Application of aberration metrics to microflow imaging

Atherosclerotic plaques can lead to serious conditions such as cerebral infarction upon rupture; therefore, the diagnosis of plaque vulnerability is essential. It has been suggested that neovascularization occurs within plaques and is associated with increased vulnerability. If neovessels within plaques can be visualized using microvascular flow imaging, which enables depiction of slow blood flow, it may be possible to assess plaque vulnerability. In this study, we investigated a method to enhance the contrast of blood flow images. A programmable ultrasound system (RSYS-0016, Microsonic) was used with a commercial linear probe (UST-5412, FUJIFILM). Plane waves were transmitted at −10, 0, and 10 degrees, and each direction was insonified twice, resulting in six transmissions per frame (1736 fps). For clutter filtering, a method based on singular value decomposition was employed. We compared the conventional correlation-based metric, phase coherence factor (PCF) [Camacho et al., TUFFC, 2009], with the signal-to-noise ratio (SNR) factor [Hasegawa et al., Ultrason Imaging, 2020] previously proposed by our group. The evaluation was performed using in vivo measurement of the carotid artery. When the dynamic range was adjusted so that the visibility of blood flow signals was comparable, the SNR factor provided better visibility of microvascular flow compared to PCF.