Chao-Jen Lai

University of Texas MD Anderson Cancer Center, Houston, Texas, United States

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Publications (47)51.74 Total impact

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    ABSTRACT: In using flat panel detectors (FPD) for cone beam computed tomography (CBCT), pixel gain variations may lead to structured nonuniformities in projections and ring artifacts in CBCT images. Such gain variations can be caused by change in detector entrance exposure levels or beam hardening, and they are not accounted by conventional flat field correction methods. In this work, the authors presented a method to identify isolated pixel clusters that exhibit gain variations and proposed a pixel gain correction (PGC) method to suppress both beam hardening and exposure level dependent gain variations.
    Medical physics. 09/2014; 41(9):091913.
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    ABSTRACT: PURPOSE Digital tomosynthesis (DTS) is able to crudely separate the overlapping anatomical structures on chest radiography. However, the resolution in the posterior-anterior (PA) view is quite limited due to the nature of DTS. Furthermore, the spillover of high-contrast objects from off-fulcrum planes generates artifacts. We demonstrate that by applying a dual-view DTS on chest imaging, the spatial resolution in PA direction is improved and the artifacts are alleviated. METHOD AND MATERIALS We proposed a dual-view DTS technique in which projection images are acquired in two orthogonal views and reconstructed using an iterative method. We acquired cone beam CT images of an anthropomorphic chest phantom with 300 projections and an angular step size of 1.2°. The experiment was carried out using 120kVp, 40mA continuous mode x-ray, with source-to-detector distance of 180cm. The flat panel detector had a pixel size of 0.2mm. The dual-view DTS images were extracted from the CBCT images with different angular ranges and step sizes to investigate the effects of these parameters. We extracted the DTS data with the angular ranges of 30° and 60° respectively, each of which was using 1.2° and 2.4° angular step sizes. Single-view DTS images were also extracted and reconstructed for comparison. Visual inspection as well as the quantitative analysis of root-mean-squared-deviation (RMSD) and the thick slice thickness were used for evaluation. RESULTS The dual-view DTS images were fairly accurate in depicting the shape and dimensions of the anatomy in the fulcrum, especially for the sagittal views. The RMSD values calculated on different regions demonstrated the improvement of the image quality in dual-view DTS over single-view DTS. The thick slice thickness was greatly reduced for dual-view DTS. Our results also demonstrated that the RMSD and thick slice thickness with 60° angular range was better than with 30° angular range, and the a smaller angular step size with the same angular range resulted better quality images. CONCLUSION With a similar number of projections, dual-view DTS can render more accurate 3D images than single-view DTS does. This work was supported in part by research grants CA124585 and CA138502 from NIH-NCI. CLINICAL RELEVANCE/APPLICATION The applying of dual-view DTS technique provides better image quality and spatial resolution in chest imaging.
    Radiological Society of North America 2013 Scientific Assembly and Annual Meeting; 12/2013
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    ABSTRACT: Purpose: To measure and investigate the improvement of microcalcification (MC) visibility in cone beam breast CT with a high pitch (75 μm), thick (500 μm) scintillator CMOS∕CsI flat panel detector (Dexela 2923, Perkin Elmer).Methods: Aluminum wires and calcium carbonate grains of various sizes were embedded in a paraffin cylinder to simulate imaging of calcifications in a breast. Phantoms were imaged with a benchtop experimental cone beam CT system at various exposure levels. In addition to the Dexela detector, a high pitch (50 μm), thin (150 μm) scintillator CMOS∕CsI flat panel detector (C7921CA-09, Hamamatsu Corporation, Hamamatsu City, Japan) and a widely used low pitch (194 μm), thick (600 μm) scintillator aSi∕CsI flat panel detector (PaxScan 4030CB, Varian Medical Systems) were also used in scanning for comparison. The images were independently reviewed by six readers (imaging physicists). The MC visibility was quantified as the fraction of visible MCs and measured as a function of the estimated mean glandular dose (MGD) level for various MC sizes and detectors. The modulation transfer functions (MTFs) and detective quantum efficiencies (DQEs) were also measured and compared for the three detectors used.Results: The authors have demonstrated that the use of a high pitch (75 μm) CMOS detector coupled with a thick (500 μm) CsI scintillator helped make the smaller 150-160, 160-180, and 180-200 μm MC groups more visible at MGDs up to 10.8, 9, and 10.8 mGy, respectively. It also made the larger 200-212 and 212-224 μm MC groups more visible at MGDs up to 7.2 mGy. No performance improvement was observed for 224-250 μm or larger size groups. With the higher spatial resolution of the Dexela detector based system, the apparent dimensions and shapes of MCs were more accurately rendered. The results show that with the aforementioned detector, a 73% visibility could be achieved in imaging 160-180 μm MCs as compared to 28% visibility achieved by the low pitch (194 μm) aSi∕CsI flat panel detector. The measurements confirm that the Hamamatsu detector has the highest MTF, followed by the Dexel detector, and then the Varian detector. However, the Dexela detector, with its thick (500 μm) CsI scintillator and low noise level, has the highest DQE at all frequencies, followed by the Varian detector, and then the Hamamatsu detector. The findings on the MC visibility correlated well with the differences in MTFs, noise power spectra, and DQEs measured for these three detectors.Conclusions: The authors have demonstrated that the use of the CMOS type Dexela detector with its high pitch (75 μm) and thick (500 μm) CsI scintillator could help improve the MC visibility. However, the improvement depended on the exposure level and the MC size. For imaging larger MCs or scanning at high exposure levels, there was little advantage in using the Dexela detector as compared to the aSi type Varian detector. These findings correlate well with the higher measured DQEs of the Dexela detector, especially at higher frequencies.
    Medical Physics 10/2013; 40(10):101915. · 2.91 Impact Factor
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    ABSTRACT: 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.
    Proc SPIE 02/2012;
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    ABSTRACT: 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. 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). 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. 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.
    Medical Physics 12/2011; 38(12):6429-42. · 2.91 Impact Factor
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    ABSTRACT: The scan equalization digital mammography (SEDM) technique combines slot scanning and exposure equalization to improve low-contrast performance of digital mammography in dense tissue areas. In this study, full-field digital mammography (FFDM) images of an anthropomorphic breast phantom acquired with an anti-scatter grid at various exposure levels were superimposed to simulate SEDM images and investigate the improvement of low-contrast performance as quantified by primary signal-to-noise ratios (PSNRs). We imaged an anthropomorphic breast phantom (Gammex 169 "Rachel," Gammex RMI, Middleton, WI) at various exposure levels using a FFDM system (Senographe 2000D, GE Medical Systems, Milwaukee, WI). The exposure equalization factors were computed based on a standard FFDM image acquired in the automatic exposure control (AEC) mode. The equalized image was simulated and constructed by superimposing a selected set of FFDM images acquired at 2, 1, 1/2, 1/4, 1/8, 1/16, and 1/32 times of exposure levels to the standard AEC timed technique (125 mAs) using the equalization factors computed for each region. Finally, the equalized image was renormalized regionally with the exposure equalization factors to result in an appearance similar to that with standard digital mammography. Two sets of FFDM images were acquired to allow for two identically, but independently, formed equalized images to be subtracted from each other to estimate the noise levels. Similarly, two identically but independently acquired standard FFDM images were subtracted to estimate the noise levels. Corrections were applied to remove the excess system noise accumulated during image superimposition in forming the equalized image. PSNRs over the compressed area of breast phantom were computed and used to quantitatively study the effects of exposure equalization on low-contrast performance in digital mammography. We found that the highest achievable PSNR improvement factor was 1.89 for the anthropomorphic breast phantom used in this study. The overall PSNRs were measured to be 79.6 for the FFDM imaging and 107.6 for the simulated SEDM imaging on average in the compressed area of breast phantom, resulting in an average improvement of PSNR by ∼35% with exposure equalization. We also found that the PSNRs appeared to be largely uniform with exposure equalization, and the standard deviations of PSNRs were estimated to be 10.3 and 7.9 for the FFDM imaging and the simulated SEDM imaging, respectively. The average glandular dose for SEDM was estimated to be 212.5 mrad, ∼34% lower than that of standard AEC-timed FFDM (323.8 mrad) as a result of exposure equalization for the entire breast phantom. Exposure equalization was found to substantially improve image PSNRs in dense tissue regions and result in more uniform image PSNRs. This improvement may lead to better low-contrast performance in detecting and visualizing soft tissue masses and micro-calcifications in dense tissue areas for breast imaging tasks.
    Medical Physics 12/2011; 38(12):6489-501. · 2.91 Impact Factor
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    ABSTRACT: PURPOSE To compare scan equalization digital mammography (SEDM) with full-field digital mammography (FFDM) for detection of microcalcifications. METHOD AND MATERIALS An anthropomorphic breast phantom (RMI 169) was imaged with a FFDM system at various exposure levels to obtain simulated SEDM images. 200-250 micron calcium carbonate grains, overlapping with 50% adipose/50% glandular simulated breast tissue slabs, were imaged and their attenuation profiles were measured. Slabs with different thicknesses were used to simulate different breast compositions using the primary signal levels as the reference. Attenuation profiles of microcalcifications were used to model and digitally insert simulated microcalcifications into three dense tissue regions in both the FFDM and SEDM images. For each microcalcification size, 16 FFDM images and 16 SEDM images were generated. The images were randomly displayed and reviewed by four readers. The readers were asked to identify and locate microcalcifications with the free-response paradigm. Sensitivity and false positives per image were used to quantify and compare the performances of the SEDM and FFDM image sets. RESULTS The SEDM images resulted in slightly higher sensitivity than the FFDM images for both microcalcification sizes (0.61 vs. 0.58 and 0.82 vs. 0.80). However, the differences were not found to be statistically significant (p-value = 0.08 and 0.25, respectively). On the other hand, the SEDM images resulted in seemingly much lower false positives/image than the FFDM images (3.08 vs. 3.44 and 0.85 vs. 1.02). However, the differences were not statistically significant (p-value = 0.13 and 0.36, respectively). CONCLUSION The SEDM technique helps improve image signal-to-noise ratios and low contrast performance in dense tissue regions of the breast. We have demonstrated the improvement in visibility of microcalcifications with limited sample size. (This work was supported in part by grants CA104759, CA124585, and CA138502 from NIH-NCI, a grant EB00117 from NIH-NIBIB, and a subcontract from NIST-ATP). CLINICAL RELEVANCE/APPLICATION SEDM may help improve the detection of abnormalities in the dense tissue regions with similar entrance exposure as compared to FFDM.
    Radiological Society of North America 2011 Scientific Assembly and Annual Meeting; 11/2011
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    ABSTRACT: PURPOSE Preliminary studies demonstrated that digital tomosynthesis imaging with 2D scanning patterns (2D DTS) may help reduce the artifacts by more effectively blurring the off-fulcrum objects. While applied to breast imaging, 2D DTS may be subjected to line artifacts due to truncation of projection views in part of the projection volume due to limitation of the scanning geometry. In this study we demonstrated that with proper constraints in reconstruction, the truncation artifact can be effectively suppressed. METHOD AND MATERIALS The exposures were assumed to be made from 6x6 source locations over a 578 mm wide square region. The source-to-object and source-to-image distances were assumed to be 500 mm and 600 mm, respectively. Projection images were computed using a distance-driven algorithm. The object was a digital breast phantom constructed from segmented cone beam CT images of a mastectomy breast specimen. The X-ray absorption coefficients were taken of mono-energetic X-ray at 19 keV. The objects were reconstructed using the iterative expectation-maximization (EM) algorithm. Maximum absorption limit and total variation minimization (TV) were applied as constraints. The evaluations of the reconstruction were by visual inspection and the root mean squared deviation (RMSD) of the reconstructed images from the breast model. RESULTS With the TV and maximum absorption limit constraints added, the line artifacts were found to largely disappear in the reconstructed images. The accuracy of the CT numbers was also found to have improved as indicated by the reduction of RMSD by approximately 9%. CONCLUSION Our results demonstrated that the truncation artifacts could be eliminated by using proper constraints, such as TV and maximum absorption limit, in 2D DTS reconstruction. In addition, the accuracy of reconstruction could also be improved. This work was supported in part by research grants: CA104759 and CA124585, EB000117 from NIBIB, CA138502A1, and a subcontract from NIST-ATPs. CLINICAL RELEVANCE/APPLICATION The elimination of truncation artifacts and improvement of reconstruction accuracy provide better image quality in 2D DTS breast imaging.
    Radiological Society of North America 2011 Scientific Assembly and Annual Meeting; 11/2011
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    ABSTRACT: 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. 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. 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. 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.
    Medical Physics 02/2011; 38(2):589-97. · 2.91 Impact Factor
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    ABSTRACT: To investigate and compare the scatter rejection properties and low-contrast performance of the scan equalization digital radiography (SEDR) technique to the slot-scan and conventional full-field digital radiography techniques for chest imaging. A prototype SEDR system was designed and constructed with an a-Se flat-panel (FP) detector to improve image quality in heavily attenuating regions of an anthropomorphic chest phantom. Slot-scanning geometry was used to reject scattered radiation without attenuating primary x rays. The readout scheme of the FP was modified to erase accumulated scatter signals prior to image readout. A 24-segment beam width modulator was developed to regulate x-ray exposures regionally and compensate for the low x-ray flux in heavily attenuating regions. To measure the scatter-to-primary ratios (SPRs), a 2 mm thick lead plate with a 2-D array of aperture holes was used to measure the primary signals, which were then subtracted from those obtained without the lead plate to determine scatter components. A 2-D array of aluminum beads (3 mm in diameter) was used as the low-contrast objects to measure the contrast ratios (CRs) and contrast-to-noise ratios (CNRs) for evaluating the low-contrast performance in chest phantom images. A set of two images acquired with the same techniques were subtracted from each other to measure the noise levels. SPRs, CRs, and CNRs of the SEDR images were measured in four anatomical regions of chest phantom images and compared to those of slot-scan images and full-field images acquired with and without antiscatter grid. The percentage reduction of SPR (percentage of SPRs reduced with scatter removal/ rejection methods relative to that for nongrid full-field imaging) averaged over four anatomical regions was measured to be 80%, 83%, and 71% for SEDR, slot-scan, and full-field with grid, respectively. The average CR over four regions was found to improve over that for nongrid full-field imaging by 259%, 279%, and 145% for SEDR, slot-scan, and full-field with grid, respectively. The average CNR over four regions was found to improve over that for nongrid full-field imaging by 201% for SEDR as compared to 133% for the slot-scan technique and 14% for the antiscatter grid method. Both SEDR and slot-scan techniques outperformed the antiscatter grid method used in standard full-field radiography. For imaging with the same effective exposure, the SEDR technique offers no advantage over the slot-scan method in terms of SPRs and CRs. However, it improves CNRs significantly, especially in heavily attenuating regions. The improvement of low-contrast performance may help improve the detection of the lung nodules or other abnormalities and may offer SEDR the potential for dose reduction in chest radiography.
    Medical Physics 01/2011; 38(1):23-33. · 2.91 Impact Factor
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    ABSTRACT: PURPOSE Digital tomosynthesis (DTS) chest imaging is currently being studied as an adjunct of conventional chest radiography for better separation of abnormality from the overlapping anatomy. Current DTS techniques have employed a linear or curvilinear scanning pattern to generate limited angle projection views. The aim of this work is to demonstrate and compare a newly developed 2D scanning technique with the 1D technique for DTS chest imaging using computer simulation. METHOD AND MATERIALS With this new DTS technique, the X-ray sources is scanned over a square area to more effectively blur out objects outside the fulcrum. A digital chest phantom was constructed from segmented CT images and used for image simulation. Projection images were computed using a ray tracing based algorithm. 25 scanning patterns, including linear and square ones, were evaluated by computing and comparing the resulting contrast to noise ratios (CNRs) and the artifact spread functions (ASFs). RESULTS For fixed angular range and number of projection views, DTS imaging technique with 2D scanning patterns was found to more effectively blur the off-fulcrum objects than that with 1D scanning patterns. It was also found to exhibit higher CNRs and lower ASFs. CONCLUSION DTS imaging with 2D scanning patterns was demonstrated to improve over DTS imaging with 1D scanning pattern by reducing the contrast of the artifacts from off-fulcrum and increasing the contrast of in-fulcrum objects. CLINICAL RELEVANCE/APPLICATION DTS chest imaging with 2D scanning patterns may help improve the detection of abnormalities, including lung nodules, over regular digital radiography or DTS imaging with 1D scanning patterns.
    Radiological Society of North America 2010 Scientific Assembly and Annual Meeting; 11/2010
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    ABSTRACT: To study the effects of overlapping anatomy on microcalcification detection at various incident exposure levels. Images of an anthropomorphic breast phantom (RMI 169) overlapping with simulated microcalcifications ranging from 150 to 212 mum in size placed in two breast density regions, fatty and heterogeneously dense, were acquired with an a-Si/a-Se flat panel based digital mammography system (Selenia) operated with Mo-Mo target/filter combination at 28 kVp. The mammograms were exposed with 20, 30, 40, 60, 80, 120, 160, 240 and 325 mAs for varying the exposure level. A 4-AFC study was performed for evaluation of the detection performance. Four 400×400-pixel images were displayed as 2×2 array on a LCD flat panel based review workstation. One of the four images contained a cluster of five microcalcifications and was randomly placed in one of the four quadrants. A physicist was asked to select the image containing the microcalcifications and to report the number of visible microcalcifications. The fraction of correct responses was computed with two different criteria: (1) the selected images contained one or more microcalcifications, and (2) the selected images contained 4 or 5 visible microcalcifications. The statistical significance of the differences in fractions for different exposure levels and regions was evaluated. The results showed that, if visibility of one or more microcalcifications is required, the fractions of correct responses were 1 for all size groups and most exposure levels in both fatty and heterogeneously dense regions. If a visibility of 80% or more of the microcalcifications was required, the fractions of correct responses significantly decreased in both regions. The results indicated that microcalcification detection in the fatty region appeared to be mainly limited by the quantum noise, and that in the heterogeneously dense region may be limited by both the anatomic noise and the quantum noise.
    Proc SPIE 03/2010;
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    ABSTRACT: In this study, we demonstrated volume of interest (VOI) scanning technique in dual resolution cone beam CT (CBCT) breast imaging. A paraffin cylinder with a diameter of 130 mm was used to simulate breast. A wire phantom with a diameter of 15 mm was constructed as VOI. The phantom contains 8 vertically aluminum wires of various diameters surrounded by paraffin. The wire phantom was inserted into the breast phantom 45 mm away from the center. The phantoms were first scanned with a bench top experimental CBCT system at a low exposure level with the detector operated in a binning mode. Then a VOI mask was placed between the x-ray source and the phantoms. The phantoms were scanned again with high exposure level and the detector operated in the non-binning mode. The VOI mask was moved to follow the wire phantom during the whole CT scan to limit the exposures to cover the VOI only. The low resolution and high resolution images were then combined together for reconstruction with FDK algorithm. Visual review of the regular and dual resolution CBCT images shows that thinnest resolvable wire in the dual resolution CBCT images has a diameter of 152 mum. The thinnest resolvable wire in regular CBCT images has a diameter of 254 mum. The estimated dose to the phantom for dual resolution CBCT is 123% of that with regular CBCT at low exposure level. The dual resolution CBCT technique greatly enhances the CT image quality while still remains a low exposure level to the phantom.
    Proc SPIE 03/2010;
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    ABSTRACT: Registration and superimposition of images acquired from two different detectors is essential to dual-resolution cone beam CT. In this study we implemented and tested a method of integration, which is to register and superimpose the high resolution volume of interest (VOI) only images to the low resolution full field images. First, we acquired two images sets: One is low exposure low resolution full field images acquired with a low resolution detector; the other is high exposure high resolution volume of interest (VOI) images acquired with a high resolution detector and VOI mask. To locate the VOI positions in full field images, the third images set with VOI mask but without phantom was acquired with the low resolution detector. In the third images set, high contrast VOI boundaries were located and used to determine positions of the VOI in full field images. Then high resolution VOI images were superimposed with the full field images to generate integrated images set. Integrated images set was tested by subtraction from full field images set and then used to reconstruct images using regular FDK algorithm. In the reconstructed images, five Al wires (as small as 152 mum) can be clearly seen in the VOI.
    Proc SPIE 03/2010;
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    ABSTRACT: It is well recognized in projection radiography that low-contrast detectability suffers in heavily attenuating regions due to excessively low x-ray fluence to the image receptor and higher noise levels. Exposure equalization can improve image quality by increasing the x-ray exposure to heavily attenuating regions, resulting in a more uniform distribution of exposure to the detector. Image quality is also expected to be improved by using the slot-scan geometry to reject scattered radiation effectively without degrading primary x-rays. This paper describes the design of a prototype scan equalization digital radiography (SEDR) system implemented with an amorphous silicon (a-Si) thin-film transistor (TFT) array-based flat-panel detector. With this system, slot-scan geometry with alternate line erasure and readout (ALER) technique was used to achieve scatter rejection. A seven-segment beam height modulator assembly was mounted onto the fore collimator to regulate exposure regionally for chest radiography. The beam modulator assembly, consisting of micro linear motors, lead screw cartridge with lead (Pb) beam blockers attached, position feedback sensors and motor driver circuitry, has been tested and found to have an acceptable response for exposure equalization in chest radiography. An anthropomorphic chest phantom was imaged in the posterior-anterior (PA) view under clinical conditions. Scatter component, primary x-rays, scatter-to-primary ratios (SPRs) and primary signal-to-noise ratios (PSNRs) were measured in the SEDR images to evaluate the rejection and redistribution of scattered radiation, and compared with those for conventional full-field imaging with and without anti-scatter grid methods. SPR reduction ratios (SPRRRs, defined as the differences between the non-grid full-field SPRs and the reduced SPRs divided by the former) yielded approximately 59% for the full-field imaging with grid and 82% for the SEDR technique in the lungs, and 77% for the full-field imaging with grid and 95% for the SEDR technique in the subdiaphragm. The SEDR technique demonstrated a substantial improvement in PSNRs over the anti-scatter grid technique. The improvements of PSNRs varied with the regions and are more pronounced in heavily attenuating regions.
    Physics in Medicine and Biology 11/2009; 54(22):6959-78. · 2.70 Impact Factor
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    ABSTRACT: With volume-of-interest (VOI) cone-beam computed tomography (CBCT) imaging, one set of projection images are acquired with the VOI collimator at a regular or high exposure level and the second set of projection images are acquired without the collimator at a reduced exposure level. The high exposure VOI scan data inside the VOI and the low exposure full-field scan data outside the VOI are then combined together to generate composite projection images for image reconstruction. To investigate and quantify scatter reduction, dose saving and image quality improvement in VOI CBCT imaging, a flat panel detector-based bench-top experimental CBCT system was built to measure the dose, the scatter-to-primary ratio (SPR), the image contrast, noise level, the contrast-to-noise ratio (CNR) and the figure of merit (FOM) in the CBCT reconstructed images for two polycarbonate cylinders simulating the small and the large phantoms. The results showed that, compared to the full field CBCT technique, radiation doses for the VOI CBCT technique were reduced by a factor of 1.20 and 1.36 for the small and the large phantoms at the phantom center, respectively, and from 2.7 to 3.0 on the edge of the phantom, respectively. Inside the VOI, the SPRs were substantially reduced by a factor of 6.6 and 10.3 for the small and the large phantoms, the contrast signals were improved by a factor of 1.35 and 1.8, and the noise levels were increased by a factor of 1.27 and 1.6, respectively. As a result, the CNRs were improved by a factor of 1.06 and 1.13 for the small and the large phantoms and the FOM improved by a factor of 1.4 and 1.7, respectively.
    Physics in Medicine and Biology 11/2009; 54(21):6691-709. · 2.70 Impact Factor
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    ABSTRACT: In this study, the authors investigated the feasibility of a dual resolution volume-of-interest (VOI) cone beam breast CT technique and compared two implementation approaches in terms of dose saving and scatter reduction. 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 microm 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. 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. These results indicate potential improvement of the image quality inside the VOI with reduced breast dose both inside and outside the VOI.
    Medical Physics 09/2009; 36(9):4007-14. · 2.91 Impact Factor
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    ABSTRACT: Images of mastectomy breast specimens have been acquired with a bench top experimental Cone beam CT (CBCT) system. The resulting images have been segmented to model an uncompressed breast for simulation of various CBCT techniques. To further simulate conventional or tomosynthesis mammographic imaging for comparison with the CBCT technique, a deformation technique was developed to convert the CT data for an uncompressed breast to a compressed breast without altering the breast volume or regional breast density. With this technique, 3D breast deformation is separated into two 2D deformations in coronal and axial views. To preserve the total breast volume and regional tissue composition, each 2D deformation step was achieved by altering the square pixels into rectangular ones with the pixel areas unchanged and resampling with the original square pixels using bilinear interpolation. The compression was modeled by first stretching the breast in the superior-inferior direction in the coronal view. The image data were first deformed by distorting the voxels with a uniform distortion ratio. These deformed data were then deformed again using distortion ratios varying with the breast thickness and re-sampled. The deformation procedures were applied in the axial view to stretch the breast in the chest wall to nipple direction while shrinking it in the mediolateral to lateral direction re-sampled and converted into data for uniform cubic voxels. Threshold segmentation was applied to the final deformed image data to obtain the 3D compressed breast model. Our results show that the original segmented CBCT image data were successfully converted into those for a compressed breast with the same volume and regional density preserved. Using this compressed breast model, conventional and tomosynthesis mammograms were simulated for comparison with CBCT.
    Proc SPIE 02/2009;
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    ABSTRACT: Breast density has been recognized as one of the major risk factors for breast cancer. However, breast density is currently estimated using mammograms which are intrinsically 2D in nature and cannot accurately represent the real breast anatomy. In this study, a novel technique for measuring breast density based on the segmentation of 3D cone beam CT (CBCT) images was developed and the results were compared to those obtained from 2D digital mammograms. 16 mastectomy breast specimens were imaged with a bench top flat-panel based CBCT system. The reconstructed 3D CT images were corrected for the cupping artifacts and then filtered to reduce the noise level, followed by using threshold-based segmentation to separate the dense tissue from the adipose tissue. For each breast specimen, volumes of the dense tissue structures and the entire breast were computed and used to calculate the volumetric breast density. BI-RADS categories were derived from the measured breast densities and compared with those estimated from conventional digital mammograms. The results show that in 10 of 16 cases the BI-RADS categories derived from the CBCT images were lower than those derived from the mammograms by one category. Thus, breasts considered as dense in mammographic examinations may not be considered as dense with the CBCT images. This result indicates that the relation between breast cancer risk and true (volumetric) breast density needs to be further investigated.
    Proc SPIE 02/2009;
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    ABSTRACT: In this work, we investigated the visibility of microcalcifications in CCD-based cone beam CT (CBCT) breast imaging. 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, ranging from 140-150 to 200-212 mum in size, were used to simulate the microcalcifications. Groups of 25 same size microcalcifications were arranged into 5 × 5 clusters. Each cluster was embedded at the center of a smaller (15 mm diameter) cylindrical paraffin phantom, which were inserted into a hole at the center of the breast phantom. The breast phantom with the simulated microcalcifications was scanned on a bench top experimental CCDbased cone beam CT system at various exposure levels with two CCD cameras: Hamamatsu's C4742-56-12ER and Dalsa 99-66-0000-00. 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 independently. The ratios of visible microcalcifications were recorded and averaged over all readers. These ratios were plotted as the function of measured image signal-to-noise ratio (SNR) for various scans. It was found that 94% visibility was achieved for 200-212 mum calcifications at an SNR of 48.2 while 50% visibility was achieved for 200-212, 180-200, 160-180, 150-160 and 140-150 mum calcifications at an SNR of 25.0, 35.3, 38.2, 42.2 and 64.4, respectively.
    Proc SPIE 02/2009;