Cascaded systems analysis of photon counting detectors

Abstract

Purpose: Photon counting detectors (PCDs) are an emerging technology with applications in spectral and low-dose radiographic and tomographic imaging. This paper develops an analytical model of PCD imaging performance, including the system gain, modulation transfer function (MTF), noise-power spectrum (NPS), and detective quantum efficiency (DQE). Methods: A cascaded systems analysis model describing the propagation of quanta through the imaging chain was developed. The model was validated in comparison to the physical performance of a silicon-strip PCD implemented on an experimental imaging bench. The signal response, MTF, and NPS were measured and compared to theory as a function of exposure conditions (70 kVp, 1-7 mA), detector threshold, and readout mode (i.e., the option for coincidence detection). The model sheds new light on the dependence of spatial resolution, charge sharing, and additive noise effects on threshold selection and was used to investigate the factors governing PCD performance, including the fundamental advantages and limitations of PCDs in comparison to energy-integrating detectors (EIDs) in the linear regime for which pulse pileup can be ignored. Results: The detector exhibited highly linear mean signal response across the system operating range and agreed well with theoretical prediction, as did the system MTF and NPS. The DQE analyzed as a function of kilovolt (peak), exposure, detector threshold, and readout mode revealed important considerations for system optimization. The model also demonstrated the important implications of false counts from both additive electronic noise and charge sharing and highlighted the system design and operational parameters that most affect detector performance in the presence of such factors: for example, increasing the detector threshold from 0 to 100 (arbitrary units of pulse height threshold roughly equivalent to 0.5 and 6 keV energy threshold, respectively), increased the f 50 (spatial-frequency at which the MTF falls to a value of 0.50) by ∼30% with corresponding improvement in DQE. The range in exposure and additive noise for which PCDs yield intrinsically higher DQE was quantified, showing performance advantages under conditions of very low-dose, high additive noise, and high fidelity rejection of coincident photons. Conclusions: The model for PCD signal and noise performance agreed with measurements of detector signal, MTF, and NPS and provided a useful basis for understanding complex dependencies in PCD imaging performance and the potential advantages (and disadvantages) in comparison to EIDs as well as an important guide to task-based optimization in developing new PCD imaging systems. C

Volumetric CT with sparse detector arrays (and application to Si-strip photon counters)

“…As reported in Xu et al (2014), such conditions are well below the count rate limit of the PCD (3 × 10 6 x-rays/pixel/s), well within the linear range of detector response free from pulse pileup effects, and give a zero-frequency detective quantum efficiency (DQE) of ~0.6.…”

“…The high threshold separates events assigned to low- and high-energy bins to allow dual-energy imaging. The detector also implements coincidence detection by which events triggering two adjacent pixels within a time window of ~20 ns (Xu et al 2014) are treated as a single count and assigned to the high-energy bin of the pixel that received the higher pulse (higher deposited energy). In the current study, the detector was operated in a photon counting mode without energy discrimination.…”

“…Modeling of other sources of non-ideality specific to PCDs (e.g. coincidence or charge sharing (Taguchi et al 2010, Xu et al 2014)) is beyond the scope of this study.…”

Signal and noise characteristics of a CdTe-based photon counting detector: cascaded systems analysis and experimental studies

“…Unlike the serial cascade model developed for Si-based PCD, 2 both serial 3 and parallel cascade models 4,5 need to be used for CdTe-based PCD due to the high K-fluorescence yield (denoted as ω ) of CdTe (86%) and the relative high energy of K-fluorescence photons of CdTe (> 23 keV). If a single x-ray photon interacts with the CdTe layer, it will experience either path A (no K-fluorescence), path B (K-fluorescence emission and K-fluorescence reabsorption in CdTe), or path C (K-fluorescence emission but no K-fluorescence reabsorption).…”

Spectral CT of the extremities with a silicon strip photon counting detector

“…Photon counting x-ray detectors (PCXDs) can benefit material separation in spectral CT mainly due to the elimination of electronic noise and the ability to discriminate the energy of incoming photons and group them into energy bins [4]. Several PCXD configurations have been proposed using CZT, CdTe, or edge-on silicon strips (Si-strip) as the detection medium.…”