Clay Didier

Characterization of scatter in cone‐beam CT breast imaging: Comparison of experimental measurements and Monte Carlo simulation

It is commonly understood that scattered radiation in x-ray computed tomography (CT) degrades the reconstructed image. As a precursor to developing scatter compensation methods, it is important to characterize this scatter using both empirical measurements and Monte Carlo simulations. Previous studies characterizing scatter using both experimental measurements and Monte Carlo simulations have been reported in diagnostic radiology and conventional mammography. The emerging technology of cone-beam CT breast imaging (CTBI) differs significantly from conventional mammography in the breast shape and imaging geometry, aspects that are important factors impacting the measured scatter. This study used a bench-top cone-beam CTBI system with an indirect flat-panel detector. A cylindrical phantom with equivalent composition of 50% fibroglandular and 50% adipose tissues was used, and scatter distributions were measured by beam stop and aperture methods. The GEANT4-based simulation package GATE was used to model x-ray photon interactions in the phantom and detector. Scatter to primary ratio (SPR) measurements using both the beam stop and aperture methods were consistent within 5% after subtraction of nonbreast scatter contributions and agree with the low energy electromagnetic model simulation in GATE. The validated simulation model was used to characterize the SPR in different CTBI conditions. In addition, a realistic, digital breast phantom was simulated to determine the characteristics of various scatter components that cannot be separated in measurements. The simulation showed that the scatter distribution from multiple Compton and Rayleigh scatterings, as well as from the single Compton scattering, has predominantly low-frequency characteristics. The single Rayleigh scatter was observed to be the primary contribution to the spatially variant scatter component.

Using Mastectomy Specimens to Develop Breast Models for Breast Tomosynthesis and CT Breast Imaging

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.

Comparison of Two Methods to Develop Breast Models for Simulation of Breast Tomosynthesis and CT

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. Several teams, including ours, have fabricated prototype, bench-top flat-panel CT breast imaging (CTBI) systems. We also use computer simulation software to optimize system parameters. We are developing a methodology to use high resolution, low noise CT reconstructions of fresh mastectomy specimens in order to generate an ensemble of 3D digital breast phantoms that realistically model 3D compressed and uncompressed breast anatomy. The resulting breast models can then be used to simulate realistic projection data for both Breast Tomosynthesis (BT) and Breast CT (BCT) systems thereby providing a powerful evaluation and optimization mechanism for research and development of novel breast imaging systems as well as the optimization of imaging techniques for such systems. Having the capability of using breast object models and simulation software is clinically significant because prior to a clinical trial of any prototype breast imaging system many parameter tradeoffs can be investigated in a simulation environment. This capability is worthwhile not only for the obvious benefit of improving patient safety during initial clinical trials but also because simulation prior to prototype implementation should result in reduced cost and increased speed of development. The main goal of this study is to compare results obtained using two different methods to develop breast object models in order to select the better technique for developing the entire ensemble.

Development of an ensemble of digital breast object models

In the investigation of emerging tomographic breast imaging methods using flat-panel detectors (FPDs), digital breast object models are useful tools These models are also commonly referred to as digital phantoms We have created an ensemble of digital breast object phantoms based on CT scans of surgical mastectomy specimens Early versions of the phantoms have been used in our published research Recently we have improved some of our processing tools such as the use of 3-D anisotropic diffusion filtering (ADF) prior to segmentation, and we have evaluated breast object models generated with different methods including power spectral analysis, ROI statistics and an observation study.

Characterization of a prototype tabletop x-ray CT breast imaging system

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 Feldkamps 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.

Investigating the effect of characteristic x-rays in cadmium zinc telluride detectors under breast computerized tomography operating conditions.

A number of research groups have been investigating the use of dedicated breast computerized tomography (CT). Preliminary results have been encouraging, suggesting an improved visualization of masses on breast CT as compared to conventional mammography. Nonetheless, there are many challenges to overcome before breast CT can become a routine clinical reality. One potential improvement over current breast CT prototypes would be the use of photon counting detectors with cadmium zinc telluride (CZT) (or CdTe) semiconductor material. These detectors can operate at room temperature and provide high detection efficiency and the capability of multi-energy imaging; however, one factor in particular that limits image quality is the emission of characteristic x-rays. In this study, the degradative effects of characteristic x-rays are examined when using a CZT detector under breast CT operating conditions. Monte Carlo simulation software was used to evaluate the effect of characteristic x-rays and the detector element size on spatial and spectral resolution for a CZT detector used under breast CT operating conditions. In particular, lower kVp spectra and thinner CZT thicknesses were studied than that typically used with CZT based conventional CT detectors. In addition, the effect of characteristic x-rays on the accuracy of material decomposition in spectral CT imaging was explored. It was observed that when imaging with 50-60 kVp spectra, the x-ray transmission through CZT was very low for all detector thicknesses studied (0.5-3.0 mm), thus retaining dose efficiency. As expected, characteristic x-ray escape from the detector element of x-ray interaction increased with decreasing detector element size, approaching a 50% escape fraction for a 100 μm size detector element. The detector point spread function was observed to have only minor degradation with detector element size greater than 200 μm and lower kV settings. Characteristic x-rays produced increasing distortion in the spectral response with decreasing detector element size. If not corrected for, this caused a large bias in estimating tissue density parameters for material decomposition. It was also observed that degradation of the spectral response due to characteristic x-rays caused worsening precision in the estimation of tissue density parameters. It was observed that characteristic x-rays do cause some degradation in the spatial and spectral resolution of thin CZT detectors operating under breast CT conditions. These degradations should be manageable with careful selection of the detector element size. Even with the observed spectral distortion from characteristic x-rays, it is still possible to correctly estimate tissue parameters for material decomposition using spectral CT if accurate modeling is used.

Comparison of Scatter/Primary Measurements with GATE Simulations for X-Ray Spectra in Cone Beam CT Mammography

It is commonly understood that scattered radiation in CT degrades the reconstructed image. Scatter correction relies on precise measurements or Monte Carlo simulation results in experimental conditions. Previous studies on both experimental measurements and Monte Carlo simulations have been reported in diagnostic radiology and conventional mammography. Rapidly developed cone beam CT mammography significantly differs from conventional mammography in breast shape, geometry, etc. aspects, which are amongst the most important factors that impact the scatter fraction. The Geant4 based Monte Carlo simulation package GATE has been successful in the application of PET and SPECT with its implemented precise modeling of various physics processes. However, there exist little reports in the literature about the GATE application in X-ray CT. We present some preliminary results on the experimental measurements of the scatter to primary ratios in cone beam CT mammography setting using the beam stop and aperture methods. The measurements were used to compare and validate the performance of the GATE simulations by choosing the low energy or standard physics models for electromagnetic processes including Compton and Rayleigh scatterings. Aided by precise GATE simulations, we discovered a significant scatter component from sources other than the studied object in scatter measurement using conventional methods. We developed a new strategy of measurement to subtract this background scatter.

WE‐E‐L100J‐03: Scatter Measurements and Simulations in X‐Ray Cone Beam CT Breast Imaging

Purpose: To compare and validate Monte Carlo simulation using the GATE software with experimental measurements for scattered radiation in X‐ray cone beam CT breast imaging . In addition, to determine the scatter distribution for single‐order, and multiple‐order incoherent and coherent scatter using validated Monte Carlo simulation.Methods: This study used a bench‐top cone‐beam CT breast imagingsystem using a flat‐panel detector. A cylindrical phantom with equivalent composition of 50% fibro‐glandular and 50% adipose tissues was used. Scatter distributions were measured by beam stop and aperture methods. A lead strip was positioned between the X‐ray source and the breast phantom in the beam stop method. Its inverse version was used in the aperture method. We computed the scatter contribution in the breast phantom by subtracting background scatter. The background scatter fraction was measured without the phantom. The Geant4‐based simulation package GATE was used to model X‐ray photon interactions in the phantom and detector. Two implemented electromagnetic interactions packages, standard and low energy models, were compared for computing efficiency and physics ingredients. A structured breast phantom was used in GATE simulations to determine the characteristics of various scatter components which cannot be separated in measurements. Results: Measurements by the two methods are consistent within 5% after background subtraction, and agree with the low energy model simulations. A hybrid model in GATE, with photoelectric process from the standard model and Compton and Rayleigh scattering processes from the low energy model, is computationally efficient while maintaining physics accuracy. The GATE simulations in the Bakic phantom show that the multiple‐order scatter distribution, as well as single‐order Compton scatter, has predominantly low‐frequency characteristics. Single‐order Rayleigh scatter was observed to be the primary contribution to the spatially variant scatter component.

Quantitative comparison of weighted Feldkamp FBP full-scan and half-scan algorithms for contrast-enhanced CT breast imaging

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.

Investigation of the use of iodinated contrast agent in a proposed flat-panel CT mammography system

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.