Preliminary study on the evaluation of tissue elasticity for peripheral nerves using ultrasound elastography in healthy subjects.

It is difficult to objectively and quantitatively evaluate peripheral nerves, and no method has been established for this purpose.

In this study, we measured the Cross-Sectional Area (CSA) of the ulnar nerve using Real-time Tissue Elastography (RTE), a technique that can quantitatively evaluate tissue stiffness using the Strain Ratio (SR) of the tissue, which is classified as a type of Strain Elastography (SE). We measured CSA of the ulnar nerve and examined SR as an index of ulnar nerve stiffness in 30 healthy adults (15 males and 15 females). Intra-examiner reliability (relative reliability [ICC] and absolute reliability) was examined using the ulnar nerve at the level of elbow canal as the measurement site, and SR and CSA were compared at 0° elbow extension and 90° flexion. Tissue elasticity was quantified by calculating SR of the ulnar nerve as ratio (B /A) of the strain value (A) of nerve to strain value (B) of the acoustic coupler, a hardness reference material with constant Young's modulus．

The ICC (1, 3) of SR for each limb position ranged from 0.78 to 0.94, and the ICC (1, 3) of CSA was 0.99. Neither additive nor proportional errors were observed for the two limbs. The sensitivity of the measurement to true change was examined using MDC₉₅, the 95% confidence interval of minimal detectable change (MDC). The SR of the ulnar nerve was significantly higher at 90° flexion of the elbow than at 0° in both men and women. On the other hand, the CSA of the ulnar nerve was 6.19 ± 2.72 mm² at 90° flexion compared with 6.58 ± 2.76 mm² at 0° elbow in males, and was significantly lower at 90° flexion than at 0° elbow. In women, the values were 6.90 ± 2.47 mm² at 0° elbow and 6.49 ± 2.47 mm² at 90° flexion, which were significantly smaller at 90° elbow flexion than at 0° elbow flexion.

We quantitatively evaluated the tissue elasticity of the ulnar nerve in two different elbow joint limb positions and found that the ulnar nerve was significantly stiffened by elbow flexion in both men and women.

These results suggest that the relative and absolute reliability of the stiffness evaluation using RTE as an index of tissue elasticity of the ulnar nerve is high, and that this evaluation method can be clinically applied to study subjects with functional impairment in the future.