Background:Optimal fluid status is an important issue in hemodialysis. Clinical evaluation of volume status and different diagnostic tools are used to determine hydration status in these patients. However, there is still no accurate method for this assessment. Purpose:To propose and evaluate relative lean water signal (LWSrel) as a water-fat MRI-based tissue hydration measurement. Study Type:Prospective. Population:A total of 16 healthy subjects (56 & PLUSMN; 6 years, 0 male) and 11 dialysis patients (60.3 +/- 12.3 years, 9 male; dialysis time per week 15 +/- 3.5 hours, dialysis duration 31.4 +/- 27.9 months). Field Strength/Sequence:A 3 T; 3D spoiled gradient echo. Assessment:LWSrel, a measurement of the water concentration of tissue, was estimated from fat-referenced MR images. Segmentations of total adipose tissue as well as thigh and calf muscles were used to measure LWSrel and tissue volumes. LWSrel was compared between healthy subjects and dialysis patients, the latter before and after dialysis. Bioimpedance-based body composition monitor over hydration (BCM OH) was also measured. Statistical Tests:T-tests were used to compare differences between the healthy subjects and dialysis patients, as well as changes between before and after dialysis. Pearson correlation was calculated between MRI and non-MRI biomarkers. A P value < 0.05 was considered statistically significant. Results:The LWSrel in adipose tissue was significantly higher in the dialysis cohort compared with the healthy cohort (246.8% +/- 60.0% vs. 100.0% +/- 10.8%) and decreased significantly after dialysis (246.8 +/- 60.0% vs. 233.8 +/- 63.4%). Thigh and calf muscle volumes also significantly decreased by 3.78% +/- 1.73% and 2.02% +/- 2.50% after dialysis. There was a significant correlation between changes in adipose tissue LWSrel and ultrafiltration volume (r = 87), as well as with BCM OH (r = 0.66). Data Conclusion:MRI-based LWSrel and tissue volume measurements are sensitive to tissue hydration changes occurring during dialysis.