System and method for non-invasive determination of tissue wall viscoelasticity using ultrasound vibrometry
View Patent ↗System and method for determining viscoelasticity of curved tissue walls using ultrasound bladder vibrometry (UBV). The UBV is a non-invasive technique utilizing, in a specific case, a focused ultrasound radiation force to excite Lamb waves in a curved bladder wall and pulse-echo techniques to track the tissue deformation propagating through such curved wall. Cross-spectral analysis is used to calculate the wave velocity, which is directly related to the elastic properties of the bladder wall.
1. A method for characterizing a parameter representing a mechanical property of a tissue wall, the method comprising:
with a transducer, detecting ultrasonic energy reflected by multiple locations along the tissue wall that is subject to stress to form ultrasonic echo data, wherein said tissue wall having geometric boundaries and defining a volume enclosed by the tissue wall, said tissue wall separating a fluid material contained in said volume from a rigid material outside of the tissue wall, and
determining dispersion data representing a mechanical deformation of said tissue wall from said echo data.
2. A method according to claim 1 , wherein said determining includes
calculating a dispersion characteristic of phase velocity of said mechanical deformation propagating along the tissue wall, and
fitting said dispersion characteristic with an anti-symmetric Lamb wave or shear-wave function to account for attenuation, of said mechanical deformation in the tissue wall, that is caused by said geometric boundaries.
3. A method according to claim 2 , further comprising causing said mechanical deformation by applying an excitation input to said tissue wall along a direction of excitation.
4. A method according to claim 2 , wherein said tissue wall is spatially curved, wherein said fitting includes introducing a correction for a spatial displacement caused by said mechanical deformation and associated with a curvature of said tissue wall, said correction being a result of comparison between a first motion of the tissue wall and a second motion of the tissue wall, the first motion occurring in a direction parallel to the excitation direction, the second motion occurring perpendicularly to a surface of said curved tissue wall.
5. A method according to claim 1 , wherein said determining includes spectrally analyzing said echo data to determine dispersion data representing said mechanical deformation, which mechanical deformation corresponds to a motion of a spatially curved tissue wall along a surface of said tissue wall.
6. A method according to claim 1 , wherein said determining includes calculating a dispersion characteristic of phase velocity of said mechanical deformation, which propagates along the tissue volume, with the use of a rheological function to account for attenuation, of said mechanical deformation in the tissue wall, that is caused by viscosity of the tissue wall.
7. A method according to claim 6 , wherein said calculating includes calculating said dispersion characteristic with the use of a rheological function employing elasticity of said tissue wall and viscosity of said tissue wall as separate factors.
8. A method according to claim 1 , wherein said detecting ultrasonic energy includes causing said stress by applying to the tissue wall at least one of (i) an acoustic radiation force, (ii) an electro-mechanical input; and (iii) a mechanical input.
9. A method according to claim 1 , wherein said tissue wall is spatially curved, and further comprising estimating a viscoelasticity parameter of said tissue wall with the use of Lamb-wave or shear-wave calculation based on said dispersion data.
10. A method according to claim 1 , further comprising determining, based on the dispersion data, said parameter representing a mechanical property of the tissue wall in association with pressure produced by said tissue wall onto said fluid material, wherein said parameter representing a mechanical property of the tissue wall includes one or more of: (i) a phase velocity of a wave associated with said mechanical deformation, (ii) a group velocity of a wave associated with said mechanical deformation; (iii) a wave-peak velocity associated with said mechanical deformation; (iv) elasticity of the tissue wall, (v) a viscosity of the tissue wall, and (vi) compliance of the tissue wall.
11. A method according to claim 1 , wherein said determining dispersion data includes performing spectral analysis of said echo data, and tissue wall includes a wall of a bladder of a living subject.