Device and method for measuring elasticity of a human or animal organ and for two- or three-dimensional representation thereof

0001The present invention relates to a device and a method for measuring the elasticity of a human or animal organ, or more generally any viscoelastic medium presenting an ultrasonic signal after ultrasonic illumination and the consecutive establishment of a representation in two or three elasticity dimensions. It applies in particular, but not exclusively, to the measurement of the elasticity of a human breast, the advantage of this technique is that the pathological nature of the tissues is often related to their elasticity.

0002The French patent application is known in the prior art <patcit id="pcit0001" dnum="FR2733142"><text>FR 2733142</text></patcit> which discloses a device for measuring elasticity performing a 2-dimensional measurement but also capable of performing a three-dimensional measurement. However, this device does not have a scanning means capable of scanning the bar (s) in two perpendicular directions.

0003We also know American patents <patcit id="pcit0002" dnum="US6176827B"><text>US 6176827</text></patcit>, <patcit id="pcit0003" dnum="US5099848A"><text>US 5099848</text></patcit>, <patcit id="pcit0004" dnum="US2002010398A"><text>US 2002/010398</text></patcit>, <patcit id="pcit0005" dnum="US6277074B"><text>US 6,277,074</text></patcit>, <patcit id="pcit0006" dnum="US5474070A"><text>US 5474070</text></patcit> which all disclose solutions for making only a two-dimensional measurement, sometimes with a fixed bar (cf. <patcit id="pcit0007" dnum="US6176827B"><text>US 6176827</text></patcit>)

0004At present, there is no ultrasonic device for measuring elasticity on the market making it possible to view said measurement in two or three dimensions.

0005Furthermore, concerning the two-dimensional elasticity measurement, the article is known "<nplcit id="ncit0001" npl-type="s"><text>Shear Modulus Imaging with 2D Transient Elastography "by Sandrin, L., Tanter, M., Catheline, S., and Fink, M., Ultrason. Ferroelectr. Freq. Control., Vol. 49 (4), pp. 426- 435 (2002</text></nplcit>) describing a technique for measuring elasticity and the two-dimensional representation of this measurement. The resolution of the inverse problem, that is to say consisting in going back to the parameters which describe the viscoelastic medium which one seeks to measure, is here imperfect because the displacement is not known in the three directions of space . Indeed, according to the calculation algorithms linked to the measurements made by the device presented in this article, operators will be forced to formulate hypotheses to solve the elasticity calculations, but practice has shown that these hypotheses are rarely justified.

0006We also know the American patent <patcit id="pcit0008" dnum="US2002010398A"><text>US 2002/010398</text></patcit> which discloses an ultrasound imaging method and system for determining the speed of propagation of shear waves generated in tissue.

0007We also know the international patent application N °<patcit id="pcit0009" dnum="WO0070362A"><text>WO 0070362</text></patcit> which describes a system using magnetic resonance elastography (MRE), in which a viscoelastic area (such as the human chest) is excited by mechanical waves. The object of this invention is based on the assumption that the results of measurements by ERM are time-independent solutions of partial differential equations describing with precision the behavior of mechanical waves in a viscoelastic material (including for longitudinal waves and in a reflective environment). For this, the Young's modulus contained in these equations can be calculated. In addition, it is proposed here to use (predominantly) longitudinal waves, these being able to penetrate the human chest, which is not the case with transverse waves.

0008In the device of this patent application, obtaining the elasticity card requires a great deal of time. Furthermore, the cost of implementing this device is very high.

0009In addition, it is clear that measuring the elasticity over larger dimensions constitutes a considerable advantage, this significant improvement in the state of the art being obtained by means of the device according to the invention.

0010The prior art is also constituted by the international patent application <patcit id="pcit0010" dnum="WO0055616A"><text>WO-A-00/55616</text></patcit> which describes an imaging method for observing the propagation of a low frequency shear pulse wave simultaneously at a multitude of points in a diffusing viscoelastic medium. To this end, ultrasonic compression waves are emitted in this device at an ultra-rapid rate which make it possible to obtain a succession of images of the medium, then the images thus obtained are processed in deferred time by cross-correlation, to determine at each point of each image the movements of the medium during the propagation of the shear wave.

0011This invention is not satisfactory because it requires considering two hypotheses:<ul id="ul0001" list-style="dash" compact="compact"><li>the second derivative of the displacement is considered null in the direction orthogonal to the plane,</li><li>the medium is assumed to be perfectly incompressible.</li></ul>

0012The invention therefore more particularly aims to remedy the drawbacks of the systems of the prior art.

0013The invention is a device as defined in claim 1.

0014Thanks to these features, the invention therefore makes it possible to propose a device which makes it possible to obtain a mapping of the elasticity of the medium to be measured in two or three dimensions, by virtue of a relatively simple and inexpensive system relative to existing solutions.

0015The ultrasonic bar includes a plurality of transducers for the acquisition of ultrasonic signals.

0016According to one possibility offered by the invention, the excitation means will consist of a mechanical vibration which may be transverse, longitudinal or more generally a mixture of the two.

0017Advantageously, the excitation means may consist of one or more hyperthermia transducers because the rise in temperature generates displacements on the ultrasound images, either with the transducer (s) used for the acquisition. ultrasonic signals or one or more transducers arranged around the viscoelastic medium.

0018According to another possibility offered by the invention, the excitation means will consist of a remote palpation using the radiation pressure, either with the transducer (s) used for the acquisition of the ultrasonic signals or one or more transducers arranged around the viscoelastic medium.

0019Advantageously, the device according to the invention will be controlled by at least one control means, for example a computer, a microcomputer or a central unit.

0020Advantageously, the ultrasonic bar will be a bar making it possible to focus in a plurality of different elevation points; in this case, the scanning being carried out by ultrasonic focusing.

0021It should be recalled here that the bar is able not only to focus along a plane but also in elevation relative to this plane, in the example according to the horizontal plane parallel to the previous one and slightly offset.

0022It is also recalled, for the sake of facilitating the understanding of the invention, that a 0 D ultrasound array emits along a linear dimension x, that a 1 D array emits along a two-dimensional plane x, y and finally that 'A 2 D strip, usually constituted by a multitude of ultrasonic transducers of square shape distributed in a 2 D matrix, makes it possible to emit ultrasound in a volume according to the three dimensions x, y and z.

0023According to one embodiment of the invention, the space existing between the ultrasonic bar and the aforementioned viscoelastic medium will be constituted at least in part by water or any other element capable of ensuring the free passage of the ultrasonic waves.

0024Advantageously, the assembly consisting of the ultrasonic transducers and their on-board electronics will be connected to the control and processing means by a very high speed digital link, for example of the LVDS type.

0025According to one embodiment of the invention, the device according to the invention will comprise two ultrasonic bars.

0026According to one embodiment of the device of the invention, the two bars will be immersed in an airtight container filled with a liquid, for example water.

0027Advantageously, the hermetic container will be connected to a rotation means able to rotate said container.

0028According to one possibility offered by the invention, the hermetic container may comprise a plurality of orifices into which are respectively introduced a mechanical vibrator and / or an ultrasonic transducer.

0029Advantageously, the orifices on or in the hermetic housing will be located at 90 ° (degrees) from each other or from one another.

0030According to another embodiment of the invention, the device according to the invention will comprise three bars capable respectively of measuring the tissue velocities in the directions y, x and z.

0031The invention also relates to a method as defined in claim 20.

0032Advantageously, the step of displacing the bar will be repeated as many times as necessary for the acquisition of all of the ultrasonic data before proceeding to the step of calculating the ultrasonic images.

0033It should be noted that the step of acquiring ultrasonic data also makes it possible to acquire the data necessary for obtaining a conventional ultrasonic image, that is to say using a conventional "beamforming". Indeed, the image or images thus obtained constitute information relevant to 2D or 3D on the morphology of the organ studied, this information is completely complementary to the elasticity parameter.

0034Advantageously, during the step of calculating tissue velocities, the second derivatives of the longitudinal component of this velocity along the three orthogonal directions of space can be measured.

0035In the same way, during the step of calculating tissue velocities, the spatial derivatives of the three components, along the three directions of space, of said velocity can be measured.

0036Embodiments of the invention will be described below, by way of nonlimiting example, with reference to the appended drawings in which:<ul id="ul0002" list-style="dash"><li>the <figref idref="f0001">figure 1</figref> illustrates the movement of an ultrasound bar of the device according to the invention provided with a simple mechanical scanning means;</li><li>the <figref idref="f0001">figure 2</figref> illustrates the movement of an ultrasound bar of the device according to the invention provided with a double mechanical scanning means;</li><li>the <figref idref="f0001">figure 3</figref> illustrates the operation of a 1.5 D bar of the device according to the invention provided with a means of scanning by ultrasonic focusing in elevation;</li><li>the <figref idref="f0002">figure 4</figref> illustrates the device according to the invention provided with a 1.5 or 1.75 D bar, capable of focusing in elevation;</li><li>the <figref idref="f0002">figure 5</figref> illustrates the device according to the invention provided with a star transducer in which the transducers are spatially distributed;</li><li>The <figref idref="f0002">figure 6</figref> illustrates the device according to the invention in the process of measuring the elasticity of the breast of a patient;</li><li>The <figref idref="f0003">figure 7</figref> schematically illustrates an embodiment of the device according to the invention.</li></ul>

0037The appended figures do not represent the complete device. This device comprises the usual elements for carrying out elasticity measurements of a human or animal organ, that is to say in particular at least one bar, or probe, ultrasonic comprising a plurality of transducers, electronic equipment capable of ensuring the acquisition of ultrasonic signals, a control and data processing means such as a computer or the like and an excitation means capable of generating low frequency displacements.

0038The invention relates to the use of a mechanical scanning means which makes it possible to ensure the scanning of the aforesaid ultrasonic strip. This allows, thanks to the method of the invention, to measure parameters which are not accessible with the devices of the prior art, in particular that described in patent N °<patcit id="pcit0011" dnum="FR9903157"><text>FR 9903157</text></patcit>. The parameters thus obtained are the second derivative of the displacement along the elevation, that is to say the direction perpendicular to the plane of the image, and the two missing components of the displacement vector.

0039In the following, we have chosen to illustrate the invention by taking as a human or animal organ, a breast but any other organ, ideally static, can be the subject of a measure of elasticity thanks to the device and to the method according to the present invention provided, of course, to present an ultrasonic signal after it has been illuminated with ultrasonic signals. However, in the event that the internal movements of the body cannot constitute a low-frequency stress that can be used for the process, it is preferable that this organ be stationary so as not to disturb the measurement.

0040The method according to the invention carries out the steps below in the following chronological order:<ol id="ol0001" compact="compact"><li>1. generation of a request, or signal, low frequency,</li><li>2. acquisition of ultrasonic data,</li><li>3. displacement of the bar thanks to the scanning means,</li><li>4. calculation of ultrasound images,</li><li>5. calculation of tissue velocities, also called displacement between successive images,</li><li>6. possibly calculation of tissue deformation rates,</li><li>7. finally, data inversion, which makes it possible to recover the parameters of the medium measured.</li></ol>

0041It should be noted that the calculation steps, ie steps 4 to 6, can start as soon as the ultrasonic bar scans the viscoelastic medium, that is to say that these steps take place ideally during the movement of said bar.

0042During the step of generating the demand, or low frequency signal, a low frequency signal is transmitted by means of excitation preferably just after the start of the ultrasonic acquisitions. This signal has a frequency, f, between 5 Hz and 1000 Hz. The low frequency vibration causes propagation in the tissues of the viscoelastic medium of low frequency elastic waves whose propagation depends on the elasticity of the medium.

0043The various means that can be used to generate low-frequency movements can consist of a mechanical vibration, produced by a vibrator which can in particular be one or more vibrating plates 20, piston (s) and / or bar (s). In the same way, the excitation means able to generate a shear wave could consist of a palpation at a distance using the radiation pressure either with the transducer (s) used for the acquisition of the signals. ultrasonic, or one or more transducers arranged around the object to be imaged.

0044During the step of acquiring the ultrasound data, N ultrasound acquisitions are carried out at a rate 1 / T typically between 100 Hz and 100,000 Hz. The acquisition of the ultrasound data is done by emitting with the ultrasonic transducers a pulse brief ultrasound which is reflected by the particles contained in the medium. The ultrasonic signal, called "speckle", is recorded by the same ultrasonic transducers over a period which can vary between 1 µs and 10 ms. This operation is repeated a number N of times at the rate 1 / T.

0045Then comes the step of moving the ultrasound or ultrasound bar. At this level, the scanning consisting in moving said strip will be carried out in three different ways according to the number and the type of ultrasonic strip used.

0046Thus, the device according to the invention can in particular be equipped with:<ul id="ul0003" list-style="dash" compact="compact"><li>a single unidirectional ultrasonic strip 1,</li><li>two ultrasonic bars 5, 6 or a bar then moved along two axes,</li><li>a 1.5 D type ultrasonic strip 9.</li></ul>

0047In the case of a single unidirectional ultrasonic strip 1, shown in the <figref idref="f0001">figure 1</figref>, the ultrasound strip 1 is moved by a distance of between 10 μm and 10 mm. At least one scan is carried out in one direction. For example, it is scanned in the direction z, formed by the plane 2, moving from Δz, formed in the figure by the two planes 3 and 4.

0048In the case of two ultrasonic strips 5 and 6, shown in the <figref idref="f0001">figure 2</figref>, or equivalent to a bar moved along two axes 7 and 8, two bars 5 and 6 are used (or only one successively). This scan provides access to all components of the tissue velocity vector.

0049In the case of a 1.5 D 9 type ultrasonic strip, shown on the <figref idref="f0001">figure 3</figref>, mechanical scanning is avoided, the result being the same with a star transducer; these two elements making it possible to focus at three different elevation points. In the case of a 1.5D 9 array, the displacement along z is obtained by modifying the focusing laws so as to change the elevation of the plane of the image.

0050During the step of calculating the ultrasound images, the ultrasound images are constructed using a summation-delay algorithm like that described in the patent. <patcit id="pcit0012" dnum="WO0055616A"><text>WO-A-00/55616</text></patcit> previously mentioned or other types of rapid beamforming such as the technique in the space of space frequencies (see article by <nplcit id="ncit0002" npl-type="s"><text>Lu, J., "2D and 3D High Frame Rate Imaging with Limited Diffraction Beams", IEEE Trans. Ultrasound. Ferroelectr. Freq. Contr., Vol. 44, N ° 4, 1997</text></nplcit>.).

0051During the step of calculating tissue velocities, also called displacement between successive images, tissue velocities or displacements between two successive ultrasonic shots, but not necessarily consecutive, are measured either by intercorrelation, described in the patent <patcit id="pcit0013" dnum="WO0055616A"><text>WO-A-00/55616</text></patcit>, by Doppler, or by autocorrelation, described in particular in the article by <nplcit id="ncit0003" npl-type="s"><text>Kasai C., Namekawa K., Koyano A. and Omoto R. "Real-time two-dimensional blood flow imaging using an autocorrelation technique", IEEE Trans. Sonics Ultrasound., Vol. 35, pp. 458-464 (1985</text></nplcit>), and more generally by any other displacement measurement technique.

0052Using a simple mechanical scan shown in the <figref idref="f0001">figure 1</figref>, at least the next component x of the tissue velocity V is accessed_x at each point in the middle located in the image area. Using an algorithm of the type described in the articles of<nplcit id="ncit0004" npl-type="s"><text>Konofagou, EE, Ophir, J., "A new elastographic method for estimation and imaging of lateral displacements, lateral strains, corrected axial strains and Poisson's ratios in tissues", Ultrasound in Med. &amp; Biol. 24, No 8, pp. 1183-1199 (1998</text></nplcit>) and <nplcit id="ncit0005" npl-type="s"><text>Tanter, M., Bercoff, J., Sandrin, L., Fink, M., "Ultrafast compound imaging for 2D motion vector estimation: application to transient elastography", Ultrason. Ferroelectr. Freq. Control.</text></nplcit>, we can also access the lateral component of tissue speed V_y. Using a mechanical double scan as shown in the<figref idref="f0001">figure 2</figref>, we access the three components of tissue speed: the bar 6 makes it possible to measure V_x and V_y and the bar 5 makes it possible to measure V_y and V_z. Precision on the estimation of V_y is increased by calculating the half-sum of the estimates with the two bars 5 and 6.

0053During the optional step of calculating tissue strain rates, the tissue strain rate is obtained by deriving V_z, also noted v (z, t) along the direction of the component considered, here relative to the depth: <maths id="math0001"><math display="block"><msub><mi>ε</mi><mi>i</mi></msub><mfenced><mi>z</mi><mi>t</mi></mfenced><mo>=</mo><mfrac><mrow><mo>∂</mo><msub><mi>υ</mi><mi>i</mi></msub><mfenced><mi>z</mi><mi>t</mi></mfenced></mrow><mrow><mo>∂</mo><mi>i</mi></mrow></mfrac></math><img file="EP1538987B1_D0001.tif" /></maths> with i = x, y or z

0054The data inversion stage consists in raising or recovering the parameters which describe the viscoelastic medium. If we consider the linear and isotropic medium, these parameters are two in number. One can choose the shear modulus µ and the compression modulus λ. In practice in soft tissue λ is of the order of Gpa and varies very little. µ is around Kpa. The elasticity or Young's modulus is equal in first approximation to 3µ. Thus, it is necessary to determine the shear modulus µ which constitutes the most significant parameter of the viscoelastic medium measured.

0055In the case of a simple mechanical scan, that is to say comprising a unidirectional bar 1, all the components of the tissue velocity vector are not known. Data can be inverted using the following equation:<maths id="math0002"><math display="block"><mi>ρ</mi><mo>⁢</mo><mfrac><mrow><msup><mo>∂</mo><mn>2</mn></msup><mo>⁢</mo><msub><mi>υ</mi><mi>i</mi></msub></mrow><mrow><mo>∂</mo><msup><mi>t</mi><mn>2</mn></msup></mrow></mfrac><mo>=</mo><mi>μ</mi><mfenced><mi>x</mi><mi>y</mi><mi>z</mi></mfenced><mo>⁢</mo><mfenced open="[" close="]"><mfrac><mrow><msup><mo>∂</mo><mn>2</mn></msup><mo>⁢</mo><msub><mi>υ</mi><mi>i</mi></msub></mrow><mrow><mo>∂</mo><msup><mi>x</mi><mn>2</mn></msup></mrow></mfrac><mo>+</mo><mfrac><mrow><msup><mo>∂</mo><mn>2</mn></msup><mo>⁢</mo><msub><mi>υ</mi><mi>i</mi></msub></mrow><mrow><mo>∂</mo><msup><mi>y</mi><mn>2</mn></msup></mrow></mfrac><mo>+</mo><mfrac><mrow><msup><mo>∂</mo><mn>2</mn></msup><mo>⁢</mo><msub><mi>υ</mi><mi>i</mi></msub></mrow><mrow><mo>∂</mo><msup><mi>z</mi><mn>2</mn></msup></mrow></mfrac></mfenced></math><img file="EP1538987B1_D0002.tif" /></maths> where i = x, y or z

0056To pose the equation above, it was necessary to make the assumption that the elastic waves traversing the medium are purely shear waves. In practice, this assumption is false because the tissues are not perfectly incompressible, which has the consequence that any shear wave is necessarily accompanied by a compression wave.

0057The desired parameter, µ (x, y, z), is obtained by discretizing this equation. In elastography, we generally have one of the three coordinates v_x, v_y or v_z. Suppose it is v_x. To discretize this equation it is necessary to be able to calculate the second derivatives in the three directions and in time:<maths id="math0003"><math display="block"><mrow><mo>{</mo><mtable columnalign="left"><mtr><mtd><mfrac><mrow><msup><mo>∂</mo><mn>2</mn></msup><mo>⁢</mo><mi>υ</mi></mrow><mrow><msup><mo>∂</mo><mn>2</mn></msup><mo>⁢</mo><msup><mi>x</mi><mn>2</mn></msup></mrow></mfrac><mo>=</mo><mfrac><mrow><mi>V</mi><mo>⁢</mo><mfenced><mi>j</mi><mo>,</mo><mi>k</mi><mo>+</mo><mn>1</mn><mo>,</mo><mi>l</mi><mo>,</mo><mi>m</mi></mfenced><mo>+</mo><mi>V</mi><mo>⁢</mo><mfenced><mi>j</mi><mo>,</mo><mi>k</mi><mo>-</mo><mn>1</mn><mo>,</mo><mi>l</mi><mo>,</mo><mi>m</mi></mfenced><mo>-</mo><mn>2</mn><mo>⁢</mo><mi>V</mi><mfenced><mi>j</mi><mi>k</mi><mi>l</mi><mi>m</mi></mfenced></mrow><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msup><mi>x</mi><mn>2</mn></msup></mrow></mfrac></mtd></mtr><mtr><mtd><mfrac><mrow><msup><mo>∂</mo><mn>2</mn></msup><mo>⁢</mo><mi>υ</mi></mrow><mrow><msup><mo>∂</mo><mn>2</mn></msup><mo>⁢</mo><msup><mi>y</mi><mn>2</mn></msup></mrow></mfrac><mo>=</mo><mfrac><mrow><mi>V</mi><mo>⁢</mo><mfenced><mi>j</mi><mo>,</mo><mi>k</mi><mo>⁢</mo><mn>1</mn><mo>,</mo><mi>l</mi><mo>+</mo><mn>1</mn><mo>,</mo><mi>m</mi></mfenced><mo>+</mo><mi>V</mi><mo>⁢</mo><mfenced><mi>j</mi><mo>,</mo><mi>k</mi><mo>,</mo><mi>l</mi><mo>-</mo><mn>1</mn><mo>,</mo><mi>m</mi></mfenced><mo>-</mo><mn>2</mn><mo>⁢</mo><mi>V</mi><mfenced><mi>j</mi><mi>k</mi><mi>l</mi><mi>m</mi></mfenced></mrow><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msup><mi>y</mi><mn>2</mn></msup></mrow></mfrac></mtd></mtr><mtr><mtd><mfrac><mrow><msup><mo>∂</mo><mn>2</mn></msup><mo>⁢</mo><mi>υ</mi></mrow><mrow><msup><mo>∂</mo><mn>2</mn></msup><mo>⁢</mo><msup><mi>z</mi><mn>2</mn></msup></mrow></mfrac><mo>=</mo><mfrac><mrow><mi>V</mi><mo>⁢</mo><mfenced><mi>j</mi><mo>,</mo><mi>k</mi><mo>,</mo><mi>l</mi><mo>,</mo><mi>m</mi><mo>+</mo><mn>1</mn></mfenced><mo>+</mo><mi>V</mi><mo>⁢</mo><mfenced><mi>j</mi><mo>,</mo><mi>k</mi><mo>,</mo><mi>l</mi><mo>,</mo><mi>m</mi><mo>-</mo><mn>1</mn></mfenced><mo>-</mo><mn>2</mn><mo>⁢</mo><mi>V</mi><mfenced><mi>j</mi><mi>k</mi><mi>l</mi><mi>m</mi></mfenced></mrow><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msup><mi>z</mi><mn>2</mn></msup></mrow></mfrac></mtd></mtr><mtr><mtd><mfrac><mrow><msup><mo>∂</mo><mn>2</mn></msup><mo>⁢</mo><mi>υ</mi></mrow><mrow><msup><mo>∂</mo><mn>2</mn></msup><mo>⁢</mo><msup><mi>t</mi><mn>2</mn></msup></mrow></mfrac><mo>=</mo><mfrac><mrow><mi>V</mi><mo>⁢</mo><mfenced><mi>j</mi><mo>+</mo><mn>1</mn><mo>,</mo><mi>k</mi><mo>,</mo><mi>l</mi><mo>,</mo><mi>m</mi></mfenced><mo>+</mo><mi>V</mi><mo>⁢</mo><mfenced><mi>j</mi><mo>-</mo><mn>1</mn><mo>,</mo><mi>k</mi><mo>-</mo><mn>1</mn><mo>,</mo><mi>l</mi><mo>,</mo><mi>m</mi></mfenced><mo>-</mo><mn>2</mn><mo>⁢</mo><mi>V</mi><mfenced><mi>j</mi><mi>k</mi><mi>l</mi><mi>m</mi></mfenced></mrow><mrow><mi mathvariant="normal">Δ</mi><mo>⁢</mo><msup><mi>t</mi><mn>2</mn></msup></mrow></mfrac></mtd></mtr></mtable></mrow></math><img file="EP1538987B1_D0003.tif" /></maths> where V (j, k, l, m) = v (jT, k.Δx, y = 1.Δy, m.Δz).

0058It is therefore necessary not only to know the displacement v_x in the image plane, but also know it around the image plane to be able to estimate the second derivative perpendicular to the image plane: ∂v²/ ∂z². In French patent N °<patcit id="pcit0014" dnum="FR9903157"><text>FR 9903157</text></patcit> and publications in pulse elastography, the second derivative perpendicular to the plane of the image is eliminated from the equation because it cannot be measured experimentally. Indeed, v is only measured in the plane (x, y), only v (x, y) is known. ∂v²/ ∂z² cannot be determined. A known hypothesis consists in posing:<maths id="math0004"><math display="block"><mfrac><mrow><msup><mo>∂</mo><mn>2</mn></msup><mo>⁢</mo><mi>υ</mi></mrow><mrow><mo>∂</mo><msup><mi>z</mi><mn>2</mn></msup></mrow></mfrac><mo>=</mo><mn>0</mn></math><img file="EP1538987B1_D0004.tif" /></maths>

0059The equation is simplified by <maths id="math0005"><math display="block"><mi>ρ</mi><mo>⁢</mo><mfrac><mrow><msup><mo>∂</mo><mn>2</mn></msup><mo>⁢</mo><msub><mi>υ</mi><mi>i</mi></msub></mrow><mrow><mo>∂</mo><msup><mi>t</mi><mn>2</mn></msup></mrow></mfrac><mo>=</mo><mi>μ</mi><mo>⁢</mo><mfenced open="[" close="]"><mfrac><mrow><msup><mo>∂</mo><mn>2</mn></msup><mo>⁢</mo><msub><mi>υ</mi><mi>i</mi></msub></mrow><mrow><mo>∂</mo><msup><mi>x</mi><mn>2</mn></msup></mrow></mfrac><mo>+</mo><mfrac><mrow><msup><mo>∂</mo><mn>2</mn></msup><mo>⁢</mo><msub><mi>v</mi><mi>i</mi></msub></mrow><mrow><mo>∂</mo><msup><mi>y</mi><mn>2</mn></msup></mrow></mfrac></mfenced></math><img file="EP1538987B1_D0005.tif" /></maths> where i = x, y or z.

0060It can be resolved without knowing the displacements in the planes located on either side of the image plane in z + Δz and z-Δz.

0061The hypothesis of nullity of the second derivative perpendicular to the plane of the image is particularly restrictive and does not make it possible to solve the opposite problem under good conditions since it is highly improbable that ∂v²/ ∂z² be zero. Thanks to the device according to the invention, the missing derivative can be obtained.

0062Two solutions are thus considered to measure v (x, y, z) and calculate ∂v²/ ∂z² : <ul id="ul0004" list-style="dash" compact="compact"><li>either a 1.5 D 9 bar or a star transducer is used to focus at three different elevation points,</li><li>or the acquisition is reproduced three times by successively moving the bar in z-Δz, z and z + Δz with Δz chosen judiciously so as to be close to the resolutions obtained in x and y (Δz ≅ Δx and Δy).</li></ul>

0063If you use a 1.5 D 9 or 1.75 D bar, shown on the <figref idref="f0002">figure 4</figref>, it is possible to produce images 10 in three planes of the image and calculate the displacements in these three planes situated for example in z-Δz, z and z + Δz. the maximum rate is nevertheless here reduced by a factor of 3. It is also possible to use a star transducer 11 in which the transducers 12 are spatially distributed, as shown in the<figref idref="f0002">figure 5</figref>.

0064In the second solution, consisting in reproducing the acquisition three times by successively moving the strip in z-Δz, z and z + Δz, it should be noted that care must be taken that the viscoelastic medium to be measured does not have moved between two acquisitions and that the applied low frequency stress is synchronized for each elevation position.

0065In the case of a double mechanical scan, that is to say comprising either two bars 5, 6 or a bar moving along two axes, all the components of the tissue velocity vector are known. A more general case (compressible medium) consists in using the Navier equation which is written:<maths id="math0006"><math display="block"><mi>ρ</mi><mo>⁢</mo><mfrac><mrow><msup><mo>∂</mo><mn>2</mn></msup><mo>⁢</mo><mover><mi>υ</mi><mo>→</mo></mover></mrow><mrow><mo>∂</mo><msup><mi>t</mi><mn>2</mn></msup></mrow></mfrac><mo>=</mo><mfenced><mi>λ</mi><mo>+</mo><mi>μ</mi></mfenced><mo>⁢</mo><mover><mo>∇</mo><mo>→</mo></mover><mfenced><mover><mo>∇</mo><mo>→</mo></mover><mn>.</mn><mover><mi>υ</mi><mo>→</mo></mover></mfenced><mo>+</mo><mi>μ</mi><mo>⁢</mo><msup><mover><mo>∇</mo><mo>→</mo></mover><mn>2</mn></msup><mo>⁢</mo><mover><mi>υ</mi><mo>→</mo></mover></math><img file="EP1538987B1_D0006.tif" /></maths>

0066We can refine this result using the following equation: <maths id="math0007"><math display="block"><mi>ρ</mi><mo>⁢</mo><mfrac><mrow><msup><mo>∂</mo><mn>2</mn></msup><mo>⁢</mo><msub><mi>υ</mi><mi>i</mi></msub></mrow><mrow><mo>∂</mo><msup><mi>t</mi><mn>2</mn></msup></mrow></mfrac><mo>=</mo><mfrac><mo>∂</mo><mrow><mo>∂</mo><msub><mi>x</mi><mi>i</mi></msub></mrow></mfrac><mfenced open="[" close="]"><mi>λ</mi><mo>⁢</mo><mfrac><mrow><mo>∂</mo><msub><mi>υ</mi><mi>j</mi></msub></mrow><mrow><mo>∂</mo><msub><mi>x</mi><mi>j</mi></msub></mrow></mfrac></mfenced><mo>+</mo><mfrac><mo>∂</mo><mrow><mo>∂</mo><msub><mi>x</mi><mi>j</mi></msub></mrow></mfrac><mo>⁢</mo><mfenced open="[" close="]"><mi>μ</mi><mo>⁢</mo><mfenced><mfrac><mrow><mo>∂</mo><msub><mi>υ</mi><mi>i</mi></msub></mrow><mrow><mo>∂</mo><msub><mi>x</mi><mi>j</mi></msub></mrow></mfrac><mo>+</mo><mfrac><mrow><mo>∂</mo><msub><mi>υ</mi><mi>j</mi></msub></mrow><mrow><mo>∂</mo><msub><mi>x</mi><mi>i</mi></msub></mrow></mfrac></mfenced></mfenced></math><img file="EP1538987B1_D0007.tif" /></maths> where v₁ = v_x, v₂ = v_y, v₃ = v_z, x₁ = x, x₂ = y and x₃ = z.

0067We then have a system of three equations and two unknowns: λ (x, y, z) and µ (x, y, z) because the density ρ varies very little in the tissues.

0068With the above equation, we understand why neglecting the tissue velocities linked to compression waves is a source of error. Certainly the tissue velocities linked to the compression waves are low compared to those generated by the shear wave, however their contribution cannot be neglected because the factor λ coefficient is large compared to the compression term. The discretization of this equation can be achieved if the three components of the tissue velocity vector are known. Indeed, this equation involves couplings between the evolutions of tissue velocities in all directions.

0069The invention proposes to use an assembly or device as shown in the <figref idref="f0002">figure 6</figref>. This device makes it possible to measure the three components of the tissue speed vector in the organ studied by successively scanning the medium along three different axes 13, 14 and 15. The bar 16 makes it possible to measure the tissue speeds along the direction y denoted u_y, the bar 17 to measure u_x and the bar 18 to measure u_z. The use of an algorithm for measuring transverse displacements can make it possible to reduce the number of scanning zones from three to two by eliminating for example the bar 18. The displacement u_z would then be determined with both the bar 16 and the bar 17 which would allow an average u_z= (u_zB1 + u_zB2) / 2.

0070A synchronization system makes it possible to move the transducer 12 between two acquisitions, an acquisition comprising the generation of elastic shear waves and the acquisition of ultrasonic signals. The displacement of the system can be carried out for example with a stepping motor or an electrodynamic actuator.

0071This acquisition sequence must be reproduced as many times as there is a shot in the image. Using three bars 16, 17, 18 each taking 128 different positions, the system requires 384 separate acquisition sequences. The environment studied can then be segmented into 128³ voxels 19 of cubic shape. The acquisition rate of the ultrasonic signals is between 100 and 100,000 shots per second.

0072If we suppose that in the studied medium, the shear waves propagates at 1 m / s and that the main dimension of this medium is 12.8 cm and that the voxels have for dimension 1mm³. The propagation of the shear wave in such an environment and over a length of 12.8 cm lasts 128 ms. For a typical rate of 1,000 shots per second, 128 ultrasonic shots will have to be taken to follow the propagation of the shear wave. We can then estimate that after 150 ms the acquisition ends. Suppose that the ultrasonic device moves after 500 ms and that a second series of 128 ultrasonic shots is made. If one uses three ultrasonic bars in order to access the three components of the displacement, it will take approximately 3 minutes (384 times 500 ms) to acquire the whole of the data necessary to the resolution of the opposite problem. This measurement time can be reduced by interlacing the ultrasonic shots, 128 shots would be necessary, therefore one minute of acquisition.

0073In the case of scanning, one of the difficulties consists in keeping a good coupling between the transducer and the medium studied for the entire duration of the scanning. In the case where the surface of the medium is planar, the scanning can be carried out using an ultrasonic coupler, for example a water-based gel. When this is not possible or when the surface of the medium is "uneven" we suggest immersing the viscoelastic medium in water. This scenario is represented on the<figref idref="f0002">figure 6</figref> where the breast 21 of a patient is immersed in a tank 22, parallelepipedal having transparent windows with ultrasound, filled with water.

0074As we have seen previously, the device according to the invention requires at least one ultrasound bar. It also requires electronic equipment for ultrasonic acquisition consisting of ultrasonic transmitters and receivers, digital-analog and analog-to-digital converters, memories, digital and analog transmission lines, etc. To this electronics dedicated to the digitization of ultrasonic signals, there is generally added a processing unit which can for example be a PC type computer associated with a user interface. The elements mentioned in this paragraph are not represented in the various figures but are perfectly known to those skilled in the art.

0075Ultrafast ultrasound imaging techniques generally use only a limited number of ultrasonic emissions to illuminate the entire medium to be imaged. They therefore have the disadvantage of sending less energy into the environment than a standard ultrasound system. Consequently, the signal-to-noise ratio drops and the dynamic range of the ultrasound image decreases, which leads to a degradation of the raw ultrasound data and is reflected in the chain of algorithms to degrade the elasticity measurements in terms of sensitivity, resolution. , etc.

0076To overcome this drawback, the device of the invention takes part of the aforesaid electronic equipment in proximity, that is to say typically at a distance of less than 50 centimeters, from the ultrasonic bar with the consequences:<ul id="ul0005" list-style="bullet" compact="compact"><li>increasing the sensitivity of the system,</li><li>increasing the transmitted energy,</li><li>the simplification of the connections between the motorized sensor part (bar + on-board electronic equipment) and the data processing unit (PC or on-board PC card or DSP processor or etc.),</li><li>greater noise immunity.</li></ul>

0077These modifications cause a reduction in the mobility of the bar which would not be compatible with standard use in ultrasound because the ultrasound bars must be light and handy. It is important to note that in the case which concerns us, the mobility of the bar is in any case limited by the stroke of the sweep. The weight of the bar is of less importance because the bar is not handled. It is motorized.

0078According to a possibility offered by the invention, the device proposes to place near, or typically less than 50, of the bar the analog part of emission and reception, that is to say the amplifiers of emission and reception, keeping a transmission of analog signals of medium levels between the sensor part and the processing unit. In this way, the path of strong analog transmission signals (after amplification) and that of weak reception signals (before amplification) are reduced, therefore the reception sensitivity is increased and the transmission of energy on transmission is improved. .

0079According to another possibility offered by the invention, the device proposes also placing in the proximity, always typically less than 50 cm (centimeter), of the bar the analog-digital (ADC) and digital-analog (DAC) converters (for the 'transmission and reception) and to connect the sensor part and the processing unit by a very high speed digital link (LVDS type for example). The structure of the device according to the invention thus produced leads to the following improvements:<ul id="ul0006" list-style="bullet" compact="compact"><li>the signal to noise ratio is increased by housing the entire analog part at the source. Strong (transmitting) and weak (receiving) analog signals are concentrated at the sensor and no longer travel the distance between the processing unit and the sensor part.</li><li>The noise received and the noise emitted are reduced because the connection between the processing unit and the sensor part becomes purely digital.</li></ul>

0080The connection between the control / processing means and the sensor part is simplified in terms of number of wires.

0081If it is assumed that an operator uses a 128-element strip, 8-bit 50 MHz converters (DAC and ADC) for transmission and reception. If transmission and reception are separated in time and all the channels are active, the digital data transfer rate reaches 128 × 8 × 50 = 51.2 Gbps (Giga bits per second). Currently, only 17 high speed 3.125 Gbps digital connections are needed to transmit this data in real time. By comparison, an analog solution would require 128 two-wire connections.

0082On the <figref idref="f0003">figure 7</figref>, the device according to the invention is shown in a new assembly. In this variant of the device, two ultrasonic probes 23 and 24 are used, and are immersed in an airtight container 26, filled with water or another suitable liquid.

0083The sealed container or housing is capable of rotating, for example by a quarter of a turn, so that the probe 23 can not only scan in the direction X but also in the direction Z. The ultrasonic probe 24 scans only in the direction Z The acquisition of ultrasonic signals is therefore done in three stages:<ul id="ul0007" list-style="dash" compact="compact"><li>scanning in the X and Z directions thanks to the two probes 23 and 24,</li><li>rotation of the hermetic container or case 26 for example by a quarter turn, that is 90 degrees,</li><li>scanning in direction Z, using probe 23.</li></ul>

0084During acquisitions, mechanical vibrators 25, inserted in orifices present on the periphery or the circumference of the hermetic container 26, can be used to generate low frequency stresses. One of the two mechanical vibrators, or the two mechanical vibrators, represented on the<figref idref="f0003">figure 7</figref> can / can be replaced by a hyperthermia probe and / or an ultrasonic transducer used in remote palpation mode. In the example chosen to illustrate the invention, the two orifices present in or on the hermetic housing 26 are located 90 degrees from each other, that is to say that the linear mechanical vibrators will be arranged perpendicular relative to each other, so that even after a quarter-turn (90 °) rotation of the container 26, the mechanical vibrators still extend in the same directions, i.e. - say the same lines as before.

0085The invention is described in the foregoing by way of example. It is understood that a person skilled in the art is able to produce different variants of the device and of the method for measuring the elasticity of a human or animal organ and the subsequent establishment of a two or three-dimensional representation of the elasticity, in particular concerning the arrangement or the arrangement of the various elements constituting said device or the order as well as the importance of the stages of said process, without going beyond the scope of the patent.