Ultrashort echo-time (UTE) imaging can detect short- and ultrashort-T2 tissue components. e.g. tendons, ligaments, and cortical bone. Multi-echo UTE is used to generate the tissue attenuation maps required for quantitative MRI PET, as short T2 skull is visible on the first echo but not on the second. For accurate classification, it is assumed that these are registered. Here we show that geometric scaling issues of fat and water may be different between the 1st and 2nd echoes – and hence lead to erroneous tissue classification.