Quantitative image quality analysis of a nonlinear spatio-temporal filter

Digital temporal and spatial filtering of fluoroscopic image sequences can be used to improve the quality of images acquired at low X-ray exposure. In this study, we characterized a nonlinear edge preserving, spatio-temporal noise reduction filter, the bidirectional multistage (BMS) median filter of Arce [1], To assess image quality, signal detection and discrimination exp eriments mere performed on stationary targets using a four-alternative forc ed-choice paradigm, A measure of detectability, d', was obtained for filter ed and unfiltered noisy image sequences at different signal amplitudes. Fil tering gave statistically significant, average d' improvements of 20% (dete ction) and 31% (discrimination). A nonprewhitening detection model modified to include the human spatio-temporal visual system contrast-sensitivity un derestimated enhancement, predicting an improvement of 6%, Pixel noise stan dard deviation, a commonly applied image quality measure, greatly overestim ated effectiveness giving 67% improvement in d'. We conclude that human tes ting is required to evaluate filter effectiveness and that human perception models must be improved to account for the spatio-temporal filtering of im age sequences.