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Using Scattered XRays to Improve the Estimation Accuracy of Attenuation Coefficients: A Fundamental Analysis
Naohiro TODA Tetsuya NAKAGAMI Yoichi YAMAZAKI Hiroki YOSHIOKA Shuji KOYAMA
Publication
IEICE TRANSACTIONS on Fundamentals of Electronics, Communications and Computer Sciences
Vol.E101A
No.7
pp.11011114 Publication Date: 2018/07/01 Online ISSN: 17451337
DOI: 10.1587/transfun.E101.A.1101 Type of Manuscript: PAPER Category: Measurement Technology Keyword: Xray CT, scattered Xrays, accuracy, Monte Carlo simulation, CramerRao lower bound,
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Summary:
In Xray computed tomography, scattered Xrays are generally removed by using a postpatient collimator located in front of the detector. In this paper, we show that the scattered Xrays have the potential to improve the estimation accuracy of the attenuation coefficient in computed tomography. In order to clarify the problem, we simplified the geometry of the computed tomography into a thin cylinder composed of a homogeneous material so that only one attenuation coefficient needs to be estimated. We then conducted a Monte Carlo numerical experiment on improving the estimation accuracy of attenuation coefficient by measuring the scattered Xrays with several dedicated toroidal detectors around the cylinder in addition to the primary Xrays. We further present a theoretical analysis to explain the experimental results. We employed a model that uses a Tjunction (i.e., Tjunction model) to divide the photon transport into primary and scattered components. This division is processed with respect to the attenuation coefficient. Using several Tjunction models connected in series, we modeled the case of several scatter detectors. The estimation accuracy was evaluated according to the variance of the efficient estimator, i.e., the CramerRao lower bound. We confirmed that the variance decreases as the number of scatter detectors increases, which implies that using scattered Xrays can reduce the irradiation dose for patients.

