Rayleigh Wave Viscoelasticity Imaging Using Laser Speckle Contrast Imaging

The elasticity of biological tissues is closely related to the occurrence and development of many diseases.Elasticity measurement of biological tissues based on ultrasound,magnetic resonance and optical imaging has received substantial attention in recent years.Laser speckle contrast imaging(LSCI)has the advantage of non-contact and wide-field,and it has a high sensitivity of motion detection.It provides a new way for the quantitative elastography of biological tissues.However,LSCI has a low imaging depth and a slow imaging speed.In previous studies,the transmission-mode LSCI is used for elasticity measurement by detecting the propagation velocity of the shear wave inside the sample.The synchronization between the excitation and the acquisition is required to increase the equivalent frame rate of imaging.The transmission-mode structure has a limited application in clinical scenarios.The synchronous acquisition method makes the system complicated.Most importantly,the elasticity measurement would be inaccurate if the viscosity of biological tissues is ignored.This thesis proposes some methods to resolve such issues,and establishes the method and system of viscoelasticity imaging based on the reflection-mode LSCI.Elasticity and viscosity imaging simultaneously of biological tissues by using a lowframe-rate LSCI is realized.The main innovative results are listed as follow:(1)A Rayleigh wave elasticity measurement method and system based on LSCI is developed.The transmission-mode LSCI method cannot measure the elasticity of the sample with a large thickness or a strong scattering,which is not convenient for practical application.The idea of measuring tissue elasticity by detecting Rayleigh wave propagating on the surface of samples is proposed.By experiment,the propagation velocity of Rayleigh wave measured on agarose phantoms under harmonic excitation is consistent with that measured under impulse excitation with a correlation coefficient of 0.998.The Rayleigh wave velocity increases linearly with the concentration of agarose(R=0.994).It demonstrates that the proposed method can accurately measure the elasticity of tissuemimicking phantoms.(2)An elastography method with LSCI using the aliasing effect is proposed.Usually cameras with high frame rate and complex synchronization systems are required in the elastography.A method to reconstruct the propagation velocity of Rayleigh wave by the aliased wave detected with a low-frame-rate LSCI is proposed.The quantitative elastography of biological tissues is realized by using a frame rate of only 10 frames per second.The proposed method is validated by experiments on agarose phantoms and ex vivo biological tissues.Elasticities are compared with those measured by traditional rheometer and by the "delayed sampling" method.The correlation coefficients are 0.999 and 0.998 respectively.When the sliding calculation window of the line fitting process is 1.95 mm,the contrast to noise ratio and spatial resolution of elastography are 3.71 and 1.53 mm respectively.(3)A method of simultaneous viscosity and elasticity imaging using LSCI is proposed.The viscosity of biological tissues causes the inaccurate measurement of the elasticity,and the viscosity of biological tissues needs to be measured.A method to measure the viscoelasticity simultaneously by obtaining the dispersion curve of Rayleigh wave velocity using LSCI is proposed.The proposed method is validated by experiments on oil-in-gelatin phantoms and ex vivo biological tissues.The elasticity and viscosity of biological tissue are imaged simultaneously by using LSCI for the first time.The shear elasticity modulus and the shear viscosity modulus measured of phantoms are compared with those measured by traditional rheometer.The correlation coefficients of are 0.844 and 0.979 respectively.