Echotexture and Correlated Histologic Analysis of Peripheral Nerves Important in Regional Anesthesia

Background and Objectives: Peripheral nerves in different body locations display different echotextures on ultrasound imaging, and knowledge of peripheral nerve echotexture is helpful for locating target nerves. However, the degree of echogenicity is often difficult to characterize. We aimed to define objectively the degree of echogenicity of peripheral nerves using grayscale measurements and compare nerve echotexture with matched histologic samples. Methods: Ultrasound images of peripheral nerves in 12 body locations were obtained in 20 healthy subjects using linear 8- to 12-MHz and curved 3- to 5-MHZ transducers. Corresponding nerve segments from 2 cadavers were imaged in vitro before they were sectioned for histologic examination. Nerve echogenicity was assessed by an objective grayscale (G) and a subjective echogenicity index (SEI) determined by experienced evaluators. The results of G and SEI in selected peripheral nerves were compared and correlated with histologic morphometry. Results: There is a close correlation between SEI and G (P < 0.05). Mixed echogenicity was seen in 30% of the peripheral nerves; 25.4% were predominantly hypoechogenic, and 44.5% hyperechogenic. Nerves in the neck and upper arm are more frequently hypoechoic, whereas those in the leg are more frequently hyperechoic. Histologically, differences in echogenicity are dependent on fascicle diameter and on nerve fascicular pattern, that is, differing ratios of fascicle number to total nerve area. Conclusions: This study suggests that grayscales can be used to objectively determine echogenicity and shows that grayscale measurements match well with subjective visual grading. Histologic analysis showed that both ratio of total fascicular area to whole nerve area and fascicular pattern are important determinants of echogenicity.