With recent advancements in imaging technology including the development of clinical 24 MHz/33 MHz probes,
high-resolution ultrasonography has emerged as an excellent tool for imaging of peripheral nerves.
Sonographic evaluation of superficial peripheral nerves is best performed with high-frequency probe (18 MHz and 24 MHz probes are used at our institution) whereas evaluation of deeper peripheral nerves may require a relatively lower frequency probe (ie.
14 MHz).
Ultrasound can also be used for image guidance for perineural steroid injections.
Advantages of ultrasound include that it is relatively inexpensive as well as portable and allows for dynamic imaging as well as easy comparison to the contralateral side. Ultrasound is also a valuable modality for evaluation of patients who are unable to undergo magnetic resonance imaging (MRI).
Furthermore,
ultrasonography is not limited by coil constraints or metal artifact. In one study,
ultrasonography was found to have greater sensitivity (93% vs 67%) and equal specificity (86%) to MRI in the detection of peripheral nerve pathologies.1
Disadvantages of ultrasound include the dependence on the examiner’s technical skill and knowledge of peripheral nerve anatomy.
In complex cases,
sonography may be more time consuming for the radiologists compared to MRI. Furthermore,
ultrasonography may be limited by the patient’s body habitus.
Ultrasound can be used to detect pathology of peripheral nerves such as neuritis,
nerve entrapment,
extrinsic compression,
nerve sheath tumors,
and traumatic laceration.2 Evaluation of the median nerve at the wrist for carpal tunnel syndrome and the ulnar nerve at the elbow for cubital tunnel syndrome are common indications for ultrasound.3 In the setting of acute trauma or suspected iatrogenic injury,
ultrasound can be used to assess the degree of peripheral nerve injury and thereby guide management decisions.
A study conducted on cadaveric median,
ulnar,
and radial nerves found that ultrasound detects transection with 89% sensitivity and 95% specificity.4
In addition to evaluating the major peripheral nerves such as the median,
ulnar,
and tibial nerves,
high resolution ultrasound can also be used to evaluate smaller peripheral nerve branches and even very small cutaneous nerves. Familiarity with high resolution sonographic evaluation of small peripheral nerves is becoming increasingly important for the practicing musculoskeletal radiologist.
In general,
ultrasound evaluation of peripheral nerves should include assessment of nerve size,
course,
and echogenicity.
Space-occupying lesions of the nerve or adjacent to the nerve should be evaluated with additional color Doppler imaging.
Abnormal findings of peripheral nerves that can be identified on high-resolution ultrasonography include loss of fascicular architecture,
altered echogenicity,
caliber change,
discontinuity,
extrinsic impingement,
mass lesion,
and scar encasement.