Method for measuring an ultrasonic attenuation parameter guided by harmonic elastography, probe and device for the implementation of the method

Abstract

A method for measuring an ultrasonic attenuation parameter guided by harmonic elastography including applying, using a vibrator included in a probe in contact with a viscoelastic medium, of a continuous low frequency vibration, the continuous low frequency vibration generating an elastic wave within the viscoelastic medium and generating, during the propagation of the elastic wave, using an ultrasonic transducer in contact with the viscoelastic medium, a series of ultrasonic acquisitions, the series of ultrasonic acquisitions including groups of ultrasonic acquisitions, the groups of ultrasonic acquisitions being generated with a repetition rate, each group of ultrasonic acquisitions including at least one acquisition; the ultrasonic attenuation parameter being measured from the ultrasonic acquisitions realised during the application of the continuous low frequency vibration.

Claims

1. A method for measuring an ultrasonic attenuation parameter guided by harmonic elastography comprising applying, using a vibrator comprised in a probe in contact with a viscoelastic medium, a continuous low frequency vibration, the continuous low frequency vibration generating an elastic wave within the viscoelastic medium and generating, during the propagation of the elastic wave, using an ultrasonic transducer in contact with the viscoelastic medium, a series of ultrasonic acquisitions, said series of ultrasonic acquisitions including groups of ultrasonic acquisitions, the groups of ultrasonic acquisitions being generated with a repetition rate, each group of ultrasonic acquisitions including at least one acquisition, the ultrasonic attenuation parameter being measured from the ultrasonic acquisitions.

2. The method for measuring an ultrasonic attenuation parameter guided by harmonic elastography according to claim 1, further comprising determining, from the series of ultrasonic acquisitions, at least one property of the propagation of the elastic wave within the viscoelastic medium.

3. The method for measuring an ultrasonic attenuation parameter guided by harmonic elastography according to claim 2, wherein the property of the propagation of the elastic wave within the viscoelastic medium is used to compute a real time positioning indicator of the probe compared to the viscoelastic medium to study.

4. The method for measuring an ultrasonic attenuation parameter guided by harmonic elastography according to claim 3, further comprising displaying in real time the real time positioning indicator.

5. The method for measuring an ultrasonic attenuation parameter guided by harmonic elastography according to claim 2, wherein the propagation property of the measured elastic wave is selected from one of amplitude of the elastic wave, phase of the elastic wave, phase velocity of the elastic wave, elasticity of the viscoelastic medium, Young's modulus of the viscoelastic medium and shear modulus of the viscoelastic medium.

6. The method for measuring an ultrasonic attenuation parameter guided by harmonic elastography according to claim 1, wherein the application of a continuous low frequency vibration is only triggered if the contact force between the vibrator and the viscoelastic medium is above a predetermined lower threshold.

7. The method for measuring an ultrasonic attenuation parameter guided by harmonic elastography according to claim 1, wherein the application of a continuous low frequency vibration is only triggered if the contact force between the vibrator and the viscoelastic medium is above a predetermined lower threshold and below a predetermined upper threshold.

8. The method for measuring an ultrasonic attenuation parameter guided by harmonic elastography according to claim 1, wherein the series of ultrasonic acquisitions is formed by a repetition of groups including at least two ultrasonic acquisitions having an intra-group repetition rate comprised between 500 Hz and 10 kHz and a repetition rate comprised between 10 Hz and 10 kHz.

9. The method for measuring an ultrasonic attenuation parameter guided by harmonic elastography according to claim 1 wherein the repetition rate is lower than a frequency of the continuous low frequency vibration.

10. The method for measuring an ultrasonic attenuation parameter guided by harmonic elastography according to claim 1, wherein the ultrasonic attenuation parameter is an instantaneous parameter and a quality coefficient associated with the measurement of the instantaneous ultrasonic attenuation parameter is computed from a property of the propagation of the elastic wave within the viscoelastic medium and/or properties of the ultrasonic wave.

11. The method for measuring an ultrasonic attenuation parameter guided by harmonic elastography according to claim 10, wherein an average ultrasonic attenuation parameter is computed from a plurality of instantaneous ultrasonic attenuation parameters and quality coefficients associated with each instantaneous ultrasonic attenuation parameter.

12. A probe for measuring an ultrasonic attenuation parameter guided by harmonic elastography comprising: a vibrator configured to apply to the viscoelastic medium a continuous low frequency vibration, the continuous low frequency vibration generating an elastic wave within the viscoelastic medium; an ultrasonic transducer configured to emit a series of ultrasonic acquisitions, said series of ultrasonic acquisitions including groups of ultrasonic acquisitions, the groups of ultrasonic acquisitions being generated with a repetition rate, each group of ultrasonic acquisitions including at least one acquisition; a system adapted to compute and display in real time a positioning indicator of the probe, said positioning indicator being computed from a propagation property of the elastic wave, said propagation property of the elastic wave being determined from the series of ultrasonic acquisitions; the ultrasonic attenuation parameter being measured from the ultrasonic acquisitions realised during the application of the continuous low frequency vibration.

13. The probe for measuring an ultrasonic attenuation parameter guided by harmonic elastography according to claim 12, said probe being further configured to apply the continuous vibration when the contact force between the probe and the viscoelastic medium is above a predetermined value.

14. The probe for measuring ultrasonic attenuation parameter guided by harmonic elastography according to claim 12, wherein the transducer is borne by the vibrator.

15. The probe for measuring an ultrasonic attenuation parameter guided by harmonic elastography according to claim 13 configured to compute an average ultrasonic attenuation parameter, said average ultrasonic attenuation parameter being computed from a multiplicity of instantaneous ultrasonic attenuation parameters and quality coefficients associated with the instantaneous ultrasonic attenuation parameters.

16. A device for measuring an ultrasonic attenuation parameter guided by harmonic elastography comprising: a probe for measuring of the ultrasonic attenuation guided by harmonic elastography according to claim 12, and a central unit connected to the probe and including at least computing system for processing reflected ultrasonic signals, a display and a control and/or input system.

Description

LIST OF FIGURES

(1) Other characteristics and advantages of the invention will become clear from the description that is given thereof below, for indicative purposes and in no way limiting, with reference to the appended figures among which:

(2) FIG. 1 illustrates the steps of the hybrid elastography method according to the invention;

(3) FIG. 2 schematically shows the vibrations applied by the vibrator and the ultrasonic acquisitions during the implementation of the method according to the invention illustrated in FIG. 1;

(4) FIG. 3 schematically shows a particular embodiment of the elastography method illustrated in FIG. 1, called stroboscopic mode;

(5) FIG. 4 shows the results obtained by implementing the part of the method according to the invention relative to the positioning of the vibrator;

(6) FIG. 5 represents a hybrid elastography probe according to the invention;

(7) FIG. 6a represents a particular embodiment of the hybrid elastography probe according to the invention;

(8) FIG. 6b represents a hybrid elastography device according to the invention.

DETAILED DESCRIPTION

(9) FIG. 1 illustrates the steps of the hybrid elastography method P according to the invention.

(10) The method P according to the invention includes a step CW of applying a continuous low frequency vibration using a vibrator comprised in a probe in contact with the viscoelastic medium.

(11) The central frequency of the continuous vibration is comprised between 5 and 500 Hz.

(12) The step CW of the method P further includes the generation, by the ultrasonic transducer, of a series of ultrasonic acquisitions. The series of ultrasonic acquisitions includes groups of ultrasonic acquisitions. The groups of ultrasonic acquisitions are emitted with a repetition rate comprised between 5 Hz and 500 Hz, each group including at least one ultrasonic acquisition.

(13) The repetition rate of the ultrasonic groups is also called inter-group repetition rate.

(14) An ultrasonic acquisition includes the emission of an ultrasonic shot followed by the detection and the recording of the reflected ultrasonic signals or echoes.

(15) The application of a continuous vibration to the viscoelastic medium generates an elastic wave within said medium. The elastic wave includes a superimposition of shear waves and compression waves. The study of the properties of this elastic wave makes it possible to obtain information concerning the correct positioning of the probe with regard to the viscoelastic medium.

(16) The viscoelastic medium to characterise diffuses at least partially the ultrasonic shots. It is thus possible to detect the ultrasonic signals reflected during the emission of the first series of ultrasonic acquisitions.

(17) The detection of the reflected ultrasonic signals may be carried out using the same ultrasonic transducer used for the emission.

(18) An ultrasound attenuation parameter may be determined from the reflected ultrasonic signals. For example the value CAP_I of the ultrasonic attenuation corresponding to a given ultrasonic acquisition may be determined. The value CAP_I is also called individual or instantaneous value of the ultrasonic attenuation or instantaneous ultrasonic attenuation parameter.

(19) The reflected ultrasonic signals detected during the step CW are successively processed during a step of determination of at least one property of the propagation of the elastic wave within the viscoelastic medium CW_P.

(20) Typically, during this step, the reflected ultrasonic signals are correlated with each other so as to measure the displacements of the viscoelastic medium brought about by the elastic wave generated by the application of the continuous vibration, according to a known technique in the field of elastography and more generally ultrasounds.

(21) From the displacements measured within the viscoelastic medium, it is possible to compute properties of the elastic wave such as the amplitude and the phase as a function of the position within the viscoelastic medium. The position of a point within the viscoelastic medium is measured as the distance between the ultrasonic transducer and said point computed along the direction of propagation of the ultrasounds emitted by the transducer. For this reason the position of a point within the viscoelastic medium is generally called depth.

(22) It is also possible to determine other parameters of the elastic wave within the viscoelastic medium, such as the phase velocity or the attenuation of the elastic wave.

(23) The variations in the amplitude and the phase of the elastic wave as a function of depth within the tissue may be computed. By making an adjustment between the theoretical model and the measured properties, it is possible to extract an adjustment quality parameter. From this adjustment quality parameter and/or other properties of the elastic wave, it is possible to compute a positioning indicator RT_IP of the probe with respect to the tissue to characterise.

(24) For example, one of the theoretical models used provides a linear variation of the phase lag at the central frequency of the elastic wave with depth in the medium to characterise. In this case, the adjustment is a linear adjustment and the adjustment quality parameter translates the linearity of the phase as a function of depth in the medium. A possible indicator is the determination coefficient R.sup.2 giving the quality of the prediction of the linear regression of the curve of the phase lag as a function of depth in the studied depth range.

(25) According to one embodiment, the step CW_P of determination of at least one property of the elastic wave within the tissue is carried out at the same time as the step of application of the continuous vibration CW and detection of the first reflected ultrasonic signals.

(26) Thanks to the method P according to the invention it is thus possible to measure in real time the properties of the elastic wave within the tissue and to obtain in real time the positioning indicator of the probe RT_IP.

(27) Advantageously, a low repetition rate makes it possible to reduce the size of the data recorded during the step of generation of the series of ultrasonic acquisitions CW and to process these data in real time to obtain the positioning indicator RT_IP. The value of this indicator is typically comprised between 0 and 1. The value 0 corresponds to a poor indicator and the value 1 to a good indicator.

(28) Advantageously, the positioning indicator is provided to the operator to help him to find a satisfactory measurement point. It may be provided for example (non-exhaustive list) in the form of a display of a coloured indicator, in the form of a more or less long bar, etc.

(29) A quality coefficient of the measurement of the ultrasonic attenuation, CAP_C, is also computed from the ultrasonic signal. The value of this coefficient is typically comprised between 0 and 1. The value 0 corresponds to a low quality and the value 1 to a high quality.

(30) The coefficient CAP_C is associated with the individual value of the ultrasonic attenuation CAP_I obtained from the ultrasonic data during acquisition.

(31) The quality coefficient CAP_C may for example be computed from only the properties of the ultrasonic signal. It may also be a combination of the quality of the ultrasonic signal and the properties of the elastic wave.

(32) According to one embodiment, the individual value of the ultrasonic attenuation is only conserved if the positioning indicator RT_IP is correct. In the case where the positioning indicator RT_IP is incorrect, the corresponding coefficient CAP_C is for example set at zero.

(33) According to one embodiment, the continuous vibration is only triggered if the contact force F between the vibrator and the viscoelastic tissue is above a predetermined lower threshold. The value of the threshold is typically 1 N.

(34) Advantageously, this lower threshold ensures sufficient coupling between the probe and the viscoelastic medium.

(35) According to one embodiment, the continuous vibration is only triggered if the contact force F between the vibrator and the viscoelastic tissue is below an upper predetermined threshold. The value of the threshold is typically 10 N.

(36) Advantageously, this upper threshold ensures that the vibration is not deformed and that the studied medium is not damaged.

(37) On account of the continuous vibratory movement of the vibrator, the determination of the contact force F between the vibrator and the medium is more complex than in the case of a standard transient elastography method. In the presence of the continuous low frequency vibration, the contact force between the vibrator and the viscoelastic medium is given by the following formula:
F=k(x+A×cos(2π.sub.ηlowt))

(38) In this formula x is the displacement of the vibrator, k the elastic constant of the spring placed in the probe, A the amplitude of the continuous vibration and flow the continuous vibration frequency.

(39) The force F may be measured using a force sensor placed on the hybrid elastography probe. Successively by applying a low pass filter to the signal thereby measured, it is possible to eliminate the low frequency part and to deduce the average contact force:
F.sub.average=k(x)

(40) Advantageously the value of the average force applied is provided to the operator so that he adapts it in order that the low frequency vibration and the acquisition of data continue.

(41) Advantageously the individual values CAP_I are accumulated in a memory and used to compute an average value CAP_M. CAP_M may be computed in several ways.

(42) For example:

(43) $\mathrm{CAP\_M}=\frac{{.Math.}_{i=1}^N\left(\mathrm{CAP\_I}\left(i\right)\times \mathrm{CAP\_C}\left(i\right)\right)}{{.Math.}_{i=1}^N\left(\mathrm{CAP\_C}\left(i\right)\right)}$

(44) The values CAP_C are then used to weight the individual measured values CAP_I. The value CAP_M is conserved at the end of the examination as being the measured ultrasonic attenuation value. The unit of the value CAP_M is for example dB/m.

(45) FIG. 2 schematically shows: The continuous low frequency vibration cSW applied by the vibrator during the step CW illustrated in FIG. 1; The series of ultrasonic acquisitions PA formed by the groups G of acquisitions and generated by the ultrasonic transducer during the step CW illustrated in FIG. 1.

(46) During the step of application of a continuous vibration CW, the vibrator oscillates at a frequency comprised between 5 and 500 Hz, with an amplitude comprised between 10 μm and 5 mm.

(47) Advantageously, thanks to the low amplitude and to the low frequency of the continuous vibration, an operator can easily maintain the probe in contact with the viscoelastic medium.

(48) At the same time as the application of the continuous low frequency vibration, the ultrasonic transducer emits ultrasonic acquisitions PA formed by groups G of ultrasonic acquisitions. In the example illustrated in FIG. 2, each group G includes two ultrasonic acquisitions.

(49) The groups G of ultrasonic acquisitions are emitted with a repetition rate LPRF comprised between 10 Hz and 500 Hz or inter-group repetition rate or simply repetition rate. The ultrasonic acquisitions belonging to a same group G are emitted with an intra-group repetition rate HPRF comprised between 500 Hz and 10 kHz. The ultrasonic transducer also detects the ultrasonic signals reflected during the generation of the ultrasonic acquisitions PA, as explained with reference to the step CW illustrated in FIG. 1. From the first series of ultrasonic acquisitions PA, it is possible to compute, by a step of correlation CORR between ultrasonic signals belonging to the same group G, the displacements of the viscoelastic medium. Said displacements of the viscoelastic medium are generated by the propagation of the elastic wave generated by the continuous vibration applied by the vibrator.

(50) It is important to note that a large number of possible ultrasonic sequences for the implementation of this method exist and that the elements indicated do not in any way constitute an exhaustive list of possible fields.

(51) Advantageously, by applying a correlation technique to the ultrasonic acquisitions belonging to a same group G—and thus brought closer in time—it is possible to detect small displacements of the order of 1 μm to 10 μm.

(52) As explained with reference to the step CW_P illustrated in FIG. 1, the displacements of the viscoelastic medium are next used to compute the properties of the elastic wave such as its amplitude and its phase as a function of depth in the medium. By comparing the measured properties with a theoretical model it is possible to deduce in real time a positioning indicator RT_IP.

(53) For example, the positioning indicator may be linked to the linearity of the phase of the elastic wave as a function of depth in the medium to characterise. The indicator then depends on the quality of the adjustment of the evolution of the phase as a function of depth by a straight line.

(54) For example, the positioning indicator may be linked to the decrease in the amplitude of the elastic wave as a function of depth in the medium to characterise. The indicator then depends on the quality of the fit in 1/Z.sup.n where Z is the depth and n an integer coefficient comprised between 1 and 3.

(55) For example, the value of the real time positioning indicator RT_IP is comprised between 0 and 1, with values close to 1 if the probe is correctly positioned with respect to the viscoelastic medium of interest.

(56) FIG. 3 shows a particular embodiment of the steps CW and CW_P of the method P according to the invention, called stroboscopic mode.

(57) The continuous sinusoidal line schematically represents the continuous vibration cSW applied by the first vibrator. The continuous vibration cSW has for example a central frequency cSWF of 50 Hz corresponding to a period of 20 ms.

(58) The continuous vertical lines represent the groups G of ultrasonic acquisitions forming the first series of ultrasonic acquisitions PA. The groups G are emitted with a first repetition rate LPRF. According to the stroboscopic acquisition mode, the first repetition rate LPRF is smaller than the central frequency of the continuous vibration cSWF.

(59) The intra-group repetition rate is comprised between 500 Hz and 100 kHz, which makes it possible to measure small displacements of the order of 1 μm.

(60) The white circles and the arrows along the continuous vibration cSW correspond to the samplings made by each group G of ultrasonic acquisitions.

(61) Thanks to the fact that the repetition rate LPRF of the groups G is less than the central frequency of the continuous vibration cSW, it is possible to sample in a complete manner the continuous vibration cSW at the end of several oscillation periods, as is illustrated by the white circles.

(62) Advantageously, the stroboscopic mode makes it possible to sample in a complete manner the continuous vibration cSW while using a low first repetition rate LPRF.

(63) The use of a low repetition rate makes it possible to process the reflected signals in real time and thus to obtain the positioning indicator RT_IP in real time.

(64) FIG. 4 schematically shows the results obtained by implementing the part of the method P according to the invention relative to the positioning of the vibrator.

(65) The graph CW Disp shows the displacement (or any other movement parameter such as the velocity, the deformation, the deformation rate) of the viscoelastic medium in a region of interest ROI as a function of depth Z in the medium and of time T. The displacements are represented using a false colour scale, the lighter colours representing a displacement along the positive direction of the axis D. The displacements are caused by the continuous low frequency vibration applied by the vibrator and are measured by the ultrasonic transducer UT placed in contact with the surface of the medium, in Z=0.

(66) From the displacements measured CW Disp in the region of interest ROI within the viscoelastic medium, it is possible to extract in real time information RT_Info concerning the elastic wave propagating within the medium and generated by the continuous vibration. Examples of such properties are amplitude A and phase Ph of the elastic wave as a function of depth within the medium.

(67) By comparing the values of A and Ph measured with predetermined thresholds it is possible to determine a positioning indicator of the vibrator with respect to the viscoelastic medium.

(68) Alternatively, it is possible to obtain an adjustment quality parameter AJ between the measured quantities A and Ph and a theoretical model describing the amplitude and phase of an elastic wave propagating within the medium. In this case, the positioning indicator is obtained from the adjustment quality parameter AJ. For example, an adjustment quality parameter is the determination coefficient R.sup.2 giving the quality of the prediction of the linear regression of the curve of the phase lag as a function of depth in the studied depth range.

(69) According to one embodiment, the adjustment quality parameter AJ is comprised between 0 and 1.

(70) Once calculated, the positioning indicator may be displayed in the form of a number or a letter or by using a colour scale. Alternatively, the positioning indicator may be a simple visual indication of the disc type colourful. Alternatively, the positioning indicator may be a simple visual indication of “Positioning OK” type indicating that the operator can trigger the transient elastography step.

(71) According to one embodiment, the propagation velocity of the elastic wave is conserved as a measurement of the elasticity of the medium.

(72) During the implementation of the method P according to the invention, the graphs CW_Disp, RT_Info and the positioning indicator of the vibrator are computed and displayed concomitantly.

(73) Advantageously, thanks to the structure of the series of ultrasonic acquisitions, the positioning indicator RT_IP as well as the graph RT_Info may be computed and displayed in real time.

(74) FIG. 5 schematically represents a probe PR for the measurement of the ultrasonic attenuation guided by harmonic elastography.

(75) The probe PR includes: A vibrator VIB configured to apply to the viscoelastic medium a continuous low frequency vibration, the continuous low frequency vibration generating an elastic wave within the viscoelastic medium; An ultrasonic transducer TUS configured to emit a series of ultrasonic acquisitions, said series of ultrasonic acquisitions including groups of ultrasonic acquisitions, the groups of ultrasonic acquisitions being generated with a repetition rate, each group of ultrasonic acquisitions including at least one acquisition.

(76) According to the embodiment illustrated in FIG. 1, the ultrasonic transducer TUS is mounted on the axis of the vibrator VIB.

(77) According to one embodiment, the probe PR includes computing means for computing in real time the positioning indicator RT_IP from the ultrasonic acquisitions.

(78) According to one embodiment, the probe PR includes means for computing and displaying the real time positioning indicator RT_IP.

(79) According to one embodiment, the refresh rate of the display of the positioning indicator is greater than 5 Hz.

(80) Advantageously, the display of the real time positioning indicator at the level of the probe allows the operator to optimise the positioning of the probe without diverting his eyes from the probe and from the body of the patient. This simplifies the operation of positioning the probe.

(81) According to one embodiment, the ultrasonic transducer TUS may be fixed to the body of the probe using a tip PT.

(82) The vibrator VIB makes the probe PR oscillate. During this oscillation, the ultrasonic transducer TUS is pushed against the viscoelastic medium applying the continuous low frequency vibration and creating the elastic wave within the medium.

(83) According to one embodiment, the vibrator VIB for the application of the low frequency vibration includes a vibratory ring placed around the ultrasonic transducer TUS or around the probe tip PT.

(84) According to one embodiment, the probe tip PT is moveable and may be actuated by the vibrator VIB. The ultrasonic transducer TUS is then pushed against the viscoelastic medium to apply the vibration, along the direction of the arrow of FIG. 5

(85) According to a second embodiment, illustrated in FIG. 6a, the probe PR is an inertial probe without external moving parts. In this case, the movement of the vibrator VIB within the probe PR leads to the movement of the probe and the continuous or pulsed vibration is once again applied by pushing the transducer TUS against the viscoelastic medium.

(86) The axis of movement of the vibrator VIB is preferably an axis A of symmetry of the ultrasonic transducer TUS. For example, the ultrasonic transducer TUS may have a circular section, the axis A passing through the centre of the ultrasonic transducer TUS.

(87) According to one embodiment, the probe PR includes control means TOG for triggering the measurement.

(88) The probe PR according to FIGS. 5 and 6a thus includes a vibrator VIB intended to apply the continuous low frequency vibration.

(89) According to one embodiment, the diameter of the ultrasonic transducer is comprised between 2 and 15 mm.

(90) According to one embodiment, the central frequency of the ultrasonic transducer is comprised between 1 MHz and 15 MHz.

(91) According to one embodiment, the ultrasonic transducer TUS is a convex abdominal probe.

(92) According to one embodiment, the probe includes a positioning indicator which is triggered when the probe is correctly positioned. This indicator may be a visual indicator, for example a change of colour of diodes. Alternatively, the indicator may be a sound or haptic indicator such as a change of type or amplitude of a vibration.

(93) FIG. 6b illustrates a hybrid elastography device DEV according to the invention.

(94) The device DEV according to the invention includes: A probe PR according to the invention; A central unit UC connected to the probe PR.

(95) The central unit may comprise: Computing means for processing the reflected ultrasonic signals; A screen SC for displaying the results obtained at the different steps of the method P according to the invention; Control or input means ENT for the control of the device by the operator.

(96) The central unit UC may be connected to the probe PR by a wire link or by wireless communication means.

(97) According to one embodiment, the screen SC is suited for the display of the results illustrated in FIG. 4. The screen SC may also display in real time the position indicator RT_IP computed during the step CW _P of the method P according to the invention.

(98) According to one embodiment, the central unit includes means configured to trigger automatically the application of a low frequency pulse on the basis of the value of the positioning indicator RT_IP computed and displayed in real time.

(99) According to one embodiment, the central unit is comprised in the probe PR.