Reliability of anterior cruciate ligament (ACL) graft signal intensity measurement on MRI: a novel method with intra- and interrater analysis over a 1-week interval

Purpose: To determine the intra- and interrater reliability of the anterior cruciate ligament (ACL) graft signal-to-noise quotient (SNQ) assessed on magnetic resonance imaging (MRI) using a standardized coronal-oblique plane and region-of-interest (ROI) measurement protocol. Material and methods: Twelve adults following ACL reconstruction underwent MRI graft assessment. Five ROIs were placed along the intra-articular portion of the ACL graft. Posterior cruciate ligament (PCL) and background ROIs were used for SNQ normalization (SNQ = [ACL signal - PCL signal]/background signal). Two radiologists independently performed two blinded measurement sessions, 1 week apart. Reliability included intraclass correlation coefficients (ICCs), 95% confidence intervals (CIs), standard error of measurement (SEM), and smallest detectable difference (SDD). Results: Intrarater ICCs for individual ROIs ranged from 0.67 to 0.90, with the highest reliability at R2 (ICC = 0.90; 95% CI: 0.70-0.97) and the lowest at R5 (ICC = 0.67; 95% CI: 0.20-0.89). Corresponding SEM values ranged from 0.31 to 1.01 and SDD from 0.87 to 2.79. Interrater ICCs ranged from 0.52 to 0.83, again highest at R2 (ICC = 0.83; 95% CI: 0.65-0.92) and lowest at R5 (ICC = 0.52; 95% CI: 0.15-0.76). Interrater SEM values ranged from 0.51 to 1.23 and SDD from 1.43 to 3.43. Averaging SNQ across all five ROIs improved reliability, yielding excellent intrarater agreement (ICC = 0.90; 95% CI: 0.67-0.97; SEM = 0.28; SDD = 0.76) and good interrater agreement (ICC = 0.85; 95% CI: 0.67-0.93; SEM = 0.34; SDD = 0.96). Conclusions: The proposed MRI-based SNQ protocol demonstrates excellent intrarater and good interrater reliability over a 1-week interval. ROI averaging meaningfully enhances reproducibility and provides actionable SDD thresholds for detecting ACL graft signal change.