PURPOSE: The aim of this study was to assess the inter-rater reproducibility and inter-method comparability of hip alpha angle measurements on magnetic resonance imaging (MRI)/magnetic resonance arthrography (MRA) and plain radiographs in patients with femoroacetabular impingement syndrome (FAIS). METHODS: A cross-sectional study of patients who were diagnosed with symptomatic FAIS underwent preoperative MRI/MRA with axial oblique and/or radial plane imaging and had preoperative radiographs with anterior-posterior (AP), 45° Dunn and 90° Dunn views. Alpha angle measurements were performed independently by two musculoskeletal radiologists. Inter-rater reproducibility and inter-method comparability between MRI/MRA images and radiographic views were assessed using the intraclass correlation coefficient (ICC) with 95% confidence interval (CI). RESULTS: Ninety-seven patients were included of whom 93 (95.8%) received axial oblique plane images and 54 (55.6%) had radial plane MRI/MRA images. Inter-rater reproducibility was excellent (ICC >
0.9) for all planes on MRI/MRA and radiographs. MRI/MRA axial oblique images had poor (ICC 0.39, 95% CI [0.09, 0.59]), moderate (ICC 0.57, 95% CI [0.18, 0.75]) and moderate (ICC 0.64, 95% CI [0.20, 0.81]) comparability with AP, 45° Dunn and 90° Dunn, respectively. MRI/MRA radial plane images had equivocal (0 included in all CIs) comparability with AP (ICC 0.66), 45° Dunn (ICC 0.35) and 90° Dunn (ICC 0.14) radiographs. On average, alpha angle measurements were significantly higher with radial images and lower with axial oblique images, when compared to all radiographic views (p <
0.05), except axial oblique versus 45° Dunn views, where angles measured on axial oblique were significantly larger. CONCLUSION: Alpha angle measurements taken on axial oblique MRI/MRA images show moderate comparability to radiographic 45° Dunn and 90° Dunn views despite negative bias to measurements taken on radiographic AP and 45° Dunn view. Larger alpha angles were appreciated on MRI/MRA radial and axial oblique views compared to radiographic views supporting the inclusion of MRI/MRA alpha angle measurements to properly identify deformity. LEVEL OF EVIDENCE: Level II.