Abstract:
A switchable birdcage coil functioning as a transmitter coil and a separate RF surface coil functioning as the receiver coil are utilize in NMR and MRI apparatus. To prevent the switchable birdcage coil from absorbing energy by coupling to the receiver coil or by absorbing power from the RF field produced by the precessing nuclear spins, one or more of the reactive elements of the birdcage coil are switched from a normal reactive impedance state to a high impedance state. The high impedance state is formed by switchably combining the reactive element with a complementary reactive element thereby forming parallel resonant high impedance circuit that is tuned to resonate at the NMR frequency.

Description:
FIELD OF THE INVENTION 
     The invention is in the field of magnetic resonance imaging (MRI) using nuclear magnetic resonance (NMR) phenomenon and relates to the use of an actively switchable birdcage coil providing better signal-to-noise ratio. 
     BACKGROUND OF THE INVENTION 
     Birdcage coils are commonly used in NMR and MRI instrumentation to produce the RF field over the sample or the object being imaged. The birdcage coils are described, for example, in the U.S. Pat. Nos. 4,689,548 and 4,694,255. The conventional birdcage coil consists of a number of evenly spaced leg conductive elements interconnecting a pair of ring conductive elements. Each conductive element includes at least one reactive element that may be a capacitive or inductive element. There are two basic designs of birdcage coils: high-pass birdcage coils having inductive leg elements and capacitive ring elements, and low-pass birdcage coils having capacitive leg elements and inductive ring elements. There are also “band pass” or hybrid versions that use a combination of capacitive and inductive elements as leg or ring elements. 
     Basically the birdcage coil is a linear network of identical cells connected together so that the last cell in a ring is connected to the first cell. From the spatial point of view, each cell comprises a pair of ring elements coupled to a leg element forming a “ladder” network. When excited by RF energy, waves propagate along the network. For some particular frequencies the waves combine constructively corresponding to the resonant modes of the network. For the resonance of interest, the phase of the current in each adjacent leg is shifted by an angle φ=2π/N, and the amplitude of the current in each leg follows the cosine relationship:
 
 I   n   =I  cos(2 πn/N ),
 
where N is a number of cells and n=1, 2, . . . N.
 
     In a typical experiment one or more pulses of radio frequency (RF) magnetic field are applied to the sample or object in the probe to excite a nuclear resonance signal. This is followed by a reception period where the transmitter is silent and the receiver is activated to detect and record any response signal produced by the nuclei. In some systems the same coil or resonator is used to produce the transmit RF magnetic field and to receive the response signal of the nuclei. In other systems including the systems described here, a birdcage coil is used to excite a nuclear resonance signal and a separate coil or coils are used to detect the response signal produced by the nuclei. Residual coupling between the transmitter and receiver coils reduces the sensitivity during the receive mode. The small NMR currents in the receiver coil windings induce currents in the transmitter coil windings causing a loss in sensitivity since the power is absorbed and not available for signal detection. Direct coupling of the RF fields produced by the nuclei also induce currents in the transmitter coil causing a loss in sensitivity. 
     In attempt to solve the problem switching diodes were utilized to detune or disable the transmit or body coil in MRI as disclosed in the U.S. Pat. No. 4,763,076. The diodes were connected in series with the transmitter coil and must be forward biased during the transmit mode. When the diodes were forward biased by a DC current flowing from an anode to a cathode, the diodes provided a path for the RF currents. The diodes were reverse biased to detune or disable the transmitter circuit. A reverse biased diode provides RF signal isolation between its anode and cathode. Radio frequency choke coils or traps may be used in the lines for conducting the DC current to the switching diode and preventing RF currents from flowing on the lines. 
     A birdcage coil described in the U.S. Pat. No. 4,833,409 comprises a circuit for dynamically disabling it to allow for localized coil to receive the NMR signals. Each end ring of the birdcage coil is coupled to a shield surrounding the birdcage coil by four switchable impedance circuits equidistantly spaced around each end ring. When activated, the circuit provides a low impedance path between the coil and ground. This detunes the cells that are coupled to the impedance switch thereby affecting the tuning of the birdcage resonator. Though the tuning of the four cells that are coupled to the switch elements are affected, and the birdcage coil as a whole would no longer produce a resonance, currents are still induced in the individual cell inductive elements. In spite of the fact that these currents are not in a circuit that resonates at the NMR frequency, voltages are still induced in the loops and the resulting currents are smaller, but not zero or near zero because of the still finite impedance of the cell elements at the NMR frequency. 
     SUMMARY OF THE INVENTION 
     A birdcage coil of the present disclosure is used to produce the transmitter RF magnetic field over the sample and addresses the prior art problems by blocking essentially all residual currents that arise from any RF voltage that may be induced into its cells. The birdcage coil comprises a plurality of identical cells connected there between in a ladder pattern. Each cell has an upper ring reactive element, a lower ring reactive element, and a leg reactive element interconnecting reactive elements the upper and lower rings respectively. The reactive elements of one or more cells are connected to a complementary reactive element forming a parallel resonant circuit therewith. The parallel resonant circuit is coupled to PIN diodes for switching between a transmit and a receive modes of operations and tuned to a frequency of the NMR signals. The birdcage coil may have high-pass, low-pass or hybrid configurations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects of the present invention will become better understood from the following detailed description with reference to the drawings in which: 
         FIG. 1  is a schematic diagram of a prior art high-pass birdcage coil. 
         FIG. 2  is a circuit diagram of a prior art high-pass birdcage coil. 
         FIG. 3  is a simplified circuit diagram illustrating a general feature of the switchable birdcage coil of the present invention when the circuit is activated. 
         FIG. 4  is a circuit diagram of first embodiment of the switchable birdcage coil. 
         FIG. 5A  is a circuit diagram of an alternative embodiment of the switchable birdcage coil. 
         FIG. 5B  is a circuit diagram of the embodiment of  FIG. 5A  with reversed polarity of the PIN diodes and their connections to the bias supply. 
         FIG. 6  is a circuit diagram of yet another embodiment of the present invention. 
     
    
    
     The items in the drawings are labeled as follows: 
     
       
         
               
               
               
             
           
               
                   
               
               
                 Item 
                 Component 
                 Value (when applicable) 
               
               
                   
               
             
             
               
                 10 
                 Birdcage coil 
                   
               
               
                   
                 (high-pass birdcage coil) 
               
               
                 11 
                 Upper ring 
               
               
                 12 
                 Lower ring 
               
               
                 13 
                 Leg inductors 
                 L1 
               
               
                 15 
                 Ring capacitor 
                 C1 
               
               
                 16 
                 Ring Capacitor 
                 C2 
               
               
                 20 
                 Electrical network 
               
               
                   
                 (high-pass birdcage coil) 
               
               
                 30 
                 Simplified circuit 
               
               
                 31 
                 Inductor 
                 L2 
               
               
                 40 
                 Circuit diagram 
               
               
                   
                 (switchable high-pass birdcage coil) 
               
               
                 44 
                 PIN diode 
               
               
                 45 
                 Source terminal 
               
               
                 46 
                 Bias supply 
               
               
                 47 
                 Return terminal 
               
               
                 48 
                 RF choke coil 
                 L3 
               
               
                 49 
                 RF choke coil 
                 L4 
               
               
                 50 
                 Switchable birdcage coil 
               
               
                 51 
                 Switchable birdcage coil 
               
               
                 55 
                 RF choke coil 
                 L3 
               
               
                 60 
                 Switchable birdcage coil 
               
               
                   
                 (Low-pass birdcage coil circuit) 
               
               
                 61 
                 Upper ring 
               
               
                 62 
                 Lower ring 
               
               
                 63 
                 Leg capacitor 
                 C3 
               
               
                 64 
                 PIN diode 
               
               
                 65 
                 Inductor 
                 L5 
               
               
                 66 
                 Inductor 
                 L6 
               
               
                 67 
                 RF choke coil 
                 L4 
               
               
                 68 
                 RF choke coil 
                 L3 
               
               
                   
               
             
          
         
       
     
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning toward the drawings,  FIG. 1  is a diagram of a prior art high-pass birdcage coil. Birdcage coil  10  comprises upper ring  11  and lower ring  12 , each with N equally spaced capacitors around a respective ring. Upper ring capacitors  15 , each has a capacitance value C 1 , and the low ring capacitors  16 , each has a capacitance value C 2 . Leg inductors  13  have an inductance value L 1  and are extended between each pair of the upper ring capacitors and a corresponding pair of the low ring capacitors. The birdcage coil is formed as a linear network of cells. In each cell an upper ring capacitor  15  is connected to one end of a leg inductor  13 , and a low ring capacitor  16  is connected to the other end of the same leg inductor  13 . These elements are coupled together to form an electrical network  20  as illustrated in  FIG. 2 . The birdcage coil is resonant at frequencies where the total phase shift of currents around the network combine constructively, i.e. where the total phase shift around the network is a integer multiple of 2π. The normally used resonance occurs when the phase shift around the network is exactly 2π. 
     According to the teaching of the U.S. Pat. No. 4,833,409 eight switchable impedance circuits are provided to achieve a partial reduction of the currents induced in a birdcage coil. Each switchable impedance circuit is controlled by the application of a forward or reverse bias on the PIN diode. With a forward bias current the impedance of the diode is low, and with reverse bias it is high. The switchable impedance circuits are coupled to the birdcage coil at four equidistant points around a top ring and ground, and four equidistant points around a bottom ring and ground. When the switchable impedance circuit is in a low impedance state, the birdcage coil is effectively grounded at four points around the top ring and four points around the bottom ring. Though the overall resonance response of the birdcage coil is reduced, currents are still induced in each of these grounded inductive sections and represent a loss. The value of the current I is:
 
 I=V /(ω L 1), where
 
V is the induced voltage,
 
ωL 1  is the impedance of the coil,
 
ω is the NMR frequency,
 
L 1  is the inductance of the cell.
 
     The advantage of the present work over the prior art is illustrated by  FIG. 3  showing the features of the first embodiment. It is understood (though not shown) that the network is closed: the last cell n=N is connected to the first cell n=1 in a way as shown in  FIG. 2 . Simplified circuit  30  shown in  FIG. 3  illustrates the network configuration of one embodiment switching is activated during the receive mode thereby putting the birdcage coil in the receive mode. Inductors  31 , each having an inductance L 2 , are switched to be in parallel with the ring capacitors  16  each having capacitances C 2 . The values of L 2  and C 2  are chosen to resonate at the NMR frequency ω, i.e. L 2 C 2 =ω −2 . The impedance of the parallel combination of L 2  and C 2  is very high at the NMR frequency. The value of the impedance at resonance is ωL 2 Q 2 , where Q 2  is the quality factor of inductor  31  with inductance L 2  at the NMR frequency. The value of current, I 1 , induced in leg inductors  13  is equals approximately to V/(ωL 2 Q 2 ). The ratio of the current I 1  induced in leg inductor of the present embodiment to the current value I of the prior art (U.S. Pat. No. 4,833,409) is: I 1 /I=L 1 /(QL 2 ). The value of inductances L 2  is expected to be comparable or larger than the value of inductance L 1 . The quality factor Q 2  typically is greater than 50 so that the decrease of current through L 1  when the switch of the present embodiment is activated is a factor of 50 or more. At the end of the transmit period the switch is deactivated and the birdcage coil of the present embodiment functions as electrical network  20  shown in  FIG. 2 . 
       FIG. 4  is a circuit diagram of a high-pass birdcage coil implementing switch activating features noted in connection with the description of  FIG. 3 . The high-pass birdcage coil comprises the upper ring  11  with ring capacitors values C 1  and the lower ring  12  with ring capacitors values C 2  and leg inductors  13  with inductance values L 1 . PIN diode  44  is a switching diode, for example the MP4P7461F-1072T manufactured by Tyco Electronics, USA. This is a non-magnetic diode, which operates in high magnetic fields without perturbing the magnetic field homogeneity. A forward voltage (anode of PIN diode is positive with respect to the cathode) of 1 volt produces a forward current of 100 ma and a series resistance of 0.1 ohm at 100 MHz. A reverse voltage of 1 volt produces a parallel resistance of 30,000 ohms. As shown in  FIG. 4 , the PIN diodes  44  are controlled by bias supply  46 , furnishing approximately +1 volt between source terminal  45  and return terminal  47  to activate the switch to a low resistance state, and −1 volt to deactivate the switch to a high resistance state. The source terminal  45  is coupled by RF choke coils  48  to the anodes of PIN diodes  44  and the cathode of the PIN diodes are coupled to the return terminal  47  of bias supply  46  by RF choke coil  49  and, in some modifications of the embodiment, through one or more inductors  31 . The RF choke coils  48  and  49  are low resistance inductors, which block radio frequency current while passing direct current and couple the bias supply  46  to the PIN diodes  44 . Since the RF choke coils have sufficiently high impedance at RF frequencies so they do not perturb the RF operations of the birdcage coil. 
     During the transmit mode, bias supply applies a zero or negative voltage between source terminal  45  and return terminal  47 , deactivating PIN diodes  44  to their high parallel resistance or non-conductive state, essentially isolating inductors  31  from capacitors  16 . Even with the large RF voltages that may appear during the transmit phase, only one of the diodes of each diode pair between an inductor  31  and its corresponding capacitor  16  is reverse biased by any RF voltage induced in inductor  31 , so that one of the diodes is in non-conducting stage preventing any current flow in inductors  31  during the transmit phase. This circuit configuration prevents transmit RF voltage peaks form activating both diodes of a pair simultaneously. When the RF voltage switches ON one diode of the pair, the other is switched OFF even when the bias supply is applying a small or zero voltage to the diode. During the transmit mode, the RF operation of birdcage coil  40  of  FIG. 4  is similar to the birdcage coil presented by the electrical network  20  of  FIG. 2 . 
     The transmit mode is followed by a receive mode where the transmitter is silent and receiver is activated. A surface coil is used to receive the NMR response of the sample or object. The surface coil is placed very close to the sample of region of the object to obtain maximum sensitivity. It is desirable to minimize the coupling between the transmitter coil and surface receiver coil. During the receive mode the small NMR currents in the receiver coil windings induce currents in the transmitter coil windings causing a loss in sensitivity as the power is absorbed and not available for signal detection. Additionally during the receive mode, any RF magnetic flux through the windings of the birdcage coil that arise from the nuclei as well as from coupling with the surface coil will induce a voltage in the windings of the birdcage coil. To the extent that this induced voltage produces a current in the windings, a loss in signal power takes place. To minimize this loss it is desired that the impedance in series with this voltage be as high as possible. 
     To maximize this series impedance during the receive mode, bias supply  46  is activated to apply a positive voltage between source terminal  45  and return terminal  47  producing a forward bias voltage to the PIN diodes  44  causing them to exhibit a low series resistance thereby coupling inductors  31  to ring capacitors  16  forming parallel resonant circuits. As mentioned above these parallel resonant circuits are tuned to resonate at the NMR frequency ω and exhibit high impedance at this frequency. During this phase of operation, the simplified circuit  30  of  FIG. 3  illustrates the RF operation of circuit  40  of  FIG. 4  with the birdcage coil switched to its passive mode. 
     The circuit of  FIG. 5A  shows yet another embodiment of the switchable birdcage coil  50 . The high pass birdcage coil comprise the ring capacitors  15  with values C 1  in the upper ring  11 , capacitors  16  with values C 2  in the lower ring  12  and leg inductors  13  with inductance values L 1 . RF choke coils  55  with inductance L 3  and RF choke coil  49  with inductance L 4  have a low DC resistance as do leg inductors  13  and inductor  31  thereby enabling secure connection of bias supply  46  to PIN diodes  44 . When switchable birdcage coil  50  of  FIG. 5A  is activated by a +1V between source terminal  45  and return terminal  47  of bias supply  46 , PIN diodes  44  switch to a low resistance state thereby connecting inductors  31  across capacitors ring  16  forming parallel resonant circuits. The resonant frequency of these circuits is tuned to the NMR frequency providing a high impedance to be in series with leg inductors  13  thereby greatly reducing any residual currents caused by its coupling with surface coils or by direct coupling to the NMR nuclei. The PIN diodes  44  are activated to their conductive state by a current produced by a positive voltage on active terminal  45  of bias supply  46 . The current flows from source terminal  45  through RF choke coils  55  and leg inductors  13  to the anodes of PIN diodes  44  and continues from the cathodes through RF choke coil  49  to return terminal  47  of bias supply  46 . The current return path for some of the diodes may include one or more inductors  31 . 
     During the transmit mode of the experiment the diodes are switched off by a zero or negative voltage on the source terminal  45  of bias supply  46 . Even though large RF currents may flow through the high pass birdcage coil, when the RF voltage across one diode of a pair turns it ON, the voltage across the other diode member of the pair is in the opposite direction and turns it OFF, thereby preventing both diodes being ON simultaneously to form a parallel resonant circuit. The RF choke coils  49  and  55  have sufficient inductance and low stray capacitance that their impedance is sufficiently high so they do not appreciably disturb the RF operation of the birdcage coil. 
     Switchable birdcage coil  51  shown in  FIG. 5B  is similar to switchable birdcage coil  50  of  FIG. 5A ; however the difference is in the polarity of diodes. PIN diodes of switchable birdcage coil  51  have the reversed polarity in comparison to the switchable birdcage  50 . Source terminal  45  is still DC coupled to the anodes of PIN diodes  44  and the cathodes of PIN are connected to return terminal  47  of bias supply  46 . 
     A precaution to be observed in circuit layout of the circuits of  FIG. 4 ,  FIGS. 5A and 5B  is that the resonant circuits formed by a parallel combination of capacitor  16  and inductor  31  coupled together by two diodes  44  form a closed loop. It is important that the area of this loop be made as small as possible or oriented to prevent it from trapping RF magnetic flux from the precessing nuclei or by coupling to the surface coil used for receiving the NMR signals. 
     Switchable birdcage coil  60  shown in  FIG. 6  is yet another embodiment of the invention, where a switchable low-pass birdcage coil is used as the transmitter coil and a surface coil or array is used to detect the NMR response. The switchable birdcage coil  60  is actively switched to a receive mode during the receive mode of the experiment thereby preventing it from absorbing energy by coupling to the surface coil or directly to the NMR response of the nuclei. A conventional low-pass birdcage coil comprises an upper ring  61  and a lower ring  62 , each with N equally spaced inductors  65  around each ring. The inductors in both rings have the same inductance values, L 5 . Leg capacitors  63  have a capacitance value C 3 . 
     According to the present invention, the switchable low-pass birdcage circuit  60  includes PIN diodes  64 , inductors  66 , RF choke coils  67  and  68 , and bias supply  46 . During the receive mode, the PIN diodes  64  are switched ON to their conductive state, coupling inductors  66  to their adjacent leg capacitors  63  thereby forming a parallel resonant circuit. The resonant frequency of the circuit is tuned to the NMR frequency, the parallel resonant circuit forms a very high impedance thereby greatly reducing the circulating current between the upper and lower inductive rings. The diodes are switched on by passing a current from the source terminal  45  of bias supply  46  through RF choke coils  67  and possibly one or more ring inductors  65  onto the anodes of PIN diodes  64 . The current passes through the diode, turning it to a conductive state, and out the cathode and back to the return terminal  47  of bias supply  46 . In this process the current also passes through RF choke coils  68  and, possibly, through inductor  66 . The resonant circuit formed by the parallel combination of leg capacitor  63 , with capacity C 3 , and inductor  66 , with inductance L 6 , resonates at the NMR frequency ω. The inductance L 6  is selected to satisfy the resonance equation L 6 C 3 =ω −2 . In this mode of operation the upper ring  61  and the lower ring  62  are decoupled from each other by the high parallel impedance of the resonant circuit thereby greatly reducing any currents in the cells produced by their coupling to the surface coils or directly to the NMR nuclei. 
     During the transmit period, the low-pass birdcage coil is in the transmit mode and bias supply  46  applies a zero or negative voltage on active terminal  45 . Terminal  45  is connected to the anodes of the PIN diodes  64  through RF choke coils  67 , and one or more ring inductors  65  thereby causing the diodes to become non-conducting. The cathode of PIN diodes is connected to the return terminal  47  of bias supply  46  through RF choke coil  68  and for one diode of the pair the return path includes inductor  66 . Even though large RF currents may flow through the low pass birdcage coil, when the RF voltage across one diode of a pair turns it ON, the voltage across the other diode member of the pair is in the opposite direction and turns it OFF, thereby preventing both diodes being on simultaneously and forming a parallel resonant circuit. 
     A precaution to be observed in circuit layout of the circuit of  FIG. 6  is that the resonant circuits formed by the parallel combination of leg capacitor  63  and inductor  66  when coupled together by the two diodes  64  form a closed loop. It is important that the area of this loop be made as small as possible or oriented to prevent it from trapping RF magnetic flux from the precessing nuclei or by coupling to the surface coil used for receiving the NMR signals. 
     Although the invention has been described herein in its preferred form, those skilled in the art will recognize that many changes and variations may be made thereto without departing from the spirit and scope of the invention as defined in the claims. For example other types of diodes other than PIN diodes way be used to perform the switching, and in some circuits transistors or integrated circuits may be used. It is also noted, that though the invention has been illustrated by the exemplary embodiments, where a switching of one reactive component in each rung from its normal impedance to a high impedance state was provided by forming a parallel resonant circuit that resonates at the NMR frequency, it would be obvious to those skilled in the art that it may not be necessary to apply this switching to every rung of the birdcage coil. In many systems sufficient reduction of losses may be obtained by providing the switching to less than to every rung. 
     Although the invention has been illustrated with a high pass and a low pass birdcage coils, it may also be used in band pass or hybrid versions of birdcage coils that use combinations of capacitive and inductive elements as leg or ring elements.