HK1140441B - Method for optimising wave focalisation through an aberration insertion member - Google Patents

Description

Method for optimizing the focusing of a wave passing through an element introducing aberrations

Technical Field

The present invention relates generally to the field of wave focusing within a medium. In particular, the present invention relates to imaging procedures requiring focusing of waves through highly inhomogeneous media.

Background

Applications for such imaging are numerous and varied. In particular, they include underwater acoustics, telecommunications, geophysics, nondestructive testing of materials, medicine, and the like. For example, in the medical field, particularly in the case of focused ultrasound, focused waves are used for imaging and therapy.

In each of these applications there is a problem of aberrations introduced by the heterogeneous medium.

In fact, aberrations are very disadvantageous as long as the wave propagates with high intensity, or as long as it is important that the wave only passes through a certain zone, even if the use of a focused wave is rejected in certain applications, since it is not possible to achieve an accurate focusing. For example, in the field of therapy, certain applications actually require very precise focusing in order not to widen the region of action of the focused wave.

Currently, the precision required in this application can be achieved, but at the expense of high execution complexity and, in the case of therapy, of wound therapy.

In fact, in the context of brain treatment, in order to be able to focus high intensity ultrasound waves accurately, a first solution described in document FR 2843874 is to make use of the results of imaging of the patient's brain, obtained by performing a tomography scan (computed tomography or CT scan) before the treatment. The three-dimensional information of the skull structure is then used to simulate aberrations and to correct the emitted signals during treatment. These digital simulations generally require a long time, which is contradictory to real-time work.

The patient must then be re-repositioned (repositioning) in the magnetic resonance imaging application, allowing monitoring of the treatment in real time, typically by elevated temperature imaging.

The patient must then be re-relegated to the same reference basis as was used during the CT scan in order to enable the structures to be identically located. This involves a re-homing process which is often relatively complex. In particular, a stereotactic structure screwed to the head of the patient is used.

A second solution is based on temporal return of waves. It involves implanting a miniature ultrasound probe that emits ultrasound waves into a surgical tool used during a biopsy. During the removal of tissue, the probe emits ultrasound waves near the tumor, which are captured and recorded by the network of ultrasound transducers as the probe is withdrawn from the skull.

If the biopsy indicates a need for treatment, the recorded signal is temporarily returned. During their re-transmission, the returned signals are automatically focused in the region in which the biopsy is performed, i.e. on the tumor region. However, this has to be re-homing the patient accurately with respect to the network of transducers, and this again is a very difficult operation.

Then, in practice, to perform the treatment, the beam is electronically moved point by point near the initial focal point to treat the entire tumor.

The last solution is traumatic and therefore has all the drawbacks inherent to traumatic solutions.

Therefore, at present it is not advantageous in the medical context to use focused waves, in particular focused high-intensity ultrasound, as long as the elements that introduce aberrations are located in the path of the waves. However, ultrasound has some major advantages, one of which is that it works at any time in case of recurrence or recurrence, and the other is that it works for children, for whom treatment options are more limited than for adults.

It should also be noted that the use of high intensity ultrasound is an operation that is free from radiation and is only affected by local temperature increases.

Therefore, attention is paid to all methods for expanding the field of use of ultrasonic waves (more generally, expanding the field of use of radiationless waves). In particular, those methods that eliminate the influence of aberrations are attracting attention among the above-described methods.

Disclosure of Invention

The main object of the present invention is therefore to enable an accurate focusing of the wave even in the presence of an aberrating element, while avoiding the drawbacks presented by the two existing solutions, namely the complexity of a repositioning with risk of error between the two imaging procedures, and, in the case of biopsy guidance, the invasive nature of the procedure.

To this end, the invention proposes a method for optimizing the focusing of a wave in a region of interest of a medium, in which the wave is subjected to an initial, undefined phase shift induced by a network having N sources(1. ltoreq. n.ltoreq.N) aberration-inducing elements are transmitted to the medium, the method using M-1 successive corrections of the transmitted wave, each correction causing a perturbation, M perturbations being measured, these measurements being used to derive optimum focus characteristics, the method comprising the steps of:

a) transmitting waves via a network having N sources(wherein j represents a complex number, wherein j represents²1), the waveExhibits a spatial phase distribution alpha_nAnd amplitude distribution A_n(1. ltoreq. N. ltoreq.N) and propagates to a region of interest in the medium;

b) spatial phase distribution alpha of waves transmitted simultaneously by multiple sources in a network_nAnd/or amplitude distribution A_nM-1(M > 1) corrections (1. ltoreq. N. ltoreq.N), each correction corresponding to a simultaneous correction of the phase and/or amplitude of the plurality of sources in the network, and each correction resulting in the transmission of step a);

c) at the phase alpha_nAnd/or amplitude distribution A_nEach time M (1. ltoreq. m.ltoreq.M) is corrected, the measurement is made in the region of interest [6 ]]At least one perturbation I caused by said wave_m(1≤m≤M)；

d) From perturbation measurements I_mTo derive an optimal transmit phase distribution that maximizes the perturbation induced in the region of interestAnd/or amplitude distribution

The method proposes to adjust the focus using a measurement of at least one perturbation induced in the theoretical focal zone by the focused wave, so as to optimize the phase shift in all, or at least in a large part, of the transducers generating the focused wave. The perturbation is generated remotely in the area of interest through a network of transmitters.

The term "perturbation" refers to a reversible or irreversible modification of a particular physical parameter of a medium resulting from an interaction between a wave and the medium related to energy density, or to the average energy transmitted by the wave in a region of interest or to the reversible or irreversible modification of the energy density within the region of interest.

An important consequence of this restrictive definition of the perturbation is that the amplitude of the perturbation is linked only to the amplitude of the wave, or only to the energy of the wave in the region of interest, and not to its phase at any moment. For example, perturbation energy is a local temperature increase caused by the absorption of waves by the medium, the radiation force exerted by the waves or even the presence of bubbles, energy back-diffused by the region of interest, the density of acoustics in the region of interest, the movement or local deformation of the medium caused by ultrasonic, electromagnetic or optical energy.

Since the perturbation is caused by several sources in the network simultaneously within the medium, rather than by a single source sequentially within the medium, it is possible to perform a measurement of the perturbation. This ensures that the power emitted into the medium is sufficient to cause perturbations that can be discriminated by the measurement system, and that corrections effected simultaneously on multiple sources produce perturbations that can be distinguished from one another. After successive corrections of the phase and/or amplitude distribution, the optimal focus is derived from the successive shots each time achieved, using several sources in the network.

The term "region of interest" refers to a region of tissue where waves need to be focused to spatially obtain waves of maximum energy. Typically, by wave focusing, energy peaks can be obtained at the region of interest, wherein the region of interest is defined to be in the range of several hundred μm depth and lateral dimensions in the ultrasound field. More generally, for a given process using waves, the region of interest has dimensions close to the wavelengths used.

The invention is particularly useful in the context of brain therapy, where it is necessary to locate a zone of tissue necrosis (typically a tumour) very precisely, but not beyond, so as not to cause damage to healthy tissue. The invention thus relates in particular to a treatment by a high intensity focused ultrasound beam.

In this context, by means of the invention, using a single transducer network for focus correction, a focus correction can be achieved before treatment, for treatment, or rather for imaging.

Furthermore, the present invention provides operational qualities and convenience heretofore unavailable because it does not interfere with trephinery nor surgical instruments.

In particular, the invention proposes a simple non-invasive treatment of brain injuries. Since these treatments are advantageous in terms of cost and risk, the present invention meets the current socioeconomic requirements in the field of near surgical interventions.

According to a further advantageous characteristic, the method according to the invention further comprises: step c') at phase α_nAnd/or amplitude A_nEach time M (1. ltoreq. M. ltoreq.M) of the distribution is corrected, a measurement is made in the region of interest [6 ]]At least one perturbation I of a point other than_me(ii) a And step d) subsequently, a derivation step is carried out, based on the perturbation measurement I_mAnd perturbation measurement I_meDerivation of regions of interest [6]Perturbation I caused by_mMaximizing and minimizing perturbation or perturbations I outside the region of interest_meMinimized optimal transmit phase distributionAnd/or amplitude distribution

This additional advantageous property allows an even more concentrated energy transfer to the region of interest in the medium and not out of the region of interest. Thus, this characteristic makes it possible to control the range of perturbations, which is ideally confined to the region of interest.

More generally, at each correction, several measurements of the perturbation are carried out at several points in the medium.

According to a first embodiment of the invention, the spatial phase distribution α is modified by successive iterations of the phases applied to at least a plurality of sources_n(N is 1. ltoreq. n.ltoreq.N), wherein the iteration which produces the largest perturbation is selected as the optimum phase distribution alpha_n。

Based on this characteristic, an empirical iteration is performed during which an optimal phase distribution to be applied to a plurality of test sources is selected from the phase distributions of the tests. The measurement of the intensity of the perturbations induced in the focal zone at each iteration and the selection of the phase distribution of the sources that causes the largest perturbation allows us to select the optimal phase or amplitude distribution of the sources.

According to an advantageous characteristic, due to the phase distribution a_nBy usingDecomposed into K vectors V_k(1. ltoreq. K. ltoreq.K) where d_kIs a real number, each specific vector V_kDescribing the phase emitted by N sources, by applying to each vector V_kCarry out d_kValue S_kCorrection, the phase correction a being effected for all N sources_n(1. ltoreq. N. ltoreq.N), whereby M is equal to

This characteristic is used to select and/or scale all sources simultaneously to form a plurality of sources. This method has two advantages. First, according to the method of the present invention, the vector is defined using spatial variations from the aberrations, thereby enabling phase correction that directly takes into account the characteristics of these variations. Second, by using all transmitter transmissions, we avoid implementing phase corrections of amplitude that do not significantly affect the measured perturbation intensity. Therefore, the method enables us to significantly reduce the number of correction iterations of the method. For each vector V_kCarry out d_kThe next iteration is used to determine d_kD of_kGenerating a vector V_kThe optimal phase distribution. With subsequent vector V_k+1Related iteration d_k+1The revision of (c) is used to converge towards the optimal focus on all sources in the network.

Advantageously, in the radical V_kK '(K' < K) vector subspace of (a) to achieve a phase distribution α_nIn whichThen M is equal to

This property is used to select a certain amount of spatial frequency from a set of vectors describing a reduced space of spatial frequencies. This serves to reduce the time required to optimize the phase distribution.

In an advantageous implementation, the vector V is determined from K periodic functions of the space_kEach coefficient V_knIs determined by the value of the function K (1. ltoreq. K. ltoreq. K) related to the position of the source n.

A periodic function (which may be a sinusoidal function) is used to simply describe a particular space of spatial frequencies.

According to another advantageous implementation characteristic, the spatial amplitude distribution a is modified by successive modifications of at least a plurality of sources_n(N is more than or equal to 1 and less than or equal to N), and the iteration generating the maximum perturbation is selected as the optimal amplitude distribution

This additional property is used to add an optimization of the amplitude as a function of the spatial frequency in addition to the optimization phase.

According to an additional characteristic, the correction of the amplitude distribution can be effected continuously, alternately or simultaneously with the correction of the phase distribution.

According to a second embodiment, amplitude A_nCorrection and phase alpha of_nIs defined as a matrix form H_pnP vectors H of (1. ltoreq. p.ltoreq.P and 1. ltoreq. n.ltoreq.N)_pIn combination of (1), whereinWherein each value c_pIs a complex number.

According to this characteristic, each correction of the phase distribution and/or amplitude distribution corresponds to the complex number c_pA change in the value of (c). Due to the pair passing vector H_pSeveral sources are designated to make the correction in common so that the characteristic is used to generate a detectable perturbation.

From this characteristic, the optimal phase distribution is determined by means of a perturbation measurement which is used as determinant in a theoretical equation relating the phase and energy of the perturbation.

The matrix may be an orthogonal matrix, such as a hadamard matrix.

According to an advantageous characteristic, in at least two vectors H corresponding to the basis_p1And H_p(1. ltoreq. P. ltoreq.P) and for each source (n) isBy a phase value x_rR of (A) to (B)_pCorrection of ≧ 2, M corrections of the phase distribution and/or amplitude distribution are carried out, wherein a vector is phase-shifted x_rThen M is equal toThe phase distribution and amplitude distribution are transmittedEach sum of vectors and obtained perturbation measurement I_mSum matrix H_pnThe inversion of (2) is determined.

Using this property by means of theoretical equations, including applying a phase shift to one of the two vectors to obtain a vector of two different emissions H_p1And H_pPhase and amplitude shift caused by the aberration element therebetweenBy making several measurements of the perturbation, the perturbation being formed by the sum H of several values x_p1+H_p.e^jxIs caused by the continuous emission of.

By inverting the matrix H_pnAnd using for each vector H_pObtained byTo obtain a phase shiftDerivation of the distribution of (c):

then, by reversing the phaseObtaining the optimal phase distribution of the emission on each source directly

In an advantageous application of the invention, the focusing medium is a biological medium.

According to the invention, the perturbation I induced and measured in said medium_mSelected from: local movement, local velocity, stress, local temperature change in the medium.

Depending on the nature of the wave and the nature of the medium, one of these types of perturbations will be granted permission. For example, in the case of ultrasound, the method may be implemented with a wave intensity that allows a force to be generated in the medium due to the ultrasound radiation force, but at such a wave intensity that it does not cause damage to the medium, which would otherwise conflict with the situation during treatment.

According to another particular characteristic of the invention, the induced perturbation I is achieved by means of an imaging process_mThe intensity of (2) is measured.

Monitoring the intensity of the perturbations using the imaging process enables us to directly and reliably monitor the focal position in relation to the imaged structure of the medium. In the case of treatment, this can also be used to view the lesion simultaneously and then, if desired, monitor the effect of the treatment in real time. This also eliminates the need to move the patient between the imaging phase and the treatment phase. Naturally, this characteristic also allows to directly position the area to be treated without the need to re-home the patient according to a reference basis corresponding to the existing imaging steps.

For example, magnetic resonance imaging is performed after a micrometer movement caused by an ultrasound beam. In this way, by guidance and monitoring of the treatment with a single imaging procedure, it is possible to more flexibly and quickly implement a treatment protocol for brain tumors.

The imaging process can be selected from: a magnetic resonance imaging procedure, an ultrasound imaging procedure, a tomographic X-ray imaging procedure, and an optical imaging procedure.

The magnetic resonance imaging procedure is particularly suitable for monitoring changes in organic tissue, which is preferably chosen for medical applications due to the higher accuracy of the improved wave focusing, in addition to the fact that the imaging is used for implementing the invention, wherein it allows for continuously and/or simultaneously monitoring the treatment effect, or which allows for simultaneously imaging a region of interest to be examined.

Thus, the invention is implemented in a rather novel way to focus ultrasound waves in a medium which is also imaged by magnetic resonance imaging. The main advantage of these two processes is that they are fully compatible and suitable for simultaneous use.

Advantageously, the perturbation I induced in the region of interest is achieved by imaging the region of interest itself_mThe measurement of (2).

In a preferred embodiment of the invention, the perturbation I is induced and measured in said medium_mCaused by ultrasonic, acoustic or electromagnetic radiation forces.

The perturbation can be measured not only as movement but also as heat. In particular, the type of measurement is selected according to the nature of the medium.

The invention also covers a transmission system comprising a) a network of N sources]For rendering a spatial phase α_nAnd amplitude distribution A_n(1. ltoreq. N. ltoreq.N) focused wavesEmitted into a region of interest in a medium, said system using a method for optimizing focusing in a medium according to the invention, which is advantageous when said focused wave is emitted through an aberration-inducing element causing an initial, non-constant phase shift, for which purpose said system comprises means for performing, during the emission of the focused wave in the medium, the following steps:

b) said spatial phase distribution a of waves transmitted simultaneously by a plurality of sources in a network_nAnd/or amplitude distribution A_n(N is greater than or equal to 1 and less than or equal to N) M corrections, each correction corresponding to a simultaneous correction of the phase and/or amplitude of a plurality of sources of the network;

c) in phase alpha_nAnd/or amplitude A_nMeasuring perturbations I caused by said waves in said region of interest at each correction (m) of the distribution_m(1≤m≤M)；

d) From perturbation measurements I_mTo derive the perturbation I caused in the region of interest_mMaximized optimal transmit phase distributionAnd/or amplitude distribution

According to an advantageous characteristic of the invention, the source is an ultrasound emitter.

According to a preferred implementation, the different steps of the method are determined by instructions of a computer program.

The invention therefore also comprises a computer program pertaining to a data medium, which program is adapted to be executed in a system for transmitting focused waves, wherein the program comprises instructions adapted to carry out the steps of the method according to the invention.

The program may use any programming language, be it in the form of source code, object code, or an intermediate code between source and object code, such as in a parallel assembly form or in any other form as desired.

The invention also includes a data medium readable by a system according to the invention, the data medium comprising the computer program instructions described above.

The data medium may be any entity or device capable of storing a program, for example, the carrier may comprise storage means such as a ROM, such as a CD ROM or a microelectronic circuit ROM, or even any other means for magnetic reading, such as, for example, a magnetic disk (floppy disk) or a hard disk or memory card.

Second, the data medium may be a transmissible medium such as an electrical or optical signal, which can be routed by radio communication via electrical or optical cable, or by any other means. In particular, the program according to the invention may be downloaded via an internet-type network.

Alternatively, the data medium may be an integrated circuit in combination with a program, the circuit being adapted to perform or for performing the method in question.

Drawings

Further features and advantages of the invention will emerge from the following description, with reference to the accompanying drawings, which illustrate an embodiment example without any limiting characteristics. In the drawings:

FIG. 1 depicts an ultrasound probe that is advantageously used in the practice of the present invention;

FIG. 2 illustrates a scheme for optimizing focus according to the method of the present invention;

FIG. 3 shows the frequency domain of the targeted spatial variation of the aberration due to passing through the aberration-introducing element;

fig. 4 is an example of correcting aberrations caused by passing through an element that introduces aberrations using the focus optimization method according to the present invention.

Detailed Description

Fig. 1 shows an ultrasound probe 1 which is advantageously used for carrying out the invention. The ultrasound probe 1 shown in this figure is particularly used to optimize the focusing of ultrasound waves in the brain.

Indeed, the invention is particularly useful in this context, since the skull inevitably causes phase aberrations in the focused beam. These aberrations prevent accurate positioning of the focal zone, which is a particular problem in medical applications where a high degree of accuracy is required.

However, the skull is transparent to ultrasound. Accordingly, there is an opportunity to make improvements to correct aberrations due to passing through the skull so that ultrasound can be used despite the aberrations to image or treat brain regions.

Thus, the ultrasound probe 1 comprises a chassis 2 forming part of a sphere, the radius of curvature of which is selected according to the proposed application. In selected examples, for example, for brain imaging and therapy, the radius of curvature is 120 mm. The chassis 2 includes a prescribed number N of piezoelectric transducers 3.

Thus, in one practical embodiment, N-512 piezoelectric transducers 3 are mounted on the chassis 2. They constitute many wave sources. For ease of representation, only a few transducers are shown in fig. 1, which is merely schematic. Each individual transducer element 3 is used to emit ultrasound waves in a continuous and/or intermittent manner. In an embodiment example, each individual element 3 has a diameter of 8mm and emits a sine wave continuously with a central frequency of 1 MHz.

In general, when a probe emits an ultrasound beam, the phase of the wave emitted by each transducer element of the probe is calculated individually, so as to achieve focusing in a region called the focal zone of the medium.

However, the phase of the initial calculation to perform focusing cannot calculate the aberration caused by the inhomogeneous medium in which the wave is focused. These aberrations are practically unknown. The invention serves to attenuate or even eliminate the effect of these aberrations.

Fig. 2 shows the use of an ultrasound probe 1, as shown in fig. 1, for imaging or therapy within a medium 4, in this case the brain comprised in the skull being the element that introduces the aberration 6. The transmission of waves from the transducer 3 to the skull 5 and brain 4 is effected by a medium 7 having a suitable impedance.

The desired focus, shown by the dashed line, determines the focal area 6. When there is no phase correction, we notice focus degradation, as represented by the dotted line in fig. 2. This degradation is caused by the presence of the aberration-inducing element 5, which aberration-inducing element 5 is constituted by the skull lying in the path of the ultrasound beam emitted by the probe 1.

At the focal zone 6, after propagating through the skull, the contribution (p) of the nth transducer element 3 to the total acoustic field realized in the focal zone 6 can be expressed as:

wherein the content of the first and second substances,is the phase shift, A, caused by the introduction of the aberrating medium 5_nIs the amplitude of the emission, alpha_nRepresenting the phase, D, corresponding to transducer element n in the phase distribution_nIs an attenuation factor caused by an absorption phenomenon when passing through the skull 5 and by a reduction in amplitude by diffraction, f is the frequency of the emitted wave, K_wIs wave vector, R_cIs the radius of curvature of the transmission system.

Thus, the total sound field is calculated by the following equation:

this total sound field produces a perturbation I in the medium 4.

In the present invention, the perturbation intensity I is monitored at the focal zone 6 by means of measurements made in the medium 4. The measurement of the perturbation intensity I is advantageously achieved using an imaging device 8, such as magnetic resonance imaging or even echography.

However, these imaging procedures do not allow for direct and remote measurement of the additional pressure caused by the ultrasound field. On the other hand, in the case of ultrasound, the local ultrasound energy can be evaluated directly.

The ultrasonic energy is proportional to the square of the additional pressure caused by the ultrasonic field. However, it is noted that accessing local ultrasound energy does not allow direct access to phase information and therefore does not allow direct access to the induced phase shift. In addition, with these imaging procedures, it is possible to perform optimization by measurement of the amplitude only.

In an advantageous embodiment, the local ultrasound energy is sufficient to induce movement in the tissue, but does not cause any damage. This motion caused by the application of the ultrasonic field to the focal zone is then measured by imaging the tissue located in the focal zone.

Such measurements are well known to those skilled in the art and those skilled in the art may also refer to the measurement set forth in the entitled "Visco-elastic sheath properties of in vivo cleavage dispersions by MR elastomer", Sinkus R, Tanter M, XYDEas (T), et al, MAGNETRESONANCE IMAGING 23 (2): 159 sp.iss.si, data of month 2 2005 to see the characteristics in the case of magnetic resonance imaging.

In particular, when the ultrasound imaging process is used to measure perturbations, all known techniques for measuring motion can be used, including doppler measurements, inter-signal group correlation techniques, measurement of phase changes in the fourier field, and the like.

When a magnetic resonance imaging procedure is used to monitor the perturbations I, a magnetic resonance imaging sequence is used after the motion at the focus, which is sensitive to changes in motion over time. These magnetic resonance imaging sequences are used to acquire motion samples in a very short time between 1 and 10 milliseconds.

For example, similar sequences to those known in diffusion MRI can be employed. In particular, a two-dimensional map of the motion produced by the ultrasound transducer can be created using a temporal integration of the measured motion over the time the magnetic field gradients are applied during a modified MRI diffusion sequence. In each voxel of the map, the motion corresponds to the integration of the short-time motion of the voxel during the application of the force.

By observing the motion, the ultrasonic radiation force induced in the focal zone is deduced. The force density is then expressed by the expression f 2aIe_zWhere I is the sound intensity, a is the ultrasound absorption coefficient of the medium, c is the propagation velocity of sound in the tissue, e_zIs the propagation direction vector.

Assuming that the wave is locally flat at the focus, the sound intensity in the focal zone passesIs shown in which<P²>Is the time average of the acoustic energy mentioned above and Z is the acoustic impedance of the medium.

In the embodiment examples described below, a measure representing the perturbation is selected as the acoustic energy I.

According to the method of the invention, the invention uses the measured perturbation I_mFluctuations in amplitude, such as the sound intensity in the focal zone 6 when the phase delay on each transducer element 3 of the ultrasound probe 1 is modified m times.

The focused waves are also selected such that they cause a temperature rise within the medium. In this case, the perturbation measurement by temperature rise can be realized with other specific sequences, in particular by magnetic resonance imaging.

In fact, the invention proposes to correct the phase shifts on a set of transducers while also measuring the perturbations I induced in the zone of interest_mThereby determining an optimal phase distribution from the focus point.

Advantageously, also a perturbation I caused outside the region of interest 6 is achieved_mOne or more additional measurements of (a). This is particularly easy when using a magnetic resonance imaging apparatus, since the imaging allows one to observe the phenomenon over a relatively extended region.

Two embodiments are shown below. Both embodiments use known modifications of the purpose-related principle and the phase distribution to determine the optimal phase distribution using a measure of the perturbation caused in the medium.

The first embodiment uses iterative correction of the phase within the phase profile. The phase distribution is then advantageously described by means of a space vector describing the phase of the wave transmitted for each N (1. ltoreq. n.ltoreq.N) of the network of N sources. Then, the phase of element n is determined byExpression of wherein d_kIs a real number.

Preferably, the spatial frequencies corresponding to the most commonly observed variations in aberrations caused by the aberration-introducing element are selected as the basis.

However, for targeted medical applications, the phase aberration caused by biological structures varies relatively slowly in space. This is particularly the case for human skull bones. Thus, by modifying the values associated with the minority vectors corresponding to the minority spatial frequencies, an optimal solution for the phase shift can be obtained. Thus, by only correcting the value related to one of the vectors, the principle allows us to modify the phase in several transducers, or even in all transducers.

For example, the decomposition of the spatial variation of the aberrations normally caused by the skull is achieved by performing a Discrete Cosine Transform (DCT) based on the spatial frequencies FX, FY. This decomposition is shown in fig. 3. In the figure, it can be seen that the frequency space (FO) occupied by the most common spatial frequencies in the frequency space (EF) is located in the low frequency region.

Thus, by taking into account the frequency characteristics caused by the aberrations, a new basis vector is then advantageously determined, which is different from the standard basis assigned one by one for each transducer, so as to select each pair of non-zero spatial frequencies (FX, FY) along the X and Y axes of the network of transducers 3, respectively, and construct a corresponding spatial vector.

In one example embodiment, the vector is constructed in the following manner:

whereinAndis an integer describing a set of spatial frequencies, (X)_n，Y_n) Is the location of the nth transducer on the probe in a two dimensional coordinate system (e.g., describing all integers between 2 and 10). They may also be used to describe the same spatial frequency FX ═ FY in two or other dimensions describing unique frequencies FX and FY.

The phase profile is then modified M times based on these sinusoidal vectors, where the number M may be less than the number of transducers (N).

Due to phase writing of the source (n)Wherein d is_kIs real, the optimization of the phase distribution therefore involves determining a set of values d_kEach value d_kAnd vector V_kAnd (4) correlating. For this purpose, for example, by applying the value V_kIncreasing by a factor of S, e.g. 0.1 radian increments, we apply several values V to each of these vectors in succession_k. At each correction, a measurement of the resulting perturbation is achieved.

The quantity S may be related to all vectors V_kIs the same as, or may depend on V_kThe vector of interest. It is then denoted S_k。

Once for a vector, the d that causes the largest perturbation has been determined_kAnd the value is corrected according to a correction rule, and the same operation is realized on other vectors by applying an optimal value to the vector and the like.

In pair withAll or part of the vectors of spatial frequencies (FX, FY) perform this operation. Thus, realizeAnd (6) correcting. If the amount of the increment (S) is applied to all vectors V_kSimilarly, the total amount of correction is M S.K.

For each correction m, a perturbation I is implemented_mAt least one measurement of (a). Then, select d_kValue of d_kValue and maximum perturbation observed I_mThe correction (m) of (1) corresponds to.

In this embodiment, therefore, by optimizing these vectors V_kWith the phase distribution of each, we successively optimize the focus over all spatial frequencies of the aberrations, with each vector describing a particular frequency pair (FX, FY) in space of spatial frequencies.

Then, using each optimal distribution for each vector, we get the optimal spatial distribution over all spatial frequencies of interest

In fact, with respect to each vector V_kAt the end of the optimization, the optimum phase distribution is modified by the value d which causes the maximum perturbation_kThe sum of the vectors involved. By combining the optimization effects of all vectors, the following phases are exhibited at each transducer:maximum perturbation is obtained, where d_kIs the optimum value. Then, partial aberration caused by the aberration element is corrected. All iterations may be repeated if necessary to improve the optimization.

Furthermore, when one or more perturbation measurements I are carried out outside the region of interest 6_meThese measurements can then be used to select the optimal phase distribution, except for the maximum perturbation I in the region of interest 6_mFurthermore, the optimal phase distribution allows for a minimum perturbation I outside the region of interest 6_me. In particular, the minimal perturbation is the object of the second iteration for improving the optimization effect.

In addition, in the case where the skull 5 causes amplitude aberration, second-order correction can be performed by amplitude correction in addition to the optimization of the phase distribution.

This correction is advantageously effected after a first optimization of the phase distribution. In this case, after the optimization of the amplitude, a second optimization of the phase distribution can advantageously be achieved.

An example of implementing this second order correction comprises varying the amplitude coefficient A 'in successive steps, typically between 0.1 and 1'_kThe coefficient being applied to each wave constructed from a phase distribution defined by each vector V_kDescription, each vector V_kThe expression is as follows:wherein A is_nIs the default amplitude for each source. With S ' different coefficients A ' implemented in a similar manner for phase variation '_kA number of transmissions of, thus, the quantity I or the next quantityThe maximization is performed in order to optimize the ratio between the intensity at the focus and the intensity emitted by the source. For example, as long as S ' increments are implemented for all vectors, an additional number M ═ S ' is implemented in number 'And K is corrected.

In addition, by testing several phase distributions and several amplitude factors in succession, it is possible to achieve an optimization of the perturbations in the focal zone, for each vector of the space describing the spatial frequency of the aberrations, the minimum perturbation outside the focal zone exhibiting, where appropriate, an optimized focus, regardless of the phase aberrations induced by the skull 5.

The use of a vector describing the space of spatial frequencies at which aberrations are caused by the aberrating elements enables us to phase correct all transducers simultaneously, and hence enables us to provide sufficient energy to the medium so that small increments of perturbation are measurable.

In fact, the tissue movement caused by the ultrasonic radiation force is typically very small, on the order of between 10 and 100 μm. Thus, the main limitation is the minimum movement that can be measured by the imaging process. The minimum movement is typically about 1 μm. Thus, the energy supplied to the medium and the resulting perturbation should be sufficiently distinguishable under the sensitivity of the measurement.

By using phase correction for all sources, the invention thus allows us to deliver sufficient energy to the medium not only for generating detectable perturbations, but also to distinguish energy differences associated with different phase shift combinations, thus allowing optimization.

The selected imaging procedure and implementation device determine the time needed to perform the measurement corresponding to one iteration of the phase distribution. For example, according to ultrasound techniques, this time is about 1ms, and with magnetic resonance imaging techniques, this time is about 10ms if we limit themselves to measurements in a single 3D volume. Thus, using MRI techniques, we obtain a maximum total time of 10 minutes, the total number of iterations will be limited to 60000.

The first embodiment described above has the advantage of using a small number of spatial frequencies, approximately 10, in each direction X and Y, thus allowing fast iterative convergence to a scheme very close to the optimum correction. About 1000 to 2000 iterations are sufficient. Using conventional imaging procedures, this corresponds to an optimization time of about 3 minutes, which is clinically very satisfactory.

In each step of the iteration, instead of transducer by transducer, the phase is modified according to all elements or at least a large number of elements, allowing to create a modification that is distinguishable from the sound intensity at the focus, thus allowing to rely less on the measurement sensitivity of the ultrasound energy in the focal zone.

Subsequently, a simulation method is proposed, which is influenced by the theoretical causative party of the aberration represented by the spatial distribution of the phase aberration in fig. 4 a. The process converges in about 4000 steps, with V for each feature vector_k10 spatial frequencies are measured in each direction and S-30 phase shift (hence K-100-10). In fig. 4b, the amplitude I criteria of the acoustic energy normalized with respect to the optimal energy obtained with the ideal correction is considered as a function specifying the number k of each vector.

Here, optimization is performed twice in succession for all the spatial vectors. Therefore, here we perform M-S · K corrections. The amplitude I of the acoustic energy at the focus can be seen_{Standard of merit}Increase significantly during processing. Thus, the method according to the invention allows convergence to an ideal focus.

The resulting correction law is shown in fig. 4 c. Very close to the theoretical aberration law introduced for the simulation and presented in fig. 4 a.

According to a first embodiment, the invention is a test method according to which we optimize the acoustic energy generated in the medium, finding the phase distribution, which will enable us to reduce the effect of aberrations on the wave focusing.

In a second embodiment of the invention, a direct conversion of the perturbation measurement is used to determine the optimal phase distribution.

In fact, from a plurality of measurements of the intensity of the perturbation produced by the interference of the waves emitted by the two transducers, it is theoretically possible to evaluate the optimal phase shift between the two transducers. In fact, the intensity of the two beams superimposed in amplitude is modulated by the cosine of the relative phase difference between the two transducers.

Thus, when two monochromatic beams are usedAndat one point of interference, the intensity produced is:

thus, it is possible to determine the phase by modulating the intensityIf we add an additional phase shift x to the signal S₂Then we get the intensity modulated by the cosine, which is expressed as a function of x:

based on at least three measurements of I (x), x is corrected in each measurement, so it is theoretically possible to calculate A, B and

thus, the phase of the transmitted wave with respect to each transducer is corrected at least twice in unique increments of x.

One way to calculate this phase shift is to correct x for the R phase_rMeasurement of the intensity R (R > 1) is achieved, and then the system of linear equations is directly converted into three unknowns, A,And

in particular if for I (x)_r) We also achieve I (x) per measurement of_r+ π) measurement, then:

we can measure two sums:

the linear system of these two equations includes two unknowns, i.e.Andby simply solving the system, we can obtain information about phaseInformation, phase ofIs the phase shift induced by the aberration-inducing element between the two transducers.

The basis of the N actual transducers is the standard basis. The aim of the invention is to evaluate the phase shift and amplitude of the transducer in this standard basis:

however, due to the low amplitude of each transducer, it is difficult to measure the intensity of the modulation produced by both transducers. Thus, the present invention includes variations to implement the basis, which would allow multiple transducers to operate simultaneously. A variety of groups can be used, one being the hadamard group:

making H equal to H_pn(avec1 ≦ p ≦ Per1 ≦ N ≦ N) as the basis change matrix. Thus, the basis is composed of each vector H_pThe amplitude and phase of the transmission can be expressed as described by each vector H_pComplex vector of emitted amplitude and phase:

and thus H_p＝H_pnT_n＝B_pe^jβpIn which B is_pAnd beta_pIs applied to the vector H_pThe values of amplitude and phase for each set of transducers for a given vector.

Therefore, the temperature of the molten metal is controlled,

if we use basis vectorsAs a reference (phase 0 and amplitude 1), then for each vector H_p(1≤p≤P)，β_pIs composed of aberration elements and vectorsPhase comparison vector H_pThe induced phase shift. In this case, for x_rBy transmitting the vector sum continuously over a network of N sourcesBy applying a perturbation I to the specimen_m(x_r) At least two (thus, R.gtoreq.2) and preferably three (R.gtoreq.3) distinct values x are measured_rCause perturbation I_m(x_r) Can evaluate each vector H_pIs/are as followsThen, a vector H is obtained_pReference vector ofThe amplitude and phase of the correlation actually observed in the medium.

Then, by applying x to each vector_rR correction of (2) to realize the correction of P vectors H_pOf each ofAnd (6) evaluating. Here we consider R similar for all vectors.

However, this number of corrections (R) is the vector H_pIs thus expressed as R_p. Thus, achieving a phase and/or amplitude distribution overallAnd (5) correcting. When for all vectors H_pIn the sense that R_pWhen R, M is P · R.

Then, in order to obtain the phase and amplitude respectively caused by the aberrometer for each of the N sources, it is necessary to return to the standard base. For this reason, it is sufficient to invert the matrix H. In the case where the matrix cannot be inverted, then using only the imaginary inverted matrix obtained by decomposition into singular values, we obtain the phase and amplitude induced by the aberrometer for each of the N sources:

then, by inverting the phase shift:obtaining an optimal phase of transmission for each of N sources

It is also possible here to use a perturbation measurement I realized outside the region of interest 6_me. In this case, we try to make the perturbation I_meAnd (4) minimizing.

In the use of perturbation I_meAfter inverting the matrix, the aberrations caused by the aberration elements are obtained and these aberrations can be used to apply a phase distribution, which is used to optimize the perturbation I in the region of interest 6_mAnd is used for perturbation I outside the region of interest 6_meMinimization is performed.

The basis H is chosen such that the measurement noise is due to the inverse matrix H_pnBut slightly enlarged. For example, a matrix H is next selected whose eigenvectors with the largest components are the eigenvectors corresponding to the high-intensity transmissions of the network of multiple sources.

Finally, it is seen that various embodiments are possible using the principles of the present invention. In particular, for aberrations of the aberration-introducing element, a variety of vector bases describing particular spatial frequencies may be used.

Claims (21)

1. For in-medium [4]]Region of interest [6 ]]In a method of transmitting waves, wherein said waves are transmitted by N sources [3 ]]The resulting network passes through elements that introduce aberrations [5 ]]Is emitted to the medium [4]]Said aberration-inducing element [5 ]]Introducing an initial, indeterminate phase shiftN is greater than or equal to 1 and less than or equal to N, using M-1 successive corrections of the emitted wave, each correction causing a perturbation, measuring M perturbations, these measurements being used to derive optimal focusing characteristics, said measurements beingThe method comprises the following steps:

a) via N sources [3 ]]Component network transmission waveWherein j²= 1, waveExhibiting a spatial phase α_nAnd amplitude A_nDistribution, 1 ≦ N ≦ N, and the wave propagates to the medium [4 ≦ N]Region of interest [6 ]]；

b) For a plurality of sources [3 ] in the network]Said spatial phase a of the simultaneously transmitted waves_nAnd/or amplitude distribution A_nPerforming M-1 times of correction, N is more than or equal to 1 and less than or equal to N, and M>1, each correction corresponding to the plurality of sources to said network [3]And/or amplitude, and each correction causes the transmission of step a);

c) at the phase alpha_nAnd/or amplitude distribution A_nIs measured in the region of interest [6 ] each time m is corrected]At least one perturbation I caused by said wave_m，1≤m≤M；

d) Transmitting with perturbation measurement I as measured from_mDerived maximized transmit phase distributionAnd/or amplitude distributionThe wave of, the perturbation I_mIn the region of interest [6 ]]Is caused by (1).

2. Method according to claim 1, characterized in that it comprises a step c') of alpha at said phase_nAnd/or amplitude A_nEach time the distribution is corrected for m, measuring in said region of interest [6 ]]At least one perturbation at a point other thanM is more than or equal to 1 and less than or equal to M; subsequently, a transmission step d) is carried out, based on the perturbation measurement I_mAnd perturbation measurementDeriving to be in the region of interest [6 ]]In the perturbation I_mMaximizing and minimizing perturbation or perturbations outside said region of interestMinimized optimal transmit phase distributionAnd/or amplitude distribution

3. Method according to claim 1, characterized by applying to at least a plurality of sources [3 ]]To modify the spatial phase distribution a by successive iterations of the phase of_nN is 1. ltoreq. n.ltoreq.N, where the optimum phases are distributedThe iteration that generates the largest perturbation is selected.

4. A method according to claim 3, characterized in thatDistributing the phase a_nDecomposed into K vectors V_kK is not less than 1 and not more than K, wherein d_kIs a real number, each specific vector V_kDescribing the data from the N sources [3 ]]Phase of transmission by pair for each vector V_kD of_kValue S_kSecondary correction, the phase correction a being effected on all N sources_nN is not less than 1 and not more than N, then M is equal to

5. The process according to claim 4, characterized in that at the base V_kIn a subspace formed by K' vectors, to realize a phase distribution alpha_nCorrected of (2), K'<K is whereinThen M is equal to

6. Method according to any one of claims 4 and 5, characterized in that said vector V is defined according to K periodic functions of said space_kEach coefficient V_knDetermined by the value of the function K associated with the location of the nth source, K is 1 ≦ K ≦ K.

7. The method according to any one of claims 3 to 5, characterized by operating on at least a plurality of sources [3 ]]Performing continuous correction to correct spatial amplitude distribution A_nN is more than or equal to 1 and less than or equal to N, and the optimal amplitude is distributedThe iteration that produces the largest perturbation is selected.

8. Method according to claim 7, characterized in that the correction of the amplitude distribution is effected sequentially, alternately or simultaneously with the correction of the phase distribution.

9. The method of claim 1, wherein the amplitude A is_nAnd the phase alpha_nIs in the form of a matrix H_pnP vectors H defined_pP is not less than 1 and not more than P and N is not less than 1 and not more than N, whereinWherein each value C_pIs a complex number.

10. The method of claim 9, wherein the matrix H is_pnAre orthogonal.

11. The method of claim 10, wherein the matrix H is_pnIs a hadamard matrix.

12. A method according to any one of claims 9 to 11, wherein the sum at each source n isBy a phase value x_rR of (A) to (B)_pCorrection for more than 2 times, M times of correction of the phase distribution and/or amplitude distribution, wherein theAt least two vectors H corresponding to said bases_p1And H_p1 < P < P, where a vector is phase shifted by a phase x_rThen M is equal toThe optimal phase distribution and amplitude distribution are obtained by summing the obtained perturbations I for each of the transmitted vectors_mMeasurement of (D) and matrix H_pnThe inversion of (2) is determined.

13. A method according to claim 1, characterized in that the emitted wave is an acoustic, ultrasonic, electromagnetic or light wave.

14. A method according to claim 1, wherein said focusing medium [4] is a biological medium.

15. Method according to claim 1, characterized in that the perturbation I induced and measured in the medium_mSelected from: local movement, local velocity, pressure, locally induced temperature change in the medium, light intensity.

16. Method according to claim 1, characterized by the fact that it is carried out by means of an imaging device [8]]Implementing the induced perturbation I_mThe measurement of (2).

17. The method according to claim 16, wherein the imaging device [8] is selected from: magnetic resonance imaging apparatuses, ultrasonic imaging apparatuses, and tomographic X-ray imaging apparatuses, and optical imaging apparatuses.

18. The method according to claim 16 or 17, characterized by passing through said region of interest [6 ]]The imaging of itself is carried out in said region of interest [6 ]]In the perturbation I_mThe measurement of (2).

19. Method according to claim 1, characterized in that the perturbation I induced and measured in the medium_mCaused by ultrasonic, acoustic or electromagnetic radiation forces.

20. For in-medium [4]]Region of interest [6 ]]In which said waves are generated by N sources [3 ]]The resulting network passes through elements that introduce aberrations [5 ]]Is emitted to the medium [4]]Said aberration-inducing element [5 ]]Introducing an initial, indeterminate phase shiftN is more than or equal to 1 and less than or equal to N, and the device enablesMeasuring M perturbations with M-1 successive corrections of the transmitted wave, each correction causing a perturbation, these measurements being used to derive optimal focusing characteristics, the apparatus comprising:

for via N sources [3 ]]Component network transmission waveWherein j is²= 1, waveExhibiting a spatial phase α_nAnd amplitude A_nDistribution, 1 ≦ N ≦ N, and the wave propagates to the medium [4 ≦ N]Region of interest [6 ]]；

For multiple sources [3 ] in the network]Said spatial phase a of the simultaneously transmitted waves_nAnd/or amplitude distribution A_nA module for performing M-1 corrections, N is more than or equal to 1 and less than or equal to N and M>1, each correction corresponding to the plurality of sources to said network [3]And/or amplitude, and each correction causes the transmission of step a);

for at the phase alpha_nAnd/or amplitude distribution A_nIs measured in the region of interest [6 ] each time m is corrected]At least one perturbation I caused by said wave_mM is more than or equal to 1 and less than or equal to M;

for transmitting with perturbation measurement I as from_mDerived maximized transmit phase distributionAnd/or amplitude distributionOf said wave, said perturbation I_mIn the region of interest [6 ]]Is caused by (1).

21. The apparatus of claim 20, wherein the transmitted waves are ultrasonic waves.