The Application of Multiple Energy Bins in Compton and Photoelectric Sinogram Decomposition
Abstract: There is
growing interest in developing X-ray computed tomography (CT) imaging systems with
improved ability to discriminate material types, going beyond the attenuation imaging
provided by most current systems. Dual-energy CT (DECT) systems can partially address
this problem by estimating Compton and photoelectric (PE) coefficients of the materials
being imaged, but DECT is greatly... read moredegraded by the presence of metal or other materials
with high attenuation. Here we explore the advantages of multi-energy CT (MECT) systems
based on photon-counting detectors. The utility of MECT has been demonstrated in medical
applications where photon-counting detectors allow for the resolution of absorption
K-edges. Our primary concern is aviation security applications where K-edges are rare.
We simulate phantoms with differing amounts of metal (high, medium and low attenuation),
both for switched-source DECT and for MECT systems, and include a realistic model of
detector energy resolution. We extend the DECT sinogram decomposition method of Ying et
al. to MECT, allowing estimation of separate Compton and photoelectric sinograms. We
furthermore introduce a weighting based on a quadratic approximation to the Poisson
likelihood function that deemphasizes energy bins with low signal. Simulation results
show that the proposed approach succeeds in estimating material properties even in
high-attenuation scenarios where the DECT method fails, improving the signal to noise
ratio of reconstructions by over 20 dB for the high-attenuation phantom. To numerically
evaluate the performance of the weighting based MECT, the Cram´er-Rao lower bound
(CRLB) is employed to determine the upper bound of signal-to-noise ratio (SNR) for both
Compton and PE coefficients in the sinogram domain. A prominent improvement can be seen
by using MECT compared to the conventional switched-source DECT. The upper bound
eventually converges as the number of energy bins increases. Finally, we have done
simulations for a specific geometry built by American and Science Engineering with one
source and a right-triangle shaped array of detectors capable of distinguishing multiple
energy bins. The result shows a better performance by using MECT. In summary, our work
demonstrates the potential of using photon counting detectors and multiple energy bins
for stably recovering material properties even when high attenuation is present, thus
enabling the development of improved scanning
Thesis (M.S.)--Tufts University, 2016.
Submitted to the Dept. of Electrical Engineering.
Advisor: Brian Tracey.
Committee: Eric Miller, Shuchin Aeron, and Aaron Couture.
Keyword: Electrical engineering.read less