Diffusion imaging may provide intracellular muscle denervation characterization complimentary to T2- and fat fraction (FF) mapping. Non-invasive muscle diameter assessment was performed using multi-shell diffusion MRI with a cylindrical model. The apparent fiber diameter (AFD) was obtained using a dictionary method mapping restricted and free fractions to AFD. 16 subjects with suspected muscle denervation involving the brachial plexus and 4 healthy control subjects were imaged. The denervated AFD was smaller than non-denervated and healthy controls by 11 µm and 23 µm respectively. AFD in healthy controls were 67-101 µm, which compare well against histology values from the literature (50-90 µm).