Method and apparatus for elasticity imaging

Claims

What is claimed is:

1. A method of identifying a region within a tissue portion having a different elasticity than the surrounding tissue of the tissue portion comprising:

causing a deformation of the tissue portion to create stress and strain in the tissue portion;

determining at least one property of at least one of the created stress and strain in the tissue portion when the tissue portion is deformed to yield a pattern of the at least one property of the created stress and strain in the tissue portion; and

evaluating the pattern of the at least one property in the tissue portion by the further steps of:

defining a model of the tissue portion with homogeneous tissue and with given boundary conditions for the tissue portion;

calculating the pattern of the at least one property for the defined model;

comparing the pattern obtained in the determining step and the calculated pattern to obtain the difference between the patterns, this difference indicating the presence and location of a differing elasticity region of tissue within the tissue portion; and

calculating elasticity characteristics of the tissue portion by varying a spatial distribution of modulus of elasticity in the defined model to minimize the difference between the pattern from the determining step and that calculated for the defined model thereby solving an inverse mechanical problem and obtaining spatial distribution of elasticity modulus in the tissue portion.

2. The method of claim 1 wherein the determining step further comprises:

determining a resistance pressure pattern sensed at locations along a surface of the tissue portion when the tissue portion is deformed to yield the pattern of the stress on the tissue portion.

3. The method of claim 2 wherein the pressure determining step further comprises:

measuring the resistance pressure at a plurality of locations on each of at least two accessible surfaces of the tissue portion to yield a pressure pattern for each accessible surface.

4. The method of claim 1 wherein the determining step further comprises:

determining strain in the tissue portion by comparing images of an internal structure of the tissue portion before and after deformation to yield the pattern of strain in the tissue portion.

5. The method according to claim 1 wherein the determining step includes the step of motion tagging of internal tissue particles to yield the pattern of strain in the tissue portion.

6. The method of claim 5 the determining step further comprising:

imaging the tissue portion and evaluating an image of an internal structure of the tissue portion before and after deformation of the tissue portion to identify boundaries of regions of tissue comprising the tissue portion by determining the ratios of strain characteristic values for regions of the tissue portion having different elasticity.

7. The method of claim 6 wherein changes in modulus of elasticity of the tissue portion along selected reference planes is determined by imaging tissue particles and tagging tissue particles and determining shifting of particles after deformation.

8. The method of claim 5 wherein the step of calculating the pattern forming part of the evaluating step further comprises:

calculating a strain value for each tagged tissue particle along an axis of measurement of the tissue portion;

calculating a ratio of the calculated strain value of a first tissue particle relative to the calculated strain value of a second tissue particle adjacent to the first tissue particle but spaced therefrom along the measurement axis, the ratio being calculated for each pair of adjacent tissue particles along the measurement axis;

identifying a profile of ratios of strain value of adjacent tissue particles having a calculated strain value ratio equal to non-unity, the profile of ratios indicating the boundary between a different elasticity region and the remaining tissue of the tissue portion.

9. The method of claim 8 wherein the axis of measurement is substantially parallel to an axis of force used for causing deformation of the tissue portion.

10. The method of claim 8 wherein the axis of measurement is at a non-parallel angle to an axis of force causing deformation of the tissue portion.

11. The method of claim 8 including the step of determining a pressure resisting deformation at a plurality of locations on a surface of the tissue portion to yield a pattern of stress on the tissue portion.

12. The method of claim 1 wherein the determining step further comprises:

determining changes in both of the properties of stress and strain in the tissue portion when the tissue portion is deformed by measuring pressure at locations on a surface portion to provide the stress pattern, and imaging internal tissue structure after deformation to provide the pattern of stress and the pattern of strain in the tissue portion.

13. The method of claim 1 wherein the deformation causing step further comprises:

causing an additional local deformation in a second surface of the tissue portion opposite the first surface, the second deformation occurring over a substantially smaller area than a region of the first mentioned deformation by probing to additionally compress parts of the tissue portion.

14. The method of claim 1 wherein the deformation causing step further comprises:

causing shear deformation in the tissue portion by laterally shifting one part of the tissue portion relative to another part of the tissue portion on a support structure.

15. The method of claim 14, wherein said support structure is internal skeletal structure.

16. The method of claim 15 wherein the internal skeletal structure is a rib cage and the one part of the tissue portion is adjacent a surface of the tissue overlying the rib cage and is shifted in direction along the rib cage.

17. The method of claim 1 wherein the deformation causing step further comprises:

tilting a support member for the tissue portion while deformation is being caused in the tissue portion.

18. The method of claim 1 wherein the deformation causing step further comprises:

supporting the tissue portion on a surface of a support member while providing a force on a surface of the tissue portion urging the tissue portion toward the support member;

tilting the support member in first and second directions about an axis transverse to the direction of force provided while measuring pressure applied at a plurality of locations on the surface of the tissue portion.

19. The method of claim 1 wherein the tissue portion is a tubular conduit of tissue and the deformation includes expanding the tubular conduit.

20. The method of claim 19 wherein the tubular conduit is of size to receive an expandable probe, and said determining step includes sensing pressure at a plurality of locations on an interior surface of the tubular conduit.

21. The method of claim 19 wherein the tubular conduit is a blood vessel, and the determining step includes providing a tissue imaging device on an interior of the blood vessel.

22. The method of claim 1, wherein the step of causing a deformation of the tissue portion comprises muscle activity affecting and deforming the tissue portion.

23. The method of claim 1 wherein the step of calculating the pattern forming part of the evaluating step further comprises:

calculating deformation induced changes of a ratio for each of a plurality of tissue particles of a horizontal dimension of each tissue particle over a vertical dimension of the same tissue particle to yield a strain characteristic value for each tissue particle;

comparing strain characteristic values of the plurality of tissue particles to identify any region of such tissue particles having strain characteristic values substantially different than the strain characteristic values of tissue particles of a remaining part of the tissue portion, the region corresponding to the location and approximate boundary of a different elasticity region of tissue in the tissue portion.

24. The method of claim 23 and further comprising:

determining the relative elasticity between the different elasticity tissue region and the remaining tissue portion by comparing the different elasticity region strain characteristic value to the remaining tissue portion strain characteristic value.

25. The method of claim 1 wherein said evaluating step includes the step of analyzing under a known condition of deformation changes in the pattern caused by natural mechanical activity of human tissue, to determine structural and dynamic features of human tissue.

26. An apparatus for determining variations in elasticity of bodily tissue comprising:

means for applying pressure to an accessible surface of said tissue to provide at least two conditions of compression loading of tissue to be examined;

means for measuring at least one of the parameters of stress and strain under the at least two different conditions of loading for establishing a pattern of at least one of the parameters in the tissue comprising members that have portions extendable to engage the tissue with a force, and wherein said members comprise a plurality of members in an array across a surface portion of the tissue; and

means for evaluating an elasticity of the tissue from changes in the pattern of at least one of the parameters of stress and strain from at least two different conditions of loading.

27. The apparatus of claim 26 wherein the loading comprises compression loading of the tissue to be examined, and the means for measuring at least one parameter comprises means for determining a surface stress pattern on at least part of the accessible surface from the two different conditions of loading, and the means for evaluating the elasticity of the tissue from the surface stress pattern.

28. The apparatus of claim 27 wherein said means for determining the surface stress pattern comprises an array of pressure sensors for obtaining a surface stress pressure pattern of the compressed tissue.

29. The apparatus of claim 26 wherein the parameter measured is strain and the means for evaluating the elasticity of the tissue comprises means for imaging internal tissue structure at the different conditions of loading to evaluate internal strain patterns by detecting changes in boundaries between different internal tissue portions and means for evaluating the spatial distribution of the elasticity modulus from the strain pattern.

30. The apparatus of claim 29 wherein said means for imaging internal structure comprises means for ultrasonic imaging of the internal structure.

31. The apparatus of claim 29 wherein the means for imaging internal structure comprises a magnetic resonance imaging device.

32. The apparatus of claim 29 wherein said means for imaging internal structure comprises means for providing x-ray imaging of internal tissue.

33. The apparatus of claim 26 wherein the parameter measured is strain, and the means for evaluating the elasticity of tissue comprises means to evaluate internal strain patterns by tagging motion of compressed tissue particles and determining changes in locations of tagged tissue particles at the different conditions of loading.

34. The apparatus of claim 33 wherein said means for imaging internal structure comprises means for ultrasonic imaging of the internal structure.

35. The apparatus of claim 33 wherein the means for imaging internal structure comprises a magnetic resonance imaging device.

36. The apparatus of claim 26 wherein said portions of said members are individually actuatable to provide for a preselected pattern of displacement and forces across the surface portion of the tissue to be examined.

37. The apparatus of claim 36 wherein said members comprise fluid pressure actuated cylinders having first and second piston assemblies, said piston assemblies being substantially concentric, and one piston being annular and surrounding the other, and both pistons being individually actuatable to provide variations in pressures on the tissue portion.

38. The apparatus of claim 37 and means for individually actuating the fluid pressure actuated cylinders in a preselected sequence in localized areas across the portion of the tissue being analyzed to create areas of greater load and less load, and means for imaging the internal structure of the tissue during the individual loading sequence.

CROSS REFERENCE TO RELATED APPLICATION

References is hereby made to co-pending application Ser. No. 07/823,155, filed Jan. 21, 1992 and entitled METHOD AND DEVICE FOR MECHANICAL TOMOGRAPHY OF TISSUE.

BACKGROUND OF THE INVENTION

The present invention relates to an apparatus for determining tissue elasticity in various parts of the body and using such information as a diagnostic tool in the detection of abnormalities of tissue, such as those caused by cancer or other lesions. The "hardness" of tumors can be quantified in terms of the surrounding tissue elastic properties.

Diagnosing early formation of tumors or lesions, particularly those caused by cancer, has been a problem that has been attempted to be solved using various techniques, such as ultrasonic imaging, nuclear magnetic resonance imaging, x-rays, and the like. Each of these techniques have limitations, including the application of radiation to the body, which may be harmful to the body being tested.

One approach attempts to determine the relative stiffness or elasticity of tissue by applying ultrasound imaging techniques while vibrating the tissue at low frequencies. See. e.g., R. M. Lerner et al., Sono-Elasticity: Medical Elasticity Images Derived From Ultrasound Signals in Mechanically Vibrated Targets, Acoustical Imaging, Vol. 16, 317 (1988), Robert M. Lerner et al., "Sonoelasticity" Images Derived from Ultrasound Signals in Mechanically Vibrated Tissues, Ultrasound in Med. & Biol. Vol. 16, No. 3, 231 (1990) and K. J. Parker et al., Tissue Response to Mechanical Vibrations for "Sonoelasticity Imaging", Ultrasound in Med. & Biol. Vol 16, No. 3, 241 (1990).

A variety of other methods have been proposed for measuring the mechanical characteristics, e.g., elasticity, inside soft tissues. One method includes using an ultrasonic wave as a probing wave to observe the mechanical responses of tissues due to cardiac pulsation. The mechanical responses are observed using the ultrasonic wave and then information regarding the mechanical characteristics are estimated on patterns of small movements in the tissue in response to cardiac pulsation. See R. J. Dickinson and C. R. Hill, Measurement of Soft Tissue Motion Using Correlation Between A-Scans, Ultrasound in Med. and Biol. Vol. 8, 263 (1982) and M. Tristam et al., Ultrasonic Study of In Vivo Kinetic Characteristics of Human Tissues, Ultrasound in Med. and Biol. Vol. 12, 927 (1986). The technique uses Fourier analysis to objectively differentiate different tissue types in pathologies based on numerical features of the time-course of a correlation coefficient between pairs of A-Scans recorded with a particular time separation. Tissue oscillations resulting from ventricular contraction and pressure pulses in the descending aorta are measured to derive patterns of movement. Fourier series transformation is used to analyze the data to quantitate the kinetic behavior of the tissue in vivo. M. Tristam et al., Application of Fourier Analysis to Clinical Study of Patterns of Tissue Movement, Ultrasound in Med. and Biol. Vol. 14, 695 (1988).

Another method for estimating the mechanical properties of desired points inside the tissue has been proposed in which low-frequency vibration is applied to the surface and the wave velocity inside the tissue is measured by using the simultaneously transmitted ultrasonic waves. The difference of velocity near the measuring point allows one to derive the elastic property of the tissue. See T. A. Krouskop et al., A Pulsed Doppler Ultrasonic System for Making Non-Invasive Measurement of Mechanical Properties of Soft Tissue, 24 J. Rehab. Res. Dev. Vol. 24, 1 (1987).

Another method of evaluating the elasticity of tissue includes applying low-frequency vibration (e.g., several hundred Hz) to the surface while measuring both the amplitude and phase of internal vibration based on Doppler frequency modulation of simultaneously transmitted probing ultrasonic waves. The amplitude and phase maps are used to observe information that relates to the viscoelastic properties of the tissues. Y. Yamakoshi et al., Ultrasonic Imaging of Internal Vibration of Soft Tissue Under Forced Vibration, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 7, No. 2, Page 45 (1990).

In another method, the wave forms of liver dynamics caused by aortic pulsation and vessel diameter variations are observed by a signal processing technique for analyzing radio frequency M-mode signal patterns of movement (in the liver) in response to the arterial pulsation. The wave forms are used to determine tissue characteristics (displacement, velocity, and strain) as a function of time in small deformations in tissue due to the arterial pulsation. Wilson and Robinson, Ultrasonic Measurement of Small Displacements and Deformations of Tissue, Ultrasonic Imaging Vol. 4 (1982) 71-82.

Another method recently proposed for measuring and imaging tissue elasticity is described in Ophir et al., U.S. Pat. No. 5,107,837. This method includes emitting ultrasonic waves along a path into the tissue and detecting an echo sequence resulting from the ultrasonic wave pulse. The tissue is then compressed (or alternatively uncompressed from a compressed state) along the path and during such compressing, a second pulse of ultrasonic waves are sent along the path into the tissue. The second echo sequence resulting from the second ultrasonic wave pulse is detected and then the differential displacement of selected echo segments of the first and second echo sequences are measured. A selected echo segment of the echo sequence, i.e., reflected RF signal, corresponds to a particular echo source within the tissue along the beam axis of the transducer. Time shifts in the echo segment are examined to measure compressibilities of the tissue regions. This technique is further described in Ophir et al., Elastography: A Quantitative Method for Imaging the Elasticity of Biological Tissues, Ultrasonic Imaging 13, 111 (1991). See also J. Ophir et al., A Transaxial Compression Technique (TACT) For Localized Pulse-Echo Estimation of Sound Speed in Biological Tissues, Ultrasonic Imaging 12, 35 (1990).

It is desirable to have the capability to investigate tissue elasticity changes, which may indicate precursors of tumors or actual tumors without subjecting the patient to radiation. There is also a need for equipment that is easy to use and which requires relatively low capital investment. It also would be desirable to use currently available non-invasive imaging modalities, such as ultrasound, magnetic resonance imaging (MRI), computer aided tomograph (CAT) scanning, and the like.

SUMMARY OF THE INVENTION

The present invention relates to a system of devices which will deform tissue to permit a method of analyzing the elasticity of such deformed tissue using noninvasive techniques. The apparatus includes devices for applying a pressure to living tissue which causes deformation and permits determining the presence and location of tissue that has different elastic characteristics from surrounding tissue. The method of identifying a region of the tissue having a different elasticity than the surrounding tissue includes causing a mechanical deformation of a tissue portion and determining patterns of at least one of the properties of stress and strain in the deformed tissue portions to identify the presence and location of the differing elasticity regions of tissue.

A pattern of stress or strain in a limited area of tissue, together with the geometrical relationship of this limited area to a support member, a deformation member and neighboring anatomical features of that tissue, provides a way of postulating boundary conditions and calculating stress and strain patterns in the region of interest. The obtained relationship contains information about elastic modulus in the region of interest.

The apparatus utilizes various deformation techniques, such as direct pressure heads, moving "fingers" that simulate palpation of tissue, rollers that will roll across a surface of the tissue to be analyzed, and pads, and also may include internal sources of stress or strain such as changes in pressures caused by the variations in blood pressure as well as in muscle contraction.

Various imaging modalities, such as ultrasound, CAT scan, magnetic resonance imaging, and similar techniques that are presently available (or which may be developed for examining internal structures of tissue without invasion of the tissue) can be used in conjunction with the deformation techniques applied to the tissue for analyzing elasticity of the tissue.

As will be shown, various programs can be used for deformation sequences so that automatic changing or cycling of small areas of compression will occur, at the same time that the imaging modalities are operating. In this way, computerized imaging can be used for evaluating stress and strain patterns in the imaged tissue, calculating relative elasticities of the regions of interest and then projecting three-dimensional representation of an area of tissue with the different elastic properties indicated on a screen.

Intracavity elasticity can be analyzed with internal probes inserted into bodily cavities. The deformation of bodily conduits can be examined by using a force sensor array positioned annularly around a central fluid-containing system. By using a rubber-type jacket under the force sensors, pressure variations can be exerted on the internal walls of bodily conduits while the stress pattern can be determined and changes in the elasticity characteristics of the tissue around the conduit can be calculated. Thus, this unit eases the analysis of tumors being formed in the vicinity of the colon, particularly prostate tumors. In addition, the unit can be used as an intrauterine device to determine formations of lesions or the effect of scarring.

While various devices are illustrated for causing deformation of tissue from the exterior, other such devices can be used. The orientation of the devices as well as the pressure exerted, the size of the area being loaded, and other factors can be varied to suit existing conditions based on continuing examinations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a model of soft "tissue" illustrating a device for loading incorporating pressure sensors used in the present invention;

FIG. 2 is the device of FIG. 1 after loading the tissue, and illustrating a typical pressure curve across a surface of the tissue;

FIG. 3 is similar to the tissue compression in FIG. 2, with the effect of a presence of a tumor in the tissue illustrated;

FIG. 4 is an illustration of the structure shown in FIG. 3, with a piston deforming tissue from a side opposite from the pressure plate;

FIG. 5 is a schematic illustration of loading parameters for a model tissue being examined and a tumor in such tissue;

FIG. 5A is a plot of calculated pressure relationships across the surface at differing ratios of moduli of elasticity ratio between surrounding tissue and a tumor;

FIG. 6 is a graphical representation of the calculated relationship between pressure ratios and moduli of elasticity ratios for a loading structure shown in FIG. 5;

FIG. 7 is a schematic representation similar to that shown in FIG. 5 with certain loading parameters illustrated;

FIG.