Abstract:
The present invention provides a system and method wirelessly transmitting MR signals from receive coils of an RF coil assembly to a remotely located receiver system. By utilizing wireless telemetry, ghosting, SNR problems, and standing waves on shields typically associated with cabled receive coils are avoided. Furthermore, by incorporating a rechargeable battery in place of DC cables, the coaxial cable conducting large currents can be eliminated. The present invention incorporates a transmitter that transmits a modulated MR signal to a receiver at the end of the bore of the magnet of the MRI system. Modulating the MR signals with a carrier frequency enables wireless transmission of the modulated signal to the remote receiver. Preferably, the modulated signal is transmitted using a 900 MHz carrier frequency. The receiver then demodulates the received signal and transmits the resulting signal to a system control for subsequent processing and image reconstruction.

Description:
BACKGROUND OF INVENTION  
         [0001]    The present invention relates generally to magnetic resonance imaging (MRI) and, more particularly, to a wireless RF module for wirelessly transmitting acquired MR signals from a receive coil of an MRI system.  
           [0002]    When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B 0 ), the individual magnetic moments of the spins 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 an RF magnetic field (excitation field B 1 ) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, M Z , may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment M t . A signal is emitted by the excited spins after the excitation signal B 1  is terminated and this signal may be received and processed to form an image.  
           [0003]    When utilizing these 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 signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.  
           [0004]    Generally, the RF coil assembly of an MRI system includes a transmit coil to create the B 1  field and a receive coil used in conjunction with the transmit coil to detect or receive the signals from the excited spins in the imaged object. Typically, each receive coil of the RF coil assembly is connected to the receive chain of the MRI system via a coaxial transmission line or cable. Because of the proximity of coaxial cables for the receive coils with respect to one another, ghosting and signal-to-noise (SNR) related problems can occur and to prevent standing waves on the cable shields during the transmit pulse.  
           [0005]    Additionally, the receive coils of the RF coil assembly are typically supplied power through a series of DC cables. During the transmit pulse with the transmit coil, large voltages and currents can be induced in the DC cables and the shields of the coaxial cables.  
           [0006]    It would therefore be desirable to have a system and method capable of wirelessly transmitting an MR signal from the receive coil of an RF coil assembly to a remote receiver module as well as a system absent of the DC cables typically connected to the receive coil which can further increase overall safety of the device.  
         BRIEF SUMMARY OF INVENTION  
         [0007]    The present invention provides a system and method overcoming the aforementioned problems by providing wireless transmission of MR signals from receive coils of an RF coil assembly to a remotely located receiver system. By utilizing wireless telemetry, ghosting and SNR problems typically associated with cabled receive coils are avoided. Furthermore, by incorporating a rechargeable battery in place of DC cables, concerns regarding contact with a coaxial cable conducting large currents are negated. The present invention incorporates a transmitter that transmits a modulated MR signal to a receiver remote from the imaging bay of the MRI system. Modulating the MR signals with a carrier frequency enables wireless transmission of the modulated signal to the remote receiver. Preferably, the modulated signal is transmitted using a 900 MHz carrier frequency. The receiver then demodulates the received signal and transmits the resultant signal to a system control for subsequent processing and image reconstruction.  
           [0008]    In accordance with one aspect of the present invention, a wireless RF module for an MRI apparatus is provided. The module includes a modulator configured to modulate a carrier signal with an MR signal in an RF coil of the MRI apparatus. A transmitter is provided and configured to transmit the modulated MR signal. A receiver is wirelessly connected to the transmitter and configured to receive the modulated MR signal for subsequent data processing and image reconstruction.  
           [0009]    In accordance with another aspect of the present invention, an MRI apparatus comprises an MRI system having a number of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field. The MRI apparatus further includes an RF transceiver system and an RF coil assembly configured to wirelessly transmit an MR signal to the RF transceiver system.  
           [0010]    In accordance with a further aspect of the present invention, an MRI system comprises means for positioning a subject to be scanned within a bore of magnet assembly for MR data acquisition. The MRI system further includes means for impressing a polarizing magnetic field about the bore of the magnet and means for exciting nuclei in the subject. The MRI system further comprises means for sensing signals resulting from the excited nuclei in the subject and means for wirelessly transmitting the signals to a receiver means. Means for reconstructing at least one image of the subject from the signals received by the receiver means is also provided.  
           [0011]    Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0012]    The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention.  
         [0013]    In the drawings:  
         [0014]    [0014]FIG. 1 is a schematic block diagram of an MRI system incorporating the present invention.  
         [0015]    [0015]FIG. 2 is a schematic block diagram of a wireless RF module for use with an MRI system. 
     
    
     DETAILED DESCRIPTION  
       [0016]    The present invention will be described with respect to a whole body RF coil assembly of an MRI system having a transmit coil to create a B 1  field and a receive coil used in conjunction with the transmit coil to detect or receive the signals from excited spins of nuclei in an imaged object.  
         [0017]    Referring to FIG. 1, the major components of a preferred magnetic resonance imaging (MRI) system  10  incorporating the present invention are shown. The operation of the system is controlled from an operator console  12  which includes a keyboard or other input device  13 , a control panel  14 , and a display screen  16 . The console  12  communicates through a link  18  with a separate computer system  20  that enables an operator to control the production and display of images on the display screen  16 . The computer system  20  includes a number of modules which communicate with each other through a backplane  20   a.  These include an image processor module  22 , a CPU module  24  and a memory module  26 , known in the art as a frame buffer for storing image data arrays. The computer system  20  is linked to disk storage  28  and tape drive  30  for storage of image data and programs, and communicates with a separate system control  32  through a high speed serial link  34 . The input device  13  can include a mouse, joystick, keyboard, track ball, touch activated screen, light wand, voice control, or any similar or equivalent input device, and may be used for interactive geometry prescription.  
         [0018]    The system control  32  includes a set of modules connected together by a backplane  32   a.  These include a CPU module  36  and a pulse generator module  38  which connects to the operator console  12  through a serial link  40 . It is through link  40  that the system control  32  receives commands from the operator to indicate the scan sequence that is to be performed. The pulse generator module  38  operates the system components to carry out the desired scan sequence and produces data which indicates the timing, strength and shape of the RF pulses produced, and the timing and length of the data acquisition window. The pulse generator module  38  connects to a set of gradient amplifiers  42 , to indicate the timing and shape of the gradient pulses that are produced during the scan. The pulse generator module  38  can also receive patient data from a physiological acquisition controller  44  that receives signals from a number of different sensors connected to the patient, such as ECG signals from electrodes attached to the patient. And finally, the pulse generator module  38  connects to a scan room interface circuit  46  which 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  46  that a patient positioning system  48  receives commands to move the patient to the desired position for the scan.  
         [0019]    The gradient waveforms produced by the pulse generator module  38  are applied to the gradient amplifier system  42  having G x , G y , and G z  amplifiers. Each gradient amplifier excites a corresponding physical gradient coil in a gradient coil assembly generally designated  50  to produce the magnetic field gradients used for spatially encoding acquired signals. The gradient coil assembly  50  forms part of a magnet assembly  52  which includes a polarizing magnet  54  and a whole-body RF coil assembly  56 . Preferably, assembly  56  includes a transmit coil (not shown) to create a B 1  field and a receive coil (not shown) used in conjunction with the transmit coil to detect or receive the signals from excited spins of nuclei in the imaged object.  
         [0020]    A transceiver module  58  in the system control  32  produces pulses which are amplified by an RF amplifier  60  and coupled to the transmit coil of RF coil assembly  56  by a transmit/receive switch  62 . The resulting signals emitted by the excited nuclei in the patient are sensed by the receive coil of RF coil assembly  56  and wirelessly transmitted to a receiver  63 . The received signals are then input to a preamplifier  64 . The amplified MR signals are demodulated, filtered, and digitized in the receiver section of the transceiver  58 . The transmit/receive switch  62  is controlled by a signal from the pulse generator module  38  to electrically connect the RF amplifier  60  to the coil assembly  56  during the transmit mode and activate a transmitter (not shown) to wirelessly transmit the MR signals to receiver  63  during the receive mode, as will be described with respect to FIG. 2. The transmit/receive switch  62  can also enable a separate RF coil (for example, a surface coil) to be used in either the transmit or receive mode. As will be described in greater detail below, the transmitter includes a number of components to facilitate wireless transmission of MR signals to receiver  63 . A rechargeable battery  65  is also provided to provide cableless power to the transmitter and its respective components.  
         [0021]    The MR signals picked up by the receive coil of RF coil assembly  56  and transmitted to receiver  63  are digitized by the transceiver module  58  and transferred to a memory module  66  in the system control  32 . A scan is complete when an array of raw k-space data has been acquired in the memory module  66 . This raw k-space data is rearranged into separate k-space data arrays for each image to be reconstructed, and each of these is input to an array processor  68  which operates to Fourier transform the data into an array of image data. This image data is conveyed through the serial link  34  to the computer system  20  where it is stored in memory, such as disk storage  28 . In response to commands received from the operator console  12 , this image data may be archived in long term storage, such as on the tape drive  30 , or it may be further processed by the image processor  22  and conveyed to the operator console  12  and presented on the display  16 .  
         [0022]    Referring now to FIG. 2, an RF module  70  for wirelessly transmitting MR signals detected of an imaged object to a receiver  63  for subsequent processing is schematically illustrated. Module  70  picks up the signals from a receive coil  72  and transmits the signal to a wireless receiver  63 . A rechargeable battery  65  is also provided and preferably located in the receive coil  72  to provide power to module  70  and its components. The battery  65  may be a charged battery and charged at a remote location thereby eliminating the need for charging the battery while in the system. This also avoids any down time to the system resulting from charging the battery. Notwithstanding the above, a non-rechargeable battery may also be used. Preferably, the transmit pulse from the transmit coil may be picked up, rectified by a rectifier (not shown), and straightened by a capacitor (not shown) to provide the requisite power to module  70  and to keep the battery charged in accordance with well known rectifying techniques.  
         [0023]    To achieve wireless transmission of signals from the bore of the magnet of the MRI system, FIG. 1, module  70  includes a preamplifier  74  proximate the receive coil  72  and configured to receive the MR signal therefrom. Preferably, preamplifier  74  is located on a surface of the receive coil  72 . The preamplifier  74  inputs the MR signal to a modulator  76 , such as a diode circuit, wherein the MR signal is modulated with a carrier signal from a local oscillator  78  that may be located on the receive coil as well. Modulator  76  amplitude modulates the MR signal with the carrier signal from oscillator  78 . Preferably, the carrier signal has a frequency approximate to the 900 MHz frequency range.  
         [0024]    The modulated MR signal is then fed from modulator  76  to a transmitter  80 , preferably a 900 MHz transmitter. In anticipation of reduced or inadequate signal strength for wireless transmission, module  70  includes a second preamplifier  82  that amplifies the signal from transmitter  80 . In this embodiment, a matching circuit  84  is also provided which transmits the amplified modulated signal to a 900 MHz antenna  86 . If the strength of the MR signal is sufficient for wireless transmissions, module  70  may be configured absent preamplifier  82 . Typically, the MR signal need only travel a few meters, therefore, a module  70  absent component  82  is likely, but a module incorporating component  82  to provide additional signal strength for wireless transmission across several meters is contemplated.  
         [0025]    The antenna  86  then transmits the modulated signal to a receiver  63  located preferably at the end of the bore of the magnet and configured to receive the signal and subsequently feed the signal to a data processor via a preamplifier  64  and transceiver  58 , FIG. 1, for subsequent processing and image reconstruction. Alternately, receiver  63  may be incorporated with transceiver  58  of FIG. 1 by implementing an antenna stub on the transceiver. Further, receiver  63  includes demodulation circuitry to demodulate the received signal. The receiver, however, may feed the received signal to a demodulator (not shown) for signal demodulation.  
         [0026]    It is noted that FIG. 2 shows oscillator  78  connected to transceiver  58 . This connection is to show the need to have phase coherence between the two local oscillators. The phase coherency can be performed by determining the phase of the RF pulse.  
         [0027]    Additionally, the present invention is applicable with known imaging protocols and techniques. Further, the present invention may be utilized as a kit to retrofit existing cabled MRI systems to thereby take advantage of the benefits heretofore described.  
         [0028]    In one embodiment of the present invention, a wireless RF module for an MRI apparatus is provided. The module includes a modulator configured to modulate a carrier signal with an MR signal in an RF coil of the MRI apparatus. A transmitter is provided and configured to transmit the modulated MR signal. A receiver is wirelessly connected to the transmitter and configured to receive the modulated MR signal for subsequent data processing and image reconstruction.  
         [0029]    In another embodiment of the present invention, an MRI apparatus comprises an MRI system having a number of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field. The MRI apparatus further includes an RF transceiver system and an RF coil assembly configured to wirelessly transmit an MR signal to the RF transceiver system.  
         [0030]    In a further embodiment of the present invention, an MRI system comprises means for positioning a subject to be scanned within a bore of magnet assembly for MR data acquisition. The MRI system further includes means for impressing a polarizing magnetic field about the bore of the magnet and means for exciting nuclei in the subject. The MRI system further comprises means for sensing signals resulting from the excited nuclei in the subject and means for wireless transmitting the signals to a receiver means. Means for reconstructing at least one image of the subject from the signals received by the receiver means is also provided.  
         [0031]    The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.