Transient Flow Vector Distribution of Cavitation Bubbles in a Vessel Induced by Pulsed Focused Ultrasound Thrombolysis Based on Pyramid LK Optical Flow Method

Previous studies have explored the use of pulsed high-intensity focused ultrasound (pHIFU) alone to non-invasively break down blood clots through mechanical mechanisms, the main of which is believed to be acoustic cavitation. Thus, it is important to monitor the activity and motion of cavitation bubbles during ultrasound thrombolysis. In this paper, pyramid LK optical flow method (PLK-OFM) is employed to obtain the transient flow vector distribution (FVD) of cavitation bubbles in a vessel induced by pulsed focused ultrasound thrombolysis (pFUT). PLK-OFM combines the pyramid layered algorithm with LK optical flow method to estimate large and discontinuous displacement between the adjacent images. A transparent wall-less flow phantom with a diameter of 4 mm and porcine blood clot was employed to mimic an embolism and its flow environment. To enhance cavitation activity, a multiple pulse sequence was performed, of which the pulse duration, duty cycle and the number pulse are 20 us, 1:20 and 20, respectively. The results showed that bubble cloud firstly emerged at the focal region, then expanded along the vessel axis with an initial velocity, and gradually dissolved. The velocity of bubbles immediately after pFUT treatment was highest with both forward and reverse motions, and the axial mean velocity of cavitation bubbles in the focus region was about 16.23±2.85 cm/s. The transient PVD of cavitation bubbles in the vessel during pFUT can be obtained accurately by this PLK-OFM, suggesting the possibility to monitor the motions of cavitation bubbles. Moreover, this method may be useful for parameters optimization to improve thrombolysis efficiency by cavitation enhancement.