Metrics of system performance are used to assess the abilities and safety of x-ray imaging systems. The detective
quantum efficiency (DQE) is used as a measure of "dose efficiency" but, when applied to fluoroscopic systems,
requires a measurement of the temporal modulation transfer function (MTF) to account for the effects of system
lag. It is shown that the temporal MTF is exposure-rate dependent, and hence must be measured under the
specific exposure conditions of interest. We develop a small-signal approach to temporal MTF measurements
using a semi-transparent moving slanted edge. Using an x-ray image intensifier-based bench-top system, we show
that there is a 50% overstatement of the DQE when not properly accounting for lag. The small-signal approach
is used to calculate a lag-free fluoroscopic DQE that agrees with a radiographic DQE measurement under the
same exposure-rate conditions. It was found that the temporal MTF did not change within measured precision
over normal fluoroscopic conditions, and the radiopaque falling-edge results were consistent with the small-signal
temporal MTF. This approach could be implemented in a clinical setting with access to raw (linear or linearized)
fluoroscopic image data and could be generalized for use on pulsed-exposure systems.
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.