2.2. Technique

Ultrasound examination was performed by an L18-4, MHZ linear-array transducer (EPIQ Elite SW 5.0.1, Ultrasound system: Philips, Bothell). A radiologist (MAB, 19 years of experience) performed all examinations, images were reviewed by neurologist (MK). All the participants were examined in the sitting position, with the participant resting his arm on the thigh, the elbow extended, slightly abducted, and the wrist supinated. The ulnar nerve was first identified at the medial side of the mid forearm (in short axis 10 cm above the wrist crease). Then the cross-sectional area (CSA) was measured in mm². For the SWE measurements, each subject was scanned 3 times with removal of the probe from the skin between measurements. To increase the reliability of the reported stiffness values, a confidence map was used to mask areas below a specific confidence level. Large amount of gel was used with light touch of the probe to decrease pressure effect on the skin. First, the ulnar nerve was identified in short axis and SWE measurements were taken, then the probe was rotated 90° to acquire longitudinal SWE measurements. In each examination, and after identifying the nerve, the probe was held stationary for 3 to 4 seconds and a 2 mm diameter region of interest (ROI) circle was placed within the hyperechoic epineurium. After viewing the color map, real-time shear wave images were recorded with color coding. The readings consisted of median elasticity, maximum elasticity, and average elasticity with standard deviation (SD) and were reported in kPa. The color scale was mapped to a 0 kPa to 200 kPa range. The spectrum of scale colors ranges from blue for softer tissues through red for stiffer tissues (Figs. ​(Figs.1,1, ​,22).

Short axis view of the ulnar nerve shear wave elastography, with confidence map on the left, color map scale on the right, for measurement of stiffness in kPa. FDP = flexor digitorum profundus muscle, FDS = flexor digitorum superficialis muscle.

Long axis view of the ulnar nerve shear wave elastography, with confidence map on the left, color map scale on the right, for measurement of stiffness in kPa.