| Cardiovascular disease is one of the important common threats to human health, which is the leading cause of death in the Chinese population. The trend of cardiovascular disease is not optimistic."Chinese Health Statistics in2012" issued by the Ministry of Health of China, showed that cardiovascular disease mortality rate in2011was257.0/100,000people, accounting for41.52%ratio of death. Cardiovascular disease mortality is always the cause of death of residents in the first place, and showed a rising trend. If it was not controlled, by2030the prevalence of coronary heart disease would increase by3.7times than in2000. Therefore, the control of cardiovascular disease has become a top priority to improve people’s health in the21st century. Cardiovascular disease epidemic situation in China has the following characteristics:there is rapid growth of cardiovascular disease morbidity and mortality; cardiovascular disease morbidity and mortality has obvious regional differences; target population is turning young; cardiovascular mortality in rural areas is close to or over the city. Atherosclerotic cardiovascular disease is main cause of death and disabling, including coronary heart disease, stroke, abdominal aortic aneurysm and peripheral arterial disease. The occurrence of atherosclerosis development is a lengthy process. Effective control of risk factors will delay or prevent the development of atherosclerotic lesions into clinical cardiovascular disease. Bending, bifurcation and arterial stenosis, such as the aortic arch, carotid artery, coronary artery, abdominal aorta, femoral artery branch and other branches are predilection sites of atherosclerosis. Studies have shown that hemodynamic factors played a very important role in atherosclerosis, and the flow-generated shear stress was one of the key factors. Atherosclerosis is the development of a comprehensive hemodynamic process. Mutations in the arterial partial hemodynamic will lead to a series of vascular physiological variation in this region, which induce the emergence of atherosclerotic plaques or promote plaque development.Conventional diagnostic techniques such as ECG, cardiovascular ultrasound, myocardial enzymes and brain natriuretic peptide played an important role in the diagnosis of cardiovascular disease. They are still widely used and sequentially to improve. The emergence and development of new technologies, such as angiography, intravascular ultrasound(IVUS), cardiac multidetector CT and optical coherence tomography(OCT) has opened up new ideas for the diagnosis and clinical intervention. Currently, the measurement methods of blood flow velocity imaging are generally ultrasound Doppler and magnetic resonance imaging(MRI). Doppler only provides one-dimensional velocity information. In order to measure the shear force, it should be assumed that the flow rate of the blood vessels is a standard laminar flow velocity distribution and the distribution has a parabolic shape. And the characteristics of vascular flow rate can be established only in the shape of the rules. There will be a large error of the flow velocity distribution for the complex shape(curved, branching) within the vessel. Ultrasound Doppler provides one-dimensional velocity and depends on the angle between the ultrasound beam and the local velocity vector, and may have some errors. MRI provides high spatial resolution, but is limited by its low time resolution and high inspection costs.Ultrasonic particle image velocimetry(Echo PIV) technique is based on identifying and tracking flow tracers(ultrasound contrast agents, microbubbles) with high frame rate ultrasound imaging, and computing local velocity vectors by the use of an optimized PIV algorithm. With the known frame rate of the ultrasound system, the flow velocity of the blood can be calculated from the displacement of particles in cross-correlation calculation field of two consecutive image frames. Compared to the traditional measurement of blood flow, ultrasonic particle image velocimetry technique can overcome the angle dependence of blood flow velocity and direction of the Doppler probe. It is a real-time, non-invasive and two-dimensional measurement of blood flow velocity. Echo PIV could be qualified for the special shape of complex vessels, while Doppler method may be difficult to measure. Since Echo PIV method is based on the displacement microbubbles of the particle image to calculate the speed of blood, it is not only applicable to ordinary vessels, but also for special shapes to complex blood vessels. Moreover it can display real-time two-dimensional blood flow velocity. On the use of cost, ultrasonic particle image velocimetry method is lower than the MRI method. And Echo PIV is a faster imaging method. However, Echo PIV is suitable for drug delivering with microbubbles, inspection and other routine clinical applications. These advantages are in favor of its promotion and application. Currently, Echo PIV technique is still in the laboratory testing stage. The main problems that Echo PIV could not apply for the clinical application include: How to control the concentration of microbubbles after the injection; How to avoid microbubbles making damage to human body; The accuracy of Echo PFV is greatly depended on concentration of microbubble; The size of interrogation window affects the calculation of the image cross-correlation which may lead to different results of displacement; How to eliminate errors when measuring the flow with high speed gradient; How to filter spurious vectors generated in the calculation process, and so on.In this paper, the conventional Echo PIV algorithm is improved by combining a multiple iterative algorithm, sub-pixel method, filter and interpolation method, and spurious vector elimination algorithm, according to the velocity gradients. So the accuracy and efficiency of Echo PIV algorithm will be improved. In vitro vascular phantom experiments, it was validated that ultrasonic particle image velocimetry combined with a multiple iterative algorithm, sub-pixel method, filter and interpolation method, and spurious vector elimination algorithm could improve the accuracy and efficiency of flow velocity measurement with conventional Echo PIV algorithm. The improved Echo PIV algorithm could be applied to measure the flow velocity of the blood vessel phantoms. After the completion of the vascular phantom test, animal experiments were prepared and conducted which was closer to reality. SD rats were used as experimental animals in this study.Purpose:Hemodynamic changes in vascular stenosis could affect blood flow component of flow distribution which changed the nature of vascular smooth muscle cells. Observation and analysis of blood flow in the heart disease have important clinical significance of treatments of arterial stenosis. Blood flow velocity, shear stress conditions of rat artery stenosis models was observed by Echo PIV. Plaques growth, the risk of thrombosis and arterial stenosis were discussed combining with the results of hemodynamic and histopathological analysis of arterial stenosis before and after medication.Method:Sixty healthy male SD rats were used in this study. They weighed about340-360g, an average of351.24±9.32g. The rats were randomly divided into6groups. Each group had10rats. Five groups were subjected to balloon dilatation processing(PTCA balloon catheters2.0F) and were fed with high-fat diet to construct atherosclerosis model. The remaining group was used as the negative control group C. One out of the five model groups was not treated with drugs and used as the model group M. One model group was treated with the Rosiglitazone and was named as the positive control group R. Three model groups were treated with Tetramethylpyrazine with low, median and high dose and were named as T1, T2, T3, respectively. Since the animals died during the experiment occurred, and finally rats surviving was47with the successful acquisition of ultrasound particle image. On the21st day after surgery, the rat LCCA was imaged by Vevo2100system. Ultrasonography was operated by the same physician. The rats were anesthetized and fixed on the test bench. The left neck hairs were removed. The lumen diameter, the wall thickness, Doppler flow velocity should be measured and recorded. Thereafter, SonoVue was prepared with5ml saline. And then, it had been fully shaking. It took0.2ml injection with lml syringe via the right femoral vein, and then washed with0.1ml saline. A sequence of1000ultrasound contrast images were acquired and recorded at5、10min after the injection. The interrogation window was set to be from24×10to48×16pixel*pixel with a50%overlap and three iterations. The velocity distribution area was displayed by Matlab2009. Result:In this study,47rats contrast images had been processed. The mean blood flow velocity and peak velocity of each group were recorded respectively using Echo PIV and ultrasound Doppler velocimetry within three cardiac cycles. There was no statistically significant difference between two velocimetry methods by t test (P>0.05). The Echo PIV-measured peak and average velocities within several cardiac cycles are about5-10%and2-8%below the values measured by the ultrasonic spectral Doppler, respectively. Blood flow velocity curve using four polynomial had better results(the relevant index R=0.92). Vascular shear stress was calculated from the fitted curve of velocity distribution. The shear stress in the central region of normal vascular was minimum value, and increased from the central to the sides of the vessel wall. Five rats were selected in the negative control group C. They were divided into five groups(low-speed areas, medium and low speed areas, medium speed areas, medium and high speed areas and high-speed areas), according to the Doppler mean flow velocity in different locations of the carotid artery. The statistical average shear stress of three cycles(approximately120Fig.) in each groups were calculated with the application of Echo PIV and polynomial fitting. Experimental results showed that there was a positive correlation between periodic average shear stress and the average flow velocity in position of normal rat carotid arteries. The proposed calculation method of shear stress was applied for the experiment with47rats in this paper. The mean shear stress, the maximum and minimum of shear were statistically recorded for each group in three cardiac cycles vascular. There was statistically significant difference of hemodynamic parameters between the model group M and the negative control group C by t test(P<0.05). Hemodynamic parameters in the group T2with medium dose were closest to those in the negative control group C. There was no statistically significant difference between C and T2by t test(P>0.05). Hemodynamic parameters were compared with the pathological results of each group. The results showed that the group T2and R had a better recovery after drug treatment compared to other drug treatment groups. The intimal hyperplasia was smallest and the inner diameter of the stenosis is relatively small in the group T2and R. The imitative distribution of shear stress and blood flow velocity near the plaque provided important hemodynamic information to assess the risk of rupture of plaque growth. The shear force distribution and abnormal blood flow can be observed around the large or small plaques area calculated by Echo PIV. According to the observation of the flow velocity and shear distribution in the plaque region within several cardiac cycles, the shear stress is high in the region of a proximal artery within plaques, which might easily lead to plaques growth and intimal hyperplasia, and the stability of plaques. On the other hand, the shear stress is low in areas of the distal artery of the plaque, which might easily lead to lipid deposition to induce intimal hyperplasia. Changes of vascular shear stress have important clinical implications for cardiovascular disease. Vascular stenosis and plaque rupture is closely associated with shear stress. But the interaction mechanism of shear and plaque growth is not clear yet. Ultrasonic particle image velocimetry provides a new imaging method in vascular shear stress. It further to explore the prevention and treatment of restenosis and vascular plaque rupture mechanism. |
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