Patent Publication Number: US-2013237807-A1

Title: Apparatus and method for generating mechanical waves into living bodies, system and method for mapping an organ or tissue and system and method for characterising the mechanical properties of said organ or tissue

Description:
The present invention relates to an apparatus and method for generating mechanical waves into human or animal organs or tissues. It also relates to a system and method for mapping an organ or tissue and system and method for characterising the mechanical properties of said organ or tissue. 
     The present invention relates to the field of magnetic resonance imaging (MRI) and, more specifically, to devices and methods for implementing magnetic resonance elastography (MRE). 
     Over the last fourteen years, MRE has become a useful non invasive technique to determine the mechanical properties of human or animal organs or tissues. MRE provides additional valuable diagnostic means to differentiate healthy and diseased tissues. It was successfully applied to characterize tumors in the breast and fibrosis in the liver. This emerging technique effectively extends palpation to remote organs or tissues that physicians cannot directly access provided mechanical waves can be generated in said organs or tissues. 
     By measuring the induced oscillating tissue displacements over time, MRE characterizes the induced mechanical wave which propagates in the targeted organs or tissues and which locally depends on the mechanical properties of said organs or tissues. The sensitivity of the technique relies both on the hardware and software capabilities of the MRI unit and on the local wave amplitude as produced in the said organs or tissues. 
     In many applications, mechanical waves are produced by physically vibrating the surface of the subject or animal with electromechanical or piezoelectric devices. A number of different vibrators have been developed to produce the mechanical waves required to perform MRE. For the breast and liver, mechanical waves may be produced by directly applying a vibrator onto the skin. For the brain, the head may be periodically tilted in a head-rocker system or the subject may bite a vibrating bar to yield propagating waves in the brain. 
     The known systems offer limited comfort for the subject since they imply vibrating the body or physically hitting the body. Besides, the propagation of the mechanical waves through the body tissues and bones is difficult such that the mechanical wave is largely attenuated before it reaches the targeted organ or tissue. Hence, the targeted organ or tissue are not efficiently vibrated and MRE outcomes are reduced. 
     It is an object of the present invention to provide a method and an apparatus in order to efficiently vibrate a human or animal organ or tissue by generating mechanical waves with larger amplitudes therein. 
     It is an object of the present invention to provide a method and an apparatus to vibrate a human or animal organ or tissue that is easier to implement in the MRE environment and more comfortable for the subject. 
     It is an object of the present invention to provide a system and method for mapping a human or animal organ or tissue without any undesired artifact from the vibration method and apparatus. 
     It is another object of the present invention to provide a system and method for characterizing the mechanical properties of said organ or tissue with increased sensitivity. 
     The invention is disclosed as recited in the appended claims. 
     Such objects are accomplished through an apparatus for inducing a mechanical wave in at least one region and/or organ and/or tissue of a human or animal body, said apparatus comprising:
         means for generating a pressure wave of a given frequency in a gaseous medium, and   waveguide means for guiding, in a gaseous medium, said pressure wave from said generating means to a human or animal body.       

     According to the invention, the generated wave is transmitted to the human or animal body in a gaseous medium without a mechanical transmission by means of solid media. 
     The present invention makes it possible to excite an organ and/or a tissue and/or a region of a human or animal subject with a mechanical wave in a more comfortable fashion for the subject. Indeed the apparatus according to the invention makes it possible to transmit a mechanical wave to an organ or tissue of a subject without any physical hit or friction on the subject&#39;s body or without making the whole body or skull of the subject vibrate through the MRI table, with a bite-bar, or a head-rocker. 
     Moreover, the apparatus according to the invention uses natural paths in the subjects body to guide the pressure wave down to the organ or region of interest. 
     According to the present invention, the amplitude of the mechanical waves propagating through the subject&#39;s organ or tissue have larger amplitudes compared to the techniques of the prior art. 
     The apparatus according to the present invention is less complicated, easier to set up, and less intrusive compared to the systems of the prior art. 
     Moreover, the apparatus makes it possible to more precisely transmit the pressure waves to the organ or tissue. 
     The apparatus according to the invention may also comprise adapting means, arranged at the extremity of the guiding means at the human or animal body&#39;s side, for adapting said extremity of the guiding means to a surface or an airway input of said body. 
     Hence, the most part of the generated pressure wave may be transmitted from the generating means to the subject&#39;s body so the attenuation of the mechanical wave remains limited. 
     The adapting means may have a shape adapted to any part of the body of said human or animal subject and more specifically to:
         an eye of said human or animal subject,   the nose of said human or animal subject,   the mouth of said human or animal subject, or   the anus of said human or animal subject.       

     Thus, the pressure wave may be sent to the organ or tissue of the subject via the eye, the nose, the mouth, or the anus of the subject. 
     Internal airway cavities reached through the nose, the mouth, or the anus of the human or animal subject are particularly interesting because they may represent a resonant chamber where the pressure wave could be amplified as more wave energy enters the cavity. Thereof, extra-thoracic upper airways also provide for the pressure wave natural waveguides towards remote organs like, for example, the lung, the hearth, the brain, or even the more remote pituitary gland. 
     The gaseous medium, in which the pressure wave is generated and guided from the generating means to the subject&#39;s body, may be air or any other gas mixture that may be used to ventilate the human or animal subject and which may include labeled gas for medical imaging, like helium-3 or sulfur hexafluoride for MRI. 
     The means for generating the pressure wave may comprise for example:
         a loudspeaker,   an electromechanical vibrator, or   a piezoelectric element.       

     The generating means may also comprise an amplifier associated with a function generator connected to the loudspeaker, the electromechanical vibrator, or the piezoelectric element. 
     The waveguide means may comprise a rigid or flexible tubular waveguide, which length and diameter are determined according to the frequency of the pressure wave such that the attenuation of the pressure wave remains very low between the generating means and the subjects&#39; body. 
     The amplitude of the pressure wave is ultimately set at the generating means such that losses between the generating means and the subjects&#39; body can be compensated. 
     Advantageously, the apparatus according to the invention may comprise a pressure wave adapter adapting the output of the generating means to the input of the waveguide means. 
     Such an adapter is needed when there is a difference in the dimensions or the shape of the generator, for example a loudspeaker, and the waveguide to limit impedance mismatch and power losses on the way to the subjects&#39; body. 
     The invention also provides a system for mapping of at least one region and/or tissue and/or organ of the body of a human or animal subject, said system comprising:
         an apparatus according to the invention for vibrating said organ and/or tissue and/or region,   magnetic resonance imaging means for imaging the displacements of said organ and/or tissue and/or region while said organ and/or tissue and/or region is vibrated.       

     Magnetic resonance imaging (MRI) means are well known by the person having ordinary skills in the art. Such imaging means will not be detailed here. 
     According to the invention, when the organ and/or tissue and/or the region is vibrated, MRI means are used to synchronously image the oscillatory displacements of the tissues at different instants of the period of the mechanical wave. 
     MRI means may take two or three dimensional images of the organ, the tissue, or the region. Thus, the invention provides two or three dimensional synchronised mapping of the displacements of the targeted organ, tissue, region at different instants of period of the mechanical wave.
         The spatial resolution of the MRI mapping may be isotropic since there is no a priori preferred spatial direction. The spatial resolution is taken according to the mechanical wavelength, which is expected at a given frequency of the mechanical wave in the imaged region, tissue, or organ. For example in the brain, at 50 Hz, the spatial resolution may be chosen between 1×1×1 and 3×3×3 mm 3 .       

     The temporal resolution over the period of the mechanical wave may usually be between ¼ to ⅛ of this period such that four to eight sets of three dimensional displacement maps are acquired. Each set represents a snapshot of the propagation of the mechanical wave through the organ, tissue, or targeted region at different instants over the period of the mechanical wave. 
     The invention also provides a system for characterising the mechanical properties of at least one region and/or tissue and/or organ of the body of a human or animal subject, said system comprising:
         a system according to the invention providing a set of displacement maps over a given mechanical period of said organ and/or tissue and/or region   at least one computer executable program for characterising the mechanical properties of said organ and/or tissue and/or region.
 
The invention also provides a method for inducing a mechanical wave in at least one region and/or tissue and/or organ of a human or animal body, said method comprising the following steps:
   generating, by generating means, a pressure wave of a given frequency in a gaseous medium, and   guiding said pressure wave from said generating means to said human or animal body in a gaseous medium.       

     Such a method may be used to excite a human or animal subject&#39;s eye, brain, heart, airways, lung, prostate, or uterus, by transmitting the pressure wave to said brain, heart, airways, or lung via the mouth or the nose of the subject, to said prostate or uterus via the anus of the subject. 
     The invention also provides a method for mapping an organ and/or tissue and/or region of a human or animal subject&#39;s body, said method comprising the following steps:
         exciting said organ and/or tissue and/or region according to the invention,   magnetic resonance imaging of said organ and/or tissue and/or region while said organ and/or tissue and/or region is excited.       

     Such a method may be used to map a human or animal subject&#39;s eye, brain, heart, airways, lung, prostate, or uterus. 
     The invention also provides a method for characterizing the mechanical properties of at least one region and/or tissue and/or organ of a human or animal subject&#39;s body, said method comprising the following steps:
         mapping tissue displacements of said organ and/or tissue and/or region according to the invention, and   analysing said displacement maps to characterize the mechanical properties of at least a part of said organ and/or tissue and/or region.       

     Such a method may be used to characterize the mechanical properties of a human or animal subject&#39;s eye, brain, heart, airways, lung, prostate, or uterus. 
     The invention also provides a method for characterizing an organ and/or tissue and/or region of a human or animal subject&#39;s body, said method comprising the following steps:
         mapping tissue displacement fields of said organ and/or region according to the invention   analysing said displacement fields to characterize the tissue anisotropy or fibre orientation of at least a part of said organ and/or tissue and/or region.       

     The characterizing method according to the invention may also comprise a step for analysing the displacement fields and tissue anisotropy to characterize the anisotropic mechanical properties of at least a part of said organ and/or tissue and/or region. 
    
    
     
       The new and inventive features believed characteristics of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative detailed embodiment when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  schematically illustrates an apparatus according to the invention; 
         FIG. 2  schematically illustrates a mapping system according to the present invention; 
         FIG. 3  illustrates a system according to the invention for characterizing the mechanical properties of an organ and/or tissue and/or a region of a human or animal subject&#39;s body; 
         FIG. 4  schematically illustrates a method for characterizing at least one region of the body and/or organ and/or tissue of a human or animal subject according to the invention method; 
         FIGS. 5-7  illustrate the results obtained thanks to the present invention on the brain of a human subject; 
         FIGS. 8-10  illustrates the results obtained thanks to the present invention on the pituitary gland of a human subject; 
         FIG. 11  illustrates the results obtained thanks to the present invention&#39;on the upper airways of a human subject; 
         FIGS. 12-14  illustrate the results obtained thanks to the present invention on preserved Bioquest® pig lungs; 
         FIGS. 15-17  illustrate the results obtained in vivo on rat brain thanks to the present invention; 
         FIG. 18  schematically illustrates the steps of a method according to the invention; 
         FIG. 19  illustrates, in the acquired central slice of a rat brain, the dependence of the total wave amplitude and the wavelength with respect to the excitation frequency; 
         FIGS. 20-22  illustrate the results obtained in the brain of six rats excited at 521 Hz thanks to the invention; 
         FIGS. 23 to 26  illustrate the results obtained thanks to the present invention on the brain of a human subject at 43 Hz and 113 Hz; 
         FIGS. 27 and 28  illustrate the results obtained with mouth-throat MRE acquisition in humans with guided pressure wave according to the invention; and 
         FIGS. 29 and 30  illustrate the results obtained with hyperpolarized helium-3 MRE in rat lungs according to the invention. 
     
    
    
     In the following specifications, elements common to several figures are referenced through a common identifier. 
       FIG. 1  schematically illustrates an example of an apparatus  100  according to the invention. 
     The apparatus  100  comprises means  102  for generating a pressure wave and a waveguide  104  to guide the pressure wave from the generating means to the subject&#39;s body. 
     The apparatus  100  also comprises an adaptation hose  106  arranged on the extremity  108  of the waveguide  104  at the human or animal body&#39;s side, for adapting this extremity  108  of the waveguide  104  to a surface or a cavity of said body, for example to an eye, to the mouth, or to the nose of the subject. 
     On the other extremity  110  of the waveguide  104 , the apparatus comprises a compression hemisphere  112  to match the output of the generating means  102  to the extremity  110  of the waveguide  104 . 
     The means for generating the pressure wave comprise:
         a function generator  114  for generating a burst of electrical sine wave or multiple frequency wave over the frequency range 10-500 Hz;   an amplifier, more particularly and audio amplifier  116 , amplifying the electrical signal generated by the function generator  114 , and   a loudspeaker  118  to produce a pressure wave by transduction of the amplified electrical signal received from the amplifier  116 .       

     The generated pressure wave is then directed to the waveguide  104  by the compression hemisphere  112 . The compression hemisphere  112  is connected on the one hand to the output of the loudspeaker  118  and on the other hand to the extremity  110  of the waveguide. 
     According to a non limitative example, the different elements of the apparatus have the following specifications.
         Function generator  114 : Function generator Tektronix AFG 3021B
           Sine wave in burst mode: 2-10 mVpp at 10-500 Hz   Frequency range: 1 μHz-12.5 MHz   Harmonic distortion: &lt;−70 dBc for 10 Hz to 20 kHz   Function: Generation of MRI-triggered burst of sine wave or multiple frequency wave over the exploration frequency range—10-500 Hz here.   Constraints: Programmable to allow arbitrary wave shape like multiple frequency wave.   
           Audio amplifier  116  : Audio amplifier McCRYPT PA 12000
           Frequency range: 10-30 000 Hz   Power RMS at 4Ω: 2×450 W   Function: Amplification of the generated wave to supply the loudspeaker with required power.   Constraints: Adaptation to the loudspeaker impedance   
           Loudspeaker  118 : Audio loudspeaker Monacor® SPH-135/AD
           Diameter: 135 mm   Power RMS: 40 W   Impedance: 8 A   Sensibility: 89 dB/1 W/1 m   Efficiency: 0.4%   Frequency range: 39-6000 Hz   Resonance frequency: 39 Hz   DC Resistance: 5.5 A   Equivalent Air Volume: 26 L   Maximal linear displacement: 1.75 mm   Weight: 1.1 kg   Function: Transduction of electrical signal to pressure wave.   Requirements: Efficient transduction and large maximal linear displacement over the exploration frequency range to induce accordingly large amplitude pressure wave.   
           Compression hemisphere  112 : Altuglas® hemisphere
           Diameter: 200 mm   Circular output: 20 mm   Function: Adaptation of the loudspeaker-generated pressure wave to the transmission tube cross-section.   Requirements: A small volume to maximize compression and a smooth transition shape to limit wave reflexion.   Other shapes may be used such as conic, exponential or hyperbolic shapes.   
           Waveguide  114  (Transmission tube) Altuglas® tube
           Length: 1740 mm   Inner diameter: 17 mm   Outer diameter: 20 mm   Function: Transmission of the pressure wave from the magnetic loudspeaker to the imaging site in the magnetic field of the MRI imager.   Requirements: The length of the tube must be adapted to the excitation frequency. Its length, in addition to the length of the adaptation hose, must correspond to an odd multiple of a quarter wavelength of the pressure wave such that the amplitude of the pressure wave is maximal at the output.   
           Adaptation hose  106 : Flexible silicone hose Masterflex®
           Length: 200 mm   Inner diameter: 20 mm   Outer diameter: 28 mm   Function: Transmission of the pressure wave along different orientations and coupling to the measured system—subject or sample.   Requirements: To limit the reflections of the pressure wave, the hose must be adapted to the waveguide (transmission tube) diameter. This diameter must be kept along the different orientations and, for a human, subject, under the pressure of the lips and the teeth at the mouth entrance.   
               

     For the subject&#39;s protection and comfort, a breathing filter (not represented) like an Intersurgical® clear-guard 1644131 and a mouthpiece (not represented) like an Intersurgical® 1930 may be added at the end of the adaptation hose  106  before the subject. 
     Reducing means (not represented) may be used to adapt the setup to smaller subjects like animals. A non limitative example of such a reducer may have the following specifications: Reducer (not represented): A plastic adaptation piece Intersurgical° 1968
         Input diameter: 22 mm   Output diameter: 6 mm   Function: Adaptation of the setup to smaller systems like small animals.   Requirements: A smooth transition shape to limit wave reflexion.       

       FIG. 2  schematically illustrates an example of a mapping system  200  according to the present invention. 
     The mapping system comprises an apparatus  100  for exciting an organ and/or a region of a subject  202  with a pressure wave as represented on  FIG. 1 . 
     The adaptation hose  106  of the apparatus  100  is put in the mouth of the subject  202 . 
     The subject is placed in magnetic resonance imaging means  204 . 
     Computer means  206  are connected to the function generator  114  and to magnetic resonance imaging means  204 . 
     The computer means  206  control the function generator  114  and the magnetic resonance imaging means  204  so that the function generator  114  and the magnetic resonance imaging means  204  are triggered synchronously. 
     The pressure wave is generated and is sent to the organ of the subject  202 . The pressure wave causes a mechanical wave which propagates in the organ, tissue, or region of the subject. During the propagation of the mechanical wave in the organ, the magnetic resonance imaging means  204  acquire images of said organ, tissue, or region. 
     The tissue displacements of the targeted organ, tissue, or region of the body is imaged slice by slice. The slices have a thickness of 1.6 to 8 mm, for example 2 mm. A three dimensional displacement map is obtained by combining the images of all slices. 
     The images are sent to the computer means  206 . The computer means comprise a display screen  208  on which the images taken by the magnetic resonance imaging means  204  may be displayed. 
       FIG. 3  schematically illustrates an example of a system  300  for characterizing the mechanical properties of an organ and/or a tissue and/or a region of a subject&#39;s body according to the present invention. 
     The system  300  comprises a mapping system  200  as represented on  FIG. 2 . 
     The system  300  also comprises a analyzing module  302  for analyzing the images taken by, the mapping system  200  and characterizing the mechanical properties of the imaged organ. 
     The images taken by the magnetic resonance imaging means  202  and sent to the computer means  206  are transferred to the analyzing module  302 . In the analyzing module  302 , the phase of the images is unwrapped to yield displacement maps at the different instants according to the imaging sequence parameters. Movies of the propagating mechanical waves may then be processed as shown in the presentation of the results. The local wavelength of the mechanical waves is inferred from the displacement maps to finally deduce the viscoelastic moduli of the studied organ, tissue, or region of the subject&#39;s body. 
       FIG. 4  schematically illustrates a method for characterizing at least one region of the body and/or organ and/or tissue of a human or animal subject according to the invention method. 
     The method  400  of  FIG. 4  comprises a step  402  for generating a pressure wave. 
     The generated pressure wave is guided from generating means to the body of the subject in a gaseous medium at step  404 . This pressure wave generates mechanical displacements in the subjects body in the targeted region, organ and/or tissue. 
     The targeted region, organ and/or tissue is imaged with magnetic resonance imaging means at step  406 . 
     The taken images are then analyzed at step  408  to realize a displacement mapping of the targeted region, organ and/or tissue. 
     The displacement mapping in/of/around the targeted region is analysed in step  410  to determine the mechanical properties of the targeted region, organ and/or tissue. 
       FIGS. 5-7  illustrate the results obtained thanks to the present invention on the brain of a human subject. 
       FIG. 5  illustrates displacement maps along the three motion encoded directions (U x , U y , and U z  in μm) in a central slice of the brain of a healthy subject at four over eight different instants of the mechanical cycle at 50 Hz. 
       FIG. 6  illustrates wave amplitudes given along the three Motion encoded directions (AX, AY, AZ) as well as the resulting total amplitude (Atot) in μm for six over 43 acquired slices in a full brain MRE acquisition. The corresponding average magnitude image is also given for reference (bottom row) in arbitrary units. Field of view=146×256×129 mm 3 , voxel=3×3×3 mm 3 , TR=4301 ms, 8 dynamics. 
       FIG. 7  illustrates maps of corresponding processed wavelength (in mm), dynamic shear modulus (G d  in kPa), and loss shear modulus (G 1  in kPa) given with the corresponding average magnitude image as a reference (bottom row). 
       FIGS. 8-10  illustrates the results obtained thanks to the present invention on the pituitary gland of a human subject. 
       FIG. 8  illustrates displacement maps along the three motion encoded directions (U x , U y , and U z  in μm) in a central slice of the pituitary of a healthy subject at four over eight different instants of the mechanical cycle at 126 Hz. 
       FIG. 9  illustrates wave amplitudes given along the three motion encoded directions (AX, AY, AZ) as well as the resulting total amplitude (Atot) in μm for 3 over 7 acquired slices in a pituitary MRE acquisition. The corresponding average magnitude image is also given for reference (bottom row) in arbitrary units. Field of view=32×32×32 mm 3 , voxel=1.4×1.4×1.4 mm 3 , TR=889 ms, 8 dynamics. 
       FIG. 10  illustrates maps of corresponding processed wavelength (in mm), dynamic shear modulus (G d  in kPa), and loss shear modulus (G 1  in kPa) given with the corresponding average magnitude image as a reference (bottom row). 
       FIG. 11  illustrates the results obtained thanks to the present invention on the upper airways of a human subject.  FIG. 10  illustrates displacement maps along the three motion encoded directions (U x , U y , and U z  in μm) in a central slice of the upper airways (from the mouth down to the trachea) of a healthy subject at four over eight different instants of the mechanical cycle at 54 Hz. 
       FIGS. 12-14  illustrate the results obtained thanks to the present invention on preserved Bioquest® pig lungs. 
       FIG. 12  illustrates displacement maps along the three motion encoded directions (U x , U y , and U z  in μm) in a central slice of the lung of a healthy subject at four over eight different instants of the mechanical cycle at 140 Hz. 
       FIG. 13  illustrates wave amplitudes, given along the three motion encoded directions (AX, AY, AZ) as well as the resulting total amplitude (Atot) in μm for 3 over 20 acquired slices in a full lung MRE acquisition. The corresponding average magnitude image is also given for reference (bottom row) in arbitrary units. Field, of view=320×320×80 mm 3 , voxel=4×4×4 mm 3 , TR=857 ms, 8 dynamics. 
       FIG. 14  illustrates maps of corresponding processed wavelength (in mm), dynamic shear modulus (G d  in kPa), and loss shear modulus (G 1  in kPa) given with the corresponding average magnitude image as a reference (bottom row). 
       FIGS. 15-17  illustrate the results obtained in vivo on rat brain thanks to the present invention. 
       FIG. 15  illustrates displacement maps along the three motion encoded directions (U x , U y , and U z  in μm) in a central slice of the brain of a healthy animal at four over eight different instants of the mechanical cycle at 520 Hz. 
       FIG. 16  illustrates wave amplitudes given along the three motion encoded directions (AX, AY, AZ) as well as the resulting total amplitude (Atot) in μm for six over 20 acquired slices in a full brain MRE acquisition. The corresponding average magnitude image is also given for reference (bottom row) in arbitrary units. Field of view=20×20×17 mm 3 , voxel=0.8×0.8×0.8 mm 3 , TR=2937 ms, 8 dynamics. 
       FIG. 17  illustrates maps of corresponding processed wavelength (in mm), dynamic shear modulus (G d  in kPa), and loss shear modulus (G 1  in kPa) are given with the corresponding average magnitude image as a reference (bottom row). 
       FIG. 18  schematically illustrates a method for a characterizing at least one region of the body and/or organ and/or tissue of a human or animal subject according to the invention method. 
     The method  1800  of  FIG. 18  comprises a step  1802  for generating a pressure wave. 
     The generated pressure wave is guided from generating means to the body of the subject in a gaseous medium at step  1804 . This pressure wave generates mechanical displacements in the subjects body in the targeted region, organ and/or tissue. 
     The targeted region, organ and/or tissue is imaged with magnetic resonance imaging means at step  1806 . 
     The taken images are then analyzed at step  1808  to realize a displacement mapping of the targeted region, organ and/or tissue. 
     The displacement mapping is then analysed at step  1810  to determine tissue anisotropy or fibre orientation in/of/around the targeted region, organ and/or tissue. 
     The displacement mapping and tissue anisotropy in/of/around the targeted region is analysed in step  1810  to determine the anisotropic mechanical properties of the targeted region, organ and/or tissue. 
       FIG. 19  illustrates, in the acquired central slice of a rat brain, the dependence of the total wave amplitude and the wavelength with respect to the excitation frequency at 331 Hz, 425 Hz, and 521 Hz with field of view=20×20×17 mm 3 , voxel=0.8×0.8×0.8 mm 3 , TR=2937 ms, 8 dynamics. As expected, the total wave amplitude and the wavelength decrease with the frequency. 
       FIGS. 20-22  illustrate the results obtained in the brain of six rats excited at 521 Hz thanks to the invention. In  FIGS. 20-22  bimodal Gaussian fits to the data are added for visualization of the distributions. 
     More particularly,  FIG. 20  illustrates the reproducibility of the distribution of wavelength obtained in the brain of six rats excited at 521 Hz,  FIG. 21  illustrates the reproducibility of the distribution of shear storage modulus (kPa) obtained in the brain of six rats excited at 521 Hz, and  FIG. 22  illustrates the reproducibility of the distribution of shear loss modulus (kPa) in the brain of six rats excited at 521 Hz. 
       FIGS. 23 to 26  illustrate the results obtained thanks to the present invention on the brain of a human subject at 43 Hz and 113 Hz. 
       FIG. 23  illustrates the displacement amplitudes given along the three motion encoded directions (AX, AY, AZ) as well as the resulting total amplitude (Atot) in μm for seven over 43 acquired slices in a full brain MRE acquisition. The corresponding average magnitude image is also given for reference (bottom row) in arbitrary units with the following parameters: field of view=154×264×118 mm 3 , voxel=2.75×2.75×2.75 mm 3 , f=43 Hz. 
       FIG. 24  illustrates the maps of corresponding processed wavelength (in mm), dynamic shear modulus (G d  in kPa), and loss shear modulus (G 1  in kPa) given with the corresponding average magnitude image as a morphological reference (bottom row) with the following parameters: field of view=154×264×118 mm 3 , voxel=2.75×2.75×2.75 mm 3 , f=43 Hz. 
       FIG. 25  illustrates the displacement amplitudes given along the three motion encoded directions (AX, AY, AZ) as well as the resulting total amplitude (Atot) in μm for seven over 43 acquired slices in a full brain MRE acquisition. The corresponding average magnitude image is also given for reference (bottom row) in arbitrary units, with the following parameters: Field of view=154×264×118 mm 3 , voxel=2.75×2.75×2.75 mm 3 , f=113 Hz.  FIG. 26  illustrates the maps of corresponding processed wavelength (in mm), dynamic shear modulus (G d  in kPa), and loss shear modulus (G 1  in kPa) given with the corresponding average magnitude image as a morphological reference (bottom row) with the following parameters: field of view=154×264×118 mm 3 , voxel=2.75×2.75×2.75 mm 3 , f=113 Hz. 
       FIGS. 27 and 28  illustrate the results obtained with mouth-throat MRE acquisition in humans with guided pressure wave according to the invention. 
       FIG. 27  illustrates the displacement amplitudes given along the three motion encoded directions (AX, AY, AZ) as well as the resulting total amplitude (Atot) in μm for six over 28 acquired slices in a full mouth-throat MRE acquisition, with the following parameters: field of view=112×256×56 mm 3 , voxel=2×2×2 mm 3 , f=109 Hz. The corresponding average magnitude image is also given for reference (bottom row) in arbitrary units.  FIG. 27  illustrates the maps of corresponding processed wavelength (in mm), dynamic shear modulus (G d  in kPa), and loss shear modulus (G 1  in kPa) given with the corresponding average magnitude image as a morphological reference (bottom row). 
       FIGS. 29 and 30  illustrate the results obtained with hyperpolarized helium-3 MRE in rat lungs according to the invention, with the following parameters: field of view=80×40×30 mm 3 , voxel=1.25×1.25×1.25 mm 3  and f=290 Hz. 
       FIG. 29  illustrates the displacement amplitudes given along the three motion encoded directions (AX, AY, AZ) as well as the resulting total amplitude (Atot) in μm for four over 20 acquired slices in a full lung hyperpolarized helium-3 MRE acquisition. The corresponding average magnitude image is also given for reference (bottom row) in arbitrary units.  FIG. 30  illustrates the maps of corresponding processed wavelength (in mm), dynamic shear modulus (G d  in kPa), and loss shear modulus (G 1  in kPa) given with the corresponding average magnitude image as a morphological reference (bottom row). 
     The present invention may be applied to the following organs, tissues or parts of a subject&#39;s body: eyes, face, brain, neck, airways, lung, heart, prostate, breast, liver, abdomen, etc. 
     While the invention has been particularly shown and described mainly with reference to preferred embodiments, it will be understood that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.