Physics of Medical Imaging

Evaluation of position-estimation methods applied to CZT-based photon-counting detectors for dedicated breast CT

[+] Author Affiliations
Andrey Makeev, Stephen J. Glick

University of Massachusetts Medical School, Department of Radiology, 55 Lake Avenue North, Worcester, Massachusetts 01655, United States

Martin Clajus, Scott Snyder

NOVA R&D, Inc., 833 Marlborough Avenue #200, Riverside, California 92507, United States

Xiaolang Wang

Toshiba Medical Research Institute USA, Inc., 706 North Deerpath Drive, Vernon Hills, Illinois 60061, United States

J. Med. Imag. 2(2), 023501 (Apr 28, 2015). doi:10.1117/1.JMI.2.2.023501
History: Received August 6, 2014; Accepted March 19, 2015
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Abstract.  Semiconductor photon-counting detectors based on high atomic number, high density materials [cadmium zinc telluride (CZT)/cadmium telluride (CdTe)] for x-ray computed tomography (CT) provide advantages over conventional energy-integrating detectors, including reduced electronic and Swank noise, wider dynamic range, capability of spectral CT, and improved signal-to-noise ratio. Certain CT applications require high spatial resolution. In breast CT, for example, visualization of microcalcifications and assessment of tumor microvasculature after contrast enhancement require resolution on the order of 100μm. A straightforward approach to increasing spatial resolution of pixellated CZT-based radiation detectors by merely decreasing the pixel size leads to two problems: (1) fabricating circuitry with small pixels becomes costly and (2) inter-pixel charge spreading can obviate any improvement in spatial resolution. We have used computer simulations to investigate position estimation algorithms that utilize charge sharing to achieve subpixel position resolution. To study these algorithms, we model a simple detector geometry with a 5×5 array of 200μm pixels, and use a conditional probability function to model charge transport in CZT. We used COMSOL finite element method software to map the distribution of charge pulses and the Monte Carlo package PENELOPE for simulating fluorescent radiation. Performance of two x-ray interaction position estimation algorithms was evaluated: the method of maximum-likelihood estimation and a fast, practical algorithm that can be implemented in a readout application-specific integrated circuit and allows for identification of a quadrant of the pixel in which the interaction occurred. Both methods demonstrate good subpixel resolution; however, their actual efficiency is limited by the presence of fluorescent K-escape photons. Current experimental breast CT systems typically use detectors with a pixel size of 194μm, with 2×2 binning during the acquisition giving an effective pixel size of 388μm. Thus, it would be expected that the position estimate accuracy reported in this study would improve detection and visualization of microcalcifications as compared to that with conventional detectors.

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© 2015 Society of Photo-Optical Instrumentation Engineers

Citation

Andrey Makeev ; Martin Clajus ; Scott Snyder ; Xiaolang Wang and Stephen J. Glick
"Evaluation of position-estimation methods applied to CZT-based photon-counting detectors for dedicated breast CT", J. Med. Imag. 2(2), 023501 (Apr 28, 2015). ; http://dx.doi.org/10.1117/1.JMI.2.2.023501


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