Room: Room 205
Purpose: The low efficiency of clinical electronic portal imaging devices (EPIDs) contributes to poor image quality. We characterize a novel, inexpensive, high efficiency scintillating glass (LKH-5) placed on the Varian AS-1200 platform. The new detector is further evaluated for megavoltage (MV) cone-beam computed tomography (CBCT).
Methods: A prototype LKH-5 detector was experimentally characterized for modulation transfer function (MTF) and noise power spectrum (NPS), via slanted slit and flood field image analysis, respectively. These metrics combined with a Monte Carlo generated estimation of the linac fluence were used to estimate the detector detective quantum efficiency (DQE). For application to low-dose MV-CBCT images, the NPS arising from electronic noise was characterized by analyzing paired dark frame images. The DQE(0) were estimated for low per-projection doses by fitting the quantum NPS to a linear trend and adding the measured electronic NPS. MV-CBCT scans were acquired at an approximate CDTIw of 2.7 cGy using both standard (GOS) and LKH-5 EPIDs. Both were compared to a kV-CBCT scan acquired from an on-board imager (OBI) with equivalent CTDIw.
Results: For the LKH-5 EPID, the measured MTF falls to 50% and a first zero at frequencies of roughly 0.25 and 0.70 cycles/mm, respectively. The NPS exhibit linearity with dose and at quantum noise limited doses, the DQE(0) approaches 10-11%. Accounting for electronic noise, the DQE(0) falls to ~80% at a per-projection dose of 10-5 MU. The contrast and resolution of MV-CBCT with the LKH-5 exceeds that of the standard EPID and approaches the quality of kV-CBCT with the same CTDIw.
Conclusion: The LKH-5 imager demonstrates reduced MTF but large improvements in NPS, resulting in DQE(0) approximately 5-10 times greater than the standard (GOS) EPID. Despite the reduced MTF, small object detectability and qualitative analysis of line-pair features is vastly improved, enhancing suitability for MV-CBCT applications.
Funding Support, Disclosures, and Conflict of Interest: This work was supported, in part, by award number R01CA188446 from the National Institutes of Health and a research grant from Varian Medical Systems, Inc.