Geant4 Application For Tomographic Emission Research Articles

FAST (fast analytical simulator of tracer)-PET: an accurate and efficient PET analytical simulation tool

Objective. Simulation of positron emission tomography (PET) images is an essential tool in the development and validation of quantitative imaging workflows and advanced image processing pipelines. Existing Monte Carlo or analytical PET simulators often compromise on either efficiency or accuracy. We aim to develop and validate fast analytical simulator of tracer (FAST)-PET, a novel analytical framework, to simulate PET images accurately and efficiently. Approach. FAST-PET simulates PET images by performing precise forward projection, scatter, and random estimation that match the scanner geometry and statistics. Although the same process should be applicable to other scanner models, we focus on the Siemens Biograph Vision-600 in this work. Calibration and validation of FAST-PET were performed through comparison with an experimental scan of a National Electrical Manufacturers Association (NEMA) Image Quality (IQ) phantom. Further validation was conducted between FAST-PET and Geant4 Application for Tomographic Emission (GATE) quantitatively in clinical image simulations in terms of intensity-based and texture-based features and task-based tumor segmentation. Main results. According to the NEMA IQ phantom simulation, FAST-PET’s simulated images exhibited partial volume effects and noise levels comparable to experimental images, with a relative bias of the recovery coefficient RC within 10% for all spheres and a coefficient of variation for the background region within 6% across various acquisition times. FAST-PET generated clinical PET images exhibit high quantitative accuracy and texture comparable to GATE (correlation coefficients of all features over 0.95) but with ∼100-fold lower computation time. The tumor segmentation masks comparison between both methods exhibited significant overlap and shape similarity with high concordance CCC > 0.97 across measures. Significance. FAST-PET generated PET images with high quantitative accuracy comparable to GATE, making it ideal for applications requiring extensive PET image simulations such as virtual imaging trials, and the development and validation of image processing pipelines.

A deep neural network for positioning and inter-crystal scatter identification in multiplexed PET detectors: a simulation study

Objective. High-resolution positron emission tomography (PET) relies on the accurate positioning of annihilation photons impinging the crystal array. However, conventional positioning algorithms in light-sharing PET detectors are often limited due to edge effects and/or the absence of additional information for identifying and correcting scattering within the crystal array (known as inter-crystal scattering). This study explores the feasibility of deep neural network (DNN) techniques for more precise event positioning in finely segmented and highly multiplexed PET detectors with light-sharing. Approach. Initially, a Geant4 Application for Tomographic Emission (GATE) simulation was used to study the spatial and statistical properties of inter-crystal scatter (ICS) events in finely segmented LYSO PET detectors. Next, a DNN for crystal localisation was designed, trained and tested with light distributions of photoelectric (P) and Compton + photoelectric (CP) events simulated using optical GATE and an analytical method to speed up data generation. Using the statistical properties of ICS events, an energy-guided positioning algorithm was then built into the DNN. The positioning algorithm enables selection of the unique or first crystal of interaction in P and CP events, respectively. Performance of the DNN was compared with Anger logic using light distributions from simulated 511 keV point sources placed at different locations around a single PET detector module. Main results. The fraction of events forward and backward scattered in the LYSO detector was 0.54 and 0.46, respectively, whereas naïve application of the Klein–Nishina formulation predicts 70% forward scatter. Despite coarse photodetector data due to signal multiplexing, the DNN demonstrated a crystal classification accuracy of 90% for P events and 82% for CP events. For crystal positioning, the DNN outperformed Anger logic by at least 34% and 14% for P and CP events, respectively. Further improvement is somewhat constrained by the physics—specifically, the ratio of backward to forward scattering of gamma rays within the crystal array being close to 1. This prevents selecting the first crystal of interaction in CP events with a high degree of certainty. Significance. Light sharing and multiplexed PET detectors are common in high-resolution PET, yet their traditional positioning algorithms often underperform due to edge effects and/or the difficulty in correcting ICS events. Our study indicates that DNN-based event positioning has the potential to enhance 2D coincidence event positioning accuracy by nearly a factor of 3 compared to Anger logic. However, further improvements are difficult to foresee without additional information such as event timing.

PET digitization chain for Monte Carlo simulation in GATE

Objective. We introduce a versatile methodology for the accurate modelling of PET imaging systems via Monte Carlo simulations, using the Geant4 application for tomographic emission (GATE) platform. Accurate Monte Carlo modelling involves the incorporation of a complete analytical signal processing chain, called the digitizer in GATE, to emulate the different count rates encountered in actual positron emission tomography (PET) systems. Approach. The proposed approach consists of two steps: (1) modelling the digitizer to replicate the detection chain of real systems, covering all available parameters, whether publicly accessible or supplied by manufacturers; (2) estimating the remaining parameters, i.e. background noise level, detection efficiency, and pile-up, using optimisation techniques based on experimental single and prompt event rates. We show that this two-step optimisation reproduces the other experimental count rates (true, scatter, and random), without the need for additional adjustments. This method has been applied and validated with experimental data derived from the NEMA count losses test for three state-of-the-art SiPM-based time-of-flight (TOF)-PET systems: Philips Vereos, Siemens Biograph Vision 600 and GE Discovery MI 4-ring. Main results. The results show good agreement between experiments and simulations for the three PET systems, with absolute relative discrepancies below 3%, 6%, 6%, 7% and 12% for prompt, random, true, scatter and noise equivalent count rates, respectively, within the 0–10 kBq·ml−1 activity concentration range typically observed in whole-body 18F scans. Significance. Overall, the proposed digitizer optimisation method was shown to be effective in reproducing count rates and NECR for three of the latest generation SiPM-based TOF-PET imaging systems. The proposed methodology could be applied to other PET scanners.

Virtual cylindrical PET for efficient DOI image reconstruction with sub-millimetre resolution

Objective. Image reconstruction in high resolution, narrow bore PET scanners with depth of interaction (DOI) capability presents a substantial computational challenge due to the very high sampling in detector and image space. The aim of this study is to evaluate the use of a virtual cylinder in reducing the number of lines of response (LOR) for DOI-based reconstruction in high resolution PET systems while maintaining uniform sub-millimetre spatial resolution. Approach. Virtual geometry was investigated using the awake animal mousePET as a high resolution test case. Using GEANT4 Application for Tomographic Emission (GATE), we simulated the physical scanner and three virtual cylinder implementations with detector size 0.74 mm, 0.47 mm and 0.36 mm (vPET1, vPET2 and vPET3, respectively). The virtual cylinder condenses physical LORs stemming from various crystal pairs and DOI combinations, and which intersect a single virtual detector pair, into a single virtual LOR. Quantitative comparisons of the point spread function (PSF) at various positions within the field of view (FOV) were compared for reconstructions based on the vPET implementations and the physical scanner. We also assessed the impact of the anisotropic PSFs by reconstructing images of a micro Derenzo phantom. Main results. All virtual cylinder implementations achieved LOR data compression of at least 50% for DOI PET reconstruction. PSF anisotropy in radial and tangential profiles was chiefly influenced by DOI resolution and only marginally by virtual detector size. Spatial degradation introduced by virtual cylinders was most prominent in the axial profile. All virtual cylinders achieved sub-millimetre volumetric resolution across the FOV when 6-bin DOI reconstructions (3.3 mm DOI resolution) were performed. Using vPET2 with 6 DOI bins yielded nearly identical reconstructions to the non-virtual case in the transaxial plane, with an LOR compression factor of 86%. Resolution modelling significantly reduced the effects of the asymmetric PSF arising from the non-cylindrical geometry of mousePET. Significance. Narrow bore and high resolution PET scanners require detectors with DOI capability, leading to computationally demanding reconstructions due to the large number of LORs. In this study, we show that DOI PET reconstruction with 50%–86% LOR compression is possible using virtual cylinders while maintaining sub-millimetre spatial resolution throughout the FOV. The methodology and analysis can be extended to other scanners with DOI capability intended for high resolution PET imaging.

Depth-encoding using optical photon TOF in a prism-PET detector with tapered crystals.

High-resolution brain positron emission tomography (PET) scanner is emerging as a significant and transformative non-invasive neuroimaging tool to advance neuroscience research as well as improve diagnosis and treatment in neurology and psychiatry. Time-of-flight (TOF) and depth-of-interaction (DOI) information provide markedly higher PET imaging performance by increasing image signal-to-noise ratio and mitigating spatial resolution degradation due to parallax error, respectively. PET detector modules that utilize light sharing can inherently carry DOI information from the multiple timestamps that are generated per gamma event. The difference between two timestamps that are triggered by scintillation photons traveling in opposite directions signifies the event's depth-dependent optical photon TOF (oTOF). However, light leak at the crystal-readout interface substantially degrades the resolution of this oTOF-based depthencoding. We demonstrate the feasibility of oTOF-based depth encoding by mitigating light leak in single-ended-readout Prism-PET detector modules using tapered crystals. Minimizing light leak also improved both energy-based DOI and coincidence timingresolutions. The tapered Prism-PET module consists of a 16 16 array of 1.5 1.5 20 lutetium yttrium oxyorthosillicate (LYSO) crystals, which are tapered down to 1.2 1.2 at the crystal-readout interface. The LYSO array couples 4-to-1 to an 8 8 array of 3 3 silicon photomultiplier (SiPM) pixels on the tapered end and to a segmented prismatoid light guide array on the opposite end. Performance of tapered and non-tapered Prism-PET detectors was experimentally characterized and evaluated by measuring flood histogram, energy resolution, energy-, and oTOF-based DOI resolutions, and coincidence timing resolution. Sensitivities of scanners using different Prism-PET detector designs were simulated using Geant4 application for tomographic emission (GATE). For the tapered (non-tapered) Prism-PET module, the measured full width at half maximum (FWHM) energy, timing, energy-based DOI, and oTOF-based DOI resolutions were 8.88 (11.18)%, 243 (286)ps, 2.35 (3.18)mm, and 5.42 (13.87)mm, respectively. The scanner sensitivities using non-tapered and tapered crystals, and 10 rings of detector modules, were simulated to be 30.9 and 29.5kcps/MBq,respectively. The tapered Prism-PET module with minimized light leak enabled the first experimental report of oTOF-based depth encoding at the detector module level. It also enabled the utilization of thinner (i.e., 0.1mm) inter-crystal spacing with barium sulfate as the reflector while also improving energy-based DOI and timingresolutions.

Modelization of extended axial field-of-view PET scanners to analyze the performance improvement

Positron Emission Tomography (PET) offers non-invasive insights into physiological processes. However, in clinical practice conventional PET scanners face limitations in comprehensive imaging due to their restricted axial field of view (FOV).This study explores the benefits and challenges of extending PET scanner AFOV using Monte Carlo simulation with GATE 9.1. The Biograph Vision Quadra scanner is a novel system based on the Digital Biograph Vision 600 scanner. Both devices use silicon photomultipliers and 3.2 x 3.2 × 20 mm Lutetium Oxyorthosilicate crystals (LSO). However, the 32 detector rings of the Quadra scanner enable a four-fold expansion in the Axial Field Of View (FOV) compared to Vision. As a result, higher sensitivity, and an enhancement of noise equivalent count rate and image quality are achieved. The purpose of this study is to analyze the effect of increasing the number of detector rings and the axial FOV on the scanner performance. To accomplish this, four different geometries are modeled using GATE (GEANT4 Application for Tomographic Emission). Based on the Biograph Vision 600 model, the length and the number of detector rings are gradually extended to reach the length of the Quadra scanner. Thus, four geometries with 8 (25.6 cm), 16 (51.2 cm), 24 (76.8 cm) and 32 (102.4 cm) detector rings are simulated, keeping the crystal configuration and the dimensions in the transaxial plane. The study evaluates sensitivity, Noise Equivalent Count Rate (NECR), and image quality parameters. Results indicate a significant increase in sensitivity and NECR with FOV extension, underscoring enhanced performance. However, artifact occurrence, particularly concentric circular patterns and background homogeneity changes, accentuates with extended AFOV. Normalization algorithms, particularly component-based normalization, mitigate these artifacts. This study contributes to the understanding of the advantages and limitations of extending PET scanner AFOV, offering insights into the effectiveness of component-based normalization for improving PET image quality.

Contribution to dose control at the Moroccan Boukhalef ionization facility: Design by Monte Carlo simulation of a container for agricultural products irradiation

In facilities that manage sources of ionizing radiation, controlling the absorbed dose delivered to a target is of paramount importance. This factor is essential as it directly reflects the quality of irradiation within such facilities. The BOukhalef Ionization Station (SIBO), part of the Moroccan National Institute for Agronomic Research (INRA), presents an example of one among several applications of ionizing radiation worldwide. It houses a60Co gamma irradiator used for conducting scientific research in the field of agriculture. The objective of this work is to contribute to dose control at SIBO by designing a container for agricultural products irradiation. This Container will enable a more uniform distribution of absorbed dose within the irradiated products. In this study, the Monte Carlo simulation GATE (Geant4 Application for Tomographic Emission) code is employed to flexibly adjust the dimensions of the container, and to calculate the dose homogeneity factor in the irradiated products. By selecting a cylindrical container for the irradiated products, this study shows a significant enhancement in the homogeneity factor through the reduction of the cylinder's thickness (external radius minus internal radius) when transitioning from a solid cylinder to a hollow one. Indeed, the homogeneity factor improves significantly from 2.65 to 1.64 when transitioning from a solid cylinder with a radius of 20 cm and a height of 20 cm, to a hollow cylinder with a height of 10 cm, an internal radius of 20 cm, and an external radius of 28.3 cm.

New GATE Digitizer Unit for versions post v9.3

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.

Monte Carlo simulation of two Siemens Biograph PET/CT system using GATE: Image quality performance

The utilization of Monte Carlo simulation is a highly effective method in advancing research within the field of nuclear medicine. An important software tool for simulating and modeling cutting-edge systems for Positron Emission Tomography (PET) is the Geant4 Application for Tomographic Emission (GATE). This research specifically concentrates on two Siemens PET scanners: the Siemens Biograph True point (True V) and Siemens Biograph mcT 20 excel. Our primary objective was to validate the GATE V8.2 simulation in accordance with the NEMA (National Electrical Manufacturers Association) NU 2-2018 protocol, using a parallel computing platform on a local cluster of computers. This cluster was managed by the open-source package TORQUE version 6.1.0, which is based on the original wrapper PBS.The results of this validation process for the Siemens Biograph True point (True V) scanner were highly promising. The scatter fraction (SF), peak noise equivalent count rate (NECR), and sensitivity all demonstrated a remarkable level of agreement, with deviations within 10.02%, 5.4%, and 1.57%, respectively. Additionally, the spatial resolution was found to deviate by less than 0.59 mm. Image evaluations demonstrate high-quality results for both Siemens Biograph True point (True V) and Siemens Biograph mcT 20 excel scanners, validating the GATE simulation. The study on acquisition time provides insights into optimizing computing simulation time.This research demonstrates the effectiveness of Monte Carlo simulation, facilitated by GATE, in accurately modeling advanced PET systems. The validation results affirm the reliability of GATE V8.2 in accordance with NEMA standards for the Siemens Biograph True point (True V) scanner. The high-quality images obtained from GATE-validated scanners underscore the potential of this simulation approach in advancing nuclear medicine research. Additionally, the study on acquisition time provides valuable considerations for optimizing simulation efficiency.

Simplified image‐based dosimetry using planar images and patient‐specific S‐values

AbstractBackgroundSingle time point measurement approach and hybrid dosimetry were proposed to simplify the dosimetry process. It is anticipated that utilizing patient‐specific S‐value would enable more accurate dosimetry assessment based on imaging compared to using the conventional MIRD S‐values.PurposeWe performed planar image‐based dosimetry scaled with a single SPECT image for the entire treatment cycle using patient‐specific S‐values (PSS dosimetry) of organs. PSS dosimetry could further simplify the dosimetry procedure compared with a conventional 2D planar/3D SPECT hybrid dosimetry, as PSS dosimetry requires only one SPECT/CT image for the treatment of the entire cycle, whereas the conventional hybrid dosimetry requires a SPECT/CT image for each treatment cycle.Methods177Lu‐DOTATATE SPECT/CT and planar image datasets acquired from Seoul National University Hospital (SNUH, Seoul, Republic of Korea) were utilized for the evaluation. Images were acquired 4, 24, 48, and 120 h after patients’ intravenous injection of 177Lu‐DOTATATE. Dose estimations based on a Monte Carlo (MC) simulation using the Geant4 Application for Emission Tomography (GATE) (v.8.2) were considered as the reference. Planar image‐based dosimetry scaled with a single SPECT image was performed using the patient‐specific S‐value (PSS). Briefly, the CT image was considered as the patient's anatomical reference and PSSs were quantified using the multiple voxel S‐value (VSV) method. Then, PSS dosimetry was performed by obtaining activity information from sequential planar images and a scaling factor derived from a single SPECT/planar image pair. Hybrid dosimetry using sequential planar images and a single SPECT image was performed for comparison. The absorbed doses of the kidneys, bone marrow (BM) in the lumbar spine, liver, and spleen calculated using the PSS and hybrid dosimetries were compared with the reference MC results.ResultsThe mean differences (MDs) of the self‐absorption S‐values between S‐value of OLINDA/EXM and PSS for the kidneys, liver, and spleen were −0.04%, −2.39%, and −2.62%, respectively. However, the differences in the self‐absorption S‐values were significantly higher for the BM (84.99%) and the remainder of the body (ROB) (280.84%). The absorbed doses estimated by the PSS and hybrid dosimetries showed relatively high errors compared with MC simulation result, regardless of the organ. In contrast, the PSS and hybrid dosimetries produced similar dose estimates. For the entire cycles of the treatment, the MDs of absorbed doses between PSS and hybrid dosimetries were −3.31%, −6.04%, 3.37%, and −2.17% for the kidneys, BM, liver, and spleen, respectively. Through a correlation analysis and the Wilcoxon signed‐rank test, we concluded that there was no significant difference between the results obtained by the two dosimetry methods.ConclusionsAs the PSS was derived using CT images with actual anatomical information and organ‐specific volume of interest (VOI), PSS dosimetry provided reliable results. PSS dosimetry was robust in estimating the absorbed dose for the later treatment cycles. Therefore, PSS dosimetry outperformed hybrid dosimetry in terms of dose estimation for a greater number of treatment cycles.

Calculation of Organ Dose Distribution (in-field and Out-of-field) in Breast Cancer Radiotherapy on RANDO Phantom Using GEANT4 Application for Tomographic Emission (Gate) Monte Carlo Simulation.

Organ dose distribution calculation in radiotherapy and knowledge about its side effects in cancer etiology is the most concern for medical physicists. Calculation of organ dose distribution for breast cancer treatment plans with Monte Carlo (MC) simulation is the main goal of this study. Elekta Precise linear accelerator (LINAC) photon mode was simulated and verified using the GEANT4 application for tomographic emission. Eight different radiotherapy treatment plans on RANDO's phantom left breast were produced with the ISOgray treatment planning system (TPS). The simulated plans verified photon dose distribution in clinical tumor volume (CTV) with TPS dose volume histogram (DVH) and gamma index tools. To verify photon dose distribution in out-of-field organs, the point dose measurement results were compared with the same point doses in the MC simulation. Eventually, the DVHs for out-of-field organs that were extracted from the TPS and MC simulation were compared. Based on the implementation of gamma index tools with 2%/2 mm criteria, the simulated LINAC output demonstrated high agreement with the experimental measurements. Plan simulation for in-field and out-of-field organs had an acceptable agreement with TPS and experimental measurement, respectively. There was a difference between DVHs extracted from the TPS and MC simulation for out-of-field organs in low-dose parts. This difference is due to the inability of the TPS to calculate dose distribution in out-of-field organs. Based on the results, it was concluded that the treatment plans with the MC simulation have a high accuracy for the calculation of out-of-field dose distribution and could play a significant role in evaluating the important role of dose distribution for second primary cancer estimation.

GATE Monte Carlo simulation toolkit for medical physics

The GATE toolkit (GEANT4 Application for Tomographic Emission) is a GEANT4-based (GEometry ANd Tracking) platform for Monte Carlo simulations in medical physics. GATE applications can be divided into two main axes: radiation-based medical imaging and radiotherapy/dosimetry. The accurate modeling of the first one is crucial for system design and optimization as well as for development and refinement of image analysis algorithms. The importance of the precise simulation of the second is essential for characterisation of external beam radiotherapy (proton therapy and carbon ion therapy) and absorbed dose assessment. Within this paper, we discuss the main features of GATE and give a general view on applications, followed by insights into future development perspectives.

Study on the PET image quality according to various scintillation detectors based on the Monte Carlo simulation

Purpose: Positron emisson tomography (PET) is a crucial medical imaging scanner for the detection of cancer lesions. In order to maintain the improved image quality, it is crucial to apply detectors of superior performance. Therefore, the purpose of this study was to compare PET image quality using Monte Carlo simulation based on the detector materials of BGO, LSO, and LuAP. Materials and Methods: The Geant4 Application for Tomographic Emission (GATE) was used to design the PET detector. Scintillations with BGO, LSO and LuAP were modelled, with a size of 3.95 × 5.3 mm2 (width × height) and 25.0 mm (thickness). The PET detector consisted of 34 blocks per ring and a total of 4 rings. A line source of 1 MBq was modelled and acquired with a radius of 1 mm and length of 20 mm for 20 seconds. The acquired image was reconstructed maximum likelihood expectation maximization with 2 iteration and 10 subsets. The count comparison was carried out. Results and Discussion: The highest true, random, and scatter counts were obtained from the BGO scintillation detector compared to LSO and LuAP. Conclusion: The BGO scintillation detector material indicated excellent performance in terms of detection of gamma rays from emitted PET phantom.

A-Si EPID Modeling in GATE for Monte Carlo Investigation on the Optical Properties of Gd<sub>2</sub>O<sub>2</sub>S:Tb and Lu<sub>2</sub>SiO<sub>5</sub>:Ce Scintillators

A detailed a-Si Electronic Portal Imaging Device (EPID) was implemented in GATE (Geant4 application for tomographic emission) toolkit for Monte Carlo simulations, employing the standard electromagnetic processes and optical photon processes. The composition of the scintillating material is varied using the conventional terbium-doped gadolinium oxysulfide phosphor (Gd2O2S:Tb) and cerium-doped lutetium oxyorthosilicate (Lu2SiO5:Ce) which is widely used in Positron Emission Tomography (PET) detectors due to its supremacy in aiding the detection of low-energy photons. It was found that the number of optical photons produced is higher when using the Lu2SiO5:Ce scintillator resulting in a slightly better signal-to-noise ratio (SNR). The scattering within the EPID components was also investigated where the majority of the detected secondary particles were created in the scintillator. The results also show that the copper plate layer of the EPID contributes to additional Compton electrons and bremsstrahlung and annihilation photons to the measurements in the scintillator and photodiode layer. The glass substrate, graphite plates, electronic components, and aluminum bottom cover are also found to contribute a huge fraction of backscattered particles to the measurements at the photodiode layer.

Monte Carlo Simulation Study on the Depth-Dose Distribution of Thermoplastic Reinforced Composites for Potential Applications in External Beam Radiotherapy

External beam radiotherapy is a crucial method in treating cancer as it requires a high level of accuracy in patient positioning. Hence, commercial thermoplastic masks have a significant role during radiotherapy treatment as it is an efficient way of accurately positioning the patient without risking their safety and comfort. However, these commercial masks are expensive due to the use of costly compounds and additives, and several studies have concluded that commercial masks can significantly increase the surface dose. This study examined the effects of various thermoplastic reinforced composites for an alternative economic mask in terms of depth-dose distribution through Monte Carlo simulations in GEANT4 Application for Tomographic Emission (GATE). The simulations were varied by incident beams and their energies, material composition of the phantom, and varying thermoplastic reinforced composites. In general, the results indicated that PCL/PLA (polycaprolactone/polylactic acid), PLA/PU (polylactic acid/polyurethane), PCL/RH (polycaprolactone/rice husk), LDPE/PALF (low-density polyethylene/pineapple leaf fiber), and PP/PALF (polypropylene/pineapple leaf fiber) composites showed relatively accurate dose delivery in the target volume, which the slight difference will accumulate the 5% marginal error in treatment planning systems.

A Method for <i>In Vivo</i> Dosimetry Using an Electronic Portal Imaging Device

The implementation of advanced radiotherapy techniques such as Intensity-Modulated RadiationTherapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT) allows much more accuratecontrol of the irradiation area and more conformal dose delivery. Since these treatments are characterizedby high dose gradients and hence small error margins, they also require advanced and highlyeffective treatment verification techniques. Fortunately, an imaging modality, originally intended forset-up verifications, called the Electronic Portal Imaging Device (EPID) was reported to have hugepotential for dosimetry and treatment verification applications. However, methods and algorithmspreviously developed for EPID dosimetry are not yet widely adopted in cancer centers due to complexitiesin the implementation. These motivate more investigation and development of new methodsto encourage cancer centers to practice EPID dosimetry and improve patient safety.This research generally aims to develop an in-vivo dosimetry method that can be used in the future toapproximate the actual dose delivered inside the target by utilizing the transmitted fluence detectedby the EPID. In this study, Monte Carlo simulations are carried out using the Geant4 Application forTomographic Emission (GATE) to implement a virtual set-up composed of a water target irradiatedwith a monoenergetic 2 MeV photon beam and an EPID model to detect the total transmitted fluence.A mathematical model has been proposed to calculate the target absorbed dose by utilizing the transmittedprimary fluence. This involves back-projection of the transmitted fluence to determine fluencevalues at certain depths inside the irradiated homogeneous medium. This is followed by the calculationof the collision kerma using the back-projected fluence and the values of the linear attenuation coefficientand mass energy absorption coefficient in the National Institute of Standards and Technology(NIST) database. The results show accurate prediction of the fluence values inside the target, calculationof the corresponding values of the collision kerma, and determination of the proportionalityconstant β that relates the collision kerma to the target absorbed dose.

Quantitative validation of Monte Carlo SPECT simulation: application to a Mediso AnyScan GATE simulation

BackgroundMonte Carlo (MC) simulations are used in nuclear medicine imaging as they provide unparalleled insight into processes that are not directly experimentally measurable, such as scatter and attenuation in an acquisition. Whilst MC is often used to provide a ‘ground-truth’, this is only the case if the simulation is fully validated against experimental data. This work presents a quantitative validation for a MC simulation of a single-photon emission computed tomography (SPECT) system.MethodsAn MC simulation model of the Mediso AnyScan SCP SPECT system installed at the UK National Physical Laboratory was developed in the GATE (Geant4 Application for Tomographic Emission) toolkit. Components of the detector head and two collimator configurations were modelled according to technical specifications and physical measurements. Experimental detection efficiency measurements were collected for a range of energies, permitting an energy-dependent intrinsic camera efficiency correction function to be determined and applied to the simulation on an event-by-event basis. Experimental data were collected in a range of geometries with ^{99text {m}}Tc for comparison to simulation. The procedure was then repeated with ^{177}Lu to determine how the validation extended to another isotope and set of collimators.ResultsThe simulation’s spatial resolution, sensitivity, energy spectra and the projection images were compared with experimental measurements. The simulation and experimental uncertainties were determined and propagated to all calculations, permitting the quantitative agreement between simulated and experimental SPECT acquisitions to be determined. Statistical agreement was seen in sinograms and projection images of both ^{99text {m}}Tc and ^{177}Lu data. Average simulated and experimental sensitivity ratios of (0.991 pm 0.011) were seen for emission and scatter windows of ^{99text {m}}Tc, and (0.897 pm 0.014) and (0.839 pm 0.014) for the 113 and 208 keV emissions of ^{177}Lu, respectively.ConclusionsMC simulations will always be an approximation of a physical system and the level of agreement should be assessed. A validation method is presented to quantify the level of agreement between a simulation model and a physical SPECT system.

Monte Carlo Dosimetry Validation for X-Ray Guided Endovascular Procedures

Endovascular treatment is continuously gaining ground in vascular surgery procedures. However, current patient radiation dose estimation does not take into account the exact patient morphology and organs' composition. Monte Carlo (MC) simulation can accurately estimate the dose by recreating the irradiation process generated during X-ray-guided interventions. This study aimed to validate the MC simulation models by comparing simulated and measured dose distributions in endovascular aortic aneurysm repair (EVAR) procedures. We conducted a clinical study in patients treated for EVAR. Patient dose measurements were taken with passive dosimeters using Optically Stimulated Luminescence technology in 4 specific anatomical points on the skin: xiphoid process, pubic symphysis, right and left iliac crest. Dose measurements were compared to the corresponding simulated doses with the Geant4 Application for Emission Tomography (GATE) and GPU Geant4-based Monte Carlo Simulations (GGEMS) MC simulations softwares. The MC simulation took as input the computed tomography scan of the patient and the parameters of the imaging system (orientation angles, tube voltage, and aluminum filtration) and gives as output the three-dimensional (3D) dose map for each patient and angulation. A good agreement with real doses was found for doses simulated by the MC GATE method (P<0.0001; r=0.97; 95% confidence interval [CI] [0.96-0.98]), as well as for doses simulated by the GGEMS method (P<0.0001; r=0.96; 95% CI [0.94-0.97]). The mean relative error for all measurements was 5±5% in the MC GATE group and 6±5% in the GGEMS group. Process execution on GGEMS (6sec) was faster than the GATE MC simulation (5hr). Considering the current imaging settings, this study shows the potential of using the GATE and GGEMS MC simulations platforms to model the 3D dose distributions during EVAR procedures.

Monte Carlo model of a prototype flat-panel detector for multi-energy applications in radiotherapy.

The incorporation of multi-energy capabilities into radiotherapy flat-panel detectors offers advantages including enhanced soft tissue visualization by reduction of signal from overlapping anatomy such as bone in 2D image projections; creation of virtual monoenergetic images for 3D contrast enhancement, metal artefact reduction and direct acquisition of relative electron density. A novel dual-layer on-board imager offering dual energy processing capabilities is being designed. As opposed to other dual-energy implementation techniques which require separate acquisition with two different x-ray spectra, the dual-layer detector design enables simultaneous acquisition of high and low energy images with a single exposure. A computational framework is required to optimize the design parameters and evaluate detector performance for specific clinical applications. In this study, we report on the development of a Monte Carlo (MC) model of the imager including model validation. The stack-up of the dual-layer imager (DLI) was implemented in GEANT4 Application for Tomographic Emission (GATE). The DLI model has an active area of 43×43 cm2 , with top and bottom Cesium Iodide (CsI) scintillators of 600 and 800μm thickness, respectively. Measurement of spatial resolution and imaging of dedicated multi-material dual-energy (DE) phantoms were used to validate the model. The modulation transfer function (MTF) of the detector was calculated for a 120 kVp x-ray spectrum using a 0.5mm thick tantalum edge rotated by 2.5o . For imaging validation, the DE phantom was imaged using a 140 kVp x-ray spectrum. For both validation simulations, corresponding measurements were done using an initial prototype of the imager. Agreement between simulations and measurement was assessed using normalized root mean square error (NRMSE) and 1D profile difference for the MTF and phantom images respectively. Further comparison between measurement and simulation was made using virtual monoenergetic images (VMIs) generated from basis material images derived using precomputed look-up tables. The MTF of the bottom layer of the dual-layer model shows values decreasing more quickly with spatial frequency, compared to the top layer, due to the thicker bottom scintillator thickness and scatter from the top layer. A comparison with measurement shows NRMSE of 0.013 and 0.015 as well as identical MTF50 of 0.8 mm1 and 1.0 mm1 for the top and bottom layer respectively. For the DE imaging of the DE-phantom, although a maximum deviation of 3.3% is observed for the 10mm aluminum and Teflon inserts at the top layer, the agreement for all other inserts is less than 2.2% of the measured value at both layers. Material decomposition of simulated scatter-free DE images gives an average accuracy in PMMA and aluminum composition of 4.9% and 10.3% for 11-30mm PMMA and 1-10mm aluminum objects respectively. A comparison of decomposed values using scatter containing measured and simulated DE images shows good agreement within statistical uncertainty. Validation using both MTF and phantom imaging shows good agreement between simulation and measurements. With the present configuration of the digital prototype, the model can generate material decomposed images and virtual monoenergetic images.

Dosimetric assessment of Gadolinium-159 for hepatic radioembolization: Tomographic images and Monte Carlo simulation

A common therapeutic radionuclide used in hepatic radioembolization is yttrium-90 (90Y). However, the absence of gamma emissions makes it difficult to verify the post-treatment distribution of 90Y microspheres. Gadolinium-159 (159Gd) has physical properties that are suitable for therapy and post-treatment imaging in hepatic radioembolization procedures. The current study is innovative for conducting a dosimetric investigation of the use of 159Gd in hepatic radioembolization by simulating tomographic images using the Geant4 application for tomographic emission (GATE) Monte Carlo (MC) simulation. For registration and segmentation, tomographic images of five patients with hepatocellular carcinoma (HCC) who had undergone transarterial radioembolization (TARE) therapy were processed using a 3D slicer. The tomographic images with 159Gd and 90Y separately were simulated using the GATE MC Package. The output of simulation (dose image) was uploaded to 3D slicer to compute the absorbed dose for each organ of interests. 159Gd were able to provide a recommended dose of 120 Gy to the tumour, with normal liver and lungs absorbed doses close to that of 90Y and less than the respective maximum permitted doses of 70 Gy and 30 Gy, respectively. Compared to 90Y, 159Gd requires higher administered activity approximately 4.92 times to achieve a tumour dose of 120 Gy. Thus; this research gives new insights into the use of 159Gd as a theranostic radioisotope, with the potential to be used as a90Y alternative for liver radioembolization.