Patent Publication Number: US-7592813-B2

Title: Wireless RF coil power supply

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   The present application is a continuation of and claims priority of U.S. Ser. No. 10/907,582 filed Apr. 6, 2005, the disclosure of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention relates generally to magnetic resonance imaging (MRI) and, more particularly, to a wireless RF coil power supply for an RF module configured to acquire MR signals from a receive coil of an MRI system. 
   When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), 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 B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image. 
   When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) 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 NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. 
   Generally, the RF coil assembly of an MRI system includes a transmit coil to create the B1 field and a receive coil used in conjunction with the transmit coil to detect or receive the signals from the excited spins in an 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. Additionally, the receive coils of the RF coil assembly are typically supplied power through the coaxial cables. As the number of receive coils increases, the number of coaxial cables increases to match; thus, a large bundle of coaxial cables results that can become uncomfortable for an imaging patient when laid across the patient and difficult to manage or maneuver. 
   Further, interactions such as parallel resonance and parasitic capacitance between the transmit coil and the coaxial cables can cause standing waves and induced current in the coaxial cables. Current induced in the coaxial cables can cause the coaxial cables to become extremely heated, which furthers patient uncomfortability. 
   It would therefore be desirable to have a system capable of supplying wireless power to an RF receive coil assembly as well as a system wirelessly connecting the RF receive coil assembly to a receiver of an MR scanner. 
   BRIEF DESCRIPTION OF THE INVENTION 
   The present invention is directed to a system and method overcoming the aforementioned problems by providing a wireless power supply arranged to provide power to operate an RF coil assembly. The wireless power supply operates without the use of a battery or a wired connection external to a bore of a magnet assembly of an MRI system. In one embodiment, the present invention incorporates a coil configured to pick up and convert RF signals into electrical energy. In another embodiment, a photovoltaic cell is configured to convert light energy into electrical energy. 
   Therefore, in accordance with one aspect of the invention, an MR system is disclosed that includes an RF coil operable to transmit or receive RF signals and located within a bore of a magnet, and a converter to convert RF signals to digital signals. The MR system further includes a power supply that provides power to at least operate the RF coil and converter. The power supply is operable without use of a battery and without use of a wired connection external to the bore of the magnet. 
   In accordance with another aspect of the invention, an MR assembly is disclosed that includes an RF coil operable in at least one of a transmit mode and a receive mode, and configured to be located within a bore of a magnet. A transmitter is operably connected to the RF coil and wirelessly transmits MR signals acquired by the RF coil when operating in a receive mode. The MR assembly also includes a power supply that provides power to at least operate the transmitter and the RF coil. The power supply has at least one photovoltaic cell and a fiber optic cable to receive a beam of light from a light source external to the bore of the magnet and translate the beam of light to the power supply. 
   In accordance with a further aspect of the present invention, an MR apparatus includes a first RF coil for transmitting an RF signal inside a magnet bore and a second RF coil placed adjacent to an imaging subject positioned inside the magnet bore. The second RF coil operates in a receive mode to receive MR signals from the imaging subject. A signal converter is included to convert MR signals into digital signals. The MR apparatus further includes a rechargeable power supply connected to the second RF coil and the signal converter that supplies power thereto and a pickup coil connected to the rechargeable power supply and to recharge the power supply with electrical energy generated from the RF signal. 
   Various other features and advantages of the present invention will be made apparent from the following detailed description and the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention. 
     In the drawings: 
       FIG. 1  is a schematic block diagram of an MR imaging system incorporating the present invention. 
       FIG. 2  is a schematic block diagram of an RF module incorporating a wireless power supply having a photocell according to one embodiment of the present invention. 
       FIG. 3  is a schematic block diagram of an RF module incorporating a wireless power supply having a rectifier bridge and an energy storage device according to another embodiment of the present invention. 
       FIG. 4  is a schematic block diagram of an RF module incorporating a wireless power supply having a pickup coil according to yet a further embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   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 B1 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. However, one skilled in the art will appreciate that the present invention is also applicable with local and surface coils. 
   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. 
   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. 
   The gradient waveforms produced by the pulse generator module  38  are applied to the gradient amplifier system  42  having Gx, Gy, and Gz 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 , a bore  55 , and a whole-body RF coil assembly  56 . Preferably, assembly  56  includes a transmit coil to create a B1 field and a receive coil used in conjunction with the transmit coil to detect or receive the signals from excited spins of nuclei in the imaged object. 
   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 . Transceiver module  58  wirelessly transmits phase information to a frequency converter (shown in  FIGS. 2-4 ) inside bore  55  via a wireless transmitter  65 . The resulting signals emitted by the excited nuclei in the patient may be sensed by the receive coil of RF coil assembly  56  and wirelessly transmitted to a wireless receiver  63 . The received signals are then input into the transceiver module  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. 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. 
   The MR signals picked up by the receive coil of RF coil assembly  56  and transmitted to wireless receiver  63  are 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, 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 . 
   Referring now to  FIG. 2 , a digital RF module  70  for receiving the signals from excited spins of nuclei in the imaged object and for wirelessly transmitting the signals to wireless receiver  63  for subsequent processing is schematically illustrated. A receive coil  72  detects the signals from the imaged object. A preamplifier  74  amplifies the detected signals received from receive coil  72 . 
   A frequency converter  78  downconverts the signals to reduce the required bandwidth of the ADC used in digitization of the signals from the digital RF module  70  to the wireless receiver  63 . Downconversion requires phase information from the transmit pulse carrier. In a preferred embodiment, transceiver module  58  wirelessly transmits the phase information to a wireless receiver  80 , which supplies the phase information to the frequency converter  78 . 
   Wireless transmission, as used herein, comprises a transmission medium without electrically conductive wires. In this way, the transmission medium does not contain electrically conductive wires that adversely interact with RF pulses from the transmit coil. The wireless transmission, being free of electrically conductive wires, prevents the RF pulses from the transmit coil from inducing currents on electrically conductive wires placed in the vicinity of an imaging patient. Modes of wirelessly transmitting signals include RF signals transmitted through the air and light signals transmitted between an optical transmitter and receiver pair across fiber optic cables. Other modes of transmitting signals without the use of electrically conductive wires are similarly contemplated and are considered within the scope of the present invention. 
   Still referring to  FIG. 2 , the downconverted signals are digitized by an analog-to-digital converter (ADC)  82 . The digital signals are then wirelessly transmitted by a wireless transmitter  84  to the wireless receiver  63 . The wireless transmitter  84  and the wireless receiver  63  communicate without the use of electrically conductive wires as described above. In a preferred embodiment, a signal modulator  86  converts the electrical signals into either RF pulses for transmitting the signals via RF signals or light signals for transmitting the signals via fiber optic cable. 
   Power to the components  72 - 86  of the digital RF module  70  is generated wirelessly and without the use of a battery, which converts chemical energy into electrical energy. In one embodiment and as shown in  FIG. 2 , a power supply  88  includes a light source  90  optically connected to a photocell array  92  via a fiber optic cable  94 . Fiber optic cable  94  has a plurality of fiber strands designed to transfer light from the light source  90  to the photocell array  92 . Photocell array  92  includes an array of photovoltaic cells that converts visible light, infrared radiation and/or ultraviolet radiation into direct current (DC). Light source  90  is preferably a high intensity light source optically coupled to the photocell array  92  that supplies visible light, infrared radiation, or ultraviolet radiation to the photocell array  92 . Light source  90  can be located inside or outside of the bore  55 . A voltage regulator  93  regulates the voltage from the photocell array  92 . 
   A power bus  95  connects power supply  88  to receive coil  72  to provide a voltage reference. Power bus  95  further connects power supply  88  to preamplifier  74 , frequency converter  78 , wireless receiver  80 , ADC  82 , wireless transmitter  84 , signal modulator  86 , and other components in digital RF module  70  that require electrical power. Power supply  88  supplies power to power bus  95  for power distribution thereacross. 
     FIG. 3  shows a rechargeable power supply to provide power to the digital RF module  70  in accordance with another embodiment of the present invention. A power supply  96  is configured to derive electrical power directly from the receive coil  72  itself. During the transmit mode of the coil assembly transmitter, the receive coil  72  has a voltage induced therein that does not represent image data. As such, the induced voltage caused by the transmit coil of the coil assembly  56  is transmitted to power supply  96  over an electrical connection  97  and is rectified by a rectifier bridge  98  and stored in a capacitor  100  or other energy storage device. In a preferred embodiment, capacitor  100  includes at least one UltraCap for storing the rectified voltage. The rectifier bridge  98  includes a plurality of diodes configured to rectify the RF induced voltage. The capacitor  100  receives and stores the rectified voltage. The capacitor  100  is connected to a voltage regulator  102 , which controllably discharges and powers the components  74 - 86  over a power bus  104  during the receive mode of the coil assembly. 
   It is contemplated that rectifier bridge  98  may draw enough current out of receive coil  72  to cause an imaging artifact. As such, a separate pickup coil  106  can be used as shown in  FIG. 4 . Pickup coil  106  is located inside digital RF module  70  and away from with the imaging patient. Pickup coil  106  is preferably a multi-turn loop of wire in which an RF voltage is induced by the transmit mode of the coil assembly transmitter. During the transmit mode of the coil assembly transmitter, the pickup coil  106  has a voltage induced therein. As such, the induced voltage caused by the transmit coil of the coil assembly  56  is rectified by a rectifier bridge  108  and stored in a capacitor  110  or other energy storage device. Power from capacitor  110  is regulated by a voltage regulator  112  and is supplied to the components  74 - 86  over a power bus  114 . Power bus  114  also connects power supply  96  to receive coil  72  to provide a voltage reference. Pickup coil  106  is located away from receive coil  72  such that distortion to the uniformity of the transmit field near receive coil  72  is reduced. 
   In an alternative embodiment, pickup coil  106  is a multi-turn loop of wire in which voltage is induced by gradient fields. In this case, pickup coil  106  is constructed to be sensitive to a low KHz range where the main frequency associated with the leading and trailing edges of the gradient pulses is located. The induced voltage is rectified by rectifier bridge  108 , stored in a capacitor  110 , and regulated by voltage regulator  112  for supplying power to the components  74 - 86  over power bus  114 . 
   The present invention is directed to an apparatus whereby a batteryless power system provides power to the components of a digital RF module. The batteryless system avoids the typical wired connections external to the bore of the magnet assembly of conventional MRI systems. As such, patient discomfort typically caused by placing a large bundle of wires across the patient is eliminated. Also, in one preferred embodiment, fiber optic cables are advantageously used to supply power. Moreover, these fiber optic cables advantageously output less heat compared to conventional wire-based power supplies. 
   Therefore, in accordance with one embodiment of the invention, an MR system is disclosed that includes an RF coil operable to transmit or receive RF signals and located within a bore of a magnet, and a converter to convert RF signals to digital signals. The MR system further includes a power supply that provides power to at least operate the RF coil and converter. The power supply is operable without use of a battery and without use of a wired connection external to the bore of the magnet. 
   In accordance with another embodiment of the invention, an MR assembly is disclosed that includes an RF coil operable in at least one of a transmit mode and a receive mode, and configured to be located within a bore of a magnet. A transmitter is operably connected to the RF coil and wirelessly transmits MR signals acquired by the RF coil when operating in a receive mode. The MR assembly also includes a power supply that provides power to at least operate the transmitter and the RF coil. The power supply has at least one photovoltaic cell and a fiber optic cable to receive a beam of light from a light source external to the bore of the magnet and translate the beam of light to the power supply. 
   In accordance with a further embodiment of the present invention, an MR apparatus includes a first RF coil for transmitting an RF signal inside a magnet bore and a second RF coil placed adjacent to an imaging subject positioned inside the magnet bore. The second RF coil operates in a receive mode to receive MR signals from the imaging subject. A signal converter is included to convert MR signals into digital signals. The MR apparatus further includes a rechargeable power supply connected to the second RF coil and the signal converter that supplies power thereto and a pickup coil connected to the rechargeable power supply and to recharge the power supply with electrical energy generated from the RF signal. 
   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.