Abstract:
A magnetic resonance system has a base body embodying a magnet system that generates magnetic fields in an excitation region, a patient bed that is movable, with a patient thereon through the base body, a local coil that is operable to detect magnetic resonance signals from the patient, and an evaluation device that evaluates the magnetic resonance signals detected by the local coil. A base body coupling element, at a predetermined base body location, is connected to the evaluation device and inductively or capacitively couples with a patient bed coupling element, located at a predetermined patient bed location, that is connected to the local coil. The magnetic resonance signals are fed from the local coil to the evaluation device via the patient bed coupling element and the base body coupling element.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention concerns a magnetic resonance system of the type having a base body and a patient bed, wherein the base body contains a magnet system that generates magnetic fields in an excitation region to excite nuclear spins in a patient in the excitation region, and wherein the patient bed is movable in a travel direction relative to the base body. 
   2. Description of the Prior Art 
   Magnetic resonance systems of the above type are generally known. 
   Conventionally the acquisition of the magnetic resonance signals emitted by the examination subject ensues either with a whole-body antenna or with local coils. If the magnetic resonance signal is acquired by a whole-body antenna, the magnetic resonance signal can be acquired from the entire excitation region. The acquisition is possible only with relatively low signal-to-noise ratio (SNR). Therefore in many cases local coils are also used in magnetic resonance systems, often multiple local coils. The local coils are arranged in proximity to the examination subject (normally a person) and can therefore acquire with good SNR, even if only from a small part of the excitation region. Moreover, due to the spatial resonance by the arrangement of the local coils the spatial coding with gradient fields can as such be extended. The required measurement time for an acquisition can thus be reduced. 
   In order to be able to utilize the advantages of local coils for imaging of the entire body of a person from head to foot, a large number of local coils is required that are placed on the patient in a number of planes arranged along the travel direction of the patient bed. These planes are often designated as levels. 
   The excitation region typically exhibits a length of approximately 40 to 60 cm as viewed in the patient bed travel direction. Only a portion of the body of the person can therefore ever be imaged, namely the portion that is located in the excitation region. For this reason the patient bed with the patient located thereupon must be moved bit by bit through the excitation region. The local coils located in the excitation region must respectively be activated and be connected with an evaluation device for evaluation of magnetic resonance signals. The other local coils can be deactivated. It is thereby possible to manage with a relatively low number of reception channels even though many local coils are present. 
   In conventional magnetic resonance systems the aforementioned type of examination is conducted by connecting all local coils, via a corresponding number of plugs at the patient bed and via a movable cable harness, to an evaluation device that is arranged at the base body i.e., the stationary part of the MR apparatus. Due to the attenuation of the long, thin cables that are used, pre-amplifiers must be arranged in the local coils. Each local coil must furthermore have a detuning circuit in order to be able to be deactivated upon non-usage and given transmission. Complicated common mode chokes (known as sheath wave barriers) must also be inserted into the long cable harness in order to be able to limit induced voltages that occur in the transmission mode. 
   For the back region of the patient it is possible to arrange the acquisition array in a fixed manner under the movable patient bed in the excitation region. In this case the number of these local coils must only be sufficient for the excitation region. If the examination concerns the back region of the patient, it is therefore possible to save many local coils, their cabling and the channel selection. The distance from the examination subject is increased only by the relatively slight thickness of the patient bed itself, which is most cases quite tolerable. 
   By contrast, this procedure cannot be used or can just barely be realized on the top side of the examination subject since the thicknesses of the patient and the various body regions of the patient are very different. Primarily for thin patients or, for example, at the head or legs, an acquisition coil array permanently installed in the excitation region would be far removed from the body surface, such that the advantage of the local coils (namely a high spatial resolution and a good SNR) would be lost. 
   A plug connection for local coils that operates without contact (namely via inductive coupling) is known from DE 101 30 615 C2. This teaching already represents an advance since a galvanic contact between the local coil and the evaluation device is no longer required for coupling of a local coil to the evaluation device. However, as before the requirement of an active plugging of the connection by an operating personnel exists. The local coil must also be manually connected to the evaluation device or disconnected therefrom. 
   From DE 35 00 456 C2 it is known to couple a local coil with the whole-body antenna. Here a contactless coupling is in fact realized. However, this coupling is only possible for a single coil, and even then only given a suitable orientation of the local coil. The teaching of DE 35 00 456 C2 thus cannot be extended to a number of local coils. Here as well the local coil must be actively connected to the whole-body antenna or disconnected therefrom. 
   From EP 0 437 049 A2 it is known to directly, inductively couple one local coil to another coil that is arranged in the immediate proximity of the local coil. Again, the local coil must be actively connected to the evaluation device or disconnected from it. 
   A magnetic resonance system with a local coil is known from DE 197 51 017 A1, wherein the local coil is inductively coupled with a decoupling coil which is connected with the evaluation device via electrical conductors. 
   A magnetic resonance system with a local coil is known from U.S. Pat. No. 5,243,289, wherein the local coil is connected with coupling elements which are inductively coupled with an inductor, and the inductor is connected with the evaluation device. The degree of the inductive coupling can be adjusted by variation of the relative position (distance and/or overlap) of the inductor and the coupling elements. 
   A magnetic resonance system with a local coil is known from DE 39 35 082 D1, wherein the local coil is connected to a plug connection that is arranged at the patient bed. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to further develop as magnetic resonance system of the aforementioned type such that a local coil can be automatically coupled with an evaluation device when it is located in the excitation region and is otherwise caused to be decoupled from the evaluation device. 
   The object is achieved via a magnetic resonance system of the aforementioned type wherein
         a base body coupling element that is connected with an evaluation device for evaluation of magnetic resonance signals is arranged at the base body at a predetermined base body location,   a patient bed coupling element that is connected with a local coil for acquisition of a magnetic resonance signal is arranged at a predetermined patient bed location at the patient bed, and   the base body coupling element and the patient bed coupling element are arranged and fashioned such that the magnetic resonance signal acquired by the local coil can be fed via the patient bed coupling element and the first base body coupling element to the evaluation device when and as long as the patient bed has moved by a predetermined segment of the travel region.       

   Naturally, the segment of the travel region must be suitably selected but, this is possible without further measures. 
   When the patient bed is moved through the travel region and only the patient bed coupling element is present at the patient bed, the base body coupling element remains unutilized when the patient bed has not moved by the segment. Therefore (if the aforementioned base body coupling element is designated as a first lease body coupling element and the segment of the travel region is designated as a first segment of the travel region) a second patient bed coupling element that is connected with a second local coil for acquisition of a magnetic resonance signal is advantageously arranged at the patient bed at a predetermined second patient bed location. In this case the second patient bed coupling element is arranged and fashioned such that the magnetic resonance signal acquired by the second local coil can be fed to the evaluation device via the second patient bed coupling element and the first base body coupling element when and as long as the patient bed has moved by a predetermined second segment of the travel region. 
   The first and the second segments of the travel region can be disjoint, thus can be spaced from one another or only border one another. However, the predetermined first segment and the predetermined second segment advantageously meet one another in an overlap region. A sliding transition from the first local coil to the second local coil then ensues given movement of the patient bed. It is optimal when the first segment and the second segment are of equal size and the overlap region is approximately half as large as the first segment. 
   In an analogous manner, the first patient bed coupling element remains unutilized when the patient bed is not moved by the first segment and only the first base body coupling element is present at the base body. A second base body coupling element that is connected with the evaluation device is therefore advantageously arranged at the base body at a predetermined second base body location. The second base body coupling element is then arranged and fashioned such that the magnetic resonance signal acquired by the first local coil can be fed to the evaluation device via the first patient bed coupling element and the second patient bed coupling element when and as long as the patient bed has moved by a predetermined third segment of the travel region. The third segment can be identical with the second segment. 
   When the first base body coupling element is connected with the evaluation device via a preamplifier, the number of preamplifiers can be minimized. One preamplifier per local coil is then no longer required, rather only one preamplifier per usable acquisition channel. 
   The excitation region normally extends in the travel direction over an excitation region length that is a multiple of the size of the first segment. The multiple can (but does not have to) be an integer multiple of the size of the first segment. 
   The travel region likewise normally exhibits a travel region length that is a multiple of the excitation region length. Here it is in fact possible but not absolutely necessary that the multiple is an integer multiple. 
   The first patient bed coupling element is advantageously fashioned such that it detunes the first local coil when the first local coil cannot be coupled to the evaluation device. An automatic detuning of the first local coil then ensues. 
   The coupling elements (thus the first base body coupling element and the first patient bed coupling element, possibly also second etc. base body and patient bed coupling elements) can alternatively be fashioned as inductive coupling elements or as capacitive coupling elements. In both cases a number of advantageous embodiments are possible. 
   In an embodiment of the first coupling elements as inductive coupling elements, for example, it is possible to provide only a simple detuning capacitor between the first patient bed coupling element and the first local coil. However, in this case a degradation of the SNR by 10 to 20% must be accepted. A capacitive transformation circuit comprising a number of capacitors is therefore advantageously arranged between the first patient bed coupling element and the first local coil. The losses of SNR can be limited to approximately 2% by means of such a transformation circuit. 
   The first patient bed coupling element advantageously has a first patient bed conductor loop and a second patient bed conductor loop that respectively generate a temporally variable magnetic field upon feed of a magnetic resonance signal from the first local coil to the evaluation device, which temporally variable magnetic field is oriented perpendicular to the travel direction, whereby the magnetic fields generated by the patient bed conductor loops are oriented inversely relative to one another at every point in time. The two patient bed conductor loops are then essentially decoupled from the magnet system and are also essentially insensitive for the emitted magnetic resonance signal. The two patient bed conductor loops can thereby alternatively be connected in series or in parallel to one another. 
   The first base body coupling element is fashioned analogous to the first patient bed coupling element. Here the two base body conductor loops can also alternatively be connected in series or in parallel to one another. The interconnection of the base body conductor loops (in series or in parallel) can thereby be selected independent of the interconnection of the patient bed conductor loops. 
   As already mentioned, the magnet system comprises a whole-body antenna by means of which an essentially homogeneous, radio-frequency magnetic field can be generated in the entire excitation region. The whole-body antenna in all cases has transmission elements that are oriented parallel to the travel direction. A particularly space-saving arrangement of the first base body coupling element therefore results when the first base body coupling element is arranged between two immediately adjacent transmission elements or is integrated into one of the transmission elements. 
   In the latter case (integration into one of the transmission elements) it is advantageous for the first and the second whole-body conductor loops have segments running parallel to the travel direction, with capacitors arranged in these segments and the capacitors being dimensioned such that an excitation current oscillating in the appertaining transmission element causes no signal in the first base body coupling element. The first base body coupling element is then decoupled from the appertaining transmission element. 
   When the coupling elements are fashioned as capacitive coupling elements, it is preferred that the coupling elements are respectively fashioned as a pair of narrow coupling strips. For example, the coupling strips can respectively be approximately 2×10 cm in size and, viewed transverse to the main surface of the coupling strips, the coupling strips of the first patient bed coupling element can be spaced by approximately 0.3 to 1.0 mm from those of the first base body coupling element. 
   The coupling strips of the first patient bed coupling element are advantageously adjacent to one another on their narrow sides. An unavoidable parasitic capacitive interaction of the coupling strips of the patient bed coupling element with one another is then minimal. The coupling strips of the first base body coupling element are naturally fashioned similarly. 
   When a choke is connected in parallel with the first patient bed coupling element, an automatic detuning of the local coil is ensured for the case that the appertaining local coil cannot be coupled to the evaluation device. 
   A protective circuit is advantageously arranged between the first base body coupling element and the evaluation circuit, which protective circuit compensates the series blind resistance of the first local coil, the first patient bed coupling element and the first base body coupling element upon feeding of a magnetic resonance signal from the first local coil via the first patient bed coupling element and the first base body coupling element to the evaluation circuit, and which protective circuit detunes the first base body coupling element when no patient bed coupling element interacts with the first base body coupling element. 
   Independent of the embodiment of the first base body coupling element as a capacitive or as an inductive coupling element, it is in principle possible to arrange the first base body coupling element outside of the excitation region. However, the first base body coupling element is advantageously arranged within the excitation region. 
   The first base body coupling element, independent of its concrete embodiment, can likewise advantageously be detuned by means of a blocking circuit. The blocking circuit can, for example, be fashioned as an inductor that is connected via a typical PIN diode. 
   Furthermore it is possible to operate the local coil not only as an acquisition coil but rather also as a transmission coil. In this case a signal splitter that is connected with an RF driver element is arranged between the first base body coupling element and the evaluation circuit. A magnetic resonance excitation signal emitted by the RF driver element can be fed via the first base body coupling element and the first patient bed coupling element into the first local coil when and as long as the patient bed has moved by the first segment. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  schematically illustrates a magnetic resonance system. 
       FIG. 2  is a section through the magnetic resonance system of  FIG. 1 . 
       FIG. 3  is a perspective view of a patient bed and base body coupling elements. 
       FIG. 4  is a further section through the magnetic resonance system of  FIG. 1 . 
       FIG. 5 , schematically illustrates arrangements of the patient bed coupling elements relative to base body coupling elements. 
       FIG. 6  illustrates degrees of coupling between a patient bed coupling element and adjacent base body coupling elements. 
       FIG. 7  illustrates degrees of coupling of adjacent patient bed coupling elements with a base body coupling element. 
       FIG. 8  schematically illustrates the signal flow from a local coil to an evaluation device. 
       FIGS. 9 and 10  respectively show a local coil and a patient bed coupling element with a capacitive transformation circuit. 
       FIG. 11  is an exemplary embodiment of inductive coupling elements. 
       FIG. 12  shows a whole body antenna with base body coupling elements. 
       FIG. 13  shows a further embodiment of the arrangement of  FIG. 12 . 
       FIG. 14  shows a detail from the embodiment of  FIG. 3 . 
       FIG. 15  schematically illustrates the signal flow from a local coil to an evaluation circuit. 
       FIGS. 16 and 17  illustrate different embodiments of coupling elements. 
       FIG. 18  shows a further embodiment of the inventive arrangement of coupling elements. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   According to  FIG. 1 , a magnetic resonance system has a base body  1 . The base body  1  embodies a magnet system by means of which magnetic fields can be generated in an excitation region  2 . 
   The magnet system includes at least one basic magnet  3  for generation of a temporally static basic magnetic field that is spatially at least essentially homogeneous within the excitation region  2 . The magnet system furthermore includes a whole-body antenna  4  by means of which a radio-frequency magnetic field can be generated that is at least essentially homogeneous in the entire excitation region  2 . The magnet system normally additionally includes gradient magnets for generation of gradient fields and a shielding magnet. 
   The magnetic resonance system according to  FIG. 1  furthermore has a patient bed  5 . The patient bed  5  can be moved in a travel direction z over a travel region relative to the base body  1 . The travel region is determined such that—as viewed in the travel direction z—each point of the patient bed  5  can be positioned in the excitation region  2 . Since the excitation region  2  normally extends over an excitation region length l which is approximately 40 to 60 cm in the travel direction z and the patient bed exhibits a length L on the order of 2 m, the travel region length of the patient bed  5  is thus inevitably a multiple of the excitation region length l. 
   Due to the mobility of the patient bed  5 , an examination subject  6  (normally a person  6 ) can be brought into the excitation region  2  by corresponding movement of the patient bed  5  when said examination subject  6  is arranged on the patient bed  5 . If the examination subject  6  is introduced into the excitation region  2 , it can thus be excited to emit a magnetic resonance signal via corresponding activation of the magnet system (in particular of the whole-body antenna  4 ) and generation of suitable magnetic fields corresponding with this. 
   It is possible to acquire the emitted magnetic resonance signal by means of the whole-body antenna  4  and to feed it to an evaluation device  7  by which the magnetic resonance signal can be evaluated. However, only a qualitatively low-grade reconstruction of the examination subject  6  is possible in this manner. Local coils  8  by means of which a significantly higher-grade magnetic resonance signal can be acquired (even if only over a small volume per local coil  8 ) are therefore normally arranged on the examination subject  6 . In contrast to the prior art, the local coils are not connected with the evaluation device  7  via a cable. The lines between the local coils  8  and the evaluation device  7  are therefore indicated only with dashes in  FIG. 1 . The type and manner of the connection of the local coils  8  to the evaluation device  7  is the subject matter of the present invention. 
   As can be seen from  FIG. 1  and as shown in more detail in  FIG. 2 and 3 , many local coils  8  are normally arranged on the examination subject  6 . Those local coils  8  that are arranged at essentially the same height (as viewed in the travel direction z) thereby respectively form what is known as a level. Depending on the situation of the individual case, the levels can cover the entire body of the examination subject  6 . 
   Each local coil  8  is connected with a patient bed coupling element  9 . A 1:1 association thereby exists. Each local coil  8  is thus connected with a single patient bed coupling element  9  and, in reverse, each patient bed coupling element  9  is also connected with only a single local coil  8 . Each patient bed coupling element  9  is arranged at a predetermined point of the patient bed  5 . Insofar as it is necessary, these points are subsequently called patient bed points since they are defined with regard to the patient bed  5 . 
   The patient bed coupling elements  9  are arranged in a plurality of rows  10  (see in particular  FIG. 3 ). Each row  10  extends in the travel direction z over a length that is at maximum as large as the length L of the patient bed  5 . Each row  10  of patient bed coupling elements  9  can therefore extend at maximum over approximately 2 m as viewed in the travel direction z. Within each row  10  the patient bed coupling elements  9  follow one another with a small interval a. The interval a is typically 8 to 15 cm, in particular 10 to 12 cm. 
   Furthermore, base body coupling elements  11  that are connected with the evaluation device  7  are arranged at the base body  1  for connection of the local coils  8  to the evaluation device  7 . The base body coupling elements  11  are arranged at predetermined points of the base body  1 . Insofar as it is necessary, these points are subsequently called base body points since they are defined with relation to the base body  1 . 
   As is apparent from  FIGS. 3 and 4 , the base body coupling elements  11  are also arranged in rows  12 . According to  FIG. 4 , each row  12  of base body coupling elements  11  likewise extends in the travel direction z, however only over a smaller length than the patient bed  5 , namely essentially over the excitation region length l. Each row  12  of base body coupling elements  11  interacts with one of the rows  10  of patient bed coupling elements  9 . 
   From  FIG. 4  it is also apparent that the base body coupling elements  11  are arranged in the excitation region  2 . Due to the circumstance that a plurality of base body coupling elements  11  are arranged one after another in the excitation region  2  as viewed in the travel direction z, from  FIG. 4  it is thus also apparent that an interval a of the base body coupling elements  11  is significantly smaller (as viewed in the travel direction z) than the excitation region length l. The interval a of the base body coupling elements  11  from one another is generally the same as the interval a of the patient bed coupling elements  9  from one another. 
   The arrangement of the rows  10  of patient bed coupling elements  9  and the arrangement of the rows  12  of base body coupling elements  11  are matched to one another such that the local coils  8  associated with the respective row  10  of patient bed coupling elements  9  can feed the magnetic resonance signals acquired by them to the evaluation device  7  via the patient bed coupling elements  9  of the respective row  10  and the base body coupling elements  11  of the corresponding row  12  of base body coupling elements  11 . This naturally only applies when the respective patient bed coupling element  9  of a row  10  of patient bed coupling elements  9  is arranged in the active region of one of the base body coupling elements  11  of the corresponding row  12  of base body coupling elements  11 . This is subsequently explained in detail in connection with  FIG. 5 . 
   A transmission channel is represented in solid lines in  FIG. 5 . According to  FIG. 5 , a local coil  8  is thereby connected with a patient bed coupling element  9 . The patient bed coupling element  8  couples with a base body coupling element  11 . The base body coupling element is connected with the evaluation device  7  via a preamplifier  13 . 
   In all cases a number of such transmission channels exist at a specific point in time. This is represented with dashed lines in  FIG. 5  for two further transmission channels, but only the transmission channel indicated in solid lines is considered first in the following. The elements  8 ,  9 ,  11  of this transmission channel are subsequently designated as first elements  8 ,  9 ,  11 , thus as a first local coil  8 , first patient bed coupling element  9  and first base body coupling element  11 . 
   When the patient bed  5  is moved over its travel region, the first patient bed coupling element  9  at some point reaches a position that corresponds to the first patient bed coupling element  9  that in  FIG. 5  is drawn in dashes above the first patient bed coupling element  9 . In this movement position a coupling would in fact possibly exist with the base body coupling element  11  drawn in dashes in FIG above the first base body coupling element  11 , however not with the first base body coupling element  11 . A degree of coupling k 1  of the first patient bed coupling element  9  with the first base body coupling element  11  is thus zero in this travel position (see  FIGS. 6 and 7 ). 
   When the patient bed  5  is now moved further in the travel direction z, the degree of coupling k 1  with which the first patient bed coupling element  9  couples with the first base body coupling element  11  increases gradually toward a maximum value. This state is reached when the first patient bed coupling element  9  and the first base body coupling element  11  are situated precisely opposite one another corresponding to the representation from  FIG. 5 . After this the degree of coupling k 1  gradually decreases again to zero. 
   The segment of the travel region in which the degree of coupling k 1  is greater than zero, is subsequently called the first segment. The magnetic resonance signal can only be fed from the first local coil  8  via the first patient bed coupling element  9  and the first base body coupling element  11  to the evaluation device  7  only when and as long as the patient bed  5  has been moved by this segment of the travel region. The first segment exhibits a size that is at maximum twice as large as the interval a of the base body coupling elements  11  from one another. It is thus significantly smaller than the excitation region length l. This amounts to a multiple of the size of the first segment. 
   The patient bed coupling elements  9  are normally all designed identically. The base body coupling elements  11  are also normally all designed identically. Therefore, not only can the magnetic resonance signal that is acquired by the first local coil  8  be acquired via the first base body coupling element  11 , but rather also the magnetic resonance signals of other local coils  8  insofar as their patient bed coupling element  9  is arranged in the same row  10  as the first patient bed coupling element  9 . The patient bed  5  must merely be moved by another segment (subsequently called the second segment) that is specific for the respective other patient bed coupling element  9 . The magnetic resonance signal that is acquired by the first local coil  8  can be transferred in an analogous manner not only via the first base body coupling element  11  but also via another base body coupling element  1  to the evaluation device  7  when this other base body coupling element  11  is arranged in the same row  12  of base body coupling elements  11  as the first base body coupling element  11 . It is only necessary to move the patient bed by another segment (subsequently called the third segment) that is specific to the respective base body coupling element  11 . 
   In addition to the degree of coupling k 1 , a degree of coupling k 2  is drawn in  FIG. 6  with which the patient bed coupling element  9  (which is drawn dashed in  FIG. 5  above the first patient bed coupling element  9 ) couples with first base body coupling element  11 . The first segment and the second segment clearly overlap one another in an overlap region  14 . Furthermore, the overlap region  14  is approximately half as large as the first segment. Due to the similar design of the coupling elements  9 ,  11  and the regular spacing a of the coupling elements  9 ,  11  from one another, the first and the second segments are also equally large. 
   In addition to the degree of coupling k 1 , a degree of coupling k 3  is drawn in  FIG. 7  with which the patient bed coupling element  9  couples with first base body coupling element  11  that, in  FIG. 5 , is drawn dashed below the first base body coupling element  11 . The first segment and the third segment also clearly overlap one another in an overlap region  15  that is approximately half as large as the first segment. Furthermore, the third segment is as large as the first segment. The third segment is in particular identical with the second segment. 
   As already mentioned, the patient bed coupling elements  9  are in all cases designed identically among one another. This is if anything absolutely necessary at least per row  10  of patient bed coupling elements  9 . The base body coupling elements  11  are also in all cases designed identically, whereby here as well the same design is necessary within each row  12  of base body coupling elements  11 . Rows  10 ,  12  of patient bed coupling elements  9  and base body coupling elements  11  interacting with one another must also be fashioned such that the respective coupling elements  9 ,  11  can interact. When the design of a single patient bed coupling element  9  and of a single base body coupling element  11  is subsequently described, this specification is therefore exemplary for all patient bed coupling elements  9  and all base body coupling elements  11  at least of the respective pair of rows  10 ,  12  of coupling elements  9 ,  11 . 
   According to  FIG. 8 , the patient bed coupling element  9  and the base body coupling element  11  are fashioned as inductive coupling elements  9 ,  11 , for example. The local coils  8  therefore exhibit an inductance L 1 , the patient bed coupling element an inductance L 2  and the base body coupling element  11  an inductance L 3 . The local coil  8  is thereby tuned to the Larmor frequency of the magnetic resonance system by means of a capacitor  16  that exhibits a capacitance C 1 . 
   When the patient bed  5  is moved such that the patient bed coupling element  9  couples with none of the base body coupling elements  11 , the capacitor  16  and the patient bed coupling element  9  form an oscillating circuit that is resonant at the Larmor frequency of the magnetic resonance system. The patient bed coupling element  9  is therefore fashioned such that it detunes the local coil  8  when the local coil  8  cannot be coupled to the evaluation device  7 . For protection against a possible malfunction of the patient bed coupling element  9  it is possible to install a safety element (for example a typical fuse) into the local coil  8  if applicable. 
   The base body coupling element  11  should likewise be deactivated in an analogous manner when no patient bed coupling element  9  is situated opposite it. A controllable blocking circuit  17  is therefore associated with the base body coupling element  11 . In the simplest case the blocking circuit  17  includes a capacitor  18 , a coil  19  and a PIN diode  20 . The capacitor  18  exhibits a capacitance C 3 , the coil  19  an inductance L 4 . If the PIN diode  20  is activated, the coil  19  and the capacitor  18  form a trap circuit that is resonant at the Larmor frequency of the magnetic resonance system. The blocking circuit  17  therefore separates the preamplifier  13  and the base body coupling element  11  from one another. The base body coupling element  11  is thus decoupled from the preamplifier  13  at the Larmor frequency, thus can be detuned by means of the blocking circuit  17 . 
   In contrast to this, when the patient bed  5  is moved such that the base body coupling element  11  couples with the patient bed coupling element  9  differentiation must be made between transmission case and acquisition case. 
   In the transmission case the blocking circuit  17  is activated. The base body coupling element  11  therefore does not couple with the patient bed coupling element  9 , such that the patient bed coupling element  9  furthermore detunes the local coil  8 . 
   In contrast to this, in the acquisition case the blocking circuit  17  is not activated, such that the local coil  8  is coupled to the preamplifier  13  via the patient bed coupling element  9  and the base body coupling element  11 . The inductance L 4  of the coil  19  is selected such that even in this case the local coil  8  is loaded only at high resistance. 
   The unit of  FIG. 8  composed of the local coil  8 , capacitor  16  and patient bed coupling element  9  is functional but exhibits a relatively low SNR. According to  FIGS. 9 and 10 , a capacitive transformation circuit  21  that comprises a plurality of capacitors  22  is therefore advantageously arranged between the patient bed coupling element  9  and the local coil  8 . The capacitors  22  drawn in  FIGS. 9 and 10  with solid lines are thereby absolutely necessary; the capacitors  22  drawn with dashed lines are merely optional. The degradation of the SNR to 1 to 2% can be limited with the embodiments according to  FIGS. 9 and 10 . 
   The patient bed coupling element  9  should be designed such that it does not couple with the excitation field of the whole-body antenna  4 . For this reason the patient bed coupling element  9  according to  FIG. 11  advantageously comprises a first and a second patient bed conductor loop  23 ,  24 . When a magnetic resonance signal is fed from the local coil  8  to the patient bed coupling element  9 , this magnetic resonance examination signal respectively induces a loop current l 1 , l 2  in both the first and the second patient bed conductor loop  23 ,  24 . Corresponding magnetic fields naturally correspond with the loop currents l 1 , l 2 . Since the magnetic resonance signal and thus also the loop currents l 1 , l 2  are radio-frequency, the magnetic fields are temporally variable. 
   The patient bed conductor loops  23 ,  24  have segments  25  through  27  that run parallel to the travel direction z. The patient bed conductor loops  23 ,  24  therefore extend essentially in a plane which contains the travel direction z. The temporally variable magnetic fields are thus oriented perpendicular to the travel direction z. However, since the loop currents  11 , l 2  flow inversely in the patient bed conductor loops  23 ,  24 , the corresponding magnetic fields are oriented inversely at every point in time. 
   According to  FIG. 11 , the two patient bed conductor loops  23 ,  24  are connected parallel to one another but, they can also be connected in series with one another. 
   The base body coupling elements  11  can likewise be fashioned like the patient bed coupling elements  9  (see also  FIGS. 12 and 13 ). The individual elements of the base body coupling element  11  are therefore not explained in detail. For differentiation from the corresponding elements of the patient bed coupling element  9 , they are provided with a prime. The interconnection of the two base body conductor loops  23 ′,  24 ′ (in series or in parallel) can thereby be the same as in the patient bed coupling element  9 . However, it can also be different from that interconnection. Independent of this, however, the magnetic fields generated by the patient bed conductor loops  23 ,  24  induce temporally variable induction currents l 1 ′, l 2 ′ in the base body conductor loops  23 ′,  24 ′ that are inversely oriented relative to one another at every point in time. 
   According to  FIGS. 12 and 13 , the whole-body antenna  4  comprises a number of transmission elements  29  that are oriented parallel to the travel direction z. According to  FIG. 12 , the base body coupling elements  11  are respectively arranged between two immediately adjacent transmission elements  28 . According to  FIG. 13 , the base body coupling elements  11  are integrated into the transmission elements  28 . 
   Normally either the embodiment according to  FIG. 12  or the embodiment according to  FIG. 13  are resorted to. However, in principle a combined embodiment is also possible, thus that a portion of the base body coupling elements  11  is arranged between the transmission elements  28  and another portion of the base body coupling elements  11  is integrated into the transmission elements  28 . 
   Given this embodiment according to  FIG. 13  (see additionally  FIG. 14  as well) capacitors  29  are also arranged in the segments  25 ′ through  27 ′ of the base body conductor loops  23 ′,  24 ′, which segments  25 ′ through  27 ′ run parallel to the travel direction z. The capacitors  29  are dimensioned such that the base body coupling elements  11  satisfy two conditions. 
   The excitation current IA must be distributed to the segments  25 ′ through  27 ′ such that it evokes no signal in the base body coupling element  11 . Given the embodiment of  FIG. 13 and 14 , the capacitances of the two outer capacitors  29  of a base body coupling element  11  must therefore be half as large as the capacitance of the middle capacitor  29  of the appertaining base body coupling element  11 . Moreover, the capacitors  29  must exhibit in total an effective capacitance that corresponds to the capacitance C 3  of the capacitor  18  from  FIG. 8 . 
   As an alternative to the embodiments according to  FIGS. 8 through 14 , it is naturally also possible that the base body coupling elements  11  and the patient bed coupling elements  9  are fashioned as capacitive coupling elements  9 ,  11 . This is schematically presented in  FIG. 15 . 
   Given embodiment in the form of capacitive coupling elements  9 ,  11 , according to  FIG. 15  a protective circuit  30  is advantageously arranged between the base body coupling element  11  and the evaluation circuit  7  (or respectively the preamplifier  13 ). According to  FIG. 15 , the protective circuit  30  comprises one or (as shown) two chokes  31  as well as a blocking circuit  32 . The blocking circuit  32  corresponds to the blocking circuit  17  from  FIG. 8  and is therefore not explained in detail in the following. 
   The protective circuit  30  has two functions. It compensates the series blind resistance of the local coil  8 , the patient bed coupling element  9  and the base body coupling element  11  in the event that a magnetic resonance signal is fed from the local coil to the evaluation device  7  via the patient bed coupling element  9  and the base body coupling element  11 . Additionally, it detunes the base body coupling element  11  in the event that no patient bed coupling element  9  interacts with the base body coupling element  11 , such that the base body coupling element  11  is not resonant at the Larmor frequency of the magnetic resonance system. 
   Furthermore, a choke is connected in parallel with the patient bed coupling element  9  such that the patient bed coupling element  9  and the choke  33  form a radio-frequency trap circuit at the Larmor frequency of the magnetic resonance system. Given capacitive coupling, the patient bed coupling element  9  is also fashioned such that it detunes the local coil  8  when the local coil  8  cannot be coupled to the evaluation device  7 . 
   The coupling elements  9 ,  11  (this applies both for the patient bed coupling elements  9  and for the base body coupling elements  11 ) are advantageously respectively fashioned as a pair of narrow coupling strips  34  according to  FIGS. 16 and 17 . The coupling strips  34  of each coupling element  9 ,  11  are thereby advantageously adjacent to one another at their narrow sides to minimize the unavoidable parasitic capacitance between them. 
   The embodiments of the present invention described in the preceding exclusively concern the transfer of a magnetic resonance signal form the local coils  8  to the evaluation device  7 . The local coils  8  are thus operated as acquisition coils. However, according to  FIG. 18  it is also possible to operate the local coils  8  as transmission coils. This applies independent of whether the coupling elements  9 ,  11  are fashioned as capacitive or inductive coupling elements  9 ,  11 . 
   According to  FIG. 18 , a signal splitter  35  is arranged between the base body coupling element  11  and the evaluation circuit  7 . The signal splitter  35  is connected with an RF driver element  36 . It is thus possible to feed a magnetic resonance excitation signal that is emitted by the RF driver element  36  into the local coil  8  via the base body coupling element  11  and the patient bed coupling element  9 . This naturally applies only when the patient bed  5  is moved such that corresponding coupling elements  9 ,  11  couple with one another (see  FIGS. 6 and 7 ). 
   The following features of the present invention are mentioned briefly in conclusion:
         The connection lines from the local coils  8  to the patient bed coupling elements  9  are advantageously relatively short. It is therewith ensured that the local coils  8  are also actually arranged in the excitation region  2  (and thus can acquire a magnetic resonance signal) when they are coupled to the evaluation device  7  via one of the base body coupling elements  8 .   The base body coupling elements  11  are advantageously permanently arranged on the base body  1 . However, they can also be connected with the base body  1  such that they can be detached.   It is possible that the local coils  8  and the patient bed coupling elements  9  are combined into inseparable units. In this case the patient bed  5  advantageously comprises guides so that the patient bed coupling elements  9  can be positioned exactly. However, the patient bed coupling elements  9  can also be separable from the local coils  8 . In this case it is possible (but not necessary) to arrange the patient bed coupling elements  9  on the patient bed  5  in a fixed manner. The connection between the local coils  8  and the patient bed coupling elements  9  can in this case be fashioned corresponding to DE-C2-101 30 615, for example.   Further local coils  37  are recognizable under the patient bed  5  in  FIG. 2 . These local coils  37  can be connected with the evaluation device  7  corresponding to the present invention. However, this is only required when these local coils  37  are arranged at the patient bed  5 . However, it is also possible to arrange these local coils  37  stationary at the base body  1 , above the patient bed  5 .       

   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.