Shear wave imaging method and installation for collecting information on a soft solid

Abstract

This shear wave imaging method, for collecting information on a target region (R) of a soft solid (S), comprises at least the steps a) of generating at least one shear wave (SW) in the target region, and b) of detecting a propagation pattern of the shear wave in the target region. Step a) is realized by applying to particles of the target region (R) some Lorentz forces resulting from an electric field (E) and from a magnetic field (B). At least one of the electric field (E) and the magnetic field (B) is variable in time, with a central frequency (f.sub.o) between 1 Hz and 10 kHz. Alternatively, both the electric and magnetic fields (E, B) are variable in time, with a central difference frequency (f.sub.o) between 1 Hz and 10 kHz. The shear wave imaging installation comprises a first system (4, 7) for generating at least one shear wave (SW) in the target region (R) and a second system (10) for detecting a propagation pattern of the shear wave. The first system includes first means (4) to apply an electric field (E) through the target region (R) and second means (7) to apply a magnetic field (B) through the target region. The first and second means are configured to apply to particles of the target region some Lorentz forces resulting from the electric field (E) and the magnetic field (B), where at least one of these fields is a quantity variable in time, with a central frequency (f.sub.o) between 1 Hz and 10 kHz, or both fields are quantities variable in time, with a central difference frequency (f.sub.o) between 1 Hz and 10 kHz.

Claims

1. Shear wave imaging method for collecting information on a target region of a soft solid, the method comprising at least the following steps: a) generating at least one shear wave in the target region, b) detecting a propagation pattern of the at least one shear wave in the target region characterized in that step a) is realized by applying to particles of the target region some Lorentz forces resulting from an electric field and from a magnetic field, at least one of the electric field and the magnetic field is variable in time, with a central frequency between 1 Hz and 10 kHz or both the electric and magnetic fields are variable in time, with a central difference frequency between 1 Hz and 10 kHz.

2. The method according to claim 1, wherein during step a), a first one of the electric field and the magnetic field is variable in time and the second one of the electric field and the magnetic field is constant in time.

3. The method according to claim 2, wherein during step a), the electric field is variable in time and the magnetic field is constant in time.

4. The method according to claim 2, wherein during step a), the magnetic field is variable in time and the electric field is constant in time.

5. The method according to claim 1, wherein during step a), both the electric field and the magnetic field are variable in time, at frequencies between 1 and 10 kHz, or both the electric field and the magnetic field are variable in time at high frequencies, with a central difference frequency between 1 Hz and 10 kHz.

6. The method according to claim 1, wherein the central frequency of the variable field or the central difference frequency is between 5 and 1000 Hz.

7. The method according to claim 1, wherein the electric field is generated by two electrodes located on both sides, on one side or within the soft solid and connected to a source of electric current with an intensity between 1 A and 1 A.

8. The method according to claim 1, wherein the electric field is generated by a second magnetic field, variable in time.

9. The method according to claim 1, wherein the soft solid is a biological tissue of human, animal or vegetal origin, preferably an organ, or an artificial medium.

10. The method according to claim 1, wherein during step b), measurement of the shear wave propagation speed occurs via a sensor assembly based on ultrasound technology.

11. The method according to claim 1, wherein during step b), measurement of the shear wave propagation speed occurs via a sensor assembly based on magnetic resonance imaging (MRI) technology.

12. The method according to claim 11, wherein the magnetic field used during step a) is generated by the sensor assembly based on magnetic resonance imaging (MRI) technology.

13. The method according to claim 1, wherein during step b) a source of the at least one shear wave is detected.

14. Shear wave imaging installation for collecting information of a target region of a soft solid, said shear wave imaging installation comprising: a first system for generating at least one shear wave in the target region a second system for detecting a propagation pattern of the at least one shear wave characterized in that the first system includes first means to apply an electric field through the target region and second means to apply a magnetic field through the target region and in that the first and second means are configured to apply to particles of the target region Lorentz forces resulting from the electric field and the magnetic field, where at least one of the electric field and the magnetic field is a quantity variable in time, with a central frequency (f.sub.0) between 1 Hz and 10 kHz, or both the electric and magnetic fields are quantities variable in time, with a central difference frequency between 1 Hz and 10 kHz.

15. The installation according to claim 14, wherein the first means includes a set of electrodes installed on both sides, on one side or within the soft solid and connected to a source of electric current with an intensity between 1 a and 1 A.

16. The method of claim 6, wherein the central difference frequency is between 50 and 150 Hz.

17. The method of claim 9, wherein the biological tissue is an organ.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood on the basis of the following description which is given in correspondence with the appended figures and as an illustrative example, without restricting the object of the invention. In the annexed figures:

(2) FIG. 1 is a schematic representation of an installation according to a first embodiment of the invention;

(3) FIG. 2 is a view at a larger scale of detail II on FIG. 1;

(4) FIG. 3 is a schematic representation similar to FIG. 1, for an installation according to a second embodiment of the invention;

(5) FIG. 4 is a view at a larger scale of detail IV on FIG. 3;

(6) FIG. 5 is a schematic representation similar to FIG. 1, for an installation according to a third embodiment of the invention; and

(7) FIG. 6 is a schematic representation similar to FIG. 1, for an installation according to a fourth embodiment of the invention.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

(8) The installation 2 represented on FIGS. 1 and 2 includes an alternative current generator 4 which is connected to a first electrode 42 and to a second electrode 44 located on either sides of an electrically conductive soft solid S housed in a box 5. Box 5 is optional and can be omitted if soft solid S stands on its own. Soft solid S can be a portion of an animal body, such as a muscle. In such a case, no box 5 is needed. Soft solid S is considered to be electrically conductive insofar as its conductivity is between 10.sup.3 and 1 Siemens per meter (S/m) for frequencies between 1 Hz and 10 kHz. In other words, at least some particles of soft solid S are electrically loaded, with a positive or negative charge q.sup.+ or q.sup..

(9) One considers a region R of soft solid S to be studied by elastography. In the example of FIGS. 1 and 2, region R corresponds almost to the totality of solid S located in box 5, between electrodes 42 and 44. Alternatively, region R can correspond to a smaller portion of solid S.

(10) Since an alternative current I is applied to electrodes 42 and 44, an alternative electric field {right arrow over (E)} is generated between these electrodes, across region R of soft solid S.

(11) Generator 4 delivers a current with an amplitude I equal to 100 mA, with a sinusoidal shape and a frequency f equal to 100 Hz.

(12) In practice, amplitude I of the current can be selected between 1 A and 1 A and frequency f can vary in a range between 1 Hz and 10 kHz. Preferably, frequency f is chosen in the range 5 to 1000 Hz, most preferably in the range 50 to 150 Hz, whereas the value of 100 Hz gives good experimental results.

(13) One defines the central frequency f.sub.o of a variable signal as the arithmetic mean value of the Fourier transform of this signal. Such a definition is known, e.g., from J. T. Taylor and Q. Huang in CRC Handbook of electrical filters (1997).

(14) Frequency f can be a central frequency f.sub.o in the sense that any modulated signal can be used for the electric field E. Any wave or signal mixing that result in a central frequency as mentioned here-above is suitable for the methods and installations of the invention.

(15) In the present example, as current I delivered by generator 4 is sinusoidal, electric field {right arrow over (E)} is sinusoidal, with the same frequency f, and the central frequency f.sub.o of electric field {right arrow over (E)} equals the central frequency of the current delivered by generator 4.

(16) Alternatively, the current delivered by generator 4 is not sinusoidal but has the form of a square wave or of one or several picks. In such a case, central frequency f.sub.o can be calculated via a Fourier transform, as explained here-above.

(17) Region R of soft solid S is also subjected to a constant or permanent magnetic field {right arrow over (B)}. This permanent magnetic field {right arrow over (B)} is generated across soft solid S by a permanent magnet 7. Alternatively, one can use an electromagnet instead of permanent magnet in order to generate magnetic field {right arrow over (B)}. The field strength of magnetic field of {right arrow over (B)} is between 10 mT and 10 T. The direction of magnetic field {right arrow over (B)} is perpendicular to the plane of FIG. 1, that is perpendicular to electric field {right arrow over (E)}. In this example, magnetic field {right arrow over (B)} is directed away from the viewer of FIGS. 1 and 2.

(18) Reference 6 denotes a closed envelope, preferably shielded against magnetic perturbations, inside which magnetic field {right arrow over (B)} is generated.

(19) Coming now to FIG. 2, one considers a particle P.sub.1 of soft solid S which is loaded with a positive electric charge q.sup.+. One considers the case where electric field {right arrow over (E)} is directed from electrode 42 towards electrode 44, as shown by the arrow in solid line on FIG. 2. Because of electric field {right arrow over (E)}, particle P.sub.1 is subjected to an electric force {right arrow over (F.sub.E1)} which equals q.sup.+.Math.{right arrow over (E)} and is parallel to electric field {right arrow over (E)} and oriented in the same direction because electric charge q.sup.+ is positive. This corresponds to the first part of equation 1.

(20) Under the effect of this electric force, particle P.sub.1 moves in the same direction and has a velocity {right arrow over (V)}.sub.1 oriented in the same direction. Because of this velocity {right arrow over (V)}.sub.1 and of the second part of equation 1, particle P.sub.1 is also subjected to a magnetic force {right arrow over (F.sub.M1)} which equals q.sup.+.Math.{right arrow over (V)}.sub.1 {right arrow over (B)} and which is oriented to the right on FIG. 2. Thus, particle P.sub.1 is subjected to a Lorentz force {right arrow over (F.sub.L1)} oriented to the right and towards electrode 44 on FIG. 2.

(21) When the polarity between electrodes 42 and 44 is inverted, then electric field {right arrow over (E)} is oriented as shown by the arrow in dashed line on FIG. 2 and, for the same reasons as explained previously, particle P is subjected to a Lorentz force {right arrow over (F.sub.L1)} which is the sum of an electric force {right arrow over (F.sub.E1)} and a magnetic force {right arrow over (F.sub.M1)} respectively oriented in opposite directions to forces {right arrow over (F.sub.E1)} and {right arrow over (F.sub.M1)}.

(22) {right arrow over (F.sub.E1)} equals q.sup.+.Math.{right arrow over (E)} and {right arrow over (F.sub.L1)} equals q.sup.+.Math.{right arrow over (V)}.sub.1 {right arrow over (B)} where {right arrow over (V)}.sub.1 is the velocity of particle P.sub.1 under the effect of electric field {right arrow over (E)} oriented towards electrode 42.

(23) Thus, because of the fact that electric field {right arrow over (E)} is variable in time, particle P.sub.1 is subjected to alternative Lorentz forces {right arrow over (F.sub.L1)} and {right arrow over (F.sub.L1)}, which generates shear waves as shown by axis lines SW at the bottom of FIG. 2.

(24) If one considers another particle P.sub.2 with a negative electric charge q.sup. as represented on top of FIG. 2, then the force distribution will be reversed to the force distribution for particle P.sub.1. Nevertheless, this particle P.sub.2 will also be subjected to alternative Lorentz forces {right arrow over (F.sub.L2)} and {right arrow over (F.sub.L2)}. This will also contribute to the shear wave generation represented by axis lines SW at the bottom of FIG. 2.

(25) In summary, positively loaded particles P.sub.1 and negatively loaded particles P.sub.2 of soft solid S have roughly the same behavior and <<shake>> because of alternatively changing Lorentz forces, which generates shear waves SW in soft solid S.

(26) As explained here-above, the propagation speed of the shear waves SW can be considered as representative of the shear elastic modulus of soft solid S in region R, irrespective of whether or not the region is elastically homogeneous.

(27) Installation 2 also includes an ultrasonic probe 10 which can be of any known type, e.g. of the type mentioned in U.S. Pat. No. 6,770,033. This probe 10 is connected to an ultrasound scanner 12 which is provided with a speckle tracking module, so that, as known in the art, this scanner is capable of measuring a propagation speed of shear waves SW within region R.

(28) An elastography method implemented with installation 2 is now described: first, one defines target region R of soft solid S as the portion of this soft solid located between electrodes 42 and 44, within envelope 6. When the generator 4 is actuated, this region R is subjected to electric field {right arrow over (E)} and magnetic field {right arrow over (B)}. As mentioned here-above, this results in submitting positive and negative particles P.sub.1 and P.sub.2 to variable Lorentz forces, which generates shear waves SW in the target region R. Depending on the distribution and strength of electric field {right arrow over (E)} and magnetic field {right arrow over (B)}, one or several shear waves is/are generated. This or these shear waves can then be detected by ultrasonic probe 10 which sends a corresponding electronic signal S.sub.10 to ultrasound scanner 12. The speckle tracking module of ultrasound scanner 12 detects the propagation speed of shear waves SW in ant direction.

(29) If region R is elastically homogeneous, the propagation speed of shear waves SW within region R is constant and a measure of this speed allows to determine, via the speckle tracking module of ultrasound scanner 12, the shear elastic modulus of sift solid S within this region.

(30) If region R is elastically heterogeneous, the different propagation speeds of shear waves SW with region R can be measured and a map of the corresponding shear elastic modulus values, within region R, can be established.

(31) Thus, the method of the invention is an elastography method and it is very efficient to detect any elasticity inhomogeneity, even in a deep region of tissue R, which is relatively far away from its boundaries considered as the portion of tissue R close to electrodes 42 and 44.

(32) The frequency used for working ultrasonic probe 10 is between 100 kHz and 100 MHz, preferably between 2 MHz and 20 MHz, with a pulse repetition frequency between 10 Hz and 100 kHz, preferably between 1 kHz and 5 kHz.

(33) In the second, third and fourth embodiments of the invention respectively represented on FIGS. 3 to 6, the same pieces of equipment as in the first embodiment have the same reference numbers. Here-after, only the differences with the first embodiment are listed.

(34) In the embodiment of FIGS. 3 and 4, electrodes 42 and 44 are located on one side of soft solid S.

(35) If an electrical impedance inhomogeneity 8 is present in region R of soft solid S, then the behavior of its positive and negative particles under the effect of Lorentz forces is different from the behavior of the positive and negative particles P.sub.1 and P.sub.2 of the rest of tissue T. Shear waves SW8 generated in the region of this inhomogeneity 8 have a different pattern than shear waves SW, as shown in FIG. 4.

(36) According to another method of the invention, it is possible to locate, thanks to ultrasound scanner 12, the source region of shear waves SW8, which can be identified as an impedance inhomogeneity. On this basis, it is possible to draw a map or tomographic image of the electrical conductivity of soft solid S within region R. Thus, an electric tomography method is implemented.

(37) If soft solid S is elastically and electrically heterogeneous, then a combination of the elastography and electric tomography methods mentioned here-above in reference to FIGS. 2 and 4 can be implemented.

(38) In the embodiment of FIG. 5, one uses a Magnetic resonance imaging (MRI) system assembly 20 to measure shear waves propagation speed in the target region R of a soft solid S. MRI assembly 20 includes a MRI probe 202 and an MRI magnetic field generator 204, which generates a permanent or constant magnetic field {right arrow over (B)}.

(39) An electric field {right arrow over (E)} is generated as in the first embodiment, by a sinusoidal current generator 4 which is connected to electrodes 42 and 44.

(40) In this embodiment, one takes advantage from the fact that a permanent magnetic field is generated for MRI subassembly 20, by field generator 204. This permanent magnetic field is used as the constant magnetic field {right arrow over (B)} of the first embodiment to generate the shear waves, in conjunction with electric field {right arrow over (E)}.

(41) With this embodiment, one can implement an elastography method, similar to the one mentioned in reference to FIG. 2, and/or an electric tomography method, similar to the one mentioned in reference to FIG. 4.

(42) In the embodiment of FIG. 6, a variable magnetic field {right arrow over (B)} is generated, with a central frequency f.sub.o between 1 Hz and 10 kHz, preferably in the same range as the one mentioned for the electric field {right arrow over (E)} of the first embodiment. The strength of this variable magnetic field is between 10 mT and 10 T. The pulse shape of the magnetic field {right arrow over (B)} can be sinusoidal, square or have any arbitrary wave form. This variable magnetic field {right arrow over (B)} is created by an electro-magnet 17.

(43) A static or constant electric field {right arrow over (E)} is generated between two electrodes 42 and 44 via a generator of DC current 14. The static electrical current generated by generator 14 can have an intensity between 1 A and 1 A.

(44) As mentioned with respect to the first embodiment, the positive and negative particles of a region R of soft solid S located between electrodes 42 and 44 are subjected to variable Lorentz forces, which generates shear waves.

(45) An ultrasonic probe 10 is connected to an ultrasound scanner 12 and measures the propagation speed of the shear waves, as in the first embodiment, in order to implement an elastography method. Alternatively or in addition, an electric tomography method can be implemented on the basis of the detection of sources of shear waves corresponding to impedance inhomogeneities. Alternatively, a MRI subassembly can be used, instead of ultrasonic probe 10 in the fourth embodiment.

(46) According to an alternative embodiment which is not represented on the figures, one can use two variable fields, namely a variable electric field {right arrow over (E)} and a variable magnetic field {right arrow over (B)}, each of them having a central frequency f.sub.o between 1 Hz and 10 kHz. According to still another alternative embodiment which is not represented, each one of electric E and magnetic field {right arrow over (B)} is a high frequency field, preferably a field modulated in amplitude, frequency and/or phase. In such a case, the respective frequencies of these fields, which can be as high as several MHz, are chosen so that their central difference frequency f.sub.o, computed on the basis of their difference frequency f, is in the range 1-10 kHz, preferably 5 to 1000 Hz, more preferably between 50 and 150 Hz.

(47) According to another non represented alternative embodiment, the electrical field {right arrow over (E )} can be generated by a second magnetic field {right arrow over (B)}, because a variable magnetic field generates an electrical field, leading to eddy currents in the soft solid S.

(48) According to still another embodiment which is not represented, electrodes 42 and 44 can be installed within region R, instead of on one side, either sides or around this solid for the embodiments shown on the figures.

(49) The invention is explained here-above when fields {right arrow over (E)} and {right arrow over (B)} are perpendicular to each other. It can also be implemented with non perpendicular fields {right arrow over (E)} and {right arrow over (B)}, provided that they are not collinear.

(50) Irrespective of the actual method used to generate the shear waves SW, as explained here-above, the ultrasonic probes 10 or the MRI subassembly 20 can detect an abnormality, such as a cancerous tumor, as an inhomogeneity 8. In a zone of target region R where such a cancerous tumor exists, soft solid S is not as soft as in the other zone, so that shear waves tend to propagate quicker than in the other zones. This difference in shear waves propagation speed can then be interpreted as an indication that a cancerous tumor is present or might be present in this region.

(51) The invention is not limited to the detection of tumors in organs. It can be used to characterize different types of soft solids, animal or vegetal soft solids and soft material in cosmetic or food industry. The invention can also be used to characterize non metallic portions of prostheses.

(52) In order to increase the accuracy of the method of the invention, metallic particles can be injected in the soft solid. Alternatively, a conductive liquid, such as salty water, can be injected in the bladder of a patient in order to facilitate generation of shear waves in the prostate, the stomach, the liver or the pancreas.

(53) The invention can be implemented at a macroscopic level, as explained here-above, and also at a microscopic level. In particular, a biological cell can be considered as soft solid for implementing the method of the invention.

(54) The embodiments and variants considered here-above can be combined in order to generate new embodiments of the invention.