Peripheral Nerve Imaging
– by Dr. Alice Smith, Radiologist –
Peripheral nerve imaging (PNI) with MR demonstrates anatomic detail that can help in localization and treatment planning. Accurate clinical history, a thorough physical exam and electrodiagnostic testing allow for the determination of location and severity of a peripheral nerve disorder; however, they do not provide the anatomic detail necessary for precise localization. The visualization of fasicular structure is possible with MRI which exploits the differences in water content and connective structure of fascicles and perineurium compared with surrounding epineurium. (Figure 1)
Peripheral nerve imaging is capable of demonstrating normal versus abnormal nerves, and provides anatomic detail utilizing a combination of fat suppressed T2-weighted, T1- weighted and proton density images. Contrast is utilized in cases of suspected neoplastic involvement and inflammation. PNI assists in the evaluation and treatment planning of trauma, entrapment neuropathy, nerve sheath tumors, metastases and inflammatory demyelinating polyradiculoneuropathy. For example, identification of a completely normal course and caliber for a nerve under suspicion, argues against surgical treatment. In the setting of trauma, PNI is beneficial in the assessment of nerve continuity and neuroma formation. (Figure 2) In addition, identification of anatomic variants, such as those involving the sciatic nerve, plays a role in improving the safety of surgical procedures for the release of pelvic sciatic nerve entrapment.
PNI can also help differentiate radiation induced neuritis and recurrent disease after treatment. Radiation neuritis results in a uniform symmetric thickening and enhancement of fibers and trunks and T2 hyperintensity (Figure 3), whereas recurrent disease typically will demonstrate an asymmetric enhancing mass.
PNI with MRI also allows for assessment of the involved musculature. An increase in T2 signal can be seen in denervated muscles as early as 4 days post injury. Muscular volume loss and fatty infiltration indicate prolonged denervation, which is one of the important factors in negatively influencing functional outcome.
Limitations of PNI include the assessment of small nerves, and it has limited utility if imaging is performed 2 years after onset of symptoms.
To order peripheral nerve imaging, order imaging of the body part in which the suspected injured nerve is involved. For example, if the distal sciatic nerve is suspected, order a lower extremity MR and specify nerve imaging in the request. For questions, contact Alice Smith, MD at firstname.lastname@example.org.
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