The Preliminary Mechanical Study Of Isolated Carotid Plaque Using Ultrasonic Velocity Vector Imaging

Objective: To evaluate the changes of mechanical parameters at carotid atherosclerotic plaque and the nearby reference carotid wall before and after the grip strength experiment. Try to explore a new ultrasonic method of quantification assessment for the plaque stability.Method: 110 patients with 119 isolated plaques underwent the high frequency ultrasound scanning. The single-slice gray-scale two-dimensional short axis views of carotid artery at the site of plaque in 64 patients with 71 isolated plaques and the multi-slices gray-scale two-dimensional views (include 3 short axis views and longitudinal axis views) of carotid artery at the site of plaque in 46 patients with 48 isolated plaques were achieved for three cardiac cycles separately. A dedicated commercial Velocity Vector Imaging (VVI) workstation was employed for the off-line two-dimensional gray-scale short axis image analysis and the exploration of mechanical parameter changes (i.e., peak strain) at the sites of plaque intima and their counterpart in the same and nearby reference short axis view separately (i.e., single-slice short axis views of the thickest plaque: three points at plaque and three opposite counterpart points, total: six points.multi-slices short axis views of superior, middle and inferior plaque separately: three points at plaque and three opposite counterparts on each view, total: eighteen points). Three another points were defined on the longitudinal axis view of the longest dimension of the plaque at superior, middle and inferior sites separately. The mechanical parameters at different points of carotid intima and the blood pressure were calculated automatically and measured before and after grip strength test. The dynamic two-dimensional velocity vectors on the gray-sclae images, strain-time curves at different points of the plaque and reference wall, the color anatomic M-mode and reconstructed three-dimensional M-mode strain images were generated automatically by the VVI work station. Wilcoxom test was used for the comparison of the peak train and the absolute diference of the peak strain between the observation points and their counterparts, P<0.003 and P<0.017 was recongnized as statistically significant difference.Results: The chaos and asynchrony of intima velocity vectors at the sites of plaque significantly differed from those synchronized velocity vectors at the nearby reference wall were clearly demonstrated. The color anatomic M-mode and reconstructed three-dimensional M-mode strain images also showed this obvious asynchronized strain distribution at the site of the carotid plaque. For single-slice short axis views: a significant difference (P <0.003) of the peak strain between the points at plaque shoulders and its reference counterparts was explored on the strain-time curve. The peak strain at the points of the two plaque shoulders was dissymmetry (P <0.017), and the peak strain at the top point of plaque was lower than the higher peak strain of the two plaque shoulders (P =0.000). For the multi-slices views: the peak strains at the points of plaque shoulders also were dissymmetry on each short axis views of the plaque (P <0.017) before and after the grip strength experiment. At the same time,there was not significant difference of the peak strains at the nearby reference intima points(P>0.05). The absolute difference between the lower peak strain of the two plaque shoulders and that of the top point of plaque was higher than that between the two points of the carotid atherosclerotic plaque shoulders(P<0.017) on all the three short axis views of the plaque. The absolute difference between the higher peak strain of the two plaque shoulders and that of the top point of the plaque was higher than that between the points of the carotid atherosclerotic plaque shoulders(P<0.017) on the middle short axis views of the plaque before and after the grip strength experiment. For the long axis views: there was a significant difference between the peak strain absolute difference from the point of upstream site to the point of downstream site and the that between the points at upstream and downstream sites to the middle point of the plaque respectively before and after grip strength tes(tP=0.000). The peak strain at the higher strain value side of the plaque was significantly higher than that at the lower strain value side. There was a significant difference between the peak strain difference from the higher strain value side to the middle point of the plaque and the peak strain difference from the lower strain value side to the middle point of the plaque(P<0.017). Conclusion: The mechanical asynchrony of the plaque intima and the absolute difference of peak strains between the points of the plaque and reference wall could be used as a quantitative scanning method for the plaque stability. The quantification of the peak strain dissymmetry at the shoulders and upstream and the downstream sites of plaque could be used as a potential predictive method for the probability assessment of a rupture of the unstable plaque and a consequent stroke in future.