A moving slanted-edge method to measure the temporal modulation transfer function of fluoroscopic systems

• Published in: Medical physics (Lancaster) 2008-06, Vol.35 (6), p.2473-2484
• Main Authors: Friedman, S. N., Cunningham, I. A.
• Format: Article
• Language: English
• Subjects: CORRECTIONS; detective quantum efficiency; diagnostic radiography; edge; edge spread function; Environmental Exposure; FLUOROSCOPY; Fluoroscopy - methods; Fourier transforms; FREQUENCY RANGE; Image detection systems; IMAGE INTENSIFIERS; Image sensors; Laboratories; lag; line spread function; Linear Models; Medical image noise; Medical imaging; MODULATION; modulation transfer function; Modulation transfer functions; NOISE; optical transfer function; Phosphors; QUANTUM EFFICIENCY; RADIOLOGY AND NUCLEAR MEDICINE; Reproducibility of Results; spatiotemporal phenomena; temporal MTF; Time measurement; TRANSFER FUNCTIONS; X RADIATION; x-ray imaging; X-Rays; X‐ray detectors; X‐ray optics
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.2919724

Lag in fluoroscopic systems introduces a frame-averaging effect that reduces measurements of image noise and incorrectly inflates measurements of the detective quantum efficiency (DQE). A correction can be implemented based on measurements of the temporal modulation transfer function (MTF). We introduce a method of measuring the temporal MTF under fluoroscopic conditions using a moving slanted edge, a generalization of the slanted-edge method used to measure the (spatial) MTF, providing the temporal MTF of the entire imaging system. The method uses a single x-ray exposure, constant edge velocity, and assumes spatial and temporal blurring are separable. The method was validated on a laboratory x-ray image intensifier (XRII) system by comparison with direct measurements of the XRII optical response, showing excellent agreement over the entire frequency range tested ( ± 100 Hz ) . With proper access to linearized data and continuous fluoroscopy, this method can be implemented in a clinical setting on both XRII and flat-panel detectors. It is shown that the temporal MTF of the CsI-based validation system is a function of exposure rate. The rising-edge response showed more lag than the falling edge, and the temporal MTF decreased with decreasing exposure rate. It is suggested that a small-signal approach, in which the range of exposure rates is restricted to a linear range by using a semitransparent moving edge, would be appropriate for measuring the DQE of these systems.