![]() Further optimization, achieved by implementing a material-specific regularization parameter, reduced percent errors to be within ☐.5%. ![]() For tissue substitute materials in a cylindrical phantom, percentage errors of RSPs were reduced from a range of -8% to +4% to be within ☒%. While ~50% of WEPL pixels were rejected due to severe range mixing in pRG, RSPs of >90% CT voxels could still be optimized if multiple pRG projections, e.g. ![]() Tikhonov regularization was applied under the assumption that HU-converted RSPs are inaccurate, but the inaccuracy is within a few percent. Pixels in pRGs that exhibited severe proton range mixing were rejected for the optimization. RSPs of individual xCT voxels were optimized iteratively by minimizing the difference between the measured WEPLs and the calculated WEPLs by ray tracing with HU-converted RSPs. X-ray CT of the phantom was acquired and co-registered with the pRG acquisition coordinates. By rotating the phantom, multiple pRG projections were acquired at angles from 0 to 358° with an increment of 2°. Water equivalent path lengths (WEPL) in the pRG were derived. Time-resolved dose rate functions (DRF) were measured by the imager placed downstream of a test phantom consisting of tissue substitute materials. To reduce range uncertainties induced by HU-converted RSPs, this study investigates optimizing the RSP of individual voxels in xCT iteratively based on multi-projection proton radiography (pRG) acquired using a single amorphous silicon flat panel imager. In proton therapy, range uncertainties induced by the conversion from x-ray CT (xCT) Hounsfield units (HU) to relative stopping power (RSP) compromise the precision of dose delivery.
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