LAUSR

BACKGROUND Arrays of thick segmented crystalline scintillators are useful x-ray converters for image-guided radiation therapy using electronic portal imaging and cone-beam megavoltage computed tomography. 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 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 code developed in MATLAB for simulation of two-dimensional 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 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 9 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 9 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, e.g., in nuclear medicine, or in two-dimensional detection systems for quality assurance of MR-linacs. This article is protected by copyright. All rights reserved.