Abstract:
A magnetic resonance elastography (“MRE”) driver that can produce shear waves in a subject without relying on mode conversion of longitudinal waves is disclosed. More specifically, the MRE driver includes a pneumatic driver located remotely from a magnetic resonance imaging (“MRI”) system which is operable in response to an applied electrical current to oscillate, a pressure-activated driver that is positioned on a subject in the MRI system, and a tube that is in fluid communication, at one end, with the pneumatic driver. The pressure-activated driver includes a base plate and a driver plate having a region between them that receives the tube. Oscillations of the pneumatic driver produce a pressure wave in the tube that causes the driver plate to vibrate. The driver plate rests against the subject of interest to apply a corresponding shear oscillatory force to the subject during the MRE examination.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/229,776, filed on Jul. 30, 2009, and entitled “Shear Mode Pressure-Activated Driver for Magnetic Resonance Elastography.” 
     
    
     FIELD OF THE INVENTION 
       [0002]    The field of the invention is magnetic resonance imaging (“MRI”) systems and methods. More particularly, the invention relates to drivers for use in magnetic resonance elastography (“MRE”). 
       BACKGROUND OF THE INVENTION 
       [0003]    Magnetic resonance imaging (“MRI”) uses the nuclear magnetic resonance (“NMR”) phenomenon to produce images. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B 0 ), the individual magnetic moments of the nuclei in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B 1 ) that is in the x-y plane and that is near the Larmor frequency, the net aligned moment, M z , may be rotated, or “tipped,” into the x-y plane to produce a net transverse magnetic moment M xy . A signal is emitted by the excited nuclei or “spins,” after the excitation signal B 1  is terminated, and this signal may be received and processed to form an image. 
         [0004]    When utilizing these “MR” signals to produce images, magnetic field gradients (G x , G y , and G z ) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. 
         [0005]    The measurement cycle used to acquire each MR signal is performed under the direction of a pulse sequence produced by a pulse sequencer. Clinically available MRI systems store a library of such pulse sequences that can be prescribed to meet the needs of many different clinical applications. Research MRI systems include a library of clinically-proven pulse sequences and they also enable the development of new pulse sequences. 
         [0006]    The MR signals acquired with an MRI system are signal samples of the subject of the examination in Fourier space, or what is often referred to in the art as “k-space.” Each MR measurement cycle, or pulse sequence, typically samples a portion of k-space along a sampling trajectory characteristic of that pulse sequence. Most pulse sequences sample k-space in a raster scan-like pattern sometimes referred to as a “spin-warp,” a “Fourier,” a “rectilinear,” or a “Cartesian” scan. The spin-warp scan technique employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of MR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (“2DFT”), for example, spatial information is encoded in one direction by applying a phase encoding gradient, G y , along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient, G x , in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse, G y , is incremented, ΔG y , in the sequence of measurement cycles, or “views” that are acquired during the scan to produce a set of k-space MR data from which an entire image can be reconstructed. 
         [0007]    There are many other k-space sampling patterns used by MRI systems. These include “radial”, or “projection reconstruction” scans in which k-space is sampled as a set of radial sampling trajectories extending from the center of k-space. The pulse sequences for a radial scan are characterized by the lack of a phase encoding gradient and the presence of a readout gradient that changes direction from one pulse sequence view to the next. There are also many k-space sampling methods that are closely related to the radial scan and that sample along a curved k-space sampling trajectory rather than the straight line radial trajectory. 
         [0008]    An image is reconstructed from the acquired k-space data by transforming the k-space data set to an image space data set. There are many different methods for performing this task and the method used is often determined by the technique used to acquire the k-space data. With a Cartesian grid of k-space data that results from a 2D or 3D spin-warp acquisition, for example, the most common reconstruction method used is an inverse Fourier transformation (“2DFT” or “3DFT”) along each of the 2 or 3 axes of the data set. With a radial k-space data set and its variations, the most common reconstruction method includes “regridding” the k-space samples to create a Cartesian grid of k-space samples and then performing a 2DFT or 3DFT on the regridded k-space data set. In the alternative, a radial k-space data set can also be transformed to Radon space by performing a 1DFT of each radial projection view and then transforming the Radon space data set to image space by performing a filtered backprojection. 
         [0009]    It has been found that MR imaging can be enhanced when an oscillating stress is applied to the object being imaged in a method called MR elastography (“MRE”). The method requires that the oscillating stress produce shear waves that propagate through the organ, or tissues to be imaged. These shear waves alter the phase of the MR signals, and from this the mechanical properties of the subject can be determined. In many applications, the production of shear waves in the tissues is merely a matter of physically vibrating the surface of the subject with an electromechanical device such as that disclosed in U.S. Pat. No. 5,592,085. Shear waves may also be produced in the breast and prostate by direct contact with the oscillatory device. Also, with organs like the liver, the oscillatory force can be directly applied by means of an applicator that is inserted into the organ. 
         [0010]    A number of driver devices have been developed to produce the oscillatory force needed to practice MRE. As disclosed in U.S. Pat. Nos. 5,977,770; 5,952,828; 6,037,774 and 6,486,669, these typically include a coil of wire through which an oscillating current flows. This coil is oriented in the polarizing field of the MRI system such that it interacts with the polarizing field to produce an oscillating force. This force may be conveyed to the subject being imaged by any number of different mechanical arrangements. Such MRE drivers can produce large forces over large displacement, but they are constrained by the need to keep the coil properly aligned with respect to the polarizing magnetic field. In addition, the current flowing in the driver coil produces a magnetic field that can alter the magnetic fields during the magnetic resonance pulse sequence resulting in undesirable image artifacts. 
         [0011]    Another approach is to employ piezoelectric drivers as disclosed in, for example, U.S. Pat. Nos. 5,606,971 and 5,810,731. Such drivers do not produce troublesome disturbances in the scanner magnetic fields when operated, but they are limited in the forces they can produce, particularly at larger displacements. Piezoelectric drivers can also be oriented in any direction since they are not dependent on the polarizing magnetic field direction for proper operation. 
         [0012]    Yet another approach is to employ an acoustic driver as described in, for example, U.S. Pat. No. 7,034,534, in which the acoustic driver is located remotely from the MRI system and is acoustically coupled by a tube to a passive actuator positioned on the subject being imaged. The passive actuator does not disturb the magnetic fields and it may be positioned on the subject and oriented in any direction. 
         [0013]    Generally, pneumatic drivers generate shear waves by directly coupling the driver to the object of interest (e.g. the chest wall) or by driving a plate that is mechanically coupled to the specimen under investigation. Both techniques share a common driver: a modified audio speaker driven by a waveform generator and amplifier. Previous pneumatic drivers have included, for example, a thin cylinder with one face constructed of thick plastic and the other constructed of a thin, plastic. For example, the thin plastic face may be around one-sixteenth of an inch. A thin tube connected to the thick plastic face introduces sound waves into the cylinder that, in turn, cause the thin plastic face of the driver to vibrate. The vibrations result from the pressure wave traveling down the tube that is connected to an audio speaker. 
         [0014]    While such a configuration has been successfully used in a variety of applications, it produces shear waves in the subject under investigation by the process of mode conversion of a longitudinal wave into a shear wave. Mode conversion produces complex wave patterns due to the fact that shear waves are generated at tissue interfaces whose geometry is often complex and multilayered. Additionally, these tissue interfaces serve as multiple sources for shear wave production, resulting in constructive and destructive interference patterns that are difficult to model and separate. Additionally, longitudinal waves introduce bulk motion into the subject, which produces phase errors that affect the estimation of the shear wavelength and, hence, stiffness estimates. 
         [0015]    It would therefore be desirable to have a magnetic resonance elastography driver that produces shear waves in a subject under examination without relying on the mode conversion of longitudinal waves. By directly producing shear waves in a subject without relying on mode conversion of longitudinal waves, the complexity of the MRE inversion process would be substantially reduced, and errors introduced by the use of longitudinal waves would be mitigated. 
       SUMMARY OF THE INVENTION 
       [0016]    The present invention is a magnetic resonance elastography (“MRE”) driver that can produce shear waves in a subject without interfering with the magnetic resonance imaging (“MRI”) system and without the mode conversion of longitudinal waves. More specifically, the MRE driver includes a pneumatic driver located remotely from the MRI system that is operable in response to an applied electrical current to oscillate, a pressure-activated driver that is positioned on a subject in the MRI system, and a tube that is in fluid communication, at one end, with the pneumatic driver. The pressure-activated driver includes a base plate and a driver plate arranged in spaced relation to each other such that a region is defined therebetween to receive the elastic tube. Oscillations of the pneumatic driver produce a pressure wave in the tube that causes the driver plate in the pressure-activated driver to vibrate. The driver plate rests against the subject of interest to apply a corresponding shear oscillatory force to the subject during the MRE examination. 
         [0017]    The foregoing and other aspects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a block diagram of a magnetic resonance imaging (“MRI”) system that employs the present invention; 
           [0019]      FIG. 2  is a graphic representation of an exemplary magnetic resonance elastography (“MRE”) pulse sequence employed by the MRI system of  FIG. 1 ; 
           [0020]      FIG. 3  is a block diagram of a portion of the MRI system of  FIG. 1  showing an MRE driver array, wave generator, and amplifier assembly; 
           [0021]      FIG. 4  is a pictorial view of an exemplary pneumatic driver used in the system of  FIG. 1 ; 
           [0022]      FIG. 5A  is a graphic representation of a configuration of the MRE driver system of the present invention; 
           [0023]      FIG. 5B  is a graphic representation of alternative configuration of the MRE driver system shown in  FIG. 5A ; 
           [0024]      FIGS. 6A and 6B  are pictorial representations of an exemplary sliding tube system for adjusting a phase delay in a pressure wave conveyed to the MRE driver system of  FIGS. 5A and 5B ; 
           [0025]      FIGS. 7A and 7B  are pictorial representations of an exemplary valve system for adjusting a phase delay in a pressure wave conveyed to the MRE driver system of  FIGS. 5A and 5B ; 
           [0026]      FIG. 8  is a pictorial representation of an exemplary manifold system for adjusting a phase delay in a pressure wave conveyed to a coupled MRE driver system; 
           [0027]      FIG. 9A  is a cross-sectional view of the MRE driver system of  FIG. 5A ; and 
           [0028]      FIG. 9B  is a cross-sectional view of an alternative configuration of the MRE driver system of  FIG. 5A . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    Referring particularly to  FIG. 1 , the preferred embodiment of the invention is employed in a magnetic resonance imaging (“MRI”) system  100 . The MRI system  100  includes a workstation  102  having a display  104  and a keyboard  106 . The workstation  102  includes a processor  108 , such as a commercially available programmable machine running a commercially available operating system. The workstation  102  provides the operator interface that enables scan prescriptions to be entered into the MRI system  100 . The workstation  102  is coupled to four servers: a pulse sequence server  110 ; a data acquisition server  112 ; a data processing server  114 , and a data store server  116 . The workstation  102  and each server  110 ,  112 ,  114  and  116  are connected to communicate with each other. 
         [0030]    The pulse sequence server  110  functions in response to instructions downloaded from the workstation  102  to operate a gradient system  118  and a radiofrequency (“RF”) system  120 . Gradient waveforms necessary to perform the prescribed scan are produced and applied to the gradient system  118 , which excites gradient coils in an assembly  122  to produce the magnetic field gradients G x , G y , and G z  used for position encoding MR signals. The gradient coil assembly  122  forms part of a magnet assembly  124  that includes a polarizing magnet  126  and a whole-body RF coil  128 . 
         [0031]    RF excitation waveforms are applied to the RF coil  128 , or a separate local coil (not shown in  FIG. 1 ), by the RF system  120  to perform the prescribed magnetic resonance pulse sequence. Responsive MR signals detected by the RF coil  128 , or a separate local coil (not shown in  FIG. 1 ), are received by the RF system  120 , amplified, demodulated, filtered, and digitized under direction of commands produced by the pulse sequence server  110 . The RF system  120  includes an RF transmitter for producing a wide variety of RF pulses used in MR pulse sequences. The RF transmitter is responsive to the scan prescription and direction from the pulse sequence server  110  to produce RF pulses of the desired frequency, phase, and pulse amplitude waveform. The generated RF pulses may be applied to the whole body RF coil  128  or to one or more local coils or coil arrays (not shown in  FIG. 1 ). 
         [0032]    The RF system  120  also includes one or more RF receiver channels. Each RF receiver channel includes an RF amplifier that amplifies the MR signal received by the coil  128  to which it is connected, and a detector that detects and digitizes the I and Q quadrature components of the received MR signal. The magnitude of the received MR signal may thus be determined at any sampled point by the square root of the sum of the squares of the I and Q components: 
         [0000]        M =√{square root over ( I   2   +Q   2 )}  Eqn. (1);
 
         [0033]    and the phase of the received MR signal may also be determined: 
         [0000]    
       
         
           
             
               
                 
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                   Eqn 
                   . 
                   
                       
                   
                    
                   
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         [0034]    The pulse sequence server  110  also optionally receives patient data from a physiological acquisition controller  130 . The controller  130  receives signals from a number of different sensors connected to the patient, such as electrocardiograph (“ECG”) signals from electrodes, or respiratory signals from a bellows or other respiratory monitoring device. Such signals are typically used by the pulse sequence server  110  to synchronize, or “gate,” the performance of the scan with the subject&#39;s heart beat or respiration. 
         [0035]    The pulse sequence server  110  also connects to a scan room interface circuit  132  that receives signals from various sensors associated with the condition of the patient and the magnet system. It is also through the scan room interface circuit  132  that a patient positioning system  134  receives commands to move the patient to desired positions during the scan. 
         [0036]    The digitized MR signal samples produced by the RF system  120  are received by the data acquisition server  112 . The data acquisition server  112  operates in response to instructions downloaded from the workstation  102  to receive the real-time MR data and provide buffer storage, such that no data is lost by data overrun. In some scans, the data acquisition server  112  does little more than pass the acquired MR data to the data processor server  114 . However, in scans that require information derived from acquired MR data to control the further performance of the scan, the data acquisition server  112  is programmed to produce such information and convey it to the pulse sequence server  110 . For example, during prescans, MR data is acquired and used to calibrate the pulse sequence performed by the pulse sequence server  110 . Also, navigator signals may be acquired during a scan and used to adjust the operating parameters of the RF system  120  or the gradient system  118 , or to control the view order in which k-space is sampled. The data acquisition server  112  may also be employed to process MR signals used to detect the arrival of contrast agent in a magnetic resonance angiography (“MRA”) scan. In all these examples, the data acquisition server  112  acquires MR data and processes it in real-time to produce information that is used to control the scan. 
         [0037]    The data processing server  114  receives MR data from the data acquisition server  112  and processes it in accordance with instructions downloaded from the workstation  102 . Such processing may include, for example: Fourier transformation of raw k-space MR data to produce two or three-dimensional images; the application of filters to a reconstructed image; the performance of a backprojection image reconstruction of acquired MR data; the generation of functional MR images; and the calculation of motion or flow images. 
         [0038]    Images reconstructed by the data processing server  114  are conveyed back to the workstation  102  where they are stored. Real-time images are stored in a data base memory cache (not shown in  FIG. 1 ), from which they may be output to operator display  112  or a display  136  that is located near the magnet assembly  124  for use by attending physicians. Batch mode images or selected real time images are stored in a host database on disc storage  138 . When such images have been reconstructed and transferred to storage, the data processing server  114  notifies the data store server  116  on the workstation  102 . The workstation  102  may be used by an operator to archive the images, produce films, or send the images via a network to other facilities. 
         [0039]    Referring particularly to  FIG. 2 , an exemplary pulse sequence, which may be used to acquire magnetic resonance (“MR”) data according to an embodiment of the present invention, is shown. The pulse sequence is fundamentally a 2DFT pulse sequence using a gradient recalled echo. Transverse magnetization is produced by a selective 90 degree radiofrequency (“RF”) excitation pulse  200  that is produced in the presence of a slice select gradient, G z , pulse  201  and followed by a rephasing gradient pulse  202 . A phase encoding gradient, G y , pulse  204  is then applied at an amplitude and polarity determined by the view number of the acquisition. A read gradient, G x , is applied as a negative dephasing lobe  206 , followed by a positive readout gradient pulse  207 . An MR echo signal  209  is acquired 40 milliseconds after the RF excitation pulse  200  during the readout pulse  207  to frequency encode the 256 digitized samples. The pulse sequence is concluded with spoiler gradient pulses  212  and  213  along read and slice select axes, and a rephasing gradient pulse  211  is applied along the phase encoding axis (“G y -axis”). As is well known in the art, this rephasing pulse  211  has the same size and shape, but opposite polarity of the phase encoding pulse  204 . The pulse sequence is repeated 128 times with the phase encoding pulse  204  stepped through its successive values to acquire a 128-by-256 array of complex MR signal samples that comprise the data set. 
         [0040]    An alternating magnetic field gradient is applied after the transverse magnetization is produced and before the MR signal is acquired. In the pulse sequence illustrated in  FIG. 2 , the read gradient, G x , is used for this function and is alternated in polarity to produce bipolar, gradient waveforms  215 . The frequency of the alternating gradient  215  is set to the same frequency used to drive the MRE transducer, and it typically has a duration of 25 milliseconds. At the same time, the pulse sequence server  110  produces sync pulses as shown at  217 , which have the same frequency as and have a specific phase relationship with respect to the alternating gradient pulses  215 . These sync pulses  217  are used to produce the drive signals for the magnetic resonance elastography (“MRE”) transducer to apply an oscillating stress  219  to the patient. To insure that the resulting waves have time to propagate throughout the field of view, the sync pulses  217  may be turned on well before the pulse sequence begins, as shown in  FIG. 2 . 
         [0041]    The phase of the MR signal  209  is indicative of the movement of the spins. If the spins are stationary, the phase of the MR signal is not altered by the alternating gradient pulses  215 , whereas spins moving along the read gradient axis (“G x -axis”) will accumulate a phase proportional to their velocity. Spins which move in synchronism and in phase with the alternating magnetic field gradient  215  will accumulate maximum phase of one polarity, and those which move in synchronism, but 180 degrees out of phase with the alternating magnetic field gradient  215  will accumulate maximum phase of the opposite polarity. The phase of the acquired MR signal  209  is thus affected by the “synchronous” movement of spins along the G x -axis. 
         [0042]    The pulse sequence in  FIG. 2  can be modified to measure synchronous spin movement along the other gradient axes (G y  and G z ). For example, the alternating magnetic field gradient pulses may be applied along the phase encoding axis (“G y -axis”) as indicated by dashed lines  221 , or they may be applied along the slice select axis (“G z -axis”) as indicated by dashed lines  222 . Indeed, they may be applied simultaneously to two or three of the gradient field directions to “read” synchronous spin movements along any desired direction. 
         [0043]    MRE may be implemented using most types of MR imaging pulse sequences. Gradient echo sequences can be readily modified to incorporate the alternating gradient as illustrated in the above-described embodiment. In some cases, however, the characteristics of a gradient echo sequence may not be ideal for a particular application of the technique. For example, some tissues (such as those with many interfaces between materials with dissimilar magnetic susceptibilities) may have a relatively short T 2 * relaxation time and, therefore, may not provide enough signal to obtain a noise-free image at the required echo delay time. In this setting, a spin echo implementation of the invention may be ideal, because for a given echo delay time (“TE”), this pulse sequence is much less sensitive to susceptibility effects than a gradient echo sequence. When a spin echo pulse sequence is used, the alternating magnetic field gradient can be applied either before and/or after the 180 degree RF inversion pulse. However, if the alternating gradient is applied both before and after the RF inversion pulse, the phase of the alternating magnetic field gradient must be inverted 180 degrees after the RF inversion pulse in order to properly accumulate phase. 
         [0044]    The physical properties of tissue are measured using MRE by applying a stress and observing the resulting strain. For example a tension, pressure, or shear is applied to a subject and the resulting elongation, compression, or rotation is observed. By measuring the resulting strain, elastic properties of the tissue such as Young&#39;s modulus, Poisson&#39;s ratio, shear modulus, and bulk modulus can be calculated. Moreover, by applying the stress in all three dimensions and measuring the resulting strain, the elastic properties of the tissue can be completely defined. 
         [0045]    The attenuation of the strain wave can be estimated by observing the rate at which the strain decreases as a function of distance from the stress producing source. From this, the viscous properties of the gyromagnetic medium may be estimated. The dispersion characteristics of the medium can be estimated by observing the speed and attenuation of the strain waves as a function of their frequency. Dispersion is potentially a very important parameter for characterizing tissues in medical imaging applications. 
         [0046]    Referring to  FIG. 3 , the present invention is an MRE driver that may be placed on the subject  300  and energized to produce an oscillating stress. It includes a pressure-activated driver  302 , which is positioned over the region of interest in the subject  300  and is connected by means of an inlet tube  304  to a remotely located pneumatic driver assembly  306 . The pneumatic driver assembly  306  is remote from the bore  150  of the magnet assembly  124  in the sense that it is away from the strong magnet fields produced by the magnet assembly  124  where its operation is not impeded by those fields, and where its operation will not perturb the MRI system magnetic fields. The pneumatic driver assembly  306  is electrically driven by a waveform generator and amplifier  308 , which in turn is controlled by the pulse sequence server  110  in the MRI system control  310 . The MRI system control  310  directs the MRI system to perform an MRE scan by driving the RF coil  128 , and the gradient coils  122  in the magnet assembly  124  to perform a series of pulse sequences, while enabling the waveform generator  308  at the proper moment during each pulse sequence to apply an oscillatory stress to the subject  300 . 
         [0047]    Referring particularly to  FIG. 4 , the pneumatic driver assembly  306  includes a loudspeaker  400  mounted on one side of a thin enclosure  402 . An exemplary loudspeaker  400  is a speaker, such as a 15 inch speaker manufactured by Resonant Engineering and sold as Model SE15. Such a speaker has a resonant frequency of 30 hertz (“Hz”) and can handle 1000 watts (“W”) peak power or 600 W root-mean-square (“rms”). The enclosure  402  is constructed of a rigid material such as polycarbonate, and is, for example, a rectangular enclosure having dimensions of 18 inches by 18 inches by 1.5 inches. The enclosure  402  large opening  404  wall  406  and a flange  408  on the loudspeaker  400  fasten together such that the speaker  400  directs acoustic energy directly into the enclosure  402 . 
         [0048]    One end of the tube  304  connects to an opposing wall  410  of enclosure  402  and is in fluid communication with its interior by an output opening  412 . As a result, the acoustic energy produced by the loudspeaker  400  is directly coupled to one end of the inlet tube  304  through the thin enclosure  402 . 
         [0049]    The inlet tube  304  is made of a material which is flexible, but which is not elastic. The inlet tube  304  is non-elastic such that it does not stretch in response to the variations in air pressure caused by the acoustic energy it conveys. The flexibility enables it to be fed along a winding path between the subject in the magnet and the remote site of the pneumatic driver assembly  306 . For example, the inlet tube  304  may be twenty feet long and have an inside diameter of one inch. Exemplary such inlet tubes  304  are made of a vinyl material sold under the trademark “TYGON” (Saint-Gobain Corporation, La Défense, France) and have a wall thickness of approximately one-eighth inch. As a result, the acoustic energy is efficiently conveyed from the driver assembly  306  to the pressure-activated driver  302 . 
         [0050]    Referring now to  FIG. 5A  a graphic representation of an embodiment of the pressure-activated MRE driver system of the present invention is shown. Such embodiment will be discussed in brief herein, and in more detail below. The embodiment shown in  FIG. 5A  includes a single tube  512 , a base plate (not shown), a moveable plate  504 , and recoil system (not shown).  FIG. 5B  shows an alternative configuration, in which a second tube  514  is introduced that is 180 degrees out of phase from the first tube  512 . The phase delay produced when employing this second tube  514  provides an ability to double the shear wave amplitude by providing force on the top plate during both positive and negative phases of a sinusoidal pressure waveform. In  FIG. 5B , it is also possible to change the phase delay by modifying the effective length of the tube  512 , thereby providing the generation of beat modes. Additionally, other means for altering the phase delay may be employed, including adding a sliding “trombone” type tube system, and a valve or shunt system that effectively increases or decreases the length of the tube  512 . For example, a sliding tube system or a valve system can be added to the inlet tube  304  so that the effective length of the tube  512  can be adjusted. Alternatively, separate pneumatic drivers could be employed, and driven such that there is a phase delay between the two sources. 
         [0051]    An exemplary sliding tube system defining a pressure wave path is illustrated in  FIGS. 6A and 6B . The sliding tube system  600  includes a first tube portion  602  extending along an axis of extension  604  and a second tube portion  606  also extending along the axis of extension  604 . The first and second tube portions,  602  and  606 , are slidably positioned with respect to each other such that either can be slid along the direction of the axis of extension  604  while maintaining a fluidly sealed interior  608  therebetween. A cylindrical seal  610  interposed between the first and second tube portions,  602  and  606 , engages a flange  612  on the second tube portion  606 , thereby preventing the first and second tube portions,  602  and  606 , from disengaging each other. By sliding the first and second tube portions,  602  and  606 , with respect to each other, the effective length of the tube  512  is increased or decreased, thereby providing a phase delay in the conveyance of the pressure wave in the tube system  600  to the driver system. 
         [0052]    An exemplary valve system is illustrated in  FIGS. 7A and 7B . The valve system  700  acts as an effective by-pass that creates two path lengths for the sound wave. The valve system includes a first tube  702  defining a first wave path, such as a short path, and a second tube  704  defining a second wave path, such as a long path. Disposed between the first and second tube paths,  702  and  704 , are two or more valves  706 . As explained later, more than two valves  706  can be employed with multiple different tubes defining multiple different wave paths, thereby providing a configuration in which the length of the tube  512  can be effectively changed to any multiple or fraction of the wavelength of interest. When in an open position ( FIG. 7A ), the valves  706  provide a pathway, identified by dashed arrow  708 , for a pressure wave to be conveyed to the driver system along the first tube  702 . However, when the valves  706  are placed in a closed position ( FIG. 7B ), a pathway, identified by dashed arrow  710 , for a pressure wave to be conveyed to the driver system along the second tube  704  is provided. 
         [0053]    In another configuration, illustrated in  FIG. 8 , a manifold system is utilized to provide a phase delay to a pressure wave conveyed to an MRE driver system by using the manifold system to select a tube of a particular length. The manifold system  800  includes an input manifold  802  and an output manifold  804  interposed between the inlet tube  304 , for example, after the inlet line is split by a T-valve. Alternatively, the manifold system is provided to a second inlet tube attached either to the same or a different pneumatic driver as inlet tube  304 . Each manifold  802 ,  804 , connects to a plurality of tubes, such as tubes  806 ,  808 , and  810 . Together, the manifolds  802 ,  804  and the tubes,  806 ,  808 ,  810 , define a plurality of different wave paths through which a pressure wave can be conveyed to the driver system. For convenience, the tubes  806 ,  808 ,  810  may be coiled into loops and stacked concentrically, as illustrated in  FIG. 8 . 
         [0054]    Referring particularly now to  FIG. 9A , the aforementioned embodiment of the pneumatic driver system shown in  FIG. 5A  is shown in cross-section along view  9 . The MRE driver includes a base plate  502  and a moveable top plate  504 . Extending from the base plate  502  are a plurality of protrusions  506  that extend from the base plate  502  towards the moveable plate  504 . A plurality of protrusions  508  also extend from the moveable plate  504  towards the base plate  502 . The protrusions,  506  and  508 , may be ridges or a series of pegs, for example, and can be formed as continuous portions of the plates,  502  and  504 , or affixed thereto, such as by nylon screws or the like. The base plate  502  and moveable plate  504  are arranged in a spaced relation such that the protrusions,  506  and  508 , define a plurality of channels  510  configured to receive a tube  512 . 
         [0055]    By way of example, the protrusions,  506  and  508 , are configured such that the protrusions  506  extending from the base plate  502  are interleaved with the protrusions  508  extending from the moveable plate  504 . Further, the protrusions  506  extending from the base plate  502  may not be present along the full length of the base plate  502 , just as the protrusions  508  extending from the moveable plate  504  may not be present along the full length of the moveable plate  504 . In such a configuration, the plurality of channels  510  formed by the protrusions,  506  and  508 , form a single, continuous and winding passage in which the tube  512  is placed. 
         [0056]    An acoustic pressure wave traveling down the tube  512  provides a positive pressure within the tube  512  that moves the moveable plate  504  of the driver system via contact with the protrusions  508  formed thereon. For example, sinusoidal pressure waveforms within the tube  512  create shear motion of the moveable plate  504  relative to the base plate  502 . In an alternative configuration, a second tube  514  is positioned to lie in another plurality of channels  516  formed by the protrusions,  506  and  508 . In such a configuration, the second tube  514  is positioned such that it is 180 degrees out of phase with the first tube  512 . As noted above, the phase delay produced when utilizing this second tube  514  provides an opportunity to double the shear wave amplitude by providing force on the moveable plate  504  during both positive and negative phases of a sinusoidal pressure waveform. In general, it is also possible to change the phase delay by modifying the length of the tube  512 , thereby creating a configuration in which beat modes are produced. It should be appreciated that the tube  512  need not be circular in cross-section. In fact, and in the alternative, a more ovular cross section may be advantageous since such a configuration will direct the outward force of the pressure wave in a more discriminatory manner. Thus, as used herein, the term “tube” refers to any such elongated enclosure having flexible or semi-flexible walls that define a region in which a fluid can pass, whether circular in cross-section or not. 
         [0057]    The moveable plate  504  should have sufficient elastic recoil to ensure that when the pressure within the tube  512  is zero, or negative, the moveable plate  504  will return to its original position. To achieve this, a recoil system, such as, for example, elastic straps  516 , are disposed on either ends of both plates,  502  and  504 , in a neutral position. This recoil system provides a longitudinal recoil of the top plate  504  after it is displaced by a positive pressure waveform passing through the tube  512 . In addition, the recoil system is selected such that it provides a support for the weight of the moveable plate  504  and subject. For example, when the recoil system includes elastic strips  516 , the thickness of the elastic strips  516  is selected such that the elastic strips  516  also provide the desired support. 
         [0058]    Referring particularly now to  FIG. 9B , an alternative configuration of the pneumatic driver system shown in  FIG. 5A  is shown in cross-section along view  9 . This alternative configuration includes a recoil system including thinner elastic strips  518  and a set of magnetic resonance (“MR”) compatible bearings  520 . These MR compatible bearings  520  are coupled to the base plate  502 , for example at the protrusions  506 , and act to reduce friction during operation of the driver and to support the weight of the moveable plate  504  and subject. In particular, a second set of protrusions  522  are formed in the moveable plate  504  and extending toward the base plate  502 . This second set of protrusions  522  forms a plurality of spaces  524  in which the bearings  520  may be placed. 
         [0059]    It is an aspect of the present invention to provide a driver system for producing a shear stress on a subject while performing a magnetic resonance elastography (“MRE”) scan with a magnetic resonance imaging (“MRI”) system, the shear stress producing a shear wave in the subject that has a substantially doubled amplitude than produced by other configurations of the driver system. This alternative configuration includes another tube  514  that conveys a pressure wave that is 180 degrees out of phase with the pressure wave conveyed by the tube  512 . The addition of this out-of-phase pressure wave being conveyed to the protrusions  508  extending from the moveable plate  504  results in a driving force on the driver system during both positive and negative phases of a sinusoidal pressure waveform. In general, and as noted above, it is also possible to change the phase delay between the two pressure waves by modifying the length of the tube  512 , thereby creating an instance where beat modes are produced and conveyed to the protrusions  508 . 
         [0060]    The present invention has been described in terms of one or more preferred embodiments, and it should be appreciated that many equivalents, alternatives, variations, and modifications, aside from those expressly stated, are possible and within the scope of the invention.