosynthesis and cone beam CT. In this study, the accuracy and computation speed of a ray‐driven PBP operator were investigated. To simulate x‐ray tomographyimaging, a ray‐driven projector was developed and implemented on a 64 computing node PC Cluster. Each x‐ray path is represented by sampling points in the object. Their μ values are summed up to compute the integral attenuation along the path. To evaluate the accuracy of the reprojection algorithm as the function of sampling ratio (Sampling length / voxel length), reprojections obtained with a significantly smaller sampling ratio were used as reference. To minimize the additional loss of accuracy from pixelization, the pixel size of the projection images was selected to be one third of the voxel size projected back to the image plane. Error images were formed by subtracting the re‐projection from the reference re‐projection and used to compute the percentage errors. SART were implemented for image reconstruction in tomosynthesis and CBCTimaging. To evaluate its accuracy, CBCTimagesreconstructed with 300 projection views over 360 degree were compared to the original CBCTimages. Errors were computed from the subtraction images and plotted together the computing time as the function of the sampling ratio. With reduced sampling ratio, the accuracies of both the re‐projection and SART algorithms were improved at the expense of longer computing time. The improvement was accelerated with smaller sampling ratios. Aliasing artifacts were visible when the sampling ratios were greater than 0.5. We have demonstrated that the accuracy of the re‐projection and SART reconstructions improved with reduced the sampling ratio at the expense of longer computing time for a ray‐driven PBP. The tradeoff between accuracy and computing time should be determined by the imaging requirement.

Purpose: To estimate and study the dose saving and scatter reduction properties of the VOI cone beam CT technique. Method and Materials: To implement the VOI scanning technique, a filter with a circular or rectangular opening is inserted between the x‐ray source and the patient to deliver a higher level x‐ray exposure inside the VOI and a lower exposure level outside the VOI in acquiring the projection images. This technique is expected to result in higher image quality inside the VOI with lower entire patient dose and significantly lowered scatter‐to‐primary ratios (SPRs) within the VOI. Modeling the patient as a cylinder of soft tissue, we have developed Monte Carlo simulation programs based on the Geant4 package to estimate the dose to the patient and the SPRs at the detector input. Simulation was performed at selected locations to estimate the variation of the dose level and SPRs from inside to the outside of the VOI. Simulation was also performed to estimate the reduction of the dose levels and SPRs with the VOI scanning technique as compared to full field cone beam CT.Results: Our simulation results show that with the exposure level reduced by the VOI filter, dose levels for the entire scan were significantly reduced both inside and outside the VOI. Scatter intensities at the detector input were also shown to decrease significantly both inside and outside the VOI in the projection images.Conclusion: Our simulation study successively demonstrated the ability of the VOI scanning technique to reduce patient dose and x‐ray scatter for improved image quality. Acknowledgement: This work was supported in part by a research grant EB000117 from the NIH‐NIBIB and a research grant CA104759 from the NIH‐NCI. The author also would like to thank Dr. Ioannis Sechopoulos and Yu Chen for consulting assistance in Geant4 programming.

Purpose: The dosimetric uncertainties in junction regions are one of the major concerns in treatments involving field-junctions, especially tangential breast and supraclavicular fossa radiotherapy fields. The uncertainties are linked to dose calculation algorithms, set-up, jaw/couch position uncertainties, and choice of mono-versus multiple-isocentric techniques. The purpose of this study was to comprehensively investigate the effects of these common variants to understand dose non-uniformity in junction regions. Methods: To understand the effects from dose algorithms and setup uncertainties, a typical tangential/superclav field combination was planned to an anthropomorphic Rando phantom using the Pinnacle-9.0 treatment-planning system and delivered 6 times on different days. 3 films (EBT2) were placed in coronal plane across the field-junction. Pinnacle CCC-9.0, Eclipse AAA-10.0 and AcurosXB-11.0 were used for calculation. To understand the effects from the linac's mechanical uncertainties, junctions were generated with two open fields using both mono-isocentric and two-isocentric techniques. Junction-fields were delivered on 6 different linacs to EBT2 in a solid water phantom. Impacts of couch position accuracy/reproducibility, and jaw-position accuracy/reproducibility were analyzed. Results: In general, the major contributor to the dose uncertainties in junction region is the jaw position accuracy. Among the 6 linacs investigated in this study, the uncertainty of jaw position can be up to ±3mm, subsequently leading to about 25% hot/cold of dose distributions in junction region. The impact of mono versus two-isocentric technique is relatively small (within 5%), because the patient-couch positioning was found to have good accuracy and reproducibility. All dose algorithms are found to be in close mutual agreement (within 5%) for most regions of the junction. Jaw positioning has good short-term reproducibility, but might change over months. Conclusion: Dose inhomogeneities at field-junctions are more strongly dependent on jaw-positioning accuracy compared to setup, dose-calculation, and other source of errors.Impact:Guidance to improve dose uniformity in field-junction. This work was funded by National Institutes of Health grants 2R44CA105806-02, CA010953

Purpose: The novel deterministic radiation transport algorithm, Acuros XB (AXB), has shown great potential for accurate heterogeneous dose calculation. However, the clinical impact between AXB and other currently used algorithms still needs to be elucidated for translation between these algorithms. The purpose of this study was to investigate the impact of AXB for heterogeneous dose calculation in lung cancer for intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT). Methods: The thorax phantom from the Radiological Physics Center (RPC) was used for this study. IMRT and VMAT plans were created for the phantom in the Eclipse 11.0 treatment planning system. Each plan was delivered to the phantom three times using a Varian Clinac iX linear accelerator to ensure reproducibility. Thermoluminescent dosimeters (TLDs) and Gafchromic EBT2 film were placed inside the phantom to measure delivered doses. The measurements were compared with dose calculations from AXB 11.0.21 and the anisotropic analytical algorithm (AAA) 11.0.21. Two dose reporting modes of AXB, dose-to-medium in medium (Dm,m) and dose-to-water in medium (Dw,m), were studied. Point doses, dose profiles, and gamma analysis were used to quantify the agreement between measurements and calculations from both AXB and AAA. The computation times for AAA and AXB were also evaluated. Results: For the RPC lung phantom, AAA and AXB dose predictions were found in good agreement to TLD and film measurements for both IMRT and VMAT plans. TLD dose predictions were within 0.4%–4.4% to AXB doses (both Dm,m and Dw,m); and within 2.5%–6.4% to AAA doses, respectively. For the film comparisons, the gamma indexes (±3%/3 mm criteria) were 94%, 97%, and 98% for AAA, AXB_Dm,m, and AXB_Dw,m, respectively. The differences between AXB and AAA in dose–volume histogram mean doses were within 2% in the planning target volume, lung, heart, and within 5% in the spinal cord. However, differences up to 8% between AXB and AAA were found at lung/soft tissue interface regions for individual IMRT fields. AAA was found to be 5–6 times faster than AXB for IMRT, while AXB was 4–5 times faster than AAA for VMAT plan. Conclusions: AXB is satisfactorily accurate for the dose calculation in lung cancer for both IMRT and VMAT plans. The differences between AXB and AAA are generally small except in heterogeneous interface regions. AXB Dw,m and Dm,m calculations are similar inside the soft tissue and lung regions. AXB can benefit lung VMAT plans by both improving accuracy and reducing computation time.

Purpose: To evaluate the heterogeneity corrected dose calculations from the Acuros XB (AXB), a novel deterministic dose calculation algorithm based on grid‐based Boltzmann transport equation solver (GBBS), for IMRT and VMAT plans. Methods: The Radiological Physics Center's lung phantom was used to create clinically equivalent IMRT and VMAT plans (RapidArc) with the Eclipse planning system 10.0 that were delivered using a Varian 23 iX. Absolute doses and relative dose distributions were measured with thermoluminescent dosimeters(TLDs) and radiochromic film. The measured dose distributions were compared with calculated doses from both AXB (11.0.3) and AAA (10.0.24) dose calculation algorithms. The AXB calculated dose‐to‐water and dose‐to‐medium were both compared to measurements. Gamma analysis (±7%/4mm, ±5%/3mm, and ±3%/3mm) was used to quantify correspondence between AXB dose distributions and the film measurements. The computation time between AAA and AXB were also evaluated. Results: For TLD point doses, both AAA and AXB heterogeneity corrected dose calculations are within 5% inside the PTV for both IMRT and VMAT plans. The agreements observed between the measured and calculated doses for both AXB dose reporting methods are better than those observed with the AAA algorithm. The gamma analysis showed that the differences between AAA, AXB and film measurement met the RPC ±7%/4 mm criteria. The percent of pixels passing rate for both the AXB dose to medium and AXB dose to water are higher than AAA. The computation time between AAA and AXB are comparable for IMRT plans but AXB is significantly faster (4 times) than AAA for VMAT plans. Conclusions: The AXB implemented in the Eclipse planning system calculates a more accurate heterogeneity corrected dose than the AAA algorithm as compared to measurement in lung and improve the calculation speed for VMAT radiotherapy. Work supported by grants CA10953, CA81647, 2R44CA105806‐02, CA016672 (NCI, DHHS).

Purpose: Acuros XB (AXB), a novel deterministic dose calculation algorithm based on grid‐based Boltzmann transport equation solver (GBBS), was recently implemented in Eclipse treatment planning system (TPS). The purposes of this study were to verify the dosimetric performance of AXB by (1) comparing measured and calculated data and (2) comparing AXB with the existing Eclipse anisotropic analytical algorithm (AAA). Methods: Clinically equivalent intensity modulated radiation therapy(IMRT) and volumetric modulated arc therapy (VMAT) plans were created in Eclipse for the Radiological Physics Center (RPC) head and neck IMRT phantom using the RPC phantom dose prescription specifications. Each plan was calculated with two different algorithms, AXB 11.0.91 and AAA 10.0.24. Each treatment plan was delivered to the RPC phantom three times for reproducibility using a Varian Clinac21iX. Dose distributions were measured with thermoluminescent dosimeters(TLDs) and radiochromic film (GafChromic® EBT2). Gamma analysis (7%/4mmDTA) was used to quantify the agreement between AXB and AAA calculated dose distributions and those measured using films. The computation times for AAA and AXB were also evaluated. Results: In this study, good agreement was observed between measured and calculated doses both from AAA and AXB (dose‐to‐medium in medium). Comparing the calculations with TLD measurements, agreement for both AAA and AXB were < 5% (ranges from 0.2% to 4.6% for AAA, 0.1% to 3.6% for AXB) for IMRT and VMAT plans. The gamma analysis results for both AAA and AXB met the RPC 7%/4 mm criteria (over 90% passed). The AAA and AXB computation times were comparable for IMRT but AXB was ∼3 times faster than AAA for VMAT. Conclusions: The AXB was determined to be accurate using the RPC IMRT H&N phantom measurements and in good agreement with the AAA while having shorter calculation times for VMAT as compared to the AAA.

Purpose: To investigate the effects of under‐sampled and limited angle‐sampled projections on microcalcification visibility in cone beam breast CT.Materials and Methods: Different sizes of calciumcarbonate grains were embedded in a gelatin cylinder to simulate microcalcifications in dense breasts. The phantom was then imaged at 80 kVp with a bench‐top CBCTsystem, which employs a‐Si/CsI based flat panel detector with a pixel size of 194 microns operated in the non‐binning mode. For scans with under‐sampling technique, projections corresponding to 30, 50, 60, 75, 100, and 150 equally spaced views were acquired over 360 degrees with adjusted mAs per image to keep the total mAs the same. The resulting CBCTimages were compared to those obtained with 300 projections, which have been considered adequate for breast imaging. For scans with limited angle sampling technique, an angular range of 60, 72, 90, and 120 degrees were chosen to acquire projections with an increased mAs per image while a reduced mAs per image was used to acquire projections for views outside this angular range to generate a complete projection image set with the same total mAs. Feldkamp algorithm with a ramp filter was used for image reconstruction. The reconstructed images were sequentially displayed on a review workstation and reviewed by a reader at two different times. The reader was asked to indicate the visible microcalcifications, of which the ratios were calculated over all readings. Result: The visibility ratio was found to increase with the size range of the calcifications with both scanning techniques. It was found that the number of projections had no significant effect on the calcification visibility. However, the visibility ratio increased with the angular range for high exposure projections. (supported in part by research grants CA104759 and CA124585 from NCI and EB000117 from NIBIB, and a subcontract from NIST‐ATP)

Purpose: Monte‐Carlo (MC) simulation was used to estimate doses for breast models derived from segmented CBCTimages of mastectomy breast specimens. The results were compared to those estimated for half‐ellipsoidal and cylindrical breast phantoms to investigate the accuracy in using them to represent real breasts in dose measurement or estimation. Method and Materials: A previously validated EGSnrc MC package was used to form a half cone beam to simulate cone beam CT at 80kVp. CBCTimages of 10 mastectomy breast specimens, covering a 130.5 mm diameter, 97.9 mm height, were segmented and binned into 100×100×75 image sets for MC simulation. Half‐ellipsoidal and cylindrical breast phantoms, both with a diameter of 13 cm and a height of 10cm and a breast density of 0%, 20%, 40%, 60%, 80% and 100%, were constructed and used in MC simulation. DOSXYZnrc was used to track 2×108 incident photons to simulate 300 projection images over 360°. The resulting radiation dose was tallied for each voxel to study the dose distribution and average dose. Results: Our study indicates that average doses estimated for the segmented breast models were similar to those for the ellipsoidal phantoms but significantly different from those for the cylindrical phantoms. Dose distributions for the ellipsoidal and cylindrical phantoms were found to be similar along a radius in the coronal direction close to the chest. However, those along the rotating axis were found to be different. Doses for the cylindrical phantoms were found to be smaller by as much as 56% in the slice closest to the nipple. Conclusion: It was found that the ellipsoidal phantoms provided a better representation for patient breasts than the cylindrical phantoms in MC dose estimation. This indicates that an ellipsoidal phantom of same dimension and average density should be used in MC dose estimation or actual dose measurement.

Purpose: By using Monte Carlo(MC) dose calculation to investigate the variation of radiation dose with the breast density (tissue composition) in cone beam breast CT, both globally and on a voxel‐by‐voxel basis. Method and Materials: To simulate cone beam breast CTimaging, we first used Rep78 to build an x‐ray spectrum for an x‐ray tube with tungsten target and a HVL of 4.08mm Al. A collimator was defined in BEAMnrc and used to form a half‐cone beam for pendant geometry used in cone beam breast CT. For validation, we compared MC dose calculations with ionization chamber measurements at 4 different locations inside the 11 cm diameter, 12 cm high cylindrical phantom under the same imaging conditions. After normalizing for the iso‐center dose, the deviations were within 3.7%. 3 cone beam CTimages were first corrected and filtered to minimize the cupping artifacts and then filtered to reduce the noise level. The processed images were then segmented using threshold method to separate the dense tissue from the adipose tissue. The segmented image sets were binned into a 100×100×80 image sets with a pixel size of 1mm for MC calculation. DOSXYZnrc was used to track 1×109 incident photons distributed over 300 projection images over 360°. The resulting radiation dose was tallied for each voxel and averaged over all voxels in the breast. Results: Results from MC calculations indicated that radiation dose was significantly higher in dense tissue structures and lower in the adipose tissue. The regions of higher dose were found to match the 3D dense tissue structures. The average breast dose was found to linearly vary with the breast density computed from the breast CT data. Conclusion: Our MC study has shown a close relationship between the radiation dose and the breast density both globally and on a voxel‐by‐voxel basis.

Purpose: To implement an a‐Se FP detector based SEDR system for chest imaging and to investigate scatter rejection properties and low‐contrast performance of the system.Method and Materials: A prototype SEDR system was implemented with an a‐Se FP detector to improve image quality in heavily attenuating regions. Slot‐scan imaging geometry was used to reduce x‐ray scatter without attenuating primary x‐rays by modifying FP's readout for electronic aft‐collimation. A 24‐segment fan‐beam width modulator assembly was developed to equalize x‐ray exposures at detector input for more uniform image SNRs. A 2‐mm thick lead plate with a 2‐D array of aperture holes was used to measure primary signals which were then subtracted from those obtained without the lead plate to determine scatter components. A 2‐D array of aluminum beads (3mm) was used as low‐contrast objects to measure the contrast ratios (CRs) and contrast‐to‐noise ratios (CNRs) for evaluating low‐contrast performance in chest images. Two images acquired with same techniques were subtracted from each other for measuring the noise levels. SPRs, SNRs, CRs and CNRs of the SEDR images were measured and compared with those of slot‐scan and full‐field images acquired with and without anti‐scatter grid. Results: Both SEDR and slot‐scan techniques resulted in lower SPRs, improved SNRs, CRs, and CNRs than anti‐scatter grid technique in all regions. The improvement is more pronounced in heavily attenuating regions: SEDR technique produced the best CNRs and the lowest SPRs, followed in order by slot‐scan and anti‐scatter grid techniques. Conclusion: SEDR technique can reject scatter effectively without attenuating primary x‐rays. Furthermore, it improves image SNRs and CNRs in heavily attenuating regions by modulating x‐ray intensities regionally and compensating for x‐ray attenuation by patient's anatomy.Acknowledgement: This work was supported in part by grants CA104759 and CA124585 from NIH‐NCI, a grant EB00117 from NIH‐NIBIB, and a subcontract from NIST‐ATP.

Purpose: To compare the visibility of micro‐calcifications in cone‐beam breast CT with that in conventional CT.Material and method: CalciumCarbonate grains (ranging from 180–200 to 250–280 microns) were used to simulate calcifications. Two bowl shaped plastic containers filled with paraffin were used to simulate a breast. The phantom was scanned with a bench‐top experimental CBCTsystem, configured for breast imaging. The system consists of a conventional tungsten target x‐ray tube (G‐1592) and an a‐Si:H/CsI flat panel detector (Paxscan 4030CB). The source to image and source to iso‐center distances are 100 and 75 cm, corresponding to a magnification factor of 1.33 at the iso‐center. A conventional CTsystem (GE Discovery STE PET/CT Scanner) was used to scan one phantom by itself and with two phantoms on an anthropomorphic chest phantom. Result: The visibility of the calcifications in CBCTimages was found to be significantly better than that in the conventional CTimages. Calcifications as small as 180–200μm were visible in the CBCTimages. While calcifications of 212–224μm were visible in the conventional CTimages when only breast phantom was scanned. None of the calcifications studied (280 μm or smaller) were visible when the two breast phantoms were placed on the anthropomorphic chest phantom for scan. Conclusion: Our results indicated that CBCT is capable of imaging calcifications as small as 180 μm. This is due to the intrinsic high spatial resolution of the detector used and the pendant geometry used which allowed the breast to be scanned and imaged by itself. The conventional CT is intrinsically lower in spatial resolution. An important disadvantage is the inclusion of the entire chest in the scan which results in degraded spatial resolution as well as overall higher radiation dose to the patient.

Purpose: To assess the influence of quantum and anatomic noises on the detection of simulated microcalcifications (MCs) on digital mammograms.Materials and Methods: Images of an anthropomorphic breast phantom (RMI 169) overlapping with simulated MCs ranging from 160 to 224 micron in size placed at various locations were acquired with an aSi/aSe flat panel based digital mammography system (Selenia) operated with Mo‐Mo target/filter combination at 28 kVp. These mammograms were exposed with 10, 20, 40, 80, 160, and 325 mAs. All images were displayed randomly on a review workstation and reviewed by five readers independently. The readers were asked to count the number of visible MCs. The ratio of visible MCs was computed for each different combination of MC size, reader and mAs setting. The performance differences in the ratios averaged from all readers between mAs levels was assessed by performing Student t‐test to compute the p values. Results: It was found that in the dense tissue region the visibility reached 90% at 10, 33 and 37 mAs for 200–212, 160–180 and 180–200 micron MCs, respectively; and at 10 mAs for 180–200 and 200–212 micron MCs and at 15 mAs for 160–180 micron MCs in the fatty region. The lower visibility in the dense tissue region may be due to the fact that the photon flux was lower in the dense tissue regions therefore requiring higher mAs setting to achieve sufficient image SNRs for detection of MCs. Our results also show that in the dense tissue region, the visibility of 180–200 micron MCs stayed at 90% at 80 mAs. This may indicate that the anatomical noise was limiting the detectability of 180–200 micron MCs. This work was supported in part by grants CA104759 and CA124585 from NIH‐NCI, a grant EB00117 from NIH‐NIBIB, and a subcontract from NIST‐ATP.

Purpose: To compare breast density measurements using CBCT with those based on mammograms.Materials and Methods: 19 mastectomy specimens were imaged with a bench top aSi/CsI flat panel based CBCT system. Scans were performed at 80 kVp with a voxel size of 0.29 mm and an estimated dose level of 24 mGy at the isocenter. The reconstructed images were corrected and segmented to separate glandular tissue from adipose tissue. The voxel numbers for glandular tissue and the entire breast were counted from the segmented images and used to calculate the volume based breast density. The breast density was also calculated based on mammograms using a commercial software package. The number of pixels for dense tissue regions was counted and divided by that for the entire breast to compute the area based breast density. These two breast densities were used for comparison. The volume based breast densities were also converted into BI‐RADS categories for comparison with those from mammographic reading as recorded in the diagnostic report. Results: The volume based breast densities were mostly equal or lower than the mammographic ones (fig. 1 and Table 1) with a correlation (coefficient = 0.73) between the two. Comparison based on the BI‐RADS categories, more breasts with volume based measure were classified as categories 1 and 2 while most breasts with mammographic reading were classified as category 3 (Fig. 2) with a correlation coefficient of 0.55. Conclusion: It was found that breast densities calculated from CBCTimages were significantly lower than those from mammograms for the 19 patients studied. Implication to the use of CBCT breast density as a risk factor for breast cancer remains to be investigated. This work was supported in part by grants CA104759 and CA124585 from NIH‐NCI, a grant EB00117 from NIH‐NIBIB, and a subcontract from NIST‐ATP.

Purpose: To investigate the visibility of microcalcifications (MCs) with an aSi/aSe flat panel detector (FPD14, Anrad Corporation) based cone beam CT(CBCT)system.Method and materials: A bench top experimental CBCTsystem was configured for breast imaging and used to image a breast phantom with embedded simulated MCs of various sizes. A paraffin cylinder with a diameter of 135 mm and a height of 40 mm was used to simulate a 100% adipose breast. Calcium carbonate grains, ranging from 125 to 425 μm in size, were used to simulate the microcalcifications. 25 MCs from same size group were arranged into a 5 × 5 cluster and embedded at the center of a 15 mm diameter paraffin rod to make an MC insert, which may be inserted into a hole at the center of the breast phantom for imaging. The breast phantom with MCs was scanned at various exposure levels. 300 projection images were acquired over 360° and reconstructed with Feldkamp's backprojection algorithm using a ramp filter. The images were reviewed by 6 readers. The ratios of visible MCs, averaged over all readers, were plotted as the function of the square root of measured dose level and image signal‐to‐noise ratio (SNR).Results: It was found that 90% visibility was achieved for 224 – 250 μm calcifications at an SNR of 16.8 or an isocenter dose of 5.2 mGys. 70% visibility was achieved for 224 – 250, 212 – 224, 200 – 212, 180 – 200, 160 – 180, 150 – 160 and 124 – 140 μm calcifications at an SNR of 16.8, 16.8, 25.1, 25.1, 44.5, 44.5 and 52.6, respectively. Conclusions: Percentage of visible MCs increased with the imageSNR and isocenter dose. Higher SNRs and isocenter dose are required for imaging smaller MCs.

Purpose: Microcalcifications (MCs) play an important role in early detection of breast cancer. This study is to describe a technique to automatically identify and segment the calcifications from the breast anatomy in cone beam CT(CBCT)images.Method and Materials: Projection images of mastectomy breast specimens were acquired with a bench‐top experimental CBCT system and reconstructed using FDK algorithm with a regular ramp filter. The reconstructed images were first corrected for the non‐uniform cupping artifacts due to x‐ray scatter and beam hardening. The corrected CBCTimages had more uniform background but were still too noisy for threshold segmentation. To reduce the noise level, the images were processed with a noise averaging filter in z direction followed by a total variation filter to reduce the noise level while keeping the edge sharpness and contrast of calcifications. To automatically detect the calcifications with proper threshold level, a Gaussian curve fitting was used to fit the histogram of the CBCTimage. Since the CT numbers of calcifications are usually significantly above those for breast tissue, cutoff point of the Gaussian curve was used as the threshold value to segment the calcifications. Results: With this method, the calcifications in the entire breast were successfully segmented from the breast tissue background. The results agreed well with those identified by visualization. Conclusion: The calcifications in CBCT breast images can be automatically detected by threshold based segmentation method with the nonuniform cupping artifacts corrected for and the noise level properly reduced.

Purpose: To use Monte Carlo simulation method to investigate and compare the overall radiation dose and dose distribution for breast imaging using multi‐slice CT and cone beam CT(CBCT) techniques. Method and Materials:CBCTimages of a breast specimen have been combined with multi‐slice CTimages of a patient chest to form a 210×206×110 model for a patient lying in prone position with both breasts freely protruding downward. Image segmentation was applied to separate the images into areas for adipose tissue, soft tissue,muscle, air and bone. Rep78 was used to compute the spectra for 120 and 80 kVp x‐rays with a HVL of 4.65 and 4.08 mm Al, respectively, for simulating multi‐slice CT and CBCT, respectively. A collimator was added to simulate the x‐ray fan beam used in multi‐slice CT and a half‐cone beam for CBCT. To simplify the simulation, the table was assumed to shift by one fan beam width after the gantry makes one full rotation. 2×107photons were used to calculate the dose for multi‐slice CT with DOSXYZnrc and 1×109photons for CBCT scan of the same breast phantom. The results of the dose calculations were rescaled for 240 mAs and 25 mAs, respectively. Results: The results of Monte Carlo calculations for multi‐slice CT indicated that dose levels in the heart and bone structures were about twice as high as those in the breasts. The breast dose from multi‐slice CT was from 0.8 to 2 times as high as those in cone beam breast CT.Conclusion: Results from our Monte Carlo calculations indicated that breast doses were found to be higher with a typical multi‐slice CT scan of the chest. An additional dose problem for multi‐slice CT is full exposures to the rest of the chest, incurring high radiation doses in the heart and bone structures.

Purpose: To investigate the effects of voxel size on the visibility of simulated microcalcifications (MCs) in cone bream breast CT.Method and materials:Calciumcarbonate grains (ranging from 140–150 to 200–212 μm), were used to simulate MCs. 25 MCs from same size group were arranged into a 5 × 5 cluster and embedded at the center of a 15 mm diameter paraffin rod. Rods with MCs from various size groups were constructed and scanned with a bench top experimental CBCT system. A CCDdetector (C4742‐56‐12ER, Hamamatsu Photonics K.K) operated in 2×2 binning mode was used to acquire 300 projection images with a pixel size of 48 μm over 360°. Imaging geometry was configured with a magnification factor of 1.16 to result in a voxel size of 42 μm to match the spatial resolution limit. The acquired images were further binned into image sets of larger pixels (96, 144, 192, 240 and 288 μm). The image sets were then reconstructed with Feldkamp's backprojection algorithm using a ramp filter. The reconstructed image sets were reviewed by 6 readers independently. The visibility, computed as the ratio of visible MCs and averaged over all readers, was plotted as the function of voxel size to quantify the effects of voxel size on the visibility of MCs.Results: Based on the visibility versus voxel size plot, 90% visibility was achieved for 200 – 212, 180 – 200, 160 – 180, 150 – 160 and 140 – 150 μm calcifications at a voxel size of 249, 249, 206, 166 and 125 μm, respectively. The visibility versus voxel size plot may be used to determine the minimum pixel size required to achieve a specific visibility level for MCs of certain sizes. Conclusions: We have demonstrated that the minimum voxel size required to achieve a specific visibility level decreased with the MC size.

Purpose: To describe and demonstrate the use of an improved segmentation technique to estimate breast density from cone beam breast CT (BCT) images.Method and Materials: To compute the breast density, the dense tissue must be separated from the adipose tissue in the BCT images. To accomplish this task, the BCT images were first processed with a previously reported post‐reconstruction method to correct for the beam hardening and scatter induced cupping artifacts. With this method, the adipose signals were extracted from the coronal view CTimage and used to form a 3‐D background map which was then subtracted from the original images for correction. The corrected data were edge enhanced and smoothed to reduce the noise levels in the images. The results were processed by threshold segmentation to separate the dense tissue from the adipose tissue and form a 3‐D dense tissue map which was then used to compute the dense tissue volume and the breast density. Results: For demonstration, the proposed technique was applied to cone beam CTimages of mastectomy breast specimens to estimate the breast density. The cupping artifacts were successively corrected for and the dense tissue was successfully separated from the adipose tissue with the segmented dense tissue structures matching well with those visualized in the images.Conclusion: We have successfully implemented a method to use cone beam breast CTimages to estimate the breast density. The technique was successfully demonstrated with cone beam CTimages of mastectomy breast specimens. This technique lends itself to more accurate and consistent measurement of breast density which may be used an indicator for breast cancer risk. This work was supported in part by grants CA104759 and CA124585 from NIH‐NCI, a grant EB00117 from NIH‐NIBIB, and a subcontract from NIST‐ATP.

Purpose: To investigate scatter rejection properties and low‐contrast performance of a flat‐panel based SEDR system for chest imaging.Method and Materials: A prototype SEDR system was developed to improve the image quality especially in heavily attenuated regions. Slot scan imaging geometry was used to reduce x‐ray scatter without attenuating primary x‐rays. Regional beam width modulation was used to equalize the x‐ray exposures at the detector input for more uniform image signal‐to‐noise ratios. A low‐dose chest phantom (pre‐scan) image was acquired with the slot‐scan technique to determine the equalization factors for SEDR imaging. A steel bar was placed in front of the chest phantom to measure the scatter component and scatter‐to‐primary ratios (SPRs) across the phantom image. Two images acquired with the same techniques were subtracted from each other for measuring the noise levels. SPRs, SNRs, and contrast‐to‐noise ratios (CNRs) of the SEDR images were measured and compared with those of the slot‐scan images and full‐field images acquired with and without anti‐scatter grid. Results: The SEDR technique resulted in lower SPRs in heavily attenuated regions like retrocardum and mediastinum than the slot‐scan and full‐field techniques. They resulted in similar SPRs in lungs as the slot‐scan technique. Both the SEDR and slot‐scan methods produced better SNRs than the anti‐scatter grid method in full‐field imaging. The SEDR technique resulted in the best CNRs, followed in order by the slot‐scan and anti‐scatter grid techniques. The improvement of CNR was more pronounced in heavily attenuated regions. Conclusion: The SEDR technique can effectively reject scatter without having to attenuate the primary x‐rays. Furthermore, it can improve image SNRs and CNRs by regionally compensating for x‐ray attenuation by patient's anatomy. Acknowledgement: This work was supported in part by grants CA104759 and CA124585 from NIH‐NCI, a grant EB00117 from NIH‐NIBIB, and a subcontract from NIST‐ATP.

Purpose: To evaluate and compare the spatial resolution properties of three different cone beam CTsystems using an aluminum wire phantom. Method and Materials: A wire phantom was constructed to evaluate and compare different cone‐beam CTsystems for their spatial resolution. The phantom contains 12 aluminum wires of various sizes (51, 76, 102, 127, 152, 178, 203, 229, 254, 279, 305, and 356 μm in diameter) suspended in air in the axial direction and positioned along a 7.2 cm diameter circle around the rotating axis. The wire phantom was imaged with cone‐beam CTsystems using three different detectors: an aSi/aSe flat panel detector (Anrad FPD14), an aSi/CsI flat panel detector (Varian PaxScan 4030CB) and a CCD/CsI detector (Hamamatsu C4742). Images were acquired in both continuous and pulsed x‐ray modes and in various binning modes (1×1, 2×2, 3×3 and 4×4). 300 projection images were acquired over 360° for each scan. Feldkamp's filtered backprojection algorithm with a pure ramp filter was used for 3D reconstruction. Results: Careful examination of the reconstructed images shows that wires as small as 100 μm were resolved with Anrad and Varian detectors. However, despite the smaller pixel size of the Anrad detector,images obtained with the Varian detector showed slightly better quality. It was found that binning did not affect the visibility but degraded quality was observed with increased binning size. Due to motion blur, images obtained with continuous x‐rays showed lower spatial resolution.Conclusion: It was found that the spatial resolution of cone‐beam CTsystem does not solely depend on detector's resolution. Using pulse mode X‐ray shows a better performance in cone‐beam CTsystem than continuous mode. This work was supported in part by grants CA104759 and CA124585 from NIH‐NCI, a grant EB00117 from NIH‐NIBIB, and a subcontract from NIST‐ATP.

The magnitude of scattered radiation and dose efficiency inside and outside the VOI was investigated for VOI cone beam computed tomography(CBCT) technique. For experiments, a bench‐top CBCT system with a flat‐panel digital detector was constructed. Two cylindrical polycarbonate phantoms (11 cm and 15 cm in diameter) were used to simulate breasts. To implement the VOI scanning technique, a lead filter with a rectangular opening was placed between the x‐ray source and the phantoms. The x‐ray tube voltage setting was 80 kVp. The phantoms were imaged without and with the VOI filter for the open filed and the VOI field, respectively. Slot scanning technique with varying slot width was used to measure SPR values at the detector plane. Dose measurement was performed using TLD dosimeters at five different locations for the 11‐cm phantom and six locations for the 15 cm phantom. The results showed that the SPR value inside the VOI were about 0.37 and 0.039 for the open field and the VOI field, respectively, for the 11 cm phantom. This indicates that the SPR can be reduced by a factor of 9 inside the VOI. The results also showed that dose increases from the center toward to the edge for the open field, but decreases for the VOI field. The dose can be reduced by a factor of 3 or more outside the VOI and by a factor of 1.6 at the center of the phantom for the 11 cm phantom (this work was supported in part by a grant CA104759 from the NCI and a research grant EB00117 from the NIBIB).

Purpose: To evaluate and compare dose saving and scatter reduction properties of VOI‐centered and breast‐centered approaches to VOI cone beam CT with Monte Carlo simulation.Method and Materials: To implement the VOI scanning technique, a mask with an opening is inserted between the x‐ray source and the patient to deliver higher exposures inside the VOI and lower exposures outside the VOI during the scan. There are two approaches to implement this technique. One is to center the breast with the rotation axis and move the VOI mask to track the VOI. The other is to center the VOI with the rotation axis, thus keeping the VOI mask fixed during the scan. A cylindrical breast model with a VOI close to the breast boundary was constructed to evaluate and compare the two approaches with Geant4 based Monte Carlo simulation. Dose distribution in the breast and scatter‐to‐primary ratios (SPRs) at the detector input were evaluated and compared to those estimated for full field cone beam CT.Results: Both VOI scanning techniques significantly reduce the radiation doses and scatter intensities both inside and outside the VOI. However, dose saving with the breast‐centered approach was approximately twice as much as that with the VOI‐centered approach both inside and outside the VOI. SPRs inside the VOI were similar for the two approaches and varied little with rotation degree. Conclusion: The breast‐centered scan appears to be the preferred approach to VOI cone beam CT because it resulted in substantially better dose saving while similar scatter reduction as compared to the VOI centered approach. Effort to implement this technique is underway to demonstrate this technique and to validate the simulation results. Acknowledgement: This work was supported in part by grants CA104759 and CA124585 from NIH‐NCI, a grant EB00117 from NIH‐NIBIB, and a subcontract from NIST‐ATP.

Purpose: To investigate and compare the anti‐scatter grid and slot scanning methods for their effects on the observers' performance in digital chest imaging. Materials and Methods: Anthropomorphic chest phantom was imaged with an a‐Si/a‐Se flat‐panel based digital radiography system. The system was operated in both the slot scanning and full‐field modes. An anti‐scatter grid was used. Imaging technique was 120 kVp and 0.15 to 16 mAs for both modes. After that, the grid was removed and same imaging technique was applied to acquire phantom images for both modes. 1‐cm diameter computer‐simulated nodule with a nominal peak contrast ratio of 5% was used to generate simulated nodules at hilum and sub‐diaphragm locations by applying SPR values for all images. 4‐AFC experiment was conducted to measure the ratio of correct observations as a function of the exposure level for various imaging conditions and locations. 24 sets of four‐512×512 images were generated for each exposure level, resulting in generating 192 sets for each imaging condition and location. These images were displayed randomly on a review workstation. The ratios of correct observations versus exposure levels were computed for various imaging conditions and locations. Result: With low exposure, the observers' performance improved with the exposure level but reached the maximum at ∼4 mAs for all techniques. At the hilum location, slot scan imaging without grid performed the best, followed by full‐field imaging without grid. The use of an anti‐scatter grid in both full‐field and slot scan modes seemed to degrade the performance significantly. Conclusion: Improvement with the exposure level has been observed for all imaging techniques. However, the observers' performance reached the maximum at different rates depending on the nodule location and techniques. (This work was supported in part by a research grant EB000117 from the NIH‐NIBIB and a research grant CA104759 from the NIH‐NCI).

Purpose: To investigate scatter rejection and low‐contrast performance of an electronic aft‐collimation based slot‐scan imaging technique for chest imaging.Method and Materials: The slot‐scan imaging technique was implemented with a 1.6 cm wide fan‐beam and a modified flat‐panel (FP) detector for electronic aft‐collimation. During the scan, the leading edge line of the scanning fan‐beam is reset to erase the scatter accumulated while the trailing edge line is read out to acquire the image signals following the fan‐beam exposure. Two images acquired with the same techniques were subtracted from each other for measuring the noise levels. A 2‐mm thick lead plate with a 2‐D array of holes was used to measure the primary signals which were then subtracted from those obtained without the lead plate to determine the scatter components. A 2‐D array of aluminum beads (3mm in diameter) is used as the low‐contrast objects to measure the contrast ratios (CRs) and contrast‐to‐noise ratios (CNRs) for evaluation of the low‐contrast performance in chest images.Results: The slot‐scan imaging method resulted in lower average scatter‐to‐primary ratios (SPRs) and improved CRs, primary signal‐to‐noise ratios (PSNRs) and CNRs than the anti‐scatter grid method. Slot‐scan imaging used in conjunction with grid resulted in further reduction of the SPRs and improvement of CRs. However, in most cases, the PSNRs were degraded and CNRs were at the same level or even degraded. Conclusion: Slot‐scan imaging with electronic aft‐collimation can effectively reject scatter without having to attenuate the primary x‐rays. It resulted in almost doubled CNR improvement as compare to the anti‐scatter grid method. Further improvement by using slot‐scan imaging with an anti‐scatter grid was found to be limited. Conflict of Interest: This work was supported in part by a research grant EB00117 from the NIH‐NIBIB and the research grants CA51248 and CA104759 from the NIH‐NCI.

Purpose: To report on the use of a new scatter measurement technique to characterize the scatter rejection properties and to further correct for the in‐slot scatter in slot scan imaging with a high frame rate flat panel detector.Method and Materials: A new slot scan imaging technique was developed with a high frame rate (7.5 fps) flat panel detector. 300 images were acquired as an x‐ray fan beam, defined by a slot collimator placed in front of the tube, was scanned across an anthropomorphic chest phantom. Time‐intensity curves were formed from the image sequence for all pixels. They were integrated over the fan beam passing frames to form the scatter rejected image signals and over all frames to form the open field image signals. By assuming the primary component to be constant during the fan beam exposure, the scatter components were extracted from the time‐intensity curves to estimate the in‐slot scatter using an iterative background estimation algorithm. The results were used to measure the scatter signals and to correct for in‐slot scatter. Results: Slot scan images of an anthropomorphic chest phantom were acquired and formed for various projected fan beam widths. 2D maps of scatter‐to‐primary ratios and contrast‐to‐noise ratio (CNR) improvement factors were generated and plotted for the slot‐scan images and compared with those for the full field, non‐grided images . In the lungs, CNR was improved by 30 to 50%, and in the mediastinum, CNR was improved by 60 to 150%. Conclusion: We have described and demonstrated the use of a new scatter measurement technique to estimate and correct for the in‐slot scatter on a pixel‐by‐pixel basis in slot scan imaging with a high frame rate flat panel detector. This work was supported in part by research grants EB00117 from the NIH‐NIBIB and CA51248 and CA104759 from the NIH‐NCI.

Purpose: To describe and discuss about a technique to simulate digital mammograms and tomosynthesisimages with cone beam CTimages of mastectomy breast specimens. Method and Materials: To allow x‐ray images to be simulated, cone beam breast CTimages of a mastectomy breast specimen were acquired at 80 kVp and used to generate a 3‐D map of linear attenuation coefficients for mammographic x‐rays. The map was then deformed to generate a new map for a compressed breast. A software re‐projector was developed and used to compute projection images for a stationary detector and a linearly shifted x‐ray source. A special re‐projector was developed to preserve the contrast of microcalcifications (MCs). The resulting images were then used to reconstructtomosynthesismammograms using various reconstruction algorithms. Results: We have successively used cone beam CTimages of mastectomy breast specimens to generate 3‐D maps of attenuation coefficients to model compressed breasts and to use them to simulate digital mammograms and tomosynthesisimages. It was found that MCs clearly visible in cone beam CTimages were not visible in regular mammograms but faintly visible in tomosynthesisimages. The MCs were clearly visible in both MC enhanced mammograms and the tomosynthesisimagesreconstructed from them. Conclusions: Cone beam CTimages may be used to model a compressed breast for simulating mammograms and tomosynthesisimages. Cone beam CTimages are superior to regular or tomosynthesismammograms in depicting MCs within their resolution limit. MC enhanced re‐projection, however, could be useful in displaying the cone beam CTimages with the more conventional appearances of regular and tomosynthesismammograms while maintaining the contrast and visibility of MCs in the CTimages. This work was supported in part by a research grant EB00117 from the NIH‐NIBIB and a research grant CA104759 from the NIH‐NCI.