BACKGROUND. Gastric volumes, emptying, and motility can be measured by magnetic resonance imaging (MRI). Quantitative assessment of gastric volumes by MRI involves segmenting the stomach from surrounding structures. Subjective manual segmentation of a single time point dataset with ~100 images takes ~3 hours. Our objective was to replace this rate-limiting manual segmentation with a semi-automated process designed to foster widespread utilization of gastric MRI. METHODS. Gastric images were acquired with a 3D gradient echo MRI sequence at 5, 10, 20, and 30 minutes after ingestion of Ensure (300 mL) labeled with gadolinium in 20 healthy subjects (age 35 ± 2y, 14 women) and 29 patients with dyspepsia (age 43 ± 4y, 23 women). Gastric emptying was measured by scintigraphy in 27 patients; 8 had normal, 11 had rapid, and 8 had delayed gastric emptying. Manual and semi-automated analysis were performed with AnalyzeTM. In the semi-automated quantitative approach, coarse regions of food and air in each scan are computed automatically using histogram based multilevel thresholding. Morphological opening (erosion and dilation with 9x9x3 structuring element) and seeded region growing are used to isolate food and air. If the air extraction failed, the process was repeated after applying a local grayscale minimum filter to the volume. Finally, the volumes of food and air were computed and compared with the manual method (Fidler J, NGM 2009). Investigators processing these images were blinded to the results of the manual analysis. The agreement between volumes computed by the two methods was assessed using Lin's concordance correlation coefficient (CCC), volume differences and the Bland Altman test. RESULTS. At all time points, the volume of gastric contents (and air) measured by manual and semi-automated techniques exhibited good agreement (Table). Differences were not related to the average postprandial volumes measured by manual and semi-automated techniques (ie, Bland Altman test was not significant). The time required for semi-automated segmentation ranged from 204s (95% CI 177 232s) for images acquired postprandially at 5 minutes to 233s (95% CI 189 276) for images at 20 minutes, which is much shorter than the ~ 3 hours required for manual segmentation. CONCULSIONS. The volume of gastric contents and air on MR images can be measured by a semi-automated process with comparable accuracy and far better efficiency than a manual process. This technique should substantially enhance the utilization of gastric MRI in clinical practice and research. However, this semi-automated algorithm is customized to analyze images acquired by a specific MRI technique and may be need to be modified for images acquired by different sequences. Since the semi-automated technique relies upon a bright signal within the stomach, it cannot analyze fasting images. Comparison of Gastric Volumes Measured by Manual and Semi-automated Techniques