System and method for model-independent quantification of tissue viscoelastic properties using ultrasound
A model-independent method for producing a viscoelastic tissue property metric using ultrasound is provided. A mechanical stress, such as an acoustic force, is applied to a tissue using an ultrasound system and tissue displacement resulting from the applied acoustic force is measured. From the tissue displacement measurements, a complex modulus, such as a relative complex modulus, is extracted. A loss tangent is calculated from the extracted complex modulus. Using the calculated loss tangent, viscoelastic tissue property metrics may be calculated.
1. A method for producing a viscoelastic tissue property metric using ultrasound, the steps of the method comprising:
a) applying a mechanical stress to a tissue using an ultrasound system;
b) measuring over a time period, a tissue displacement, u(t), resulting from the applied mechanical stress using the ultrasound system;
c) calculating with the computer system, a loss tangent from real and imaginary parts of relative complex modulus values computed from the tissue displacement, u(t), measured over the time period; and
d) calculating with the computer system, at least one viscoelastic tissue property metric using the calculated loss tangent.
2. The method as recited in claim 1 in which step d further includes calculating a real part of a wavenumber using a frequency and wave speed of shear waves generated in the tissue by the applied mechanical stress, and using the real part of the wavenumber to calculate the at least one viscoelastic tissue property metric.
3. The method as recited in claim 1 in which step d includes calculating an imaginary part of a wavenumber using the loss tangent and using the imaginary part of the wavenumber to calculate the at least one viscoelastic tissue property metric.
4. The method as recited in claim 1 in which step d includes:
i) calculating a real part of a wavenumber using a frequency and wave speed of shear waves generated in the tissue by the applied mechanical stress;
ii) calculating an imaginary part of a wavenumber using the loss tangent and the calculated real part of the wavenumber; and
iii) calculating the at least one viscoelastic tissue property metric using the calculated real and imaginary parts of the wavenumber.
5. The method as recited in claim 1 in which the at least one viscoelastic tissue property metric is at least one of a storage modulus and a loss modulus.
6. The method as recited in claim 1 in which the tissue displacement is measured using ultrasound energy to detect the tissue displacement.
7. The method as recited in claim 6 in which the mechanical stress applied in step a) is generated by the ultrasound energy used to detect the tissue displacement in step b).
8. An ultrasound system for measuring a viscoelastic property of a tissue, comprising:
an ultrasound transducer configured to transmit ultrasound energy and to receive acoustic signals responsive to the transmitted ultrasound energy;
a processor in communication with the ultrasound transducer, the processor being configured to:
direct the ultrasound transducer to apply an acoustic force to a tissue;
receive signals responsive to the acoustic force from the tissue;
measure over a period of time, tissue displacement, u(t), from the received signals;
calculate a loss tangent from real and imaginary parts of relative complex modulus values computed from the tissue displacement, u(t), measured over the period of time; and
calculate at least one viscoelastic tissue property metric using the calculated loss tangent.
9. The ultrasound system as recited in claim 8 in which the processor is configured to calculate a real part of a wavenumber using a frequency and wave speed of shear waves generated in the tissue by the applied acoustic force, and to use the real part of the wavenumber to calculate the at least one viscoelastic tissue property metric.
10. The ultrasound system as recited in claim 8 in which the processor is configured to calculate an imaginary part of a wavenumber using the calculated loss tangent and to use the imaginary part of the wavenumber to calculate the at least one viscoelastic tissue property metric.
11. The ultrasound system as recited in claim 8 in which the processor is configured to:
calculate a real part of a wavenumber using a frequency and wave speed of shear waves generated in the tissue by the applied acoustic force;
calculate an imaginary part of a wavenumber using the calculated loss tangent and the calculated real part of the wavenumber; and
calculate the at least one viscoelastic tissue property metric using the calculated real and imaginary parts of the wavenumber.
12. The ultrasound system as recited in claim 8 in which the processor is configured to calculate the at least one viscoelastic tissue property metric as at least one of a storage modulus and a loss modulus.