SU‐GG‐J‐156: Quantitative Analysis of Onboard SPECT Imaging Using Compact Gamma Cameras

Abstract

Purpose: Single photon emission computed tomography (SPECT) imaging on‐board radiation therapy machines may enhance radiation therapy. Use of a light‐weight, compact detector would be desirable because of maneuverability, yet is challenged by truncation and consequently reconstruction artifacts. We hypothesize that, for a sizeable volume surrounding the tumor target, and with properly chosen detector trajectories, compact detectors can provide image quality that is comparable to that of full‐size SPECT detectors. Method and Materials: On‐board SPECT imaging was computer simulated for detectors with active surface widths of 21.2 and 40.0 cm. Detector trajectories were selected such that the common volume encompassed an 8‐cm‐diameter ROI surrounding each tumor in the torso of a NCAT phantom. For deep tumors, two common volumes — interior and partially exterior — were investigated. Radiotracer distribution was modeled for 99mTc‐Sestamibi. Noise‐free and noisy projection images were generated with an analytical simulator that models non‐uniform attenuation, collimator & detector efficiency, and spatially‐varying spatial resolution. Images were reconstructed by OSEM. In reconstructed images, root mean square error and recovered tumor activity were analyzed as a function of detector width and detector trajectory. Results: Image truncation reduced overall image quality. For example, RMS error over an entire noise‐free image slice was 18 – 35% worse with the 21‐cm‐wide detector versus the larger detector. However, for regions within the common volume for both detector widths, RMS errors differed by less that 2%. Similar results were observed for noisy images. Moreover, the recovered fraction of tumor activity was comparable, except for when the smaller‐detector common volume was interior — the recovered activity was reduced by 11%. Conclusion: Preliminary results show that an 8‐cm‐diameter ROI can be reconstructed using 21 or 40‐cm‐wide detectors with comparable RMS errors and recovered tumor activities, supporting the proposal that on‐board target localization could be accomplished using compact SPECT detectors.