Comparison of baseline drifts using three reflector blocks versus using a single reflector block for the calibration of wall-mounted Respiratory Gating for Scanner (RGSC) camera integrated with a CT.

Abstract

The calibration of the Respiratory Gating for SCanner (RGSC) system is critical to achieve better and more stable accuracy. The current procedure for a wall-mounted RGSC system has a relatively large residual error. To compare the baseline drifts in the image acquisition of DIBH using three reflector blocks versus using a single reflector block in the calibration of a wall-mounted RGSC camera system. Varian provides a calibration plate with three rows of calibration points: each row is separated by 15cm longitudinally and by 10cm laterally. In Varian's single-block calibration method, the reflector block was first placed on the center point of the calibration plate and aligned with the scanner isocenter. The calibration took a picture of the block, then placed the block on the other eight points sequentially. In the proposed three-block method, we placed three reflector blocks on the center row, with the center block aligned with the isocenter, and we took a picture of the center block by manually blocking the other two blocks in calibration. By moving the couch longitudinally in or out 15cm, the calibration goes through all nine points. Monte Carlo simulation was done using Matlab to analyze the calibration matrix eigenvalue characteristics. For a typical scan length of 40cm of DIBH, the residual baseline drift in simulated DIBH is 0.02±0.03 versus 0.30±0.12cm for three-block calibration and single-block calibration, respectively. To achieve 0.5mm tolerance for the eigenvalue, the laser and reflector box should be within ±3mm uncertainties based on the eigenvalue simulation. Three-block calibration method effectively removes baseline drift caused by couch movement in DIBH/4D CT scan for the wall-mounted camera while the single-block calibration method still has significant residual baseline drift.