BACKGROUND OF THE INVENTION

The present invention relates to the general field of methods and devices for imaging a visco-elastic medium, including at least one method for studying movements that exist in the visco-elastic medium. More particularly, the invention relates to methods comprising an excitation step during which an internal mechanical stress is generated in an excitation zone, followed by a step of imaging by acquiring signals during the movements generated in the visco-elastic medium in response to the internal mechanical stress in an imaging zone which includes the excitation zone.

In a particularly advantageous application, the invention relates to imaging the visco-elastic properties of human organs. In this medical application, the invention serves in particular to detect the presence of a liquid zone in an organ such as the breast, the liver, the ovary, etc. and to characterize that zone.

Echographic examination of the breast already makes it possible to deduce certain characteristics of observed lesions from information about the morphology and the echographic texture of those lesions. In particular, it is possible to distinguish between a cystic lesion and a solid mass on the basis of echogenicity values that reveal the acoustic specificity of liquid. A typical cyst appears on the echographic image as a zone of geometrically regular shape that does not generate echoes, given that the liquid filling the cyst contains few diffusers, whereas the echogenicity of tissue beyond the cyst appears to be clearly enhanced since the incident beam is attenuated more weakly in the cyst than in tissue.

It is also possible to estimate the risk of a solid lesion being malignant as a function of its morphology. An image having sharp outlines with few lobes and a major axis parallel to the skin is more likely to be associated with a lesion that is benign. Conversely, an image that is irregular, e.g. being star-shaped, with outlines that are poorly defined, and with strong posterior shadowing is very likely to be malignant.

Unfortunately, those morphological criteria can be insufficient since they are not completely reliable, particularly with small lesions. This lack of specificity has led to new echographic tools emerging that give quantifiable functional information that is more correlated with pathological states.

For example there are elastographic techniques that seek to measure the mechanical properties of tissue for the purpose of characterizing lesions more finely and systematically.

The rheology of the medium can also be analyzed in order to distinguish solid and liquid natures, and it is known in particular to generate radiation pressure in a well-defined zone of interest in order to generate therein, if the zone is liquid, a flow of that liquid which can then be imaged by echography.

The image obtained reveals movements in the zone of interest. When movement is observed on the resulting image, then the presence of liquid has been detected.

Such a method of detecting the presence of a liquid merely by detecting movement leads to lesions being classified in a manner that is not very robust, in particular with cysts that are complex, that are often echogenic, and with content that is viscous, as are encountered in about 50% of cases. Indeed, the movement induced in the radiation pressure zone in a cyst of that type remains very similar to the mechanical response of a visco-elastic solid. The presence or absence of movement is thus not a distinguishing criterion. However, it is specifically with cysts of that type that the morphological criteria given by echography usually turn out to be insufficient.

Furthermore, known methods are capable only of testing a particular zone of the medium as selected by the user. Indeed, the method is specific, and it is not practical to envisage implementing the known method continuously while imaging a medium.

Finally, the known method serves only to detect the presence of liquid, and does not provide any graduation in the rheology of lesions. Amongst other things; these lesions may be cysts that are viscous, milky, calcified, hemorrhagic, or indeed made up of sediments, and they can thus present rheological properties that are distinct and varied.

OBJECT AND SUMMARY OF THE INVENTION

The main aim of the present invention is thus to mitigate such drawbacks by proposing an elastographic technique that makes it possible, in particular within a single lesion, to distinguish between liquid and solid components, the invention consisting in a method as set out in the introductory part and furthermore comprising a step of calculating a quantitative index associated with the rheological properties of the visco-elastic medium at at least one point situated at a given depth outside the excitation zone, said quantitative index being representative of a comparison between signals acquired during the movements generated in response to the mechanical stress at at least one point of the excitation zone situated at the given depth, and signals acquired during the movements generated in response to the mechanical stress at at least the point of the imaging zone situated outside the excitation zone.

Such a method is based on characterizing relative movements of different zones of the visco-elastic medium, the zones being distinguished by the way in which they respond to the mechanical stress. The method proposes systematically calculating a quantitative index to reveal the presence or absence of a propagating mechanical wave in the medium outside the excitation zone. The method of the present invention thus makes it possible to observe a broad zone of the medium, by giving a map of the quantitative index at a plurality of points. This quantitative index is advantageously a similarity index.

The signals acquired for calculating the quantitative index are preferably a displacement field, or a movement field, or ultrasound noise or “speckle.” Acquiring such fields is well known to the person skilled in the art.

The method of the invention makes it possible to identify solid and liquid zones on the basis of the wave propagation criterion, which is a robust physical criterion. The invention then enables rheology to the characterized over the entire imaging zone. With the invention, all of the characterizations are performed without prior knowledge of the zone in which the medium is imaged.

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

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

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

It is then the presence or the absence of propagation of a shear wave generated by said mechanical vibration that enables solids and liquids to be distinguished. It should be observed that generating such a shear wave is advantageous since the method of the invention is then performed in parallel with measuring the elasticity of the visco-elastic medium using methods that are known, e.g. from patent WO2004/0210838. In addition, generating such a shear wave makes it possible to apply mechanical excitation to a medium in depth, and thus to probe rheology deeply within organs. This imparts another advantage to this type of stimulation.

In an advantageous application, the excitation step is coupled with treatment. The stress is then advantageously generated by a beam used for therapeutic purposes, e.g. a focused ultrasound wave beam.

Furthermore, according to the invention, the excitation that enables movement to be generated in the medium may be the result of a variety of mechanisms, including in particular one or more biological mechanisms, or indeed external vibration at low frequency. Such mechanisms need to generate internal mechanical stress that is localized in an excitation zone.

In a particular feature of the invention, the imaging step comprises a sub-step of emitting ultrasound waves into the imaging zone at a repetition frequency that is sufficient for measuring the dynamics of the movements generated by the internal mechanical stress; a sub-step of detecting and recording the ultrasound echoes generated within the medium as acquired signals; and a sub-step of estimating movement from the detected echoes for at least two successive emissions of ultrasound waves and from the ultrasound wave emission frequency, the quantitative index being calculated from said estimated movement.

In a known manner, the ultrasound waves generated during this imaging step are reflected by tissue diffusers on the lines of propagation of the excitation wave. With such a feature, it is possible in particular to use the same array of transducers for exciting and for imaging the medium.

In an advantageous implementation, at least two “directional” quantitative indices are calculated for two points situated at the same given depth outside the excitation zone, one on either side of the excitation zone, said quantitative indices being representative of two comparisons between signals acquired during the movements generated in response to the mechanical stress at at least one point of the excitation zone situated at the given depth, and signals acquired during the movements generated in response to the mechanical stress at at least the two points of the imaging zone situated outside the excitation zone.

Such a feature makes it possible to specify the properties of the medium at a given depth on either side of a given excitation zone. It is then in particular possible, when the excitation zone is localized at an interface between two zones of different echogenicities, to reveal different visco-elastic characteristics on either side of the interface.

In a particular implementation of the invention, the quantitative index is calculated at the boundary of a region of the medium defined by echography in order to test the permanent or temporary character thereof in the surrounding medium.

Such an implementation makes it possible to test for possible sliding of a solid lesion within tissue. This fixed or rolling aspect of the lesion within the surrounding tissue is likewise an important criterion for characterizing the lesion.

Advantageously, a secondary quantitative index associated with a point of the excitation zone at a given depth is calculated as a function of spatial variations of the quantitative indices of points situated outside the excitation zone along a line at said given depth.

Such a characteristic makes it possible to determine regions having different visco-elastic properties along said line, for example in order to determine the extent of a zone of tissue necrosis or a zone of complete coagulation induced by High Intensity Focused Ultrasound “HIFU” while performing HIFU treatment on a lesion.

Advantageously, a secondary quantitative index is calculated for a plurality of points at different depths in the excitation zone.

Such a characteristic makes it possible to determine the visco-elastic properties of the medium as a function of depth. This can make it possible in particular to define the outlines of a zone containing liquid.

Advantageously, the quantitative index or the secondary quantitative index calculation is reiterated at different instants.

Such a characteristic makes it possible to track variation in the visco-elastic properties of the medium as a function of time. In particular, this can make it possible to track variation in the size of a necrosis induced by HIFU or indeed by radio-frequency treatment as a function of treatment time.

Advantageously, a “time” quantitative index is calculated as a function of time variations in a quantitative index.

Such a time index can be calculated for a simple quantitative index and also for a secondary quantitative index.

Advantageously, the method can be reiterated by moving the excitation zone so as to measure quantitative indices throughout a zone of interest in the visco-elastic medium.

Advantageously, the method includes a step of constructing an echographic image of the visco-elastic medium; and the method further comprises a step of displaying a quantitative index or a magnitude derived from said index as brightness or color values on the echographic image that is obtained.

In an advantageous implementation, knowledge of the quantitative index (indices) is used to calculate parameters for a treatment and its progress by tracking modification of the medium, in particular the size of induced coagulation or tissue necrosis. By way of example, the treatment performed may be High Intensity Focused Ultrasound treatment (HIFU) or treatment by radio frequencies.

It is also possible to use the quantitative index (indices) for calculating parameters of other types of localized treatment, in particular radio-frequency treatment.

The invention also relates to a device for imaging a visco-elastic medium comprising excitation means for generating an internal mechanical stress in an excitation zone, imaging means for acquiring signals to image movements generated by the mechanical stress in the visco-elastic medium in response to the internal mechanical stress in an imaging zone that includes the excitation zone. The device according to the invention furthermore comprises calculation means for calculating a quantitative index associated with the rheological properties of the visco-elastic medium at at least one point situated at a given depth outside the excitation zone, said quantitative index being representative of a comparison between signals acquired during the movements generated in response to the mechanical stress at at least one point of the excitation zone situated at the given depth, and signals acquired during the movements generated in response to the mechanical stress at at least the point of the imaging zone situated outside the excitation zone.

Advantageously, the excitation means are an array of transducers that can also serve as imaging means.

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

Consequently, the invention is also directed to a computer program on a data medium, said program being capable of being implemented in an imaging device and comprising instructions suitable for implementing the following steps: excitation, during which an internal mechanical stress is generated in an excitation zone; imaging of the medium by acquiring signals during movements generated by the mechanical stress in the visco-elastic medium in response to the internal mechanical stress in an imaging zone including the excitation zone; and calculating a quantitative similarity index associated with the rheological properties of the visco-elastic medium on the basis of a comparison between signals acquired during the movements generated in response to the mechanical stress at at least one point of the excitation zone, and signals acquired during the movements generated in response to the mechanical stress at at least the point of the imaging zone situated outside the excitation zone.

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

The invention also provides a data medium readable by an imaging device and including computer program instructions as specified above.

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

Furthermore, the data medium may be a transmission medium such as an electrical or optical signal, that can be conveyed via an electrical or optical cable, by radio, or by other means. In particular, the program of the invention may be downloaded from a network of the Internet type.

Alternatively, the data medium may be an integrated circuit in which the program is incorporated, the circuit being adapted to execute or to be used in the execution of the method in question.

