Paraspinal muscle fat infiltration has been related with low back pain. This measurements are typically evaluated using T2w images, however, the accuracy of this method needs a proper validation, since inhomogeneities may produce severe signal changes. In this work, we developed and validated an OsiriX plugin which allows to segment infiltrated fat in T2w images. This tool also allowed us for validating the use of T2w images, considering Dixon fat images as gold-standard. To validate our plugin, we evaluated 5 cross sectional areas (L1-S1) of 4 paraspinal muscle groups for T2w images of 37 patients. To validate T2w images, we analyzed 10 healthy volunteers and 10 patients. We found that T2w segmentation with our OsiriX plugin is a reliable and an accurate method to evaluate the fat infiltration in paraspinal muscles.