Abstract:
A magnetic resonance (MR) probe head comprises a detecting device ( 3 ) with at least one antenna system which is cryogenically cooled by a cooling device, and a cooled preamplifier ( 16 ) in a preamplifier housing ( 15   a ) which is spatially separated from the detecting device ( 3 ), and a thermally insulating connecting means via which the detecting device ( 3 ) and the preamplifier housing are connected, wherein the connecting means ( 15   c ) comprises at least one cooling line ( 9, 9   a   , 9   b ) for supplying and/or returning a cooling fluid, and with at least one RF line ( 10 ) for transmitting the electric signals. The connecting means is implemented as a mechanically flexible connecting line ( 8 ) with mechanically flexible RF lines ( 10 ) and cooling lines ( 9, 9   a   , 9   b ) disposed therein. The MR probe head is easy to handle and can be highly sensitive, wherein the detecting device can be quickly installed, removed and put into operation.

Description:
[0001]    This application claims Paris Convention priority of DE 10 2006 020 772.6 filed May 3, 2006 the complete disclosure of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The invention concerns a magnetic resonance (MR) probe head with a detecting device, comprising at least one antenna system which is cryogenically cooled by a cooling device, and a cooled preamplifier in a preamplifier housing which is disposed at a spatial separation from the detecting device, and with a thermally insulating means which connects the detecting device to the preamplifier housing, wherein the connecting means comprise at least one cooling line for supplying and/or returning a cooling fluid, and at least one radio frequency (RF) line for transmitting the electrical signals. 
         [0003]    A probe head of this type is disclosed e.g. in reference [3] (see e.g.  FIG. 5  thereof) and is used for detecting NMR signals from a sample. The receiver coil is thereby cooled. This receiver coil may e.g. be a coil made from a normally conducting metal, e.g. copper or aluminium, or from superconducting materials, in particular, high-temperature superconductors (HTSC). 
         [0004]    The conventional cooled systems for NMR detection can be classified into the following categories: 
         [0005]    a) systems which are directly cooled by LN2. LN2 is thereby inserted into a container and measurements are performed until the coolant is consumed (evaporated). Such an arrangement is mentioned e.g. in reference [1] and is schematically shown in  FIG. 5 . 
         [0006]    b) systems which are cooled by a cold head in the direct vicinity of the receiver coil, generally using a so-called pulse tube cold head, which is compatible with the requirements for the magnetic field and vibration. A configuration of this type is disclosed in reference [2] and schematically shown in  FIG. 6 . 
         [0007]    c) systems which are cooled by a cold head, wherein the cold head is located at a certain distance from the probe head itself. This cold head is typically a Gifford-McMahon cooler, but may also be a pulse tube cooler. The probe head is generally cooled with a flexible transfer line to bridge the separation from the cryocooler and to also keep vibrations away from the probe head. A heat transporting medium circulates in this transfer line (usually cold, gaseous helium). The probe head itself typically contains the receiver coils and the preamplifier, which are connected to each other via RF lines. The probe head is thereby designed as a rigid unit. Such a configuration is disclosed in reference [3] and represented in  FIG. 7   b  (horizontal magnet) or in  FIG. 7   a  (vertical magnet). 
         [0008]    It must be noted that this concerns only reception of the NMR signals. This is also the process which must be controlled with maximum precision to obtain a maximum S/N ratio, and depends to a critical degree on attenuation, temperatures and noise of the elements involved. 
         [0009]    The nuclei must be excited prior to measurement by at least one RF pulse. This can be realized either by the receiver coil itself or through a separate transmitter coil. The transmitter coil may either also be cooled or be at room temperature. Further systems and devices for generating the nuclear excitation are not discussed herein for reasons of simplicity. 
         [0010]    The above-mentioned conventional MR probe heads are discussed below: 
         [0011]    a) the configurations in accordance with a) ( FIG. 5 ) have a relatively simple construction but are disadvantageous in that liquid nitrogen  60  must be refilled into a thermally insulated housing  61  and the temperature of the receiver coil  5  is only reduced to 77 K. It is thereby important that the preamplifier  58  and also the RF line  59  (usually in the form of a coaxial cable) between the receiver coil  5  and the preamplifier  58  are also normally at room temperature. In consequence thereof, the preamplifier  58  produces an unnecessary amount of noise and the RF line  59  additionally impairs the S/N ratio, for the following reasons: Firstly, the NMR signal itself is weakened by attenuation in the RF line  59 . Secondly, the RF line  59  itself generates an undesired noise signal due to its dissipation. This is higher, the higher the RF line  59  attenuation and the higher its physical temperature. These disadvantageous effects are multiplicative. An RF line  59  at room temperature has the following disadvantages: its signal losses are high, and its physical temperature is also high, thereby providing two counts for of high noise emission. For this reason, the above-mentioned effects will be substantial. 
         [0012]    b) The configuration of b) ( FIG. 6 ) is a potentially better solution in view of cooling. The system may thereby be very compact, wherein cooling takes place in the vicinity of the receiver coil  5  to facilitate obtaining a low temperature. The system is also a closed system which requires only electric energy for driving the compressor  56  required for operation of a pulse tube cold head  54  in a thermally insulated housing  55  and is connected to the pulse tube cold head  54  via a flexible pressure line  57 . In particular, no cryogenic liquids must be refilled. 
         [0013]    The configuration has one serious drawback: the preamplifier  58  must either be operated at room temperature and/or outside of the magnet  1 . In the first case, its noise temperature is not optimum, but excessively high. In the latter case, the preamplifier  58  could also be cooled. There are no conventional preamplifiers with satisfactory function which perform perfectly at cold temperatures (&lt;&lt;77K) in the magnetic fields (&gt;&gt;1T) used for NMR, in particular, high field NMR. 
         [0014]    When the preamplifier  58  is outside of the magnet  1 , it is conventionally connected to a receiver coil  5  via a correspondingly long RF line  59 . This RF line  59  is at room temperature, and therefore has the disadvantages mentioned under b) above, leading to deterioration of the S/N ratio. 
         [0015]    There is another problem. In order to cool the preamplifier  58 , it must either be cooled by LN2, which is not very user-friendly and opposes the original idea of a closed system, or a second pulse tube cold head is used only for the preamplifier  58 . This is very expensive. In any case, the problem of the non-cooled RF line  59  remains. 
         [0016]    c) the configuration in accordance with c) ( FIG. 7   a ,  FIG. 7   b ) is technically the best of the conventional configurations and potentially offers the highest performance, since the RF receiver coil  5  is operated in the room temperature bore  2  of the magnet  1  at a very low temperature (e.g. 20K) and the preamplifier  16  is operated at a such a low temperature to assume proper function (e.g. 77K). Also the RF line  52  between the receiver coil  5  and the preamplifier  16  is at a very low temperature. The temperature typically changes along the RF line  52  from 20 K to 77 K, wherein the geometry (in particular the cross-section of the cables of the RF line  52 ) is selected to obtain an optimum between minimizing the RF line losses and minimizing the heat input into the cold receiver coil  5  produced by the heat conductivity of the RF line  52 . Thus, the S/N ratio of the system is nearly optimum. After amplification with the cooled preamplifier  16 , the signal is finally output to a signal output  17  at the output of the preamplifier  16  for further signal processing. The overall cooling can be performed with one single cryocooler  20  (Gifford-McMahon or pulse tube cold head). 
         [0017]    This configuration has been prior art for some time for high-resolution probe heads of NMR spectroscopy ( FIG. 7   a ) [3]. Prototypes for MRI imaging have also been built according to this principle ( FIG. 7   b ) [4]. This configuration has the serious drawback that the entire probe head is long, which produces the following problems: 
         [0018]    1. Installation may become impossible under unfavorable spatial conditions. These problems are shown in  FIG. 8 . The separation D 1  between the magnet  1  and the wall  62  must be sufficient to permit installation and removal of the system into and from the magnet. This minimum distance is substantially determined by the sum of the lengths of the actual detecting device  3 , the preamplifier housing  15   a , and the rigid connecting means  15   c.    
         [0019]    2. The entire probe head including typically large preamplifier  16  and associated housing  15   a  is quite heavy, such that installation/removal typically requires two persons. 
         [0020]    3. Due to the large weight and bulkiness of probe head and connected cooling line, installation or removal of such a probe head in the cold state is very difficult and not practicable. The transfer line  19  ( FIGS. 7   a ,  7   b ) which extends between the cryocooler  20  and the probe head, typically contains four cooled pressure lines (continuing cooling lines  21   a - 21   d ) and is quite heavy and inflexible. Installation/removal in the cold state is desired and would permit cooling the probe head outside of the room temperature bore of the magnet prior to use, and insertion of it into the room temperature bore only directly before the measurement. After the measurement, the probe head could be removed in the cold state and be left cold until subsequent use or allowed to heat up outside. This is not practicable for the above-mentioned reasons, and for this reason, the probe head must be typically inserted into the room temperature bore  2  in a warm state (i.e. at room temperature), and then connected to the transfer line  19  and the cryocooler  20 . Only then can the cooling process start, which typically takes at least four hours. During this time, the magnet  1  is blocked and unproductive. Removal is effected under similar conditions. The warm-up phase typically takes at least two hours. These times cannot be tolerated in view of the required utilization rate of an MRI system. 
         [0021]    Prior art provides either relatively compact, cooled systems without optimum performance or high-performance systems with optimum little noise. These may, however, be fundamentally unsuitable for installation into narrow spaces, or be very difficult to handle. 
         [0022]    It is therefore the underlying purpose of the present invention to propose an MR probe head which is easy to handle, is highly sensitive, and permits rapid installation, removal and rapid start of operation of the detecting device. 
       SUMMARY OF THE INVENTION 
       [0023]    This object is achieved in accordance with the invention in that the connecting means is designed as a mechanically flexible connecting line with mechanically flexible RF and cooling lines. 
         [0024]    The detecting device of the inventive MR probe head is both small and lightweight, since the detecting device is cooled by an external cooling device and therefore contains no cold head or coolant supply container. The space required for installation and removal of the MR probe head is minimized by the flexible design of the connecting means and the RF and cooling lines. The detecting device, which forms part of the probe head, which is operated in the direct vicinity of the object to be investigated, and which receives the RF signals, can thereby be quickly installed into the magnet and put into operation. After termination of the measurements, the detecting device can be quickly and simply shut-down and removed from the room temperature bore of the magnet. 
         [0025]    The inventive MR probe head is advantageously a probe head for magnetic resonance imaging (MRI) applications or for NMR (nuclear magnetic resonance) spectroscopy. In particular, in MRI applications, rapid handling is particularly advantageous, since the investigation methods and therefore the detecting device must be frequently changed. The inventive MR probe head can reduce unproductive standstill times of the very expensive magnet system. 
         [0026]    With particular advantage, the inventive MR probe head has two cooling lines which are part of a first cooling circuit, instead of cooling the MR probe head with one single cooling line, wherein the coolant evaporates on the components to be cooled. In this case, the coolant can be recooled or also optionally reliquified and reused. 
         [0027]    In a particularly preferred embodiment of the invention, one or several RF lines are associated with each single cooling line, where they each form, together with the associated cooling line, one line package. 
         [0028]    The lines within one line package are tightly arranged, such that temperature is balanced between the individual lines within the line package. 
         [0029]    In a further development of this embodiment, the cooling line and the RF lines, associated with the cooling line, of one single line package, are thermally coupled to each other, preferably in regular separations, via coupling elements having good thermal conducting properties. These coupling elements may be produced e.g. from copper. 
         [0030]    In a further development, at least one of the line packages contains at least one elastic reinforcing element, in particular, a wire having high mechanical integrity to reduce the mechanical tensile and bending strain of the RF and cooling lines. The reinforcing element may e.g. be made from stainless steel. 
         [0031]    In a preferred embodiment, the individual line packages are twisted, preferably at least once, about the axis of the connecting line along the entire length of the connecting line. 
         [0032]    With particular advantage, the RF lines and/or the elastic reinforcing elements within the packages are twisted with the cooling line between two coupling elements through substantially 360 degrees or an integer multiple thereof, in particular, wound about the cooling line. The overall length of all lines between the coupling elements thereby remains substantially the same when they are bent. Therefore, the lines do not bulge or bend when the connecting line is bent. The cooling line may also be twisted about the RF lines and/or the elastic reinforcing elements, or both together. The cooling line may also be previously twisted about the RF line. 
         [0033]    The RF line of the inventive MR probe head advantageously comprises a flexible coaxial cable preferably with polytetrafluoroethylene (PTFE) insulation, wherein the outer conductor consists of a combination of a metal braiding and a spirally wound metal foil having good electric conducting properties. 
         [0034]    With particular advantage, the cold parts of the connecting line, in particular, the cooling lines and the RF lines, are disposed in a vacuum within at least one flexible outer jacket for thermal insulation. In this fashion, the heat input into the cold parts of the MR probe head is minimized, in particular, through super insulation. The connecting line may also comprise two parallel hose lines as an outer jacket, wherein one hose line houses the feed line and the other the return line, each comprising a number of associated RF lines. This configuration is advantageous in that each of the two hose lines may have a smaller diameter and thereby be more flexible. However, an embodiment with one single outer jacket is advantageous, since its thermal insulation is improved due to a smaller overall surface for a given cross-section and constitutes a simpler, more robust and neater configuration (seen from the outside) while also exhibiting an isotropic bending behavior. 
         [0035]    The electronics of the cooled preamplifier are advantageously cooled by a second cooling circuit, which is separate from the at least one cooling line. 
         [0036]    In a particularly preferred embodiment, the first and the second cooling circuits are cooled by the same cooling device, such that the costs and also the space for a further cooling device are obviated. 
         [0037]    Helium is preferably provided as a cooling fluid for cooling the components of the MR probe head. 
         [0038]    The cooling device may advantageously be a Gifford-McMahon cooler. 
         [0039]    The cooling device may alternatively be a pulse tube cooler. 
         [0040]    The antenna system is advantageously operated at a lower temperature than the preamplifier. 
         [0041]    In a further development of this embodiment, the antenna system is operated at a temperature in the range of 4 to 40 K and the preamplifier is operated at a temperature in the range of 40 to 100 K. 
         [0042]    In a particularly preferred embodiment of the inventive MR probe head, the separation between the preamplifier and the last coupling element on the side of the preamplifier is larger than the average separation between two coupling elements. There is no thermal contact between the RF line and the cooling line along the path between the preamplifier and the last coupling element on the preamplifier side, such that the temperature of the RF lines corresponds substantially to the temperature of the cooling lines or the temperature T C  of the receiver coils, and the temperature transition to the preamplifier temperature T P  takes place only shortly before the preamplifier. While a sufficiently dense (tightly arranged) configuration of coupling elements cools the RF line in a large area along the RF line, this coupling is no longer provided on the above-described path for the temperature transition to the preamplifier, such that the temperature can rise steadily. 
         [0043]    With respect to handling, the mass of the parts of the detecting device is advantageously less than 5 kg. 
         [0044]    In a particularly preferred embodiment of the inventive MR probe head, the preamplifier comprises several parallel preamplifier channels. 
         [0045]    It may thereby be advantageous for the preamplifier to comprise several, in particular 2, 3, 4, 6, 8, 12 or 16 parallel channels for the same nucleus to be examined, in particular, 1H. 
         [0046]    In a further embodiment of the invention, the preamplifier comprises channels for amplifying the signals of the nuclei 31P, 13C, 23Na, 19F, 170, 129Xe or 2H. 
         [0047]    The invention also concerns an MR system comprising the above-described inventive probe head. 
         [0048]    The invention can be utilized with particular advantage when the MR system is an MRI or an NMR system. 
         [0049]    The connecting line of the inventive MR probe head including its inner components may be sufficiently flexible that the space required in the extension of the room temperature bore for installation and removal of the detecting device is less than the separation between the magnet center and the end of the magnet. Moreover, the inventive MR probe head also permits installation and removal of the detecting device in the cold state, in particular, due to the inventive design of the connecting line. In this fashion, MR systems can be realized with maximum efficiency, in particular, in view of the signal/noise (S/N) ratio and simple handling. 
         [0050]    Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below may be used individually or collectively in arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but have exemplary character for describing the invention. 
     
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0051]      FIG. 1  shows an inventive MR probe head in the operating position and during installation/removal into/from the magnet; 
           [0052]      FIG. 2  shows a schematic view of an inventive MR probe head; 
           [0053]      FIG. 3   a  shows a detailed view of an inventive MR probe head; 
           [0054]      FIG. 3   b  shows a detailed view of a line package of an inventive MR probe head with coupling elements; 
           [0055]      FIG. 3   c  shows a detailed view of a line package of an inventive MR probe head with reinforcing elements; 
           [0056]      FIG. 4   a  shows a temperature dependence of an RF line of an inventive MR probe head in dependence on its running length; 
           [0057]      FIG. 4   b  shows the construction of an RF cable for an RF line of an inventive MR probe head; 
           [0058]      FIG. 5  shows a conventional MR probe head with LN2 cooling of the receiver coil; 
           [0059]      FIG. 6  shows a conventional MR probe head in a horizontal magnet with a pulse tube cooler in the room temperature bore of the magnet for cooling a receiver coil; 
           [0060]      FIG. 7   a  shows an NMR probe head with a rigid connecting means in accordance with prior art in a vertical magnet for high-resolution NMR spectroscopy; 
           [0061]      FIG. 7   b  shows an MRI probe head with a rigid connecting means in accordance with prior art in a horizontal magnet for MRI applications; and 
           [0062]      FIG. 8  shows an MR probe head corresponding to  FIG. 7   b  in the operating position and during installation/removal into/from the magnet; 
           [0063]      FIG. 9   a  shows an inventive NMR probe head with flexible connecting line in a vertical magnet for high-resolution NMR spectroscopy; and 
           [0064]      FIG. 9   b  shows an inventive MRI probe head with flexible connecting line in a horizontal magnet for MRI applications. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0065]    The inventive probe head for receiving NMR signals is schematically shown in  FIG. 1  and comprises the following components: 
         [0066]    a) a detecting device  3  with at least one cryogenically cooled antenna system. The antenna system of the inventive MR probe head comprises at least one antenna, wherein such an antenna may consist of at least one receiver coil  5 , or a resonator, or another type of RF oscillating circuit which is sensitive to electromagnetic fields. The detecting devices of the present type are mainly MRI systems for the examination, in particular, imaging of objects, and in particular of living beings. The inventive device may also be used for systems for NMR spectroscopy or also for combined methods such as e.g. localized spectroscopy. 
         [0067]    b) a cooled preamplifier  16  in a preamplifier housing  15   a  which is spatially separated from the NMR detecting device  3 . 
         [0068]    c) a flexible connecting line  8  which is thermally insulated from the surroundings for connecting the NMR detecting device  3  to the preamplifier housing  15   a , wherein the flexible connecting line  8  has cooling lines  9  (see  FIGS. 3   a ,  3   b ) for supplying or returning a cooling fluid, and also RF lines  10  for transmitting the electric signals. 
         [0069]    Due to the flexibility of the connecting line  8 , the free separation D 2  required from the outside, e.g. between a wall  62  and the outlet of the room temperature bore  2  of the magnet  1 , which must be present to insert and remove the NMR detecting device into and from the room temperature bore of the NMR magnet, has a minimum size. 
         [0070]    The resulting advantage can be noticed especially in horizontal MRI magnets which are disposed e.g. close to the wall  62  for reasons of space. 
         [0071]      FIG. 1  shows the inventive MR probe head in a horizontal magnet, wherein both the receiver coil  5  and the preamplifier  16  are cooled to a minimum possible temperature, with their RF line  10  also being kept at a very low temperature. The system solves the object of the invention through the flexible design of the connecting line  8  with a cooled flexible RF line  10  between the receiver coil  5  and the preamplifier  16 . This permits very good handling, thereby keeping all performance advantages, since the detecting device  3 , i.e. the end of the probe head that contains the receiver coil(s)  5  is small, light-weight, and can be easily handled due to the inventive highly flexible connecting line  8 . (The compact size and small weight can be realized, since in contrast to prior art in accordance with a) or b), the detecting device  3  need not contain a cold head nor a coolant supply container). In particular, in a corresponding design of the system which is explained in more detail below, the system can be inserted and removed into and from the magnet  1  in the cold state at any time. This eliminates all disadvantages of variant c) and the advantages of this variant remain practically unchanged. The inventive device therefore combines all advantages of the existing systems, but eliminates their disadvantages. 
         [0072]    The realization of the inventive configuration has not been considered up to now due to the extensive technical requirements. It is schematically shown in  FIG. 2  and in detail in  FIG. 3   a . Both figures show the probe head in the stretched position. This meets the following technical requirements for both the system and the connecting line  8  at the same time: 
         [0073]    1. The connecting line  8  has a flexible outer jacket, in which the cooling and RF lines extend and are insulated from the surroundings by a vacuum which is contracted preferentially minimally during evacuation. 
         [0074]    2. The cooling line  9  (at least two cooling lines  9   a ,  9   b  are required for a first cooling circuit  14 ) for cooling the coil are flexible. They must withstand a pressure on the order of magnitude of 20 to 30 bars while thereby remaining flexible. In one preferred embodiment, they can also be bent in the cold state. 
         [0075]    The desired bending radius is on the order of 30 cm, which is very small. The cooling lines  9 ,  9   a ,  9   b  must be sufficiently resistant to fatigue caused by repeated handling, and thereby withstand sufficient bending cycles. 
         [0076]    3. The RF lines  10  (at least one, preferably four or more) are flexible, preferably also in the cold state, and have little RF loss. Even after many cycles, they must not show any fatigue ruptures and only minimum signal crosstalk. In order to minimize the RF loss and noise, it must be possible to operate the RF lines  10  at a minimum possible temperature. 
         [0077]    4. The connecting line  8  and the antenna system have excellent thermal insulation. 
         [0078]      FIG. 2  shows the schematic view of the magnet  1  with a test object  51  and the installed detecting device  3  of the probe head, which contains a vacuum  13   a . The flexible connecting line  8  is thereby shown in a stretched state. On the left-hand side, the detecting device  3  is shown and has a vacuum-tight housing which is insulated from the vacuum  13   a  in the detecting device  3 . It contains the antenna system in the form of four receiver coils  5  and a heat exchanger  7  and a heat-conducting connection  6  between the heat exchanger  7  and the receiver coils  5  for cooling the receiver coils  5 . In principle, several heat exchangers  7  may also be provided. The flexible connecting line  8  is shown in the center. The preamplifier housing  15   a  is shown on the right, including the cooled preamplifier  16  (without further details). The flexible RF line  10  is shown by a dashed line, like in all other figures. 
         [0079]      FIG. 3   a  shows the system in more detail. The connecting line  8  comprises the vacuum-tight outer jacket, in the present case in the form of a corrugated bellows  8   a , two cooling lines  9   a  and  9   b , and RF lines  10  and is also shown in a stretched state. The receiver coils  5  form an array of four coils and are cooled by the heat exchanger  7 . The cooling lines  9   a ,  9   b  connected to the heat exchanger  7  are guided through the connecting line  8  into the preamplifier housing  15   a . Although the cooling lines  9   a ,  9   b  are tubes or thin corrugated bellows, they are only represented as a line in this illustration and also in the other figures. They extend into a coupling part  18   a  of a coupling on the preamplifier side, which can connect the preamplifier housing  15   a  to a cryocooler  20  (not shown in  FIG. 3   a ). Such a coupling is disclosed e.g. in [5] and [3]. The cold lines  9   a ,  9   b ,  10  within the connecting line  8  are thermally insulated by a vacuum insulation  13   b  as well as optional additional super insulation (not shown). 
         [0080]    Each of the four receiver coils  5  shown in  FIG. 3   a  is connected to one of the cooled preamplifiers  16  via one of the inventive flexible RF lines  10 . In this embodiment, the four preamplifiers  16  may be connected to a thermal stage (heat sink)  23  within a second cooling circuit  22  with continuing cooling lines  21   c ,  21   d  of a coupling part  18   b  on the cryocooler side which is shown e.g. in  FIG. 7   a  and described in reference [6]. 
         [0081]    The temperature of the receiver coil  5  is e.g. 20K, the temperature of the preamplifier  16  is e.g. 77K. A particularly preferred embodiment of the inventive connecting line  8  has the following structure which is described in more detail below:
       the outer jacket consists of a corrugated bellows  8   a , which is preferably reinforced by a wire braiding. This ensures good flexibility, wherein, however, the contraction through evacuation is kept at an acceptable level.   cooling: The RF lines  10  are not directly guided from the receiver coils  5  to the preamplifier  16 , but are actively cooled along the path under well-defined conditions, which are explained in more detail below. This cooling is performed by at least one of the present cooling lines  9  that extend to the receiver coil  5 . Each cooling line  9   a  and/or  9   b  may thereby be associated with a number of flexible RF lines  10 . It is thereby not important which cooling line  9   a ,  9   b  is used. The feed (cooling line  9   a ) has a slightly lower temperature than the return (cooling line  9   b ). In the simplest case, only one flexible RF line  10  is e.g. connected to the feed line  9   a . In a more complex case, the four RF lines  10  may e.g. be cooled with the feed  9   a , and also four by the return  9   b . This produces eight parallel, cooled RF lines.   The lines  9   a ,  9   b ,  10  are thermally coupled to each other by coupling elements  11 . Such a coupling element  11  typically consists of a Cu cylinder to which the cooling line  9   a  or  9   b , the outer conductor  10   b  and/or the inner outer jacket  10   c  ( FIG. 4   b ) of the flexible RF line  10  and optionally further elements are mechanically or thermally connected, preferably through soldering.   The RF lines  10  are preferably twisted in a defined periodic fashion between two coupling elements  11  ( FIG. 3   b ). This is explained in more detail below.   the cooling line  9  or the line package  4  is additionally reinforced by suitable reinforcing elements  12 , e.g. steel wires ( FIG. 3   c ).   the lines are thermally insulated from the outer jacket which is at room temperature, through vacuum insulation  13   b  ( FIG. 3   a ) and super insulation, and optionally supported by spacers (not shown).       
 
         [0088]    The temperature dependence of one of the RF lines  10  is shown in  FIG. 4   a.    
         [0089]    The temperature of the flexible RF lines  10  is kept very low along a predetermined length which corresponds in practice to the overall length of the connecting line  8 , starting from the receiver coils  5 , and optionally further thermal elements (heat-conducting connection  6 , heat exchanger  7 ) (shown in the illustration as a black box on the left). This temperature corresponds approximately to the temperature T C  of the receiver coils  5 , but may also be slightly less, since the flexible RF line  10  is directly cooled by the cooling line(s)  9   a ,  9   b  ( FIG. 3   a ). The feed  9   a  thereby always has a lower temperature than the receiver coil  5 . Cooling of the RF line  10  is maintained through more or less regular thermal connection of the RF line  10  through the coupling elements  11  along the entire length between the coupling elements. Thereby, the RF line  10  nevertheless remains highly flexible. The separations between the coupling elements  11  can be easily dimensioned such that even the temperature between the coupling elements  11  also rises only on the order of magnitude of 1K, due to the sufficient thermal longitudinal conductivity of the RF line  10  and the minimum heat input from the outside when good vacuum insulation is used. The temperature dependence of the RF lines  10  between the coupling elements  11  is approximately a square function. For this reason, the maximum excess temperature can be easily reduced when required by arranging the coupling elements  11  more tightly. 
         [0090]    The thermal connections to the coupling elements  11  terminate along a precisely selected section just before the preamplifier  16  or its thermal stage  23  (on the right in  FIG. 4   a ). This produces a continuous transition between the temperature of the cooling line  9   a ,  9   b  and the preamplifier temperature T p . The exact dependence is given by the heat conductivity of the RF line  10  as a function of the temperature and by the external heat input, and is only schematically shown in  FIG. 4   a . The length of path of this transition is dimensioned such that the total additional RF loss or the noise input of the warmer part of the RF line  10  becomes as low as possible without forming an excessive heat bridge between the cooling lines  9   a  and  9   b  and the preamplifier  16 , since the shorter this transition, the lower the total negative loss effects, but the higher the thermal conductivity along the transition length. An excessive heat conductivity would unnecessarily increase the temperature of the first cooling circuit which comprises i.a. the cooling lines  9   a ,  9   b , and thereby finally also the temperature of the receiver coil(s)  5 . The length of the transition path can be optimally selected such that the overall performance is only minimally influenced. This optimization is not critical, and substantially independent of the length L of the RF line, since the temperature of the cooled conducting part located between the coupling elements  11  is always minimum, and only a short part of the RF line  10  forms the temperature transition. This thermal configuration yields extremely low RF losses even when the connecting lines  8  are long, which is accompanied by only minimum noise increase, thereby hardly influencing the efficiency of the system. 
         [0091]    A particular embodiment which is described below is used as the actual RF line  10 . The RF line  10  is thereby realized in the form of a coaxial cable. Common coaxial conductors are either completely rigid or semi-rigid. These two embodiments are absolutely unsuitable for the inventive MR probe head, since they would already break after a few bending cycles due to the required bending radii. On the other hand, there are coaxial cables with a wire braiding as outer jacket. These are sufficiently flexible but are, in principle, disadvantageous, since their shielding effect is not very good, typically only approximately 40 dB due to the holes in the shielding braiding. This is unfavorable, since when several cables extend closely next to each other, crosstalk impairs the mutual decoupling of the channels, in particular, in an array system. 
         [0092]    One advantageous component of the inventive configuration is therefore the use of a particular coaxial cable which has an additional inner outer jacket  10   c  of a spirally wound metal foil, which is surrounded by an outer conductor  10   b  in the form of a conventional wire braiding. Such a cable is schematically shown in  FIG. 4   b . When used for this purpose, such cables have the very favorable property that they are still sufficiently flexible and also have very high shielding values (&gt;90 dB) and have a very good RF quality and therefore low loss at low temperatures. They may be provided with a PTFE jacket  10   a  which is sufficiently flexible and does not break, even at low temperatures. The flexibility of the jacket may additionally be increased if required by dividing it into short segments, such that the RF line can be kept highly flexible even in the cold state. This PTFE jacket  10   a  reliably keeps the coaxial cable together even at low temperatures. The inside of the cable is designed by a conventional insulator  10   d , in particular of PTFE. The inner conductor  10   e  is either a strand or an individual conductor. 
         [0093]    A cable of this type is sufficiently flexible for the critical outer layers  10   a - 10   c , and the inner components  10   d ,  10   e  are not critical in view of the intended bending strain due to the small diameters. In this fashion, the RF line  10  has all properties required for the intended application: It is flexible, has very low damping values at low temperatures and good crosstalk damping, and is also sufficiently resistant to bending fatigue. 
         [0094]    A further essential feature of the inventive connecting line is that the RF lines  10  are not guided parallel to their associated cooling lines  9  but are twisted relative to the cooling line  9  between two coupling elements  11  through approximately 360 degrees or an integer multiple thereof. The cooling and RF lines form a line package  4 . This is shown in  FIG. 3   b . As a result thereof, when the connecting line  8  is bent, the overall length of the section of the RF lines  10  between two coupling elements  11  and between them and the cooling line  9  remains the same, since the length differences of the parts facing towards the bending center and the parts located on the outside are, to first order, balanced during twisting. In consequence thereof, the RF lines  10  do not bulge or bend when the connecting line  8  is bent. Without this measure, the service life of the RF lines  10  would be unpredictably reduced through repeated local bending, and the cooling line  9  would also be exposed to unnecessary local forces. 
         [0095]    The inventive connecting line advantageously comprises one further feature: Two or more defined rigid or elastic reinforcing elements  12  (longitudinal elements, e.g. steel wires) can be mounted in a similar fashion about the cooling line  9 , which is already the case in the RF lines  10 . This is shown in  FIG. 3   c . These reinforcing elements  12  are dimensioned such that they are just sufficiently flexible for the intended bending radii and forces. They then form a natural guidance of the overall cold structure of the line package (consisting of the respective cooling line  9  and the RF lines  10  associated therewith) and ensure that it is not locally bent. This is important to ensure that there are only steady, uniform bending radii of the cold elements at any location. This again substantially reduces maximum local stresses that could cause premature fatigue fractures. 
         [0096]    The line packages  4  are thermally insulated and mechanically supported against the outer jacket. Moreover, these line packages  4  are also advantageously twisted with each other, at least once through 360 degrees along the length of the flexible connecting line  8 . The thermal insulation is realized e.g. through initial individual wrapping and then common wrapping with super insulation foil. Spacers may optionally be periodically used to support the cold parts relative to the outer jacket, advantageously starting from the hard coupling elements  11 . 
         [0097]    The above-described features produce a flexible connecting line  8  with excellent properties. For testing the flexibility in the cold state, 1000 bending cycles were carried out at a temperature of approximately 20 K and a bending radius of 30 cm. No failure was observed. 
         [0098]    A connecting line of this type permits implementation of an MR system with a cooled preamplifier which is connected to a cold detecting device through a flexible, cold line. A cold NMR/MRI measuring head with maximum performance can be inserted into the magnet in a simple manner like a conventional room temperature measuring coil using the above-described device. 
         [0099]      FIGS. 9   a  and  9   b  show two possible MR system configurations.  FIG. 9   a  shows an NMR system for high-resolution NMR spectroscopy or micro imaging.  FIG. 9   b  shows an MRI system for MR imaging. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           1  magnet 
           2  room temperature bore of the magnet 
           3  detecting device 
           4  line package 
           5  receiver coil 
           6  heat conducting connection to cool the receiver coil 
           7  heat exchanger for cooling the receiver coil 
           8  flexible connecting line between the detecting device and the preamplifier 
           8   a  outer jacket of the flexible connecting line 
           9  flexible cooling line to cool the coil 
           9   a  flexible cooling line to cool the coil (feed) 
           9   b  flexible cooling line to cool the coil (return) 
           10  flexible RF line 
           10   a  outer jacket (PTFE) of the flexible RF line 
           10   b  outer conductor (braiding) of the flexible RF line 
           10   c  inner outer jacket (metal foil) of the flexible RF line 
           10   d  insulator (PTFE) of the flexible RF line 
           10   e  inner conductor of the flexible RF line 
           11  coupling element between the cooling line and the RF line for cooling the RF line 
           12  reinforcing element (steel wire) 
           13   a  vacuum in the detecting device 
           14  first cooling circuit 
           15   a  preamplifier housing as part of the probe head 
           15   c  rigid connecting means 
           16  cooled preamplifier 
           17  signal output at the output of the preamplifier 
           18   a  coupling part on the preamplifier side between the preamplifier housing and the cryocooler 
           18   b  coupling part on the cryocooler side between the preamplifier housing and the cryocooler 
           19  transfer line between the cryocooler and the probe head 
           20  cryocooler 
           21   a  continuing cooling line 
           21   b  continuing cooling line 
           21   c  continuing cooling line 
           21   d  continuing cooling line 
           22  second cooling circuit 
           23  thermal stage (heat sink) of the cooled preamplifier(s) 
           51  test object 
           52  rigid RF line within the probe head 
           54  pulse tube cold head 
           55  thermally insulated housing for pulse tube system 
           56  compressor for pulse tube cold head 
           57  flexible pressure line between the compressor and the pulse tube cold head 
           58  preamplifier at room temperature 
           59  RF line between the RF coil and the preamplifier (at room temperature) 
           60  LN2 for cooling the receiver coil 
           61  thermally insulated housing (foamed polystyrene) 
           62  wall 
         D 1  required separation magnet/wall for rigid probe head 
         D 2  required separation magnet/wall for flexible probe head 
         T p  preamplifier temperature 
         T c  temperature of receiver coil 
         L running length along the flexible RF line (coordinate) 
       
     
       LIST OF REFERENCES 
       [0000]    
       
         [1] WO 2003/050554 A3 
         [2] DE 196 48 253 A1 
         [3] DE 199 46 371 C1 
         [4] “Cryogenic probe setup for routine MR imaging on small animals at 9.4 T”, R. Haueisen, D. Marek, M. Sacher. F. Kong, K. Ugurbil, S. Junge, ESMRM Basel, 16 Sep. 2005 
         [5] U.S. Pat. No. 5,829,791 
         [6] U.S. Pat. No. 5,889,456.