RF ultrasound based estimation of pulsatile flow induced microdisplacements in phantom

Abstract

Mechanical stimulus is key component to estimate tissue stiffness. Few techniques have been developed to induce external mechanical stimulus into tissues. We hypothesize that the natural tissue motion due to cardiovascular activity could be employed for this purpose and with decrease of tissue stiffness increase their motion amplitude. The assessment of elastic sub-millimeter tissue displacements is one of the leading developments for ultrasonic characterization of tissue stiffness. The objective of this study was to investigate the feasibility to parametrize the phantom material response to pulsatile flow. The displacements were evaluated in tissue-mimicking phantoms with known stiffness. The two agar phantoms, having vessel imitating channel with controlled pulsatile flow inside, were manufactured (agar concentrations 6 and 3 g/l in distilled water, predicted Young modulus was 10 and 7 kPa respectively). The pulse water flow in channel was produced by centrifugal pump MultiFlow (Gampt) with period of 1 s. The length of channel was 19 cm embedded in the tissue mimicking agarose gel. Linear array transducer L14-5 (5–14 MHz) driven by scanner SonixTouch (Ultrasonix) was used for the echoscopy of phantom and ultrasound (US) radiofrequency (RF) data acquisition. The collected beam formed B-mode RF data (120 fps) were used for the displacements estimation applying phase-correlation and sub-sample techniques. The pulsation of channel diameter and displacements of material were estimated at a few distances from channel border in all phantoms. The pulsation of diameter and displacements of material were parametrized extracting double amplitudes. Amplitudes of displacements of material were normalized respective to pulsation amplitude of channel diameter. The relation of amplitude parameters with concentration of agar was evaluated. It was determined that displacement is correlated with stiffness: with decrease of tissue stiffness the motion amplitude is increasing. The method may provide the technological background for future studies characterizing in vivo tissue stiffness from vascular pulsations generated displacements.