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 (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). 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.

Quantitative MRI Differentiates Electromyography Severity Grades of Denervated Muscle in Neuropathy of the Brachial Plexus

“…Specifically, T 2 increases and AFD decreases were observed to correlate with lower MUR, with most comparisons between grades shown to be significant. While the current T 2 results parallel previous findings, 15 AFD and FF may provide additional characterization of atrophy and fat infiltration. Also, the bias corrections for T 2 and diffusion used here obviated the need for a normal muscle reference that may be unavailable for analysis.…”

“…MRI may be sensitive to early muscle denervation; following sciatic nerve transection in rats, T 2 value changes in muscle have been detected as early as 24 hours, 13 and similar T 2 w muscle signal changes have also been observed as early as 4 days following peripheral nerve injury in humans 14 . In a preliminary study of subjects with muscle denervation, quantitative T 2 ‐mapping provided superior ability to distinguish “none” vs. “decreased” or “discrete” MUR grades 15 as compared to qualitative T 2 w imaging, using adjacent normal muscle T 2 as a reference. To detect muscle fatty infiltration observed in chronic neuropathies, 14 low signal on fat‐suppressed images may be utilized; quantitatively, elevated fat fraction (FF) in fat‐water imaging more commonly used in imaging of myopathies 16 may also apply to imaging chronic muscle denervation.…”

Diffusion MRI fiber diameter for muscle denervation assessment

“…To better quantify muscle changes, quantitative techniques such as T 2 -(4,5) and fat fraction (FF)-mapping (5,6) have been employed to assess the extent of extracellular edema and fatty infiltration, respectively. Quantitative T 2 -mapping may also provide superior correlation to EMG as compared to qualitative imaging (7).…”

Brachial Plexus Magnetic Resonance Neurography

“…68 To complement nerve assessment of muscle denervation, quantitative muscle MRI assessments of the shoulder muscles may be additionally performed during BP MRN; in particular, T2-mapping and diffusion-based muscle diameter metrics may help differentiate muscle denervation severity. [69][70][71] CONCLUSION Brachial plexus MRN is challenging, but by scanning unilaterally at 3.0 T, selecting the appropriate type and placement of surface coils, incorporating motion artifact reduction and vascular suppression techniques, and optimizing contrast and fat suppression parameters, image quality can be significantly enhanced. Improved image quality increases diagnostic confidence and thereby influences decision making for patient management.…”