Quantitative <scp>MRI</scp> Differentiates Electromyography Severity Grades of Denervated Muscle in Neuropathy of the Brachial Plexus

Abstract

Background Quantitative MRI (qMRI) metrics reflect microstructural skeletal muscle changes secondary to denervation and may correspond to conventional electromyography (EMG) assessments of motor unit recruitment (MUR) and denervation. Hypothesis Differences in quantitative T 2 , diffusion‐based apparent fiber diameter (AFD), and fat fraction (FF) exist between EMG grades, in patients with clinically suspected neuropathy of the brachial plexus. Study Type Prospective. Population A total of 30 subjects (age = 37.5 ± 17.5, 21M/9F) with suspected brachial plexopathy. Field Strength/Sequence 3‐Tesla; qMRI using fast spin echo (T 2 ‐mapping), multi‐b‐valued diffusion‐weighted echo planar imaging (for AFD), and dual‐echo Dixon gradient echo (FF‐mapping) sequences. Assessment qMRI values were compared against EMG grades (MUR and denervation). qMRI values (T 2 , AFD, and FF) were obtained for five regional shoulder muscles. A 4‐point scale was used for MUR/denervation severity. Statistical Tests Linear mixed models and least‐squares pairwise comparisons were used to evaluate qMRI differences between EMG grades. Predictive accuracy of EMG grades from qMRI was quantified by 10‐fold cross‐validated logistic models. A P value &lt; 0.05 was considered statistically significant. Results Mean (95% confidence interval) qMRI for “full” MUR were T 2 = 39.40 msec (35.72–43.08 msec), AFD = 78.35 μm (72.52–84.19 μm), and FF = 4.54% (2.11–6.97%). Significant T 2 increases (+8.36 to +14.67 msec) and significant AFD decreases (−11.04 to −21.58 μm) were observed with all abnormal MUR grades as compared to “full” MUR. Significant changes in both T 2 and AFD were observed with increased denervation (+9.59 to +15.04 msec, −16.25 to −18.66 μm). There were significant differences in FF between some MUR grades (−1.45 to +2.96%), but no significant changes were observed with denervation ( P = 0.089–0.662). qMRI prediction of abnormal MUR or denervation was strong (mean accuracy = 0.841 and 0.810, respectively) but moderate at predicting individual grades (accuracy = 0.492 and 0.508, respectively). Data Conclusion Quantitative T 2 and AFD differences were observed between EMG grades in assessing muscle denervation. Level of Evidence 2 Technical Efficacy Stage 1

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