A method to measure the temporal MTF to determine the DQE of fluoroscopy systems

Abstract

Fluoroscopic procedures can result in significant radiation exposures to patients. To maximize the patient benefit-to-risk ratio, systems must be designed to produce the highest possible image quality for a given patient exposure, and quality assurance programs must be designed to ensure these standards are maintained. While the detective quantum efficiency (DQE) is often used in radiography to quantify "dose efficiency," attempts to measure the DQE of fluoroscopic systems have produced nonsensical results due to system lag reducing measured noise power spectrum (NPS) values. Methods involving the use of the system temporal modulation transfer function (MTF) have been proposed to remove this effect. However, these methods are not easily implemented in a clinical setting and as a result, the DQE of fluoroscopic systems is rarely measured. We have developed a novel method to measure system temporal MTF using a moving slanted-edge method and acquiring image data while the edge is translated across the detector with constant velocity. Each pixel from a video frame is mapped to a spatiotemporal coordinate based on the distance and time from passage of the edge at that pixel. Using data acquired with both stationary and moving (45 cm/s) edges, we calculate both the spatial and temporal presampling MTF. The method has been demonstrated using a bench-top image-intensifier-based fluoroscopic system using detector exposures representative of clinical procedures. Image data was acquired by digitizing the fluoroscopic video signal. The method was validated by comparison with a direct measure of the optical decay curve of the image intensifier. After correction for the temporal effects of the video integration time, excellent agreement was obtained between the two methods. It is concluded that the moving slanted-edge method provides a practical method for measuring the temporal presampling MTF of a fluoroscopic system.