Abstract:
An implantable parametric circuit enables local signal amplification and wireless transmission of RF signals in connection with magnetic resonance imaging systems. The parametric circuit detects RF signal detected during magnetic resonance imaging procedure, amplifies the detected RF signal, and transmits the amplified RF signal in a wireless manner to an external pick-up coil. The parametric amplifier is also configured to use another RF signal generated by an external source as the primary power source. As a result, implanted or catheter coils could be used as a wireless signal transducer without the need for a battery or a power connection.

Description:
BACKGROUND 
       [0001]    Magnetic resonance imaging (MRI) is a well-established medical imaging and diagnostic tool. A great deal of current activity and research relates to interventional and/or intraoperative procedures conducted under MRI guidance (iMRI). For example, in many interventional and intraoperative procedures under MRI guidance, surgical devices such as long needles, guidewires, and catheters are used and it is advantageous for a surgeon to be able to image the local tissue and locate such instruments in conjunction with the magnetic resonance image. To achieve active device profiling during real time MRI, interventional devices have been provided with a radio frequency (RF) antenna, more particularly an RF coil, in the device. 
         [0002]    The RF or receive coil is typically located at the distal end of the catheter or other device and receives a signal from excited protons of blood or tissue of its vicinity when they return to equilibrium. The RF coil then sends an electrical signal directly to the MRI scanner by way of an attached coaxial cable. The coaxial cable is typically a very thin coaxial cable that runs through a lumen in the catheter. The presence of long conductive objects, such as coaxial cables, can lead to heating in such RF coils. Medical studies indicate that this effect is due to coupling of the RF field from the MRI system, primarily to the long cable (“Reduction of Resonant RF Heating in Intravascular Catheters Using Coaxial Chokes”, Mark E. Ladd et al., Magnetic Resonance in Medicine 43:61-5-619 (2000); “RF Safety of Wires in Interventional MRI: Using a Safety Index”, Christopher J. YEUNG et al., Magnetic Resonance in Medicine 47:187-193 (2002); “RF Heating Due to Conductive Wires During MRI Depends on the Phase Distribution of the Transmit Field”, Christopher J. YEUNG et al., Magnetic Resonance in Medicine 48:1096-1098 (2002); and “Safety of MRI-Guided Endovascular Guidewire Applications”, Chia-Ying LIU et al. Journal of Magnetic Resonance Imaging 12:75-78 (2000)). These studies indicate that long transmission lines, even without the RF coil, show significant heating, whereas, RF coils without the cable show no heating. 
         [0003]    To avoid heating caused by long transmission lines, there have been attempts to wirelessly transmit MRI signals received at RF coils. In most cases, the wireless transmission of signal relies on passive inductive coupling, a process which could lead to excessive sensitivity loss if the magnitude of inductive coupling is small. To improve the detection sensitivity through inductive coupling, a low noise preamplifier is sometimes connected to the RF coil. The preamplifier is used to amplify the received RF signal prior to the wireless transmission to the receiver connected to the MRI scanner. However, most transistor based low-noise amplifiers require a local DC power source. In the case of implanted or catheter coils, it can be difficult, impracticable, and/or expensive to provide a local DC power source without a wire connection. 
       SUMMARY 
       [0004]    According to one aspect, a system is provided for improved implantable MRI compatible devices that enable wireless transmission of RF signals and that do not require a local or DC power source. 
         [0005]    According to another aspect, a system is provided for wireless amplification of an RF signal generated by a magnetic resonance imaging (MRI) device. The system includes an ingestible device that includes a pre-amplification circuit. The pre-amplification circuit includes at least one resonator that resonates at a first frequency in response to a first RF signal generated by a magnetic resonance imaging device. The at least one resonator generates an input current at the first frequency. The at least one resonator resonates at a second frequency in response to a second RF signal generated at an external frequency generator second RF signal to generate a pumping current at the second frequency. The at least one resonator resonates at a third frequency to generate a third RF signal at the third frequency. The pre-amplification circuit also includes a varactor to generate an amplified current having the third frequency based on the input current and the pumping current the amplified current. The system also includes an external coil to: receive the third RF signal; and to transmit the third RF signal to an MRI device for image processing. 
         [0006]    According to another aspect, a system is provided for wireless amplification of an RF signal generated by a magnetic resonance imaging (MRI) device. The system includes an interventional device for introduction into a subject. The interventional device includes a pre-amplification circuit. The pre-amplification circuit includes at least one resonator that resonates at a first frequency in response to a first RF signal generated by a magnetic resonance imaging device. The at least one resonator generates an input current at the first frequency. The at least one resonator resonates at a second frequency in response to a second RF signal generated at an external frequency generator second RF signal to generate a pumping current at the second frequency. The at least one resonator resonates at a third frequency to generate a third RF signal at the third frequency. The pre-amplification circuit also includes a varactor to generate an amplified current having the third frequency based on the input current and the pumping current the amplified current. The system also includes an external coil to: receive the third RF signal; and to transmit the third RF signal to an MRI device for image processing. 
         [0007]    According to another aspect, an interventional device for introduction into a subject is provided. The interventional device includes a pre-amplification circuit. The pre-amplification circuit includes at least one resonator that receives a first RF signal from a magnetic resonance imaging device. The first RF signal has a first frequency. The at least one resonator resonates at the first frequency to generate an input current at the first frequency. The at least one resonator receives a second RF signal at a second frequency from an external frequency generator and resonates at the second frequency in response to the second RF signal to generate a pumping current at the second frequency. The at least one resonator resonates at a third frequency to generate a third RF signal at the third frequency. The pre-amplification circuit also includes a varactor to generate an amplified current having the third frequency based on the input current and the pumping current the amplified current. The at least one resonator further transmits the third RF signal for image processing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a block diagram that depicts an exemplary embodiment of the wirelessly powered amplification system. 
           [0009]      FIGS. 2A-2E  depicts exemplary circuit diagrams of pre-amplification circuits according to aspect of the wirelessly powered amplification system. 
           [0010]      FIG. 2F  depicts an exemplary circuit pre-amplification circuit according to aspect of the wirelessly powered amplification system. 
           [0011]      FIG. 3  is a graph illustrating sensitivity profiles. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Medical devices, such as catheters, guidewires, endoscopes, and/or other interventional surgical devices (interventional devices), are often inserted into the body of a subject undergoing imaging analysis with a magnetic resonance imaging (MRI) system, such as a MRI scanner. The wire-like conductive structures connecting the interventional device interact with the radio-frequency fields in the MRI system, and under certain conditions an RF signal, such as electrical currents and/or voltages can be induced in these structures. The RF current induced on the connection wire can cause local heating of tissue adjacent to the interventional device during the MRI procedure, which can potentially lead to undesired tissue damage within the subject. 
         [0013]    Aspects of the wirelessly powered amplification system and method described herein minimize RF heating and enable operation of a parametric amplifier by detecting and amplifying RF signals during MRI procedure and then wirelessly transmitting the amplified RF signals for analysis. Aspects of the wirelessly powered amplification system enable operation of an amplification circuit without requiring a physical connection to a power source. For example, the parametric amplifier can be embodied in an implantable/ingestible/in vivo system that enables wireless transmitting amplified RF signals for analysis of a subject, such as a patient. 
         [0014]      FIG. 1  depicts an exemplary aspect of the wirelessly powered amplification system (WPAS)  100 . The WPAS  100  is connected to a MRI system  101  and an interventional device  102 . The WPAS  100  includes a pre-amplification circuit  104 , a RF receiver  114 , and a signal generator  108 . 
         [0015]    The MRI system  101  is, for example, a MRI scanner device that enables the visualization of organs, organ function, and/or other tissue within a body of a subject  110 , such as a patient. The MRI system  101  includes a primary magnet (not shown) that generates a uniform magnetic field that is applied across the body of the subject  110  under observation. The MRI system  101  also includes smaller gradient magnets (not shown) that allow the magnetic field to be altered very precisely. The gradient magnets allow image “slices” of the body to be created. By altering the gradient magnets, the magnetic field can be specifically encoded on a selected part of the body. 
         [0016]    After the subject  110  is properly positioned for analysis by the MRI system  101 , a body coil  112  of the MRI system  101  emits a radio frequency (RF) radiation field signal, as indicated by reference character  113 . The RF signal  113  causes the nuclei within the body of the subject  110  to change their spin orientation and precess. The frequency of the energy at which this transition occurs is known as the Larmor Frequency. When the body coil  112  is not providing the RF radiation field, the spin of hydrogen nuclei change back to a lower energy state and reemits the electromagnetic energy at the RF wavelength. 
         [0017]    During MRI analysis of the subject  110 , the interventional device  102  can be introduced into the subject  110  via, for example, a vascular structure within the body of the subject  110 . According to the illustrated aspect, the interventional device  102  is an ingestible device, such as an ingestible capsule for use in examining the gastrointestinal tract. According to another aspect, the interventional device  102  includes flexible tubing or a lumen that extends along the length of the interventional device  102 . 
         [0018]    Regardless of the device type, the interventional device  102  is configured to include the pre-amplification circuit  104 . As explained in more detail below, the pre-amplification circuit  104  includes a first resonator that is matched and tuned to the Larmor frequency of the MRI system  101  to, for example, detect the RF energy signal emitted by the spin precession of the atoms within surrounding tissue and generate an input current signal. The pre-amplification circuit  104  further amplifies the detected RF signal and then wirelessly transmits the amplified RF signal to a receiver  114 , such as external pick-up coil. 
         [0019]    According to one aspect, the receiver  114  enables viewing of the interventional device  102  within the body of a subject  110  undergoing MRI analysis. For example, the amplified RF signal, as indicated by  116 , received by the receiver  114  is transmitted to the MRI system  101  to be processed and combined in an appropriate way for imaging. 
         [0020]    For example, the MRI system  101  includes a central processing unit  118 , such as a computer, processor, or other processing device, to receive and process the amplified RF signal  116  to create images to display via a user interface  120 . The user interface  120  includes, for example, one or more input devices  122 , along with one or more displays or output devices  124 . In a typical application, the input device  122  will include a conventional operator keyboard, or other operator input devices for selecting image types, image slice orientations, configuration parameters, and so forth. The display/output device  124  will typically include a computer monitor for displaying the operator selections, as well as for viewing scanned and reconstructed images. Such devices may also include printers or other peripherals for reproducing hard copies of the reconstructed images. The central processing unit  118  is also communicatively linked with the user interface  120  to receive input data from a user and/or to provide image data to the user. 
         [0021]    According to one aspect, the central processing unit  118  processes the amplified RF signal  116  to perform 2D Fourier transforms to convert the acquired data from the time domain to the frequency domain, and to reconstruct the data into a meaningful image. The amplified RF signal  116  may indicate different resonance characteristics for different tissue types. For example, the different resonance characteristics produced from a particular organ are displayed in an image in different of shades of grey, such that some body tissues show up darker or lighter as compared to other body tissues. As another example, the different resonance characteristics produced from a particular organ can be displayed in an image as different colors or displayed as different contrast of a particular color. 
         [0022]    The pre-amplification circuit  104  is powered via an external source, such as the RF signal generator  108  that produces a pumping RF signal, as indicated by  126 , at a desired pumping frequency. The pre-amplification circuit  104  includes a second resonator that is matched and tuned to the pumping frequency to generate a pumping current signal. As explained in more detail below, the pumping current signal is used to drive a non-linear capacitor, such as a varactor, to perform frequency mixing and to produce the amplified RF signal  116  as explained in more detail below in reference to  FIG. 2 . 
         [0023]    Notably, the pre-amplification circuit  104  uses the RF pumping signal  126  as the primary power source, rather than the DC current. As a result, the pre-amplification circuit  104  enables visualization of the interventional device  102  within the subject  110  without the need for a local battery or a power connection. 
         [0024]      FIG. 2A  depicts an exemplary schematic circuit diagram of the pre-amplification circuit  104 . According to this aspect, the pre-amplification circuit  104  is implemented with a parametric amplifier  202 . The parametric amplifier  202 A includes nonlinear components, and generates the amplified RF signal  116  with a power gain by mixing an RF input signal (e.g., signal  113 ) at a lower frequency with a pumping signal (e.g., signal  126 ) at a higher frequency. In this particular aspect, the parametric amplifier  202 A is a triple frequency resonator that includes three L-C meshes  204 A,  206 A, and  208 A that are each configured to resonate at three different frequencies. Stated differently, each of the L-C meshes  204 A,  206 A, and  208 A correspond to an individual resonator. 
         [0025]    The L-C mesh  204 A includes resonating components that resonate at a desired frequency, col. For example, the resonating components such as inductor  210  and capacitor  230 , are selected so that the entire circuit  104  resonates (i.e., conducts current) at the spin precession frequency, such as 132.1 MHz. As explained above, the source of the 132.1 MHz frequency may be, for example, an RF energy signal emitted by the precessing of the atomic spins within surrounding tissue. For example, the body coil  112  of the MRI  102  excites the nuclei spins to introduce Larmor precession of spins and the resonation components of the L-C mesh  204  couples with the nuclei spins to acquire the RF input signal  113 . Thus, during the precessing of atoms an input current signal  212  is induced at the resonating components at input frequency ω1. 
         [0026]    The L-C mesh  206 A includes resonating components that resonates when receiving pumping signal  126  at a pumping frequency, ω3. For example, the L-C mesh  206 A includes as inductor  240  and capacitor  242 . The pumping frequency is provided by an external source, such as RF signal generator. During application of the pumping frequency, a pumping current  216  is induced at another resonating component, inductor  214 . The inductors  214 ,  240  and capacitor  242  are selected so that the entire circuit  104  resonates at a pumping frequency, such as 633.9 MHz. The majority of the pumping current  216  flows through a varactor  218 . The varactor  218  has a high-Q at zero biased condition, and it performs frequency mixing between the input current  212  at the ω1 frequency and the pumping current  216  at the ω3 frequency, to generate an amplified current signal  220  at the difference frequency ω2. 
         [0027]    The L-C mesh  208 A includes resonating components, such as an inductor  222  and the varactor  218 , such that the amplified current signal  220  flows through the resonating components and generates an amplified output signal (e.g., amplified RF signal  116 ). According to one aspect, the inductor  222  is a rectangular copper loop. A receiving coil  224  (e.g., receiver  114 ) can be positioned external to the body and can be configured to resonate at the difference frequency at ω2 to inductively pick-up the amplified up converted output signal and provide the amplified output signal to the MRI device for processing. According to one aspect, the parametric circuit  104  has a noise level of approximately 1.0 dB and produces amplified output signal that has a stable gain up to 27 dB. 
         [0028]      FIG. 2B  depicts another exemplary schematic circuit diagram of the pre-amplification circuit  104 . In this particular aspect, the pre-amplification circuit  104  is also implemented with the parametric amplifier  202 . In this aspect, however, the receiver coil  224  is coupled to a different portion of the parametric amplifier  202 . In particular, the receiving coil  224  (e.g., receiver  114 ) can be configured to resonate at the frequency at ω1 to inductively pick-up the amplified output signal and provide the amplified output signal  116  to the MRI device for processing. Thus, in this particular aspect, although the parametric amplifier  202 B includes three L-C meshes  204 B,  206 B, and  208 B, only two frequencies at L-C meshes  204 B and  206 B are used for signal acquisition. 
         [0029]      FIG. 2C  depicts another exemplary schematic circuit diagram of the pre-amplification circuit  104 . In this particular aspect, the parametric amplifier  202 C is a double frequency resonator that includes L-C meshes  204 C,  208 C that each resonate at a different frequency. This double frequency resonator configuration can be used, for example, when the signal frequency ω1 is close but not equal to the difference frequency ω2. 
         [0030]    According this aspect, the L-C mesh  204 C includes resonating components that resonate at a desired frequency, ω1. For example, the resonating components such as inductor  210  and capacitor  230 , are selected so that the entire circuit  104  resonates (i.e., conducts current) at the spin precession frequency, such as 499.55 MHz. 
         [0031]    As described above, the pumping frequency is provided by an external source, such as RF signal generator, at a pumping frequency, ω3. During application of the pumping frequency, the pumping current  216  is induced at inductor  214 . For example, the value of the resonating components such as inductor  214  is selected so that the entire circuit  104  resonates at a pumping frequency, such as 999.16 MHz. Notably, the inductor  240  and capacitor  242 , as depicted in  FIGS. 2A and 2B , are not included in embodiment depicted in  FIG. 2C . 
         [0032]    The majority of the pumping current  216  flows through a varactor  218 . The varactor  218  has a high-Q at zero biased condition, and it performs frequency mixing between the input current  212  at the ω1 frequency and the pumping current  216  at the ω3 frequency, to generate an amplified current signal  220  at the difference frequency ω2. The varactor  218  is selected such that the difference frequency ω2 is nearly equal to the ω1 frequency. For example, ω2 in this case is 499.61 MHz, which is only 60 kHz higher than ω1 at 499.55 MHz. 
         [0033]    The L-C mesh  208 C includes resonating components, such as an inductor  222  and the varactor  218 , such that the amplified current signal  220  flows through the resonating components and generates the amplified output signal (e.g., amplified RF signal  116 ). According to one aspect, the inductor  222  is a rectangular copper loop. A receiving coil  224  (e.g., receiver  114 ) can be positioned external to the body and can be configured to resonate at the difference frequency at ω2 to inductively pick-up the amplified up converted output signal and provide the amplified output signal to the MRI device, or other measurement device for processing. 
         [0034]      FIG. 2D  depicts another exemplary schematic circuit diagram of the pre-amplification circuit  104 . In this particular aspect, the parametric amplifier  202 D is a double frequency resonator. In this aspect, the pre-amplification circuit  104  is substantially the same as the pre-amplification circuit depicted in  FIG. 2C . In particular, the pre-amplification circuit  104  includes two L-C meshes  204 D and  208 D that are configured the same as L-C meshes  204 C and  208 C, respectively. In this aspect, however, the receiver coil  224  is coupled to a different portion of the parametric amplifier  202 D. In particular, the receiving coil  224  (e.g., receiver  114 ) can be configured to resonate at the frequency at ω1 (e.g., Larmor frequency), to inductively pick-up the amplified output signal and provide the amplified output signal to the MRI device for processing. The pumping frequency is provided by an external source, such as RF signal generator, at a pumping frequency, ω3. 
         [0035]      FIG. 2E  depicts another exemplary schematic circuit diagram of the pre-amplification circuit  104 . In this particular aspect, the parametric amplifier  202 E is a single frequency resonator that includes L-C mesh  208 C. This single frequency resonator configuration can be used, for example, when the three frequencies ω1, ω2, and ω3 are close but not equal to each other. For example, ω1 is 499.55 MHz, ω2 is 499.61 MHz, and ω3 is 499.58 MHz. 
         [0036]    As described above, the pumping frequency is provided by an external source, such as RF signal generator, at a pumping frequency, ω3. During application of the pumping frequency, the pumping current  216  is induced at inductor  214 . For example, the value of the resonating components such as inductor  214  is selected so that the entire circuit  104  resonates at a pumping frequency, such as 499.58 MHz. Notably, the inductor  240  and capacitor  242 , as depicted in  FIGS. 2A and 2B , are not included in embodiment depicted in  FIG. 2C . 
         [0037]    The L-C mesh  208 E includes resonating components, such as an inductor  222  and the varactor  218 , such that the amplified current signal  220  flows through the resonating components and generates the amplified output signal (e.g., amplified RF signal  116 ). According to one aspect, the inductor  222  is a rectangular copper loop. A receiving coil  224  (e.g., receiver  114 ) can be positioned external to the body and can be configured to resonate at the frequency at ω1 to inductively pick-up the amplified up converted output signal and provide the amplified output signal  116  to the MRI device, or other measurement device for processing. In this aspect, because ω1 and ω2 are substantially same frequencies, the same receiving coil  224  can receive both signals. External circuitry (not shown) can be used to separate these two signals. 
         [0038]      FIG. 2F  depicts an exemplary pre-amplification circuit  104  that comprises the resonating components, such as described above in reference to  FIG. 2A . 
         [0039]      FIG. 3  is a graph  300  that depicts enhanced normalized sensitivity with and without parametric amplification between the resonator and an external loop at different distance separations. In particular, graph  300  depicts power sensitivity profiles  302 ,  304 ,  306 , and  308  that correspond to an exemplary resonator output loop (e.g., receiver) having dimensions 6.5×6.5 mm 2 . The bottom two profiles  302 ,  304  represent the detection sensitivity obtained without parametric amplification at ω2 and ω1 respectively. The top two profiles  306 ,  308  represent the substantially enhanced detection sensitivity obtained with parametric amplification at ω1 and ω2 respectively. Thus, there is improved sensitivity at the output loop when parametric amplification is used. 
         [0040]    Those skilled in the art will appreciate that variations from the specific embodiments disclosed above are contemplated by the invention. The invention should not be restricted to the above embodiments, but should be measured by the following claims.