In time-of-flight positron emission tomography (TOF-PET), a coincidence time resolution (CTR) below 100 ps reduces the angular coverage requirements and, thus, the geometric constraints of the scanner design. Among other possibilities, this opens the possibility of using flat-panel PET detectors. Such a design would be more cost-accessible and compact and allow for a higher degree of modularity than a conventional ring scanner. However, achieving adequate CTR is a considerable challenge and requires improvements at every level of detection. Based on recent results in the ongoing development of optimised TOF-PET photodetectors and electronics, we expect that within a few years, a CTR of about 75 ps will be be achievable at the system level. In this work, flat-panel scanners with four panels and various design parameters were simulated, assessed and compared to a reference scanner based on the Siemens Biograph Vision using NEMA NU 2-2018 metrics. Point sources were also simulated, and a method for evaluating spatial resolution that is more appropriate for flat-panel geometry is presented. We also studied the effects of crystal readout strategies, comparing single-crystal and module readout levels. The results demonstrate that with a CTR below 100 ps, a flat-panel scanner can achieve image quality comparable to that of a reference clinical scanner, with considerable savings in scintillator material.

The Digitizer Unit plays an important role in modeling using Geant4 Application for Tomographic Emission (GATE), a Geant4-based platform used for numerical simulations in medical imaging and radiotherapy. It simulates the response of the photodetection components using a sequence of analytical and semi-analytical models. The Digitizer Unit was written for the first version of GATE approximately 20 years ago. Since then, it has in parts grown in a code that can be hardly maintained. Some parts of the code were unused or duplicated; some of the functionalities were not working anymore. Therefore, the GATE Digitizer Unit update is required in order to incorporate the novelties of Geant4 to update its current version and add new features. In this article, the implementation of the new GATE Digitizer Unit (since version 9.3) is presented. Added functionalities, the impact of changes on users, the current status of the work, and perspectives are discussed.

In positron emission tomography (PET), achieving high resolution and accurate image reconstruction is crucial for effective diagnosis and research applications. This study addresses the challenges posed by the parallel flat panel architecture of a novel high-resolution time-of-flight (TOF) PET scanner. Despite its cost and size benefits, this architecture introduces significant anisotropic spread in point source distribution, particularly in the direction orthogonal to the panels. Furthermore, objects closer to one of the panels may exhibit a splitting into two components. To tackle these issues, we propose a methodology to estimate and apply a continuously spatially variant point spread function (PSF). Our approach utilizes Gaussian Mixture Models (GMM) to fit the spread of simulated point sources. This fitting process followed by parameters interpolation through Gaussian process regression (GPR), accommodates the anisotropic and spatially varying nature of the spread. We implemented a continuous shift varying convolution that was integrated into a Maximum Likelihood Estimation Maximization (MLEM) algorithm. We simulated a clinically realistic count level to assess the ability of the method to correctly recover the PSF deformations, on a Derenzo resolution phantom and a brain phantom.

Recent developments in PET instrumentation, including advances in silicon photomultipliers and electronics, are fueling the development of novel PET systems improving flexibility and reducing costs. Among the improvements, ultra-high resolution time-of-flight (TOF) enables the use of limited angle geometry without sacrificing image quality. In this work, we present simulation results for a PET system under development, with ultra-high TOF resolution and depth-of-interaction (DOI). The proposed system is arranged for long axial field-of-view scanning using flat detector panels. We evaluate the performance of the system for varying TOF and DOI resolutions while incorporating geometric corrections during reconstruction. Results demonstrate the promising performance of a 2-panel, 70 ps TOF resolution system with 2.5 mm DOI resolution, as evidenced by traditional and task-based image quality metrics.

The detection of annihilation photons in positron emission tomography (PET) is based on scintillation light detection, but an interesting alternative is detection based on Cherenkov photons. Dense Cherenkov radiators provide an opportunity for high gamma detection efficiency-due to their high stopping power and photofraction-and excellent coincidence time resolution (CTR). However, because only a few tens of Cherenkov photons follow a gamma interaction in the radiator, the detection efficiency, and the energy resolution of a pure Cherenkov detector are an issue. This work explores gamma detection efficiency and CTR of PbF2-based detectors with different surface treatments and photodetectors covering one, two, or all crystal faces. Following the detector simulation analysis, we investigate the potential performance of a full-size Cherenkov PET scanner and quantitatively compare image quality with a commercial clinical PET scanner. We demonstrate that even though pure Cherenkov scanners have basically no energy resolution, the scatter fraction of around 50% is not prohibitively large, and images comparable to the state-of-the-art clinical PET scanner can be achieved due to improved efficiency and CTR attainable with PbF2.

The shift towards early diagnosis and prevention in medicine necessitates high-performing PET imaging systems with improved sensitivity and specificity. To achieve this, additional time-of-flight measurements in PET enhance the signal-to-noise ratio and enable the construction of better-performing systems. However, the timing resolution of PET systems is limited by several factors, such as slow scintillation response time, optical photon travel time spread, number of detected scintillation photons, and timing precision of the photo-sensor and fast readout electronics. Although there have been efforts to develop new scintillators for PET, this contribution focuses on optimizing photo sensors and readout electronics. The authors integrated high-performing HD-NUV-MT silicon photomultipliers from FBK with novel low-power FastIC ASICs for fast-timing applications. They constructed a pair of 16-channel gamma detector modules and placed them in a coincidence setup, demonstrating very high coincidence timing resolution below 100 ps FWHM. This achievement enables the construction of high-performing PET detectors with incomplete sampling and simplified systems consisting of flat panel detectors that can be easily combined in larger systems. The authors simulated the performance of an imager comprising two 120 cm × 60 cm panels of segmented 20 mm thick LYSO crystal arrays read by dual-sided readout with fast timing and imaged a human XCAT phantom. They demonstrated that such a system, consisting of much less scintillator material compared to a total body PET imager with a standard opening, exhibits excellent performance. Overall, this approach provides a promising path toward developing highly sensitive and specific PET imaging systems that can aid in early disease diagnosis and prevention.

INTRODUCTION:Potential changes in patient anatomy during proton radiotherapy may lead to a deviation of the delivered dose. A dose estimate can be computed through a deformable image registration (DIR) driven dose accumulation. The present study evaluates the accumulated dose uncertainties in a patient subject to an inadvertent breathing associated motion. MATERIALS AND METHODS:A virtual lung tumour was inserted into a pair of single participant landmark annotated computed tomography images depicting opposite breathing phases, with the deep inspiration breath-hold the planning reference and the exhale the off-reference geometry. A novel Monte Carlo N-Particle, Version 6 (MCNP6) dose engine was developed, validated and used in treatment plan optimization. Three DIR methods were compared and used to transfer the exhale simulated dose to the reference geometry. Dose conformity and homogeneity measures from International Committee on Radioactivity Units and Measurements (ICRU) reports 78 and 83 were evaluated on simulated dose distributions registered with different DIR algorithms. RESULTS:The MCNP6 dose engine handled patient-like geometries in reasonable dose calculation times. All registration methods were able to align image associated landmarks to distances, comparable to voxel sizes. A moderate deterioration of ICRU measures was encountered in comparing doses in on and off-reference anatomy. There were statistically significant DIR driven differences in ICRU measures, particularly a 10% difference in the relative D98% for planning tumour volume and in the 3 mm/3% gamma passing rate. CONCLUSIONS:T he dose accumulation over two anatomies resulted in a DIR driven uncertainty, important in reporting the associated ICRU measures for quality assurance.

In this simulation study, we evaluate the performance of a limited angular coverage PET system consisting of two/four fast-timing 50 ps FWHM CTR flat-panel detectors made of 5–20 mm long pixelated lutetium oxyorthosilicate crystals. We studied image quality and count rates following the National Electrical Manufacturers Association standard, spatial resolution by imaging a Derenzo phantom and a hot rod, and investigated the sensitivity of different scanner designs. We demonstrated the possible use of such a scanner by imaging a human head and a torso of the extended cardiac-torso (XCAT) digital phantom. All the designs were compared to the reference scanner, based on Siemens Biograph Vision PET/CT scanner geometry. We show that good coincidence timing resolution (CTR) can compensate for lower detection efficiency or smaller angular coverage. Good image quality can be obtained with a simple limited-angle PET system without distortions or artefacts. Substantial degradation of the spatial resolution with increased crystal length is observed in the two-panel design due to the parallax error, but not in the four-panel design. The four-panel design simulated with a CTR of 50 ps FWHM is comparable to that of the current state-of-the-art clinical PET/CT scanner. Similar fast-timing limited-angle planar detectors could enable much less expensive total-body or single organ (dynamically selectable) imaging devices.

Positron emission tomography (PET) is one of the most important diagnostic tools in medicine, providing three-dimensional imaging of functional processes in the body. The method is based on detecting two gamma rays originating from the point of annihilation of the positron emitted by a radio-labeled agent and used to follow the human’s physiological processes. In Time-Of-Flight PET, gamma rays’ arrival time is measured in addition to their position. The coincidence timing resolution (CTR) of state-of-the-art scanners is between 200 ps and 500 ps FWHM, which can significantly improve the contrast in imaging large objects. However, increasing the sensitivity of the next-generation PET scanners requires increasing the imaging device’s timing accuracy. Using the latest advances, a multichannel system with improved CTR is becoming technologically possible. Generally, 3D images from limited angle PET scanners are distorted and have artifacts. Fortunately, with improving timing resolution of PET gamma detectors, artifact-free images can be obtained even by a very simplified detector. We were studying a simple panel PET detector consisting of gamma detectors with 50 ps coincidence timing resolution. With this new concept, the price of PET scanners for imaging single or multiple organs can be drastically decreased. We evaluated different panel detector arrangements by imaging different phantoms. The reconstructed images were compared with those obtained with the Siemens Biograph Vision, a state-of-the-art clinical PET scanner. We found comparable image quality parameters of both systems when the CTR approaches 50 ps FWHM and that good CTR can partially compensate for smaller gamma detection efficiency.

In positron emission tomography (PET) time-of-flight (TOF) information is used to improve the reconstructed image. By improving the coincidence timing resolution (CTR), the angular covarege requirements are reduced, allowing for new possibilities such as open geometry and the use of flat panel detectors. The latter come with some benefits, like lower material cost, new possibilities for modularity and such a scanner would be more compact compared to a conventional ring scanner. Currently achieved values of CTR aren’t sufficient to enable the performance of flat panel scanners to reach the standard set by currently used state-of-the-art scanners in clinical practice. In order to achieve sufficient CTR for flat panel scanners to become viable, improvements have to be made on every level of the detection chain. With some preliminary results with optimised electronics and photodetectors we expect that in a few years CTR as low as 75 ps could be achieved at the system level. In this work various designs of such flat panel scanners were simulated with image quality phantoms in order to determine their performance using the NEMA standard. The simulations were performed using GATE software and image reconstruction was performed using CASToR.

Using Cherenkov radiation in positron emission tomography (PET) has the potential to improve the time of flight (TOF) resolution and reduce the cost of detectors. In previous studies promising TOF results were achieved when lead fluoride (PbF2) crystals were used instead of a scintillator. In this work, a whole-body PbF2 Cherenkov TOF-PET scanner was simulated and optimized. Different configurations of the PbF2 crystals and their surface treatment were considered. Also evaluated was the influence of the crystal-photodetector coupling and of the detection efficiency of the photodetectors. Of special interest is a whole-body PbF2 Cherenkov TOF-PET scanner with a multi-layer detector, which improves the time resolution and reduces the parallax error, without compromising the detection efficiency. Images of a phantom were reconstructed for different configurations of the simulated whole-body PbF2 Cherenkov TOF-PET scanner and the quality of images was compared to that of a whole-body TOF-PET scanner with standard LSO scintillators. The TOF resolution of the whole-body PbF2 Cherenkov TOF-PET scanner with a multi-layer detector was 143 ps FWHM, out of which the fundamental limitation due to light production and transportation was only 22 ps FWHM.