The present invention relates to imaging methods and devices using shear waves.
More particularly, the invention relates to a method of imaging using shear waves to observe a diffusing viscoelastic medium which contains particles reflecting ultrasonic compression waves, in which method an elastic shear wave is generated in the viscoelastic medium and the displacement of the viscoelastic medium subjected to said shear wave is observed by means of at least one ultrasonic compression wave.
Document U.S. Pat. No. 5,810,731 describes an example of such a method, in which the shear wave is generated locally inside the observed viscoelastic medium, by means of the radiation pressure of a modulated ultrasound wave focussed on a point to be observed. An additional ultrasound wave is then dispatched to this focal point, the reflection of the wave making it possible to ascertain certain propagation parameters of the shear wave (in particular the dynamic viscosity of the medium and its shear modulus) in the vicinity of the abovementioned focal point.
This technique has the drawback of allowing the analysis of just a single point of the viscoelastic medium under study each time a shear wave is generated. If one wishes to obtain a complete image of the observed viscoelastic medium, it is necessary to repeat the operation a very large number of times, this involving a considerable idle time (for example, several minutes) to obtain this image.
This considerable idle time renders this prior art method impractical to use.
Moreover, such an idle time may impede the use of said method to obtain an image of a living tissue, which is always in motion.
An aim of the present invention is in particular to alleviate these drawbacks.
To this end, according to the invention, a method of the kind in question is essentially characterized in that the shear wave is generated by applying to the viscoelastic medium an excitation having the form of a low-frequency pulse which exhibits a central frequency f of between 20 and 5000 Hz, this low-frequency pulse exhibiting a duration of between 1/2f and 20/f, in that it comprises a propagation observation step in the course of which the propagation of the shear wave is observed simultaneously at a multitude of points in the observed medium, these points forming a substantially continuous observation field extending at least along a first axis, this shear wave propagation observation step consisting in:
emitting into the observed medium a succession of at least 10 shots of ultrasonic compression waves at a rate of between 100 and 100 000 shots per second,
detecting and recording in real time the echoes generated by the reflecting particles of the viscoelastic medium at each ultrasonic wave shot, these echoes corresponding (directly or indirectly) to successive images of the observed medium, and in that said method furthermore comprises a subsequent image processing step in the course of which the images thus obtained are processed at a later time at least by cross-correlation between successive images, so as to determine at each point of the observation field a motion parameter chosen between the displacement and the strain of the viscoelastic medium, in such a way as to thus obtain a succession of images showing the evolution of the motion parameter of the viscoelastic medium under the effect of the propagation of the shear wave.
By virtue of these arrangements, a film is obtained which clearly illustrates the propagation of the shear wave in the viscoelastic medium, which may make it possible for example, in medical applications, to directly tag cancerous areas in the tissues of a patient: the propagation of the shear waves in fact occurs there very differently from the neighboring areas.
This tagging is performed much more easily than through conventional observation by simple ultrasound echography, since the propagation of the shear waves is dependent on the shear modulus of the medium, itself varying greatly between an area of healthy tissues and an area of cancerous tissues: the shear modulus typically varies in a ratio of 1 to 30 between a healthy area and a cancerous area, whereas the bulk modulus, which governs the propagation of the acoustic compression waves used in ultrasound echography, varies by only the order of 5% between a healthy tissue and a cancerous tissue.
It will be noted that the film obtained illustrates the propagation of the shear wave much more clearly than the simple succession of images given by the reflecting particles of the medium, since said film makes it possible to view at each instant the areas of the observed medium which undergo motions of the same magnitude on account of the propagation of the shear wave, whereas the succession of images of the reflecting particles would make it possible to view only a haze of bright points in motion.
In preferred embodiments of the method according to the invention, recourse may furthermore possibly be had to one and/or to the other of the following arrangements:
the duration of the low-frequency pulse is between 1/2f and 2f;
the central frequency of the low-frequency pulse is between 30 and 1000 Hz;
the observed viscoelastic medium consists of a living body comprising at least one internal organ subjected to pulsatile motions, the low-frequency pulse which generates the shear wave being constituted by a pulsatile motion of said internal organ;
the observed viscoelastic medium is delimited by an outside surface and the low-frequency pulse is applied in the vicinity of this outside surface;
the low-frequency pulse is applied by a means of excitation chosen from:
an acoustic wave generated by at least one acoustic transducer,
and a shock generated locally by physical contact in the vicinity of the outside surface of the viscoelastic medium;
the ultrasonic compression wave shots are emitted and the echoes generated by the reflecting particles of the viscoelastic medium are detected by means of a bank of transducers which comprises at least one transducer and which is arranged in contact with the outside surface of the viscoelastic medium, the shear wave being applied to the viscoelastic medium by imposing a pulsatile displacement on said bank of transducers;
said motion parameter is the strain of the viscoelastic medium: this arrangement is particularly useful in the last case envisaged hereinabove, since it makes it possible to dispense with the displacement of the bank of transducers, which displacement would otherwise disturb the measurement of the displacement of the points of the observation field;
in the course of the observation of the propagation of the shear wave, between 100 and 10 000 ultrasonic compression wave shots are emitted at a rate of between 100 and 100 000 shots per second;
the observation field extends at least along a plane comprising on the one hand, the first axis and on the other hand, a second axis perpendicular to the first axis;
in the course of the propagation observation step, a bank of several acoustic transducers arranged at least along the second axis is used to emit the ultrasonic compression wave shots and detect the echoes generated by the reflecting particles of the viscoelastic medium, the echoes detected by each acoustic transducer being stored directly without prior processing in the course of the propagation observation step, and the image processing step comprising a preliminary substep of forming paths in the course of which an image of the viscoelastic medium corresponding to each ultrasonic compression wave shot is generated by combining at least some of the echoes received by the various transducers;
the image processing step is followed (immediately or otherwise) by a viewing step in the course of which a film consisting of the succession of processed images is viewed under slow motion, each point of each image exhibiting an optical parameter which varies according to the value of the motion parameter assigned to this point;
the optical parameter is chosen from the gray level and the chromatic level;
the image processing step is followed (immediately or otherwise) by a mapping step in the course of which, on the basis of the evolution of the motion parameter over the course of time in the observation field, at least one propagation parameter is calculated for the shear wave at at least some points of the observation field;
the shear wave propagation parameter which is calculated in the course of the mapping step is chosen from the speed of the shear waves, the shear modulus, the attenuation of the shear waves, the shear elasticity, and the shear viscosity. Moreover, a subject of the invention is also an imaging device using shear waves to observe a diffusing viscoelastic medium which contains particles reflecting ultrasonic compression waves, this device comprising means of excitation for generating an elastic shear wave in the viscoelastic medium and means of acquisition for observing, by means of at least one ultrasonic compression wave, the displacement of the viscoelastic medium subjected to said shear wave, characterized in that the means of excitation are adapted to apply to the viscoelastic medium an excitation having the form of a low-frequency pulse which exhibits a central frequency f of between 20 and 5000 Hz, this low-frequency pulse exhibiting a duration of between 1/2f and 20/f, and in that the means of acquisition are adapted to observe the propagation of the shear wave simultaneously at a multitude of points in the observed medium, these points forming a substantially continuous observation field extending at least along a first axis, said means of acquisition being adapted for:
emitting into the observed medium a succession of at least 10 shots of ultrasonic compression waves at a rate of between 100 and 100 000 shots per second,
detecting and recording in real time the echoes generated by the reflecting particles of the viscoelastic medium at each ultrasonic wave shot, these echoes corresponding (directly or indirectly) to successive images of the observed medium, and in that said device furthermore comprises image processing means adapted to process at a later time the images obtained by the observation means, at least by cross-correlation between successive images, so as to determine at each point of the observation field a motion parameter chosen between the displacement and the strain of the viscoelastic medium, in such a way as to thus obtain a succession of images showing the evolution of the motion parameter of the viscoelastic medium under the effect of the propagation of the shear wave.
In preferred embodiments of the device according to the invention, recourse may moreover possibly be had to one and/or to the other of the following arrangements:
the means of observation comprise a bank of transducers which includes at least one transducer and which is adapted to be arranged in contact with an outside surface delimiting the viscoelastic medium, the means of excitation being adapted to impose a pulsatile displacement on said bank of transducers;
said motion parameter is the strain of the viscoelastic medium;
the observation field extends at least along a plane comprising on the one hand, the first axis and on the other hand, a second axis perpendicular to the first axis, the bank of transducers comprising several transducers arranged at least along the second axis, control means being provided for selectively operating the device either in the mode of imaging by shear waves, or in a standard echography mode making it possible to acquire between 10 and 100 images per second.