Optical Photon Transport Research Articles

Evaluation of TOPAS MC tool performance in optical photon transport and radioluminescence-based dosimetry

Radiation therapy plays a pivotal role in modern cancer treatment, demanding precise and accurate dose delivery to tumor sites while minimizing harm to surrounding healthy tissues. Monte Carlo simulations have emerged as indispensable tools for achieving this precision, offering detailed insights into radiation transport and interaction at the subatomic level. As the use of scintillation and luminescence dosimetry becomes increasingly prevalent in radiation therapy, there arises a need for validated Monte Carlo tools tailored to optical photon transport applications. In this paper, an evaluation process of the TOPAS (TOol for PArticle Simulation) Monte Carlo tool for Cerenkov light generation, optical photon transport and radioluminescence based dosimetry is presented. Three distinct sources of validation data are utilized: one from a published set of experimental results and two others from simulations performed with the Geant4 code. The methodology employed for evaluation includes the selection of benchmark experiments, making use of opt3 and opt4 Geant4 physics models and simulation setup, with observed slight discrepancies within the calculation uncertainties. Additionally, the complexities and challenges associated with modeling optical photons generation through luminescence or Cerenkov radiation and their transport are discussed. The results of our evaluation suggests that TOPAS can be used to reliably predict Cerenkov generation, luminescence phenomenon and the behavior of optical photons in common dosimetry scenarios.

Optical photon transport simulations for SiPM based PET scanner

Positrons emitted from radioisotope quickly annihilates to two collinear photons that are captured by the array of scintillator detectors to infer about the origin of radioactivity. Coincidence collection of pair of points from the detector modules makes a line of response, their large number reveals a radioactive region of interest. Most of the medical diagnostic research is conducted with position sensitive imaging device whose performance strongly depends on the optical light collection efficiency. A versatile Lutetium–Yttrium Oxyorthosilicate (LYSO) scintillator is used to mimic the optical photon production and transport processes followed by the light collection with Silicon PhotoMultiplier sensor. Charge spectrum from a 2 × 2 × 10 mm3 single LYSO bar is obtained in GEANT4, and reproduces well the scintillation characteristics of LYSO along with photons incidence angle distribution with a single SiPM pad. This excercise is followed by the simulation performance of two PET modules comprises of 12 × 12 SiPM tile coupled with mesh of LYSO bars. The preliminarly results shows the transverse position resolution (FWHM) as 7.82 mm for a point source. Objective of this study is to search for a cheaper detector solution which will have equivalent imaging performance.

PDOZ: innovative personal electronic dosimeter for electron and gamma H *(d) dosimetry

The personal (or active) electronic dosimeters (PEDs) are devices used to determine the individual exposure to ionizing radiations and they are employed in hospitals, research laboratories and nuclear power plants. The PDOZ project is a personal electronic dosimeter able to detect, discriminate and measure the delivered dose by beta particles and gamma rays. In this paper, several Monte Carlo simulations are described. The first one is regarding the ICRU sphere, [11,12] implemented to evaluate the ambient dose equivalent, H *(10), and the fluence-to-dose equivalent conversion coefficients for gamma rays and beta particles. The second simulation is carried out to study the prototype dosimeter response to gamma rays and beta particles and, also thanks to previous one, to obtain the conversion curve necessary to calculate the ambient dose equivalent from the silicon photomultipliers counts. In the last one, instead, the performance of a prototype dosimeter, composed by a small plastic scintillator coupled to two SiPMs, is evaluated and a simulation with different radioactive sources is made whose results are compared with the experimental measurements. All simulations are carried out by Geant4 including the optical photon transport.

Quantitative Assessment on Optical Properties as a Basis for Bioluminescence Imaging: An Experimental and Numerical Approach to the Transport of Optical Photons in Phantom Materials.

Bioluminescence imaging (BLI) is a widely used technique in preclinical scientific research, particularly in the development of mRNA-based medications and non-invasive tumor diagnostics. It has become an essential tool in current science. However, the current state of bioluminescence imaging is primarily qualitative, making it challenging to obtain quantitative measurements and to draw accurate conclusions. This fact is caused by the unique properties of optical photons and tissue interactions. In this paper, we propose an experimental setup and Geant4-simulations to gain a better understanding of the optical properties and processes involved in bioluminescence imaging. Our goal is to advance the field towards more quantitative measurements. We will discuss the details of our experimental setup, the data we collected, the outcomes of the Geant4-simulations, and additional information on the underlying physical processes.

A review of the Geant4 simulation platform for applications involving optical-based sensing and dosimetry

Geant4 is a versatile simulation platform for numerous applications involving transport of ionizing radiation, including ionizing photons. Moreover, the physics needed for tracking optical photons is also embedded in the Geant4 simulation toolkit. From its introduction two decades ago there have been a variety of reports concerning the use of the Geant4 code in optical photons transport applications. Further to this, in particular for the interests herein, characterizations of radioluminescence radiation detectors have also been conducted. Noting the rapid development of such detectors, reliant on light collection and detection, there has been an implicit need for optimization of the different components forming the respective optical devices. Compared to ionizing radiation transport modeling less attention has been paid to this capability among the radiation physics community and Geant4 users. This review first describes in brief terms the physics built into Geant4 to provide for the transport of optical photons. This is followed by a retrospective on reported applications, discussion of the strengths and deficiencies of optical transport analyses by Geant4 and contemplation of possible future applications.

A generative adversarial network to speed up optical Monte Carlo simulations

Detailed simulation of optical photon transport and detection in radiation detectors is often used for crystal-based gamma detector optimization. However, the time and memory burden associated with the track-wise approach to particle transport and detection in commonly used Monte Carlo codes makes optical simulation prohibitive at a system level, where hundreds to thousands of scintillators must be modeled. Consequently, current large system simulations do not include detailed detector models to analyze the potential performance gain with new radiation detector technologies. Generative adversarial networks (GANs) are explored as a tool to speed up the optical simulation of crystal-based detectors. These networks learn training datasets made of high-dimensional data distributions. Once trained, the resulting model can produce distributions belonging to the training data probability distribution. In this work, we present the proof of concept of using a GAN to enable high-fidelity optical simulations of nuclear medicine systems, mitigating their computational complexity. The architecture of the first network version and high-fidelity training dataset is discussed. The latter is generated through accurate optical simulation with GATE/Geant4, and contains the position, direction, and energy distributions of the optical photons emitted by 511 keV gamma rays in bismuth germanate and detected on the photodetector face. We compare the GAN and simulation-generated distributions in terms of similarity using the Jensen–Shannon distance. Excellent agreement was found with similarity values higher than 93.5% for all distributions. Moreover, the GAN speeded the optical photon distribution generation by up to two orders of magnitude. These very promising results have the potential to drastically change the use of nuclear imaging system optical simulations by enabling high-fidelity system-level simulations in reasonable computation times. The ultimate is to integrate the GAN within GATE/Geant4 since numerous applications (large detectors, bright scintillators, Cerenkov-based timing positron emission tomography) can benefit from these improvements.

A new optical photon transport model for application to high aspect ratio scintillation pillars

A new Monte Carlo model of optical photon transport has been developed for application to scintillation pillars with side surfaces modeled using their measured 3D topography. The model addresses deficiencies of existing models specifically pertinent to high aspect ratio scintillator geometries. Inaccuracies with these models’ description of the reflectance of scintillation photons at media boundaries are exacerbated by the high number of surface interactions in such geometries. These models rely either on (a) user-specified parameters that are challenging to set, or (b) pre-computed look-up tables that assume geometrically unconstrained photon incidence on uniform surface finishes, an assumption that is invalid in high aspect ratio geometries. Alternatively, the new model reconstructs the side surfaces of a pillar from 3D topographic measurements and uses first-principles physics of reflection and transmission to transport photons over the surfaces’ microscopic features and throughout the entire pillar volume. It takes advantage of the recent availability of open-source software capable of complex surface modeling and ray-tracing, as well as high performance computing. The model was validated against measurements of the number of photoelectrons (PEs) generated in two silicon photomultipliers (SiPMs) located at the ends of 5 mm × 5 mm × 200 mm EJ-204 scintillation pillars with different surface finishes. It was also validated against the relative amplitudes and arrival time difference of the SiPMs’ signals. The model accurately predicts the measurements as function of the scintillation position along the pillars’ length with 2% mean-absolute-percentage error (MAPE) in the number of PEs, and 16% in the relative amplitudes. The root-mean-squared error (RMSE) in the arrival time difference is 0.114 ns. The model also predicts the experimentally observed higher scintillation light collection in pillars with smoother side surfaces. A proof-of-concept code of the model is available as open source.

A validated Geant4 model of a whole-body PET scanner with four-layer DOI detectors

The purpose of this work is to develop a validated Geant4 simulation model of a whole-body prototype PET scanner constructed from the four-layer depth-of-interaction detectors developed at the National Institute of Radiological Sciences, National Institutes for Quantum and Radiological Science and Technology, Japan. The simulation model emulates the behaviour of the unique depth of interaction sensing capability of the scanner without needing to directly simulate optical photon transport in the scintillator and photodetector modules. The model was validated by evaluating and comparing performance metrics from the NEMA NU 2-2012 protocol on both the simulated and physical scanner, including spatial resolution, sensitivity, scatter fraction, noise equivalent count rates and image quality. The results show that the average sensitivities of the scanner in the field-of-view were 5.9 cps kBq−1 and 6.0 cps kBq−1 for experiment and simulation, respectively. The average spatial resolutions measured for point sources placed at several radial offsets were 5.2± 0.7 mm and 5.0± 0.8 mm FWHM for experiment and simulation, respectively. The peak NECR was 22.9 kcps at 7.4 kBq ml−1 for the experiment, while the NECR obtained via simulation was 23.3 kcps at the same activity. The scatter fractions were 44% and 41.3% for the experiment and simulation, respectively. Contrast recovery estimates performed in different regions of a simulated image quality phantom matched the experimental results with an average error of -8.7% and +3.4% for hot and cold lesions, respectively. The results demonstrate that the developed Geant4 model reliably reproduces the key NEMA NU 2-2012 performance metrics evaluated on the prototype PET scanner. A simplified version of the model is included as an advanced example in Geant4 version 10.5.

Design of a Depth of Interaction Detector Module for Use in a Magnetic Resonance Imaging System

In this study, we designed a depth of interaction (DOI) detector module for use in a magnetic resonance imaging (MRI) system. The detector uses a semiconductor photosensor that is unaffected by a magnetic field. The detector module consists of two layers of a crystal array that have different light sharing characteristics. To verify the performance of the detector module, simulations of the optical photon transport in the crystal array were performed with a DETECT2000 tool. Various combinations of the parameters, such as the reflector used and the treatment of crystal surfaces, were simulated and the characteristics were evaluated through a reconstructed flood image. In certain combinations of the simulation conditions, all of the crystal pixels could be identified along all directions. Thus, the proposed DOI detector module could extract the three-dimensional gamma ray interaction position from the two-dimensional flood image.

Optimization of Parameters for a CsI(Tl) Scintillator Detector Using GEANT4-Based Monte Carlo Simulation Including Optical Photon Transport

GEANT4-based Monte Carlo simulation has been performed for a 2"φ × 2" L cylindrical CsI(Tl) single-crystal scintillator. The simulation includes transport of incident gamma photons as well as generation and transport of optical photons in the crystal up to the coupled photodetector. An optical photon detection spectrum was generated from the number of optical photons detected at the photodetector surface per incident primary gamma photon. This spectrum is equivalent to a conventional pulse-height spectrum, apart from the fact that its abscissa is only proportional, but not equal, to the actual pulse height (volts). This has an energy resolution of about 6.5% at 662 keV arising from the light generation and transport statistics. This spectrum was compared with an experimentally recorded pulse-height spectrum (after the abscissa of both the spectra were converted to energy scale) from a gamma spectrometer based on an in-house grown 2"φ × 2" L cylindrical CsI(Tl) crystal, thereby validating the simulation code. The effects of variation of scintillation and optical properties as well as physical and surface parameters on the simulated spectrum were subsequently studied in order to identify the important parameters. The quantum efficiency of the photodetector and the matching of its spectral response with the emission spectrum of the scintillator, reflectivity of the scintillator coating material, and the absolute light yield of the scintillator were identified as the parameters that influence the pulse-height spectrum in the most dominant way. Results of simulation presented in this paper are likely to facilitate the optimization of future detector setups.

MR Compatible Digital Three-Dimensional Readout Detector using a Convergence of Cross Point and Dual-Ended Readout Methods

We designed a magnetic resonance (MR) compatible depth-encoding detector using cross point and dual-ended readout methods with wavelength-shifting (WLS) fibers. To evaluate the performance of the novel detector module, we used the DETECT2000 simulation tool to model the transport of optical photons in the crystal array. The detector is composed of two layers of crystal arrays, three layers of WLS fiber arrays, and two sensor arrays. The identification of crystal pixels was determined by the cross point readout method using a digital positioning algorithm, and the depth information in each crystal pixel was measured by the dual-ended readout method. The average pixel identification accuracy was 93.5 % (range: 84.4 %-100 %).

New DOI Detector Using a Bottom and Side Readouts with a Cross-Arranged Scintillator Array for Positron Emission Tomography

We designed a depth-encoding positron emission tomography (PET) detector by using a bottom and side readout method with a cross-arranged scintillator array. To evaluate the characteristics of the novel detector module, we used the DETECT2000 simulation tool to perform the optical photon transport in the crystal array. The detector module consists of an M (column) × N (row) cross-arranged crystal array composed of M/3 sub-arrays consisting of N × 3 crystals. The second column of the sub-array is arranged perpendicular to the first and the third columns. The crystal is optically coupled to the crystals of the other columns; however, the surfaces between the crystals in the same column are treated as reflectors. A 6 × 5 crystal array consisting of two sub-arrays was considered for proof of concept. The two multi-pixel photon counter (MPPC) arrays are coupled to the bottom and one side of the crystal array, respectively. The x-y position is determined by the bottom MPPC array, and the side MPPC array gives depth information. All pixels in the x-y plane and the z direction were clearly distinguished.

GEANT4 simulations of small cosmic ray detection modules based on scintillators and WLS bars

This paper describes the results of GEANT4 simulations for a prototype of detection module designed as a basic element of a low-cost, reconfigurable, modular mini-array for detection of cosmic rays. The basic layout of each individual module is based on a 400 cm2 scintillator tile, optically coupled to a Wavelength Shifter (WLS) bar and a Silicon Photomultiplier (SiPM) for light collection and readout. The array will include about 50 modules in its final stage, to be employed in various geometrical configurations. GEANT4 simulations of the transport of optical photons inside the detector are here reported, with the aim of estimating the key parameters contributing to the photon collection efficiency and compare different solutions.

Optical photon transport and geometry contributions to time response of scintillation detectors

The sensitivities of both time and energy resolutions of a typical scintillation detector to major optical parameters (i.e. paint reflectivity, quantum efficiency of photomultiplier tube and attenuation coefficient) have been estimated using a dedicated Monte Carlo (MC) optical photon transport (OPT) simulation code, PHOTRACK, and the OPT capabilities of general-purpose code, FLUKA. Both cylindrical and parallelepiped geometries have been considered for the scintillator cell and lightguide. The results determine the scintillation light wavelength regions that the energy and time resolution represent enhanced sensitivities to small change/uncertainty in optical parameters.

Three-dimensional cross point readout detector design for including depth information

We designed a depth-encoding positron emission tomography (PET) detector using a cross point readout method with wavelength-shifting (WLS) fibers. To evaluate the characteristics of the novel detector module and the PET system, we used the DETECT2000 to perform optical photon transport in the crystal array. The GATE was also used. The detector module is made up of four layers of scintillator arrays, the five layers of WLS fiber arrays, and two sensor arrays. The WLS fiber arrays in each layer cross each other to transport light to each sensor array. The two sensor arrays are coupled to the forward and left sides of the WLS fiber array, respectively. The identification of three-dimensional pixels was determined using a digital positioning algorithm. All pixels were well decoded, with the system resolution ranging from 2.11 mm to 2.29 mm at full width at half maximum (FWHM).

Model-based design evaluation of a compact, high-efficiency neutron scatter camera

This paper presents the model-based design and evaluation of an instrument that estimates incident neutron direction using the kinematics of neutron scattering by hydrogen-1 nuclei in an organic scintillator. The instrument design uses a single, nearly contiguous volume of organic scintillator that is internally subdivided only as necessary to create optically isolated pillars, i.e., long, narrow parallelepipeds of organic scintillator. Scintillation light emitted in a given pillar is confined to that pillar by a combination of total internal reflection and a specular reflector applied to the four sides of the pillar transverse to its long axis. The scintillation light is collected at each end of the pillar using a photodetector, e.g., a microchannel plate photomultiplier (MCP-PM) or a silicon photomultiplier (SiPM). In this optically segmented design, the (x,y) position of scintillation light emission (where the x and y coordinates are transverse to the long axis of the pillars) is estimated as the pillar’s (x,y) position in the scintillator “block”, and the z-position (the position along the pillar’s long axis) is estimated from the amplitude and relative timing of the signals produced by the photodetectors at each end of the pillar. The neutron’s incident direction and energy is estimated from the (x,y,z)-positions of two sequential neutron–proton scattering interactions in the scintillator block using elastic scatter kinematics. For proton recoils greater than 1 MeV, we show that the (x,y,z)-position of neutron–proton scattering can be estimated with < 1 cm root-mean-squared [RMS] error and the proton recoil energy can be estimated with < 50 keV RMS error by fitting the photodetectors’ response time history to models of optical photon transport within the scintillator pillars. Finally, we evaluate several alternative designs of this proposed single-volume scatter camera made of pillars of plastic scintillator (SVSC-PiPS), studying the effect of pillar dimensions, scintillator material (EJ-204, EJ-232Q and stilbene), and photodetector (MCP-PM vs. SiPM) response vs. time. We demonstrate that the most precise estimates of incident neutron direction and energy can be obtained using a combination of scintillator material with high luminosity and a photodetector with a narrow impulse response. Specifically, we conclude that an SVSC-PiPS constructed using EJ-204 (a high luminosity plastic scintillator) and an MCP-PM will produce the most precise estimates of incident neutron direction and energy.

Characterization of a cylindrical plastic β-detector with Monte Carlo simulations of optical photons

In this work we report on the Monte Carlo study performed to understand and reproduce experimental measurements of a new plastic β-detector with cylindrical geometry. Since energy deposition simulations differ from the experimental measurements for such a geometry, we show how the simulation of production and transport of optical photons does allow one to obtain the shapes of the experimental spectra. Moreover, taking into account the computational effort associated with this kind of simulation, we develop a method to convert the simulations of energy deposited into light collected, depending only on the interaction point in the detector. This method represents a useful solution when extensive simulations have to be done, as in the case of the calculation of the response function of the spectrometer in a total absorption γ-ray spectroscopy analysis.

Geant4 simulation of optical photon transport in scintillator tile with direct readout by silicon photomultiplier

The direct coupling of silicon photomultiplier to the scintillator tile is considered to be the main option for active elements of the highly granular hadron calorimeter developed for future linear collider experiments. In this study, the response of the scintillator-SiPM system to minimum ionising particles was simulated using the optical photon transport functionality available in the Geant4 package. The uniformity of response for both flat tile and tile with dimple was estimated from the simulations and compared to the experimental results obtained in the previous studies.

Development of a novel depth-of-interaction encoding method and use of light spreading in a scintillation crystal array with single-ended readout

We designed a depth-of-interaction (DOI) positron emission tomography detector by applying a simple light-spreading method in a single-layer crystal array with a single-ended photosensor. By configuring different reflector patterns along the depth of the crystal, we were able to alter the light distribution of the gamma interaction at each depth. To evaluate the characteristics of the new DOI detector, we simulated the optical photon transport in the crystal array by using DETECT2000. Three different arrays comprising triangular-, square-, and hexagonal-shaped crystals were modeled. The 511-keV gamma-ray interactions were generated for all depths, and flood images were acquired. We confirmed that DOI layers could be distinguished in the obtained images and that the number of DOI layers depends on the number of sides of the polygonal crystals.

Monte Carlo modeling of a novel depth-encoding PET detector with DETECT2000

We designed a depth-encoding positron emission tomography (PET) detector using wavelength shifting (WLS) fibers for a low-cost and high-resolution PET. In order to characterize the theoretical performance of the newly-designed PET detector, we performed a DETECT2000 simulation of the optical photon transport in the crystal array and the WLS fibers. The detector module is made of an array of individual crystals, WLS fibers, and multi-pixel photon counters (MPPCs). The lutetium oxyorthosilicate (LSO) crystal array consists of 15 × 15 crystals with a 2 mm × 2 mm surface size and three different heights: 18, 20, and 22 mm. On every other line, scintillators 2 mm shorter in height are arranged in the y-direction on the top and the x-direction on the bottom of the array to make grooves for insertion of a WLS fiber. The total size of the detector module is 30 mm × 30 mm × 22 mm. One end of each WLS fiber is coupled to MPPCs to measure scintillating light trapped in the fiber. The depth-of-interaction (DOI) resolution and pixel identification capability are determined by using a DETECT2000 simulation. All pixels of the 15 × 15 LSO array are well decoded, and the DOI resolution ranges from 1.43 mm to 2.53 mm with an average of 2.08 mm full width at half maximum (FWHM). These results indicate that the proposed detector performs respectably for high-resolution PET scanning.