Abstract:
A circulator has a ferrite, and the circulator is arranged in the vicinity of a device that produces a static magnetic field in the environment surrounding the device, this static magnetic field giving the circulator a non-reciprocal behavior, with respect to circulation of energy among the gates of the circulator, as a result of interaction of the ferrite with the static magnetic field. A magnetic resonance apparatus embodies such a circulator, and the basic field magnet of this magnetic resonance apparatus generates the static magnetic field.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention concerns a circulator, in particular for use in radio-frequency systems of a magnetic resonance apparatus, as well as a magnetic resonance apparatus with such a circulator. 
     Magnetic resonance (MR) imaging is a known and firmly established method that is in particular used in medical imaging. A body to be examined is introduced into a strong, homogeneous, static magnetic field (known as the basic magnetic field) that causes an alignment of the nuclear spins of atomic nuclei in the body, in particular of hydrogen atoms (protons) bound to water. These nuclei are excited to a precession movement around the basic magnetic field by means of radio-frequency excitation pulses. After the end of a corresponding radio-frequency (RF) excitation pulse, the nuclear spins precess at a frequency known as the Larmor frequency, which depends on the strength of the basic magnetic field. Due to various interaction types, the nuclear spins align along the preferred direction (predetermined by the basic magnetic field) with a characteristic time curve. The time curve is, among other things, tissue dependent and can be described using characteristics known as relaxation times. An image can be generated from the spatial distribution of the nuclear spin density in connection with the respective relaxation times via computational and/or measurement analysis of the integral, radio-frequency nuclear signals. The association of the nuclear magnetic resonance signal (detectable as a result of the precession movement) with the point of its origin ensues by the application of magnetic field gradients. For this purpose, gradient fields are superimposed on the basic magnetic field and controlled such that an excitation of the nuclei occurs only in a slice to be imaged. Imaging systems based on these physical effects are also known under the designations nuclear spin tomography, nuclear magnetic resonance (NMR) tomography and magnetic resonance imaging (MRI). 
     A radio-frequency system with a radio-frequency antenna is required both for RF excitation of the nuclear spins and for detection of the nuclear spin signals. 
     At the transmitter side, the radio-frequency system has at least one radio-frequency amplifier (RFPA—radio-frequency power amplifier”) that amplifies a control signal that is thereupon conducted to a radio-frequency antenna and is fed via one or more input ports into the radio-frequency antenna. A non-optimal matching of the input port of the radio-frequency antenna leads to partial, significant return voltages or powers. In practice an optimal matching of the input port of the radio-frequency antenna is seldom present since the matching also depends on, among other things, the load of the radio-frequency antenna, which varies with the body to be examined. Therefore the signal fed into the radio-frequency antenna angle is normally at least partially reflected. The components (in particular radio-frequency power amplifier) upstream from the input port of the radio-frequency antenna must tolerate this reflected power without damage. 
     Often multi-port antennas that have at least 2 (typically 8, 16 or 32) input ports coupled among one another are used in magnetic resonance apparatuses. The control signals for the input ports are typically generated by multiple radio-frequency power amplifiers. Ideally, each input port is fed by its own radio-frequency power amplifier. In that the input ports are coupled among one another, over-coupled voltages can occur at the input ports. These voltages add to the voltages reflected at the input ports and returning, such that the radio-frequency power amplifiers which activate an input port of a multi-port antenna are exposed to particularly high loads. 
     The problem of the reflected and over-coupled voltage can be solved by an over-dimensioning of the radio-frequency power amplifier. The peak voltage of the radio-frequency power amplifier is selected well above the forward voltage that is necessary in operation, such that the peak voltage is in each case greater than the sum of the forward voltage and the voltage of the return signal. However, the cost of the radio-frequency power amplifier is increased thereby. 
     Another possibility is to arrange a circulator or a one-way conductor (isolator) at the output port of the radio-frequency power amplifier or in the radio-frequency power amplifier itself, such that the returning power does not reach the radio-frequency power amplifier and additionally load the amplifier. 
     A circulator is a passive component with at least three gates in which a power fed to one gate is presented, attenuated by a slight transmission loss, to another gate, while all remaining gates are largely decoupled; thus only the power reduced by a high suppression loss is present at those gates. The ports of the circulator are characterized by a cyclical sequence, meaning that a power presented at any of the gates is handed off to the respective next gate. An isolator is likewise a passive component with two connections (ports) that ideally passes an electromagnetic power only in one direction. 
     This non-reciprocal behavior is typically generated in circulators and isolators by a ferrite that is located in a static magnetic field that is generated by a permanent magnet surrounding the ferrite. The ferrite assumes a gyrotropic response due to the magnetic field of the permanent magnet. Radio-frequency signals that are fed to a gate are merely relayed to the next gate. The desired, non-reciprocal response occurs within a specific frequency range that can be determined by suitable dimensioning of the ferrite and the magnitude of the static magnetic field. 
     During the operation of a circulator, radio-frequency losses heat the circulator, in particular its ferrite and its permanent magnet. This leads to a change of the signal transfer with regard to amplitude and phase of the transferred signal. Problems thereby arise with regard to the necessary precision of the transmission of the radio-frequency signals in a magnetic resonance apparatus. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a circulator that has a simple design, is cost-effective to produce and with which a precise and constant signal transfer is enabled. Furthermore, an object of the invention is to provide a magnetic resonance apparatus with an improved radio-frequency system. 
     The circulator according to the invention has a ferrite, and the circulator is in proximity to an apparatus that generates a static magnetic field in its surroundings, such that the circulator is given its non-reciprocal property by interaction of the ferrite with the static magnetic field of the apparatus. The circulator exhibits its non-reciprocal property as long as the ferrite is arranged in the static magnetic field, and an interaction of the static magnetic field with the circulator thereby occurs. This means that, due to a stable arrangement of the circulator (i.e., the ferrite thereof) in the static magnetic field of the apparatus the circulator or its ferrite, the typically functioning, non-reciprocal property of the circulator can be ensured. 
     The circulator preferably is given its non-reciprocal property due to the interaction of the ferrite with only the static magnetic field of the apparatus. This means that no additional static magnetic field but only the static magnetic field that is generated by the apparatus accords the circulator the non-reciprocal functionality that is typical of a circulator. 
     In conventional circulators, the magnetic field that accords the ferrite an anisotropic permeability and the circulator its non-reciprocal functionality is generated by a magnet belonging to the circulator—typically by a permanent magnet. In contrast to this, the ferrite of the circulator according to the invention receives its anisotropic permeability due to the static magnetic field of the apparatus which, as an external apparatus, is not an actual component of the circulator. The module of the circulator that generates the magnetic field in conventional circulators thus can be fashioned significantly more simply or preferably can be entirely omitted, such that the circulator according to the invention can be produced more cost-effectively in comparison to conventional circulators. 
     Since this module in conventional circulators warms in the course of operation due to radio-frequency losses, and since this heating (as described above) negatively affects the precision and constancy of the signal transmission, the circulator according to the invention allows a particularly precise and constant signal transmission. 
     The circulator preferably includes an arrangement to cool the circulator. Because the ferrite of the circulator according to the invention is now better accessible, the arrangement for cooling can be designed comparably simply and can be arranged more efficiently in proximity to the ferrite or at the ferrite itself. For example, cooling by attachment of a cooling body or a heat pipe system can be achieved. The temperature of the circulator can hereby be kept largely constant in a simple manner during the operation, such that the signal transmission is improved with regard to its precision and constancy. 
     In one embodiment, the circulator is fashioned as a three-gate circulator. 
     In another embodiment, the circulator is fashioned as an isolator. This can be achieved by, for example, a third port of a circulator being terminated with a load installed into the circulator, such that a circulator so modified possesses only two gates that merely relay the fed power in one direction. 
     The apparatus that generates a static magnetic field in its surroundings is preferably a magnetic resonance apparatus. A magnetic resonance apparatus possesses a strong static magnetic field, such that this magnetic field that is typically used for magnetic resonance imaging is also advantageously used to the effect that the circulator receives its non-reciprocal property due to the interaction of the ferrite with the static magnetic field. 
     The frequency range of the circulator advantageously is the Larmor frequency of the magnetic resonance apparatus. In this way the circulator can be used in a radio-frequency system of the magnetic resonance apparatus. The modulation of the frequency of the circulator can be achieved by, among other things, the installation site that defines the strength of the external magnetic field, by the selection of the material of the ferrite and via the dimensioning of the adaptation circuit of the circulator. 
     The MRT apparatus according to the invention has a radio-frequency system that has a circulator as described above. As described above, the radio-frequency system that is equipped with the circulator allows a particularly precise and constant signal transmission. 
     The circulator is preferably arranged between a radio-frequency power amplifier and a radio-frequency antenna, wherein a first gate of the circulator is connected with the radio-frequency power amplifier and a second gate of the circulator is connected with a radio-frequency antenna. The transmission direction of the circulator proceeds from the first gate to the second gate. 
     As described above, components of the radio-frequency system (in particular of the radio-frequency power amplifier) are exposed to particular loads due to a non-optimal adaptation of the input protocols of the radio-frequency antenna. The use of the circulator in the radio-frequency system between the radio-frequency power amplifier and the radio-frequency antenna blocks waves that have been reflected at the radio-frequency antenna, or conducts these into a different channel so that the components of the radio-frequency system (in particular the radio-frequency power amplifier) are protected. It is possible to dimension the components that are protected from reflected waves by the circulator smaller overall since they no longer need to tolerate the additional load that would arise due to reflected waves. The radio-frequency system can hereby be produced more cost-effectively. Additionally, advantages with regard to a precise and constant signal transmission result due to the use of the circulator in the radio-frequency system of the magnetic resonance apparatus. 
     In a preferred embodiment, the circulator is arranged at an input port of the radio-frequency antenna. A wave reflected by the radio-frequency antenna is directly discharged at the input port in this way so that all further components of the radio-frequency system are protected. 
     In another embodiment, a third gate of the circulator is terminated by a load. In this case, the circulator acts as an isolator that lets the signals pass only in one direction—from the radio-frequency power amplifier to the radio-frequency antenna. The load thus can be attached to the circulator or be installed in the circulator. 
     In another embodiment, a third gate of the circulator is connected with a receiver. In this way an otherwise typical transmission-reception diplexer can be replaced by the circulator. In the transmission case, signals are conducted from the radio-frequency power amplifier via the first gate to the second gate and to the radio-frequency antenna. In the reception case, measurement signals are relayed by the radio-frequency antenna from the second gate via the third gate of the circulator to the receiver. The receiver is fashioned such that, at least in transmission operation, it represents a load, i.e. represents a fixed-power terminator that absorbs (“swamps”) power coming from the antenna. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates a magnetic resonance apparatus having a radio-frequency system embodying a circulator in accordance with the present invention. 
         FIG. 2  is a perspective view of an embodiment of a circulator according to the present invention. 
         FIG. 3  schematically illustrates a first embodiment of a radio-frequency system of a magnetic resonance apparatus embodying a circulator according the present invention. 
         FIG. 4  schematically illustrates a second embodiment of a radio-frequency system of a magnetic resonance apparatus embodying a circulator according the present invention. 
         FIG. 5  schematically illustrates a third embodiment of a radio-frequency system of a magnetic resonance apparatus embodying a circulator according the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  schematically shows the design of a magnetic resonance apparatus  1 . The components of the magnetic resonance apparatus  1  with which the actual measurement is implemented are located in a radio-frequency-shielded measurement chamber  3 . In order to examine a body by means of magnetic resonance imaging, various magnetic fields tuned as precisely as possible to one another in terms of their temporal and spatial characteristics are radiated at the body. 
     A strong magnet (typically a cryomagnet  5  with a tunnel-shaped opening) generates a static, strong basic magnetic field  7  that is typically 0.2 Tesla to 3 Tesla and more. A body (not shown) to be examined is borne on a patient bed  9  and positioned in the basic magnetic field  7 . 
     The excitation of the nuclear spins of the body ensues via magnetic radio-frequency excitation pulses that are radiated via a radio-frequency antenna (shown here as a body coil  13 ). The radio-frequency excitation pulses are generated by a pulse generation unit  15  that is controlled by a pulse sequence control unit  17 . After an amplification by a radio-frequency power amplifier  19 , they are relayed to the radio-frequency antenna. The radio-frequency system shown here is merely schematically indicated. Typically more than one pulse generation unit  15 , more than one radio-frequency power amplifier  19  and multiple radio-frequency antennas are used in a magnetic resonance apparatus  1 . 
     Furthermore, the magnetic resonance apparatus  1  has gradient coils  21  with which gradient fields for selective slice excitation and for spatial coding of the measurement signal are radiated in a measurement. The gradient coils  21  are controlled by a gradient coil control unit  23  that, like the pulse generation unit  15 , is connected with the pulse sequence control unit  17 . 
     The signals emitted by the excited nuclear spins are received by the body coil  13  and/or by local coils  25 , amplified by associated radio-frequency pre-amplifiers  27  and processed further and digitized by a receiver  29 . 
     An image processing unit  31  generates an image from the measurement data, which image is presented to a user via an operating console  33  or is stored in a memory unit  35 . A central computer  37  controls the individual system components. 
     The radio-frequency antenna with which the excitation pulses are radiated (in this case the body coil  13 ) must be adapted with optimal precision to the upstream radio-frequency system so that as little energy as possible is reflected at the input port of the radio-frequency antenna. As described above, however, an optimal adaptation of the radio-frequency antenna is not always possible. 
     If the radio-frequency antenna additionally has multiple input ports coupled among one another, as is often typical in magnetic resonance apparatuses, interfering couplings can occur between the individual antenna ports, such that the voltages over-coupled at the input ports add to a voltage reflected at the input port. 
     In order to prevent a signal returning at the input port of the radio-frequency antenna from returning to the radio-frequency system and loading this system, the input port of the radio-frequency antenna and the output port of the radio-frequency system are connected with a circulator  39 , and in fact such that a signal sent from the radio-frequency power amplifier  19  to the radio-frequency antenna is passed through largely unattenuated by the circulator  39  while a signal in the reverse direction is largely blocked. 
     In the example shown here, the circulator  39  is simultaneously connected with the receiver  29  such that the circulator  39  fulfills the function of an otherwise necessary transmission-reception diplexer. However, this is merely one specific arrangement of the circulator  39  in the radio-frequency system of the magnetic resonance apparatus  1 . Various arrangement variants are explained in detail later in  FIG. 3  through  FIG. 5 . 
     The circulator  39  is arranged in proximity to the cryomagnet  5 , and in fact such that the circulator  39  has the typical non-reciprocal property due to the interaction of the static basic magnetic field  7  with its ferrite. The circulator thus is given its non-reciprocal property as long as long as an interaction of its ferrite with the static basic magnetic field  7  occurs. The location of the attachment of the circulator  39  is thereby selected such that the magnetic field strength predominating there interacts with the circulator  39  (in particular with its ferrite) such that the circulator  39  is tuned to the Larmor frequency of the magnetic resonance apparatus  1 . 
     Those locations at which the static basic magnetic field  7  has the magnetic field strength suitable for the operation of the circulator  39  form an area that is typically rotationally-symmetrical around the longitudinal axis of the cryomagnet  5 . The circulator  39  can be arranged at multiple points of this area, advantageously at the point at which the feed or discharge cable can be fashioned optimally short and therefore cost-effectively. 
     In the event that multiple circulators  39  are used (for example to protect various input ports of a multi-port antenna), these can likewise be arranged rotationally-symmetrically around the longitudinal axis of the cryomagnet  5  since the strength of the basic magnetic field  7  remains the same at these locations. 
     The design and the interaction of the ferrites with the magnetic field is now explained in detail now in  FIG. 2 . 
     The circulator  39  shown in  FIG. 2  comprises an electrical circuit board  41  that possesses three gates  43  respectively offset by 120°. The electrical circuit board  41  in this embodiment is fashioned in a Y-shape. It can also exhibit other laminar shapes with a rotational symmetry of 120° as they are used in conventional circulators. The electrical circuit board  41  lies between two disc-shaped ferrites  45 . For their part, the ferrites  45  lie between two base plates lying at the same potential, of which only the rear base plate  47  is shown for clarity. 
     The circulator  39  is arranged in proximity to the cryomagnet  5  such that the static basic magnetic field  7  generated by the cryomagnet  5  has a component that intersects the ferrite  45  perpendicularly. In this way the ferrite  45  has the typical gyrotropic property that imparts to the circulator  39  the non-reciprocal functionality typical to it. A power presented at one gate is passed on nearly unattenuated to the next gate while the following gate is largely decoupled. 
     Since the circulator  39  no longer needs permanent magnets for its function, the circulator  39  is overall more cost-effective to produce. Moreover, the circulator  39  can be cooled better and more efficiently since it has fewer components than a conventional circulator, and these fewer components are, moreover, better accessible. In the circulator  39  shown here, a centrally arranged cooling body  49  that dissipates the heat arising in the circulator  39  from the circulator  39  into the environment is schematically indicated at the rear base plate  47 . Since the cooling of the circulator  39  can be designed more simply and efficiently, the operating temperature of the circulator  39  is subjected to fewer fluctuations in comparison to conventional circulators, such that a more precise and more constant signal transmission is achieved with the circulator  39 . 
     Additional components of the circulator  39  such as, for example, connection bushings to connect conductors to the three gates of the circulator or dielectric separator layers that surround the ferrite  45  and contribute to the electrical separation of the circuit board  41  from the base plates  47  are not shown for clarity, however do not differ from known circulators. 
     Various arrangement variants of the circulator  39  in a radio-frequency system of a magnetic resonance apparatus  1  are now explained in detail in  FIG. 3  through  FIG. 5 . The principle of possible appropriate arrangements of the circulator  39  in the radio-frequency system is primarily explained in  FIG. 3  through  FIG. 5 . The radio-frequency system itself is not limited to the forms shown in  FIG. 3  through  FIG. 5 . 
       FIG. 3  shows a schematic section from a radio-frequency system of a magnetic resonance apparatus  1  in which the circulator  39  according to the invention is respectively arranged at an input port of a radio-frequency antenna  51 . The radio-frequency system in the exemplary embodiment shown here is designed such that two different radio-frequency antennas  51  (for example a body coil and a body matrix coil) can be alternately activated via a coil diplexer  53  with the radio-frequency system. A circulator  39  is respectively arranged at each of the input ports of the radio-frequency antennas  51 . The third gate of each circulator  39  is terminated with a load  55 . 
     The signals coming from the radio-frequency power amplifier  19  are passed on by the circulators  39  to the radio-frequency antennas  51  while the energy of a wave that was reflected at the radio-frequency antennas  51  is discharged into the load  55 . In this way the circulator  39  protects the components upstream from it, such as (for example) the radio-frequency power amplifier  19 , the coil diplexer  53  or the supplying coaxial cables  57  that, due to this, can be designed more cost-effectively since they must tolerate smaller loads. The coaxial cables  57  are merely shown indicated in a section of the radio-frequency system for clarity. 
     A transmission-reception diplexer  59  is respectively located between the radio-frequency antennas  51  and the circulators  39 , such that the two radio-frequency antennas  51  can also be used as receiver antennas. In this case a signal received by the radio-frequency antennas is relayed to a receiver  29 . 
       FIG. 4  shows a schematic section of a different embodiment variant of the radio-frequency system. Here the circulator  39  is arranged at the output port of the radio-frequency power amplifier  19 . In comparison to the variant in  FIG. 3 , this variant has the advantage that only one circulator  39  is necessary in order to protect the radio-frequency power amplifier  19 . For this, the following components (such as the coaxial cable  57  leading to the radio-frequency antennas  51 , the coil diplexer  53  or the transmission-reception diplexers  59 ) must be dimensioned so that they withstand the load due to a wave reflected at the radio-frequency antennas  51 . 
     In the variants shown in  FIG. 3  and  FIG. 4 , the third gate of the circulator  39  is respectively terminated with a load  55 ; the circulator  39  is thus used as a one-way conductor (isolator). 
       FIG. 5  shows an embodiment variant in which the circulators  39  are likewise arranged at the input ports of the radio-frequency antennas  51  but are simultaneously used as transmission-reception diplexers. In this embodiment, the third gate of the circulator  39  is connected with a receiver  29  so that—in the event that the radio-frequency antennas  51  are used to receive nuclear magnetic resonance signals—the reception signal is relayed by the circulator  39  to the receiver  29  of the radio-frequency system. 
     In this case the transmission-reception diplexers  59  connected between the circulators  39  and the receiver  29  serve to discharge a power discharged by the circulators  39  in transmission operation into a load. In the reception case, they transmission-reception diplexers  59  are switched such that the signal arriving from the circulator is relayed to the receiver  29 . In that the transmission-reception diplexers  49  must only tolerate a load due to a return power, they can overall be dimensioned smaller compared with the transmission-reception diplexers  59  from  FIG. 3  or  4 , which must also tolerate the power provided by the radio-frequency power amplifier  19  in addition to the return power. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted heron all changes and modifications as reasonably and properly come within the scope of their contribution to the art.