US strain imaging successful in diagnosing carpal tunnel syndrome

In a recent study, ultrasound (US) strain imaging was successfully used to identify and map tissue movement in the carpal tunnel, differentiating normal from abnormal nerve activity in the wrist, including carpal tunnel syndrome (CTS).

Published this month in Radiology, the study was conducted by Yin-Yin Lao, PhD, of the National Tsing Hua University in Taiwan, and colleagues.

Both MRI and CT are commonly used imaging modes when diagnosing CTS, but both can be hindering because of high costs and potential radiation exposure (in CT). Recently, US has become a first step in the diagnosing of CTS because of its noninvasive nature and its ability to display anatomic structures at a submillimeter spatial resolution higher than 25 frames per second. It also can be used to examine the wrist tissue in both static and real time.

Strain imaging, also called elastography, is an US technology that yields strains from tracked motion and has become a tool for examining elastic properties and detecting abnormalities in tissue. Previous studies have found that the elasticity in CTS-afflicted median nerves are slightly stiffer than in healthy wrist tissue.

“The aim of our study was to determine the feasibility of a two-dimensional US strain imaging method for quantifying and mapping mechanical behaviors of the median nerve, flexor retinaculum, and flexor tendons within the carpal tunnel in normal and CTS disease states during active finger motion,” Lao and colleagues wrote.

The research team examined 10 wrists in healthy volunteers and 16 in volunteers confirmed to have CTS. Raw US signals were collected and correlated to estimate incremental displacements, from which strains were computed. Mean cumulative strain values and standard deviations of the spatially averaged cumulative strains in the wrists over the entire cycle of finger motion were compared.

Researchers found the mean cumulative strain values for patients with CTS and the volunteers were similar, but were able to use US to show the standard deviations of the cumulative strain for the shear strain of the flexor retinaculum was lower in patients with CTS than in the healthy volunteers. The axial strain of the median nerve was higher in the healthy volunteers than in the CTS patients through the US technique.

“Our study demonstrated that US strain imaging can be used to quantify and map tissue kinematics in the carpal tunnel and to differentiate abnormal from normal median nerves in the human wrist. It may therefore serve as a potentially useful diagnostic tool for CTS,” the authors wrote.