MicroCT with energy-resolved photon-counting detectors

Abstract

The goal of this paper was to investigate the benefits that could be realistically achieved on a microCT imaging system with an energy-resolved photon-counting x-ray detector. To this end, we built and evaluated a prototype microCT system based on such a detector. The detector is based on cadmium telluride (CdTe) radiation sensors and application-specific integrated circuit (ASIC) readouts. Each detector pixel can simultaneously count x-ray photons above six energy thresholds, providing the capability for energy-selective x-ray imaging. We tested the spectroscopic performance of the system using polychromatic x-ray radiation and various filtering materials with Kabsorption edges. Tomographic images were then acquired of a cylindrical PMMA phantom containing holes filled with various materials. Results were also compared with those acquired using an intensity-integrating x-ray detector and single-energy (i.e. non-energy-selective) CT. This paper describes the functionality and performance of the system, and presents preliminary spectroscopic and tomographic results. The spectroscopic experiments showed that the energy-resolved photon-counting detector was capable of measuring energy spectra from polychromatic sources like a standard x-ray tube, and resolving absorption edges present in the energy range used for imaging. However, the spectral quality was degraded by spectral distortions resulting from degrading factors, including finite energy resolution and charge sharing. We developed a simple charge-sharing model to reproduce these distortions. The tomographic experiments showed that the availability of multiple energy thresholds in the photon-counting detector allowed us to simultaneously measure target-to-background contrasts in different energy ranges. Compared with single-energy CT with an integrating detector, this feature was especially useful to improve differentiation of materials with different attenuation coefficient energy dependences.

Breast tissue decomposition with spectral distortion correction: A postmortem study

“…State of the art photon-counting detectors based on semiconductors such as silicon, [30][31][32][33][34] cadmium telluride (CdTe) 27,[35][36][37][38] and cadmium-zinc-telluride (CZT) 29,39-41 offer a maximum count rate over 10 7 (counts/s)/mm 2 . [42][43][44] However, the detector response may become nonlinear under such high count rates due to various artifacts such as pulse pileup, characteristic escape, and charge-sharing.…”

Image‐based spectral distortion correction for photon‐counting x‐ray detectors

“…For energies below 30 keV, the measured counts were significantly higher than the simulated spectrum, most likely due to charge sharing effect. 20 However, in the intermediate energy range, the measured counts were comparably smaller. Finally, the measured counts in higher energy region exceeded that of the simulated spectrum and extended over the maximum tube voltage of 100 kVp.…”

“…34,35 Another approach, which has been widely used in studies on photon-counting detectors, sets the highest energy thresholds at the maximum tube voltage; thus, any pulse exceeding the threshold will be counted in the last energy bin (pileup bin) and excluded from data acquisition. 6,20 However, pulse pileup cannot be effectively eliminated in this approach because it can also occur below the maximum tube voltage. Furthermore, the dose efficiency will be reduced since photons registered in the pileup bin will not contribute to the imaging task.…”

“…An analytical model has also been proposed to simulate the abnormally high counts in the low energy region for CdTe detectors by taking the effects of the photoelectron range, characteristic x-ray emission, and carrier diffusion into consideration. 20 Therefore, charge sharing artifacts can be eliminated by increasing the electronic threshold. However, such an approach comes at the cost of losing low energy photons, which may be undesirable in many applications.…”

Energy-resolved X-ray detectors: the future of diagnostic imaging

“…Multienergy discrimination is extremely sensitive on the chemical composition of the material. A seminal work has been done in 2003 by Pacella et al 43,44 in developing X-ray imaging detectors based on gas electron multipliers, with energy resolution capability. Pacella et al 45 demonstrated in 2006 that energy-resolved imaging is a powerful way to distinguish materials that cannot be discriminated with the standard (energy integrated) X-ray imaging, and even more, it could reveal the nature of the material despite the poor energy resolution of the detector.…”