Agreement of shear wave and strain elastography according to the size of the region of interest: a study of epidermal cysts

Purpose: This study aimed to evaluate the intra-observer agreement of ultrasound elastography according to region-of-interest (ROI) size in surgically confirmed epidermal cysts. Methods: This retrospective study included 201 surgically confirmed epidermal cysts that underwent two consecutive strain elastography and shear wave elastography (SWE) examinations. Circular ROIs were drawn in the lesion and adjacent fat to obtain the strain ratio, shear wave (SW) velocity, and SW elasticity. Lesions were stratified by ROI diameter into <2 mm (group 1) and ≥2 mm (group 2). For each parameter, the absolute intra-observer differences, intraclass correlation coefficients (ICCs), Bland-Altman bias, 95% limits of agreement (LoAs), and correlations with the ROI-to-lesion-diameter ratio (ROI ratio) were calculated. Results: The ICCs for SW velocity were good in group 2 and moderate in group 1, whereas the ICCs for strain ratio and SW elasticity were moderate in both groups. The mean difference in strain ratio correlated positively with the ROI ratio, but not with absolute ROI diameter, whereas mean differences in SW velocity and SW elasticity correlated negatively with both ROI diameter and ROI ratio. Group 2 demonstrated significantly smaller mean differences in SW velocity and SW elasticity than group 1 (velocity, 1.20±2.12 vs. 1.17±1.32 m/s, P=0.005; elasticity, 79.5±97.9 vs. 49.2±65.5 kPa, P=0.030). Bland-Altman analysis confirmed narrower LoAs in group 2 for all three parameters. Conclusion: An ROI diameter ≥2 mm reduced measurement differences in SWE of epidermal cysts, whereas a large ROI ratio increased measurement variability in strain ratio, supporting technique-specific ROI optimization.