Modulation transfer functions (MTFs) are used to analyze the spatial frequency transfer characteristics of medical imaging systems. By definition, accurate MTFs should not include the effects of image noise and they should not be aliased. Therefore, many techniques used to compute MTFs register and average together multiple profiles to improve both signal to noise and/or eliminate aliasing. It is demonstrated that improper registration of individual profiles can cause errors of up to 100% in the MTF. Computer modelling shows that a maximum allowable error of 2% in the MTF requires a registration precision of +/- 1/9 of a pixel 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 accurate than the minimum requirement.
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