MR Elastography-Based Slip Interface Imaging for Assessment of Myofascial Interface Mobility in Chronic Low Back...

Abnormal restrictions at myofascial interfaces are a key factor in chronic low back pain (CLBP) with myofascial pain syndrome (MPS), but no established imaging method exists for quantitative assessment. This pilot study evaluated the clinical feasibility of MR elastography (MRE)-based slip interface imaging (SII) for visualizing myofascial mobility in CLBP-MPS patients versus age-/sex-/BMI-matched healthy participants. A new SII biomarker, maximal normalized displacement discontinuity ( $D_{\mathrm{norm}}$ ), was introduced, and the previously developed normalized octahedral shear strain (NOSS) map was used for comparison. Preliminary assessments of vibration-frequency effects, repeatability, and group differences in inter-muscular mobility were conducted.

MRE was conducted at 30, 60, and 90 Hz to compare $D_{\mathrm{norm}}$ with NOSS for visualizing inter-muscular slip interfaces at the quadratus lumborum-erector spinae (QL-ES) and erector spinae-multifidus (ES-M) muscle boundaries at L3-L4. Repeatability of $D_{\mathrm{norm}}$ and NOSS was assessed at 30 Hz (identified as the best-performing frequency) using within-subject coefficients of variation and a linear mixed-effects model from within- and between-day scans. Clinical feasibility was then assessed via Wilcoxon rank-sum tests in six CLBP-MPS patients and matched controls.

$D_{\mathrm{norm}}$ at 30 Hz exhibited superior delineation of slip interfaces compared to higher frequencies and NOSS. $D_{\mathrm{norm}}$ demonstrated significantly lower variation and better repeatability than NOSS. Healthy participants showed continuous, well-defined slip interfaces, whereas CLBP-MPS patients displayed disrupted interfaces with significant mobility reduction at L4 ES-M.

SII with lower vibration frequency and $D_{\mathrm{norm}}$ map enhances visualization of lower back myofascial mobility and shows potential for assessing differences between healthy individuals and CLBP-MPS patients.