The diagnostic work-up of peripheral neuropathies has traditionally relied on the patient’s clinical history, physical examination and electrophysiological studies, which are unable to display the anatomic detail needed for precise localization and treatment planning. Technical advances are rapidly changing the clinical and instrumental approach to peripheral nerve diseases and radiological techniques such as Nerve Ultrasound and Magnetic Resonance Neurography are increasingly used in the investigation up of peripheral neuropathies as complementary tools to neurophysiology. In conventional MRI studies peripheral nerves are poorly visualized due to low contrast resolution between nerves, muscles and vessels, signal intensity variability, pulsatility artifacts and small size. These limitations have been overcome by the development of Magnetic Resonance Neurography (MRN), which amplifies the difference in the signal intensity of intact nerves relative to that of muscles. MRN is a tissue-selective imaging technique, based on T2-weighted sequences with fat suppression, acquired with surface coils and small field of view and directed at identifying and evaluating specific characteristics of nerve morphology, such as internal fascicular pattern, longitudinal variations in signal intensity and caliber, and connections and relations to other nerves or plexuses. Three-dimensional (3D) MRN represents a further refinement of conventional MRN, conferring the advantage of generating oblique and curved-planar reformations of nerve roots, peripheral nerves and plexuses. Investigation of peripheral nerves with 3T MR units offers some advantages compared with 1.5T due to the improved signal-to-noise ratio (SNR), which allows for thinner slices, greater contrast, better spatial resolution and reduced scanning time, although some technical issues should be considered in order to obtain good quality examinations.
In MRN studies normal nerves are identifiable as rounded or ovoid structures on axial images, they are typically isointense to slightly hyperintense on T2-weighted sequences, depending on the size of the nerve, on the amount of endoneurial fluid and degree of fat suppression. Diseased nerves become hyperintense relative to muscles, and look focally or globally enlarged.
MRN has been reported to be effective on the diagnostic workup of traumatic nerve injuries, nerve entrapment syndromes and nerve tumours. More recently MRN has been proposed for the evaluation of hereditary and immune mediated disorders of peripheral nerves.
Diffusion Tensor Imaging (DTI) is a novel technique which has been recently applied to the investigation of peripheral nerve disorders. The technique is sensitive to subtle changes in tissue microstructure and enables measurement of nerve integrity based on quantitative parameters such as Fractional Anisotropy (FA) Apparent Diffusion Coefficient (ADC), Mean (MD), Axial (AD) and Radial Diffusivity (RD). DTI has been extensively applied to examine the median nerve at the carpal tunnel and more recently to assess brachial, lumbar plexus and sciatic nerves, although its overall diagnostic value in clinical routine is still to be ascertained.
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