Abstract:
An implementation of an optical transmission path for NMR signals from local coils in magnetic resonance imaging employs a photomodulator that may be incorporated into a connecting optical cable to be shared among multiple local coils and to provide for connection and disconnection at an electrical interface eliminating the need for optical connectors.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable 
   CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not applicable 
   BACKGROUND OF THE INVENTION 
   The present invention relates to magnetic resonance imaging (MRI) and in particular to an interface for connecting local coils used in MRI imaging to an MRI machine. 
   Magnetic resonance imaging can provide sophisticated images of the human body by detecting faint nuclear magnetic resonance (“NMR”) signals, primarily from concentrations of hydrogen protons in the tissues of the body. In MRI, a patient is located in a strong, polarizing, magnetic field and hydrogen protons of the patient&#39;s tissues are excited into precession with a radio frequency (“RF”) pulse. A series of applied gradient magnetic fields are switched on and off to spatially encode the precessing protons by phase and frequency. A sensitive antenna is then used to detect the NMR signals which are reconstructed into images. 
   MRI machines normally provide an integral antenna as part of the magnet assembly that may be used both for the RF excitation pulse and for detecting the NMR signal. Preferably, however, the NMR signals will be detected using one or more “local coils” being one or more small antennas that may be positioned near the patient to provide for improved signal-to-noise ratio in the detection of the NMR signals. 
   Typically, a shielded cable is attached to the local coil to receive a signal from preamplifiers built into the local coil that amplify the signal before transmitting it to the MRI machine. The shielded cable may connect to a termination box on the MRI machine (a “dog house”) often at the end of the patient table, where signals from the shielded conductor are routed to the MRI processing electronics. The termination box may also provide a source of electrical power, transmitted through the shielded cable to the local coil, to power the preamplifiers. In addition, the shielded cable may conduct other electrical signals to the local coil including active decoupling signals communicating with decoupling circuits in the local coil to detune the local coil during the RF excitation pulse to prevent excessive current conduction in the local coil during that time period. The termination box may also provide a separate electrical connector for a second shielded cable passing to the local coil and conducting an RF excitation pulse to the local coil when the local coil operates both in a receive and transmit mode. 
   The area around the operating MRI machine represents a difficult electrical environment for connecting a local coil to the MRI acquisition circuitry, principally with respect to establishing a good radio frequency ground. The switched fields used during the imaging process can promote high shield currents on the shield that may cause heating and possible risk to the patient. Baluns, such as those described in U.S. Pat. No. 6,605,775 entitled: “Floating Radio Frequency Trap For Shield Currents” and hereby incorporated by reference and assigned to the assignee of the present invention, provide one method of reducing these shield currents. 
   The shielded cables passing from the local coils to the termination box are relatively bulky and inflexible, in part, as a result of the necessary physical separation required between the patient and currents in the shield (normally enforced by a thick insulator), and the inherent stiffness of the cable conductors. This later problem is exacerbated for multi-channel coils which employ separate conductors for each channel. The inflexibility and bulk of these shielded cables can cause storage problems when multiple coils must be stored on-site, for example, in the limited space of the MRI room. 
   One promising solution to the problems of shield currents and electrical interference is that of transmitting the NMR signals optically, for example, over optical fibers. However, this approach faces a number of practical problems. The first is the high cost of optical modulation circuitry suitable to provide high signal-to-noise transmission of the NMR signal, a cost that is multiplied by the number of channels of the local coil. 
   Optical connectors allowing connecting and disconnecting of the optical fiber system from the MRI machine are currently inadequate for use in the MRI environment and introduce unacceptable signal noise resulting from the extreme sensitivity of fiber connections to vibration induced changes in alignment. 
   Electrical power is still required by the optical modulator and/or preamplifier in the local coil, and cabling for this purpose offsets some of the benefit of increased flexibility of the fiber, as well as making any connector more complex, now having to handle optical and electrical signals. 
   One final problem with optical transmission of NMR signals from local coils is the large installed base of conventional local coils and MRI machines that are not “optically enabled”, accepting only electrical rather than optical signals. Such systems present an obstacle to large-scale adoption of an optical transmission system which initially would be suitable for only a small market of machines. 
   BRIEF SUMMARY OF THE INVENTION 
   The present inventors have recognized that the above obstacles to optical transmission of NMR signals can be moderated by a detachable optical cable system integrated with an optical modulator (and possibly a demodulator) so that connections between the optical cable and local coil may be made using a conventional electrical connector. In this way, the cost of the modulation circuitry can be shared among a number of coils, optical connectors are eliminated, and if the electrical connector is correctly chosen, the optical cable can be used for both new and legacy coils. 
   Specifically then, the present invention provides a local coil system having a support structure that may be positioned on or near the patient and at least one resonant electrical antenna attached to that support structure for receiving NMR electrical signals from the patient. A first electrical connector is attached to the support structure and receives the NMR signals to connect to a second electrical connector. The second electrical connector includes a photomodulator converting the NMR electrical signals to optical signals which are provided to an optical cable. A photodemodulator attaches to a second end of the optical cable to receive the optical signals and convert them back into NMR electrical signals for communication to an MRI machine. 
   It is thus one object of at least one embodiment of the invention to provide a practical method of implementing optical transmission of NMR signals from local coils by placing the photomodulator on the cable to be shared among multiple coils as connected with a standard electrical connector. 
   The optical cable may be unbroken by connectors between the first and second end. 
   It is yet another object of at least one embodiment of the invention to overcome the problem of decreased signal-to-noise ratio caused by current optical connectors. By integrating the modulator with the cable, electrical connectors can be used to disconnect the cable from the local coil and MRI machine, reducing or avoiding the need for optical connectors. 
   A third electrical connector may be used to allow the photo demodulator to communicate the NMR electrical signals to the MRI machine and the first and third connectors may have substantially identical electrical and mechanical configurations. 
   It is thus another object of at least one embodiment of the invention to provide a migration path to optically enabled local coils by providing a cable system that may work with conventional MRI machines and with legacy local coils. 
   The photomodulator may be an electrically driven light source or an electrically driven light gate. 
   Thus it is another object of at least one embodiment of the invention to provide a system that may flexibly work with different modulation types, for example, a laser diode or a Mach-Zehnder modulator. 
   The optical cable may be free from metallic electrical conductors. To this end, the system may include a light source attached to the second end of the optical cable providing an optical power signal, and the second electrical connector may further include a photocell receiving the optical power signal from the optical cable. The photocell may provide power to the photomodulator or preamplifiers associated with the coil or may provide a signal to electrically decouple the coil. 
   Thus it is another object of at least one embodiment of the invention to wholly eliminate electrical shields that reduce flexibility of the cable, and to thereby wholly eliminate shield currents such as increase electrical interference and produce undesirable heating of the patient. 
   In one embodiment, the optical cable may include metallic electrical conductors for passing power along the cable. 
   It is thus another object of at least one embodiment of the invention to provide for a low cost version of the optical transmission cable that does not require optical transmission of substantial power but which may use standard techniques to block shield currents on DC conductors. 
   These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified perspective diagram of an MRI machine having a magnet assembly and providing a patient table working with the magnet assembly and having contained optical cables that may connect a local coil either directly to the processing electronics of the MRI machine in a shield room or through the conventional electrical cabling of the table&#39;s termination box; 
       FIG. 2  is a schematic diagram of the optical cable of  FIG. 1  showing use of an electrical connector to provide an electrical connection between the local coil and the optical cable through a photomodulator and photocells integrated into the optical cable; 
       FIG. 3  is a detailed fragmentary view of a photomodulator operating to gate or intensity modulate a light signal received from the second end of the cable; 
       FIG. 4  is a mechanical diagram of a optical cable of  FIG. 2  as may work with both optically enabled local coils or legacy local coils and which may be used to retrofit existing MRI machines to optically enabled local coils; and 
       FIG. 5  is a perspective view in phantom of an adapter module that may attach to the termination box of an MRI machine to convert a standard MRI machine into use with optically enabled local coils. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 1 , an MRI machine  10  may include a magnet assembly  12  providing a homogenous polarizing magnetic field within a bore  14  of the magnet assembly  12 . 
   The bore  14  may receive a patient table  16  for supporting a patient thereupon, the patient table  16  movable through the bore  14  during the examination process. The table  16  may include a termination box  18  at one end to which signals from local coils may be connected by means of connectors on the termination box  18  (not shown). 
   The termination box  18  communicates by means of shielded electrical cable  20  through a penetrator  22  in a shielded wall of the MRI room to an MRI processing unit  23 , the latter which receives the NMR signals and reconstructs them into an image. Shielded electrical cable  20  may also carry transmit signals in the opposite direction, the transmit signals being an RF pulse transmitted to some local coils that provide transmitting as well as receiving capabilities as will be described below. 
   In the present invention, the table  16  may include a number of pockets  24  along its edges, the pockets  24  holding electrical connectors  26  communicating with optical cables  28  (as will be described further below) that may pass to the termination box  18  after conversion into electrical signals or that may pass through opening  22 ′ in the shielded wall of the MRI room to a conversion unit  30  outside the MRI room that may convert the optical signals to electrical signals for use by the MRI processing unit  23 . In both cases, the optical cables  28  pass through guideways within the table  16  to provide them with mechanical protection and to prevent them from tangling or interfering with access to a patient. The optical cables  28  may also be used outside of the table  16  for legacy MRI machines or the like. 
   An optically enabled local coil  34  will typically provide a form  36  that may be rigid or flexible, as is understood in the art, to fit about a portion of the patient. An electrical connector  32  is supported on the form  36 , or attached to the local coil  34  by means of a short connecting lead (not shown), to receive signals from one or more loop antennas  38 . 
   The electrical connectors  26  of the optical cables  28  may be attached to corresponding electrical connectors  32  to receive electrical NMR signals therefrom. Multiple local coils  34  may connect to different electrical connectors  26  or a single local coil  34  may have up to 128 multiple channels connecting to multiple electrical connectors  26 . Generally the optical cables  28  have a smaller diameter and are more flexible and lower in weight than electrical counterparts. 
   Referring now to  FIG. 3 , an example loop antenna  38  representing one channel on a local coil  34  may provide signals to a low noise preamplifier  40  contained within the local coil  34 . The preamplifier receives electrical power through a power lead  43  and provides an output signal on output lead  41 . 
   The local coil may further include active decoupling circuitry  42  that may receive an electrical signal on decoupling lead  44  to decouple the loop antenna  38  during a period when an RF excitation pulse will be received. 
   Each of leads  41 ,  43  and  44  join to electrical connector  32  which may be connected to electrical connectors  26  joined to a first end  45  of the optical cable  28 . 
   Within a housing of the electrical connectors  26 , or closely attached thereto, each of leads  41 ,  43  and  44  may connect to optical interface circuitry  55  providing a conversion between electrical signals and optical signals or vice versa. 
   Specifically, output lead  41  from the preamplifier  40  is received by a photomodulator  46  which, in a first embodiment, includes an impedance matching circuit  47  matching the output of the preamplifier  40  to the impedance of laser diode  49 . The laser diode  49  converts the electrical signals from the preamplifier  40  into a modulated light signal  50  coupled to a standard optical fiber  48  contained within the optical cable  28 . The laser diode  49  may be, for example, a constant light power in the absence of an NMR signal of approximately 10 milliwatts at a 1,550-nanometer wavelength that is linearly modulated in power to provide the required signal-to-noise ratio light signal  50 . It will be understood to those of ordinary skill in the art that other frequencies and powers may be used as dictated by the transmission window of the optical fiber  48  and dynamic range and noise floor requirements. 
   The light signal  50  is propagated along the optical fiber  48  to a second end  51  of the optical cable  28  to be received by electrical interface circuitry  95  including a demodulator  52  which may be, for example, a photodiode  53  together with the necessary biasing and impedance matching circuitry  54  providing an output signal  56 . The demodulator  52  may include filter elements, bias adjustments, and other well-known circuit features, and may be in the conversion unit  30  outside the MRI room, as described above, or may be in a housing of electrical connector  58 , or closely attached thereto, at the second end of the optical cable  28 . In the former case, the output signal  56  may proceed directly to the MRI processing unit  23  shown in  FIG. 1 . In the latter case, the output signals  56  may pass through the electrical connector  58  to be received by corresponding electrical connector  60  attached to the termination box  18  described above. 
   The electrical interface circuitry  95  at the second end  51  of the optical cable  28  may also include one or more laser diode light sources  62  and  64  coupled to optical fibers  66  and  68 , respectively. Laser diode light sources  62  and  64  may deliver approximately one watt at 620 nanometers of wavelength. The low efficiency of current laser diode light sources cause them to dissipate as much as 10 watts per diode which may be removed from the circuitry (as is displaced from the patient) by heat sinks and/or air blowers. Piezoelectric nonmagnetic blowers may be used when the second end  51  of the cable  28  is in the magnetic field of the magnet assembly  12 . 
   The optical fibers  66  and  68  carry optical power signals  70  that are received by photocells  72  and  74  at the first end  45  of the optical cable  28 . The photocells  72  and  74  may be followed by power conditioning circuitry including DC-to-DC converter modules, filters and the like to provide a source of DC power to the local coil  34 . 
   In one embodiment, DC power from photocell  72  may be received by the photomodulator  46  along lead  73  to provide for biasing current and the like, and by the low noise preamplifier  40  along lead  43  passing through electrical connectors  26  to electrical connector  32 . 
   The electrical signal from photocell  74  may provide a decoupling signal on decoupling lead  44  to decoupling circuitry  42 . Laser diode light source  64  thus will be activated to produce signal  78  when loop antenna  38  must be decoupled. Alternatively, laser light source  64  may be of lower power and may activate a photodiode (used directly as a decoupling circuit element) or to switch power from photocell  72  to the decoupling lead  44 . 
   In the embodiment of  FIG. 2 , the cable  28  is composed exclusively of optical fibers with no metallic conductors, and thus no electrical shielding is required. As a result, no shield currents are generated and no protection against heating of the patient is required. 
   Referring now to  FIG. 3  in an alternative embodiment, the photomodulator  46 ′ may be a Mach-Zehnder type photomodulator that does not require a source of electrical power, but receives light  80  along an additional optical fiber  82  and the NMR electrical signal on output lead  41  to modulate the intensity of the light  80  to produce modulated light signal  50  that is returned to the demodulator  52 . The light  80  may be supplied by a laser diode light source (not shown) similar to laser diode light sources  62  and  64 . 
   The embodiment of  FIG. 3  may also eliminate metallic conductors in the cable  28  using the light power signals  70  as described above. Alternatively, it will be understood that some metallic conductors  86  may be employed together with optical fiber  48  (and possibly optical fiber  82 ) in lieu of optical fibers  66  and  68  in a embodiment where low frequency signals and power are conducted on copper conductors while the NMR signals is transmitted optically. In this embodiment, a shield may be required and shield currents must be suppressed by conventional methods such as baluns, chokes or high resistance cable. The benefit of low electrical interference with the NMR signal on optical fiber  48  and improved flexibility to the cable by eliminating some shielding and metallic conductors is still obtained. 
   Referring again to  FIG. 2 , while only a single loop antenna  38  (and hence single channel) is shown, the invention contemplates that multiple channels may be accommodated by a given cable  28  by adding additional optical fibers while still increasing the flexibility of the cable over an electrically conductive version. 
   Referring now to  FIG. 4 , the electrical connectors  26  may be compatible with electrical connectors  90  standardly used on local coils that are not optically enabled as well as with electrical connectors  32  of the optically enabled local coil  34 . In this way, the cables  28  may be used for both types of coils facilitating the migration of hospitals from one system to the other. 
   The optical interface circuitry  55  such as the photomodulator  46  and photocells  72  and  74  may be connected with fibers  48 ,  82 ,  66  and  68  of the cable  28  by factory-made permanent connections without the need for releasable connectors because the optical cable  28  can be disconnected from the local coil  34  at the interface between electrical connectors  26  and  90  or  26  and  32 . Likewise at the second end  51  of the cable  26 , the electrical interface circuitry  95  may be connected with fibers  48 ,  82 ,  66  and  68  of the cable  28  by factory made permanent connections without the need for releasable connectors either by permanent connection to the conversion unit  30  holding the electrical interface circuitry  95 , or by the interface between electrical connectors  58  and  60 . The use of the factory controlled termination without the need for releasable optical connectors provides substantial gains in signal-to-noise ratio. 
   While the electrical interface circuitry  95  may be connected directly to the MRI machine  10 , when connectors  58  and  60  are used, they may be made mechanically identical to electrical connectors  32  and  26 , respectively, to allow the system to work with existing MRI machines  10 . 
   Referring now to  FIG. 5  in MRI machines  10  with a termination box  18 , an adapter module  100  may be developed to facilitate transition of an MRI machine  10  to optical signal communication. The termination box  18  typically provides connector  60  for handling signals received for receive local coils  34  and a connector  102  providing signals output to transmit-type local coils  34 . The adapter module  100  may therefore include a connector  104  connecting to connector  102  and providing a pass through to a connector  106  that may be received by connector  108  of the transmit coil. 
   Similarly, connector  60  may join to connector  58 , as has been described, which may provide signals to the electrical interface circuitry  95  and then to cable  28 . In parallel, connector  58  may connect to a pass-through connector  110  that may connect to connectors  90  of legacy coils or the like. 
   Importantly then, the present invention provides a migration path overcoming the compatibility problems that would otherwise occur in the transition from electrical to optical communication of the NMR signals. 
   It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.