Implementation and repeatability of preclinical continuous vibration shear wave elastography

<i>Objective</i>. The biomechanical properties of tissue are of interest in preclinical cancer research where their changes can be related to treatment response. Preclinical shear wave elastography (SWE) may be used to measure the viscoelastic properties of tumours although their small size presents challenges. Here we study the repeatability of 3D shear wave speed (SWS) measurements using continuous harmonic vibrations under different conditions in a preclinical<i>in-vivo</i>tumour model.<i>Approach</i>. Subcutaneous tumours (MDA-MB-231) grown on the flank of athymic nude mice (<i>n</i>= 4) were imaged using a system comprising a research ultrasound scanner and a mechanically translated 18 MHz linear imaging probe. Shear waves were induced in the tumours by external contactors driven at three different vibration frequencies (500, 700 and 1000 Hz), in two orientations (top and side), sedated in separate sessions using injectables or breathable anaesthesia. Measurements were repeated over three consecutive days. 3D tumour volume outlines were used to determine the spatial transformation required to register sets of 3D SWS data, allowing measurement of repeatability of the 3D pattern of SWS using normalised cross correlation.<i>Main results</i>. Analysis of variance of mean SWS measurements (2-5 m s^-1) revealed significant differences between the tumours (<i>p</i>< 0.001), and vibration frequencies (<i>p</i>< 0.001). Mean SWS was not significantly affected by the choice of anaesthetic or tumour orientation. Intratumoural SWS spatial distributions showed improved day-to-day repeatability when obtained from the same tumour (+76% increase in normalised cross correlation compared to different tumours), the same orientation (+39% compared to different orientations), and when using a side orientation at 500 Hz (+18% compared to top orientations). Breathing motion with gaseous anaesthesia was found to be slower (∼1.5 s vs ∼0.5 s period) but with greater amplitude (<0.6 vs <0.3 mm) than with injectable. Side orientation was found to reduce respiratory motion amplitude. SWS measurements and their repeatability however were not significantly affected by the choice of anaesthesia, and therefore variation in breathing motion.<i>Significance</i>. SWE with continuous vibration is a repeatable and feasible technique for<i>in-vivo</i>preclinical use.