Slit design for efficient and accurate MTF measurement at megavoltage x-ray energies

Empirical determination of the modulation transfer function (MTF) for analog and digital megavoltage x-ray imagers is a challenging task. The most common method used to determine MTF at megavoltage x-ray energies employs a long, narrow slit formed by two parallel, metal blocks in order to form a "slit beam." In this work, a detailed overview of some of the important considerations of slit design is presented. Based on these considerations, a novel, compact slit, using 19 cm thick tungsten blocks, was designed. The prototype slit was configured to attach to the accessory slot of the gantry of a linear accelerator, which greatly simplified the measurement process. Measurements were performed to determine the presampling MTF at 6 MV for an indirect detection active matrix flat panel imager prototype previously developed for megavoltage imaging applications. In addition, the effects of two important slit design parameters, material type and thickness, on the accuracy of MTF determination were investigated via a Monte Carlo-based theoretical study. Empirically determined MTFs obtained from the prototype slit closely match those from an earlier, less compact slit design based on 40 cm thick steel blocks. The results of the Monte Carlo-based theoretical studies indicate that the prototype slit achieves close-to-ideal performance in terms of accurately determining the MTF by virtue of practically 100% beam attenuation in regions other than the slit gap. Furthermore, the theoretical results suggest that it may be possible to achieve even further reductions in slit thickness without compromising measurement accuracy.