Purpose: To assess the accuracy of supraspinatus muscle fat fraction and atrophy measured on the Y-view compared with the newly proposed fossa-view sagittal MRI plane. Materials and Methods: This study included 84 patients (36 male; mean age, 65.1 ± 10.1 years) who underwent shoulder MRI with extended oblique sagittal T1-weighted and three-dimensional (3D) six-echo Dixon imaging between December 2020 and November 2022. The reference fat fraction was calculated by integrating voxel-wise Dixon values, while supraspinatus muscle volume was quantified using a 3D nnU-Net algorithm and normalized to the scapular volume to derive the standardized muscle index (SMI). Fat fraction and cross-sectional area were quantified on the Y-view and fossaview and compared with the reference values. Subgroup analyses were performed using fatty degeneration and retraction grades. Results: Agreement with the reference fat fraction was significantly higher for the fossa-view (intraclass correlation coefficient [ICC], 0.923) than for the Y-view (ICC, 0.822;p = 0.006). The fossa-view showed smaller deviations and narrower limits of agreement.For SMI, the Y-view (ICC, 0.782) showed higher agreement than the fossa-view (ICC, 0.694), although the difference was not statistically significant (p = 0.219). Subgroup analyses showed better Y-view performance at lower retraction grades and better fossa-view performance at higher grades, although the differences were not statistically significant (all p > 0.05). Conclusion Both planes reliably quantified the fat fraction with greater accuracy in the fossa-view. However, single-plane assessment of muscle atrophy was less reliable, underscoring the need for MRI evaluation of the entire muscle.