Study of Confounding Factors in Shear Wave Elastography of the Diaphragm

Changes in diaphragmatic stiffness are associated with pathological conditions, including severe pneumonia, and the application of shear wave elastography (SWE) in evaluating diaphragmatic stiffness has been studied. This study investigates confounding factors in diaphragmatic SWE to facilitate the development of a standard protocol that may be used in a clinical setting.

A total of 84 healthy volunteers were subjected to SWE to determine the shear wave velocity (SWV) and coefficient of variation (CV) of their diaphragms. The effects of transducer frequency, respiration, scanning sides, body positioning and region of interest (ROI) were investigated. 20 volunteers from the recruited volunteers were re-scanned by a second observer.

No significant differences were observed in median SWV for transducer frequencies, scanning sides and ROI locations. For respiration, median SWV was higher during inspiration than during expiration (p < 0.001). There was also a significant difference in diaphragm CV between the former and the latter (p = 0.038). The median SWV for body position was significantly higher when sitting compared with lying in the supine position (p < 0.001). Regarding CVs, these were lower for the L10-2 transducer, inspiration, left side, supine position, and at inferior ROI locations, although most were not significant except for respiration. Inter-observer reliability appeared to have a low error rate, as indicated by the intra-class correlation coefficient of 0.911.

We recommend a protocol that uses a higher frequency L18-5 transducer. Imaging should be taken in the supine position during inspiration at the right middle hemi diaphragm area.