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) 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.
