Quantitative T2 -mapping magnetic resonance imaging for assessment of muscle motor unit recruitment patterns

Abstract

Introduction: In this study, we aimed to determine whether muscle transverse relaxation time (T₂ ) 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 T₂ -weighted, fat-suppressed sequences for severity of muscle edema-like patterns and manually placed regions of interest within muscles to obtain T₂ values from T₂ -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. T₂ -relaxation times were significantly (P < .001) increased in all EMG recruitment categories compared with control muscles. T₂ differences were not significant between EMG grades of motor unit recruitment (P = .151-.702). T₂ and EMG score concordance was acceptable (Harrell's 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). T₂ 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 T₂ MR muscle mapping is a promising tool for noninvasive evaluation of the degree of motor unit recruitment loss.

Keywords: MRI; T2 mapping; electromyography; muscle; musculoskeletal.

Figure 1:

Morphological, Dixon fat-suppressed T₂-weighted images (top row) and T₂ maps (bottom row), superimposed on proton-density weighted images, for patients with varying edema pattern severity by morphological grading. (A) 72-year-old man with recent onset right foot drop status post lumbar laminectomy two weeks prior, with mild edema pattern of the tibialis anterior muscle and normal signal intensity arising from the posterior tibialis. (B) Corresponding representative axial T₂ mapping image demonstrates elevated T₂ of the anterior tibialis. (C) 42-year-old man with weakness of shoulder abduction status post glenohumeral capsular repair surgery 1.5 years prior, with moderate edema pattern of all three deltoid muscle heads and normal signal intensity of the infraspinatus. (D) Corresponding representative axial T₂ mapping image demonstrates elevated T₂ of the deltoid. (E) 54-year-old man with spontaneous onset of severe pain followed by left shoulder abduction weakness, 8 months prior and attributed to neuralgic amyotrophy, with severe edema pattern of the infraspinatus muscle and normal signal intensity of the deltoid. (F) Corresponding representative axial T₂ mapping image demonstrates elevated T₂ of the infraspinatus.