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High resolution dual detector volume-of-interest cone beam breast CT––Demonstration with a bench top system

PURPOSE In this study, we used a small field high resolution detector in conjunction with a full field flat panel detector to implement and investigate the dual detector volume-of-interest (VOI) cone beam breast computed tomography (CBCT) technique on a bench-top system. The potential of using this technique to image small calcifications without increasing the overall dose to the breast was demonstrated. Significant reduction of scatter components in the high resolution projection image data of the VOI was also shown. METHODS With the regular flat panel based CBCT technique, exposures were made at 80 kVp to generate an air kerma of 6 mGys at the isocenter. With the dual detector VOI CBCT technique, a high resolution small field CMOS detector was used to scan a cylindrical VOI (2.5 cm in diameter and height, 4.5 cm off-center) with collimated x-rays at four times of regular exposure level. A flat panel detector was used for full field scan with low x-ray exposures at half of the regular exposure level. The low exposure full field image data were used to fill in the truncated space in the VOI scan data and generate a complete projection image set. The Feldkamp-Davis-Kress (FDK) filtered backprojection algorithm was used to reconstruct high resolution images for the VOI. Two scanning techniques, one breast centered and the other VOI centered, were implemented and investigated. Paraffin cylinders with embedded thin aluminum (Al) wires were imaged and used in conjunction with optically stimulated luminescence (OSL) dose measurements to demonstrate the ability of this technique to image small calcifications without increasing the mean glandular dose (MGD). RESULTS Using exposures that produce an air kerma of 6 mGys at the isocenter, the regular CBCT technique was able to resolve the cross-sections of Al wires as thin as 254 μm in diameter in the phantom. For the specific VOI studied, by increasing the exposure level by a factor of 4 for the VOI scan and reducing the exposure level by a factor of 2 for the full filed scan, the dual-detector CBCT technique was able to resolve the cross-sections of Al wires as thin as 152 μm in diameter. The CNR evaluated for the entire Al wire cross-section was found to be improved from 5.5 in regular CBCT to 14.4 and 16.8 with the breast centered and VOI centered scanning techniques, respectively. Even inside VOI center, the VOI scan resulted in significant dose saving with the dose reduced by a factor of 1.6 at the VOI center. Dose saving outside the VOI was substantial with the dose reduced by a factor of 7.3 and 7.8 at the breast center for the breast centered and VOI centered scans, respectively, when compared to full field scan at the same exposure level. The differences between the two dual detector techniques in terms of dose saving and scatter reduction were small with VOI scan at 4× exposure level and full field scan at 0.5 × exposure level. The MGDs were only 94% of that from the regular CBCT scan. CONCLUSIONS For the specific VOI studied, the dual detector VOI CBCT technique has the potential to provide high quality images inside the VOI with MGD similar to or even lower than that of full field breast CBCT. It was also found that our results were compromised by the use of inadequate detectors for the VOI scan. An appropriately selected detector would better optimize the image quality improvement that can be achieved with the VOI CBCT technique.

Radiation doses in cone-beam breast computed tomography: a Monte Carlo simulation study.

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 < 0.01). Normalized MGDs were found to decrease with increasing glandularity. CONCLUSIONS Our results indicate that it is sufficient to use homogeneous breast models derived from CBCT generated structured breast models to estimate the average dose. This investigation also shows that ellipsoidal digital phantoms of similar dimensions (diameter and height) and glandularity to actual breasts may be used to represent a real breast to estimate the average breast dose with Monte Carlo simulation. We have also successfully demonstrated the use of structured breast models to estimate the true MGDs and shown that the normalized MGDs decreased with the glandularity as previously reported by other researchers for CBBCT or mammography.

SU-FF-I-23: Full-Scan Versus Half-Scan in Cone Beam Breast CT - a Quantitative Comparison

Purpose: To evaluate and compare half‐scan and full scan techniques for cone beam breast CT in terms of CT number, radiation dose distribution and CNR.Method and Materials:CT number comparison: A 11 cm diameter breast phantom, made of a stack of glandular and fat tissue, was analytically modeled to evaluate the difference of CT numbers reconstructed from Feldkamp algorithm based half and full scan reconstruction. Radiation dose distribution: An 11 cm diameter cylindrical breast model was constructed to evaluate the dose distribution with Geant4 based Monte Carlo simulation.CNR evaluation: An 11 cm diameter polycarbonate cylinder was imaged to simulate breast imaging. Five holes of the same size (5 mm in diameter) were drilled at different radial distance from the phantom center. The phantom, filled with iodine solution, was imaged with an aSi/CsI flat panel detector based cone beam CT scanner. Iodine CNRs were measured for different half‐scan coverage selection. Results: It has been found that the CT numbers reconstructed from half‐scan were close to those from full‐scan. The radiation doses in the half‐scan coverage can be twice than those out the coverage as the radial distance increasing. The CNRs for different half‐scan coverage were approximately identical. Conclusion: Our results show that the reconstructed data from half‐scan are comparable to that from full‐scan. The radiation doses are low for the positions out the half‐scan coverage. The CNRs vary little with half‐scan coverage. 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.

Contrast-to-noise ratio improvement in volume-of-interest cone beam breast CT

In this study, we demonstrated the contrast-to-noise ratio (CNR) improvement in breast cone beam CT (CBCT) using the volume-of-interest (VOI) scanning technique. In VOI breast CBCT, the breast is first scanned at a low exposure level. A pre-selected VOI is then scanned at a higher exposure level with collimated x-rays. The two image sets are combined together to reconstruct high quality 3-D images of the VOI. A flat panel detector based system was built to demonstrate and investigate the CNR improvement in VOI breast CBCT. The CNRs of the 8 plastic cones (Teflon, Delrin, polycarbonate, Lucite, solid water, high density polystyrene, nylon and polystyrene) in a breast phantom were measured in images obtained with the VOI CBCT technique and compared to those measured in standard full field CBCT images. CNRs in VOI CBCT images were found to be higher than those in regular CBCT images in all plastic cones. The mean glandular doses (MGDs) from the combination of a high exposure VOI scan and a low exposure full-field scan was estimated to be similar to that from regular full-field scan at standard exposure level. The VOI CBCT technique allows a VOI to be imaged with enhanced image quality with an MGD similar to that from regular CBCT technique.

SU‐C‐218‐01: Comparison of Single‐View and Dual‐View Digital Chest Tomosynthesis

Purpose: Digital tomosynthesis (DTS) has been applied to chest imaging to crudely separate objects at different depth levels. However, the spatial resolution in the posterior‐anterior (PA) direction is very poor. We implement a dual‐view DTS technique to improve the spatial resolution along the PA direction and compare the results with the single‐view DTS technique and the cone beam CT(CBCT) technique. Methods:Computer simulations and experiments were conducted with the CBCT, dual‐view DTS and single‐view DTS techniques using the same geometry. For simulations, clinical CTimages obtained at 120 kVp were used to generate a digital chest phantom, with which the projection images were computed. For imaging experiments, an anthropomorphic chest phantom was scanned with a bench top experimental CBCT system. 300 projections were obtained for CBCTimaging using the FDK algorithm for reconstruction. 51 projections covering 60 degrees in the PA direction were used for single‐view DTS imaging using the iterative algorithm for reconstruction. 25 projection over 60 degrees in the PA direction and another 25 images over 60 degrees in the lateral direction were used for dual view DTS. Results: For sagittal slice images, the CBCTimage closely resembled the original phantom image. The dual‐view DTS images were fairly accurate in depicting the shape and dimensions of the anatomy in the fulcrum, despite the presence of some artifacts. With the single‐view DTS images, the anatomy was poorly depicted in the PA direction. For coronal slice images, the single‐view DTS images appeared less blurred but were much more affected by artifacts from the off‐fulcrum objects, resulting in severe distortion. The dual‐view DTS images, while not free from artifacts, provides more accurate rendition of the anatomy.Conclusions: Our results show that the dual‐view DTS technique substantially improved the spatial resolution of the reconstructed images in the PA direction. This work was supported in part by grants CA104759, CA124585, and CA138502A1 from NIH‐NCI, a research grant CA00117 from NIH‐NIBIB, and a subcontract from NIST‐ATP.

TH‐E‐110‐10: Doses in Volume‐Of‐Interest Cone‐Beam Computed Tomography (CBCT)‐ a Monte Carlo Simulation Study

Purpose: Volume‐of‐interest (VOI) cone‐beam CT(CBCT) is a technique to image a pre‐selected VOI with enhanced image quality while maintaining an acceptable average breast dose. This technique requires a high exposure VOI scan with a low exposure full‐field scan. In this study, Monte Carlo(MC) simulation was used to estimate the doses of the full‐field scan, VOI scan and various combinations of the two for breast imaging. Methods: For MC simulation, BEAMnrc and DOSXYZnrc were used to simulate CBCT breast imaging and estimate the average doses. For validation, air kermas at the VOI and iso‐center were measured and simulated for comparison. For simulation study, a 13‐cm‐diam, 9‐cm‐high Lucite cylinder was scanned by an x‐ray source located at 88 cm away. The VOI was defined to be a 2.5‐ cm‐diam, 2.5‐cm‐high cylindrical volume located at various radial positions in the middle plane of the phantom. Two scanning techniques were simulated: (1) the breast centered scan, (2) the VOI centered scan. The former requires a moving collimator to be used during the VOI scan while the latter requires only a stationary collimator to be used. 400 million incident photons were used to simulate 300 projections over 360 degrees to achieve 3D dose maps. Results: It was found that the average VOI doses decreased by 37% in the VOI scans as compared to those in the full‐field scans. The additional dose incurred by a VOI scan (VOI centered) was 6% of the dose from a full‐field scan for the same x‐ray techniques used. Conclusions: Our results indicated that it is feasible to employ a high exposure VOI scan in conjunction with a low exposure full‐field scan to enhance the image quality within the VOI while maintaining the overall average dose to be equal to or even lower than that of a regular full‐field scan. This work was supported in part by grants CA104759, CA124585 and CA13852 from NIH‐NCI, a grant EB00117 from NIH‐NIBIB, and a subcontract from NIST‐ATP.

TU‐A‐301‐01: Microcalcifications Visibility in Cone Beam Breast CT with Various Flat Panel Detectors

Purpose: To investigate the visibility of microcalcifications (MCs) in cone beam breast CT using various flat panel detectors. Methods: We investigated the visibility of MCs in cone beam CT(CBCT) breast imaging using various flat panel detectors, including PaxScan 4030CB (aSi/CsI) by Varian MedicalSystems, FPD14 (aSi/aSe) by Anrad, C4742 (CCD/GdO2S:Tb) and C7921 (CMOS/CsI) detectors by Hamamatsu. A paraffin cylinder with a diameter of 135 mm and a thickness of 40 mm was used to simulate a 100% adipose breast. Calcium carbonate grains, from 125 – 140 μm to 224 – 250 μm in various size groups, were used to simulate the MCs. Groups of 25 same size MCs were arranged into 5 × 5 clusters. Each cluster was embedded at the center of a 15 mm diameter cylindrical paraffin phantom, which as inserted into a hole at the center of the breast phantom. The breast phantom with the simulated MCs was scanned on a bench top CBCTsystem at various exposure levels for each detector. The reconstructed images were reviewed by 6 readers independently. The MCvisibility was quantified as the fraction of visible MCs and averaged over all readers for analysis. The visibility was plotted as a function of the estimated dose level and image signal‐to‐noise ratio (SNR) for various scans and detectors. The relative detector DQEs were compared among the four detectors. Results: It was found the relationship between the visibility and size can be fitted with a Boltzmann function for all the detectors. The Varian detector has the best MCvisibility among all the detectors at the same dose level. Conclusions: The visibility of MCs increased with the isocenter dose and image SNR for all detectors.Detector with better DQE could achieve the same MCvisibility with lower radiation dose. This work was supported in part by grants CA104759, CA13852 and CA1245 85 from NIH‐NCI, a grant EB00117 from NIH‐NIBIB, and a subcontract from NIST‐ATP.

SU‐C‐301‐05: Comparative Low‐Contrast Performance of Scan Equalization Digital Mammography (SEDM) v.s. Full‐Field Digital Mammography (FFDM): A Simulation Study with Micro‐Calcifications

Purpose: To investigate the effects of exposure equalization on the noise property and visibility of micro‐calcifications using simulated SEDM imaging experiments. Method: An anthropomorphic breast phantom was imaged at various exposure levels using a full‐field digital mammography system. A lead plate with two‐dimensional array of aperture holes was used to measure primary signals which were then subtracted from those obtained without the lead plate to separate scatter components from total image signals. Exposure equalization factors were determined from a standard FFDM image and these factors were then multiplied with images acquired at various exposure levels to simulate SEDM image. Two sets of image were acquired and subtracted from each other to estimate the noise properties for different imaging techniques. Images of simulated micro‐calcifications with different sizes were composed with primary signals and then added with scatter components to form FFDM and simulated SEDM images for visualization studies. Results: SEDM resulted in reduced noise level in dense area while increased noises level in less attenuating area (fatty area or at the board of the breast where the thicknesses are reduced) compared with the standard FFDM method. The visualization of simulated micro‐calcification is seen slightly improved in dense area of the breast phantom due to improved signal‐to‐noise ratio (SNR) and contrast‐to‐noise ratio (CNR). Conclusion: SEDM technique can improve image SNRs and CNRs in dense area of the breast, hence to improve the visibility of micro‐calcification in breast images. This work was supported in part by grants CA104759, CA124585 and CA13852 from NIH‐NCI, a grant EB00117 from NIH‐NIBIB, and a subcontract from NIST‐ATP.

TU‐A‐301‐06: Image Quality, Dose Saving and Scatter Reduction in Dual‐Resolution Cone Beam CT Breast Imaging Using Two Different VOI Scanning Techniques

Purpose: To estimate the image quality, dose saving and scatter reduction in the dual‐resolution cone beam CT(CBCT) breast imaging using two different VOI scanning techniques. Methods: There are two different VOI scanning techniques in dual‐resolution CBCT breast imaging: (1) breast‐ centered with a moving VOI mask technique and (2) the VOI‐centered with a stationary VOI mask technique. In either case, the breast is first scanned with a low resolution detector at a lower exposure level. The VOI in the breast is then scanned with a high resolution detector at a higher exposure level through the VOI mask. The two image sets are combined together to reconstruct high resolution CTimages. A bench‐top experimental CBCTsystem with a flat panel detector and a high resolution CMOSdetector was built. A paraffin cylinder with a diameter of 130 mm was used to simulate breast. A wire phantom, a 15 mm diameter paraffin cylinder with 8 vertically oriented aluminum wires of various diameters, was used to test the image quality. The wire phantom was inserted into the breast phantom 45 mm away from the center and designated as the VOI. The spatial resolution in the reconstructed images, phantom dose and scatter‐to‐primary ratio were measured for both scanning techniques. Results: For both techniques, a visual review of the reconstructed images showed that the diameter of thinnest resolvable wire is 254 μm for regular CBCT and 152 μm for dual resolution CBCT. The doses for dual‐resolution CBCT can be reduced by a factor of 2 inside of the VOI and up to a factor of 6 outside the VOI. The scatter components can be reduced by a factor of 6 inside the VOI. Conclusions: Both scanning techniques were found to result in improved image quality, substantially reduced dose and scatter in dual‐resolution CBCT. This work was supported in part by grants CA104759, CA13852 and CA124585 from NIH‐NCI, a grant EB00117 from NIH‐NIBIB, and a subcontract from NIST‐ATP.

SU‐E‐I‐149: Digital Breast Tomosynthesis Using 2D Source Scanning Patterns: A Simulation Study

Purpose: Digital tomosynthesis (DTS) imaging with 2D scanning patterns may help reduce the artifacts by more effectively blurring the off‐fulcrum objects. In this study, the effects of various source scanning patterns on the reconstructed images are studied via simulation. Methods: The X‐ray source was assumed to be movable following various scanning patterns. The source‐to‐iso‐center and source‐to‐image distances were assumed to be 500 mm and 600 mm, respectively. Projection images were computed using an improved distance‐driven algorithm. The objects studied included a Shepp‐ Logan phantom and a digital breast phantom constructed from segmented cone beam CTimages. The X‐ray absorption coefficients were taken of mono‐energetic X‐ray at 19 keV. The x‐ray source was scanned with 1D (1D DTS) and 2D (2D DTS) patterns. The objects were reconstructed using the iterative expectation‐maximization algorithm. The root‐mean‐squared‐ deviation (RSMD) was calculated to indicate the deviation in terms of CT numbers of the reconstructed image to the original phantom. Results: The RMSD of the reconstructed Shepp‐Logan phantom was 243.3 for 1D DTS and 104.7 for 2D DTS. Visual inspection revealed reduced artifacts and better CT number accuracy with 2D DTS than 1D DTS. For the digital breast phantom, the accuracy of the reconstructed tissue structures was assessed using the CT number uniformity in the dense tissue and adipose tissue regions. The RMSD of the digital breast phantom was 113.1 for 1D DTS and 50.0 for 2D DTS. The comparison showed that the 2D DTS resulted in better accuracy in CT number uniformity. Conclusions: Our results demonstrated that the use of 2D source scanning patterns has the potential advantage of more effectively blurring objects in off‐fulcrum planes, thus reducing the artifacts and resulting in more accurate image reconstruction. This work was supported in part by research grants: CA104759 and CA124585, EB000117 from NIBIB, CA138502A1, and a subcontract from NIST‐ATPs.