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

Abstract

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.

Optical Photon Propagation Characteristics and Thickness Optimization of LaCl3:Ce and LaBr3:Ce Crystal Scintillators for Nuclear Medicine Imaging

“…A more accurate prediction of the radiation and optical photons transfer and escape in the material, without a priori knowledge of the experimental AE results would require simulation tools, where an accurate geometry is necessary and the radiation and the optical transfer properties as well as the photon absorption lengths can be more accurately considered [42][43][44][45][46][47][48]. Despite a degree of overestimation in higher energies by the utilized analytical methodology and the corresponding assumption in the optical photon exit layer, the presented method can still provide useful qualitative results in terms of comparing the efficiency of different crystalline materials at different thicknesses when excited by ionizing radiation.…”

Phosphors and Scintillators in Biomedical Imaging

“…In addition to analytical modeling, Monte Carlo methods, based on Mie light scattering effects within scattering media [85] for evaluating granular phosphors, GATE software Version 5.0 for crystals [86] or GEANT for granular screens [87], have been used [74,88,89].…”