Method for imaging the mechanical properties of tissue
An imaging system comprises a device to excite mechanical waves in elastic tissue, a device for measuring the resulting tissue motion at a plurality of locations interior to the tissue at a number of time instances, a computing device to calculate the mechanical properties of tissue from the measurements, and a display to show the properties according to their location. A parameter identification method for calculating the mechanical properties is based on fitting a lumped dynamic parametric model of the tissue dynamics to their measurements. Alternatively, the mechanical properties are calculated from transfer functions computed from measurements at adjacent locations in the tissue. The excitation can be produced by mechanical vibrators, medical needles or structures supporting the patient. The measurements may be performed by a conventional ultrasound imaging system and the resulting properties displayed as semi-transparent overlays on the ultrasound images.
1. A method for determining localized properties within a region under study, the method comprising:
applying a vibration signal to the region, the vibration signal simultaneously vibrating a plurality of locations within the region and having components at a plurality of frequencies;
concurrently measuring a response to the vibration signal at each of the plurality of locations within the region at each of a plurality of times; and,
from the measured responses determining localized properties of the region;
wherein determining localized properties of the region comprises providing a parametric model of the region and estimating parameters of the parametric model for which the vibration signal would yield the measured responses.
2. A method according to claim 1 wherein the parametric model comprises a one-dimensional chain of linear dynamic systems.
3. A method according to claim 2 wherein the linear dynamic systems each comprise a mass connected to masses of one or more other ones of the linear dynamic systems by a spring and a damper arranged in parallel.
4. A method for determining localized properties within a region under study, the method comprising:
applying a vibration signal to the region under study, the vibration signal simultaneously vibrating a plurality of locations within the region, and having components at a plurality of frequencies;
concurrently measuring a response to the vibration signal at each of the plurality of locations at each of a plurality of times; and,
from the measured responses determining localized properties of the region wherein determining localized properties of the region comprises determining transfer functions for portions of the region located between the plurality of locations.
5. A method according to claim 4 comprising fitting a parametric model to the transfer functions.
6. A method according to claim 1 comprising displaying an image based on the localized properties of the region.
7. A method according to claim 6 wherein the image based on the localized properties of the region is superimposed on a second image of the region.
8. A method according to claim 7 wherein the second image comprises an ultrasound image.
9. A method according to claim 7 wherein the second image comprises a computerized tomography image.
10. A method according to claim 1 wherein the region comprises tissue of a human or animal.
11. A method according to claim 10 wherein the region comprises a region within a human breast.
12. An imaging device comprising:
a vibrator;
a driver circuit connected to drive the vibrator to generate a vibration signal having components at a plurality of frequencies, the vibration signal capable of simultaneously vibrating a plurality of locations in a region of tissue;
an imaging system disposed to measure responses comprising at least one of: a tissue displacement, a tissue velocity and a tissue acceleration at the plurality of locations at a plurality of times; and,
a data processor connected to receive response data representing the responses, the data processor programmed to compute from the response data localized properties of the region by a process comprising estimating parameters of a parametric model of the region for which the vibration signal would yield the responses at the locations.
13. A method according to claim 1 wherein the response to the vibration signal comprises a time-varying displacement of the location.
14. A method according to claim 1 wherein the response to the vibration signal comprises one or more of displacement, velocity and acceleration of the location.
15. A method according to claim 1 wherein the parametric model models a plurality of masses coupled to one another by one or more viscoelastic couplings, each viscoelastic coupling comprising at least one of: a spring and a damper; wherein the estimated parameters of the parametric model comprise one or more of: a parameter corresponding to a mass of one of the plurality of masses; a parameter corresponding to a spring in one of the viscoelastic couplings, and a parameter corresponding to a damper of one of the viscoelastic couplings.
16. A method according to claim 1 wherein estimating parameters of the parametric model comprises arranging the measured responses to provide an overconstrained system of linear equations and computing an approximate solution to the system of linear equations.
17. A method according to claim 1 comprising determining image values for locations in an image based at least in part on the estimated parameters.
18. A method according to claim 17 comprising superimposing the image on an ultrasound image of the region.
19. A method according to claim 1 wherein the parametric model comprises a chain of linear dynamic systems.
20. A method according to claim 4 wherein determining the transfer functions for portions of the region comprises taking a first one of the plurality of locations to be an input of a linear dynamic system and a second one of the plurality of locations to be an output of the linear dynamic system and, from the responses corresponding to the first and second locations, calculating a corresponding transfer function.
21. A method according to claim 20 comprising, after determining the transfer functions, determining one or more properties of each of the transfer functions.
22. A method according to claim 4 wherein the properties comprise low-frequency magnitudes of the transfer functions.
23. A method according to claim 4 wherein the properties comprise averages of the magnitudes of the transfer functions at frequencies within a frequency range.
24. A method according to claim 4 comprising fitting a parametric model to the transfer functions in the frequency domain.
25. A method according to claim 4 comprising computing a coherence function corresponding to one or more of the transfer functions.
26. A method according to claim 5 comprising determining image values for locations in an image based at least in part on values of one or more parameters of the parametric model.
27. A method according to claim 4 comprising determining image values for locations in an image based at least in part on the localized properties of the region.
28. An imaging device according to claim 12 comprising:
a needle having a proximal end coupled to the vibrator and a distal end that is insertable into the region.
29. An imaging device according to claim 28 wherein the needle comprises a cannula and a stylet extending through a bore of the cannula wherein the stylet is coupled to carry the vibration signal from the vibrator to a distal tip of the stylet.
30. An imaging device according to claim 12 wherein the vibrator is connected to deliver the vibration signal by way of a support structure capable of supporting a human subject.
31. An imaging device according to claim 12 wherein the imaging system comprises an ultrasound imaging system.
32. An imaging device according to claim 30 wherein the ultrasound imaging system comprises an ultrasound probe, the vibrator comprises a first vibrator mounted on the probe, and the imaging device comprises one or more additional vibrators mounted on the probe to cancel vibrations of the probe caused by the first vibrator.
33. An imaging device according to claim 32 comprising a vibration detector disposed to measure residual vibration of the ultrasound probe and a controller configured to drive one or more of the first and second vibrators based on a feedback signal from the vibration detector to reduce vibration of the ultrasound probe.
34. An imaging device according to claim 31 wherein the ultrasound imaging system comprises an ultrasound probe with the vibrator mounted on the probe, and the imaging device comprises means for cancelling vibrations of the probe caused by the vibrator.