Abstract:
An array of two or more piezoelectric drivers generates shear waves in a region of interest of a human undergoing a MRE test. The use of the array of drivers allows for better diagnosis of disease of the humans or animals.

Description:
TECHNICAL FIELD 
       [0001]    This application relates in general to magnetic resonance elastographic systems, and in specific to systems and methods that use an array of piezoelectric drivers in magnetic resonance elastographic systems. 
       BACKGROUND 
       [0002]    Magnetic Resonance Elastography (MRE) is an MRI-based method for imaging the mechanical properties of tissue. The technique is used to depict the spatial distribution of tension in skeletal muscle, brain tissue, breast tissue, liver tissue, prostate tissue, etc. In this technique, a driver, e.g. pneumatic or electromechanical driver, is used to generate shear waves in a region of interest, such as brain, breast, liver, prostate, etc. of a human subject, while the human subject is located in a magnetic resonance imaging (MRI) system. In some instances, shear waves are generated by applying mechanical motion to the surface of the region of interest of the human subject. A mechanical actuator is coupled to the human subject, and provides cyclic motion that is synchronized to the MRI imaging sequence. Another way to generate shear waves in the tissue is to use a piezoelectric bending element. In other instances, a needle is inserted into the tissue of the animal or human subject, and the waves are generated by vibrating the needle. For more information about piezoelectric drivers, see Chan, Q. C. C. et al., “Localized Application of Shear Waves to Tissues for MR Elastography via a Needle Device,” Proceedings of the 13 th  ISMRM, Florida, USA May 7-13, 2005; Chan, C. C., et al., “Shear Waves Induced by Moving Needle in MR Elastography, Proceedings of the 26 th  Annual International Conference of the IEEE EMBS, San Francisco, Calif. USA, Sep. 1-5, 2004, pg. 1-3; Chan, Q. C. C., et al. “Needle Shear Wave Driver for Magnetic Resonance Elastography,” Magnetic Resonance in Medicine 55:1175-1179 (2006); Chen, Jun, et al., “Imaging Mechanical Shear Waves Induced by Piezoelectric Ceramics in Magnetic Resonance Elastography,” http://scholar.ilib.cn/Abstract.aspx?A=kxtb-e200606016, (downloaded Jun. 19, 2008); the disclosures of which are hereby incorporated herein by reference. 
       BRIEF SUMMARY 
       [0003]    Various embodiments as described herein may be used to improve the operations of MRE systems. Devices, systems, and methods described herein may lead to improved medical care of humans and also animals. Embodiments of the invention involve the use of an array of two or more piezoelectric drivers to generate shear waves in a region of interest of a human subject undergoing a MRE test. 
         [0004]    One embodiment of the invention involves a phased array driver for a magnetic resonance elastography system comprising: a first driver having a piezoelectric element that comprises a MRI compatible piezoelectric material; a second driver having a piezoelectric element that comprises a MRI compatible piezoelectric material; wherein the first driver and the second driver are arrayed to produce share waves in a region of interest of a human subject. 
         [0005]    Another embodiment of the invention involves a phased array driver for a magnetic resonance elastography system comprising: a first driver having a piezoelectric element that comprises a PVF2 material; a second driver having a piezoelectric element that comprises a PVF2 material, wherein the first driver and the second driver are arrayed to produce shear waves in a region of interest of a human subject. 
         [0006]    Another embodiment of the invention involves a magnetic resonance elastography system comprising: a magnetic resonance imaging (MRI) system that scans a subject; and a phased array of drivers that produce shear waves in a region of the subject from a signal, wherein each of the drivers in the array comprises a piezoelectric element having a MRI compatible piezoelectric material or PVF2 material. 
         [0007]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0009]      FIG. 1  depicts an exemplary arrangement for an MRE system, according to embodiments of the invention; 
           [0010]      FIG. 2  depicts exemplary results of a test using the system of  FIG. 1 ; 
           [0011]      FIG. 3  depicts another exemplary result of another test using the system of  FIG. 1 ; 
           [0012]      FIGS. 4A-4C  depict the components of an exemplary driver, according to embodiments of the invention; 
           [0013]      FIGS. 5A-5B  depict the components of another exemplary driver, according to embodiments of the invention; 
           [0014]      FIG. 6  depicts the components of another exemplary driver, according to embodiments of the invention; 
           [0015]      FIG. 7  depicts the components of another exemplary driver, according to embodiments of the invention; 
           [0016]      FIGS. 8A-8D  depict a comparison of the shear waves generated by a single driver and the shear wave generated by a phase array of two drivers, according to embodiments of the invention; 
           [0017]      FIGS. 9A and 9B  depict exemplary arrangements of phase array drivers, according to embodiments of the invention; 
           [0018]      FIGS. 10A and 10B  depict exemplary arrangements of phase array drivers located on a patient, according to embodiments of the invention; and 
           [0019]      FIG. 11  depicts another exemplary arrangement for an MRE system, according to embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Embodiments of the invention use one or more driver arrays to induce an oscillating stress to produce shear waves that propagate through a human to allow tissue and/or organs to be imaged. The shear waves alter the phase of the magnetic resonance signals produced by a MRI system, and from the altered phase, mechanical properties of the subject can be determined, such as the elasticity, viscosity of the tissue or organ, the density of the tissue or organ, and the size and/or shape of tissue or organ. Note that multiple tests conducted at different times, can provide changes in elasticity, density, viscosity, size, and shape over time to detect diseases at a very early stage. The information provided by MRE test(s) can be used by a practitioner, along with data from other sources, e.g. x-ray test, CT tests, ultrasound tests, PET tests, regular MRI tests, chemical tests (e.g. blood tests, etc.), to provide a more accurate diagnosis of a disease or illness of a patient at a very early stage. 
         [0021]    Data that includes the mechanical properties of the subject can allow for earlier diagnosis of diseases with increased specificity and sensitivity. The earlier and more accurate the diagnosis, the better chance of recovery for the patient. Diseases that benefit from having mechanical property data include brain diseases such as Alzheimer&#39;s disease and mild cognitive impairments, liver diseases such as cirrhosis, spleen diseases, kidney diseases such as kidney stones or tumors, pancreas diseases such as tumors, prostate diseases such as prostate carcinoma, uterine diseases such as uterine tumors, and arterial diseases such as arteriosclerosis and the like. For example, liver cirrhosis may manifest itself as a change in elasticity of the liver tissue, but not show any change in liver chemistry. Thus, detecting a change in the elasticity may lead to an earlier diagnosis and treatment of liver disease. As another example, Alzheimer disease manifests itself as a change in elasticity and density of the brain, which can be readily detected at an early stage by a MRE test. Other tests can also detect Alzheimer disease at an earlier stage, e.g. a PET scan test, however a PET scan uses radiation, which is detrimental to a patient. Any disease that manifests itself as a change in the mechanical properties of tissue or organs can be detected using embodiments of the invention. 
         [0022]    In some applications, the production of shear waves in the tissues can be accomplished by physically vibrating the surface of the subject with a pneumatic or an electromechanical device. For example, shear waves may be produced in the breast or liver or prostate by direct contact with the oscillatory driver to the surface of the human body. Also, with organs like the liver or breast, the oscillatory force can be directly applied by means of an applicator that is inserted into the organ by a needle driver. However, if possible, it is preferential to apply the force noninvasively, i.e. to the surface of the subject. 
         [0023]    The driver may comprise a piezoelectric device, which vibrates to produce the shear waves. One type is a piezoelectric material that is made especially for MRI applications. Materials for nonmagnetic bending actuators are made by Piezo Systems, Inc., 186 Massachusetts Avenue, Cambridge, Mass. 02139. Another type of piezoelectric device is uses a polyvinylidene fluoride (PVF2) membrane as the vibrating surface. Such a material is known as Pro-Wav, which is available from S. Square Enterprise Company Limited, Pro-Wave Electronics Corporation. One advantage of using the PVF2 material is that the membrane is not brittle, and is capable of conforming to different curved surfaces of the body of the patient. This provides a more accurate reading, by allowing full contact with the body of the patient, and thus better insertion of the shear waves. Another advantage of using piezoelectric drivers is that the size for the drivers are much smaller than the other types of drivers, e.g. pneumatic drivers, and thus allow for easier set up. Also the piezoelectric drivers do not suffer the power attenuation that pneumatic drivers experience, namely the air tube loses power rapidly over distance. Another advantage is that the piezoelectric drivers do not use coils which are susceptible to MRI induced eddy currents, which can produce artifacts in the images. 
         [0024]      FIG. 1  depicts an exemplary arrangement for an MRE system  100 , according to embodiments of the invention. System  100  includes a MRE driver  101 , which is a piezoelectric driver that comprises a MRI compatible piezoelectric material or a PVF2 membrane. The driver  101  is placed in contact with patient  102 , which may be a normal subject. The patient  102  with the driver is then placed into a MRI system  103 , which comprises a MRI scanner  109 . The MRI scanner  109  is controlled by MRI console  108 . The operation of the MRE system  100  produces MRE data  107 , which may be graphically viewed on a display device, not shown. The MRE driver  101  uses a signal that is produced by generator  104 , and is amplified by amplifier  105 . The oscilloscope  106  monitors the signal from the generator  104 . The signal generation of generator  104  is synchronized with the operation of the MRI system  103 . A trigger on the MRI scanner provides a signal to the generator to initiate a vibration. For example, the signal activates the generator to form ten pulses at its set frequency. 
         [0025]      FIG. 2  depicts exemplary results  200  of a test using the system of  FIG. 1 . In  FIG. 2 , the driver  101  is located on the surface of any interested region  201  of patient  102 . The driver  101  is vibrated to produce shear waves  202  in the tissue region  201 . The resulting data  200  depicts an image that shows the differences in elasticity of the region  201 . The image is formed by analysis of the MRE data produced by the test. The wave image is inverted to produce the elastogram image of the resulting data  200 . 
         [0026]      FIG. 3  depicts another exemplary results  300  of another test using the system of  FIG. 1 . In  FIG. 3 , the driver  101  is located over tissue/organ region  301  of patient  102 . In this example, the tissue region  301  includes tumors  303   ab.  The driver  101  is vibrated to produce shear waves  302  in the tissue region  201 . The resulting data  300  depicts an image that shows elasticity of the region  201 . As the shear wave passes through a tumor  303   a,  which is softer or more elastic than the surrounding region, the wave becomes shorter. As the shear wave passes through a tumor  303   b,  which is harder or less elastic than the surrounding region, the wave becomes longer. Note that the tumors have different elasticity values than the surrounding regions, and thus are readily identifiable. 
         [0027]      FIGS. 4A-4B  depict the components of an exemplary driver, according to embodiments of the invention.  FIG. 4A  depicts an exploded perspective view showing the different components of the driver.  FIG. 4B  depicts a perspective view of the driver showing the side that is placed onto the patient.  FIG. 4C  depicts a perspective view of the driver showing the side that faces away from the patient. The driver  400  includes a housing  401  that includes a fixing device  402  that connects the driver to the patient. The fixing device  402  may be Velcro™, an adhesive, a snap, or the like. The driver  400  includes mounting frame  403  that supports the piezoelectric element  404 . The element  404  may comprise a piezoelectric device composed of special made MRI compatible piezoelectric material or PVF2. The driver  400  also includes reinforcement layer  405  that supports and protects the piezoelectric element  400 , and insulation layer  406  to prevent electric current from the piezoelectric material traveling to the patient. The driver operates by receiving electricity through wires  407 . This embodiment is useful for tests involving breasts, heart, abdominal organs such as the liver, the spleen, the pancreas, a kidney, the prostate, as well as the pelvis. 
         [0028]      FIGS. 5A-5B  depict the components of another exemplary driver, according to embodiments of the invention.  FIG. 5A  depicts an exploded perspective view showing the different components of the driver.  FIG. 5B  depicts a perspective view of the driver showing the side that is placed onto the patient.  FIG. 5C  depicts a perspective view of the driver showing the side that faces away from the patient. The driver  500  includes a housing  501  that includes a fixing device  502  that connects the driver to the patient. The fixing device  502  may be Velcro™, an adhesive, a snap, or the like. The driver  500  includes the piezoelectric element  504 . The element  504  may comprise a piezoelectric device composed of a special made MRE compatible material or PVF2. The driver  500  also includes reinforcement layer  505  that supports and protects the piezoelectric element  500 , and insulation layer  506  to prevent the electric current from the piezoelectric material traveling to the patient. The driver operates by receiving electricity through wires  507 . This embodiment is useful for tests involving the head, neck, and extremities. 
         [0029]      FIG. 6  depicts the components of another exemplary driver, according to embodiments of the invention. The driver  600  includes a flexible housing  601  that includes a fixing device  602  that connects the driver to the patient. The fixing device  602  may be Velcro™, an adhesive, a snap, or the like. The driver  600  includes a PVF2 piezoelectric element within the housing. The driver operates by receiving electricity through wires  603 . This embodiment is useful for tests involving the arms or legs. 
         [0030]      FIG. 7  depicts the components of another exemplary driver, according to embodiments of the invention. The driver  700  is adapted to be used in tests involving breasts. The driver includes a flexible housing  701  that includes a fixing device  702  that connects the driver to the patient. The fixing device  702  may be Velcro™, an adhesive, a snap, or the like. The driver  700  includes two PVF2 piezoelectric elements within the housing thus allowing both breasts to be examined at the same time. 
         [0031]    The size of the drivers may be varied as needed. Some regions of a patient&#39;s body may require a larger vibration, and hence a larger driver, to produce the shear waves needed to examine the region. Some portions may be thicker or comprise tissue that is more attenuating than other regions. For example, the human brain is encased in the skull, which comprises a thick bone material. The shear waves are greatly attenuated by the skull. Thus, the vibration power needed to analyze the brain should be larger. Other regions, e.g. arms and legs, are thinner and therefore, a lower vibration power can be used. Typically, the deeper the region of interest, the greater the power should be. 
         [0032]    One embodiment of a MRE system can use a plurality of drivers in a phased array. A plurality of drivers would be located at various sites on the patient. The sites are selected according to the anatomic location of the human body to minimize interference between the waves created by the drivers and to illuminate the region of interest (ROI) wholly. The drivers may comprise MRI compatible piezoelectric materials or PVF2 material. Using a phased array of drivers increases the sensitivity of the MRE test and reduces the effects of attenuation. To reduce the wave interference induced by having multiple drivers, each driver is synchronized with the same frequency, and the same power, and triggers at the same time. 
         [0033]      FIGS. 8A-8D  depict a comparison of the shear waves generated by a single driver and the shear wave generated by a phased array of two drivers, according to embodiments of the invention. In  FIG. 8A , a single driver is used to produce the shear waves as shown. The driver  803  is arranged on the tissue as shown in  FIG. 8B . The waves produced are relatively strong near the surface, but are rapidly attenuated as the distance increases from the driver, as shown in the diagram  802 . In  FIG. 8C , an array of two drivers is used to produce the shear waves  804  as shown. The drivers  806  are arranged around the tissue as shown in  FIG. 8D . The waves produced appear to be relatively unattenuated throughout the sample, as shown in the diagram  805 . The drivers trigger at the same time, with the same power, and the same frequency, and have symmetrical locations so the shear waves constructively interfere with each other to form a stronger signal. 
         [0034]    Tests conducted on regions of the body that are relatively deep or include attenuating tissue benefit by using a phased array. The pluralities of drivers allow the shear wave to penetrate to the deeper areas, and pass through attenuating materials. The drivers of the area may be located in areas that have less attenuating materials than other regions. For example, some locations of the skull attenuate less than other areas. Knowledge of human anatomy and physiology will allow for proper placement. 
         [0035]      FIGS. 9A and 9B  depict exemplary arrangements of phase array drivers, according to embodiments of the invention.  FIG. 9A  depicts a plurality of drivers  400  of  FIGS. 4A-4C .  FIG. 9B  depicts a plurality of drivers  500  of  FIGS. 5A-5C . In each embodiment, the drivers are located on a belt that may be secured to a patient. In  FIG. 9A , only two of the four drivers will be used in a test, so the wires of the other two are disconnected. In  FIG. 9B , all four drivers are to be used, and thus all four drivers have power wires. Note that the number of drivers is by way of example only as two or more drivers may be used to form the array. Note that each driver may be shaped differently from the other drivers to accommodate different shapes, sizes and contours of patient. 
         [0036]      FIGS. 10A and 10B  depict exemplary arrangements of phase array drivers located on a patient, according to embodiments of the invention.  FIG. 11A  depicts the array of  FIG. 10A  being used on a patient. In this example, all four of the drivers are being used, and thus all four have wires to receive power.  FIG. 11B  depicts the array of  FIG. 10B  being used on a patient. In this example, only two drivers are used because the arm is small relative to other regions of the body. Note that each driver may be shaped differently from the other drivers to accommodate different shapes, sizes and contours of patient. 
         [0037]      FIG. 11  depicts another exemplary arrangement for an MRE system, according to embodiments of the invention. System  1100  includes a MRE driver  1101 , which is a piezoelectric driver that comprises a MRE compatible piezoelectric material or membrane. The driver  1101  is placed in contact with patient  1102 , which may be a normal subject. The patient  1102  with the driver is then placed into a MRI scanner  1103 . The patient  1102  with the driver  1101  and the MRI scanner  1103  are located in a shielded room  1104 . The MRI scanner  1103  is controlled by MRI console  1105 . The operation the MRE system  1100  produces MRE data  1106 , which is processed by post-processing software  1107  to produce images  1108  that may be graphically viewed on a display device  1109 . 
         [0038]    The MRE driver  1101  uses a signal that is produced by generator  1110 , and is amplified by amplifier  1111 . The oscilloscope  1112  displays the signal from the generator  1110 . The signal generation of generator  1110  is synchronized with the operation of the MRI system  1103  by signal  1114 . Typical frequencies are 60 Hz, 80 Hz, 100 Hz, or 150 Hz. The signal duration lasts through the MRE scan. 
         [0039]    This arrangement also includes an electrical-optical-electrical conversion. The driver  1101  requires an electric signal to operate. However, using metal wire to provide the signal may induce interference in the signal, because the metal wire will inductively receive EM fields generated by the MRI scanner  1103 . Thus, the scanner  1103  can interfere with the operation of the driver  1101 . The signal leaving the amplifier  1111  is converted to an optical signal by converter  1113 . Such a conversion may be accomplished by using an LED or an LED laser. The light signal is then carried on a fiber optic line to the driver  1101 . Another converter  1115  that is proximate to the driver  1101  converts the light signal back into an electrical signal. The second converter may be located next to the driver  1101  or may be integrated with the driver  1101 . The second converter may also comprise an amplifier to boost the electric signal that is being sent to the driver. The amplifier may be instead of or in addition to amplifier  1111 . Note that in this arrangement, the generator  1110 , the oscilloscope  1112 , and the amplifier  1111  comprise a single component  1115  that may be portable. 
         [0040]    Note that in this embodiment, the MRI scanner  1103  controls the activation of the signal generation  1110 . However, the MRI console  1105  gives the command to the MRI scanner  1103  to control signal generator  1110 . The generator can be controlled to change the frequency of all or some of the drivers. Thus, each of the drivers can receive the same signal frequency or may receive different signal frequencies. Note that the drivers may receive the signal frequency at the same time to have the same phase or may receive the signal at different times to have different phase. 
         [0041]    Additionally, the amplifier  1111  can be controlled to change the power of the signal being sent to all or some of the drivers. The power can be increased to all or some of the drivers. Thus, the drivers may all be operating at the same power level or may have different power levels. 
         [0042]    Note that each of the drivers in the array may be the same size or may have different sizes. Furthermore, a smaller driver located in one region may receive more power than a larger driver located in another region. Thus, the shear wave produced by the drivers may have similar wave power, because the smaller driver is receiving more power. 
         [0043]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.