HK1130415B - A method and a device for imaging a visco-elastic medium - Google Patents

Description

Method and apparatus for imaging viscoelastic medium

Technical Field

The present invention relates to the general field of methods and apparatus for imaging viscoelastic media and includes studying motion present in viscoelastic media. More specifically, the present invention relates to a process comprising the steps of: an excitation step in which an internal mechanical stress is generated in an excitation region; this is followed by an imaging step by acquiring signals during the motion generated in the viscoelastic medium in response to the internal mechanical stress in an imaging zone containing the excitation zone.

In a particularly advantageous application, the invention relates to imaging viscoelastic properties of a human organ. In this medical application, the invention is particularly useful for detecting the presence of a fluid region in an organ, such as the breast, liver, ovary, etc., and for characterizing the region.

Background

Echographic examinations of the breast have made it possible to infer specific characteristics of the lesion from information relating to the morphology and echographic texture of the lesion observed. In particular, cyst lesions (cystic lesions) and solid mass (solid mass) can be distinguished based on echogenic values that exhibit acoustic characteristics of the liquid. A typical cyst appears as a non-echogenic region of regular geometry on an echographic image, assuming that the fluid filling the cyst contains almost no scatterers, while the echo of other tissue than the cyst is significantly enhanced because the incident beam is attenuated weaker in the cyst than in the tissue.

The risk of a solid lesion being malignant can also be estimated from its morphology. Images with sharp contours with few rounded protrusions and major axes parallel to the skin are more likely to be associated with benign lesions. Conversely, an irregular (e.g., star-shaped) image with an unclear outline and a strong back shadow is very likely to be malignant.

Unfortunately, these morphological criteria are not sufficient because they are not completely reliable, especially for small lesions. The lack of specificity has led to the emergence of new echographic tools that give quantifiable functional information more relevant to pathological states.

For example, there are a number of elastography techniques that seek to measure mechanical properties of tissue in order to more accurately and systematically characterize a lesion.

The rheological properties of the medium can also be analyzed in order to distinguish between solid and liquid properties, it being known in particular to generate a radiation pressure in a precisely defined region of interest, so that a flow of liquid is generated therein if the region is liquid, which can then be imaged using echography.

The acquired image reveals the motion of the region of interest. When motion is observed on the resulting image, the presence of liquid is detected.

This method of detecting the presence of liquid by detecting motion alone results in classifying lesions in a manner that is not very robust, particularly for complex cysts with sticky substances that produce echogenic values, which is encountered in 50% of the cases. The motion induced in the region of radiation pressure in this type of cyst is very similar to the mechanical response of a viscoelastic solid. The presence or absence of motion is not a discriminating criterion. However, the morphological criteria given by echography prove to be insufficient, in particular for that type of cyst.

Furthermore, the known methods are only capable of testing specific media areas selected by the user. The method is application specific and it is not realistic to want to continuously perform the known method when imaging a medium.

Finally, the known method is only used to detect the presence of liquid and does not carry out any grading of the rheological behaviour of the lesions. In addition, these lesions may be cysts in the following cases: viscous, milky, limy, hemorrhagic, or indeed formed from precipitates, and thus they exhibited different and varying rheological properties.

Disclosure of Invention

It is therefore a main object of the present invention to alleviate this drawback by proposing an elastography technique which makes it possible, in particular, to distinguish between a liquid component and a solid component in a single lesion, the invention consisting in a method as described in the introduction and further comprising: a calculation step of calculating a quantitative indicator related to the rheological property of the viscoelastic medium at least one point in the imaging zone located at a given depth outside the excitation zone, the quantitative indicator representing a comparison between: a signal obtained during motion in response to mechanical stress in the excitation region at least one point located at the given depth, and a signal obtained during motion in response to mechanical stress in the imaging region at least the point located outside the excitation region.

This method is based on characterizing the relative motion of different regions in a viscoelastic medium, which are distinguished by the way they respond to mechanical stress. The method systematically proposes: a quantitative indicator is calculated to reveal the presence or absence of propagating mechanical waves in a medium located outside the excitation region. Thus, the method of the invention makes it possible to observe a wider area in the medium, for example by giving a map of the quantitative indicators at a plurality of points. The quantitative indicator is advantageously a similarity indicator.

The signal obtained for calculating the quantitative indicator is preferably a displacement field, or a motion field, or ultrasound noise or "speckle". Obtaining such fields is known to those of ordinary skill in the art.

The method of the invention makes it possible to identify solid or liquid regions based on a wave propagation criterion, which is a robust physical criterion. The present invention thus enables the characterization of rheological properties over the entire imaging area. With the present invention, all characterization can be performed without having to know a priori information about the region where the medium is imaged.

In an advantageous implementation, the step of comparing the acquired signals comprises: the time variations and/or amplitude variations in the obtained signals are compared.

Advantageously, the calculation step comprises calculating a maximum of a correlation function between the signals acquired for the two points considered, the quantitative indicator being a function of said maximum.

According to another particular feature of the invention, the internal mechanical stress is a mechanical vibration generated by ultrasound radiation pressure.

The discrimination between solid and liquid is then achieved by the presence or absence of propagation of shear waves generated by said mechanical vibrations. It should be observed that generating such shear waves is advantageous since the method of the invention is then performed in parallel with the measurement of the elasticity of the viscoelastic medium using known methods (for example patent WO 2004/0210838). In addition, generating such shear waves makes it possible to apply mechanical excitation deep in the medium, whereby rheological properties deep within the organ can be detected. This provides another advantage for such excitation.

In an advantageous application, the stimulation step is combined with a treatment. The stress is then advantageously generated by a beam used for therapeutic purposes, for example a focused ultrasound beam.

Furthermore, in the present invention, the excitation that produces motion in the medium may be the product of a variety of mechanisms, including in particular one or more biological mechanisms or even external vibrations of low frequency. According to the invention, this mechanism requires the generation of internal mechanical stresses located in the excitation region.

In a particular feature of the invention, the imaging step comprises: a transmission sub-step for transmitting ultrasound waves into said imaging zone at a repetition frequency sufficient to measure the dynamic characteristics of said motion generated by said internal mechanical stress; a detection and recording sub-step for detecting and recording the ultrasonic echo generated in the medium as an acquired signal; and an estimation sub-step for estimating a motion from the detected echoes of at least two successive ultrasound transmissions and from the frequency of said ultrasound transmissions, wherein said quantitative indicator is calculated from said estimated motion.

In a known manner, the ultrasound waves generated during the imaging step are reflected by tissue scatterers on the propagation line of the excitation wave. With this feature, the same transducer array can be used specifically for both excitation and imaging of the medium.

In an advantageous implementation, at least two "directions" of quantitative indicators are calculated for two points at the same given depth outside the excitation zone, one on each of the two sides of the excitation zone, said quantitative indicators representing two comparisons between: signals acquired during motion in response to mechanical stress in the excitation region at least one point located at the given depth, and signals acquired during motion in response to mechanical stress in the imaging region at least the two points located outside of the excitation region.

Such a feature makes it possible to determine the properties of the medium at a given depth on either side of a given excitation region. Thereby, different viscoelastic properties on either side of an interface between two regions having different echo values can be revealed when said excitation zone is located at the interface.

In one particular implementation of the invention, the quantitative indicator is calculated on the boundary of the echographic defined region of the medium in order to test its permanent or temporary properties in the surrounding medium.

This implementation makes it possible to test possible slippage of solid lesions in the tissue. This fixed or rolling condition of the lesion within the surrounding tissue is also an important criterion for characterizing the lesion.

Advantageously, a second quantitative indicator associated with a point at a given depth in the excitation zone is calculated as a function of the spatial variation of the quantitative indicator for a plurality of points along a straight line at the given depth, located outside the excitation zone.

Such a feature makes it possible to determine regions along the line with different viscoelastic properties, for example to determine the extent of a region of tissue necrosis or complete coagulation induced by high intensity focused ultrasound "HIFU" when performing HIFU therapy on a lesion.

Advantageously, a second quantitative indicator is calculated for a plurality of points at different depths in the excitation region.

This feature allows the viscoelastic properties of the medium to be determined as a function of depth. This enables in particular to define the contour of the area containing the liquid.

Advantageously, the calculation of the quantitative indicator or the second quantitative indicator is repeated at different times.

This feature can track changes in the viscoelastic properties of the media over time. In particular, this makes it possible to track the size of the necrosis caused in HIFU or even by radiofrequency treatment over the treatment time.

Advantageously, a "time" quantitative indicator is calculated, which is a function of the time variation in the quantitative indicator.

Such a time index may be calculated for a single quantitative index as well as for a second quantitative index.

Advantageously, the method can be repeated by moving said excitation area in order to measure a quantitative indicator in the whole region of interest in the viscoelastic medium.

Advantageously, the method comprises a construction step for constructing an echographic image of said viscoelastic medium; and the method further comprises a display step for displaying quantitative indicators or amplitudes derived from said indicators as brightness values or color values on the obtained echographic images.

In an advantageous implementation, the information of the quantitative indicators is used to calculate parameters for the course of treatment and its development, in particular the size of the coagulation or tissue necrosis caused, by tracking the modification of the medium. For example, the therapy performed may be high intensity focused ultrasound therapy (HIFU) or radio frequency therapy.

The quantitative indicators may also be used to calculate parameters for other types of local treatment, in particular radiofrequency treatment.

The present invention also provides an apparatus for imaging a viscoelastic medium, the apparatus comprising: an excitation module for generating internal mechanical stress in the excitation region; and an imaging module for acquiring signals for imaging motion resulting from mechanical stress in the viscoelastic medium in response to the internal mechanical stress in an imaging zone containing the excitation zone. The apparatus of the present invention further comprises: a calculation module for calculating a quantitative indicator relating to the rheological behaviour of the viscoelastic medium at least one point located at a given depth outside the excitation zone, said quantitative indicator representing a comparison between: a signal obtained during motion in response to mechanical stress in the excitation region at least one point located at the given depth, and a signal obtained during motion in response to mechanical stress in the imaging region at least the point located outside the excitation region.

Advantageously, the excitation module is a transducer array, which may also be used as an imaging module.

In a preferred implementation, the steps of the method are defined by computer program instructions.

The invention therefore also provides a computer program on a data medium, the program being adapted to be implemented in an imaging apparatus and comprising instructions adapted to perform the steps of: an excitation step in which an internal mechanical stress is generated in an excitation region; an imaging step of imaging the medium by acquiring signals during motion generated by mechanical stress in the viscoelastic medium in response to internal mechanical stress in an imaging zone containing the excitation zone; and a calculation step of calculating a quantitative similarity index relating to the rheological behaviour of the viscoelastic medium based on a comparison between: a signal obtained during motion in response to mechanical stress at least one point in the excitation region, and a signal obtained during motion in response to mechanical stress at least the point in the imaging region that is outside the excitation region.

The program can use any programming language, and it can be in the form of source code, object code, or a code intermediate source and object code, such as partially compiled form, or in any other desired form.

The present invention also provides a data medium readable by an imaging device comprising computer program instructions as described above.

The data medium may be any form of entity or device capable of storing the program. For example, the medium may include a storage device such as a Read Only Memory (ROM), e.g., a CD ROM or microelectronic circuit ROM, or even a magnetic recording device, e.g., a floppy disk or hard disk.

Further, the data medium may be a transmission medium, such as an electrical or optical signal, which can be transmitted by means of electrical or other means via electrical or optical cables. In particular, the program of the present invention may be downloaded from an internet-type network.

Alternatively, the data medium may be an integrated circuit in which the program is embodied, the circuit being adapted to perform, or for use in the performance of, the method of the invention.

Drawings

Further characteristics and advantages of the invention will become apparent from the following description, with reference to the attached drawings, which show embodiments with non-limiting reference. In the drawings:

FIG. 1 is a schematic representation of the use of the apparatus of the present invention;

FIG. 2 is a schematic representation of the apparatus of the present invention;

FIG. 3 illustrates a particular application of the present invention in viscoelastic media;

FIGS. 4a and 4b are schematic representations of imaging methods embodying the invention in regions containing liquid and in regions that are solid overall;

FIG. 5 is a graph showing correlation coefficients obtained for the two environments shown in FIGS. 4a and 4 b;

fig. 6 shows an example of display of a quantitative index according to the present invention.

Detailed Description

Fig. 1 is a schematic representation of an imaging device 1 according to the present invention for imaging a viscoelastic medium 2. In an advantageous implementation, the medium 2 is a biological tissue, for example a human organ or a part of an organ, such as a breast.

The apparatus 1 is connected to at least one ultrasound probe 3. Such a detector 3 may comprise a single element, or a one-or two-dimensional transducer array. When the device of the invention is used to observe a medium 2, the probe 3 is in contact with the medium 2.

The device 1 comprises an electronic module for controlling the emission of compression waves (for example ultrasound waves) by the probe 3.

The viscoelastic medium 2 diffuses such a compression wave. Specifically, an ultrasonic compression wave can be propagated in the viscoelastic medium 2, whereby an echographic image can be obtained.

Advantageously, the apparatus 1 is connected to a display module 4 capable of displaying information extracted from the imaging data.

Figure 2 shows the apparatus 1 and the detector 3 more accurately. In the example shown in the figure, the detector 3 is a linear one-dimensional transducer array comprising N elements [ T1, T2.. TN ], where N equals 128, for example. The device 1 comprises a number of channels V, preferably N channels [ V1, V2.., VN ], i.e. in this example 128 channels, which are capable of individually controlling the individual transducer elements [ T1, T2.. TN ] of the detector 3.

The software module 5 is used to control the electronic channels [ V1, V2., VN ] in order to implement the excitation sequence and the imaging sequence and also to alternate these sequences.

The memory module 6 is used to record the backscattered ultrasound signals received during the imaging sequence.

According to the invention, the device 1 is programmed to carry out the various steps of the method of the invention in order to observe the rheological behaviour of the medium 2.

The first step of the present invention is: the mechanical excitation generates internal mechanical stresses in the medium 2. Whereby excitation signals are sent to the transducer elements via electronic channels which are themselves controlled by the software module 5.

Advantageously, the internal mechanical stress is generated by using separate groups of transducer elements [ T1, T2.. TN ] or by using a single transducer element to continuously emit one or more ultrasound compression waves, which are optionally focused.

By way of example, the exciting step can be performed by focusing an acoustic beam having sufficient energy to successively excite the tissue at one or more depths.

Advantageously, the device 1 is able to increase the energy of these waves by increasing the amplitude of the pressure field or by increasing the length of the emitted wave train. In the application envisaged, the length of the wave train is advantageously in the range 1 microsecond (μ s) to 10,000 μ s.

Advantageously, the frequency of the excitation ultrasound wave is in the range of 0.1 megahertz (MHz) to 50 MHz. Furthermore, time coding of the excitation signal may be used in order to improve its penetration.

Fig. 3 shows the mechanism of the invention in medium 2. In this figure, a plurality of waves are emitted, which are focused on a plurality of points Ai located at different depths. Thus, the "excitation" zone a covering the focal points Ai of the wave is insonified. The waves used have sufficient energy to move the tissue, primarily in the z direction, thereby creating shear waves. For example, a wave train having a length in the range of 1 to 10,000 μ s is used, and the number of beams used is in the range of 1 to 50, and these different excitation sequences are transmitted at a repetition rate that oscillates in the range of 10 to 20,000 hertz (Hz). Thus, the insonified region a is excited by the ultrasonic radiation pressure.

The compressional waves can also be focused simultaneously or alternately on at least two different locations. In any case, the excitation area a is a concatenation (collocation) of a set of insonification points. It should also be observed that the use of unfocused or minimally focused waves is advantageous for covering a wider and spread excitation region.

The second step of the method of the invention is an imaging step in which ultrasound waves are continuously emitted so as to irradiate an "imaging" zone B of the medium 2 containing the excitation zone a. Preferably, the same transducer array as the excitation step [ T1, T2.. TN ] may be used by using a different excitation frequency or voltage than the excitation sequence.

Furthermore, as an alternative, it is also fully conceivable to use a second array of transducers or other elements contained in the first array, or even to use a single-element transducer. In particular, this makes it possible to process the imaging step in parallel with the excitation step.

The emission rate during the imaging step must be very high in order to observe the propagation of shear waves or currents. Typically, this corresponds to a transmission rate in the range of 0.1Hz to 20,000 Hz.

The acoustic irradiation of the medium 2 during the imaging step may be performed using focused or unfocused ultrasound waves.

During this imaging step, the ultrasound waves are reflected by the reflective particles present in the region B of the medium 2. These reflections cause ultrasonic echoes. The backscattered signals corresponding to these ultrasound echoes are then detected with the transducer array [ T1, T2.. TN ] and recorded in the memory module 6.

The effect of motion on the echographic images can then be observed. More specifically, from the acquired ultrasound echoes, an estimate of the motion in the medium is used to quantify the mechanical response of the viscoelastic medium to internal excitation stresses. To achieve this estimation of motion, the backscatter signals corresponding to a given set of reflecting particles or scatterers are compared with each other to estimate the displacement of the set of scatterers from the original or previous position.

The estimated displacement may be axial or vectorized. The displacement velocity is obtained when the displacement relative to one of the previous shots is estimated based on the delay in arrival time at the transducer element [ T1, T2.. TN ] and the ultrasound propagation velocity is assumed to be constant and known.

In which case conventional methods for estimating motion known to those skilled in the art may be used. For example, the cross-correlation function may be maximized, a frequency-type method such as estimating phase shift, or even a doppler method for estimating velocity. Vector schemes, such as the vector doppler method, or the use of optical flow are also envisaged.

These estimation methods are implemented in a software module 5, the software module 5 processing data stored in a memory module 6.

For example, software module 5 causes the execution channels to be formed so as to calculate a series of echographic images of imaging zone B, from which images motion estimates are then calculated using one-dimensional or vector algorithms known to the person skilled in the art.

The motion field calculated when estimating the motion caused by exciting internal mechanical stresses may be an absolute displacement field or a relative displacement field, i.e. a velocity.

Advantageously, for estimating the displacement, it is advantageous to perform a preliminary step of imaging the medium 2 before the excitation. The method comprises the following steps: at least one compressed ultrasound wave is transmitted and reflected echoes are received to establish a set of reference echoes.

The next step of the method of the invention is a calculation step, calculating a quantitative index for analyzing the rheological properties of the medium. It is as follows: the consistency of the displacement field caused by the excitation is estimated from between at least one point of the excitation area a and a point outside the area a in the imaging area B.

Fig. 3 shows that the calculation is carried out in the observation zone B of the viscoelastic medium 2. For local quantification of mechanical behavior in the vicinity of a point a0 belonging to an excitation region a, the present invention uses an estimate of the motion at said point a0 and an estimate of the motion at least one point B01 in the imaging region outside the excitation region a. This is particularly useful for quantifying changes in motion.

Point B01 is preferably located a short distance, e.g., 0.5 millimeters (mm) laterally, from point a0 to determine the fluid or solid properties of region a0.

In practice, the term "dots" is used to designate a smaller physical area of the medium 2 around the geometrical points of the medium 2. The size of the physical region may be variable in order to perform a trade-off between robustness of the estimation and processing time. The displacements in these critical regions can be summarized by, for example, arithmetic averaging of the pixels contained in these regions.

When the plurality of pixels or the plurality of cells for which the displacement is calculated correspond to a given physical area in the medium around a point, a quantitative index (e.g., calculated from the correlation coefficient) is calculated as a mean value of the quantitative indexes calculated for each pair of pixels or cells in the corresponding areas a and B, e.g., a mean value of the correlation coefficients calculated for each pair of pixels in the corresponding areas a and B.

According to the invention, the time variations in displacement or, similarly, in velocity observed in the respective regions around points a0 and B01 are compared and from this a quantitative indicator relating to the viscoelastic properties of medium 2 is derived.

The quantitative indicator preferably characterizes the spatial consistency of the displacement field at the two points. Spatial consistency means the presence or absence of similarity between the displacement fields obtained at different points. The coherence relates to the amplitude of the signals obtained and/or the temporal variation of these signals. The quantitative indicator may specifically be referred to as a "similarity" indicator. In order to quantify the similarity of the amplitude and the similarity of the time variation, the index may be specifically constructed by the maximum correlation coefficient between the displacement signal at a0 and the displacement signal at B01.

Fig. 4a and 4b show the situation in which the invention is implemented in a medium 2 containing a liquid pocket 8, when point a0 of the activation zone is located in the liquid pocket 8 and when point a0 of the activation zone is located outside the liquid pocket 8.

The quantitative index is the maximum correlation coefficient calculated using the following formula:

where t scans for a time period,sis the field to be investigated, which may be in particular the velocity or displacement field or may even be the ultrasound spot intensityThe field(s) is (are) such that,xis the abscissa of point B01 at the same depth as point a0, and xs is the abscissa of point a0. This coefficient represents a comparison between the time variations of the signals obtained for points a0 and B01 during the motion of the medium.

The maximum correlation coefficient may be calculated between displacement or velocity fields resulting from the time averaging of the displacement or velocity fields at points a0 and B01.

Other types of quantitative indicators may also be calculated to quantify the similarity between the displacement signals on a0 and B01. In particular, it may comprise different distances between the original or amplitude normalized signals or between optionally offset signals, e.g. p-norm, Euclidean norm (p 2), entropy criteria such as Kullback divergence, etc., or even by calculating the maximum of the cross-correlation function.

Advantageously, as shown in fig. 3, a plurality of points B0j, j 1 to M, are observed, at the same depth as point a0 and at different distances from point a0. Again, the temporal changes in displacement or similarly velocity observed from the corresponding regions around points a0 and B0j are spatially compared, and a quantitative indicator relating to the viscoelastic properties of medium 2 is derived for each point B0 j. Thus, after a given excitation, a number of points further and further away from a0 are analyzed.

Advantageously, as shown in fig. 4a and 4B, at least two directional quantitative indicators C01 and C0j ' are calculated, one on each of the two sides of the excitation area of the two points B01 and B0j ', and the two points B01 and B0j ' are located on one side of the excitation area, respectively.

The invention thus makes it possible to detect and characterize interfaces in a specific manner, since the interface between two media having different viscoelastic properties can be characterized by evaluating the mechanical response in the excitation region a or on either side. The large difference between the two directional quantitative indicators on both sides of a given excitation region indicates the presence of an interface, i.e. the presence of a sudden change in mechanical properties in the vicinity of the excitation region a.

FIG. 5 shows the values of the directional quantitative indicators constituted by the maximum correlation coefficients as described above and obtained during the imaging performed as shown in FIGS. 4a and 4B for a plurality of points B0j and B0 j', the abscissa of these points on either side of the excitation area AxUp to 20 mm. In fig. 4a, the obtained curve shows the decorrelation (decorrelation) of the field under investigation outside the excitation region a to which a0 belongs. The curve of this shape corresponds to the presence of liquid at a0.

In contrast, for FIG. 4B, the resulting curve shows the correlation maximum, which corresponds to the abscissa of points B0j and B0jxSlightly decreased.

Advantageously, the point B0j at which the quantitative indicator is at a maximum may be determined. Point a0 may then be associated with the indicator, which may be, for example, the maximum of the maximum correlation coefficient between the displacement signal at a0 and the displacement signal at point B0j { B01.., B0M }.

In particular, a change in the quantitative index with distance A0B0j may also advantageously produce a second quantitative index to characterize the rheological properties of the viscoelastic medium 2. The second quantitative indicator describes that the quantitative indicator decreases as the distance from a0 increases. Advantageously, the quantitative indicator associated with point a0 is calculated from the spatial variation of the previously calculated quantitative indicators on the line at the same depth as point a0.

For example, the gradient of the quantitative indicator at the depth of point a0 may be estimated to calculate a distance corresponding to the quantitative indicator after an n% (e.g., 90%) drop, to calculate a concavity of the quantitative indicator or some other property associated with the spatial second derivative of the quantitative indicator, etc., each of which may constitute a second quantitative indicator associated with a given depth of the excitation region a.

The calculation may then be repeated several times more, thereby calculating a second quantitative index at a plurality of points at different depths, and calculating a map of this second quantitative index in the excitation area a. In parallel, it is also possible to map the quantitative index in the imaging region B.

As shown in fig. 3, excitation area a is then subdivided into P +1 points Ai in the Z-direction { A0... AP }. For each measurement point Ai, a set of M points Bij is defined in the imaging zone B on at least one side of the excitation zone A, where j ≧ 1.

For all i ≦ P and all j ≦ M, the maximum correlation coefficient Cij between at least one velocity field or displacement time field at point Ai and at least one velocity field or displacement time field at point Bij is calculated.

Then, a second quantitative indicator on AP is repeatedly calculated at a different point A0. in the excitation area A, where P ≧ 0, based on the quantitative similarity index calculated for Bij. The second quantitative indicator to be calculated on Ai is defined, for example, as the maximum of the derivative of the correlation coefficient Cij with respect to j.

Furthermore, the coefficients Cij are calculated between velocity or displacement fields that are time-averaged over a plurality of points in the areas a and B concerned.

The calculation of the second quantitative index is repeated for each i, where i ≦ P, covering all points Ai of the excitation area A.

Thereafter, the excitation region a may be moved in a continuous excitation emission. By performing a plurality of alternating excitation and imaging sequences, and by moving the excitation and imaging regions a and B, for example laterally in the imaged medium, it is possible to displace a in depth or azimuthally so as to cover the extension region D as shown in fig. 1.

Each excitation point a of the medium 2 may then be given a binary value, for example by using a threshold value for the second quantitative indicator, and the value may be displayed by using a color coding or a display coding. An example of such a display is given in fig. 6, where the black areas correspond to the detection of a liquid pocket 8 as shown in fig. 4.

Advantageously, two "directional" second quantitative indicators can be defined around the excitation region, one being the maximum of the derivative of Cij with respect to j, where j corresponds to a point Bij on one side of Ai, and the other being the maximum of the derivative of Cij 'with respect to j, where j gives you a point Bij' on the other side of Ai. Again, this second quantitative indicator serves to reveal the presence of an interface when they are very different on either side of the excitation region.

It may also be based on the difference between the values given for the pair [ Ai; bij ] and the time variation of the quantitative index calculated successively, the "time" quantitative index is calculated. This second quantitative indicator is used in particular to give additional information about the release time of the medium, the magnitude of which is related to the viscosity of the medium.

It should be observed that a previously calculated time variation of the second indicator may also be used. It is contemplated in the present invention that such calculations may be made at multiple depths.

The second quantitative indicator may also be calculated from a spatial variation that is a function of the depth of the quantitative indicator calculated for a set of points A0... AP paired with a set of points Bij ═ B0j to BPj located at the point A0... AP.

On the same line, the spatial variation as a function of the depth of the previously calculated second quantitative indicator may also be used. Again, the calculations are performed for multiple depths.

This second quantitative indicator can be used in particular to give an indication about the depth range of the lesion, by enabling a particular test to be made as to whether a deeper interface is present.

The information of the above quantitative index makes it possible to analyze the behavior of the medium 2 inside and outside the excitation area a and to infer a specific viscoelastic behavior.

A clear distinction between liquid behavior and solid behavior can be achieved based on the fact that the motion induced in the fluid is to maintain a restricted flow in the mechanically stimulated region a and possibly in the region very close thereto. In contrast, in a solid, the excitation produces a propagating shear wave that spatially extends the mechanical response to a large distance from the source.

Based on the calculated quantitative indicators given as examples, a large value of the quantitative similarity indicator indicates a strong correlation between the fields inside and outside the source, which indicates the presence of a propagating shear wave, which is characteristic of a viscoelastic solid. Conversely, a smaller value of the quantity similarity index indicates a viscous liquid more.

Therefore, the correlation coefficient close to 1 obtained without considering the position of the point Bij means that the elastic wave propagates with little attenuation and the medium is a solid.

In a more viscous solid, the attenuation will result in a correlation coefficient that gradually decreases with increasing distance AiBij.

In contrast, in liquids, the correlation coefficient drops very rapidly with increasing distance AiBij. This is due to the fact that: that is, the excitation at Ai is very different from the liquid flow species at Bij.

Thus, by calculation, at a plurality of point pairs [ Ai; bij ], which is a characteristic of viscoelastic properties that change abruptly or otherwise in two dimensions of the imaged medium, can detect spatial variations in the quantitative index. For example, in a particular application for detecting breast lesions, lesions with well-defined boundaries (i.e., with well-defined contours) are generally benign and exhibit spatial variation in quantitative indices that are very different from penetrating malignant lesions with very poorly defined boundaries. This determination of the mechanical characteristics of the interface between healthy tissue and the lesion allows them to be characterized.

The determination of the quantitative indicator according to the invention can also be used to quantify the viscosity of the fluid, in particular the movement of the fluid extending towards the source and decaying even further in the radial direction as the viscosity of the fluid increases.

For viscoelastic solids, the spatial variation of the temporal mechanical response to a propagating shear wave quantified by the quantitative indicators calculated for each pair of points Ai and Bij in the medium becomes greater with increasing viscosity and decreasing elasticity.

Since the quantitative indicator gives an estimate of the spatial coherence between two points in the shear field, it is a relevant indicator of the release time of the medium and also an indicator to measure the quality of the elasticity. Thus, by relying on the quality criterion that the quantitative indicators can represent, an alternative quantitative elasticity estimation can be performed in parallel, for example using the method described in patent WO 2004/0210838.

Advantageously, the entire method of the present invention can be repeated continuously while imaging the medium, so that the calculated quantitative index map is updated periodically. In particular, a quantitative indicator over a discrete distance may be calculated, for example in order to track changes in the area being necrosed while the medium 2 is being subjected to HIFU treatment. Tracking the quantitative index in this manner can be used to perform automated control of the method of treating the medium.

Advantageously, an image of the quantitative indicator in the imaging zone, or an image of the amplitude associated therewith, is displayed. For this purpose, a suitable color coding may be used. Such display may be implemented in a manner superimposed on a standard echographic image, or may be implemented in a juxtaposed manner. The display may be graded, thereby enabling assessment of viscosity, for example, or it may be binary, thereby distinguishing liquid regions from solid regions. Thus, for example, a quantitative indicator may be associated with a pixel color value, and a quantitative similarity indicator for region D may be created and displayed on display module 4.

Claims (13)

1. A method for imaging a viscoelastic medium (2), comprising the steps of:

an excitation step in which an internal mechanical stress is generated in the excitation region [ A ];

an imaging step of imaging by acquiring signals during a movement generated by mechanical stress in the viscoelastic medium (2) in response to the internal mechanical stress in an imaging region [ B ] containing the excitation region [ a ];

the method is characterized in that it further comprises: a calculation step for calculating a quantitative indicator [ Cij ] related to the rheological behaviour of the viscoelastic medium (2) at least one point [ Bij ] of the imaging zone [ B ] located at a given depth outside the excitation zone [ a ], said quantitative indicator representing a comparison between: a signal obtained during motion generated in response to mechanical stress at least one point [ Ai ] in the excitation zone [ A ] located at the given depth, and a signal obtained during motion generated in response to mechanical stress at least the point [ Bij ] in the imaging zone [ B ] located outside the excitation zone [ A ].

2. The method of claim 1, wherein the signal comparison step comprises comparing time and/or amplitude variations of the signals.

3. Method according to claim 1 or 2, characterized in that said calculation step comprises calculating a maximum value of the correlation function of the obtained signals corresponding to the two points [ Ai, Bij ] considered, said quantitative indicator [ Cij ] being a function of this maximum value.

4. The method of claim 1, wherein the internal mechanical stress is a mechanical vibration generated by ultrasonic radiation pressure.

5. The method of claim 1, wherein the imaging step comprises:

a transmission sub-step for transmitting ultrasound waves into said imaging zone at a repetition frequency sufficient to measure the dynamic characteristics of said motion generated by said internal mechanical stress;

a detection and recording sub-step for detecting and recording the ultrasonic echoes generated in said medium (2) as acquired signals; and

an estimation sub-step for estimating a motion from the detected echoes of at least two successive ultrasound transmissions and from the frequency of said ultrasound transmissions, wherein said quantitative indicator [ Cij ] is calculated from said estimated motion.

6. The method according to claim 1, characterized in that at least two "directional" quantitative indicators of similarity [ Cij, Cij ' ] are calculated for two points [ Bij, Bij ' ] at the same given depth outside the excitation region [ a ], which are located on either side of the excitation region [ a ], respectively, said quantitative indicators [ Cij, Cij ' ] representing two comparisons between: signals acquired during motion generated in response to mechanical stress at least one point in the excitation zone [ Ai ] located at the given depth, and signals acquired during motion generated in response to mechanical stress at least the two points [ Bij, Bij' ] in the imaging zone located outside the excitation zone [ A ].

7. Method according to claim 1, characterized in that the quantitative indicator [ Cij ] is calculated on the boundary of the echographic defined area of the medium in order to test its permanent or temporary properties in the surrounding medium.

8. The method of claim 1, wherein a second quantitative indicator associated with a point at a given depth in the excitation region is calculated as a function of the spatial variation of the quantitative indicator [ Cij ] for a plurality of points along a straight line at the given depth that are outside the excitation region [ a ].

9. The method according to claim 8, characterized by calculating a second quantitative indicator for a plurality of points [ Ai ] at different depths in the excitation area [ A ].

10. A method according to claim 8 or 9, wherein the calculation of the quantitative indicator or the second quantitative indicator is repeated at different times.

11. The method of claim 1, wherein a "time" quantitative indicator is calculated that is a function of the time variation in the quantitative indicator [ Cij ].

12. Method according to claim 1, characterized in that it comprises a step for constructing an echographic image of said viscoelastic medium, and a display step for displaying on the obtained echographic image a quantitative indicator [ Cij ] or an amplitude derived from said indicator as a brightness value or a color value.

13. An apparatus for imaging a viscoelastic medium (2), the apparatus comprising:

an excitation module (3) for generating an internal mechanical stress in the excitation region [ A ];

an imaging module (3) for acquiring signals for imaging a motion generated by mechanical stress in the viscoelastic medium (2) in response to the internal mechanical stress in an imaging zone [ B ] containing the excitation zone [ A ];

the device is characterized in that it further comprises: -a calculation module for calculating a quantitative indicator [ Cij ] related to the rheological properties of the viscoelastic medium (2) at least one point [ Bij ] located at a given depth outside the excitation zone [ a ], said quantitative indicator [ Cij ] representing a comparison between: a signal obtained during motion generated in response to mechanical stress at least one point [ Ai ] in the excitation zone [ A ] located at the given depth, and a signal obtained during motion generated in response to mechanical stress at least the point [ Bij ] in the imaging zone [ B ] located outside the excitation zone [ A ].