Optimization of scintillator-reflector optical interfaces for the LUT Davis model

Full modulation transfer functions of thick parallel- and focused-element scintillator arrays obtained by a Monte Carlo optical transport model

BACKGROUND

Arrays of thick segmented crystalline scintillators are useful x-ray converters for image-guided radiation therapy using electronic portal imaging (EPI) and megavoltage cone-beam computed tomography (MV-CBCT). Ionizing-radiation-only simulations previously showed relatively low modulation transfer function (MTF) in parallel-element arrays because of beam divergence. Hence, a focused-element geometry (matching the beam divergence) has been proposed. The "full" (ionizing and optical) MTF performance of such a focused geometry compared to its radiation-only MTF has, however, not been fully investigated.

PURPOSE

To study the full MTF performance of such arrays in a more realistic situation in which optical characteristics are also included using an in-house detector model that supports light transport, and quantify the errors in MTF estimation when the optical stage is ignored.

METHODS

First, radiation (x-ray and electron) transport was simulated. Then, transport of the generated optical photons was modeled using ScintSim2, an optical Monte Carlo (MC) code developed in MATLAB for simulation of two-dimensional (2D) parallel- and focused-element scintillator arrays. The full-MTF responses of focused- and parallel-element geometries, for a large array of 3 × 3 mm² CsI:Tl detector elements of 10, 40, and 60 mm thicknesses, were examined. For each configuration, a composite line spread function (LSF) was calculated to obtain the MTF.

RESULTS

At the Nyquist frequency, for 10 mm-thick central elements and 60 mm-thick peripheral parallel elements, full-MTF exhibited a drop of up to 15 and 79 times, respectively, compared with radiation-only MTF. This was found to be partly attributable to the angular distribution of the light emerging from the detector-element exit face and the dependence on its aspect ratio, since the light exiting thicker scintillators exhibited a more forward-directed distribution. Focused elements provided an increase of up to nine times in peripheral-area full MTF values.

CONCLUSIONS

Full MTF was up to 79 times lower than radiation-only MTF. Focused arrays preserved full MTF by up to nine times compared to parallel elements. The differences in the results obtained with and without inclusion of optical photons emphasize the need to include light transport when optimizing thick segmented scintillation detectors. Besides their application in detector optimization for radiotherapy megavoltage photon imaging, these findings can also be useful for other segmented-scintillator-based imaging systems, for example, in nuclear medicine, or in 2D detection systems for quality assurance of MR-linacs.

The Accuracy of Cerenkov Photons Simulation in Geant4/Gate Depends on the Parameterization of Primary Electron Propagation

Energetic electrons traveling in a dispersive medium can produce Cerenkov radiation. Cerenkov photons' prompt emission, combined with their predominantly forward emission direction with respect to the parent electron, makes them extremely promising to improve radiation detector timing resolution. Triggering gamma detections based on Cerenkov photons to achieve superior timing resolution is challenging due to the low number of photons produced per interaction. Monte Carlo simulations are fundamental to understanding their behavior and optimizing their pathway to detection. Therefore, accurately modeling the electron propagation and Cerenkov photons emission is crucial for reliable simulation results. In this work, we investigated the physics characteristics of the primary electrons (velocity, energy) and those of all emitted Cerenkov photons (spatial and timing distributions) generated by 511 keV photoelectric interactions in a bismuth germanate crystal using simulations with Geant4/GATE. Geant4 uses a stepwise particle tracking approach, and users can limit the electron velocity change per step. Without limiting it (default Geant4 settings), an electron mean step length of ~250 μm was obtained, providing only macroscopic modeling of electron transport, with all Cerenkov photons emitted in the forward direction with respect to the incident gamma direction. Limiting the electron velocity change per step reduced the electron mean step length (~0.200 μm), leading to a microscopic approach to its transport which more accurately modeled the electron physical properties in BGO at 511 keV. The electron and Cerenkov photons rapidly lost directionality, affecting Cerenkov photons' transport and, ultimately, their detection. Results suggested that a deep understanding of low energy physics is crucial to perform accurate optical Monte Carlo simulations and ultimately use them in TOF PET detectors.

Integration of polarization in the LUTDavis model for optical Monte Carlo simulation in radiation detectors

OBJECTIVE

Cerenkov photons have distinctive features from scintillation photons. Among them is their polarization: their electric field is always perpendicular to the direction of propagation of light and parallel to the plane of incidence. Scintillation photons are instead considered unpolarized.

APPROACH

This study aims at understanding and optimizing the reflectance of polarized Cerenkov photons for optical Monte Carlo simulation of scintillation detectors with Geant4/GATE. First, the Cerenkov emission spectrum and polarization were implemented in the previously developed look-up-table Davis model of crystal reflectance. Next, we modified Geant4/GATE source code to account for scintillation and Cerenkov photons LUTs simultaneously. Then, we performed optical Monte Carlo simulations in BGO using GATE to show the effect of Cerenkov features on the photons' momentum at the photodetector face, using two surface finishes, with and without reflector.

MAIN RESULTS

In this work, we describe the new features added to the algorithm and GATE. We showed that Cerenkov characteristics affect their probability to be reflected/refracted and thus their travel path within a material.

SIGNIFICANCE

We showed the importance of accounting for accurate Cerenkov photons reflectance while performing advanced optical Monte Carlo simulations.