Measuring the signal-to-noise ratio in magnetic resonance imaging: a caveat

The validity of the signal-to-noise ratio (SNR) as an objective quality measure for biomedical images has been the subject of a long-standing debate. Nevertheless, the SNR is the most popularly used measure both for assessing the quality of images and for evaluating the effectiveness of image enhancement and signal processing techniques. In this correspondence, we illustrate that under certain conditions the SNR can be changed by a nonlinear transformation, and also that it is often hard to measure objectively. Therefore, the issue is not only how well the SNR correlates with image quality as perceived by a human observer (which has been the primary subject of earlier debate), but also that SNR is questionable from a quantitative measurement point of view.