METHOD AND APPARATUS FOR DETERMINING VISCOELASTIC PARAMETERS IN TISSUE

• US 20100160778A1
• Filed: 11/03/2009
• Published: 06/24/2010
• Est. Priority Date: 11/03/2008
• Status: Active Grant

• Current Assignee: University of British Columbia
• Original Assignee: University of British Columbia
• Inventors: Eskandari, Hani, Rohling, Robert N., Salcudean, Septimiu E.
• Granted Patent: US 8,394,026 B2
• Time in Patent Office: Days
• US Class Current: 600/438
• CPC Class Codes: A61B 8/00 Diagnosis using ultrasonic, sonic or infrasonic waves imaging of objects using sonar G01S15/00
• CPC Class Codes: A61B 8/13 Tomography A61B8/10, A61B8/12 take precedence; tomography for radiation diagnosis A61B6/02
• CPC Class Codes: A61B 8/485 involving measuring strain or elastic properties
• CPC Class Codes: A61B 8/587 Calibration phantoms models for medical purposes G09B23/28
• CPC Class Codes: G01S 15/8977 using special techniques for image reconstruction, e.g. FFT, geometrical transformations, spatial deconvolution, time deconvolution

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.

Claims

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 the 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 the 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 density of each of the elements 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 solving for the viscoelastic parameters comprises generating a system of equations comprising the viscosity and elasticity of each of the elements 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 is constrained to be between 0 MPa and 1 MPa.

15. A method as claimed in claim 9 wherein the viscosity is constrained to be between 0 kPa·

s and 1 kPa·

s.

16. A method as claimed in claim 1 wherein solving for the viscoelastic parameters comprises generating a system of equations comprising the viscosity and elasticity of each of the elements 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. 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 calculating strain at two locations within the tissue;

(c) estimating the difference in relaxation-times between the two locations from the difference in the phases of the strains at the two locations.

19. A method as claimed in claim 18 wherein estimating the difference in relaxation-times at a particular frequency comprises dividing the difference in phases of the strains at the two locations by the particular frequency.

20. A method as claimed in claim 18 wherein estimating the difference in relaxation-times comprises determining a transfer function relating the strain between the two locations and estimating the difference in relaxation-time therefrom.

21. A method as claimed in claim 18 further comprising fitting a power-law model to a plurality of relaxation times each corresponding to a different frequency.

22. A method as claimed in claim 20 wherein estimating the difference in relaxation-times at a particular frequency comprises dividing the difference in phase of the transfer function between the two locations by the particular frequency.

23. A method as claimed in claim 20 wherein the transfer function is a first order transfer function.

24. A method as claimed in claim 22 further comprising determining the absolute value of the relaxation-time at one of the two locations from the absolute value of the relaxation-time at the other of the two locations.

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 the absolute values for one or more of the viscoelastic parameters for each of the elements of the model are solved.

26. 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 determine tissue displacement 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 strain between two locations within the tissue from the measured displacements and to estimate the difference in relaxation-times between the two locations from the difference in the phases of the strains at the two locations.