Physical characterization and optimal magnification of a portal imaging system

One problem in radiation therapy is ensuring accurate positioning of the patient so that the prescribed dose is delivered to the diseased regions while healthy tissues are spared. Positioning is usually assessed by exposing film to the high-energy treatment beam. Unfortunately, these films exhibit poor image quality (primarily due to low subject contrast) and the development delays make film impractical to check patient positioning routinely. Therefore, we have been developing a digital video-based imaging system to replace film. The system consists of a copper plate/fluorescent screen detector, a 45° mirror, and a TV camera equipped with a large aperture lens. We have determined the signal and noise transfer properties of the imaging system by measuring its modulation transfer function and noise power spectrum and have used these values to estimate the optimal magnification for the imaging system. We have found that the optimal magnification is 2.3-2.5 when optimizing signal transfer (spatial resolution) alone; however, the optimal magnification is only 1.5-2.0 if signal-to-noise ratio transfer is considered.