The Effects of Spectral X-Ray Photon Counting Detector Parameters on Detector Performance: Thickness and Pitch

Abstract

The design of photon counting spectral imaging (x-CSI) x-ray detectors involves optimising many parameters including pixel thickness, pitch, and type of charge sharing correction algorithm (CSCA) employed, if any. The optimal value for one parameter depends on the other parameter values, as well as extrinsic variables such as application specific photon fluxes and energies of interest. No analytical approaches currently exist for optimising these parameters simultaneously. This work thus utilised our inhouse simulation framework, combining Monte Carlo and finite element methods to systematically simulate the response of 715 different CdTe based x-CSI detectors, comprising 13 different CSCAs, 5 different pixel thicknesses (1 mm-3 mm), and 11 different pixel pitches (100 µm-600 µm). Detector response to monoenergetic irradiation at 80 keV at 4 different fluxes was assessed using a range of metrics. Due to its complexity, the analysis of this work is divided into several publications, with this one focusing on the effects of pixel pitch and thickness. We were able to identify, and provide mechanistic explanations for, general trends in detector performance with varying pixel geometry that will be of interest to x-CSI detectors designers. Superficially similar spectral metrics were found to vary significantly in their sensitivity to different charge sharing mechanisms, underlining the importance of carefully selecting the evaluation metric for photon counting detectors based on their application. The parameters used here were selected based on our own interests, however this work demonstrates the utility of this framework for optimising x-CSI detector parameters for various spectral applications.

CdTe Based Energy Resolving, X-ray Photon Counting Detector Performance Assessment: The Effects of Charge Sharing Correction Algorithm Choice

“…The large data set, and multiple degrees of freedom, make analysis of the entire dataset in a single paper unwieldly and for that reason the analysis is divided across several publications. The work in this paper represents the second such submission, with the first focusing on pixel pitch and thickness at a single flux, without using a CSCA [32]. That work explained the behaviour of the "No CSCA" data for each metric in terms of CSEs and pulse pileup.…”

“…The work presented here represents a subset of a larger study into the effect of x-CSI system parameters on system performance. A previous publication [32] explored the effect of pixel pitch and thickness. Here we have examined how various classes of additive CSCAs compare at low to moderate medical fluxes.…”

“…The general shape of the APE trends in Figure 9 is that of a peak, increasing with pitch up to a point and then decreasing with pitch beyond that point. The initial increase in APE with pixel pitch was previously shown to result from a reduction of CSEs [32], resulting in more incident photons being correctly assigned their full energy. The drop in APE at still higher pixel pitches is associated with pulse pileup effects, which lead to multiple photons being summed together, and thus the count being assigned to the higher energy bin 4 (coincidence bin) rather than the lower energy bin 3 (photopeak bin).…”

An Overview of X-ray Photon Counting Spectral Imaging (x-CSI) with a Focus on Gold Nanoparticle Quantification in Oncology

“…This point is perhaps stated overly simplistically, and it should be noted that electronic noise is still present in x-CSI systems-it just manifests in a less damaging way (as a small uncertainty in photon energy rather than a large uncertainty in photon counts). Spectral overlap also still occurs in x-CSI systems, due to noise sources, partial energy deposition, and charge-sharing effects [47]. Nevertheless, these problems are less detrimental to x-CSI systems than their counterparts in EI systems for the reasons discussed above.…”

“…This is due to the presence of a range of chargesharing effects (CSEs) that collectively act to distribute charge deposited in one pixel across neighbouring pixels [69]. Fluorescence X-rays, charge cloud expansion (due to diffusion and electrostatic repulsion), and Compton scattering can all act as CSEs [70], though their relatively short ranges mean that spectral degradation from CSEs is most prominent at small pixel sizes (below a few hundred µm) [47]. Nevertheless, if left uncorrected, CSEs can result in X-ray photons registering at a broad range of different energies other than their actual one [70].…”