Objective. We investigate scintillation detectors with silicon photomultipliers (SiPMs) as alternatives to direct-conversion detectors based on CdTe/Cd1−xZnxTe (CZT) for x-ray photon-counting imaging. Here, we measure counting and spectral performance of three scintillators and compare the results with performances reported in literature for CdTe/CZT detectors for diagnostic photon-counting computed tomography (PCCT). Approach. We built 1 × 1 mm2 single-pixel detectors by coupling readily available LYSO:Ce, YAP:Ce, and LaBr3:Ce scintillators to ultrafast SiPMs. Pulse processing was optimized for rate capability rather than energy resolution. We exposed the detectors to three radioisotopes to determine energy response proportionality and energy resolution. Using an x-ray tube, we measured x-ray spectra and count rate curves, i.e. output count rate (OCR) versus input count rate (ICR). Main results. The energy resolutions of the LYSO:Ce and YAP:Ce detectors exceed 30% full-width-at-half-maximum (FWHM) at 60 keV, with YAP:Ce showing a more proportional response. For a 30 keV count-detection threshold, the maximum OCR of the YAP:Ce detector is 5.4 Mcps pixel−1 for paralyzable-like counting, while the OCR approaches 12.5 Mcps pixel−1 for nonparalyzable-like counting. The LYSO:Ce detector reaches 4.5 Mcps pixel−1 and 10 Mcps pixel−1, respectively, and the LaBr3:Ce detector 10.4 Mcps pixel−1 and 22 Mcps pixel−1. Thereby, the rate capability of the LaBr3:Ce detector is almost 80% of that reported for two CdTe/CZT detectors for diagnostic PCCT. Moreover, the LaBr3:Ce detector has high proportionality and an energy resolution of about 20% FWHM at 60 keV, which is comparable to at least one CdTe detector for diagnostic PCCT. The x-ray tube spectra measured using the scintillation detectors show reasonable agreement with incident spectra. Significance. This work indicates that LaBr3:Ce-based detectors may become an alternative to direct-conversion detectors for diagnostic PCCT, whereas LYSO:Ce- and YAP:Ce-based detectors appear better suited for applications with lower ICR, e.g. cone-beam PCCT in radiotherapy. Ways to further improve x-ray photon-counting scintillation detectors are also discussed.

Proton radiotherapy is a cancer therapy that uses ionising radiation in the form of protons, offering an alternative and complement to conventional radiotherapy with high energy X-rays. In comparison to X-rays, proton beams have a finite range in tissue and deposit dose more locally. As a consequence, proton radiotherapy has the potential to deliver a higher dose to the tumour while better sparing surrounding healthy tissue. However, the delivered dose distribution in the case of protons is much more sensitive to changes in patient anatomy compared to X-rays, which therefore requires an accurate knowledge of proton stopping power ratio (SPR) values of the tissues to be treated.....

Objective. Cone-beam computed tomography (CBCT) is used for patient positioning in proton therapy, but not directly for treatment planning due to its inferior image quality compared to fan-beam CT. One way to improve its value for proton radiotherapy might be to use CBCT setups capable of extracting spectral information, which can be realised through several hardware configurations. Here, we compare different setups w.r.t. to their capability of predicting proton stopping power ratios (SPRs). Approach. We investigate six different spectral CBCT realisations in a simulation study, namely a single-source setup with either a dual-layer detector or a photon-counting detector (PCD), a kVp-switching setup with either an energy-integrating detector (EID) or a PCD, and a dual-source setup with either EIDs or PCDs. Our figure of merit is the normalised Cramér–Rao Lower Bound (nCRLB) on SPR variance based on projection data. We take (cross)scatter into account, and compare ideal and realistic detector models to help guide future detector developments. Each setup is optimised w.r.t. source spectra, mAs ratios and energy bin settings (where applicable). Main results. Assuming a realistic detector response, setups with a kVp-switching source perform best, with the setup paired with an EID slightly outperforming the PCD-based setup (nCRLBs of 2.74 and 2.81, respectively). However, if the mAs ratio of the kVp-switching source is fixed, the performance of the kVp-switching setup with an EID is significantly degraded (nCRLB = 9.46) and outperformed by PCD-based setups, with nCRLBs of 3.27, 3.45 and 3.60 for the dual-source setup with two PCDs, the single-source setup and the kVp-switching setup with one PCD, respectively. Spectra with higher source voltage or wider spectral separation generally yield lower CRLB values, and avoiding the spectral distortion caused by charge sharing in direct-conversion PCDs promises to lower CRLB values by about a third. Significance. We present an extensive comparison of spectral CBCT setups for their application in proton radiotherapy, using a methodology that allows to compare their theoretical limit of performance without being influenced by the choice of reconstruction algorithm or the conversion scheme from Hounsfield units to SPR values.

X-ray photon-counting detectors (PCDs) are a rapidly developing technology. Current PCDs used in medical imaging are based on CdTe, CZT, or Si semiconductor detectors, which directly convert X-ray photons into electrical pulses. An alternative approach is to combine ultrafast scintillators with silicon photomultipliers (SiPMs). Here, an overview is presented of different classes of scintillators, with the aim of assessing their potential application in scintillator-SiPM based indirect X-ray PCDs. To this end, three figures of merit (FOMs) are defined: the pulse intensity, the pulse duration, and the pulse quality. These FOMs quantify how characteristics such as light yield, pulse shape, and energy resolution affect the suitability of scintillators for application in indirect PCDs. These FOMs are based on emissive characteristics; a fourth FOM (ρZ_eff^3.5) is used to also take stopping power into account. Other important properties for the selection process include low self-absorption, low after-glow, possibility to produce sub-mm pitch pixel arrays, and cost-effectiveness. It is shown that material classes with promising emission properties are Ce³⁺- or Pr³⁺-doped materials, near band gap exciton emitters, plastics, and core-valence materials. Possible shortcomings of each of these groups, e.g., suboptimal emission wavelength, nonproportionality, and density, are discussed. Additionally, the engineering approach of quenching the scintillator emission, resulting in a targeted shortening of the decay time, and the possibility of codoping are explored. When selecting and/or engineering a material, it is important to consider not only the characteristics of the scintillator but also relevant SiPM properties, such as recharge time and photodetection efficiency.

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.

Cone-beam computed tomography (CBCT) and X-ray projection radiography are commonly used in the proton therapy workflow for the verification of patient positioning. The prospect of using the CBCT images for dose calculation purposes is attractive but currently hampered by the poorer image quality compared to the planning (fan-beam) CT. Ideally, the CBCT scan with the patient's anatomy of the day would provide sufficiently accurate proton stopping power ratios (SPR) to directly replan the treatment if needed. Dual-energy fan-beam CT has been proven to increase the accuracy of calculated SPR values compared to single-energy CT. A similar outcome may therefore be expected for dual-energy/spectral CBCT. This work aims to compare two possible realizations of dual-energy CBCT, namely a rapid kVp-switching source CBCT and a photon-counting detector (PCD) CBCT with two energy bins, with respect to their suitability for extracting SPR values. To perform this comparison, we determine the Cramér-Rao Lower Bound on the variance of the estimated electron density and effective atomic number. In our simulation study, we find that for the rapid kVp-switching setup the optimum voltage pair is 80/140 kVp, and the optimum ratio of the source current at 80 kVp to the source current at 140 kVp is 2:1 (4:1) for extracting the electron density (effective atomic number). In case of the PCD-based setup, a 140 kVp (100 kVp) spectrum and energy bins of [20; 50), [50; 150) keV appear best suited for extracting electron density (effective atomic number), outperforming the kVp-switching setup by a factor of 3.8 (4.9).

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.

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.