A COSINE MODULATION ARTIFACT IN MODULATION TRANSFER-FUNCTION COMPUTATIONS CAUSED BY THE MISREGISTRATION OF LINE SPREAD PROFILES

Modulation transfer functions (MTFs) are used to analyze the spatial f requency transfer characteristics of medical imaging systems. By defin ition, accurate MTFs should not include the effects of image noise and they should not be aliased. Therefore, many techniques used to comput e MTFs register and average together multiple profiles to improve both signal to noise and/or eliminate aliasing. It is demonstrated that im proper registration of individual profiles can cause errors of up to 1 00% in the MTF. Computer modelling shows that a maximum allowable erro r of 2% in the MTF requires a registration precision of +/- 1/9 of a p ixel for each profile, if the profile was sampled at twice the cutoff frequency of the MTF. One suggested registration method, demonstrated with experimental magnetic resonance image data, is 1.5 times more acc urate than the minimum requirement.