Accuracy of Magnetic Resonance Imaging Signal Intensity Ratio Measurements in the Evaluation of Multifidus Muscle Injury and Atrophy Relative to That of Histological Examinations

Study Design. A matched-pairs animal study. Objective. To confirm the accuracy of magnetic resonance imaging (MRI) as a means of evaluating edema and fat degeneration of the multifidus muscle by comparing measurements made using MRI with those made using histological examination. Summary of Background Data. MRI is considered a reliable means of evaluating multifidus muscle edema and fat degeneration. However, it is not clear whether its results are always consistent with histological findings. Methods. Models of different degrees of multifidus injury were created at the L2-L3, L3-L4, and L4-L5 disc levels in 56 New Zealand white rabbits. These were divided into 4 groups and subjected to different processes: sham surgery, dissection and stripping of the multifidus, crushing of the muscle lasting 1 hour, and crushing of the muscle lasting 2 hours. Two rabbits per group were examined at each of the indicated points in time. Multifidus edema was assessed using fat-suppressed T2 signal intensity ratio of gross multifidus to psoas (T2R) on MRI bilaterally, wet weight and wet: dry weight ratio on the left side (edema-left), and visual edema score on the right side (edema-right). Muscle fat degeneration was detected bilaterally using the T1 signal intensity ratio of gross multifidus to psoas with MRI (T1R) and visual fat degeneration score (fat score) with histology. Results. Pearson correlation coefficient analyses showed significant correlations (P < 0.001) between left T2R and edema-left (r = 0.927), right T2R and edema-right (r = 0.868), and T1R and fat score (r = 0.804). A paired ttest demonstrated no significant differences between MRI measurements and histological changes (P = 0.999, 1.000, and 0.998). Bland-Altman plots also depicted good agreement between MRI measurements and histological changes (limits of agreement: left multifidus edema, +/- 0.75; right multifidus edema, +/- 1.01; fat degeneration, +/- 1.23). Conclusion. The MRI technique is an accuracy means of evaluating multifidus muscle injury and atrophy.