The T1w/T2w ratio is commonly used at 3T as a biomarker of pathological changes in brain's structure. However, this method faces strong B1+ inhomogeneities that induce bias in the contrast. Simulations of this bias at 3T and 7T were done using an EPG model, and reported on the anatomical contours of human brains. At 3T, only a central part of the brain provides acceptable bias in the contrast (≤5%). At 7T, more than half of the brain gives unreliable contrast values. Therefore, T1w/T2w needs to be employed with cautiousness at 3T, and might not be a reliable method at 7T.