Abstract:
A method for mapping fibrous media by propagation of ultrasound from a set transducers, wherein: a number of unfocused incident ultrasonic waves having different wavefronts are emitted; the signals reverberated by the medium toward each transducer are captured; coherent signals respectively corresponding, for each transducer, to contributions coming from different fictitious focal points in the medium are determined; and then the orientation of the fibers is determined by comparing a spatial coherence between said coherent signals, in a plurality of directions.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to methods and devices for mapping fibrous media. 
       BACKGROUND OF THE INVENTION 
       [0002]    Such a method has already been described, for example by Derode and Fink (Spatial coherence of ultrasonic speckle in composites, Derode A., M. Fink,  IEEE Trans Ultrason Ferroelectr Freq Control  1993; 40(6):666-75), which teaches the successive emission of ultrasonic waves focused by a transducer array placed on the surface of a composite material, with multiple orientations of the transducer array. For each ultrasonic wave firing, a function of the spatial coherence between signals captured by the transducers after reverberation of the transmitted ultrasonic wave is calculated, and the direction of the fibers of the composite material is determined as being the direction of the transducer array corresponding to the maximum of the spatial coherence function. 
         [0003]    This known method is suitable for a simple medium such as a composite material in which the fibers are regularly arranged; it is not appropriate for studying a more complex medium such as biological tissue. 
         [0004]    However, there is a need to map the structure of biological tissues composed of fibers, such as myocardial, muscle, and brain tissue. This structure plays a crucial role in both the mechanical function (muscle tissue) and the electrical function (brain, muscle, heart) of these tissues, and the spatial orientation of the fibers is therefore a very important parameter to determine for diagnostic purposes and for functional exploration of these bodies. 
         [0005]    For example, in brain imaging, it is very important to identify the neuronal fiber pathways that connect different brain areas. Currently, the only technology able to provide a three-dimensional image of the organization of fibers is magnetic resonance imaging by diffusion tensor (diffusion MRI). This very slow technique is used for exploration of the adult brain, but is too limited for imaging moving organs such as the heart. Also, MRI is not used to image the brains of very young children, especially premature babies who may have brain development abnormalities which unfortunately are impossible to diagnose with current techniques. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention is intended to overcome these disadvantages. 
         [0007]    To this end, the invention provides a method for mapping fibrous media, comprising: 
         [0008]    (a) a measurement step during which a set of transducers T ij  emits, in a field of view of a medium comprising fibers, a number N of unfocused incident ultrasonic waves (meaning not focused in the field of view) l having different wavefronts, and respective signals RFraw l,ij  (t) representative of ultrasonic waves reverberated by the medium are captured by the transducers T ij  from the incident waves l, 
         [0009]    (b) a step of synthesizing coherent data during which are determined, from N sets of captured signals RFraw l,ij  (t), for a number M of fictitious focal points P k  in the field of view, coherent signals RFcoherent k,ij  (t) corresponding to the signals that would have been received by the transducers T ij  if a wave focused at point P k  had been emitted by said transducers, 
         [0010]    (c) a step of mapping fibers of the medium, during which the presence and orientation of fibers at each point P k  are determined by comparing a spatial coherence between coherent signals RFcoherent k,ij  (t) in a plurality of directions. 
         [0011]    It is thus possible to map very quickly and easily the structure of biological tissues composed of fibers, such as the myocardium and other muscles and the brain, due to the fact that the backscattered signals contain information about the tissue microstructure that is not directly visible in the ultrasound image (B mode). It is the analysis of spatial coherence which reveals the orientation of the fibers, because the tissue anisotropy is found in the coherence function measured in different directions. 
         [0012]    In various embodiments of the method according to the invention, one or more of the following arrangements may possibly be used: 
         [0013]    during step (a), the set of transducers used is a two-dimensional transducer array; 
         [0014]    during step (c), an integral of a function of spatial coherence between transducers is determined in a plurality of directions, and the direction of the fibers is determined as being a direction which maximizes said integral; 
         [0015]    the incident ultrasonic waves are plane waves having different propagation directions; 
         [0016]    the incident ultrasonic waves are divergent waves (emitted by the ultrasound array as if they came from different source points); 
         [0017]    the incident ultrasonic waves are emitted successively; 
         [0018]    the incident ultrasonic waves are encoded spatiotemporally and emitted simultaneously, then the reverberated waves are captured simultaneously and separated by decoding; 
         [0019]    an image of the fibers detected in the medium is displayed; 
         [0020]    an ultrasound image of the field of view is determined and this ultrasound image is displayed with a superimposed image of the fibers; 
         [0021]    the ultrasound image is determined by beamforming the coherent signals determined in step (b); 
         [0022]    the medium to be imaged is human or animal tissue (particularly mammalian). 
         [0023]    The invention also relates to a device for implementing a mapping method as defined above, comprising a set of transducers T ij  as well as control and processing means suitable for: 
         [0024]    (a) causing the set of transducers T ij  to emit, in a field of view of a medium comprising fibers, a number N of incident ultrasonic waves l having different wavefronts, and causing the transducers T ij  to capture respective signals RFraw l,ij  (t) representative of reverberated ultrasonic waves from the incident waves l, 
         [0025]    (b) determining, from the N sets of captured signals RFraw l,ij  (t), for a number M of fictitious focal points P k  in the field of view, coherent signals RFcoherent k,ij  (t) corresponding to the signals that would have been received by the transducers T ij  if a wave focused at point P k  had been emitted by said transducers, 
         [0026]    (c) determining the presence and orientation of fibers at each point P k , by comparing functions of spatial coherence between the signals RFcoherent k,ij  (t) in a plurality of directions. 
         [0027]    Advantageously, the set of transducers is two-dimensional. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    Other features and advantages of the invention will be apparent from the following description of one of its embodiments, given by way of non-limiting example with reference to the accompanying drawings. 
           [0029]    In the drawings: 
           [0030]      FIG. 1  is a schematic view of a device for implementing a method according to an embodiment of the invention, and 
           [0031]      FIG. 2  is a block diagram of a portion of the device of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    In the various figures, the same references designate identical or similar elements. 
         [0033]      FIG. 1  shows an exemplary imaging device that operates by emitting and receiving ultrasonic compression waves, for example in the frequency range of 2 to 40 MHz. 
         [0034]    The imaging device represented in  FIG. 1  is adapted for performing synthetic ultrasound imaging of a field of view la in a fibrous medium  1 , for example tissue of a patient, particularly a muscle (myocardium or other muscle) or the brain. 
         [0035]    The imaging device comprises, for example: 
         [0036]    an array  2  of n ultrasonic transducers, for example a two-dimensional array comprising for example several hundred transducers and adapted for obtaining a three-dimensional (3D) image of the field of view  1   a;    
         [0037]    an electronics bay  3  or the like for controlling the transducer array  2  and adapted for acquiring the signals captured by the transducer array; 
         [0038]    a computer  4  or the like for controlling the electronics bay  3  and viewing the ultrasound images obtained from said captured signals. 
         [0039]    The transducer array  2  may, for example, be a planar matrix extending along two perpendicular axes X,Y, with the Z axis perpendicular to the X, Y axes denoting the depth direction in the field of view. In what follows, the transducers will be denoted T ij , i and j being the two indices denoting the position of each transducer respectively along the X and Y axes. The transducer array  2  may in particular comprise n 1  transducers in the X direction and n 2  transducers in the Y direction, with n=n 1 *n 2 . The following description uses this type of transducer array  2  for its example, but other forms of transducer array are also possible within the scope of the invention. 
         [0040]    As represented in  FIG. 2 , the electronics bay  3  may comprise for example: 
         [0041]    n analog-to-digital converters  5  (A/D ij ) connected individually to the n transducers T ij  of the transducer array  2 , 
         [0042]    n buffers  6  (B ij ) respectively connected to the n analog-to-digital converters  5 , 
         [0043]    a central processing unit  8  (CPU) communicating with the buffers  6  and the computer  4 , 
         [0044]    a memory  9  (MEM) connected to the CPU  8 , 
         [0045]    a digital signal processor  10  (DSP) connected to the CPU  8 . 
         [0046]    This device allows implementing a method for mapping the fibers of the medium  1 , which includes in particular the following three steps carried out by the CPU  8  with the assistance of the digital signal processor  9 : 
         [0047]    a) measurement (emission/reception and recording of raw data) 
         [0048]    b) synthesis of coherent data, 
         [0049]    c) analysis of fiber orientation, 
         [0050]    d) optionally, determining an image of the medium in B mode and superimposing the fiber mapping. 
       Step a: Measurement (Emission/Reception and Recording of Raw Data): 
       [0051]    The transducer array is placed in contact with the medium  1  and a number N of incident ultrasonic waves is emitted into the medium  1  by the transducers T ij  (N may be for example between 2 and 100, in particular between 5 and 10). The incident waves in question are unfocused (more specifically, not focused in the field of view) and have different respective wavefronts, meaning wavefronts of different shapes and/or different orientation. Advantageously, the incident waves are plane waves of various different inclinations, characterized by their respective angles of inclination α x , α y  relative to the Z axis, respectively in planes (X, Z) and (Y, Z), or are divergent waves emitted as if they originated from different points in space. 
         [0052]    The incident waves are generally pulses of less than a microsecond, typically about one cycle of the ultrasonic wave at the center frequency. The firing of incident waves may be spaced apart, for example by about 50 to 200 microseconds. 
         [0053]    Each incident wave encounters reflectors in the medium 1, which reverberate the incident wave. The reverberated ultrasonic wave is captured by the transducers T ij  of the array. This signal captured by each transducer T ij  comes from the medium 1 as a whole, since the incident wave is not focused at emission. Reverberant signals captured by the n transducers T ij  are then digitized by the corresponding analog-to-digital converters A/D ij  and stored in the corresponding buffers B ij . These signals stored in the buffers after each incident firing will be referred to hereinafter as raw RF data (“RF” is a term conventionally used in the field, simply because of the ultrasound frequency used). These raw RF data consist of an array of n 1 *n 2  time signals RFraw l,ij (t) respectively captured by the transducers T ij  after the firing of incident ultrasonic waves l. 
         [0054]    After each firing of incident waves l, the signals stored in the buffers B ij  are transferred to the memory  9  of the signal processor  8  for processing by said processor. At the end of step (a), the memory  9  therefore contains N arrays of raw RF signals. 
         [0055]    Note that the various incident waves could be spatiotemporally encoded, to allow simultaneous emission of some or all of the incident waves and an also simultaneous reception of the reverberated waves, which are then separated by decoding prior to storing them. 
       Step b: Synthesis of Coherent RF Data 
       [0056]    From N arrays of raw RF data, a number M of arrays of synthetic coherent RF data is calculated by the processor  8 , respectively at M points P k (x, y, z) of the field of view  1   a  (k being an integer between 1 and M, and x, y, z being the coordinates of point P k  on the X, Y, Z axes). Each of these M arrays of synthetic coherent RF data contains n i *n 2  time signals RFcoherent k,ij (t) corresponding to the signals which would respectively be captured by the transducers T ij  if the transducers were emitting a focused incident wave at point P k . 
         [0057]    The arrays of coherent RF data may be obtained for example by assuming a uniform propagation speed 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 very high frame rate ultrasonography and transient elastography” (IEEE Trans Ultrason Ferroelectr Freq Control 2009 March; 56(3): 489-506. 
         [0058]    As the direction of propagation of the plane wave corresponding to each firing l is known, and the propagation speed c is known, the processor  8  can calculate for each point P k  the propagation time τ ec (l, k) of the incident wave l to point P k , and the propagation time τ rec(l, k, i, j) of the reverberated wave from point P k  to transducer T ij , therefore the total time to travel in both directions 
         [0000]      τ( l, k, i, j )=τ ec ( l, P   k )+τ rec ( l, P   k   , i, j ).
 
         [0059]    The spatially coherent signal for transducer T ij , corresponding to virtual focal point P k , is then calculated using the formula: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    where B(l) is a weighting function for the contribution of each firing of incident waves l (in the current case, the values of B(l) may be all equal to 1). 
         [0060]    The arrays of coherent data RFcoherent k  may then possibly be refined by correcting the effects of aberrations in the medium  1 , for example as explained in the aforementioned documents EP2101191 or Montaldo et al. 
       Step c: Analysis of Fiber Orientation 
       [0061]    Next a spatial coherence is determined, for each array RFcoherent, indicative of the coherence between signals RFcoherent kij (t) for a same point P k . 
         [0062]    This spatial coherence can be measured for example by a spatial coherence function R(m) calculated using the correlations of signals c k (ij,pq) received on transducers ij and pq, by summing all correlations between pairs of remote transducers of m elements in a given direction in plane (X, Y). 
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         [0000]    where  RFcoherent k,y    is a temporal mean of RFcoherent k, ij , and T 1 , T 2  are two times. 
         [0063]    By considering only the transducers aligned with each other in a same direction of plane (X, Y) and renumbering these transducers Tq, q from 1 to Q, these correlations can be written as c(p, q) and we obtain: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0064]    The Van Cittert-Zernike theorem establishes the shape of this function R(m) in a randomly reflecting medium (therefore isotropic) for a monochromatic beam. R(m) is the spatial Fourier transform of the square of the focal spot. For a focal spot whose lateral extension is given by a function sin(ax)/x, R(m) is a triangle whose apex is at m=0 (autocorrelation) and which cancels out at m=Q. 
         [0065]    For a non-isotropic medium, additional spatial coherence is obtained when the direction of alignment of the transducers is aligned along the fibers. 
         [0066]    The integral S k  of this function in the considered direction of alignment in plane (X, Y) gives a parameter of spatial coherence, which is maximized in the fiber alignment direction. By calculating this parameter of spatial coherence in a plurality of alignment directions of the transducers, one can discover the direction producing the maximum spatial coherence parameter S k  and thus deduce the direction of the fibers at point P k . 
         [0067]    Note that the abovementioned spatial coherence functions R(m) or the spatial coherence parameters S k  could be averaged over several neighboring points P k , therefore within a small volume of the field of view around a point of interest. 
         [0068]    Another possible spatial coherence parameter is the focus criterion C k , which gives the ratio between coherent energy and incoherent backscattered energy. With the above notation, in other words by numbering from q=1 to Q the transducers aligned along a same direction in plane (X, Y), we have: 
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         [0069]    where t q  is a delay which allows rephasing all the signals RFcoherent kq (t). 
         [0070]    As in the previous case, this spatial coherence parameter is calculated in several directions for each point P k , and the direction of the fibers is determined as being the direction that maximizes parameter Ck. 
         [0071]    One can thus very quickly determine a three-dimensional mapping of fibers of the medium within the field of view  1   a.  This mapping can advantageously be presented to the user of the device in the form of cross-sectional images of the medium  1 , for example displayed on the screen of the computer  4 . If desired, these images can be calculated with restoration of continuity between fibers detected at different points P k . 
       Step d: Image Formation 
       [0072]    From the arrays RFcoherent k  calculated in step (b), it is possible to form a three-dimensional B-mode image of the field of view la by beamforming, as described for example in the aforementioned document EP2101191. 
         [0073]    It is possible to superimpose the fiber mapping determined in step (c) onto this B-mode image, and cross-sectional images of the field of view can be displayed on the screen of the computer, showing both the B-mode image and the fibers superimposed on this image.