Ultrasound imaging device and method employing continuous emission and reception

Abstract

The invention relates to an ultrasound imaging device, and to its associated method, comprising: - at least one emitter element configured to emit a continuous excitation signal (Sem) into a medium, the excitation signal being coded so as to form a succession of N overlapping ultrasonic waves that each have a respective signature (Sk), k=[1, 2, 3, …………., N], - at least one receiver element, distinct from the emitter element, and configured to receive an echo signal (Secho) simultaneously with the emission of the excitation signal emitted by the emitter element, the echo signal being generated by the excitation of the medium by the excitation signal, and - filtering means configured to determine by decoding each of the signatures, the contribution of each wave in the echo signal, so as to generate an image.

Description

Description Title: DEVICE AND METHOD FOR ULTRASOUND IMAGING WITH CONTINUOUS EMISSION AND RECEPTION The invention relates to an ultrasound imaging device and method, combining two ultrasound transducers allowing respectively to transmit and receive simultaneously and continuously an acoustic signal and to record the echoes generated by the medium. State of the art Ultrasound is an imaging technique that uses sound waves to visualize soft tissues like tendons, muscles, joints, blood vessels, and internal organs. Unlike CT scanners, which use X-rays, and MRI machines, which use radio waves, ultrasound scanners use ultrasound waves to create images. One of the main components of an ultrasound scanner is the probe, called a transducer. It typically produces a short signal (called a pulse) of ultrasound waves that travel at a known speed of around 1540 m / s. The tissues and structures that this wave encounters absorb, reflect, or refract it. When the wave returns to the transducer, the latter converts the pressure field at its interface into electrical signals. The signals are then processed and shaped to reconstruct an image of the medium during acquisition. Soft tissues and organs appear on screen in grayscale. Blood and other fluids are represented in black, while soft tissue / bone interfaces are revealed in white. High-frequency transducers can produce very detailed and well-resolved images of surface features, while low-frequency transducers can produce very detailed and well-resolved images of surface features, while frequency are better suited to produce an image of deeper parts but in less detail and with lower resolution. Generally, ultrasound scanners use a single transducer grouping together a set of piezoelectric or other elements, which operate alternately in transmission mode, each first sending an ultrasonic wave pulse, then in reception mode to receive the echoes of each pulse, all of the echoes being processed to reconstruct the image of the observed medium. In order to improve image quality, it is necessary to optimize the resolution, resolution being defined as the smallest distance that can be separated by an instrument, as well as the contrast of the image. When using ultrasound, the intrinsic resolution, which is directly related to the wavelength itself, is better for high frequencies, but there is then a greater attenuation of the ultrasound in the medium which results in a lower penetrating power. Therefore, it is necessary to transmit more energy. Increasing the transmitted energy can be done by increasing the amplitude of the emitted waves, which is not without risk for the patient. Increasing the transmitted energy can also be achieved by lengthening the pulse duration, which inevitably leads to the appearance of so-called blind zones corresponding to the closest depths in contact with the probe, and reduces the resolution. Indeed, currently ultrasound procedures do not consider the possibility of simultaneously emitting and receiving ultrasonic waves. Furthermore, improving image quality and obtaining three-dimensional (3D) images requires reducing echo acquisition times, which is not compatible with the transmission and reception operation of current transducers, which, with this operation, induce an incompressible round-trip propagation time. Known from the prior art is document US 2010 / 111217 A1 which relates to a detection system and method configured to generate a set of N complementary Golay sequences, the code generator encodes a baseband signal with the complementary Golay sequences, the complementary Golay sequences having an ideal autocorrelation and the sum of the cross-correlations of each set of complementary Golay sequences being zero.The detection system and method also include a modulator for modulating each of the N sets of coded complementary Golay sequences on separate frequencies and a transducer for transmitting the N modulated sets of coded complementary Golay sequences. The transmitting system (code generator, modulator, transducer) produces complementary GOLAY code sequences which are transmitted simultaneously using modulation and frequency division multiplexing techniques, and the receiving system (demodulator, filter) identifies the backscattered sequences, as if they had been sent successively. US 2016 / 213258 A1 relates to imaging systems and methods based on the generation and use of unpaired coded excitation signals. The use of unpaired signals allows for spatial and / or temporal and / or functional coding of the transmitted signals. In some embodiments, high-rate imaging may be achieved by using orthogonal codes for spatial and / or temporal coding, and by using a subset of transducer elements as transmitters, and another subset of elements as receivers. Coded excitation signals consist of a succession of non-overlapping waves. US 2002 / 049381 A1 referred to in the Written Opinion by D4 discloses an ultrasound imaging system for imaging ultrasound scatterers, comprising a probe (208) for transmitting ultrasound waves and detecting ultrasound echoes reflected by said ultrasound scatterers, wherein said probe comprises a first group of transducer elements, labeled transmission group (T), for transmitting ultrasonic waves, and a second separate group of transducer elements, labeled receiving group (R), for detecting ultrasonic echoes reflected by said ultrasonic diffusers. The system also comprises a processing system (202) comprising transmitting and receiving means, coupled to said probe (208), for providing an encoded signal to said transmitting group (T) and receiving signals from said receiving group (R), respectively; transmitting beamforming means (103) for focusing the ultrasonic waves onto a focusing line, receiving beamforming means (105) for forming summed received signals of signals received from the focusing line and processing means for processing these summed received signals to form decoded signals; and means for displaying an image (109) which is a function of these decoded signals. The signals emitted are not continuous signals, but simple successions of sequences (pulses) of course of extended duration.

[0009] . In order to overcome all or part of these drawbacks, the invention proposes to combine two ultrasonic transducers allowing respectively the simultaneous and continuous transmission and reception of an acoustic signal. This allows more energy to be transmitted without losing resolution and to decouple the acquisition time from the round trip time of the ultrasonic wave. Statement of the invention Therefore, the invention proposes an ultrasound imaging device comprising: - at least one emitting element configured to emit a continuous excitation signal into a medium, the excitation signal being coded so as to form a succession of N ultrasonic waves, overlapping, and each having a respective signature, - at least one receiving element, distinct from said transmitting element, and configured to receive an echo signal simultaneously with the transmission of the excitation signal by said transmitting element, the echo signal being generated by the excitation of the medium by the excitation signal, and - filtering means configured to determine by decoding each of the signatures, the contribution of each wave in the echo signal, to generate an image. Various embodiments of the invention are provided, integrating, according to all of their possible combinations, the various optional characteristics set out below. In order to ensure good separability, in terms of image resolution, of spatially close elements in the medium as well as sufficient image contrast, decoding by filtering of the "mismatched" type is preferred. According to another preferred aspect making it possible to overcome the heating phenomena of the at least one emitting element and the at least one receiving element, due to the continuous emission and reception of the ultrasonic waves, the ultrasound imaging device comprises means for cooling the at least one emitting element and the at least one receiving element. According to yet another preferred aspect allowing access to two-dimensional (2D) or three-dimensional (3D) imaging, the device comprises: - a first plurality of emitting elements configured to each emit an excitation signal into a medium, each excitation signal being coded so as to form a succession of N overlapping ultrasonic waves, each having a respective signature, - a second plurality of receiving elements, distinct from said transmitting elements, each configured to receive an echo signal simultaneously with the transmission of the excitation signal transmitted by the associated transmitting element, each echo signal being generated by the excitation of the medium by said associated excitation signal. According to a certain application aspect of the invention, the device is configured so that the N ultrasonic waves admit frequencies between 1.5 and 50 MHz, for its use in a medical diagnostic method. According to another application aspect of the invention, the device is configured so that the N ultrasonic waves admit thermal and / or mechanical indices at least of the order of 1.5 times the maximum values ​​admitted for diagnostic applications, for its use in a therapeutic method. The invention also relates to a method for imaging a medium using ultrasound, comprising: - a step of transmitting, by at least one transmitting element, a continuous excitation signal, the excitation signal being coded so as to form a succession of N overlapping ultrasonic waves, each having a respective signature, - a step of simultaneous reception, by at least one receiving element, distinct from said transmitting element, of an echo signal, the echo signal being generated by the excitation of the medium by the excitation signal, and - a filtering step to determine by decoding each of the signatures, the contribution of each wave in the echo signal, to generate an image. Various embodiments of the invention are provided, integrating, according to all of their possible combinations, the various optional characteristics set out below. According to a preferred aspect making it possible to limit the noise of estimation of the medium in the echo signal, the filtering step (F) implements a filter of the “mismatched” type. According to another preferred aspect allowing to quickly encode and decode the waves, the ultrasonic waves are non-concordant with each other, in the sense that they each present a strong autocorrelation and very weak intercorrelations with each other. According to yet another preferred aspect, the N signatures are spatio-temporal signatures, i.e. a function of the amplitude, frequency and / or phase of the ultrasonic waves of the excitation signal. In other words, the amplitude, phase and / or frequency depend on the time and location of the emitting element. According to yet another preferred aspect allowing access to a high-speed imaging method, the refresh rate of the signal(s) allowing generation of an image is greater than that based on the round-trip duration of the ultrasonic waves. According to yet another preferred aspect allowing access to two-dimensional (2D) or three-dimensional (3D) imaging, the method implements: - a first plurality of emitting elements forming a first transducer, which simultaneously carry out the emission step, - a second plurality of receiving elements, distinct from said transmitting elements, and forming a second transducer, which carry out the reception step. List of figures Other features and advantages of the invention will emerge from the detailed description of a non-limiting mode of implementation, and from the appended drawings in which: [Fig. 1] Figure 1 is a schematic view of a continuous transmission and reception ultrasound imaging device according to one embodiment of the invention. [Fig. 2] Figure 2 is a comparison between the image (in this case a line) obtained by continuous excitation versus the image obtained by excitation by a pulse of two diffusing elements, one being fixed, the other in motion. [Fig. 3] Figure 3 is a comparison between the image (in this case a line) obtained by continuous excitation versus the image obtained by excitation by a pulse of a diffusing element moving at constant speed. [Fig. 4] Figure 4 is a comparison between the image (in this case a line) obtained by continuous excitation versus the image obtained by excitation by a pulse of a diffusing element moving with acceleration then deceleration. [Fig. 5] Figure 5 is a comparison between the image (in this case a line) obtained by continuous excitation versus the image obtained by excitation by a pulse of a diffusing element moving in a rapid sinusoidal motion and an average amplitude. [Fig. 6] Figure 6 is a comparison between the image (in this case a line) obtained by continuous excitation versus the image obtained by excitation by a pulse of a diffusing element moving in a very rapid sinusoidal motion and with a high amplitude. Description of an embodiment According to the principle of the invention and as shown in Figure 1, the ultrasound imaging device comprises: - at least one emitting element configured to emit a continuous excitation signal Sem into a medium, the excitation signal being coded so as to form a succession of N overlapping ultrasonic waves, each having a respective signature Sk, k=[l, 2, 3, .... , N], - at least one receiving element, distinct from said transmitting element, and configured to receive an echo signal Secho simultaneously with the transmission of the excitation signal by said transmitting element, the echo signal being generated by the excitation of the medium by the excitation signal, as well as: - filtering means configured to determine by decoding each of the signatures, the contribution of each wave in the echo signal, these filtering means ultimately generating an image. In the case of the invention, to obtain a succession of overlapping waves, the excitation signal is emitted continuously, then cut into segments which form the overlapping waves by a sliding sampling window approach. This cutting, and therefore the overlap of successive waves, is arbitrarily fixed by the user according to the desired refresh rate and potentially the speed of the ultrasound in the medium. Using a continuous signal with a succession of overlapping sequences allows more signal to be transmitted and thus increases the signal-to-noise ratio and therefore the sensitivity of the imaging system. In this continuous signal, any segment of the excitation signal can be considered as a specific signature that can be found / identified in the receiving signal. It becomes possible to reconstruct an image of the environment explored at any time, with almost infinite temporal resolution. Both the at least one transmitting element and the at least one receiving element are elements capable of transforming electrical energy into ultrasonic energy and vice versa. The mechanism of operation of these elements can be based on the physical effect called piezoelectricity. Some crystals called piezoelectric, such as quartz or tourmaline, naturally develop electrical charges on their faces when subjected to a variation in mechanical pressure. The effect is reciprocal, that is to say that if we apply, by means of electrodes, a variation of potential (therefore of electric charges) on two opposite faces of such a crystal, its thickness will vary in one direction or another. in the other (increase or decrease) depending on the polarity of the applied potential. In turn, this variation in thickness will act on the medium like the vibration of a piston. The vibration frequency of the crystal is controlled by the frequency of the alternating variation of the applied potential difference. If this frequency is high (of the order of MHz), then an ultrasonic wave is produced. Conversely, when the ultrasonic wave reflected by the medium is received by the receiving element, the variation in acoustic pressure experienced by the piezoelectric crystal is transformed into an alternating variation in electrical potential which can then be measured and recorded at the electrodes. This is what is called recording the ultrasonic echo. Technologies other than piezoelectricity can be used, such as those using cMUT transducers. Micromachined capacitive transducers (cMUTs) are capable of converting mechanical energy, provided by ultrasonic waves, into electrical energy capable of powering very low-power electronic devices. There are different ways to view the ultrasound image. The representation of the images can be the result of an amplitude modulation of the echoes (one-dimensional ultrasound or A mode), or the result of a modulation of the intensity (or brightness) of the ultrasound spot (B mode or two-dimensional image). The TM mode (time-motion mode, which is used in the results in Figures 2 to 6) allows the movement of organs to be followed by adding a time scan to the one-dimensional B mode (a single line of fire). We therefore see the movement of more or less intense spots (representing the echoes) of the mobile organic tissue structures crossed by the ultrasonic firing line scrolling across the screen. The electronic structure of echo processing, then visualization and finally recording of images can be summarized using the stages of acquisition, signal processing and visualization. The probe delivers an alternating signal pulse when it records an ultrasonic echo. This signal, after pre-amplification, is first rectified then demodulated before being finally amplified. The purpose of rectification is to retain only a positive signal, while amplitude demodulation allows only the envelope of the signal to be retained. The main function of the amplification stage is to amplify the signals without distorting them, with the particularity of seeking to compensate for the attenuation effects of ultrasound in tissues. The gain G of an amplifier is defined as the ratio of the input voltage to the output voltage. The range of amplitudes of the received echoes is converted to gray scale. However, for better visualization of interesting echoes, i.e. those coming from the deep organs that we wish to observe, it is advisable to "compress" the gray scale more or less to offer the widest range of levels for these echoes. An analog / digital converter has the function of transforming an analog signal (i.e. a continuous variation in voltage generated by the echo) into a series of discrete digital values ​​which can then be subject to mathematical processing capable of modifying these values ​​according to needs (amplification, non-linear filtering, etc.). The factors that are likely to affect the quality of the ultrasound image are therefore essential. The so-called subjective factors are related to the correct interpretation of the section plane, the recognition of normal and abnormal structures, the recognition of induced movements (due to breathing, coughing, postural changes, etc.) and natural movements (cardiac, peristaltic, fetal movements, etc.), and the discrimination of artifacts. The so-called objective factors are related to the equipment used. a) spatial resolution: lateral and axial; it is related to the ultrasound frequency, the pulse duration and the focusing (fixed mechanical or electronic) as we have seen previously. But we must also take into account the density of the ultrasound firing lines (parallel or divergent depending on the type of mechanical scanning probe or electronic linear probe). The number of firing lines is related to the image rate (number of images per second). The higher the number of lines, the better the lateral resolution. b) the quality of the contrast or contrast resolution depends on the dynamic compression that we discussed above. Thus, the gray scale can and must be adapted according to clinical requirements. c) dynamic resolution is the ability to follow moving organs; it obviously depends on the image rate.d) noise, which corresponds to signals that do not contain any information useful to the image and which will degrade the quality of the latter. Part of the noise is a function of the electronic circuits of the ultrasound scanner itself, another part depends on the diffusion phenomena seen previously. Overall, all of the noise can be considered random and can be reduced by averaging the images. e) artifacts correspond to images that are artificial and not representative of a real anatomical structure. There are three main causes of artifacts: physical causes (multiple reflections for example), factors linked to the instrumentation (poor gain compensation for example), factors linked to the operator (moving the probe too quickly for example). Advantageously, the ultrasound imaging device comprises means for cooling the at least one emitting element and the at least one receiving element. These cooling means comprise, for example, a cooling loop in which a refrigerant circulates. This loop limits the heating of the transmitting and receiving elements which are continuously used by the excitation signals. In its basic version using a transmitting element and a receiving element, and as shown in Figures 2 to 6, the device reconstructs only one line of the image. For the purpose of performing two-dimensional (2D) or three-dimensional (3D) imaging, it is advantageous to combine a first plurality of emitting elements, then forming a first transducer, each configured to emit a continuous excitation signal into a medium, with a second plurality of receiving elements, then forming a second transducer. The receiving elements are of course distinct from said emitting elements, and are configured to each receive an echo signal simultaneously with the emission of the set of excitation signals by the first plurality of emitting elements. The first and second transducers can be combined in a housing commonly called a "probe". Now regarding the process, it includes at least: - a step of emission E, by at least one emitting element, of a continuous excitation signal Sem, the excitation signal being coded so as to form a succession of N ultrasonic waves each having a respective signature Sk, k=[l, 2, 3, . , N], - a step of simultaneous reception R, by at least one receiving element, distinct from said transmitting element, of an echo signal Secho, of said excitation signal, the echo signal being generated by the excitation of the medium by the excitation signal, and - a filtering step F to determine by decoding each of the signatures, the contribution of each wave in the echo signal, to generate an image. The method can be implemented to obtain a two-dimensional (2D) or three-dimensional (3D) image with the following steps: - a first plurality of emitting elements forming a transducer Te simultaneously carry out the emission step E. - a second plurality of receiving elements, distinct from said transmitting elements, and forming a second transducer T r proceed to the reception step (R). The excitation signal Sem of each emitting element is continuous and is modulated according to a succession of N ultrasonic waves overlapping according to the length of the codes used. The transmitted signal is subjected to modulation allowing pulse compression upon reception, in order to increase the resolution along the ultrasonic axis of the measurement as well as the signal-to-noise ratio. Waves exhibit spatiotemporal coding, i.e., a signature that is a function of their amplitude, frequency, and / or phase. In other words, waves exhibit an acoustic signature whose amplitude, phase, and / or frequency depend on time and the location of the emitting element. The transmitted signal must in fact contain a temporally variable and identifiable acoustic signature in order to be able to be found in the received echo signal. Waves can be coded using pseudo-random coding such as Golay or Gold type codes for example. Coding preferentially uses code division multiplexing (CDM), which is a multiplexing technique that uses spread spectrum communication. In spread spectrum communication, a narrowband signal is spread over a wider frequency band or multiple channels by division. It does not restrict digital signals or frequencies from the bandwidth. In order to recover the signal from the medium, as if it came from the interaction of a short pulse with the medium, a decoding operation is necessary. It is recommended to carry out decoding using filtering or another suitable technique. The length of the acoustic signature to be sought and the refresh rate or overlap between the signatures sought are the two adjustable parameters that allow the temporal resolution and the refresh rate of the signals to be set. The filtering step (F) preferentially implements a "mismatched" type filter, making it possible to decode the received echoes with respect to a portion of the transmitted signal. The latter is identified in an optimized manner, with respect to a given criterion, in the echoes to reconstruct an image by subsequently relying on the travel times. The objective is to obtain the best noise gain compared to white noise in the least squares sense. Thus, if the reference signal for the convolution is not exactly a replica, we can say that the filter is "mismatched". The "mismatched" filter, noted q, used is based on the minimization of the ISLR (integrated SideLobe Ratio) criterion on the emitted signal noted s: We introduce the output signal of the filter y: OR : columns is the matrix concatenating all portions of the signal s for all delays. The optimization problem solved to obtain this filter for a given transmission signal is: F and q being defined as follows: - q corresponds to the mismatched filter used to obtain an image associated with a refreshed signal, noted here s. This filter q is optimal because the echo signal decoded by this filter, noted here y, must have an autocorrelation function as close as possible to a Dirac function. - F is a diagonal matrix to extract the side lobes from the autocorrelation function of y. The energy of the side lobes is what is optimized to obtain q. The solution is obtained using Lagrange multipliers. The results show that the filter performs well with respect to ISLR in high-noise environments. Preferably, the ultrasonic waves are mutually non-concordant in the sense that they each exhibit strong autocorrelation and very weak cross-correlations with each other. This allows to obtain a spread function of the imaging system as similar as possible to a function of the Dirac distribution type. The "mismatched" filtering at reception also contributes to improving the real correlation properties. With the device and method according to the invention, a refresh rate at least 10 times higher than conventional approaches requiring waiting for the round-trip propagation time of the acoustic wave is obtained, since it becomes possible to reconstruct the signal in a quasi-continuous manner. A transition from 20 kHz to 200 kHz is, for example, entirely accessible. This is shown in Figures 2 to 6, in which we can observe a better spatiotemporal resolution with the ultrasound scanner according to the invention compared to a conventional pulse ultrasound scanner, whether for a static or dynamic medium, including for a medium moving at high speeds or over a very short distance. In the case where the imaging device is used for medical diagnostic purposes, i.e. as a medical ultrasound scanner, the emitted signal is coded according to N ultrasonic waves which admit frequencies between 1.5 and 50 MHz. In the case where the imaging device is used for therapeutic purposes, i.e. as a medical ultrasound scanner, the N ultrasonic waves admit thermal and / or mechanical indices at least 1.5 times higher than the maximum values ​​allowed for diagnostic applications. The thermal index (TI) is defined as the acoustic output power of the transducer divided by the estimated power required to increase the temperature of the medium being probed by 1°C. The mechanical index (MI) is defined as the maximum rarefaction pressure divided by the square root of the center frequency of the excitation signal bandwidth. Ultrasound can indeed be used to treat certain inflammatory conditions. The vibrations generated by ultrasound increase local blood circulation and help flush out inflammatory fluid. These ultrasounds facilitate the entry of nutrients and the removal of waste products from injured tissues. The complete mechanisms of the biological effects of ultrasound absorption by human or animal tissues are beginning to be understood. Ultrasound absorbed by tissues can give rise to three main effects: 1) thermal effects in which the absorption of ultrasound produces a rise in temperature of the absorbing medium. This increase in temperature is due to the viscosity of the medium which is the origin of friction forces at the molecular level. These forces are dissipative and the energy consumed is degraded into heat. In general the rise in temperature increases regularly until a plateau which reflects a thermal equilibrium corresponding to the dissipation of heat in the intra and extra cellular environments. For the ultrasound intensities used in ultrasound this corresponds to an increase of 1° to 2° C if the duration of the examination is estimated at 10 minutes, which is completely negligible. The most important parameters involved in this process are the following: acoustic intensity, focusing, tissue characteristics (viscosity, specific heat), duration of the examination. 2) Cavitation, whereby, under certain conditions, the ultrasonic beam can develop cavities or bubbles in the medium it passes through. For this to happen, it is necessary for gas or vapor molecules to be present in the medium. Cavitation is a complex phenomenon that includes the formation of cavities but also their implosion. There are two cases: (i) stable cavitation: the cavities are formed under the effect of acoustic pressure, which is generally oscillatory but of relatively low intensity, and micro-flows are observed at the periphery of the cavities. In the range of 1 to 4 MHz this effect can occur beyond the threshold of 1 W / cm 2 . (ii) transient or implosive cavitation: this is a more violent effect that only exists for high intensities (well beyond the values ​​used in diagnostic ultrasound). The implosion of gas cavities under the action of the ultrasound field can have secondary effects such as shock waves leading to the degradation of certain macromolecules, significant temperature increases, modifications of existing chemical reactions or the initiation of new reactions (analogous to ionizing radiation) or even sonoluminescence (light emission). 3) direct effects that can produce the rupture of macromolecules up to the rupture of DNA, and the acceleration of chemical reactions as would an enzyme. Changes in electrical charges on the surface of cells subjected to ultrasonic fields have also been observed in vitro. In summary, the device and method according to the invention can be used in any context in which the imaging rate is a limiting factor (high-speed 3D US imaging, aortic jet flow mapping, potentially compression wave velocity mapping, etc.). The temporal accumulation of the collected signal also allows an increase in sensitivity and makes it possible to image in anatomical areas which are not currently within the scope of ultrasound (behind the bone such as the brain, or even in the lung). This type of sequence can also be used to perform ultrasound therapy and imaging simultaneously, with the therapy signal serving as the medium for image creation. The inventiveness lies in the fact of transmitting continuously. The entire paradigm of ultrasound since its invention lies in the implementation of short transmissions ensuring good time resolution and reception with the same transducer. Transmitting long codes leads to a blind zone on the image (impossibility of transmitting and receiving at the same time) and transmitting continuously without a coding-decoding phase prohibits having the depth information of the received echoes. This approach based on two separate transducers with continuous transmission and reception is a departure from the approach of ultrasounds using pulsed signals, which use the same transducer for transmission and reception. Thus the method and the device according to the invention allow: - to obtain a considerable gain in terms of refresh rate, - to access faster ultrasound imaging rates which make ultra-fast 3D imaging possible, - to visualize and understand moving structures that are too fast for current systems, - to visualize and understand very short physical phenomena, - to transmit more signal and thus increase the signal-to-noise ratio and therefore the sensitivity of the imaging system to image where it is currently not possible because the attenuation is too strong. In addition, the ultrasound imaging device and method according to the present invention offer diverse application potential, both in the medical field and in other sectors. From a medical perspective, this technology could improve the quality of images subsequently used for diagnostic purposes. In the field of biology and biomedical research, this technology could enable real-time cellular imaging, i.e., allow the dynamic observation of cells and their movements. This technology could also enable the bioprinting of organs in three dimensions, i.e. real-time monitoring of the bioprinting process for optimal precision. However, the applications are not limited to medicine. In the underwater field, for example, the device could be used for seabed exploration, facilitating detailed mapping of underwater structures. Finally, in terms of non-destructive testing, the invention could be used in industry to inspect the quality of certain materials without altering their integrity. Nomenclature The transmitter transducer Tr transducer receiver Sem Continuous emission signal Secho Echo signal Sk coded emitted wave

Claims

Claims 1. Ultrasound imaging device comprising: - at least one emitting element configured to emit a continuous excitation signal (Sem) into a medium, the excitation signal being coded so as to form a succession of N overlapping ultrasonic waves, each having a respective signature (Sk), k=[l, 2, 3, . , N], - at least one receiving element, distinct from said transmitting element, and configured to receive an echo signal (Secho) simultaneously with the emission of the excitation signal emitted by said transmitting element, the echo signal being generated by the excitation of the medium by the excitation signal, and - filtering means configured to determine by decoding each of the signatures, the contribution of each wave in the echo signal, so as to generate an image.

2. Ultrasound imaging device according to the preceding claim, characterized in that the filtering means are configured to implement a so-called “mismatched” filter.

3. Ultrasound imaging device according to claim 1 or 2, characterized in that it comprises means for cooling the at least one emitting element and the at least one receiving element.

4. Ultrasound imaging device according to any one of the preceding claims, characterized in that it comprises: - a first plurality of emitting elements each configured to emit a continuous excitation signal into a medium, each excitation signal being coded so as to form a succession of overlapping ultrasonic waves each having a respective signature, - a second plurality of receiving elements, distinct from said transmitting elements, each configured to receive an echo signal simultaneously with the transmission of the excitation signal transmitted by the associated transmitting element, each echo signal being generated by the excitation of the medium by said associated excitation signal.

5. Ultrasound imaging device according to any one of the preceding claims, characterized in that the device is configured so that the N ultrasonic waves admit frequencies between 1.5 and 50 MHz, for its use in a medical diagnostic method.

6. Ultrasound imaging device according to any one of the preceding claims, characterized in that the device is configured so that the N ultrasonic waves admit thermal and / or mechanical indices at least of the order of 1.5 times the maximum values ​​admitted for diagnostic applications, for its use in a therapeutic method.

7. Method for imaging a medium using ultrasound comprising: - a step of emission (E), by at least one emitting element, of a continuous excitation signal (Sem), the excitation signal being coded so as to form a succession of N overlapping ultrasonic waves, each having a respective signature (Sk), k=[l, 2, 3, . , N], - a step of simultaneous reception (R), by at least one receiving element, distinct from said transmitting element, of an echo signal (Secho) of said excitation signal, the echo signal being generated by the excitation of the medium by the excitation signal, and - a filtering step (F) to determine by decoding each of the signatures, the contribution of each wave in the echo signal, to generate an image.

8. A method of imaging a medium using ultrasound according to claim 7, in which the ultrasonic waves are non-concordant with each other, in the sense that they each have a strong autocorrelation and very weak intercorrelations with each other.

9. Method for imaging a medium by ultrasound according to any one of claims 7 or 8 in which the N signatures are spatiotemporal signatures, that is to say acoustic signatures whose amplitude, phase and / or frequency depend on the time and the location of the emitting element.

10. Method for imaging a medium using ultrasound according to any one of claims 7 to 9, in which the refresh rate of the signal for generating an image is greater than that based on the round trip duration of the ultrasonic waves.

11. Method for imaging a medium by ultrasound according to any one of claims 7 to 10 in which the filtering step (F) implements a so-called “mismatched” filter.

12. Method for imaging a medium by ultrasound according to any one of claims 7 to 11 in which: - a first plurality of emitting elements forming a first transducer (Te) carry out the emission step (E), - a second plurality of receiving elements, distinct from said transmitting elements, and forming a second transducer (Tr) carry out, simultaneously with the transmission step (E), the reception step (R).

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