Abstract:
A data acquisition unit of a magnetic resonance system has an examination region therein, relative to which an examination subject is conveyed by a patient bed. The data acquisition unit has a built-in radio-frequency transmission arrangement, that radiates radio-frequency energy into the examination subject, and a built-in reception arrangement that receives radio-frequency magnetic resonance signals emitted from the examination subject as a result of excitation by the radiated radio-frequency energy. The reception arrangement operates as a resistive arrangement when the radio-frequency energy is being radiated by the transmission arrangement, and operates as a superconducting arrangement when the magnetic resonance signals are being received thereby.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention concerns a magnetic resonance system of the type having a whole body arrangement for receiving RF magnetic resonance (MR) signals. 
     2. Description of the Prior Art 
     Magnetic resonance systems are generally known that include a data acquisition unit that has an examination region that is open axially on both sides relative to a central axis thereof, the examination region being radially surrounded by an inner wall. A patient on a transport bed is movable into and out of the examination region, along the central axis of the examination region. A substantially homogenous static basic magnetic field is generated in the examination region by a basic field magnet in the data acquisition unit. Radio-frequency energy is radiated into the examination subject by a transmission arrangement that radially surrounds the examination region. The radiation radio-frequency energy causes magnetic resonance signals to be emitted from the examination subject, which are detected by a reception arrangement that also radially surrounds the examination region. 
     The RF transmission and reception arrangement that is built into the data acquisition unit is referred to as whole-body antenna arrangement, and is stationary with respect to the data acquisition unit. 
     In known systems of this type, the transmission arrangement is normally identical with the reception arrangement. This combined transmission/reception arrangement is often fashioned as a birdcage resonator. 
     A homogeneous excitation of the person located in the examination region to magnetic resonances is possible by means of magnetic resonance systems fashioned in such a way. A homogeneous reception of excited magnetic resonances from the entire examination region is also possible. Various three-dimensional reconstructions are determined using the acquired magnetic resonance signals with the use of the whole-body reception arrangement, but only at inferior quality. Therefore local coils are often used for the acquisition of magnetic resonance signals. Qualitatively significantly higher-grade reconstructions are often possible by means of local coils. However, local coils exhibit the disadvantage that they must be manually applied on the patient and also must be manually removed again. Their use is therefore relatively time-consuming. Furthermore, acquisition of magnetic resonance signals by means of an individual local coil is possible only from a small part of the entire examination region. The person must therefore be covered by means of many local coils over a large area. This is often subjectively perceived to be uncomfortable. 
     The signal strength that occurs in the acquisition mode in magnetic resonance applications is also relatively low. Significant efforts are therefore undertaken to keep the noise optimally low, thus to maximize the signal-to-noise ratio (SNR). The use of cooled local coils is one possibility for minimization of noise. A further possibility is the use of superconducting coils. Superconducting coils are, for example, described in WO-A-01/94964 as well as in the following technical articles:
         “Superconducting RF Coils for Clinical MR Imaging at Low Field” by Q. Y. Ma et al., Academic Radiology, Vol. 10, Nr. 9, September 2003, pages 978 to 987;   “Superconducting and Cold Copper MRI Coils” by L. C. Bourne, appearing in ISMRM 5 th  (1997), page 1527;   “Superconducting Coil Array for Parallel Imaging” by J. Wosik et al., appearing in Proc. Intl. Soc. Mag. Reson. Med. 13 (2005), page 678;   “High Temperature Superconducting Surface Coils with Liquid Nitrogen or Pulse Tube Refrigeration” by Markus Vester et al., appearing in ISMRM 5 th  (1997), page 1528;   “Superconducting MR Surface Coils for Human Imaging” by Q. Y. Ma et al., placed on the Internet and retrievable at http://www.supertron.com/Product/Publications/pub-3.htm.       

     In all publications cited above, small coils are always used as the coils. In one of the publications it is even explicitly stated that a noteworthy improvement of the SNR is to be expected only for coil diameters of at maximum 12 cm. Such dimensions are thus significantly smaller than the typical diameter of a whole-body transmission and reception arrangement. This diameter is normally 50 to 65 cm. 
     A system of superconducting resonators for magnetic resonance applications is known from EP 1 626 286 A1. The resonators can effect a current distribution that is nearly identical with that of a conventional birdcage resonator. The system can be dimensioned such that magnetic resonance whole-body measurements are possible. It is used both in the transmission case and in the reception case. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a magnetic resonance system that is fashioned relatively simply and by means of which a qualitatively high-grade “screening” of a person can be implemented relatively quickly. 
     The object is achieved according to the invention by a magnetic resonance system wherein the reception arrangement is fashioned such that it acts as a resistive reception arrangement in the event that the radio-frequency excitation field is generated by means of the transmission arrangement and acts as a superconducting reception arrangement in the event that excited magnetic resonance signals are received by means of the reception arrangement. 
     The transmission arrangement can be identical to the reception arrangement (i.e., the same structure that forms the transmission arrangement also forms the reception arrangement). In this case the reception arrangement has reception elements in which a reception current oscillates in a current flow direction in the reception case and an excitation current oscillates in the current flow direction in the transmission case. Viewed transverse to the current flow direction, the reception elements exhibit a superconducting partial cross-section and a non-superconducting partial cross-section. The superconducting partial cross-sections exhibit a current carrying capacity (current rating) that lies between the reception current and the excitation current. Due to this design the reception current oscillates in the superconducting partial cross-section and the excitation current oscillates in the non-superconducting partial cross-section. 
     Alternatively, the transmission arrangement can be an arrangement different from the reception arrangement. In this case the reception arrangement exhibits a current carrying capacity that is smaller than an induced current that is induced in the reception arrangement by the radio-frequency excitation field. 
     When the transmission arrangement is identical with the reception arrangement, the reception arrangement is formed by a number of superconducting reception coils. Due to the relatively low current carrying capacity of the reception coils, in this case (thus when the superconducting reception coils are also used for emission of the radio-frequency excitation field) it can be advantageous for the reception coils to have a number of windings. The reception coils can then generate a strong radio-frequency excitation field with a relatively low current. However, it is also possible for the reception arrangement to be fashioned otherwise. It can in particular be fashioned as a birdcage resonator. 
     The reception arrangement can also be formed by a number of superconducting reception coils when the transmission arrangement is an arrangement different from the reception arrangement. The transmission arrangement can be fashioned in a conventional manner. For example, it can be fashioned as a birdcage resonator. 
     As already mentioned, the superconducting reception arrangement exhibits a relatively low current carrying capacity. It is therefore possible for a detuning circuit to be associated only with the transmission arrangement with the reception arrangement having no detuning circuit. The transmission arrangement can be cooled. In this case more precise excitation pulses can be emitted. 
     The reception coils are mutually thermally shielded from the examination region and from gradient coils. It is preferable, however, to individually thermally shield the reception coils from the examination region and from the gradient coils. 
     The basic magnet can be a permanent magnet or an electromagnet. When the basic magnet is an electromagnet, it is preferably superconducting. In particular, in this case a common cooling device can be associated with the basic magnet and the reception coils. 
     A magnetic resonance signal received by the reception arrangement is preferably wirelessly taken from the reception arrangement to an evaluation device. It is then possible to completely encapsulate a cooling reservoir in which the reception arrangement is arranged. The tapping of the received magnetic resonance signal can ensue, for example, via coupling elements that are arranged inside and outside the cooling reservoir. A preamplifier device is normally arranged downstream from the reception arrangement. The preamplifier device is advantageously also cooled. The signal-to-noise ratio can thereby be optimized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates a magnetic resonance system from the side. 
         FIG. 2  is a section through the magnetic resonance system of  FIG. 1  along a line II-II in  FIG. 1 . 
         FIG. 3  illustrates a first exemplary embodiment of a transmission arrangement and a reception arrangement in accordance with the invention. 
         FIG. 4  illustrates a second exemplary embodiment of a transmission arrangement and a reception arrangement in accordance with the invention. 
         FIG. 5  schematically illustrates a number of reception coils. 
         FIG. 6  schematically illustrates a cooling circuit. 
         FIG. 7  schematically illustrates a cross-section through a reception element. 
         FIG. 8  illustrates a transmission arrangement and a reception arrangement. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to  FIGS. 1 and 2 , a magnetic resonance system comprises a base body  1 . The base body  1  exhibits an examination region  2 . The examination region  2  is normally fashioned essentially symmetrically around a central axis  3 . It is open axially relative to the central axis  3  (i.e. in the direction of the central axis  3 ) on both sides. It is bounded by an inner wall  4  of the base body  1  radial to the central axis  3  (meaning away from the central axis  3  or toward the central axis  3 ). The inner wall  4  is normally at least essentially closed tangential to the central axis  3  (i.e. around the central axis  3 ). 
     The inner wall  4  exhibits a distance “a” from the central axis  3 . The distance “a” can be constant. In this case the examination region  2  is strictly circular in cross-section (as viewed relative to the central axis  3 ). For example, the distance “a” can be constant and lie between 25 and 35 cm. This case is presented in  FIG. 2 . 
     However, the distance “a” could also be position-dependent. In this case the examination region  2  would be, for example, elliptical or oval in cross-section as viewed relative to the central axis  3 . If the examination region  2  is, for example, elliptical or oval in cross-section relative to the central axis  3 , the distance “a” can, for example, be approximately 35 cm horizontally, approximately 25 cm vertically. 
     Independent of the constancy or non-constancy of the distance “a”, the distance “a” is determined such that a transport bed  5  together with a person  6  lying on the transport bed  5  can be conveyed through the examination region  2 . 
     According to  FIG. 1 , a transport bed drive  7  is associated with the transport bed  5 . The transport bed  5  (naturally together with the person  6 ) can be conveyed through the examination region  2  by means of the transport bed drive  7 . 
     The magnetic resonance system furthermore comprises a basic field magnet  8 . A static basic magnetic field B that is at least essentially homogeneous within the examination region  2  can be generated by the basic field magnet  8 . 
     According to the representation in  FIGS. 1 and 2 , the basic magnet  8  is fashioned as a system of ring magnets  9  that is arranged concentrically relative to the central axis  3 . Elliptical or oval ring magnets  9  turning around the central axis  3  are also known. In these cases the basic magnetic field B runs parallel to the central axis  3 . However, other embodiments are also possible in which the basic magnetic field B runs perpendicular to the central axis  3 . 
     The basic field magnet  8  can in principle be fashioned in an arbitrary manner, for example as a permanent magnet or as an electromagnet. It is advantageously fashioned as a superconducting magnet. A cooling device  10  by means of which a coolant medium  11  (normally liquid air or liquid nitrogen) is cooled is therefore associated with the basic field magnet  8 . 
     A transmission arrangement  12  and a reception arrangement  13  are arranged radially outward, abutting the inner wall  4 . They radially surround the outside of the examination region  2 . Both the transmission arrangement  12  and the reception arrangement  13  are arranged stationary relative to the examination region  2 . The reception arrangement  13  is fashioned as a superconducting reception arrangement  13 . 
     A radio-frequency excitation field RF that is at least essentially homogeneous in the entire examination region  2  can be generated by the transmission arrangement  12 . The person  6  (insofar as he is located in the examination region  2 ) can therefore be excited to emit magnetic resonance signals M by means of the radio-frequency excitation field RF. The excited magnetic resonance signals M can be received by means of the reception arrangement  13 . The reception within the examination region  2  is thereby possible, independent of the precise location at which the magnetic resonance signals M are excited. By means of the reception arrangement  13 , it is thus possible to receive excited magnetic resonance signals from the entire examination region  2 . 
     The transmission arrangement  12  and the reception arrangement  13  are only schematically depicted in  FIGS. 1 and 2 . From  FIGS. 3 and 4  it is apparent that the reception arrangement  13  is fashioned not as a single-unit resonance structure but rather comprises a number of reception coils  14 . Each one of the reception coils  14  receives a magnetic resonance signal M from a portion of the examination region  2 . In their entirety the reception coils  14  cover the entire examination region  2 , and in fact do so with an essentially uniform sensitivity. The reception arrangement  13  could also be fashioned differently, for example as a birdcage resonator or as a TEM. 
     The reception coils  14  are fashioned as superconducting reception coils  14  according to  FIGS. 3 and 4 . They are even advantageously fashioned as high temperature superconductors, thus as superconductors with a transition temperature above 77 Kelvin or, respectively, 196° C. According to  FIG. 5  they are surrounded by a cooling medium  15 , normally liquid air or liquid nitrogen. In the preferred embodiment according to  FIG. 5 , each reception coil  14  comprises its own shielding  16 . By means of the shielding  16 , the reception coils are thermally shielded from their environment, in particular from the examination region  2  as well as from gradient coils (not shown). The reception coils  14  are thus advantageously individually thermally shielded from the examination region  2  and from the gradient coils. A suitable shielding  16  is, for example, described in DE-C-196 39 924. 
     It is possible for a separate coolant circuit to be associated with the reception coils  14 . When the basic field magnet  8  is also fashioned as a superconductor, a common cooling device (here the cooling device  10 ) is advantageously associated with the basic field magnet  8  and the reception coils  14 . This is schematically presented in  FIG. 6 . 
     According to  FIG. 3 , the transmission arrangement  12  is identical to the reception arrangement  13 . In particular in this case, when the reception arrangement  13  thus also serves for emission of the radio-frequency excitation field, it can be reasonable when the reception coils  14  exhibits a plurality of windings  17 . This is depicted for one of the reception coils  14  in  FIG. 3 . A stronger radio-frequency excitation field RF can thereby be generated. However, depending on the situation of the individual case it can also be sufficient when the reception coils  14  exhibit only a single conductor loop  18 . This is shown for another of the reception coils  14  in  FIG. 3 . The remaining reception coils  14  are only schematically depicted in  FIG. 3 . 
     According to  FIG. 4 , the transmission arrangement  12  is an arrangement different than the reception arrangement  13 . In this case the reception arrangement  13  also advantageously comprises a number of superconducting reception coils  14 . The reception coils  14  can alternatively exhibit a plurality of windings  17  or comprise a single conductor loop  18 , as needed. The transmission arrangement  12  can be fashioned in a conventional manner in the embodiment according to  FIG. 4 . For example, as indicated in  FIG. 4  it can be fashioned as a birdcage resonator  12 . 
     The reception arrangement  13  comprises reception elements. In the case that the reception arrangement  13  comprises a number of superconducting reception coils  14 , the reception elements are, for example, identical with the reception coils  14 . 
     In the reception case, a reception current I that is caused by the received magnetic resonance signals M oscillates in a current flow direction x in the reception elements  14 . When the reception arrangement  13  is identical with the transmission arrangement  12 , in the transmission case an excitation current I′ that generates the radio-frequency excitation field RF also oscillates in the current flow direction x in the reception elements  14 . Given this case configuration an embodiment that is subsequently explained in detail in connection with  FIG. 7  can be advantageous. 
     According to  FIG. 7 , the reception elements  14  (viewed transverse to the current flow direction x) respectively comprise a superconducting partial cross-section  19  and a non-superconducting partial cross-section  20 . The two partial cross-sections  19 ,  20  can, for example, be connected with one another similar to a bi-metal strip. The superconducting section partial cross-section  19  exhibits a current carrying capacity that is greater than the reception current I. Due to this circumstance the reception current I oscillates nearly completely in the superconducting partial cross-section  19  since this partial cross-section  19  exhibits a significantly lower resistance (due to its superconductivity) than the non-superconducting partial cross-section  20 . The current carrying capacity of the superconducting partial cross-section  19  is, however, smaller than the excitation current I′. The superconducting partial cross-section  19  is thus not superconducting with regard to the excitation current I′. It thus behaves like a “normal” resistive conductor for the excitation current I′. The resistance of the superconducting partial cross-section  19  for the excitation current I′ is significantly greater than the resistance of the non-superconducting partial cross-section  20 . The excitation current I′ therefore oscillates nearly entirely in the non-superconducting partial cross-section  20 . 
     An analogous effect can be utilized when the transmission arrangement is different from the reception arrangement  13 . In this case the reception arrangement  13  is designed such that its maximum current carrying capacity is in fact greater than the reception current I. However, the maximum current carrying capacity is selected smaller than an induced current I″ that is induced in the reception elements  14  by the radio-frequency excitation field RF. Thus the reception arrangement  13  need not have a detuning circuit (see  FIG. 8 ). A detuning circuit  21  is required only for the transmission arrangement  12  so that the transmission arrangement  12  does not impair the reception of the magnetic resonance signals M. 
       FIG. 8  shows a further advantageous embodiment that can be realized independently of whether the reception arrangement  13  has a detuning circuit or not. According to  FIG. 8 , the transmission arrangement  12  is likewise cooled. It is thus located in a cooling reservoir  22  in which it is held at a temperature that lies below the boiling point of nitrogen (thus below −196°). 
       FIG. 8  also shows two further advantageous embodiments of the inventive magnetic resonance system. These two embodiments can also be realized independent of one another, whether the transmission arrangement  12  is identical with the reception arrangement  13  or not. These two embodiments can also be realized independently of one another. 
     The reception arrangement  13  according to  FIG. 8  is completely encapsulated in the coolant reservoir  22 . The transmission of a received magnetic resonance signal M to an evaluation device  23  ensues via first coupling elements  24  and second coupling elements  25 . The first coupling elements  24  are arranged in the coolant reservoir  22  and are connected with the reception arrangement  13 . The second coupling elements  25  are arranged outside of the coolant reservoir  22  and are connected with the evaluation device  23 . The first coupling elements  24  inductively and/or capacitive interact with the second coupling elements  25 . The coupling elements  24  and  25  allow the magnetic resonance signal M received from the reception arrangement  13  to be wirelessly taken from the reception arrangement  13  and transmitted to the evaluation device  23 . 
     A preamplifier device  26  is arranged downstream of the reception arrangement  13 . The preamplifier device  26  is also arranged in the coolant reservoir  22 . It is thus also held to a temperature that lies below the boiling point of nitrogen. 
     A qualitatively higher-grade whole-body acquisition is thus possible in a simple manner by means of the inventively fashioned magnetic resonance system without having to apply a plurality of local coils on a person  6 . A “screening” of the person  6  is thus possible, in particular in a simple manner analogous to CT systems. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted heron all changes and modifications as reasonably and properly come within the scope of his contribution to the art.