SU‐E‐T‐133: A Method to Improve the Accuracy of Optical‐CT 3D Dosimetry by Correcting for Stray Light

Purpose: Radiochromic plastics have recently emerged which can yield 3D dose information over clinical volumes in high resolution. These dosimeters require a fast, accurate read‐out, and broad beam optical‐Computed‐Tomography (optical‐CT) systems have shown promise, but can be sensitive to stray light artifacts. Here we present and evaluate a new method to correct for stray light artifacts in optical‐CT by deconvolving a measured, spatially invariant, point spread function (PSF). Methods: The correction was developed for the DLOS telecentric scanner (Duke Large field of view Optical‐CT Scanner). The PSF was constructed from a series of varying sized aperture images. The spatial invariance and robustness of the PSF is key to implementation, and was thoroughly investigated, including with/without the presence of an irradiated/unirradiated dosimeter, and across the field of view. The efficacy of the correction was evaluated both on individual projection images, containing known OD variations, and onknown 3D dose distributions. Results: The dominant source of stray light in this telecentric system was determined to originate in the imaging lens. Corrections of up to 3% required for heavily dosed dosimeters. Slight variations in PSF were observed under wide conditions, but this corresponded to small uncertainty in the correction (<1%). Accounting for stray light extended the dynamic range of the system from ∼30dB to ∼60dB giving significantly better potential for CNR. Conclusions: The method is shown to accurately account for stray light artifacts. The telecentric nature of the DLOS system brings two major advantages over non‐telecentric systems. First, the general amount of stray light in the projections is much less, due to removal through the telecentric imaging lens. Second, the remaining stray light appears to behave such that the PSF is spatially invariant and robust under a variety of conditions. The latter fact enables elegant accurate stray light correction using deconvolution.