Imaging description

Like all other soft tissue tumors, peripheral nerve sheath tumors are best imaged by MRI (Figure 80.1). There is a considerable overlap in the imaging features of schwannoma and neurofibroma. These tumors are located along nerve trunks and it is often helpful to identify the nerve of origin for these tumors. On MRI normal nerves appear as tubular structures with low signal intensity on all pulse sequences. Generally, peripheral nerve sheath tumors typically have low signal intensity on T1-weighted sequences and marked signal hyperintensity on T2-weighted images (Figure 80.2). According to Suh et al. the presumptive diagnosis of a peripheral nerve sheath tumor can be made if a soft tissue mass is found either along a peripheral nerve distribution or if it is connected to a nerve bundle at its proximal and distal end. A plexiform neurofibroma may have characteristic features on CT; it shows diffuse fusiform enlargement of a peripheral nerve and/or multiple masses along the course of a peripheral nerve. It has low attenuation compared with muscle with values ranging from 15 to 20 Hounsfield units on non-enhanced scans.

Differentiation of schwannoma from a neurofibroma is of relevance to the surgeon since during resection of a schwannoma the tumor can be separated from the parent nerve, but this is not possible with a neurofibroma. Some authors attempted to identify imaging criteria that can distinguish a schwannoma from a neurofibroma. MRI can show helpful features for the differentiation of schwannoma from neurofibroma, however no single imaging sign or combination of signs allow definitive differentiation between these two tumors. MRI signs that are described more commonly in schwannomas include: (1) fascicular appearance on T2-weighted images; (2) a thin hyperintense rim on T2-weighted images; (3) schwannomas are typically encapsulated showing a hyperintense mass on T2-weighted images surrounded with a low signal capsule; (4) schwannomas often demonstrate diffuse enhancement of the lesion on fat-suppressed T1-weighted images following gadolinium injection.