![]() ![]() Muscle DWI typically uses lower diffusion-encoding b-values (300–800 s/mm 2) than those used in the brain (≥800 s/mm 2), in part because of the higher mean diffusivity (~1.6 µm 2/ms) and lower anisotropy (~0.1–0.3) in muscle ( 17). However, muscle denervation studies may be either confounded by effects of edema due to transection surgery ( 4, 13, 15, 16) or biased by increased T 2 when signal-to-noise ratio (SNR) is inadequate ( 15), motivating investigating muscle DWI and T 2 in vivo in humans. Animal ( 12, 13) and human microscopy studies ( 14) have both demonstrated that muscle denervation leads to decreased muscle diameter and increased diffusion anisotropy ( 13). More specifically, decreased muscle diameter results in reduced radial diffusivity and increased fractional anisotropy (FA). Monte-Carlo simulations have shown that muscle cell size (or diameter) most strongly influences diffusion anisotropy and mean diffusivity ( 11). In muscle, diffusion imaging has demonstrated sensitivity to microstructural changes following exercise ( 8), and in the assessment of primary myopathies ( 9, 10). DWI depicts the microscopic movement of water molecules restricted by impermeable or semi-permeable tissue boundaries. Quantitative diffusion-weighted imaging (DWI) may complement T 2- and FF-mapping to assess muscle denervation ( 4). Quantitative T 2-mapping may also provide superior correlation to EMG as compared to qualitative imaging ( 7). 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. Standard, qualitative MRN uses T 2-weighted, fat suppressed sequences ( 1) that depict muscle edema during active denervation ( 2) and preceding fatty infiltration in the chronically denervated state ( 3). MRN affords the ability to simultaneously evaluate many muscles, included deep-seated ones, which may be difficult to access by EMG. Peripheral nerve MRI may be used as an adjunct to directly visualize nerves and muscle. The diagnostic workup of peripheral neuropathies includes a physical exam and frequently, needle electromyography (EMG) to qualify the degree of muscle denervation. Peripheral neuropathies may precipitate muscle denervation, whereby loss of innervation at the neuromuscular junction results in muscle weakness. Keywords: Diffusion imaging fiber diameter muscle denervation Correlation was low between AFD and T 2 (−0.395, P<0.001) and between FA and T 2 (0.359, P<0.001).Ĭonclusions: Diffusion MRI-based AFD complements T2- and FF-mapping techniques to non-invasively assess muscle denervation. Correlation between AFD and FF (−0.331, P<0.001) was low, but correlation between FA and FF was negligible (0.197, P=0.016). Results: Mean AFD was 89.7☑3.6 µm in control, 71.6☑5.3 µm in non-denervated, and 60.7☑5.9 µm in denervated muscles and were significantly different (P<0.001) in paired comparisons and in 10/12 individual muscle region comparisons. T 2- and fat fraction (FF)-mapping were acquired for comparison. Two-sample t-tests (α=0.05) between groups were performed with Holm-Bonferroni correction. Six regions of interests (ROIs) within separate shoulder muscles were selected, yielding three groups consisting 47 control (healthy), 36 non-denervated (patients), and 68 denervated (patients) muscle ROIs. In all, 18 exams of patients with muscle denervation and 8 exams of healthy subjects were performed at 3T. Simulations were performed to determine precision, bias, and optimize dictionary parameters. An orientation-invariant dictionary approach utilized a cylinder-based forward model and multi-compartment model for obtaining restricted and free fractions. Methods: A spin-echo diffusion acquisition with multi-b-valued diffusion sampling was used. It was hypothesized that AFD differences between denervated, non-denervated and control muscles would be greater than those from standard diffusion metrics. Background: To develop and evaluate a diffusion MRI-based apparent muscle fiber diameter (AFD) method in patients with muscle denervation. ![]()
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