Purpose
While X-ray photon-counting detectors (PCDs) promise to revolutionize medical imaging, theoretical frameworks to evaluate them are commonly limited to incident fluence rates sufficiently low that the detector response can be considered linear. However, typical clinical operating conditions lead to a significant level of pile-up, invalidating this assumption of a linear response. Here, we present a framework that aims to evaluate PCDs, taking into account their non-linear behavior.

Approach
We employ small-signal analysis to study the behavior of PCDs under pile-up conditions. The response is approximated as linear around a given operating point, determined by the incident spectrum and fluence rate. The detector response is subsequently described by the proposed perturbation point spread function (pPSF). We demonstrate this approach using Monte-Carlo simulations of idealized direct- and indirect-conversion PCDs.

Results
The pPSFs of two PCDs are calculated. It is then shown how the pPSF allows to determine the sensitivity of the detector signal to an arbitrary lesion. This example illustrates the detrimental influence of pile-up, which may cause non-intuitive effects such as contrast/contrast-to-noise ratio inversion or cancellation between/within energy bins.

Conclusions
The proposed framework permits quantifying the spectral and spatial performance of PCDs under clinically realistic conditions at a given operating point. The presented example illustrates why PCDs should not be analyzed assuming that they are linear systems. The framework can, for example, be used to guide the development of PCDs and PCD-based systems. Furthermore, it can be applied to adapt commonly used measures, such as the modulation transfer function, to non-linear PCDs.@en

Due to recent development in detector technology, photon-counting computed tomography (PCCT) has become a rapidly emerging medical imaging technology. Current PCCT systems rely on the direct conversion of X-ray photons into charge pulses, using CdTe, CZT, or Si semiconductor detectors. Indirect detection using ultrafast scintillators coupled to silicon photomultipliers (SiPM) offers a potentially more straightforward and cost-effective alternative. In this work a new 2D perovskite scintillator, benzylamonium lead bromide (BZA)₂PbBr₄, is experimentally characterised as function of temperature. The material exhibits a 4.2 ns decay time under X-ray excitation at room temperature and a light yield of 3700 photons/MeV. The simulation tool developed by Van der Sar et al. was used to model the pulse trains produced by a SiPM-based (BZA)₂PbBr₄ detector. The fast decay time of (BZA)₂PbBr₄ results in outstanding count-rate performance as well as very low statistical fluctuations in the simulated pulses. These features of (BZA)₂PbBr₄, combined with its cost-effective synthesis make (BZA)₂PbBr₄ very promising for PCCT.

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Photon-counting detectors (PCD) for medical X-ray computed tomography (CT) are designed to measure the number of X-ray photons incident on a detector pixel as well as the energy of the individual X-rays. They are expected to yield improvements in image quality for a given radiation dose, and to offer opportunities for spectral imaging beyond dual-energy techniques. However, the fluence rate incident on the detector can exceed 108 mm-2 s-1 in CT, so that the detector pulses generated by the X-rays likely pile up on each other, which distorts the measurement. The semiconductors CdTe and Cd1-xZnxTe (CZT, x ≈ 0.1-0.2) are commonly considered efficient X-ray absorbers that provide sufficient rate capability (fast pulses in the order of 101 ns and a high pixel density ≥ 4 mm-2) and energy resolution (8-20% FWHM at 60 keV). In such detectors, an X-ray is converted into electron-hole pairs, which travel to (pixelated) electrodes, on which they induce a current pulse. However, the cost-effective synthesis of material of sufficient quality to make this a stable and reliable detection process appears to remain an issue. Thus, the aim of this thesis is to explore the photon-counting performance, e.g., the rate capability and energy resolution, of an alternative detector concept based on a scintillator, which converts an X-ray into a light pulse, and a silicon photomultiplier (SiPM), which detects the light. Since such a detector relies on light rather than charge transport, it may enable cost-effective manufacturing of stable and reliable PCDs...
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We investigate X-ray photon-counting scintillation detectors with silicon photomultiplier (SiPM) readout. These circumvent some drawbacks of direct-conversion detectors. We measured observed count rate (OCR) versus X-ray tube current for single-pixel detectors consisting of LYSO:Ce and YAP:Ce scintillators coupled to ultrafast SiPMs. For a 30 keV threshold, the maximum OCRs equal 4.5 Mcps/pixel (LYSO:Ce) and 5.5 Mcps/pixel (YAP:Ce) for paralyzable-like counting and 10 Mcps/pixel (LYSO:Ce) and 12.5 Mcps/pixel (YAP:Ce) for nonparalyzable-like counting. We estimate that the twice as fast LaBr3:Ce scintillator yields OCRs approaching those of CdTe/CZT-based photon-counting CT detectors. We also show energy response data and discuss dose-efficient pixel miniaturization.

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X-ray detectors with photon-counting capabilities promise to revolutionise medical imaging. For an efficient comparison of detectors of various materials and with different setup choices, reliable detector performance measures are needed. The detector point spread function (PSF) is a commonly used measure, which describes the spatial response of an X-ray detector to the irradiation of a single pixel, given the energy spectrum of the source. In the case of an energy-resolving PCD, the detector PSF is typically derived for each energy bin and characterises its resolution. Moreover, it is commonly determined under low count rate conditions, to avoid dead time and pile-up related distortions. Under these assumptions, the PSF can be determined in a straightforward manner, but does not fully characterise the detector under all conditions encountered in clinical practice. This is especially true since the number of registered counts per energy bin depends on both the incident spectrum and the fluence rate, due to pile-up and dead time. We therefore propose a new metric, the differential point spread function (dPSF), which describes the change in the output count rate due to a small change in the input spectrum, for a given combination of incident spectrum and fluence rate. The dPSF can be used to characterize the spectral and spatial performance of a PCD under high-fluence conditions, i.e. when its response becomes non-linear. We illustrate the use of the dPSF by performing a Monte-Carlo study in which we compare the response of direct-conversion and scintillationbased PCDs at different fluence rates.

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We investigate fast silicon photomultiplier (SiPM)-based scintillation detectors for X-ray photon-counting applications, e.g., photon-counting computed tomography (CT). Such detectors may be an alternative to CdTe/CdZnTe (CZT) and Si detectors, which face challenges related to availability and cost-effective growth of detector-grade material, and detection efficiency, respectively. Here, we experimentally study energy response and count rate performance of a 1 mm × 1 mm single-pixel detector consisting of the readily available LaBr3:Ce scintillator and an ultrafast SiPM. We used three radio-isotopes and an X-ray tube for the experiments. Raw detector signals were processed by a second-order low-pass filter with a cut-off frequency f_c equal to 25 MHz or 100 MHz. The detector pulse height was shown to be proportional to photon energy. We measured FWHM energy resolutions of 19.5% (f_c=25 MHz) and 21.5% (f_c=100 MHz) at 60 keV. The measured X-ray tube spectra showed signs of the expected features of such spectra. The best count rate performance was achieved using f_c=100 MHz. In case of paralyzable-like counting and a 30 keV counting threshold, the maximum observed count rate (OCR) was 10.5 Mcps/pixel. For nonparalyzable-like counting and the same threshold, the OCR appeared to approach an asymptotic value greater than 20 Mcps/pixel. These numbers are close to those of CdTe/CZT detectors highly optimized for photon-counting CT. In conclusion, we show promising spectral X-ray photon-counting performance of an LaBr3:Ce scintillation detector with SiPM readout. Depending on the application-specific requirements, miniaturization of the pixel size may be necessary, for which we discuss potential dose-efficient implementations.

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Purpose: The implementation of photon-counting detectors is widely expected to be the next breakthrough in X-ray computed tomography (CT) instrumentation. A small number of prototype scanners equipped with direct-conversion detectors based on room-temperature semiconductors, such as CdTe and CdZnTe (CZT), are currently installed at medical centers. Here, we investigate the feasibility of using silicon photomultiplier (SiPM)-based scintillation detectors in photon-counting computed tomography (PCCT) scanners, as a potential alternative to CdTe and CZT detectors. Methods: We introduce a model that allows us to compute the expected energy resolution as well as the expected pulse shape and associated rate capability of SiPM-based PCCT detectors. The model takes into account SiPM saturation and optical crosstalk, because these phenomena may substantially affect the performance of SiPM-based PCCT detectors with sub-mm pixels. We present model validation experiments using a single-pixel detector consisting of a 0.9 × 0.9 × 1.0 mm³ LuAP:Ce scintillation crystal coupled to a 1 × 1 mm² SiPM. We subsequently use the validated model to compute the expected performance of the fast scintillators LYSO:Ce, LuAP:Ce, and LaBr₃:Ce, coupled to currently available SiPMs, as well as to a more advanced SiPM prototype with improved dynamic range, for sub-mm pixel sizes. Results: The model was found to be in good agreement with the validation experiments, both with respect to energy resolution and pulse shape. It shows how saturation progressively degrades the energy resolution of detectors equipped with currently available SiPMs as the pixel size decreases. Moreover, the expected pulse duration is relatively long (~200 ns) with these SiPMs. However, when LuAP:Ce and LaBr₃:Ce are coupled to the more advanced SiPM prototype, the pulse duration improves to less than 60 ns, which is in the same order of magnitude as pulses from CdTe and CZT detectors. It follows that sufficient rate capability can be achieved with pixel sizes of 400 μm or smaller. Moreover, LaBr₃:Ce detectors can provide an energy resolution of 11.5%-13.5% at 60 keV, comparable to CdTe and CZT detectors. Conclusions: This work provides first evidence that it may be feasible to develop SiPM-based scintillation detectors for PCCT that can compete with CdTe and CZT detectors in terms of energy resolution and rate capability.

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Positron annihilation lifetime spectroscopy (PALS) and Doppler broadening positron annihilation spectroscopy (DB-PAS) depth profiling demonstrate pronounced growth of vacancy clusters at the grain boundaries of as-deposited Al-doped ZnO films deposited as transparent conductive oxide (TCO) on Cu(In,Ga)Se2 (CIGS) solar cells upon accelerated degradation at 85∘C/85% relative humidity. Quantitative fractions of positrons trapped either in the vacancy clusters at the grain boundaries or in Zn monovacancies inside the grains of ZnO:Al were obtained by detailed analysis of the PALS data using a positron trapping model. The time and depth dependence of the positron Doppler depth profiles can be accurately described using a planar diffusion model, with an extracted diffusion coefficient of 35nm2/hour characteristic for in-diffusion of molecules such as H2O and CO2 into ZnO:Al TCO films via the grain boundaries, where they react with the ZnO:Al. This leads to increased open volume at the grain boundaries that imposes additional transport barriers and may lead to charge carrier trapping and nonradiative recombination. Simultaneously, a pronounced increase in series resistance and a strong reduction in efficiency of the ZnO:Al capped CIGS solar cells is observed on a remarkably similar timescale. This strongly indicates that these atomic-scale processes of molecular in-diffusion and creation of open volume at the grain boundaries play a key role in the degradation of the solar cells.@en