Volumetric analysis of abdominal subcutaneous and visceral adipose tissue accumulation plays a crucial role in the development of metabolic disregulation. Despite the need for a feasible method of quantifying these volumes, tissue segmentation remains an inefficient, costly, and often imprecise predominantly manual process. To address the problems inherent in manual analysis methods, a novel, semi-automated image analysis method has been developed ^[1-4]. Image acquisition takes about six minutes and requires overlapping stacks of MRI fat-water separated images, now widely available as product sequences. The purpose of this study was to demonstrate the feasibility and describe preliminary observations of applying this semi-automated tissue segmentation technique to quantify and monitor longitudinal changes in abdominal adipose tissue and thigh muscle volume in obese adults during weight loss.Obese (BMI >30 kg/m²) adults scheduled for clinical weight loss surgery were prospectively recruited to participate in this HIPAA-compliant, IRB-approved study. Research subjects underwent MR exams at a total of 2-5 time points before and after weight loss surgery. Subjects were scanned in a supine position on a 3T MR scanner (GE Signa Excite HDxt, GE Medical Systems, Milwaukee, WI) with the body coil. An axial, 3D, dual spoiled-gradient-echo, fat-water separation MRI sequence was used to acquire overlapping stacks of images from base of skull to bottom of knees. Each stack acquired in a separate breath hold. A total of 476 slices were acquired for each patient at each visit. Source images were reconstructed offline, overlapping images were discarded, and a single composite stack of water and calibrated fat images was generated for each visit. The composite image stack was segmented into tissue compartments using a novel, semi-automated, 3D, non-rigid, multi-atlas segmentation method.^[1-4] Sixteen labeled atlases were registered to the image volumes. A voting scheme was then used to combine the 16 labels into a 3D segmentation of each compartment: abdominal subcutaneous adipose tissue (ASCAT, defined from the top of the T9 vertebral body to the top of the femoral head), visceral adipose tissue (VAT, defined as the adipose tissue within the abdominal cavity, and the thigh muscles (defined as gluteus, iliacus, adductor, hamstring, quadriceps femoris and sartorius). Adipose and muscle tissue volumes were then calculated automatically. Results were summarized descriptively. Eight women and two men were enrolled. One subject was scanned twice, one three times, two four times, and six five times. Subjects had mean BMI of 41.7 kg/m² (range 36.2-49.4) initially, and of 32.8 kg/m² (range 28.3-39.4) at the time of the last visit, two weeks to 30 weeks after the initial visit. There were no technical failures in any subject at any visit. At their first visit, subjects had average ASCAT, VAT, and thigh muscle volumes of 15.91, 6.31, and 9.67 L, respectively. At the end of the study patients lost an average of 5.21 L of ASCAT, 2.16 L of VAT, and 1.14 L of thigh muscle. Relative to baseline levels, the proportional reductions were 32.7, 34.2, and 11.8%, respectively. For every unit change in BMI patients lost an average of 0.56L of ASCAT and 0.25L of VAT. As a proportion of body weight, adipose tissue volumes decreased during weight loss from 0.142 to 0.122 L/kg for ASCAT, and 0.056 to 0.046 L/kg for VAT. By comparison, as a proportion of body weight, thigh muscle volume increased from 0.086 to 0.096 L/kg. Longitudinal changes in adipose tissue compartment volumes and muscle volumes are illustrated in figures 1 and 2, respectively, in one subject scanned at 5 time points over 202 days. This study demonstrates the feasibility of applying a new semi-automated tissue segmentation technique to quantify and monitor longitudinal changes in abdominal adipose tissue and thigh muscle volume in obese adults during weight loss. Acquisition and analysis of all tissue compartments for each patient at each visit was successful, despite the technical challenges inherent in MRI of obese adults. As a proportion of body weight, adipose tissue volumes decreased during weight loss, but thigh muscle volumes increased.