BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention concerns a transmission antenna for magnetic resonance applications of the type having a basic configuration known as a “birdcage” antenna. 
     2. Description of the Prior Art 
     In magnetic resonance imaging, transmission and reception antennas separated from one another are used, in particular in the head area. The antennas are typically also designated as coils. Among other things, the requirement to design the transmission antennas to be detunable results due to the use of separate transmission and reception antennas. 
     In the prior art, transmission antennas are normally fashioned as birdcage resonators. They therefore always have two ferrules as terminating elements, wherein the ferrules are fashioned identically. In particular, the ferrules are composed of ferrule segments that are coupled with one another via ferrule capacitors. In individual cases, antennas known as TEM resonators are used as an alternative to birdcage resonators. 
     The transmission antennas known in the prior art operate quite well given symmetrical examination subjects and relatively low static magnetic fields (up to approximately 1.5 Tesla). However, the quality of the radio-frequency transmission field decreases given asymmetrical examination subjects and larger static magnetic fields (for example 3 Tesla and more). 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to achieve a transmission antenna for magnetic resonance applications that deliver a qualitatively high-grade radio-frequency transmission field even given asymmetrical examination subjects and larger static magnetic fields. 
     The above object is achieved in accordance with the present invention by a transmission antenna for magnetic resonance applications, wherein the transmission antenna has multiple antenna rods, each rod extending from a first end to a second end. The antenna rods proceed substantially parallel to a central axis and are distributed around the central axis. The antenna rods are terminated at their first ends by a first terminating element, and are terminated at their second ends by a second terminating element. The first terminating element is formed as a ferrule that has a number of ferrule segments that are coupled with each other via ferrule capacitors. The transmission antenna further has a detuning circuit that detunes the transmission antenna. The detuning circuit is located at the side of the transmission antenna at the second terminating element. Either the second terminating element is formed as a completely continuous short circuit ring and the detuning circuit is arranged between the antenna rods and the second terminating element, or the second terminating element has a number of ferrule segments between which the detuning circuit is arranged, but no ferrule capacitors. The second terminating element has a larger cross-section than the first terminating element. 
     The transmission antenna has a radio-frequency shield radially, externally surrounding the antenna rods. 
     To optimize the quality of the radio-frequency transmission field, in an embodiment the antenna rods exhibit an antenna rod distance from the radio-frequency shield as viewed in the radial direction, and that the antenna rod distance varies as viewed from the first terminating element to the second terminating element. The variation can be linear. 
     Alternatively or additionally, the first terminating element and the second terminating element can exhibit (relative to the central axis) a first terminating element distance and a second terminating element distance from the radio-frequency shield, respectively, as viewed in the radial direction, with the terminating element distances being different from one another. 
     To achieve a larger cross-section, the second terminating element can have a larger axial width than the first terminating element, with the first and second terminating elements having the same radial thickness. 
     In principle, the transmission antenna can be fashioned as a whole-body antenna. However, it is preferably fashioned as a head coil. 
     The transmission antenna can in principle be tuned to any arbitrary resonance frequency. However, it is advantageously tuned to a resonance frequency that is at least as high as the Larmor frequency of hydrogen in a static magnetic field of three Tesla. 
     The detuning circuit advantageously includes PIN diodes, with the PIN diodes being switched to the conductive state for tuning the transmission antenna to the resonant frequency. 
     The transmission antenna normally has conductors to feed signals to the transmission antenna and/or to conduct signals away from the transmission antenna. The conductors are advantageously arranged on the side of the second terminating element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates a transmission antenna for magnetic resonance applications, as seen from the side. 
         FIG. 2  shows the transmission antenna of  FIG. 1  in cross-section. 
         FIG. 3  shows a first embodiment of the transmission antenna of  FIG. 1  in an “unrolled” representation. 
         FIG. 4  shows a second embodiment of the transmission antenna of  FIG. 1  in an “unrolled” representation. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to  FIGS. 1 and 2 , a transmission antenna for magnetic resonance applications possesses a number of antenna rods  1 . Each antenna rod  1  extends from a first end  1 ′ to a second end  1 ″ of the respective antenna rod  1 . 
     In a preferred embodiment, the antenna rods  1  run (at least essentially) parallel to a central axis  2 . The number of antenna rods  1  is at minimum  4 . There are normally  16  or  32 . However, other numbers of antenna rods  1  are also possible, for example 6, 8, 12, 24 or 40 antenna rods  1 . 
     In the illustrated normal case, the antenna rods  1  run exactly parallel to the central axis  2 . However, alternative embodiments are possible in which the antenna rods  1  define a direction that is only essentially parallel to the central axis  2 . In this latter cited case, the respective antenna rod  1  exhibits a direction that possesses a first partial component and a second partial component. The two partial components complement the direction of the respective rod axis  1 . 
     The first partial component is (exactly) parallel to the central axis  2 . The second partial component is orthogonal to the central axis  2 . As long as the first partial component is greater than the second partial component, the direction of the antenna rod  1  is essentially parallel to the central axis  2 . For example, a curve of the antenna rods  1  essentially parallel only to the central axis  2  can result in that the transmission antenna is fashioned slightly conical (see  FIGS. 1 and 2 ), and/or that the antenna rods  1  proceed slightly helically around the central axis  2 , similar to lands and riflings of a firearm. A combination of these two measures is also possible. 
     For example, the transmission antenna can be fashioned as a whole-body antenna. However, it is preferably fashioned as a head coil according to  FIG. 1 . 
     The terms “axial”, “radial” and “tangential” used in the following, are referenced to an axis, for example the central axis  2 . The term “axial” indicates a direction parallel to the respective axis. The terms “radial” and “tangential” mean directions in a plane orthogonal to the respective axis. The term “radial” refers to a direction in this plane that is directed toward or away from the respective axis. 
     The term “tangential” designates a direction around the axis in the plane orthogonal to the axis. If the terms “axial”, “radial” and tangential” are used without explicit reference to an axis, they refer to the central axis  2 . If they should refer to a different axis, which axis this is added. 
     The antenna rods are terminated at their first ends  1 ′ by means of a first terminating element  3 . The first terminating element  3  is fashioned as a ferrule that has a number of ferrule segments  4 . The ferrule segments  4  are coupled with one another via ferrule capacitors  5 . 
     The antenna rods  1  are furthermore terminated at their second ends  1 ″ by means of a second terminating element  6 . According to  FIGS. 1 and 2 , the second terminating element  6  is fashioned as an additional ferrule. However, this is not absolutely necessary. Independent of whether the second terminating element  6  is fashioned as an additional ferrule or not, however, the second terminating element  6  is fashioned differently than the first terminating element  3 . This is explained in detail in the following in connection with  FIG. 1 through 4 . 
     According to  FIG. 1  the transmission antenna is tuned to a resonance frequency fR. The resonance frequency fR can in principle be arbitrary. For example, the resonance frequency fR can be at least as high as the Larmor frequency fL (H, 3 T) of hydrogen in a static magnetic field of three Tesla. 
     The transmission antenna furthermore has a detuning circuit  7 . If the detuning circuit is not activated (detuned case), the transmission antenna is not tuned to the resonance frequency fR. It is thus not resonant at the resonance frequency fR; rather, it is detuned. In contrast to this, if the detuning circuit  7  is activated (tuned case), the transmission antenna is resonant at the resonance frequency fR; the transmission antenna is thus tuned to the resonance frequency fR. 
     The detuning circuit  7  is not shown in  FIGS. 1 and 2 . According to  FIGS. 3 and 4 , the detuning circuit  7  is arranged on the side of the second terminating element  6 . 
     The basic principle of the present invention was previously explained. Possible embodiments of the present invention are subsequently explained in connection with  FIG. 1 through 4 . 
     According to  FIGS. 1 and 2 , the transmission antenna possesses a radio-frequency shield  8 . The radio-frequency shield  8  radially, externally surrounds the antenna rods  1 . The first terminating element  3  exhibits a distance a from the radio-frequency shield  8  (as viewed in the radial direction), which distance a is cited in the following as a first terminating element distance a. The antenna rods  1  furthermore exhibit a distance b from the radio-frequency shield  8  (as viewed in the radial direction), which distance b is subsequently called antenna rod distance b. 
     In the embodiment of  FIGS. 1 and 2  (in which the second terminating element  6  is fashioned as an additional ferrule and therefore is in particular in element  6  different than the radio-frequency shield  8 ), the second terminating element  6  furthermore exhibits a distance c from the radio-frequency shield  8  (as viewed in the radial direction). This distance c is called the second terminating element distance c in the following. 
     It is possible that the antenna rod distance b is constant as viewed from the first terminating element  3  to the second terminating element  6 . Such an embodiment is possible independent of whether the antenna rods  1  run parallel to the central axis  2  or—as shown in  FIGS. 1 and 2  —define a frustum, for example. The transmission characteristic of the transmission antenna can, however, be optimized if the antenna rod distance b varies as viewed from the first terminating element  3  to the second terminating element  6  (as is likewise shown in  FIG. 1 ). The antenna rod distance b can hereby in particular vary linearly. However, in individual cases a different type of variation is alternatively conceivable. For example, the radio-frequency shield  8  can run straight, and the antenna rods  1  can for example run parabolically or be otherwise curved. 
     In the embodiment according to  FIGS. 1 and 2 , in which the second terminating element  6  is an element  6  different than the radio-frequency shield  8 , the second terminating element distance c can furthermore be equal to the first terminating element distance a. However, the second terminating element distance c is preferably different than the first terminating element distance a. 
     As an alternative to the embodiment according to  FIGS. 1 and 2  (and also  FIGS. 3 and 4 ), the second terminating element  6  can be identical with the radio-frequency shield  8 . In this case the definition of a second terminating element distance is not reasonable. The remaining statements—in particular with regard to the curve of the antenna rod distance b—are still valid, however. 
     As already mentioned and also shown in  FIG. 1 through 4 , the second terminating element  6  can be fashioned as an additional ferrule. In this case the additional ferrule (=second terminating element  6 ) must be fashioned differently than the ferrule (=first terminating element  3 ). For example, the additional ferrule  6  can exhibit a larger cross-section than the ferrule  3 . A larger ferrule can hereby in particular be achieved in that, although thicknesses d 1 , d 2  of the ferrules  3 ,  6  are equal (as viewed in the radial direction), the additional ferrule  6  exhibits a greater width b 2  (viewed in the axial direction) than the ferrule  3 . 
     The cross-section of the additional ferrule  6  should advantageously be significantly larger than the cross-section of the ferrule  3 . The term “significantly” means that the cross-section of the additional ferrule  6  is at least twice as large as the cross-section of the ferrule  3 . The cross-section of the additional ferrule  6  is advantageously at least three times as large (for example four to six times as large) as the cross-section of the ferrule  3 . 
     Due to its design (in particular due to the larger cross-section), the additional ferrule  6  (=second terminating element  6 ) normally exhibits a lower inductance than the ferrule  3  (first terminating element  3 ), but this is not absolutely necessary. 
     As an alternative or in addition to the dimensioning of the cross-sections different from one another, the additional ferrule  6  according to  FIGS. 3 and 4  can be fashioned as a continuous short circuit ring. The term “continuous short circuit ring” means either that the additional ferrule  6  (see  FIG. 3 ) is fashioned to be entirely continuous, thus is always and unconditionally short-circuited. In this case, the detuning circuit  7  according to  FIG. 3  is arranged between the antenna rods  1  and the additional ferrule  6  (=second terminating element  6 ). Alternatively, the term “continuous short circuit ring” can also mean that the additional ferrule  6  can in fact possess a number of ferrule segments  9  (analogous to ferrule  3 ). In this latter cited case, however, no ferrule capacitors are arranged between the ferrule segments  9  of the additional ferrule  6 . Rather, in this case the detuning circuit  7  is arranged in the second terminating element  6 . Such an embodiment is shown in  FIG. 4 . 
     The detuning circuit  7  normally has PIN diodes  10  (see  FIGS. 3 and 4 ). If the PIN diodes  10  are not connected through, the transmission antenna is advantageously not tuned to the resonance frequency fR. However, if the PIN diodes  10  are connected through, the transmission antenna is advantageously tuned to the resonance frequency fR. 
     If the PIN diodes  10  are connected through, they are electrically conductive. For this reason, in the through-connected state of the PIN diodes  10  (thus in the tuned case) the additional ferrule  6  then also acts as a continuous short circuit ring if the detuning circuit  7  is arranged in the additional ferrule  6 . 
     Signals must be supplied to the transmission antenna. These are hereby low-frequency signals. An example of such a low-frequency signal is a detuning voltage by means of which the detuning circuit  7  is controlled. However, the signals can likewise be radio-frequency signals. An example of such a signal is the transmission current that is fed into the transmission antenna. Furthermore, signals (in particular radio-frequency signals) can be conducted away from the transmission antenna. An example of such a signal is the signal received by the transmission antenna in the event that the transmission antenna can also be operated as a reception antenna. An additional example of radio-frequency signals to be conducted away are microwave signals that are received by microwave receivers which are integrated into the antenna rods  1 . The microwave receivers are not shown in the figures. 
     The supply and discharge of the aforementioned signals ensues via conductors  11 . According to  FIGS. 1 through 4 , the conductors  11  are advantageously arranged on the side of the second terminating element  6 . However, this is not absolutely necessary. 
     The transmission antenna of the present invention possesses many advantages. It is in particular simple in design, highly effective and can be flexibly dimensioned and used. 
     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.
A transmission antenna for magnetic resonance applications has a birdcage-like structure that includes antenna rods proceeding between first and second terminating elements respectively located at opposite ends of the antenna rods. A detuning circuit is located at the second terminating element. Either the second terminating element is formed as a completely continuous short circuit ring and the detuning circuit is arranged between the ends of the antenna rods and the second terminating element, or the second terminating element has a number of ferrule segments, between which the detuning circuit is arranged. The second terminating element has a larger cross-section than a the first terminating element.