Method and apparatus for determining viscoelastic parameters in tissue

Abstract

Described herein are a method and apparatus for determining viscoelastic parameters of a tissue. A vibration signal is applied to the tissue and displacements at a plurality of locations within the tissue are measured at a plurality of times. The viscoelastic parameters of the tissue, including elasticity and viscosity, can then be determined by fitting a finite element model of the tissue to the vibration signal and the measured displacements and by solving for the viscoelastic parameters of the model. A value for density of each element of the model is selected and the absolute values for the viscoelastic parameters of each of the elements in the model is determined. Alternatively, the difference in relaxation-times between two locations within the tissue can be determined from the difference in phases of the strains at the two locations.

1 . A method for determining viscoelastic parameters of a tissue, the method comprising:
 (a) applying a vibration signal to the tissue;   (b) measuring displacements at a plurality of locations within the tissue in response to the vibration signal at a plurality of times; and   (c) determining the viscoelastic parameters of the tissue by fitting a finite element model of the tissue to the vibration signal and the measured displacements and solving for viscoelastic parameters of the model, wherein a value for density of each element of the model is selected and absolute values for one or more of the viscoelastic parameters for each of the elements of the model are solved.   
     
     
         2 . A method as claimed in  claim 1  wherein the measured displacements are expressed in a frequency domain as phasors having a real and an imaginary component. 
     
     
         3 . A method as claimed in  claim 1  wherein the viscoelastic parameters that are solved include both elasticity and viscosity. 
     
     
         4 . A method as claimed in  claim 1  wherein the value for density of each element is selected to be the density of water. 
     
     
         5 . A method as claimed in  claim 3  wherein the viscoelastic parameters further comprise one or more of relaxation-time and wave speed. 
     
     
         6 . A method as claimed in  claim 1  wherein the vibration signal consists of one frequency component. 
     
     
         7 . A method as claimed in  claim 1  wherein the solving for viscoelastic parameters comprises generating a system of equations comprising a viscosity and elasticity of each of the elements of the model and wherein the system of equations is treated as being linear in the viscosity and elasticity of each of the elements. 
     
     
         8 . A method as claimed in  claim 7  wherein the viscoelastic parameters are unconstrained. 
     
     
         9 . A method as claimed in  claim 7  wherein the viscoelastic parameters are constrained. 
     
     
         10 . A method as claimed in  claim 1  wherein the system of equations is solved iteratively by minimizing a cost function. 
     
     
         11 . A method as claimed in  claim 1  wherein the system of equations is solved using instrumental variables. 
     
     
         12 . A method as claimed in  claim 9  wherein the system of equations is solved iteratively by minimizing a cost function. 
     
     
         13 . A method as claimed in  claim 9  wherein the number of equations in the system of equations is reduced by removing equations from the system of equations for which non-zero force terms are present. 
     
     
         14 . A method as claimed in  claim 9  wherein the elasticity of each of the elements is constrained to be between 0 MPa and 1 MPa. 
     
     
         15 . A method as claimed in  claim 9  wherein the viscosity of each of the elements is constrained to be between 0 kPa·s and 1 kPa·s. 
     
     
         16 . A method as claimed in  claim 1  wherein the solving for viscoelastic parameters comprises generating a system of equations comprising a viscosity and elasticity of each of the elements of the model and wherein the system of equations is treated as being non-linear in the viscosity and elasticity of each of the elements. 
     
     
         17 . A method as claimed in  claim 16  wherein the system of equations is solved iteratively by minimizing a cost function. 
     
     
         18 .- 24 . (canceled) 
     
     
         25 . An apparatus for determining viscoelastic parameters of a tissue, the apparatus comprising:
 (a) an actuator for generating a vibration signal and applying a generated vibration signal to the tissue;   (b) a driver circuit connected to drive the actuator to generate a vibration signal having at least one frequency component;   (c) an ultrasound imaging system configured to measure displacements at a plurality of locations within the tissue in response to the vibration signal at a plurality of times; and   (d) a data processor communicatively coupled to the ultrasound imaging system and configured to determine the viscoelastic parameters of the tissue including elasticity and viscosity by fitting a finite element model of the tissue to the vibration signal and the measured displacements and solving for viscoelastic parameters of the model, wherein a value for density of each element of the model is selected and absolute values for one or more of the viscoelastic parameters for each of the elements of the model are solved.   
     
     
         26 . (canceled)