A Method to Measure the Temporal MTF to Determine the DQE of Fluoroscopy Systems

摘要

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 incorrect results due to system lag reducing measured noise power spectrum (NPS) values and artificially inflating DQE values. Methods involving the use of the system temporal modulation transfer function (MTF) have been proposed to remove this effect. However, measurements of the temporal MTF from acquired image data is problematic, as a direct measure of the system decay curve from images acquired at the image frame rate of 30 Hz is seriously undersampled. 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. Image data is acquired while an attenuating edge is translated across the detector with constant velocity. Pixel values from fluoroscopic images are mapped to a new spatiotemporal coordinate based on the distance and time from passage of the edge at that pixel. Both the presampling spatial MTF and presampling temporal MTF are determined from three images: an open image (with no edge) for normalization; an image of a stationary edge; and an image of a moving edge (of order 45 cm/s). 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 sampled at 2.5 kHz. After correction for the temporal effects of 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 presampling temporal MTF of a fluoroscopic system.