We introduce a novel Monte Carlo approach for simulating charge propagation in semiconductor detectors, accounting for Coulomb repulsion and diffusion over time.

Semiconductor detectors for high-energy sensing (X/γ-rays) play a critical role in fields such as astronomy, particle physics, spectroscopy, medical imaging, and homeland security. The increasing need for precise detector characterization highlights the importance of developing advanced digital twins, which help optimize the design and performance of imaging systems. Current simulation frameworks primarily focus on modeling electron–hole pair dynamics within the semiconductor bulk after the photon absorption, leading to the current signals at the nearby electrodes. However, most simulations neglect charge diffusion and Coulomb repulsion, which spatially expand the charge cloud during propagation due to the high complexity they add to the physical models. Although these effects are relatively weak, their inclusion is essential for achieving a high-fidelity replication of real detector behavior. There are some existing methods that successfully incorporate these two phenomena with minimal computational cost, including those developed by Gatti in 1987 and by Benoit and Hamel in 2009. The present work evaluates these two approaches and proposes a novel Monte Carlo technique that offers higher accuracy in exchange for increased computational time. Our new method enables more realistic performance predictions while remaining within practical computational limits.

Photon-counting detectors based on CZT are essential in nuclear medical imaging, particularly for SPECT applications. Although CZT detectors are known for their precise energy resolution, defects within the CZT crystals significantly impact their performance. These defects result in inhomogeneous material properties throughout the bulk of the detector. The present work introduces an efficient computational model that simulates the operation of semiconductor detectors, accounting for the spatial variability of the crystal properties. Our simulator reproduces the charge-induced pulse signals generated after the X/gamma-rays interact with the detector. The performance evaluation of the model shows an RMSE in the signal below 0.70 function as a digital twin to accurately replicate the operation of actual detectors. Thus, it can be used to mitigate and compensate for adverse effects arising from crystal impurities.

CdZnTe-based detectors are highly valued because of their high spectral resolution, which is an essential feature for nuclear medical imaging. However, this resolution is compromised when there are substantial defects in the CdZnTe crystals. In this study, we present a learning-based approach to determine the spatially dependent bulk properties and defects in semiconductor detectors. This characterization allows us to mitigate and compensate for the undesired effects caused by crystal impurities. We tested our model with computer-generated noise-free input data, where it showed excellent accuracy, achieving an average RMSE of 0.43% between the predicted and the ground truth crystal properties. In addition, a sensitivity analysis was performed to determine the effect of noisy data on the accuracy of the model.

Iterative reconstruction techniques have been integrated into clinical practice for emission tomography reconstruction. While these techniques have shown promising results in image reconstruction, there remains a lack of clarity regarding which parts of the image are reliably estimated and which are not. Currently, there is a missing measure to differentiate between epistemic and aleatoric errors. One approach to address this issue is to estimate the null space of the imaging model for a given object. In this study, we utilize a previously proposed adaptation of the Landweber iteration, known as the Wilson-Barret method, for calculating the null space of a noiseless reference phantom and for a corresponding reconstruction of a simulated phantom. Using the calculated null spaces we can apply a method for creating hallucination maps, that allow derivation of the epistemic error. Our findings suggest that the hallucinated components of the image are primarily attributable to the edges or high-frequency components of the phantom as well as the center of the phantom, which is consistent with expectations.

Background Absolute quantitative myocardial perfusion SPECT requires addressing of aleatory and epistemic uncertainties in conjunction with providing image quality sufficient for lesion detection and characterization. Iterative reconstruction methods enable the mitigation of the root causes of image degradation. This study aimed to determine the feasibility of a new SPECT/CT method with integrated corrections attempting to enable absolute quantitative cardiac imaging (xSPECT Cardiac; xSC). Methods We compared images of prototype xSC and conventional SPECT (Flash3D TM ) acquired at rest from 56 patients aged 71 ± 12 y with suspected coronary heart disease. The xSC prototype comprised list-mode acquisitions with continuous rotation and subsequent iterative reconstructions with retrospective electrocardiography (ECG) gating. Besides accurate image formation modeling, patient-specific CT-based attenuation and energy window-based scatter correction, additionally we applied mitigation for patient and organ motion between views ( inter-view ), and within views ( intra-view) for both the gated and ungated reconstruction. We then assessed image quality, semiquantitative regional values, and left ventricular function in the images. Results The quality of all xSC images was acceptable for clinical purposes. A polar map showed more uniform distribution for xSC compared with Flash3D, while lower apical count and higher defect contrast of myocardial infarction (p = 0.0004) were observed on xSC images. Wall motion, 16-gate volume curve, and ejection fraction were at least acceptable, with indication of improvements. The clinical prospectively gated method rejected beats ≥20% in 6 patients, whereas retrospective gating used an average of 98% beats, excluding 2% of beats. We used the list-mode data to create a product equivalent prospectively gated dataset. The dataset showed that the xSC method generated 18% higher count data and images with less noise, with comparable functional variables of volume and LVEF (p = ns). Conclusions Quantitative myocardial perfusion imaging with the list-mode-based prototype xSPECT Cardiac is feasible, resulting in images of at least acceptable image quality.

A digital brain phantom was created from 3D CAD images and imported to a simulation of a Siemens Healthineers Symbia Evo SPECT system. The simulation was performed using GATE, a Geant-4 based Monte Carlo toolkit, which included features to import 3D CAD structures. The brain object was generated using an MRI study of a human brain and imported to GATE as a tessellated mesh filled with radiation infused water that had a radiation spectrum matching Tc-99m and spatial distribution that filled the brain volume. The simulation was configured for a dual head SPECT with a LEHR parallel collimator and NaI scintillation detectors on a brain phantom filled with water. The resulting projection.

We report the reconstructed resolution (RR) achieved by a new high-resolution multifocal collimator (SZHRX) and respective quantitative iterative reconstruction with distant dependent collimator modeling, attenuation, and scatter correction. SZHRX is primarily designed for cardiology and neurology SPECT applications using Tc99m perfusion imaging (cold-lesion detection task) and I123 DaTscan (hot-lesion detection task). It has a suitable magnification volume and designed to increase effective sensitivity >2x at 28 cm organ centered orbit, with a reconstructed resolution similar to a close non-circular orbit low-energy high-resolution (LEHR) collimator acquisition. RR is measured as the edge-response of an extended source with adjustable contrast in a warm background. The results show that the design goals were achieved: a better RR for cold spheres compared to LEHR regardless of the distances probed, and RR for hot spheres is in between the values for LEHR at 28 cm and NCO. We only show Tc99m results here.

1224 Objectives: We designed a new multifocal “SMARTZoom” class collimator [1, 2] with a suitable High Resolution and eXtended magnification imaging volume for cardiac applications at nominal radius of rotation of 28cm (“HRSX”). The HRSX collimator is a new design with smaller holes, and higher order polynomial design equation. The target resolution is set to achieve at 28cm similar resolution as the LEHR at close orbit. In the cardiology application the cardio-centric orbit is similar to current SMARTzoom acquisitions [3, 4] but yields somewhat better resolution. The sensitivity improvement at that distance and for the larger volume is in practice expected to be >2.5x over LEHR. In this abstract, we seek to demonstrate on phantoms and a clinical data set that the design goals are achieved. Methods: We use the Data Spectrum static cardiac phantom, male with normal heart insert as test object and compare the images to LEHR acquired, close orbit scan. The reconstruction is absolute quantitative for all multi-channel collimators using the xSPECT Quant approach for Tc-99m. The comparison is based on internal quantitative assessment of the phantom data and visual assessment of the clinical data set. The phantom was acquired using a 360 degree arc. The LEHR clinical acquisition was done over 32 views per detector at 5 s/view (90° rotation, 180° arc), while the HRSX acquisition was done over 25 views per detector at 1.5 s/view (104° rotation, 208° arc). Results: Image assessment shows that the design goal has been achieved. The Data Spectrum static cardiac phantom shows that the reconstructed image of the HRSX with a 28 cm orbit has similar signature to the LEHR with NCO orbit, Fig 1. This is supported by an internal tooling for automatic assessment of the heart insert variation, which shows a difference between insert widths smaller (0.95) than what we usually observe in different LEHR acquisition, details shown in Fig 2. The clinical HRSX detected 6.5 m counts in comparison to the 2.1 m from the LEHR acquisition. For comparison purposes, we subsample the LEHR acquisition to 23% to match HRSX acquisition time, and 46% to double it. The reconstruction shows the HRSX to be as good as the 46% LEHR, with comparable noise structure and resolution, clearly seen on the right ventricle, Fig 3. Conclusions: The new multifocal collimator “HRSX” achieves the design goal for use in quantitative SPECT application of nuclear cardiology. It shows a resolution similar to the LEHR, but at a sensitivity > 2.5x on average for Tc-99m. Literature: 1. Zeintl, J., et al. First Experience with SMARTZOOM Collimation in Clinical Cardiac SPECT. in Annual Congress of the European Association of Nuclear Medicine. 2009. Barcelona, Spain.2. Zeintl, J., et al., Performance Characteristics of the SMARTZOOM® Collimator. IEEE Medical Imaging Conference Record, 2011.3. Vija, A.H., et al. A method for improving the efficiency of myocardial perfusion imaging using conventional SPECT and SPECT/CT imaging systems. in Nuclear Science Symposium Conference Record (NSS/MIC), 2010 IEEE. 2010.4. Yoneyama, H., et al., Validation of Left Ventricular Ejection Fraction with the IQ*SPECT System in Small-Heart Patients. J Nucl Med Technol, 2017. 45(3): p. 201-207.