Quantitative <scp>T₂</scp>‐mapping magnetic resonance imaging for assessment of muscle motor unit recruitment patterns

Abstract

Abstract Introduction In this study, we aimed to determine whether muscle transverse relaxation time (T 2 ) magnetic resonance (MR) mapping results correlate with motor unit loss, as defined by motor unit recruitment patterns on electromyography (EMG). Methods EMG and 3‐Tesla MRI exams were acquired no more than 31 days apart in subjects referred for peripheral nerve MRI. Two musculoskeletal radiologists qualitatively graded T 2 ‐weighted, fat‐suppressed sequences for severity of muscle edema‐like patterns and manually placed regions of interest within muscles to obtain T 2 values from T 2 ‐mapping sequences. Concordance was calculated between qualitative and quantitative MR grades and EMG recruitment categories (none, discrete, decreased) as well as interobserver agreement for both MR grades. Results Thirty‐four muscles (21 abnormal, 13 control) were assessed in 13 subjects (5 females and 8 males; mean age, 46 years) with 14 EMG‐MRI pairs. T 2 ‐relaxation times were significantly ( P &lt; .001) increased in all EMG recruitment categories compared with control muscles. T 2 differences were not significant between EMG grades of motor unit recruitment ( P = .151‐.702). T 2 and EMG score concordance was acceptable (Harrellʼs concordance index [c index]: rater A, 0.71; 95% confidence interval [CI], 0.51‐0.87; rater B, 0.77; 95% CI, 0.57‐0.91). Qualitative MRI and EMG score concordance was poor to acceptable (c index: rater A, 0.60; 95% CI, 0.50‐0.79; rater B, 0.72; 95% CI, 0.55‐0.89). T 2 values had moderate‐to‐substantial ability to distinguish between absent vs incomplete (ie, decreased or discrete) motor unit recruitment (c index: rater A, 0.78; 95% CI, 0.50‐1.00; rater B, 0.86; 95% CI, 0.57‐1.00). Discussion Quantitative T 2 MR muscle mapping is a promising tool for noninvasive evaluation of the degree of motor unit recruitment loss.

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