Elastography Methods Based On The Intravascular Ultrasound Imaging

Intravascular ultrasound imaging (IVUS) is widely used in artery atheromatousplaque morphology assessment, drug therapy, non-drug intervention process, andatherosclerotic plaque vulnerability evalution. It was concerned as the gold-standardtechnique for coronary artery disease diagnosis. In1991, ultrasound elastographywas firstly presented by Dr. Ophir and his group. The basic theory of ultrasoundelastography can be concluded like: Firstly, the ultrasonic signals of the soft tissuewas captured by ultrasonic transducer before-and after applying a force to let thetissue deformed; then the displacement and the strain distribution will be calculated;finally the elastic image or strain image of detected biological tissue can be obtained.Elastography has an extensive application in clinical diagnosis of breast, prostate andother organs. In recent years, many researches have shown that ultrasoundelastography combined with IVUS can provide the parametric image of vascularwall elasticity, the biomechanical characteristics of coronary plaques and so on,which are very useful in identifying plaque component and detecting plaquevulnerability.However, the elasticity imaging technology has not yet been applied to thecommercial intravascular ultrasound imaging system. Our research group developedan ultra-high frequency intravascular ultrasound imaging probe and system (centralfrequency is50~60MHz). In this paper, we focused on the intravascular ultrasoundelasticity imaging method for atherosclerotic plaque recognition, which is shown as follow:(1) Firstly, the functional requirements of the intravascular ultrasound imagingsystem was analyzed. And some1-dimensional and2-dimensional displacementestimation methods were compared;(2) Secondly, according to our home-made intravascular ultrasound imagingsystem parameters, the application of the atherosclerosis plaque identification by aclassical1-D elastography algorithm was studied by a simulated model of the bloodvessel based on Finite Element Analysis (FEA). The radial displacement of vesselwalls caused by the different intravascular pressure was estimated. By comparing theelastograms under different pressures, the optimal pressure difference was obtained.The simulation results indicated that this method can effectively distinguish betweendifferent component plaques, which will be helpful in expanding our IVUS systemapplication field;(3) Then, the influence of different ingredients in the agar-based phantom onthe acoustic and mechanical properties was studied. The experimental scheme wasdesigned according to the orthogonal experiment method. By combining the typicallevel factors, the experiment times was greatly reduced. Then through the rangeanalysis and variance analysis, how the ingredients of phantom influence theiracoustic properties were found. These results will be useful for the phantom tests ofour ultrahigh-frequency IVUS probe and system;(4) At the end part, a modified2-D displacement estimation method forestimating soft tissue deformation was proposed. The algorithm combines theadvantages of multi-level search and single-level tracking approach, and can make atradeoff between the calculation speed and accuracy. A simulation experimentcompared with the1-dimensional approach was performed to verify the effect andreal-time performance of this modified algorithm.