MRI T2 Relaxation Time of the Intervertebral Disc: Repeatability, the Effect of Time of Day, and Unloaded Rest

Abstract

Background

T2 relaxation time is a widely used quantitative MRI measure of intervertebral disc degeneration, correlated with water and biochemical composition. T2 time is dependent on many factors, including scan parameters, signal decay model, and region(s) of interest, such that quantification of T2 time repeatability is necessary for interpreting T2 study outcomes. Further, the sensitivity of T2 time to factors such as time of day and unloaded rest has not been sufficiently quantified. The objective of this study was to evaluate repeatability, as well as time of day and unloaded rest effects on lumbar disc T2 relaxation time.

Methods

Young asymptomatic participants underwent MRI. Part 1 assessed intra-rater, inter-rater, and inter-scan Repeatability. Part 2 evaluated effects of Time of Day across four time points spanning 8 am to 5 pm, and 45 min unloaded Rest. T2 time was evaluated using a noise-corrected exponential decay model for a circular region of interest (ROI) in the nucleus pulposus (NP) and for 5 regions across a mid-sagittal line spanning the annulus fibrosus (AF), NP, and transition regions.

Results

Intra- and inter-rater repeatability was good-excellent (ICC = 0.76–0.99) across all regions. Inter-scan repeatability was good-excellent (ICC = 0.89–0.93) for the NP, while the AF had moderate-poor repeatability (ICC = 0.17–0.53). The minimal detectable change (MDC) for the NP ROI was 31 ms. Time of Day and Rest did not significantly affect NP T2 time. Across regions, all measured changes for Time of Day and Rest were less than corresponding MDCs.

Conclusions

T2 time variability is primarily driven by acquisition factors, not rater subjectivity. Time of Day and Rest T2 time changes did not exceed within-subject variability (by MDC) and therefore are not critical considerations for some study designs. The reported MDCs provide practical thresholds for interpreting individual T2 time differences.