Reply to “Letter to the Editor: Salivary gland ultrasound elastography requires interpretation of ‘normal’”

Shear wave elastography (SWE) is an innovative imaging technique that quantifies tissue stiffness, thus enabling objective assessment of this biomechanical feature. Since its initial application for the evaluation of liver cirrhosis, numerous research has shown that the shear wave velocity (SWV) of the liver is affected by multiple factors, including fibrosis, inflammation, congestion, and fatty change.^{1, 2} As a result, SWV is an output of the summation of all these pathological processes. This concept also applies to SWV of other organs and tissue, including the salivary gland.

Our study compared SWV in the salivary gland affected by sialolithiasis before and after treatment. A higher SWV before treatment was attributed to inflammation and swelling triggered by ductal obstruction. We appreciate Hoffman et al. for their comments, which specifically mention that fatty infiltration following chronic sialolithiasis may cause a decreased SWV, which might mask the effect of inflammation, fibrosis, and atrophy. No pathological specimen is available in our study cohort; therefore, we cannot clarify whether there was fatty infiltration of the gland in our patients. Sumi et al. found that fatty infiltration of the salivary gland affected by chronic sialolithiasis was more likely asymptomatic.³ Our study enrolled patients with symptomatic sialolithiasis seeking treatment. In addition, the mean duration of SWV measurement before and after treatment of sialolithiasis was a relatively short interval of 1.55 months. Therefore, we proposed that inflammation and swelling were the dominant factors for elevated SWV before treatment and lowering of SWV after stone removal, regardless of fatty infiltration.

Several studies explored normal SWV and Young's modulus in the normal salivary gland. However, no widely accepted single value has been established so far, since normal SWV varies by age, ultrasound machine, and technique utilized to acquire SWV.^{4, 5} Alternatively, measurement of the contralateral gland offers us an objective standard to compare SWV between normal and diseased glands and assess the degree of recovery after treatment.

SWE is an effective tool for quantitative assessment of various salivary gland diseases. Hoffman et al. reminded us to be aware of the complexity hidden behind a simple number (SWV) so that we could interpret these data correctly.

The authors declare no conflicts of interest.