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A Piecewise-Focused Pixelated Detector for MV Imaging

J Star-Lack

J Star-Lack1*, D Shedlock1, E Abel1, M Sun1, M Shin2, R Fahrig2, G Virshup7, (1) Varian Medical Systems, Palo Alto, CA, (2) Stanford University, Stanford, California,

TH-A-141-10 Thursday 8:00AM - 9:55AM Room: 141

Purpose:For portal imaging, high DQE detectors can be constructed from thick pixelated scintillator arrays that absorb MV x-rays. However, due to beam divergence, MTF and DQE losses can be significant for off-axis elements not focused towards the source. We present a novel focusing approach based on situating a shaped fiber optic plate (FOP) between rectilinear scintillator arrays and an amorphous silicon flat panel imager (AMFPI).

Methods:The entire FOP comprises seven wedge-shaped sections that are fused together so that the center of each section points towards the source focal spot which is located 1500 mm away. The arc-shaped FOP directs light from the scintillator assembly to the AMFPI. The scintillator assembly consists of seven identical rectilinear sub-arrays, each with a 1 degree bevel, that are close-packed end-to-end. Each 15mm thick CdWO4 sub-array comprises 66x66 elements with a pixel pitch of 0.784 mm resulting in a piecewise-focused area detector having dimensions 365 mm x 52 mm.

Results:Monte Carlo simulations of radiative and optical transport with a 6MV source predict a DQE(0) of 23%. With no beam divergence, MTF and DQE values at a spatial frequency of 0.4mm⁻¹ are 0.5 and 15% respectively. Maximum off-axis MTF and DQE losses occur at the subarray edges (divergence angle =1 degree) and are only 6% and 12% respectively at 0.4mm⁻¹. If the detector were not focused, MTF and DQE losses at 0.4mm⁻¹ would be 30% and 85% respectively. These losses would occur at edge of the detector where the beam divergence is 7 degrees.

Conclusion: A novel approach to creating a focused detector for MV portal, cone-beam or helical CT imaging is presented. Compared to previously proposed designs, all the block arrays are identical and rectilinear scintillator thus reducing costs and simplifying manufacturing processes. Assembly and experimental measurements are underway.

Funding Support, Disclosures, and Conflict of Interest: NIH Academic-Industrial Partnership NIH RO1 CA138426; Varian Medical Systems

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