Measurement of propagation wave velocity of thyroid induced by pulsation

Shear wave imaging is a technique with ultrasound that estimates tissue stiffness by measuring shear wave induced by Push Beam, which is generated by acoustic radiation force. However, the technique requires Push Beam for a duration of several hundred micron seconds to deform tissue, and a decay of estimate accuracy may be attributed to out-of-focus Push Beam generated by motions of pulsation and heartbeat. In this presentation, the propagation velocity of thyroid was measured by ultrasound spatial compound imaging with a high temporal resolution. In this proposal method, two components of particle velocity can be obtained by using spatial compound imaging, and two kinds of propagation velocity can be also estimated from the propagation appearance of each component. Verasonic ultrasound scanner (30 MHz, 16 bit, Redmond, WA) and 7.5 MHz UST-5412 Linear Probe (Hitachi Aloka Medical. Ltd.) were used to acquire RF data. Each plane wave was transmitted at 5 different angles, -18, 18, -9, 9, 0, with each time interval of 200μs to acquire echo data for making one spatial compound image. The data of each angle was processed by parallel beamforming, and a spatial compound image was obtained by superposing five B-mode images. Time interval between each spatial compound image is 1 ms. Therefore, the frame rate of the imaging was 1 kfps. A measurement area was thyroid of a 22-year-old healthy male at short axis view. An appearance of the propagation from the artery to thyroid area induced by pulsation was visualized with high-frame-rate ultrasound imaging. There were obvious difference between the shear waves along lateral direction. It's considered that mechanical properties of tissue can be observed from the propagation. According to our past research, the shear wave along muscle fibers was 7.43 ± 0.30 m/s at the frequency of 15 Hz. The muscle fiber direction can be distinguished by the proposal method. These results showed possibility of estimation of tissue stiffness by measuring the shear wave induced by pulsation.