Evaluating the impact of X-ray spectral shape on image quality in flat-panel CT breast imaging
ABSTRACT In recent years, there has been an increasing interest in exploring the feasibility of dedicated computed tomography (CT) breast imaging using a flat-panel digital detector in a truncated cone-beam imaging geometry. Preliminary results are promising and it appears as if three-dimensional tomographic imaging of the breast has great potential for reducing the masking effect of superimposed parenchymal structure typically observed with conventional mammography. In this study, a mathematical framework used for determining optimal design and acquisition parameters for such a CT breast imaging system is described. The ideal observer signal-to-noise ratio (SNR) is used as a figure of merit, under the assumptions that the imaging system is linear and shift invariant. Computation of the ideal observer SNR used a parallel-cascade model to predict signal and noise propagation through the detector, as well as a realistic model of the lesion detection task in breast imaging. For all evaluations, the total mean glandular dose for a CT breast imaging study was constrained to be approximately equivalent to that of a two-view conventional mammography study. The framework presented was used to explore the effect of x-ray spectral shape across an extensive range of kVp settings, filter material types, and filter thicknesses. The results give an indication of how spectral shape can affect image quality in flat-panel CT breast imaging.
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ABSTRACT: Dedicated CT imaging of the breast holds great promise for improving the detection and diagnosis of early stage breast cancer. However, before CT breast imaging (CTBI) can become a clinical reality, it is imperative that the issue of image degradation due to the rather large scattered radiation component be addressed. One approach to reducing scatter in CT breast imaging is with the use of anti-scatter grids. This paper describes a theoretical study analyzing the ideal linear observer signal-to-noise ratio (SNR) achieved with varying hypothetical grids. Results suggest that improvement in performance with anti-scatter grids can be attained if the primary x-ray transmission is high enough and if scatter transmission of the grid is low enough. It is shown that for detection of a 4 mm lesion in a dense breast (95% fibroglandular tissue), the AUC (area under the ROC curve) for a grid with primary transmission of 80%, and scatter-to-primary ratio of 5% would be 99%, whereas the AUC for the no grid case would be 88%. Thus, for this specific CTBI task, the ideal linear observer with a high performance grid would substantially outperform that with no grid. Similar improvement in performance with grids was observed for the detection of microcalcifications.
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ABSTRACT: Early detection is one of the most important factors in the survival of patients diagnosed with breast cancer. For this reason the development of improved screening mammography methods is one of primary importance. One problem that is present in standard planar mammography, which is not solved with the introduction of digital mammography, is the possible masking of lesions by normal breast tissue because of the inherent collapse of three-dimensional anatomy into a two-dimensional image. Digital tomosynthesis imaging has the potential to avoid this effect by incorporating into the acquired image information on the vertical position of the features present in the breast. Previous studies have shown that at an approximately equivalent dose, the contrast-detail trends of several tomosynthesis methods are better than those of planar mammography. By optimizing the image acquisition parameters and the tomosynthesis reconstruction algorithm, it is believed that a tomosynthesis imaging system can be developed that provides more information on the presence of lesions while maintaining or reducing the dose to the patient. Before this imaging methodology can be translated to routine clinical use, a series of issues and concerns related to tomosynthesis imaging must be addressed. This work investigates the relevant physical processes to improve our understanding and enable the introduction of this tomographic imaging method to the realm of clinical breast imaging. The processes investigated in this work included the dosimetry involved in tomosynthesis imaging, x-ray scatter in the projection images, imaging system performance, and acquisition geometry. A comprehensive understanding of the glandular dose to the breast during tomosynthesis imaging, as well as the dose distribution to most of the radiosensitive tissues in the body from planar mammography, tomosynthesis and dedicated breast computed tomography was gained. The analysis of the behavior of x-ray scatter in tomosynthesis yielded an in-depth characterization of the variation of this effect in the projection images. Finally, the theoretical modeling of a tomosynthesis imaging system, combined with the other results of this work was used to find the geometrical parameters that maximize the quality of the tomosynthesis reconstruction. Ph.D. Andrew Karellas, John N. Oshinski, Xiaoping P. Hu, Carl J. D’Orsi and Ernest V. Garcia