Assessment of Shear Wave Elastography Acoustic Output - a Simulation and Experimental Study

Abstract: Shear Wave Elastography (SWE) is a medical imaging modality which is able to measure tissue stiffness through the speed of ultrasound-induced shear waves. Pre- vious studies have reviewed the safety aspects of clinical ultrasound, highlighting the harmlessness of the technique. However, in arterial SWE the same type of investiga- tion has not been performed for all applications. The present work aimed to develop an experimental setup for the assessment of acoustic output, which is the pressure field generated by an ultrasound transducer. A second aim was to investigate the safety aspects of SWE with particular attention to arterial applications. In a first step, FOCUS platform was used to simulate and visualize the pressure and intensity distribution around the focal point in three dimensions. Two studies were performed at different focal depths: 15, 20, 25, 30, and 35 mm. In the first study the number of activated elements was kept constant and equal to 64. In the second study the f-number was constant at approximately 1.3. Push widths in three dimensions were compared at different depths, the push dimension did not change in a pronounced way when the f-number was kept constant, but it did when the number of elements was constant. An experimental setup was then developed, made of the programmable ultra- sound system Verasonics with a linear array transducer L7-4 to generate shear waves and a membrane-type hydrophone to analyze the distribution of peak positive pres- sure, peak negative pressure, mechanical index (MI), and spatial peak-time average intensity (ISP T A) at focal depth equal to 35 mm and voltage set at 90 V. The push di- mensions resulting from the hydrophone were compared to FOCUS results, showing similar values especially in x and y direction. To conclude, given MI and ISPTA below the safety thresholds of FDA regula- tions, the present work represents an additional step toward in vivo assessment of arterial stiffness by Shear Wave Elastography.