High quantum efficiency segmented detectors for megavoltage x -ray imaging using indirect detection active matrix flat panel imagers.

Abstract

Electronic portal imaging devices (EPIDs) based on indirect-detection, active matrix flat panel imagers (AMFPIs) have become the technology of choice for geometric verification of patient localization and dose delivery in external beam radiotherapy. However, the imaging performance of current AMFPI EPIDs is severely limited by the fact that these devices are based on relatively thin, powdered-phosphor screens that make use of only ∼2% of the incident radiation. This work reports on the investigation of a highly efficient form of EPID based on a 2D matrix of thick, optically-isolated, scintillating elements that are dimensionally matched to the pixels of an underlying active matrix array. Two types of such segmented detector designs were explored, segmented phosphors and segmented crystals. Theoretical calculations based on cascaded systems modeling and Monte Carlo simulations were performed to estimate the upper limits of imaging performance, as quantified by the frequency-dependent detective quantum efficiency (DQE), for a variety of detector configurations. Engineering prototypes of segmented phosphor and crystal detectors were fabricated and empirical characterization of imagers incorporating these detectors was performed to determine x-ray sensitivity, modulation transfer function, noise power spectrum, and DQE under radiotherapy imaging conditions. Segmented phosphor-based imagers achieved ∼3 times higher x-ray quantum efficiency compared to a conventional EPID, while exhibiting comparable MTF and zero-frequency DQE. However, the DQE performance of these prototype imagers at higher spatial frequencies was significantly lower than the conventional imager. In the case of segmented crystals, a prototype based on a 40 mm thick CsI(Tl) detector, corresponding to a quantum efficiency of ∼55%, exhibited significantly superior DQE compared to the conventional imager across all spatial frequencies, with a zero-frequency DQE of ∼22% (compared to ∼1% for the conventional imager). Furthermore, Monte Carlo-based theoretical calculations indicate that, with further optimization, segmented crystal-based imagers could achieve DQE values up to 50%. It is anticipated that the realization of such very high-DQE megavoltage imagers would enable the visualization of soft tissue structures at very low doses in megavoltage tomographic and projection imaging---a highly desirable goal for modern, image-guided radiotherapy.