Necessary Scan Coverage of the MR Neurography

Because MR neurography sequences are relatively lengthy and motion sensitive, it is not generally feasible to screen the whole extremity from nerve root to sciatic bifurcation in one sitting. Department scheduling constraints and limited patient tolerance for prolonged MR imaging generally limit imaging time to approximately 1 hour. Therefore, it is imperative to prospectively determine the most likely level of the abnormality before initiating imaging, using all available clinical and electrophysiological information. For most of the patients, the abnormality was clinically localized using pain distribution, presence of a palpable mass, and/or motor or sensory deficits to determine the necessary coverage. Where available, electrophysiological studies, including electromyography and nerve conduction velocity studies, were used to limit the area examined. If electrophysiological studies were unavailable and the physical examination could not localize the abnormality to either the lumbosacral plexus or sciatic nerve, imaging was initiated proximally at the lumbosacral plexus and continued distally into the thigh by using multiple stations through the tibioperoneal bifurcation or until the causative abnormality was revealed.

Understanding the relevant lumbosacral plexus and sciatic nerve anatomy is crucial for the correct interpretation of MR neurographic studies. The lumbar plexus is composed of the ventral rami of L1 through L4 and is anatomically located behind the psoas muscle . A minor branch of L4 combines with the ventral ramus of L5 to form the lumbosacral cord or trunk . The lumbosacral trunk descends over the sacral ala and combines with the ventral rami of S1, S2, and S3 (and a branch of S4 to form the sacral plexus. The individual sacral plexus neural components coalesce and diverge along the ventral piriformis muscle surface, making it the key anatomic landmark for locating the sacral plexus and sciatic nerve. The sciatic nerve originates from the upper division of the sacral plexus at the inferior piriformis muscle border, and exits the pelvis through the greater sciatic foramen . The sciatic nerve gives rise to the tibial and common peroneal nerves, two anatomically and functionally distinct nerves that travel together in the thigh as the sciatic nerve. In most patients, the sciatic nerve bifurcates just above the knee into separate common peroneal and tibial nerves.