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.

The refinement of high-performing scintillator compounds is critical to the advancement of time-of-flight positron emission tomography (TOF-PET). Here, we characterize 2nd, 4th, and 5th generation cerium-doped, calcium co-doped lutetium–yttrium oxyorthosilicate (LYSO) crystals developed and supplied by Luxium Solutions in terms of light yield, emission spectra, non-proportionality, and decay time. Additionally, the samples are coupled to a Philips 3200 digital photon counter (DPC) for a comparison of performance as a detector component. The detector sensitivity, energy resolution, and coincidence resolving time (CRT) of the crystals are measured at −25 °C. The 4th and 5th generation crystals are shown to have similar light yields and decay times at room temperature, and outperform the 2nd generation crystals in all measured characteristics. The detectors making use of 5th generation crystals had markedly better sensitivity, energy resolution, and CRT values than their 4th generation counterparts.

Nuclear energy emerges as a promising and environmentally friendly solution to counter the escalating levels of greenhouse gases resulting from excessive fossil fuel usage. Essential to harnessing this energy are nuclear batteries, devices designed to generate electric power by capturing the energy emitted during nuclear decay, including α or β particles and γ radiation. The allure of nuclear batteries lies in their potential for extended lifespan, high energy density, and adaptability in harsh environments where refueling or battery replacement may not be feasible. In this review, we narrow our focus to nuclear batteries utilizing non-thermal converters such as α- or β-voltaics, as well as those employing scintillation intermediates. Recent advancements in state-of-the-art direct radiation detectors and scintillators based on metal perovskite halides (MPHs) and chalcogenides (MCs) are compared to traditional detectors based on silicon and III-V materials, and scintillators based on inorganic lanthanide crystals. Notable achievements in MPH and MC detectors and scintillators, such as nano-Gy sensitivity, 100 photons/keV light yield, and radiation hardness, are highlighted. Additionally, limitations including energy conversion efficiency, power density, and shelf-life due to radiation damage in detectors and scintillators are discussed. Leveraging novel MPH and MC materials has the potential to propel nuclear batteries from their current size and power limitations to miniaturization, heightened efficiency, and increased power density. Furthermore, exploring niche applications for nuclear batteries beyond wireless sensors, low-power electronics, oil well monitoring, and medical fields presents enticing opportunities for future research and development.

X-ray photon-counting detectors (PCDs) are a rapidly developing technology used in medical imaging. Current PCDs are based on room-temperature semiconductors, such as CdTe and CZT, directly converting incident X-ray photons into electrical pulses. An alternative to this approach is the use of ultrafast scintillators in combination with silicon photomultipliers. A very interesting class of materials potentially suitable for this application is scintillators exhibiting core−valence luminescence (CVL), which typically has a decay time between 0.5 and 2 ns. In this work, two families of Cs−Cl-based compounds, Cs−Zn−Cl and Cs− Mg−Cl, are investigated for their potential application in PCDs. These families of compounds are especially interesting because most members exclusively show CVL at room temperature, resulting in a fast scintillation pulse containing no slow components. Additionally, several approaches to tailor the scintillation properties of these materials, i.e., doping with Br− and Zn²⁺, are studied. Unfortunately, all compounds show a strong drop in the CVL response in the diagnostic energy range (25−150 keV), the operational range of a PCD. PCDs based on these materials will thus be able to handle the high X-ray fluence rate of an imaging task but will not be able to sufficiently discriminate the energies of incident X-ray photons. In addition to the Cs−Zn−Cl and Cs−Mg− Cl compounds, the nonproportional response of the CVL component of BaF₂ is studied utilizing fast digitization of individual scintillation pulses in order to discriminate between processes related to the CVL and self-trapped exciton emission of BaF₂.

Direct conversion photon counting detectors (PCDs) using CdTe, CZT, or Si for the sensor material are being investigated and manufactured. Indirect conversion, scintillator-based PCDs have historically thought to be too slow for the high flux requirements of diagnostic CT. Recent scintillators investigated for e.g. PET applications are very fast and inspire us to rethink this paradigm. We evaluate the potential of a LaBr3:Ce PCD using Monte Carlo simulations. We compared a CdTe PCD and a LaBr3:Ce PCD, assuming a pixel density of 9 pixels/mm2 in each case and a surrounding 2D anti-scatter grid. A 1x1 mm2 area was illuminated by flat field X-rays and the lower bound on the noise for varying contrast types and material decomposition scenarios was calculated. For conventional imaging without material decomposition, the LaBr3:Ce PCD performed worse than CdTe because of the need to wrap pixels in reflector, which reduces geometric efficiency. For water-bone material decomposition, the two PCDs performed similarly with our assumptions on pulse shape and PCD geometry. For three-material decomposition with a K-edge imaging agent, LaBr3:Ce reduced variance by about 35% because of the elimination of charge sharing that is present in CdTe. These results motivate further exploration of scintillator-based PCDs as an alternative to direct conversion PCDs, especially with future K-edge imaging agents.

Background
Photon counting detectors (PCDs) for x-ray computed tomography (CT) are the future of CT imaging. At present, semiconductor-based PCDs such as cadmium telluride (CdTe), cadmium zinc telluride, and silicon have been either used or investigated for clinical PCD CT. Unfortunately, all of them have the same major challenges, namely high cost and limited spectral signal-to-noise ratio (SNR). Recent studies showed that some high-quality scintillators, such as lanthanum bromide doped with cerium (LaBr3:Ce), are less expensive and almost as fast as CdTe.

Purpose
The objective of this study is to assess the performance of a LaBr3:Ce PCD for clinical x-ray CT.

Methods
We performed Monte Carlo simulations and compared the performance of 3 mm thick LaBr3:Ce and 2 mm thick CdTe for PCD CT with x-rays at 120 kVp and 20–1000 mA. The two PCDs were operated with either a threshold–subtract (TS) counting scheme or a direct energy binning (DB) counting scheme. The performance was assessed in terms of the accuracy of registered spectra, counting capability, and count-rate-dependent spectral imaging-task performance, for conventional CT imaging, water–bone material decomposition, and K-edge imaging with tungsten as the K-edge material. The performance for these imaging-tasks was quantified by nCRLB, that is, the Cramér–Rao lower bound on the variance of basis line-integral estimation, normalized by the corresponding value of CdTe at 20 mA.

Results
The spectrum recorded by CdTe was distorted significantly due to charge sharing, whereas the spectra recorded by LaBr3:Ce better matched the incident spectrum. The dead time, estimated by fitting a paralyzable detector model to the count-rate curves, was 20.7, 15.0, 37.2, and 13.0 ns for CdTe with TS, CdTe with DB, LaBr3:Ce with TS, and LaBr3:Ce with DB, respectively. Conventional CT imaging showed an adverse effect of reduced geometrical efficiency due to optical reflectors in LaBr3:Ce PCD. The nCRLBs (a lower value indicates a better SNR) for CdTe with TS, CdTe with DB, LaBr3:Ce with TS, LaBr3:Ce with DB, and the ideal PCD, were 1.00 ± 0.01, 1.00 ± 0.01, 1.18 ± 0.02, 1.18 ± 0.02, and 0.79 ± 0.01, respectively, at 20 mA. The nCRLBs for water–bone material decomposition, in the same order, were 1.00 ± 0.02, 1.00 ± 0.02, 0.85 ± 0.02, 0.85 ± 0.02, and 0.24 ± 0.02, respectively, at 20 mA; and 0.98 ± 0.02, 0.98 ± 0.02, 1.09 ± 0.02, 0.83 ± 0.02, and 0.24 ± 0.02, respectively, at 1000 mA. Finally, the nCRLBs for K-edge imaging, the most demanding task among the five, were 1.00 ± 0.02, 1.00 ± 0.02, 0.55 ± 0.02, 0.55 ± 0.02, and 0.13 ± 0.02, respectively, at 20 mA; and 2.45 ± 0.02, 2.29 ± 0.02, 3.12 ± 0.02, 2.11 ± 0.02, and 0.13 ± 0.02, respectively, at 1,000 mA.

Conclusion
The Monte Carlo simulations showed that, compared to CdTe with either TS or DB, LaBr3:Ce with DB provided more accurate spectra, comparable or better counting capability, and superior spectral imaging-task performances, that is, water–bone material decomposition and K-edge imaging. CdTe had a better performance than LaBr3:Ce for the conventional CT imaging task due to its higher geometrical efficiency. LaBr3:Ce PCD with DB scheme may be an excellent alternative option for CdTe PCD.

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.

Zero-dimensional (0D) organic metal halide hybrids (OMHHs) have recently emerged as a promising class of scintillation materials, offering advantages in performance, tunability, environmental friendliness, and cost-effectiveness. While numerous 0D OMHH scintillators have been developed to date, most of them rely on radioluminescence (RL) originating from metal halide species with long decay lifetimes and are typically prepared as small-sized single crystals via a slow solution growth process. Here, we report for the first time high-performance X-ray scintillators based on facile, solution-processed amorphous 0D OMHH films that exhibit efficient RL with short decay lifetimes enabled by molecular sensitization. By reacting a rationally designed green-emitting organic halide, (4-(9,9-dimethylacridin-10(9H)-yl)phenyl)triphenylphosphonium bromide (DMAC-TPPBr), with zinc bromide (ZnBr2), we have obtained amorphous 0D (DMAC-TPP)2ZnBr4 films that exhibit outstanding optical and scintillation properties, with a high photoluminescence quantum yield of ∼85%, a short photoluminescence decay lifetime (∼12.5 ns), a high absolute light yield of ∼27,000 photons MeV−1, a radioluminescence decay lifetime of ∼8.00 ns, and a low limit of detection (LOD) of ∼9.00 nGyair s−1. By leveraging the benefits of solution-processed amorphous OMHHs with molecular sensitization, our approach paves a new pathway toward scalable, low-cost, and fast-response X-ray scintillators.

We investigate silicon photomultiplier (SiPM)-based scintillation detectors for medical X-ray photon-counting applications, where the input count rate (ICR) can reach a few Mcps/mm2 in cone-beam CT for radiotherapy, for example, up to a few hundred Mcps/mm2 in diagnostic CT. Thus, pulse pile-up can severely distort the measurement of counts and energies. Here, we experimentally evaluate the counting and spectral performance of SiPM-based scintillation detectors at 60 keV as a function of ICR/pile-up level. We coupled 0.9×0.9×3.5 mm3 LYSO:Ce and 0.9×0.9×4.5 mm3 YAP:Ce scintillators to 1.0×1.0 mm2 ultrafast SiPMs and exposed these single-pixel detectors to a 10-GBq Am-241 source. We varied ICR from 0 to 5 Mcps/pixel and studied detector performance for paralyzable-like (p-like) and nonparalyzable-like (np-like) counting algorithms, after applying a second-order low-pass filter with cut-off frequencies fc of 5, 10, or 20 MHz to the pulse trains. Counting performance was quantified by the output count rate (OCR) and the count-rate loss factor (CRLF). In addition to the traditional spectral performance measure of the fullwidth- at-half-maximum (FWHM) energy resolution at low ICR, we propose the spectral degradation factor (SDF) to quantify spectral effects of pile-up at any ICR. Best counting performance is obtained with np-like counting and fc = 20 MHz, for which the count-rate loss is at most 10% in the investigated range of ICRs, whereas p-like counting yields best spectral performance. Due to less pile-up, the fastest pulses obtained with fc = 20 MHz already provide the best SDF values at ICRs of a few Mcps/pixel, despite their worse low-rate energy resolution. Hence, spectral performance under pile-up conditions appears to benefit more from substantially faster pulses than a somewhat better low-rate energy resolution. Moreover, we show that the pulse shape of SiPM-based detectors allows to improve spectral performance under pile-up conditions using dedicated peak detection windows.

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.

Objective. Bragg peak measurements play a key role in the beam quality assurance in proton therapy. Used as base data for the treatment planning softwares, the accuracy of the data is crucial when defining the range of the protons in the patient. Approach. In this paper a protocol to reconstruct a Pristine Bragg Peak exploring the direct correlation between the particle flux and the dose deposited by particles is presented. Proton flux measurements at the HollandPTC and FLUKA Monte Carlo simulations are used for this purpose. This new protocol is applicable to plastic scintillator detectors developed for Quality Assurance applications. In order to obtain the Bragg curve using a plastic fiber detector, a PMMA phantom with a decoupled and moveable stepper was designed. The step phantom allows to change the depth of material in front of the fiber detector during irradiations. The Pristine Bragg Peak reconstruction protocol uses the measured flux of particles at each position and multiplies it by the average dose obtained from the Monte Carlo simulation at each position. Main results. The results show that with this protocol it is possible to reconstruct the Bragg Peak with an accuracy of about 470 µm, which is in accordance with the tolerances set by the AAPM. Significance. It has the advantage to be able to overcome the quenching problem of scintillators in the high ionization density region of the Bragg peak.

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).

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.

Embedding signal processing in the front-end of radiation detectors represents an approach to cope with the growing complexity of nuclear imaging scanners with increasing field of view (i.e., higher number of channels). Machine learning (ML) offers a good compromise between intrinsic image reconstruction performance and computational power. While most hardware accelerators for ML are based on digital circuits and, thus, require the analog-to-digital conversion of all individual signals from photodetectors, an analog approach allows to streamline the pipeline. We present the study of an analog accelerator implementing a neural network (NN) with 42 neurons in a 0.35- μ m CMOS process node. The specific target is the reconstruction of the position of interaction of gamma-rays in the scintillator crystal of Anger cameras used for PET and SPECT. This chip can be used stand-alone or monolithically integrated within the application specific integrated circuit (ASIC) for the filtering of current signals from arrays of silicon photomultipliers (SiPMs). Computation is performed in charge domain by means of crossbar arrays of programmable capacitor. The architecture of the 64-input ASIC and the training of the NN are presented, discussing the impact of weight quantization on 5 bits. From MATLAB and circuit simulations, consistent with ASIC topology and operations, the NN capabilities were tested using two different datasets, obtained from both simulated data and experimental data, both based on PET detector composed by a monolithic scintillator crystal readout by an 8×8 array of SiPMs. Simulations show an achievable spatial resolution better than 2-mm full-width-at-half-maximum with a 10-mm thick crystal, a max. count rate of 200kHz and the energy efficiency per inference is estimated to be of 93.5GOP/J, i.e., competitive with digital counterparts, with an energy consumption of 38nJ per inference and area of 23mm2.

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.

Objective. In radiotherapy, the internal movement of organs between treatment sessions causes errors in the final radiation dose delivery. To assess the need for adaptation, motion models can be used to simulate dominant motion patterns and assess anatomical robustness before delivery. Traditionally, such models are based on principal component analysis (PCA) and are either patient-specific (requiring several scans per patient) or population-based, applying the same set of deformations to all patients. We present a hybrid approach which, based on population data, allows to predict patient-specific inter-fraction variations for an individual patient. Approach. We propose a deep learning probabilistic framework that generates deformation vector fields warping a patient's planning computed tomography (CT) into possible patient-specific anatomies. This daily anatomy model (DAM) uses few random variables capturing groups of correlated movements. Given a new planning CT, DAM estimates the joint distribution over the variables, with each sample from the distribution corresponding to a different deformation. We train our model using dataset of 312 CT pairs with prostate, bladder, and rectum delineations from 38 prostate cancer patients. For 2 additional patients (22 CTs), we compute the contour overlap between real and generated images, and compare the sampled and ‘ground truth’ distributions of volume and center of mass changes. Results. With a DICE score of 0.86 ± 0.05 and a distance between prostate contours of 1.09 ± 0.93 mm, DAM matches and improves upon previously published PCA-based models, using as few as 8 latent variables. The overlap between distributions further indicates that DAM’s sampled movements match the range and frequency of clinically observed daily changes on repeat CTs. Significance. Conditioned only on planning CT values and organ contours of a new patient without any pre-processing, DAM can accurately deformations seen during following treatment sessions, enabling anatomically robust treatment planning and robustness evaluation against inter-fraction anatomical changes.

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.

Background: Accuracy and precision assessment in radiomic features is important for the determination of their potential to characterize cancer lesions. In this regard, simulation of different imaging conditions using specialized phantoms is increasingly being investigated. In this study, the design and evaluation of a modular multimodality imaging phantom to simulate heterogeneous uptake and enhancement patterns for radiomics quantification in hybrid imaging is presented. Methods: A modular multimodality imaging phantom was constructed that could simulate different patterns of heterogeneous uptake and enhancement patterns in positron emission tomography (PET), single-photon emission computed tomography (SPECT), computed tomography (CT), and magnetic resonance (MR) imaging. The phantom was designed to be used as an insert in the standard NEMA-NU2 IEC body phantom casing. The entire phantom insert is composed of three segments, each containing three separately fillable compartments. The fillable compartments between segments had different sizes in order to simulate heterogeneous patterns at different spatial scales. The compartments were separately filled with different ratios of 99mTc-pertechnetate, 18F-fluorodeoxyglucose ([18F]FDG), iodine- and gadolinium-based contrast agents for SPECT, PET, CT, and T1-weighted MR imaging respectively. Image acquisition was performed using standard oncological protocols on all modalities and repeated five times for repeatability assessment. A total of 93 radiomic features were calculated. Variability was assessed by determining the coefficient of quartile variation (CQV) of the features. Comparison of feature repeatability at different modalities and spatial scales was performed using Kruskal-Wallis-, Mann-Whitney U-, one-way ANOVA- and independent t-tests. Results: Heterogeneous uptake and enhancement could be simulated on all four imaging modalities. Radiomic features in SPECT were significantly less stable than in all other modalities. Features in PET were significantly less stable than in MR and CT. A total of 20 features, particularly in the gray-level co-occurrence matrix (GLCM) and gray-level run-length matrix (GLRLM) class, were found to be relatively stable in all four modalities for all three spatial scales of heterogeneous patterns (with CQV < 10%). Conclusion: The phantom was suitable for simulating heterogeneous uptake and enhancement patterns in [18F]FDG-PET, 99mTc-SPECT, CT, and T1-weighted MR images. The results of this work indicate that the phantom might be useful for the further development and optimization of imaging protocols for radiomic quantification in hybrid imaging modalities.

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.

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.