The present invention relates to a method of quantifying a lean tissue volume comprising the steps of acquiring (10) a acquired image as a water-fat separated magnetic resonance image, wherein the acquired image comprises a water image and a fat image, providing (20) a calibrated fat image (F), providing (30) a soft tissue mask (STM) defining areas of soft tissue in the acquired image, and defining (40) a region of interest (ROI) of the acquired image. The method further comprises a step of calculating (50) a lean tissue volume (LTV) by multiplying, for each volume element in the region of interest, the soft tissue mask with the volume (V vox ) of each volume element and the result of one minus the calibrated fat image, and summarizing the products of said multiplications for all volume elements in the region of interest.
