PERCEIVED NOISE VERSUS DISPLAY NOISE IN TEMPORALLY FILTERED IMAGE SEQUENCES

Temporal noise-reduction filtering of image sequences is commonly appl ied in medical imaging and other applications, and a common assessment technique is to measure the reduction in display noise variance. Theo retically and experimentally, we demonstrate that this is inadequate b ecause it does not account for the interaction with the human observer . Using a new forced-choice method we compare detectability of low-con trast objects and find a noise level for an unfiltered sequence that g ives the same detectability as the filtered sequence. We report the eq uivalent detectability noise variance ratio, or EDVR. For a digital lo w-pass filter that reduces the bandwidth by 1/2, display noise reducti on predicts an EDVR of 0.5. The measured value averaged over three sub jects, 0.93+/-0.19, compares favorably with the 0.85 predicted from a theoretical human observer model, and both are very close to the value of 1.0 expected for no filtering. Hence, the effective, perceived noi se is relatively unchanged by temporal low-pass filtering. The computa tional observer model successfully evaluates a simple low-pass tempora l filter, and we anticipate that it can be used to predict the observe r response to other image enhancement filters. (C) 1996 SPIE and IS&T.