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Biomedical Imaging
Published in Mohammad E. Khosroshahi, Applications of Biophotonics and Nanobiomaterials in Biomedical Engineering, 2017
Perfluorocarbon (PFC) droplets: Phase-change contrast agents transform liquid emulsions into microbubbles (MBs) contrast agents that can have both diagnostic and therapeutic functions. PFC droplets undergo a volumetric expansion when is subject to sufficient acoustic pressures delivered by an ultrasound transducer, called acoustic droplet vaporization (ADV) (Kripfgans et al. 2000). Similarly, when it is subject to optical irradiation, vaporization also occurs (ODV) (Strohm et al. 2010). Thus, when the laser fluence is below the vaporization threshold, the droplets remain in the liquid phase and can be used as a PA contrast agent. However, when the fluence exceeds the threshold, it induces droplet vaporization resulting in microbubbles which can be used for contrast enhanced ultrasound imaging. MBs can be composed of phospholipids, albumin, or polymer. These gas-filled can produce strong acoustic scattering relative to the surrounding tissue. In more advance form, AuMBs comprose albumin-shelled microbubbles with encapsulated gold nanorods. Wang et al. (2012) have investigated this system as PA/US dual modality contrast agent.
Effect of asymmetry on the flow behavior in an idealized arterial bifurcation
Published in Computer Methods in Biomechanics and Biomedical Engineering, 2020
Mahesh Nagargoje, Raghvendra Gupta
Bifurcating vessels are encountered in the microreactors, industrial piping networks, and arterial networks. Flow physics at the bifurcation is complex. The bifurcation geometry in an arterial network is prone to thrombus formation or atherosclerosis, and the bifurcating artery following Murray's law (Murray 1926) may be protected from thrombus formation (Lakshmanan et al. 2019). Understanding flow physics at a bifurcation is also important for gas embolotherapy, a potential cancer treatment strategy (Calderon et al. 2005, 2010; Qamar et al. 2010, 2017; Valassis et al. 2012) in which the blood supply to cancer cells is stopped using acoustic droplet vaporization. As the flow enters the bifurcation site, flow stream divides into two daughter vessels. At the bifurcation, the velocity profile becomes skewed with the higher velocity near the inner wall. At higher Reynolds numbers, flow separation is observed near the outer wall (Chandran et al. 2007). At the bifurcation, the vessel has some amount of curvature resulting in secondary or Dean vortices, commonly observed in curved channels originating from the interplay between pressure force across inner and outer side of bifurcation and centrifugal force (Dean 1927). The flow separation at the outer wall and the secondary flow affects the wall shear stress (WSS) distribution on the vessel walls. The low WSS value is often correlated with atherosclerosis (Caro et al. 1971; Zarins et al. 1983; Malek et al. 1999). Atherosclerosis is a cardiovascular disease caused by the narrowing of arteries due to deposition of fatty material on the walls of the arteries. The atherosclerotic plaque often develops on the outer walls of arterial bifurcations causing stenosis. This is so because the possibility of the development of the plaque is higher in the regions where wall shear stress (WSS) is low or Oscillatory Shear Index (OSI) is high (Zarins et al. 1983; Ku et al. 1985; Perktold and Hilbert 1986; Perktold and Resch 1990; Perktold et al. 1991). Low time-averaged WSS, high OSI, and high relative residence time (RRT) values are observed at the bifurcation, the potential site of atherosclerosis (Pinto and Campos 2016). Investigations on the flow behaviour in the carotid artery with stenosis show that stenosis changes the flow pattern downstream (Bit and Chattopadhay 2014b). It can lead to the formation of secondary stenosis (Bit et al. 2017). An aneurysm is another pathological condition that may develop because of the dilation of the vessel walls. In a recent study, it has been shown that the aneurysmal vessels are more prone to the formation of secondary stenosis as compared to primary stenosed vessels (Bit and Chattopadhyay 2014a, 2018). WSS in a bifurcating channel depends on different geometric parameters such as bifurcation angle, planarity, and asymmetry of the bifurcation (Lu et al. 2002; Chen and Lu 2004, 2006; Zhang and Dou 2015).