The influence of large x-ray scatter components in projection images remains a problem for digital breast tomosynthesis,
especially when anti-scatter grids may not be used because of dose limitation and possible source/detector
geometric limitations. Monte-Carlo simulation of scatter fits better in this situation, but the heavy computational
cost hinders its clinical application. To simplify scatter estimation, scatter is often assumed to be smooth.
However, scatter is not spatial invariant across a projection image, and where and to what degree the smoothness
could be claimed and utilized is unclear. In this study, we investigated this question via multi-resolution
analysis based on two experiments: one with direct measurements of scatter profiles in the projection images
of an anthropomorphic breast phantom; the other with scatter map obtained from Monte-Carlo simulation that
used a voxelized breast model as input. We applied 1D and 2D wavelet-based multi-resolution analyses to the
scatter profiles and maps. The first experiment indicated that a reduced number of scatter data points that
matches the true data can be extracted from densely sampled but noisy scatter profiles: a data reduction rate
of 64-128 was achieved at the inner region of the phantom, suggesting that the slowly changing scatter may be
obtained at lower sampling distances of 9.0-17.9 mm. Near the edge of the phantom a data reduction rate of 8
was achieved, corresponding to a sampling distance of 2.2 mm. Similar observations were made from the second
experiment.
Since the introduction of ASiR, its potential in noise reduction has been reported in various clinical applications.
However, the influence of different scan and reconstruction parameters on the trade off between ASiR's blurring
effect and noise reduction in low contrast imaging has not been fully studied. Simple measurements on low
contrast images, such as CNR or phantom scores could not explore the nuance nature of this problem. We
tackled this topic using a method which compares the performance of ASiR in low contrast helical imaging
based on an assumed filter layer on top of the FBP reconstruction. Transfer functions of this filter layer were
obtained from the noise power spectra (NPS) of corresponding FBP and ASiR images that share the same scan
and reconstruction parameters. 2D transfer functions were calculated as sqrt[NPSASiR(u, v)/NPSFBP(u, v)].
Synthesized ACR phantom images were generated by filtering the FBP images with the transfer functions of
specific (FBP, ASiR) pairs, and were compared with the ASiR images. It is shown that the transfer functions
could predict the deterministic blurring effect of ASiR on low contrast objects, as well as the degree of noise
reductions. Using this method, the influence of dose, scan field of view (SFOV), display field of view (DFOV),
ASiR level, and Recon Mode on the behavior of ASiR in low contrast imaging was studied. It was found that
ASiR level, dose level, and DFOV play more important roles in determining the behavior of ASiR than the other
two parameters.
Volumetric breast density was evaluated using a simulated cone beam breast CT with 80 kVp. The
breast was modeled as a cylinder with background tissue composition of 20% glandular and 80%
adipose. Various objects with different sizes and tissue compositions were embedded. Ray-tracing
algorithm was utilized to obtain projection images in a full rotation without considering scatter,
beam hardening and imaging noise. Filtered backprojection was adopted for image reconstruction with high quality. Reconstructed images had flat profiles except at large cone angle of 8.6° to 10°. They were calibrated using known linear attenuation coefficients of two image contrast objects. A 3D mapping of tissue densities could be directly computed within 5% error. Tissue volumes were obtained by counting voxels in appropriate attenuation coefficient ranges. Results of contrast objects were consistent with true volumes within 10% error. However, cone angle artifact decreased pixel values, and a reduction algorithm was required for accurate tissue assessment at large cone angles. This study indicates the possibility of excellent quantitative breast density measurements and volume assessments with cone-beam breast CT.
Partial volume effect and inhomogeneity are two major causes of artifacts in electronic cleansing (EC) for non-cathartic
CT colonography (CTC). Our purpose was to develop a novel method of EC for non-cathartic dual-energy CTC (DECTC)
using a subvoxel multi-spectral material classifier and a regional material decomposition method for
differentiation of residual fecal materials from colonic soft-tissue structures. In this study, an anthropomorphic colon
phantom, which was filled with a mixture of aqueous fiber (psyllium), ground foodstuff (cereal), and non-ionic iodinated
agent (Omnipaque iohexol, GE Healthcare, Milwaukee, WI), was scanned by a dual-energy CT scanner (SOMATON,
Siemens) with two photon energies: 80 kVp and 140 kVp. The DE-CTC images were subjected to a dual-energy EC
(DE-EC) scheme, in which a multi-spectral material classifier was used to compute the fraction of each material within
one voxel by an expectation-maximization (EM) algorithm. This was followed by a regional material segmentation
method for identifying of homogeneous sub-regions (tiles) as fecal materials from other tissue types. The results were
compared with the structural-analysis cleansing (SA-EC) method based upon the CTC images of native phantom without
fillings. The mean cleansing ratio of the DE-EC scheme was 96.57±1.21% compared to 76.3±5.56% of the SA-EC
scheme. The soft-tissue preservation ratio of the DE-EC scheme was 97.05%±0.64% compared to 99.25±0.77% of the
SA-EC scheme.
We evaluated the effects of scatter radiation on the reconstructed images in digital breast
tomosynthesis. Projection images of a 6 cm anthropomorphic breast phantom were acquired
using a Hologic prototype digital breast tomosynthesis system. Scatter intensities in projection
images were sampled with a beam stop method. The scatter intensity at any pixel was obtained
by two dimensional fitting. Primary-only projection images were generated by subtracting the
scatter contributions from the original projection images. The 3-dimensional breast was
reconstructed first based on original projection images which contained the contributions from
both primary rays and scattered radiation using three different reconstruction algorithms. The
same breast volume was reconstructed again using the same algorithms but based on primaryonly
projection images. The image artifacts, pixel value difference to noise ratio (PDNR), and
detected image features in these two sets of reconstructed slices were compared to evaluate the
effects of scatter radiation. It was found that the scatter radiation caused inaccurate
reconstruction of the x-ray attenuation property of the tissue. X-ray attenuation coefficients
could be significantly underestimated in the region where scatter intensity is high. This
phenomenon is similar to the cupping artifacts found in computed tomography. The scatter
correction is important if accurate x-ray attenuation of the tissues is needed. No significant
improvement in terms of numbers of detected image features was observed after scatter
correction. More sophisticated phantom dedicated to digital breast tomosynthesis may be needed
for further evaluation.
KEYWORDS: Breast, Monte Carlo methods, Signal attenuation, 3D image processing, Tissues, Photons, X-rays, 3D image reconstruction, 3D metrology, Sensors
We are developing a breast specific scatter correction method for digital beast tomosynthesis (DBT). The 3D breast volume was initially reconstructed from 15 projection images acquired from a GE prototype tomosynthesis system without correction of scatter. The voxel values were mapped to the tissue compositions using various segmentation schemes. This voxelized digital breast model was entered into a Monte Carlo package simulating the prototype tomosynthesis system. One billion photons were generated from the x-ray source for each projection in the simulation and images of scattered photons were obtained. A primary only projection image was then produced by subtracting the scatter image from the corresponding original projection image which contains contributions from the both primary photons and scatter photons. The scatter free projection images were then used to reconstruct the 3D breast using the same algorithm. Compared with the uncorrected 3D image, the x-ray attenuation coefficients represented by the scatter-corrected 3D image are closer to those derived from the measurement data.
A second-generation digital breast tomosynthesis system is used for a screening study comparing tomosynthesis with conventional two-view mammography with matched x-ray dose. The system acquires 15 projections of a breast at different angles using a digital detector. This work explores acquisition techniques that optimize the quality of projection images at low x-ray exposure. The system provides three target-filter combinations (Mo-Mo, Mo-Rh and Rh-Rh) and the recommended tube voltage range is from 25 to 40kVp. A thin disk was put on top of slabs of breast tissue equivalent materials (20 to 85mm). Contrast-to-noise ratio of the disk was measured from projections acquired with different kVp and target-filter combinations. The squared CNR normalized by average glandular dose was used to compare the quality/dose efficiency of different techniques. The optimal quality/dose efficiency was achieved as the detector entrance exposure was in the range of 5-30mR. Within this range, Mo-Mo gives the highest quality for 20mm; results are very close for 30mm; Rh-Rh is slightly better for 45mm and apparently better than others for 65 and 85mm. However, sufficient detector entrance exposure cannot be guaranteed for all cases due to the total dose limit and the system limit. For some cases, the detector is operated slightly off its optimal performance range. The kVp does not show an impact except for 85 mm, in which the quality/dose efficiency slightly increases at higher kVp. Rh-Rh is selected for > 40mm thickness; Mo-Mo is selected for 20mm thickness; and Mo-Rh is selected for 30 and 40mm.
Cone beam CT mammography (CBCTM) is an emerging breast imaging technology and is currently under intensive investigation [1-3]. One of the major challenges in CBCTM is to understand the characteristics of scatter radiation and to find ways to reduce or correct its degrading effects. Since the breast shape, geometry and image formation process are significantly different from conventional mammography, all system components and parameters such as target/filter combination, kVp range, source to image distance, detector design etc. should be examined and optimized. In optimizing CBCTM systems, it is important to have knowledge of how different imaging parameters affect the recorded scatter within the image. In this study, a GEANT4 based Monte Carlo simulation package (GATE) was used to investigate the scatter magnitude and its’ distribution in CBCTM. The influences of different air gaps, kVp settings, breast sizes and breast composition on the scatter primary ratio (SPR) and scatter profiles were examined. In general, the scatter to primary ratio (SPR) is strongly dependent on the breast size and air gap, and is only moderately dependent on the kVp setting and breast composition. These results may be used for optimization of CBCTM systems, as well as for developing scatter correction methods.
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