Role of Skeletal Muscle MRI in Peripheral Nerve Disorders

In diagnosing peripheral nerve disorders, the involved nerves can usually be determined based on clinical history and neurological findings with the aid of electrophysiological examinations. Despite the principle, we often encounter diagnostic challenges. In this chapter, we describe the clinical utility of magnetic resonance imaging (MRI) for the evaluation of peripheral nerve disorders. MRI can visualize pathological changes in skeletal muscles secondary to lesions of the peripheral nerve, plexus or nerve root. The lesion sites may be inferred based on the distribution of the involved muscles. After the first report in 1987 (Shabas et al., 1987), MRI has increasingly been used to evaluate denervated muscles (West et al., 1994; Fleckenstein et al., 1993; Uetani et al., 1993). In particular, studies of entrapment or compressive neuropathy have greatly contributed to the understanding of clinical-radiological correlations in peripheral nerve damage. Animal experiments have also been conducted, in which muscle MRI was examined after peripheral nerve transection. MRI has several distinct advantages over needle electromyography (EMG), including noninvasiveness, accessibility to deep muscles and interexaminer reliability (Koltzenburg and Bendszus, 2004; Bendszus et al., 2003; McDonald et al., 2000). MRI is particularly useful as needle EMG is difficult to perform on children or patients on anticoagulation. Excellent spatial resolution allows MRI to detect atrophy of the small muscles, moreover, different MRI pulse sequences show sensitivity to different stages of denervation, thus, MRI can provide valuable information about the duration of muscle denervation (Kamath et al., 2008). MRI has a potential to visualize mass lesions causing nerve damage, such as tumours, which is useful for the clinical judgment of surgical resectability (Grant et al., 2002). An abnormal MR signal in muscles is not specific to denervation and may also be seen in any condition that causes muscle edema, including severe muscle strains, blunt trauma and acute myositis. Thus, MRI findings need to be interpreted in combination with other clinical information. Previous muscle MRI studies of peripheral nerve disorders have mostly focused on entrapment or compression neuropathy (Andreisek et al., 2006; Petchprapa et al., 2010; Donovan et al., 2010). However, given its capability in visualizing pathological changes and mapping the distributions of the involved muscles, the use of MRI can be extended to a variety of peripheral nerve disorders. We will give a theoretical background of muscle MRI and describe its clinical applications in peripheral nerve disorders with some representative cases. We will also mention non-muscular features of MRI, e.g. nerve signal