In an effort to improve the early stage detection and diagnosis of breast cancer, a number of research groups have been
investigating the use of x-ray computerized tomography (CT) systems dedicated for use in imaging the breast.
Preliminary results suggest that dedicated breast CT systems can provide improved visualization of 3D breast tissue as
compared to conventional mammography. However, current breast CT prototypes that are being investigated have
limitations resulting in less than desirable spatial resolution, lesion contrast, and signal-to-noise (SNR) ratio. Another
option is a CT breast imaging system that uses a cadmium zinc telluride (CZT) based detector operating in a photon
counting mode. This paper uses a Monte Carlo simulation to evaluate the effect of characteristic x-rays on spatial and
spectral resolution for a CZT detector used for breast CT. It is concluded that using CZT of 500-750 μm would not cause
significant differences in spatial or spectral resolution, nor in stopping power as compared to using CZT with thickness
2-3 mm.
A number of groups are currently investigating tomographic imaging of the breast, but the optimal design and
acquisition parameters for such systems remains uncertain. One useful tool for investigating optimal parameters is
computer simulation software. A computer program that simulates xray transport through a breast object model
followed by signal and noise propagation through a CsI flatpanel detector has been modified, restructured and enhanced
in order to provide a fast yet sufficiently accurate research tool. The main focus of this work was to validate the
simulated response of a CsI flatpanel detector with a real detector namely, the Paxscan 2520 (Varian Medical Systems,
Salt Lake City, UT). Preliminary results indicate that the program provides comparable quantitative accuracy, that can
be used to obtain accurate and meaningful results to assist in research in tomosynthesis and CT breast imaging system
design.
KEYWORDS: Computed tomography, Sensors, Breast imaging, Tumors, Reconstruction algorithms, Breast cancer, X-ray computed tomography, Monte Carlo methods, Diagnostics, X-rays
Dedicated CT breast imaging using a flat-panel detector system holds great promise for improving the detection
and diagnosis of early stage breast cancer. It is currently unclear whether dedicated CTBI systems will be useful
for screening of the general population. Possibly a more realistic goal will be contrast-enhanced, flat-panel CTBI
to assist in the diagnostic workup of suspected breast cancer patients. It has been suggested that the specificity of
CE-CTBI can be improved by acquiring a dynamic sequence of CT images, characterizing the lesion enhancement
pattern. To make dynamic CE-CTBI feasible, it is important to perform very fast CT acquisitions, with minimal
radiation dose. One technique for reducing the time required for CT acquisitions, is to use a half-scan cone-beam
acquisition, requiring a scan of 180° plus the detector width. In addition to achieving a shorter CT scan, half-scan
acquisition can provide a number of benefits in CTBI system design. This study compares different half-scan
reconstruction methods with a focus on evaluating the quantitative performance in estimating the CT number of
iodinated contrast enhanced lesions. Results indicate that half-scan cone-beam acquisition can be used with little
loss in quantitative accuracy.
KEYWORDS: Breast, 3D modeling, Systems modeling, X-ray computed tomography, Sensors, Tumor growth modeling, Tissues, Breast imaging, Data modeling, X-rays
Dedicated x-ray computed tomography (CT) of the breast using a
cone-beam flat-panel detector system is a modality under investigation by a number of research teams. As previously reported, we have fabricated a prototype, bench-top flat-panel CT breast imaging (CTBI) system and developed computer simulation software to
model such a system. We are developing a methodology to use high resolution, low noise CT reconstructions of fresh mastectomy specimens for generating an ensemble of 3D digital breast phantoms that realistically model 3D compressed and uncompressed breast anatomy. These breast models can be used to simulate realistic projection data for both breast tomosynthesis (BT) and CT systems thereby providing a powerful evaluation and optimization
mechanism.
KEYWORDS: Breast, Tumors, 3D acquisition, Mammography, Tumor growth modeling, 3D modeling, Reconstruction algorithms, 3D image processing, Sensors, Imaging systems
In breast tomosynthesis (BT), multiple x-ray projections obtained over a limited angular span are reconstructed
to produce a three-dimensional (3D) volume. This 3D imagery can lead to reduced structural masking effects
compared to conventional mammography. Accordingly, there has been considerable interest in optimizing acquisition
and reconstruction parameters associated with BT. In this work, we evaluate the use of a scanning
model observer and localization ROC (LROC) methodology for performing a task-based optimization of the
angular span and number of projection angles for a simulated BT system. The observer was applied to extracted
slices of 3D volumes reconstructed with filtered backprojection. Both "background-known-exactly" (BKE) and
"quasi-BKE" (QBKE) tasks were conducted. The latter task attempts to account for limited observer training
by preserving structural noise in the detection task. Reduced noise in the form of fewer projections was important
with the BKE task, although wider angular spans were also advantageous. Higher sampling densities may
improve performance for the more-realistic QBKE tasks.
Planar X-ray mammography is the standard medical imaging modality for the early detection of breast cancer. Based on
advancements in digital flat-panel detector technology, dedicated x-ray computed tomography (CT) mammography is a
modality under investigation that offers the potential for improved breast tumor imaging. We have implemented a
prototype half cone-beam CT breast imaging system that utilizes an indirect flat-panel detector. This prototype can be
used to explore and evaluate the effect of varying acquisition and reconstruction parameters on image quality. This
report describes our system and characterizes the performance of the system through the analysis of Modulation Transfer
Function (MTF) and Noise Power Spectrum (NPS). All CT reconstructions were made using Feldkamp's filtered
backprojection algorithm. The 3D MTF was determined by the analysis of the plane spread function (PlSF) derived
from the surface spread function (SSF) of reconstructed 6.3mm spheres. 3D NPS characterization was performed
through the analysis of a 3D volume extracted from zero-mean CT noise of air reconstructions. The effect of varying
locations on MTF and the effect of different Butterworth filter cutoff frequencies on NPS are reported. Finally, we
present CT images of mastectomy excised breast tissue. Breast specimen images were acquired on our CTMS using an
x-ray technique similar to the one used during performance characterization. Specimen images demonstrate the inherent
CT capability to reduce the masking effect of anatomical noise. Both the quantitative system characterization and the
breast specimen images continue to reinforce the hope that dedicated flat-panel detector, x-ray cone-beam CT will
eventually provide enhanced breast cancer detection capability.
Previous studies have suggested that iodine contrast-enhanced breast tomosynthesis can be helpful in the
characterization of suspicious abnormalities. Dual-energy, contrast-enhanced breast tomosynthesis involves acquiring
low- and high-energy acquisitions after the administration of the contrast agent, and therefore can simplify the
procedure and reduce the effect of patient motion. In this study, a computer simulation is developed for use in
investigating optimal parameters for dual-energy, contrast-enhanced breast tomosynthesis. The simulation allows for the
selection of various polyenergetic x-ray spectra and x-ray filters, and models x-ray transport through a voxelized breast
phantom, as well as signal and noise propagation through an indirect CsI based imager. A compressed breast phantom
that models the non-uniform parenchymal structure of the breast is used. Irregular lesions were simulated by using a
stochastic growth algorithm. Simulations of dual-energy subtraction, contrast-enhanced breast tomosynthesis show that
using x-ray filters to form quasi-monochromatic high- and low-energy spectra above and below the iodine K-edge can
substantially reduce background structure, and increase lesion conspicuity as compared to single shot contrast-enhanced
BT.
In considering a breast CT system, it is important to note that the spectral attenuation profile of a tumor is very similar to that of fibro-glandular tissue. Preliminary evidence based on imaging breast specimens suggest that the CT number of a malignant breast tumor is very similar to that of surrounding fibro-glandular tissue. Therefore, it is expected that radiologists will probably rely more on tumor morphology to distinguish a malignant tumor from fibro-glandular tissue than an increase in contrast per se. Previous studies have shown that iodinated contrast agents can increase the effective attenuation coefficient yielded by a breast tumor thereby providing increased CT tumor contrast. In order to characterize how the intravenous administration of an iodinated contrast agent can affect the performance of CT breast imaging, a computer simulation of such a system was conducted. The two primary goals of this investigation were first to determine how mean glandular dose, choice of x-ray energy spectrum, and iodine contrast agent density affect tumor detection, and second to determine what effect Compton and Rayleigh scattering have on the variability of the attenuation coefficient yielded by CT mammography. The first goal was achieved by making use of a modified version of the Bakic (Med. Phys. 2003) digital breast phantom to model the uncompressed breast, and a 0.5 cm sphere representing a breast tumor was digitally inserted into the ductal region of this phantom. Several projection sets were generated with the tumor containing various densities of iodine contrast agent, different x-ray energy spectra, and different mean glandular dosage (MGD) levels . Slices through the tumor were extracted from the reconstructions of these projections and were used in human observer studies to determine tumor detectability. The second goal was achieved by using the GATE (Geant 4 Application for Tomographic Emission) Monte-Carlo software package to compute the scattering incident on the flat panel detector for an x-ray projection, then using the aforementioned Bakic phantom, a 0.5 cm sphere representing a breast tumor attenuation and a 3.0 mg/ml of Iodinated contrast agent were inserted at various locations with varying attenuation for 100 projection sets with scatter, and 100 projections without scatter. Histograms of the resulting effective attenuation coefficients yielded by Feldkamp filtered backprojection were plotted and compared.
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