Grading lumbar disc degeneration: a comparison between low- and high-field MRI.

BACKGROUND: More advanced disc degeneration on magnetic resonance imaging (MRI) is found in individuals with low back pain. However, it is unclear whether this grading is independent of the scanner's field strength.

PURPOSE: To compare disc degeneration on high- versus low-field MRI.

MATERIAL AND METHODS: Low back pain patients were enrolled to undergo high-field (3 T) MRI, followed by low-field (0.25 T) MRI of the lumbar spine within 3 h. Three radiologists graded the disc degeneration on Pfirrmann's grading scale with a hiatus of 3 months. A subsample was regraded 6 months later. Reproducibility was measured by weighted kappa statistics (using PROC FREQ statement with AGREE in the TABLES statement for SAS), absolute agreement (i.e. 1:1 agreement/the total number) and the difference in the prevalence (McNemar test).

RESULTS: Moderate to substantial agreement (κ = 0.52-0.62) and absolute agreement of 43.8-66.1% were found between field strengths. Low-field MRI tended to have numerically higher and lower grades than high-field MRI resulting in a significant difference in the prevalence of grades ( p < 0.001). Both field strengths resulted in a moderate to substantial inter-reader agreement (low-field: κ = 0.63, 0.63, 0.54 and high-field: κ = 0.55, 0.43, 0.53) and intra-reader agreement (high-field: κ = 0.57, 0.77, 0.67 and low-field: κ = 0.51, 0.50, 0.70). Only, the reader with the shortest experience had better agreement with high-field compared to low-field.

CONCLUSIONS: There were a significant difference in the prevalence of disc degeneration grading between 0.25 T and 3 T MRI. Therefore, field strength should be taken into consideration when comparing studies using disc degeneration grading as an outcome.