Abstract:
A system for inducing tissue vibration for magnetic resonance elastography is described. The system includes a passive actuator component, a first hose, a second hose, and a driving component. The passive actuator component is positionable proximate to a target tissue and includes a linearly movable piston assembly enclosed in a housing. The driving component includes a fluid pumping system and is configured to alternatingly pump a fluid through the first hose and through the second hose. When fluid is pumped through the first hose, the piston assembly moves in a first linear direction and, when fluid is pumped through the second hose, the piston assembly moves in the opposite direction. The alternating linear movement of the piston assembly induces vibration in the target tissue.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application No. 61/688,235, filed on May 10, 2012 and titled “HYDRAULICALLY-POWERED SYSTEM AND METHOD FOR ACHIEVING MAGNETIC RESONANCE ELASTOGRAPHY,” the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    Embodiments of the invention relate to a non-invasive medical imaging technique, such as magnetic resonance elastography (“MRE”), used in radiology to measure stiffness of soft tissues. 
         [0003]    Current MRE technology uses an acoustic driver system, developed by radiology researchers at Mayo Clinic. Current MRE technology, however, is limited to low frequency vibrations (e.g., 100 Hz or less) because it is based on pneumatic actuation. The wavelengths from the low frequency vibrations are smaller than the dimensions of the liver. Therefore, current pneumatic systems can be used to generate stable stiffness maps for the liver, which can be used to diagnose liver diseases, such as liver fibrosis. However, the wavelengths from the lower frequency vibrations are longer than the dimensions of other organs. As a result, current pneumatic systems generating low frequency vibrations cannot be used to generate stable stiffness maps for many organs such as the heart, prostate, pancreas, spleen, eye, etc. This is because the current inversion (a mathematical process to convert wave images to a stiffness map) strategies assume that the waves are propagating in a uniform infinite medium (i.e. the wavelengths are smaller compared to the dimensions of the organs of interest). 
       SUMMARY 
       [0004]    Therefore, embodiments of the invention provide a hydraulically-powered magnetic resonance elastography (“MRE”) vibration device used in conjunction with a magnetic resonance imaging (“MRI”) scanner that uses an inversion to generate stable stiffness maps for various organs. The vibration device generates high frequency vibrations, up to approximately 1000 Hz, which non-invasively penetrate deeper into tissue than current MRE technology to identify a disease and diagnose the state of the disease for various organs of a human or an animal body. 
         [0005]    In one embodiment, the invention provides a hydraulically-powered system used in conjunction with a magnetic resonance imaging (“MRI”) device and an inversion to achieve magnetic resonance elastography (“MRE”) generated stiffness maps. The hydraulically-powered system includes an application component, a driving component, and a plurality of hoses connecting the application component to the driving component. The application component (also referred to as a passive driver, a passive device, or a passive actuator) includes a piston rod assembly, and is positioned on a surface of a body to cause biological tissues under study to vibrate synchronized with the phase of the MRI signal of the MRI device. The driving component includes a processing unit, a memory storing data and firmware executable by the processing unit, and at least one pump or a combination of a pump and a valve. The driving component is configured to operate the application component at a controlled frequency, amplitude, and phase. 
         [0006]    In another embodiment, the invention provides a system for inducing tissue vibration for magnetic resonance elastography. The system includes a passive actuator component, a first hose, a second hose, and a driving component. The passive actuator component is positionable proximate to a target tissue and includes a linearly movable piston assembly enclosed in a housing. The first hose is coupled to the passive actuator component on a first side of the piston assembly and the second hose is coupled to the passive actuator component on the opposite side of the piston assembly. The driving component includes a fluid pumping system and is configured to alternatingly pump a fluid through the first hose and through the second hose. When fluid is pumped through the first hose, the piston assembly moves in a first linear direction and, when fluid is pumped through the second hose, the piston assembly moves in the opposite direction. The alternating linear movement of the piston assembly induces vibration in the target tissue. 
         [0007]    In still another embodiment, the invention provides a method of performing magnetic resonance elastography. A passive actuator component is positioned proximate to a target tissue on a patient. The passive actuator includes a linearly movable piston assembly. A fluid is then pumping alternatingly from a driving component through a first hose and a second hose. The first hose is coupled to the passive actuator component on a first side of the piston assembly such that pumping the fluid through the first hose causes the piston assembly to move in a first linear direction. The second hose is coupled to the passive actuator component on a second side of the piston assembly (opposite the first side) such that pumping the fluid through the second hose causes the piston assembly to move in a direction opposite the first linear direction. The alternating linear movement of the piston assembly induces a vibration in the target tissue. MRI data of the target tissue is acquired while the vibration is induced and a stiffness map of the tissue is generated based on the acquired MRI data. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is an illustration of a hydraulically-powered system used in conjunction with a magnetic resonance imaging (“MRI”) device. 
           [0009]      FIG. 2  is a schematic illustration of the system of  FIG. 1  including a driving component, an application component, and a plurality of hoses connecting the components. 
           [0010]      FIG. 3  is a schematic illustration of the driving component of  FIG. 2  interfacing with the MRI device. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
         [0012]      FIG. 1  illustrates an example of a hydraulically-powered magnetic resonance elastography (“MRE”) system including an application component  101 , a driving component  103 , and a plurality of hoses  105 ,  107 ,  109  connecting the application component  101  to the driving component  103 . When a patient is placed in an MRI environment  111 , the application component  101  (also referred to as a passive driver, a passive device, or a passive actuator) is adhered to the surface of a patient&#39;s body and generates vibrations perpendicular to the tissue surface or shear vibrations along the tissue surface. To prevent interference with the MRI system, the passive driver  101  is constructed of non-metallic/MR compatible components. However, in some constructions, the passive driver  101  includes a limited number of non-ferromagnetic metallic components. 
         [0013]    The driving component  103  (also referred to as an active driver) includes pump mechanisms for driving the hydraulic system. As some of these components may be constructed of metal (including ferromagnetic metals), the driving component  103  is positioned outside of the MRI environment/scanning room. As described in detail below, the driving component  103  operates a hydraulic pumping system to control the frequency, displacement amplitude, and phase of the passive driver  101 . 
         [0014]    The system of  FIG. 1  undergoes a three-stage process to produce spatial stiffness maps that estimates stiffness of biological tissues. First, the application component  101  is adhered to the surface of a human body and the driving component  103  causes the application component  101  to vibrate thereby inducing vibration of the biological tissues under study at a controlled frequency, amplitude, and phase. The MRI scanning system  111  is then used to capture data indicative of the transmitted waves in the region of interest (“ROI”). The wave/vibration data captured by the MRI scanning system  111  is then converted to spatial stiffness maps using a mathematical process called inversion. As described further below, the operation of the driving component  103  is coordinated with the phase of the MRI signal of the MRI scanning system  111 . For example, the phase of the tissue vibration is synchronized with the phase of the MRI device to obtain optimal imaging. In some constructions, the MRE system can be used to control when to start imaging with the MRI scanner or vice-versa. 
         [0015]      FIG. 2  illustrates the operational components of the hydraulically powered-vibration system in further detail. The driving component  103  includes at least one pump  201  or a combination of a pump and a valve system to provide flow and pressure of a liquid through the plurality of hoses  105 ,  107 ,  109  connected to the passive driver  101 . The fluid pumped by the driving component  103  into the passive driver  101  causes a piston  203  to move back and forth periodically to induce vibration of the passive driver  101 . The pump  201  of the driving component  103  forces fluid into the passive driver  101  through a first hose  105 . The increased pressure on one side of the piston  203  causes the piston to move in a first direction (downward in the example of  FIG. 2 ). At the same time, the fluid pump system  201  of the driving component  103  allows fluid on the opposite side of the piston  203  to drain through the second hose  109  as the piston moves. 
         [0016]    The passive driver  101  is equipped with a fiber-optic displacement transducer  205  that measures the position of the piston  203  and provides feedback to the processing unit  207  of the driving component  103 . Once the piston  203  reaches a defined displacement, the fluid pump system  201  forces fluid into the passive driver  101  through the second hose  109  and allows fluid to drain through the first hose  105 . As a result, the piston  203  is moved in the opposite direction (upward in the example of  FIG. 2 ). Although the example of  FIG. 2  includes a displacement sensor  205  that is used to control the operation of the fluid pump system  201 , other constructions can utilize other types of sensors to control the operation of the fluid pump system  201 . For example, a pressure transducer can be configured to measure the difference in pressure between the first hose  105  and the second hose  109 . The fluid pumping system  201  would then be controlled based on these measurements. 
         [0017]    The processing unit  207  of the driving component  103  controls the amplitude of the vibration induced through the passive driver  101  by monitoring the displacement of the piston  203  and causing the fluid pump system  201  to reverse the direction of piston movement when a desired amplitude is reached. The frequency of the vibration is controlled by regulating the speed at which the fluid pump  201  forces the liquid into the passive driver  101 . 
         [0018]    In some embodiments, the fluid pump system  201  of the driving component  103  includes a conventional hydraulic pump that provides consistent flow and pressure to a four-way electro-hydraulic servo valve (“EHSV”) to generate a controlled displacement waveform at the application component  101 . The valve is electronically controlled by the processing unit to open in alternating directions of flow to send pressurized hydraulic fluid through either the first hose  105  or the second hose  109  to either side of the piston. In some embodiments, the EHSV includes a conventional nozzle flapper-type electro-hydraulic servo valve. In other constructions, the valve is a voice-coil system. A conventional pump that supplies consistent flow and pressure to piezoelectric liquid valves or a modified pump that supplies timed pulses of flow can also be used to generate a controlled displacement waveform at the application component. 
         [0019]    As discussed above, the application component  103 , shown in  FIG. 2 , converts supplied hydraulic flow and pressure into displacement of a moveable surface to cause tissue vibration. The application component may take on various embodiments based on established technologies known to those skilled in the art. These include axial hydraulic actuators, such as a cylinder-piston-rod assembly, or chamber-diaphragm-rod assembly types. Other means of actuation, such as rotary actuators or hydraulic motors, could also be used to devise other embodiments of the application component. In the embodiment shown in  FIG. 2 , the application component comprises a cylinder with a piston and double-rod assembly. The rod is driven under hydraulic power by the piston such that it reciprocates in a fully-reversed linear motion. The rod, in turn, drives the part of the application component that articulates with the patient to generate a vibrational effect at the surface of the patient&#39;s body. It should be clear to those familiar with hydraulic technologies and skilled in the art that this vibrational effect could be generated by hydraulic devices of various constructions and designs. As noted above, the application component  101  in this example is non-metallic (e.g., includes plastic components), which makes it MR compatible. However, in some embodiments, the application component  101  includes at least some metallic components. 
         [0020]    As described above, a plurality of hoses distributes a non-compressible liquid between the passive actuator  101  and the driving component  103 . Pressurized hydraulic fluid supplied at the first hose  105  moves the piston and rod assembly  203  in the first direction (e.g., downward). Pressurized hydraulic fluid supplied at the second hose  109  moves the piston and rod assembly  203  in the opposite direction (e.g., upward). This motion is transferred to the surface of the passive actuator  101  to generate tissue vibrations. A third hose  107  is a low pressure return hose that allows leakage flow to return to a fluid reservoir of the driving component  103 . The return hose  107  bleeds air from the lines and the cylinder internal volumes. 
         [0021]    By using a virtually incompressible media (e.g., liquid) to drive the passive actuator  101 , the hydraulically-powered system provides many advantages over pneumatic means of generating tissue vibration. For example, pneumatic systems are limited by gas (e.g., air) compliance to a frequency on an order of  100  Hz or less. This frequency limitation limits the resolution of the MRE to the imaging of smaller tissue structures. A liquid fluid means of driving the application head does not have this limitation as the media used to convey flow and pressure has negligible compliance. Therefore, higher transmitted frequencies are possible using the hydraulically-powered vibration device. 
         [0022]    In addition, because of the virtual incompressibility of liquids, the performance of the system using the hydraulically-powered vibration device can be predicted with sufficient accuracy to allow the phase of the tissue vibrations to be adjusted to the phase of the applied MRI, which provides optimal imaging. The use of virtually incompressible media also makes it possible to generate higher forces that overcome attenuation of the transmitted energy, which results in the delivery of higher energies to the tissue of interest. Furthermore, because higher power can be transmitted by liquid fluidic means, flexibility in the design of the passive actuator  101  is accommodated. In particular, passive actuators can be implemented that transmit either longitudinal vibrations (i.e., perpendicular to the tissue surface) or shear vibrations (i.e., along the tissue surface). In general, the hydraulically-powered vibration device provides higher-frequency, phase-tuned tissue vibration that provides not only greater imaging resolution due to higher frequency vibration but also better clarity due to phase control. 
         [0023]    In some embodiments, the fluid used in the vibration device can be doped with a contrast agent (e.g., super paramagnetic iron oxide) to suppress a signal provided by the fluid in the MRI scanner that can create possible artifacts in the resulting images. Similarly, the field of view can be limited by avoiding the driver or saturation bands that dephase the signal from the fluid to prevent these artifacts. Also, in some embodiments, the passive actuator can be flexible, and can be properly sealed to prevent any fluid leaks. 
         [0024]    To further optimize the quality of vibration data acquired by the MRI scanning system  111 , the driving component  103  is configured to communication (bidirectional or unidirectional) with the controller of the MRI scanning system  111  as illustrated in  FIG. 3 . The driving component  103  includes a processing unit (such as a microcontroller) and a memory storing executable instructions and data that, when executed by the processing unit, cause the driving component to operate the fluid pump system and communicate with the MRI scanning system  111 . The MRI scanning system also includes a processing unit  303  and a memory  305 . 
         [0025]    The communication between the driving component  103  and the MRI scanning system  111  allows the vibration to be coordinated with and paced by the pulse sequencing of the MRI scanning system  111  or vice versa. As discussed above, the frequency and amplitude of the induced tissue vibration can be controlled by adjusting the speed at which fluid is pumped into the passive actuator  101  and the desired displacement of the piston and rod assembly  203 , respectively. Conversely, in some constructions, the pulse sequencing of the MRI system  111  is controlled based on the frequency and amplitude of the vibrations caused by the driving component  103 . 
         [0026]    Thus, the invention provides, among other things, a hydraulic system for inducing vibrations in target tissue so that spatial stiffness maps can be generated using magnetic resonance elastography. Various features and advantages of the invention are set forth in the following claims.