EP3024378A1

EP3024378A1 - Acoustic-electric imaging method and device

Info

Publication number: EP3024378A1
Application number: EP14755868.8A
Authority: EP
Inventors: Mickael Tanter; Mathieu Pernot; Mathias Fink; Jean Provost
Original assignee: Centre National de la Recherche Scientifique CNRS
Current assignee: Centre National de la Recherche Scientifique CNRS
Priority date: 2013-07-22
Filing date: 2014-07-21
Publication date: 2016-06-01
Also published as: JP2016527020A; FR3008806A1; WO2015011393A1; FR3008806B1; IL243744A0; US20160157728A1; JP6415555B2

Abstract

The invention relates to an acoustic-electric imaging method, which includes: (a) a measurement step during which incident ultrasonic waves having different wavefronts are emitted in a medium 1 to be imaged, and at least one electric sensor is used to capture raw electric signals Eraw₁(t) respectively during the propagation of the incident waves; (b) a step of forming an image, during which an image of the medium including an electric current map is determined from the raw electric signals Eraw₁(t).

Description

Acoustoelectric imaging method and device.

FIELD OF THE INVENTION

The present invention relates to acoustoelectric imaging methods and devices.

Organs such as the heart, skeletal muscles and the brain are constantly scanned by electrical impulses that carry information in neurons, or trigger muscle or myocardial contractions. To be able to image the propagation of these impulses is extremely important to diagnose numerous pathologies and to understand the brain mechanisms by the functional exploration of the brain.

Acousto-electrical imaging exploits the interaction between ultrasound and electrical currents to determine the value of electrical current at the points of interaction between ultrasound and tissue, typically at the focal spot of a focused ultrasound wave.

BACKGROUND OF THE INVENTION

The document US8057390 describes an example of an acousto-electrical imaging method, in which focused ultrasound waves are emitted to form, line by line, an image of the current. This acquisition process is slow, and all the more so since the electrical signals obtained are very weak, a high level of averaging is necessary. We thus obtain low image rates.

In addition, Kuchment et al. "Synthetic focusing in ultrasound modulated tomography," Inverse problem and imaging, 2009-10-01, pages 1-9, XP055116447, proposed a synthetic acousto-electrical imaging method, which provides that transducers emit one by one waves spherical. This results in a slow process. In addition, the incident ultrasonic waves have an amplitude too low .

SUMMARY OF THE INVENTION

The present invention is intended to overcome this disadvantage.

For this purpose, the invention proposes an acousto-electrical imaging method, comprising:

(a) a measuring step in which a set of transducers T ± _r is emitted in an observation field of a medium to be imaged, a number N at least equal to 2 of incident ultrasonic waves 1 no focused in the field of view and having different wave fronts, each incident ultrasonic wave being emitted by a plurality of transducers ΤΊ of the set of transducers, N being at least 2 and less than 100, and captured by at least one electrical sensor in contact with the medium to be imaged, raw electrical signals Erawi (t) respectively during the propagation of the incident waves 1,

(b) an image forming step, in which the Erawi raw electrical signals (t) obtained in step (a) are determined by an image of the medium comprising a mapping of electric currents (this is that is, a map of electrical values representative of the local current densities at each point in the middle).

Thanks to these provisions, it is possible to perform ultrafast imaging of electrical pulses in the medium observed, and possibly film the propagation of electrical pulses deep in the tissues, in real time and with a millimeter resolution.

In various embodiments of the method according to the invention, one or more of the following provisions may also be used:

during step b) at least from the N raw electrical signals Era i (t) is determined for a number M of fictitious focal points P _k in the field of observation, electrical values Ecoherent * each corresponding to the electrical signal which would have been picked up if an ultrasound wave focused at the point P _k had been emitted by said transducers;

during step (b), the raw electrical signals Erawl (t) are applied to a WT ^-1 inverse wavelet transform and then to an inverse Radon transform R ^-1 (the raw electrical signals Erawl (t) can of course undergo a preliminary treatment before the inverse Radon transform R ^-1 );

during step (b), an echographic image of the medium made with the set of transducers is superimposed on the mapping of electric currents;

during step (a), acoustic transducers RFrawi, i (t) representative of ultrasound waves reverberated by the medium respectively from the incident waves 1 are detected by the transducers (during the step ( b), from the N sets of RFrawi signals, i (t) picked up, M coherent RFcoherent acoustic signals ^, are determined _. (t) corresponding to the acoustic signals that would have been received by the transducers ΤΊ if an ultrasonic wave focused at the point P _k had been emitted by said transducers, and the ultrasound image of the medium is calculated from the coherent acoustic signals;

during step (b), the ultrasound image is determined by channeling from the coherent acoustic signals;

the medium to be imaged is a human or animal tissue. Furthermore, the invention also relates to a device for implementing an acousto-electrical imaging method, comprising a set of transducers Ti, minus an electrical sensor, and control means and treatment adapted for:

(a) causing a set of transducers Ti to transmit, in a medium to be imaged, a number N of unfocused incident ultrasonic waves 1 having different wave fronts, each incident ultrasonic wave being emitted by a plurality of transducers ΤΊ of the set of transducers, N being at least equal to 2 and less than 100, and by at least one electrical sensor in contact with the medium to be imaged, extracting raw electrical signals Erawi (t) respectively during the propagation of the incident waves 1,

(b) determining from the Erawi raw electrical signals (t) an image of the medium comprising a mapping of electric currents.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent from the following description of one of its embodiments, given by way of non-limiting example, with reference to the accompanying drawings.

On the drawings:

FIG. 1 is a schematic view of a device for implementing a method according to one embodiment of the invention, and

FIG. 2 is a block diagram of part of the device of FIG. 1.

DETAILED DESCRIPTION

In the different figures, the same references designate identical or similar elements.

FIG. 1 shows an example of an acoustoelectric imaging device adapted to image a medium 1 by emission and reception of ultrasonic compression waves (for example of frequencies between 0.2 and 40 MHz), with simultaneous measurement of electrical values.

The medium 1 to be imaged may consist in particular of tissues of a patient or of an animal, in particular a muscle (myocardium or other) or a brain.

The imaging device comprises for example:

a network 2 of n ultrasonic transducers, comprising for example a few hundred transducers and adapted to produce a two-dimensional image (2D) of an observation field (zone of interest, scanned by the ultrasonic waves) in the medium 1 to be imaged ;

an electronic rack 3 or the like controlling the array 2 of transducers and adapted to acquire the signals picked up by this array of transducers;

a computer 4 or the like to control the electronic bay 3 and to display the ultrasound images obtained from said captured signals.

The network 2 of transducers may for example be a linear array formed by a transducer array juxtaposed along an axis X, the Z axis perpendicular to the axis X designating the direction of the depth in the field of view. In the following, the transducers will be denoted Ti, where i is an index designating the rank of each transducer along the X axis. The following description will be made using this type of transducer array 2 as an example, but other forms Transducer networks are also possible in the context of the present invention, in particular two-dimensional arrays.

The device further comprises at least one electric sensor E1 (FIG. 2), constituted for example by two electrodes measuring an electric potential difference. This electrical sensor can advantageously be fixed to the transducer network 2 and adapted to come into contact with the medium 1 to be imaged at the same time as the transducers of the network 2.

The number of electric sensors El used is relatively small, generally less than 10, advantageously less than 5 and most often 1.

As shown in Figure 2, the bay electronic device 3 can include for example:

n + 1 analog / digital converters 5 (A / D ± - A / D _e ) connected individually to the n transducers ΤΊ of the transducer network 2 and to the electrical sensor El, n + 1 buffer memories 6 (Bi-B _e ) respectively connected to the n analog / digital converters 5,

a central unit 8 (CPU) communicating with the buffer memories 6 and the computer 4,

a memory 9 (MEM) connected to the central unit

8

a digital signal processor 10 (DSP) connected to the central unit 8.

It will be noted that the n + 1 analog / digital converters 5 (A / D +/- A / D _e ) can be identical, as can the n + 1 buffer memories 6 (Bi-B _e ), so that the device used can be simply a device conventionally used in ultrafast acoustic imaging.

This device makes it possible to implement an acousto-electrical imaging method of the medium 1, which notably includes the following steps, implemented by the central processing unit 8 of the processor 8 and the digital signal processor 10:

a) Measurement (transmission / reception and recording of raw data),

b) determining an image of the medium comprising a map of electrical values.

Step (a): Measurement (transmission / reception and recording of raw data):

The network 2 of transducers and the electric sensor E1 are brought into contact with the medium 1 and a number N of incident ultrasonic waves is emitted in the medium 1 by the transducers ΤΊ (N may be for example between 2 and 100, in particular between 5 and 10). The incident waves in question are unfocused (more precisely, not focused in the field of view) and have respectively different wavefronts, that is to say wavelengths of different shapes and / or different orientation. Advantageously, the incident waves may be plane or divergent waves whose respective F wavefronts (the wavefront F of a single wave is shown in FIG. 1) have different inclinations, characterized by their angles of rotation. respective inclinations Θ measured between their propagation direction V and the Z axis, or diverging waves emitted as if they came from different points of space. The example of plane waves will be considered in what follows.

Incident waves are generally pulses of less than one microsecond, typically about 1 to 10 cycles of the ultrasonic wave at the center frequency. The incident wave shots may be separated from each other for example from about 50 to 200 microseconds.

Each incident wave encounters in the middle 1 diffusers that reverberate the incident wave. The reverberated ultrasound wave is picked up by the transducers ΤΊ of the network. The signal thus captured by each transducer ΤΊ comes from the whole of the medium 1, since the incident wave is not focused in transmission. Similarly, the electric sensor El captures an electrical signal E (t) during the propagation of the incident ultrasonic wave, and this electrical signal results from the interaction between the incident wave and the medium 1 to be imaged over the entire line represented by the wavefront, at each moment of measurement.

Reverberant signals picked up by the n transducers Ti are then digitized by the corresponding analog-digital converters A / D ± and stored in the corresponding buffers Bi, while the electrical signal is digitized by the analog-digital converter A / D _e and stored in the corresponding buffer memory B _e . The signals thus stored in the buffers after each firing incident will be called hereinafter raw data. These raw data consist of n + 1 raw time signals RFrawi, i (t) and Erawi (t) picked respectively by the transducers ΤΊ and the electric sensor El after the incident ultrasound wave firing 1.

After each incident wave firing 1, the signals stored in the buffers Bi-B _e are transferred to the memory 9 of the signal processor 10 for processing by this processor. At the end of step (a), the memory 9 thus contains N matrices (vectors) of n + 1 raw signals.

Step (a) is repeated at a fast rate, for example 500 Hz or more, which is made possible by the small number N of incident waves used to make an image.

Step (b): determining an image of the medium comprising a mapping of electrical values:

Two methods will be explained below to achieve this step (b).

bl) First method: Synthesis of coherent data:

From the N raw data matrices, a number M of synthetic coherent data matrices (vectors) is calculated by the processor 8, respectively at M points P _k (x, z) of the observation field (where k is an integer between 1 and M and x, where z is the coordinates of the point Ρ _¾ on the X, Z axes. Each of these M coherent synthetic data vectors comprises n RFcoherentk time signals, i (t) corresponding to the signals that would be picked up respectively by the transducers ΤΊ if the transducers emitted an incident wave focused at the point P _k .

Consistent data matrices can be obtained for example by assuming a homogeneous propagation velocity c throughout the medium 1 for ultrasonic compression waves, according to the principle explained in particular in document EP2101191 or in the article by Montaldo et al. "Coherent plane-wave compounding for high-resolution ultrasound and transient elastography" (IEEE Trans Ultrasound Ferroelectr Freq Control 2009 Mar; 56 (3): 489-506).

Since the propagation direction of the plane wave corresponding to each firing 1 is known, and the speed of propagation c is known, the processor 8 can calculate for each point P _k the propagation time r _ec (l _/ k) of the incident wave 1 to the point P _k , and the propagation time r _rec (l, k, i) of the reverberated wave from the point P _k to the transducer ΤΊ, thus the total travel time τ ( l, k, i) = r _ec (l, P _k ) + T _REC (l, P _k , i) ·

The spatially coherent acoustic signal for the transducer Ti, corresponding to the virtual focusing point P _k , is then calculated according to the formula:

RFcoheren ^ _j = ΣB (l) RFraw _Uj (r (l, k, i, j)) (1) where B (1) is a weighting function for the contribution of each incident wave shot 1 (in common cases, the values B (l) can all be equal to 1). This RFcoherent signal _k i _j has a single value for each point Pk.

In the same way, we can calculate a coherent electrical signal Ecoherentk:

Ecoherent _k (t) = ^ B {l) Eraw _l (r (l, k, i, j)) (Ibis)

This electrical value is that which would be measured by the electric sensor El if an incident ultrasonic wave focused at P _k had been emitted, particularly if a sufficient number of incident waves are emitted to obtain an acousto-electric image, for example 40 to 100 waves. incidental to get a great image resolution.

These Ecoherent values _k are representative of the electric currents at the points P _k , in the same way as the electrical values captured in the aforementioned acousto-electrical imaging methods, and thus provide a mapping of the electric currents in the field of view.

The coherent data matrices RFcoherent _k and optionally the Ecoherentk values can then be optionally refined by correcting the effects of aberrations in the medium 1, for example as explained for example in the documents EP2101191 or Montaldo et al. Coherent plane-wave compounding for Very High Frame Rate Ultrasound and Transient Elastography "(IEEE Trans Ultrasound Ferroelectr Freq Control 2009 Mar; 56 (3): 489-506).

The mapping of the electric currents can be presented on the screen of the computer 4, possibly in superposition with an echographic image B mode of the medium 1 or another image of said medium 1, in particular an echographic image obtained from the Ecoherent matrices _k by channel formation in reception, as explained for example in the aforementioned EP2101191 document. b2) Second method: Radon transform and wavelets:

From the electrical signals Eraw _k (t), it is also possible to go directly back to the local values of electric currents at the points Pk, as will be explained below.

The raw electrical signal Eraw _k (t) can be modeled as follows:

Eraw _k = Kfif (x, y, z) AP (x, y, z) dxdydz (2)

JVOLUME

Or : K is an interaction constant of the order of 1CT ⁹ Pa ^-1 , p is the resistivity of the medium,

ΔΡ is the pressure variation

y is a coordinate along a Y axis perpendicular to the (X, Z) plane and

J is the detected current density distribution, i.e., the dot product of the current density vector by the electrode sensitivity vector of the electric sensor E1.

The emitted ultrasonic wave being an impulse plane wave, Δ Ρ (χ, γ, ζ) can be parameterized as a function of the emission angle Θ and the time t. Ignoring the Y direction, we have:

ΔΡ (χ, ζ) = AP (-qsin9 + ctcosG, qcosG + ctsinG),

where q and and are coordinates respectively in the direction of the wavefront F and in the direction of propagation V.

Considering the ultrasonic wave emitted as a pulse of Dirac, that is to say an infinitely short pulse, the acousto-electric signal becomes:

Eraw,

J (-q, sin 3 + and cos Θ, q cos Θ + and sin &) dq

Kp

Or equivalent

Eraw _k

RJ (6, ci) = \ J (x, z) .S (x, sin Θ + z, cos Θ - ct) dxdz (4)

Kp J J

where R [J] is the Radon transform.

In practice, the incident wave is not a Dirac pulse but a finite frequency band pulse signal, which will result in a convolution with respect to the variable and the Radon transform:

Eraw _k

(e, t) = W (ct) ® RJ (e, ct) (5)

Kp

where W (ct) is the emitted waveform and ® is the convolution product.

For example, a typical ultrasonic emission product noya (kernel) convolution next

where n and m can be adjusted within the frequency band of the transducer. This convolution nucleus is equivalent to a transform into ridgelettes ("ridgelet transform") [E. J. Candes, "Ridgelets: Theory and Applications," Stanford University, 1998] of current density distribution.

In practice, m = n and this convolution kernel becomes a ridgelette decomposition with the following parameters: a = ηλ, b = and and Θ.

The ridgelette decomposition has several mathematical properties such as a Parseval-Plancherel relation, a reconstruction formula, a parsimonious representation of slowly varying objects far from linear discontinuities, and can be expressed as a composition of a wavelet transform and the Radon transform.

More specifically, noting the wavelet and ridgelette transforms by W [. ], and R [. ], respectively, it can be shown that

^ ± = WT [R (J] (7)

Kp

Inversions of the wavelet transform and the Radon transform are well known problems. Indeed, there are exact inversions, respectively WT ^-1 and R ^-1 for these two transforms and we therefore have:

J = R ^l (8).

Kp

In practice, the inversion occurs in two steps: first, inverting the WT wavelet transform, then inverting the Radon R transform.

This gives a mapping of the current density in the whole field of view (zone swept by the incident waves) inside the medium to be imaged, and this after a very fast acquisition, which makes it possible to follow in real time very fast electrical phenomena, by obtaining a real film of the propagation of the electric pulses.

In addition, it is desirable to maximize the signal to noise ratio (SNR), resolution, and frame rate.

One approach is to emit incident waves as short pulses as possible, which optimizes the resolution. However, this corresponds to a low emitted energy and therefore a low SNR.

Otherwise, it is also possible to divide the frequency band into sub-bands corresponding to longer (and therefore more energetic) emissions. The result is, in theory, an increase in the SNR but a decrease in the image rate (since it takes several emissions to form an image).

Finally, a third approach is to issue a "chirp" that can be used to do pulse compression. This approach maximizes SNR while maintaining image throughput.

SNR can also be improved by limiting the effect of noise. Since the ridgelette transform is a parsimonious basis that will represent the current density distribution with a small number of large coefficients and a large number of small coefficients, denoising can be achieved simply by applying thresholding on the obtained signals. A first approach consists of a thresholding eliminating 'small' coefficients. Otherwise, it is also possible to use the physics of the problem. For example, coefficients containing mainly noise can be identified by performing cross-correlation on received signal windows for two emissions of opposite polarities. In addition these signals can be subtracted to eliminate systemic artifacts.

Several techniques exist for the inversion of the Radon transform. The most common is probably filtered back projection, which involves the application of a ramp-type filter before overhead projection (corresponding to channel formation). To avoid this step which increases the noise level, it is also possible to emit the incident ultrasonic waves in the form of a suitable pulse which will include this filter. Other strategies such as compressed sensing are also well suited.

Furthermore, the RFcoherent _k matrices can be calculated as explained in the above method b1) to further form a two-dimensional ultrasound image (B mode) of the field of view, by channel formation on reception, as explained for example in the document EP2101191 mentioned above.

This ultrasound image B mode (or another echographic image or not) of the field of view may possibly be superimposed on the mapping of the electrical values determined previously, and it is possible to display on the computer screen 4 at a time the Ultrasound image of the medium and mapping of electrical currents.

Claims

An acoustoelectric imaging method, comprising:

(a) a measurement step in which a set (2) of transducers T ± _r is emitted in an observation field of a medium (1) to be imaged, a number N at least equal to 2 d incidental ultrasound waves 1 not focused in the field of view and having different wave fronts, each incident ultrasonic wave being emitted by a plurality of transducers ΤΊ of the set (2) of transducers, N being at least 2 and less than 100, and the Erawi raw electrical signals (t) are respectively picked up by at least one electrical sensor (El) in contact with the medium to be imaged, during the propagation of the incident waves 1,

b) an image forming step, in which determination is made from the raw electrical signals Erawi (t) obtained at. step (a), an image of the medium comprising a mapping of electric currents.

2. Method according to claim 1, wherein during step b), at least from the N raw electrical signals Erawi (t), is determined for a number M of imaginary focusing points P _k in the field of observation, Ecoherent electric values _k each corresponding to the electrical signal that would have been captured if a ultrasound wave focused at the pole. P _k was emitted by said transducers,

3. The method of claim 1, wherein during step (b), is applied to raw electrical signals Erawl (t) a WT ^-1 inverse wavelet transform, then an inverse Radon transform R ^-1 .

The method of claim 1 or claim 2 or claim 3, wherein during in step (b), an echographic image of the medium made with the set (2) of transducers is superimposed on the mapping of electric currents.

5. The method according to claim 4, wherein during step (a), the transducers Ti receive acoustic signals RFrawi _f i (t) representative of ultrasonic waves reverberated by the medium respectively from the waves. incident 1,

during step (b), from N sets of collected RFrawi _f i (t) signals, M coherent acoustic signals RFcoherentk, i (t) corresponding to the acoustic signals which would have been received by the transducers ΤΊ are determined from an ultrasonic wave focused at the point P _k was emitted by said transducers, and the echographic image of the medium is calculated from the coherent acoustic signals.

The method of claim 5, wherein in step (b), the ultrasound image is determined by channeling from the coherent acoustic signals.

7. Method according to any one of the preceding claims, wherein the medium to be imaged is a human or animal tissue.

8. Device for implementing an acoustoelectric imaging method according to any one of the preceding claims, comprising a set of transducers ΤΊ, minus an electric sensor (El), and control and processing means ( 8, 10, 4) adapted for:

(a) causing a set of transducers T ± _r to transmit, in a medium (1) to be imaged, a number N of unfocused incident ultrasound waves 1 having different wave fronts, each incident ultrasonic wave being emitted by several transducers ΤΊ of the set (2) of transducers, N being at least 2 and less than 100, and capture by at least one sensor electrical contact with the medium to be imaged, raw electrical signals Erawi (t) respectively during the propagation of the incident waves 1,

(b) determining from the Erawi raw electrical signals (t) an image of the medium comprising. a mapping of electric currents.