Abstract:
In a magnetic resonance apparatus having a magnet arrangement which generates: a basic magnetic field in a horizontal direction, an imaging volume with a central region that is freely accessible in at least one horizontal direction substantially orthogonal to the basic magnetic field, upper and lower elements that are spaced apart in a vertical direction and between which the imaging volume is disposed, and a connector element that connects the upper and the lower elements, at least one gradient coil is provided which extends at least in regions of the upper element, the lower element and the connector element.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a magnetic resonance apparatus of the type having an “open” imaging volume. 
     2. Description of the Prior Art 
     The technique of magnetic resonance is a known technique for obtaining images of a body interior of an object under examination. Rapidly switched gradient fields that are produced by a gradient coil system are superimposed in a magnetic resonance apparatus on a static basic magnetic field that is produced by a basic field magnet. The magnetic resonance apparatus also has a radio-frequency system that emits radio-frequency signals into the object under examination in order to trigger magnetic resonance signals, and picks up the triggered magnetic resonance signals on the basis of which magnetic resonance images are created. 
     A gradient coil of the gradient coil system produces, for a specific direction in space a gradient field that has, in the ideal case, at least inside an imaging volume of the magnetic resonance apparatus, one field component which is collinear with the basic magnetic field. The field component has in this case a prescribable gradient that is approximately of the same magnitude at any desired instant, at least inside the imaging volume, in a fashion dependent on location. Since the gradient field is a temporally variable magnetic field, although the above is true for any instant, the strength of the gradient is variable from one instant to another instant. The direction of the gradient is generally permanently prescribed by the design of the gradient coil. Appropriate currents are created in the gradient coil in order to produce the gradient field. The amplitudes of the required currents are several 100 A. The rates of current rise and fall are several 100 kA/s. The gradient coils are connected to gradient amplifiers for the purpose of power supply. 
     A magnetic resonance machine with a C-shaped basic field magnet for producing a vertically directed basic magnetic field is known, for example, from German OS 40 37 894. The basic field magnet has two pole shoes between which an imaging volume of the magnetic resonance machine is arranged. In this case, access to the imaging volume in the horizontal direction is limited only by a yoke of the basic field magnet, and so the imaging volume is freely accessible in an angular range of approximately 270° with reference to a midpoint of the imaging volume. As a result of which the magnetic resonance machine is particularly suitable, owing to its open design, both for interoperative and intraoperative use and for examining patients with claustrophobia. In this arrangement, gradient coils designed as flat coils are laid in recesses in the pole plates in order to produce gradient fields. 
     Also known, for example from U.S. Pat. No. 4,829,252, is a magnetic resonance apparatus having a horizontally preceding basic magnetic field, with a basic field magnet essentially formed by two pole plates that extend in the vertical direction and that are spaced apart in the horizontal direction, and which are interconnected by four horizontally proceeding columns. An imaging volume situated at the center of the basic field magnet is accessible in the horizontal direction only from the two sides that are not blocked by the pole plates. The gradient coils are designed, in turn, as flat coils and are fastened on the sides of the pole plates, which face the imaging volume. 
     Also known from the above-named U.S. Pat. No. 4,829,252 is a magnetic resonance apparatus in which an imaging volume is arranged in a tunnel-like examination space such that the imaging volume is accessible only from the two tunnel openings. Here, the tunnel-like examination space is formed substantially by a hollow cylindrical, generally superconducting basic field magnet having a cavity in which a likewise hollow cylindrical gradient coil system is arranged. The basic field magnet produces a horizontally directed basic magnetic field in the examination space. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to create a highly efficient gradient coil system for a so-called open magnetic resonance apparatus having a basic magnetic field in the horizontal direction. 
     The object is achieved according to the invention in a magnetic resonance apparatus having a magnet arrangement which generates a basic magnetic field in a horizontal direction, an imaging volume with central region that is freely accessible in at least one horizontal direction substantially orthogonal to the basic magnetic field, upper and lower elements that are spaced apart in a vertical direction and between which the imaging volume is disposed, a connector element that connects the upper and the lower elements, and at least one gradient coil, which extends at least in sub-regions of the upper element, the lower element and the connector element. 
     Owing to the higher efficiency of the gradient coil composed to a comparable gradient coil of the type known in the prior art in which, in particular, the connector element is not used, advantages result with respect to the inductance of the gradient coil, the required installation volume, behavior in terms of waste heat and noise, and behavior of the gradient coil as an eddy current generator. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a magnetic resonance machine having two columns, in accordance with the invention. 
     FIG. 2 shows the basic layout of a conductor arrangement for a coil section of a z-gradient coil of the magnetic resonance apparatus of FIG. 1 in accordance with the invention. 
     FIG. 3 shows the basic layout of a conductor arrangement for a coil section of a y-gradient coil of the magnetic resonance apparatus of FIG. 1, in accordance with the invention. 
     FIG. 4 shows the basic layout of a conductor arrangement for a disk-shaped coil section of an x-gradient coil of the magnetic resonance apparatus of FIG. 1, in accordance with the invention. 
     FIG. 5 shows substantially illustrates a conductor arrangement for a coil section, arranged in one of the columns, for the x-gradient coil of the magnetic resonance apparatus of FIG. 1, in accordance with the invention. 
     FIG. 6 shows, for comparison purposes, the basic layout of a conductor arrangement for a coil section of an x-gradient coil in accordance with the prior art. 
     FIG. 7 shows the basic layout of a conductor arrangement for a coil section of a z-shielding coil of the magnetic resonance apparatus of FIG. 1, in accordance with the invention. 
     FIG. 8 shows the basic layout of a conductor arrangement for a coil section of a y-shielding coil of the magnetic resonance apparatus of FIG. 1, in accordance with the invention. 
     FIG. 9 shows the basic layout of a conductor arrangement for a disk-shaped coil section of an x-shielding coil of the magnetic resonance apparatus of FIG. 1, in accordance with the invention. 
     FIG. 10 illustrates a conductor arrangement for a coil section, arranged in one of the pillars, of the x-shielding coil of the magnetic resonance apparatus of FIG. 1, in accordance with the invention. 
     FIG. 11 is a perspective view of a magnetic resonance machine having four columns in accordance with the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a perspective view of a magnetic resonance apparatus as an exemplary embodiment of the invention. The magnetic resonance apparatus has a substantially cylindrical upper element  10  and a likewise substantially cylindrical lower element  15  that are interconnected by a first column  22  and a second column  26 . The magnetic resonance apparatus also has a support device  30  that can be used to position in an imaging volume  35  of the magnetic resonance apparatus, a region to be imaged of an object under examination that is disposed on the support device  30 . The imaging volume  35  extends around a center between the upper and lower elements  10  and  15 . 
     The magnetic resonance apparatus has a basic field magnet for producing a static basic magnetic field B 0  that proceeds in the horizontal direction and is as homogeneous as possible inside the imaging volume  35 . Parts of the basic field magnet are arranged at least in the upper and lower elements  10  and  15  of the magnetic resonance apparatus. 
     The magnetic resonance apparatus has a gradient coil system allowing rapidly switchable magnetic gradient fields to be produced that are as linear as possible inside the imaging volume  35 . The gradient coil system has at least an x-gradient coil for producing a magnetic gradient field with a gradient in the x direction, a y-gradient coil for producing a magnetic gradient field with a gradient in the y direction, and a z-gradient coil for producing a magnetic gradient field with a gradient in the z direction. 
     The z-gradient coil shown in FIG. 2 in this case is formed essentially by two identically constructed disk-shaped coil sections  42  and  44 . One of the coil sections  42  and  44  is arranged in an upper disk-shaped region  11  of the upper element  10 , and the other one of the coil sections  42  and  44  is arranged in a lower disk-shaped region  16  of the lower element  15 . The coil sections  42  and  44  are mirror images of one another with reference to the x-z plane. During operation of the gradient coil system, the coil section  42  or  44  arranged in the upper region  11  has current flowing therein in a direction opposite to the direction the coil section  44  or  42  arranged in the lower region  16 . 
     FIG. 2 shows the basic layout of a conductor arrangement for the coil sections  42  and  44  of the z-gradient coil as an exemplary embodiment of the invention. The columns  22  and  26  sectioned parallel to the x-z plane also are illustrated in FIG. 2 to assist orientation relative to the perspectively illustrated magnetic resonance apparatus in FIG.  1 . 
     The y-gradient coil of the gradient coil system shown in FIG. 3 is formed essentially by two identically constructed disk-shaped coil sections  52  and  54 . One of the coil sections  52  and  54  is arranged in the upper region  11 , and the other one of the coil sections  52  and  54  is arranged in the lower region  16 . The coil sections  52  and  54  are mirror images of one another with reference to the x-z plane. During operation of the gradient coil system, the coil section  52  or  54  arranged in the upper region  11  has current flowing therein in a direction opposite to the direction of current flow in the coil section  54  or  52  arranged in the lower region  16 . 
     FIG. 3 shows the basic layout of a conductor arrangement for the coil sections  52  and  54  of the y-gradient coil as an exemplary embodiment of the invention. The columns  22  and  26  sectioned parallel to the x-z plane also are illustrated in FIG. 3 to assist orientation relative to the perspectively illustrated magnetic resonance apparatus in FIG.  1 . 
     The x-gradient coil of the gradient coil system shown in FIG. 4 is formed essentially by four coil sections, respectively arranged in the upper region  11 , the lower region  16 , a first region  23  of the first column  22 , and a second region  27  of the second column  26 . 
     The x-gradient coil is formed by two substantially identically constructed disk-shaped coil sections  62  and  64 , one of the coil sections  62  and  64  being arranged in the upper region  11 , and the other one of the coil sections  62  and  64  being arranged in the lower region  16 . The coil sections  62  and  64  are mirror images of one another with reference to the x-z plane. During operation of the gradient coil system, the coil section  62  or  64  arranged in the upper region  11  has current flowing therein in a direction opposite to the direction of current flow in the coil section  64  or  62  arranged in the lower region  16 . 
     FIG. 4 shows the basic layout of a conductor arrangement for the disk-shaped coil sections  62  and  64  of the x-gradient coil as an exemplary embodiment of the invention. The columns  22  and  26  sectioned parallel to the x-z plane also are illustrated in FIG. 4 to assist orientation relative to the perspectively illustrated magnetic resonance apparatus in FIG.  1 . For the coil section  62  or  64  arranged in the upper region  11  a directional distribution, illustrated with arrows, of a current I taken as positive is depicted in FIG. 4. A directional distribution of the current I that is directed in the opposite direction exists for the coil section  64  or  62  arranged in the lower region  16 . 
     The x-gradient coil additionally has the further two, substantially identically constructed coil sections, one of the coil sections being arranged in the first region  23 , and the other one of the coil sections being arranged in the second region  27 . The coil sections arranged in the regions  23  or  27  of the columns  22  and  26  are constructed to be mirror images of one another with reference to a y-z plane. In contrast to the case of the disk-shaped coil sections  42  to  64 , the mirror imaging mentioned above in the case of the coil sections arranged in the columns  22  and  26  also is valid with regard to the distribution of current direction. 
     FIG. 5 shows the principle of a conductor arrangement for the coil section  66  of the x-gradient coil arranged in the first region  23  of the first column  22  as an exemplary embodiment of the invention. The course of the conductors includes vertical conductor sections  81  and  82  on a side of the first region  23  facing the imaging volume  35 , and on a side of the first region  23  averted from the imaging volume  35 . The vertical conductor sections  81  and  82  are connected by horizontal conductor sections  85 . In coordination with the current direction fixed in FIG. 4, the coil section  66 , illustrated in FIG. 5, for the current I has a direction proceeding from bottom to top for the conductor sections  81 , which is illustrated by arrows. 
     In other embodiments, the vertical conductor sections  81  are closed on paths other than those illustrated in FIG.  5 . For this purpose, for example, it is also possible for conductor sections to proceed on the two lateral surfaces, not used in FIG. 5, of the first column  22  and/or in the upper and/or lower element  10  and/or  15 . 
     The description in relation to FIG. 5 applies as well for the coil section (not illustrated), of the x-gradient coil, arranged in the second region  27  of the second pillar  26 . 
     An x-gradient coil in accordance with the prior art shown in FIG. 6 essentially formed by two identically constructed disk-shaped coil sections  72  and  74  that would be arranged in the upper and lower regions  11  and  16 , as mirror images of one another with reference to the x□z plane. FIG. 6 shows for comparison purposes the basic layout of a conductor arrangement for the coil sections  72  and  74  of the x-gradient coil in accordance with the prior art. The columns  22  and  26  sectioned parallel to the x-z plane also are illustrated in FIG. 6 to assist orientation relative to the perspectively illustrated magnetic resonance apparatus in FIG.  1 . By contrast with the coil sections  72  and  74  according to the prior art illustrated in FIG. 6, the coil sections  62  and  64  of FIG. 4, which belong to the x-gradient coil, which is exemplary for the invention, with coil sections in the pillars  22  and  26 , have a substantially lower conductor density and inductance. This is advantageous, inter alia, with regard to a required installation volume, waste heat behavior and eddy current induction for example in the basic field magnet. 
     It also should be noted that the x-gradient coil in accordance with the prior art is less efficient by a factor of approximately five by comparison with the y□gradient coil and the z-gradient coil. This disproportion is cancelled in the case of the inventive x□gradient coil, having the coil sections in the columns  22  and  26 . 
     FIGS. 7 to  10  show coil sections  42   s  to  66   s  of shielding coils, belonging to the gradient coils, for forming an actively shielded gradient coil system as further exemplary embodiments of the invention. 
     As shown in FIG. 7, a z-shielding coil belonging to the z-gradient coil is formed essentially by two identically constructed disk-shaped coil sections  42   s  and  44   s,  one of the coil sections  42   s  and  44   s  being arranged in the upper region  11  and the other one of the coil sections  42   s  and  44   s  being arranged in the lower region  16 . By comparison with the associated coil sections  42  and  44  of the z□gradient coil, the coil sections  42   s  and  44   s  of the z-shielding coil are arranged further outside in the regions  11  and  16  with reference to the imaging volume  35  and have a lower conductor density. During operation of the gradient coil system, here the coil sections  42   s  and  44   s  are energized in the opposite sense with regard to the coil section  42  or  44  arranged in the same region  11  or  16 . FIG. 7 shows for this purpose the basic layout of a conductor arrangement for the coil sections  42   s  and  44   s  of the z-shielding coil. The columns  22  and  26  sectioned parallel to the x-z plane also are illustrated in FIG. 7 to assist orientation relative to the perspectively illustrated magnetic resonance machine in FIG.  1 . 
     As shown in FIG. 8, y-shielding coil belonging to the y-gradient coil is formed essentially by two identically constructed disk-shaped coil sections  52   s  and  54   s,  one of the coil sections  52   s  and  54   s  being arranged in the upper region  11  and the other one of the coil sections  52   s  and  54   s  being arranged in the lower region  16 . By comparison with the associated coil sections  52  and  54  of the x-gradient coil, the coil sections  52   s  and  54   s  of the y-shielding coil are arranged further outside in the regions  11  and  16  with reference to the imaging volume  35  and have a lower conductor density. During operation of the gradient coil system, the coil sections  52   s  and  54   s  are energized in the opposite sense with regard to the coil section  52  or  54  arranged in the same region  11  or  16 . FIG. 8 shows for this purpose the basic layout of a conductor arrangement for the coil sections  52   s  and  54   s  of the y-shielding coil. The columns  22  and  26  sectioned parallel to the x-z plane are also illustrated in FIG. 8 to assist orientation relative to the perspectively illustrated magnetic resonance machine in FIG.  1 . 
     As shown in FIG. 9 an x-shielding coil, belonging to the x-gradient coil, of the gradient coil system, like the x-gradient coil is formed by four coil sections, respectively arranged in the upper region  11 , the lower region  16 , the first region  23  and the second region  27 . 
     The x-shielding coil has two substantially identically constructed disk-shaped coil sections  62   s  to  64   s,  one of the coil sections  62   s  and  64   s  being arranged in the upper region  11 , the other one of the coil sections  62   s  and  64   s  being arranged in the lower region  16 . By comparison with the associated coil sections  62  and  64  of the x-gradient coil, the coil sections  62   s  and  64   s  of the x-shielding coil are arranged further outside in the regions  11  and  16  with reference to the imaging volume  35  and have a lower conductor density. During operation of the gradient coil system, the coil sections  62   s  and  64   s  are energized in the opposite sense with regard to the coil section  62  or  64  arranged in the same region  11  or  16 . FIG. 9 shows for this purpose the basic layout of a conductor arrangement for the coil sections  62   s  and  64   s  of the x-shielding coil. The columns  22  and  26  sectioned parallel to the x-z plane also are illustrated in FIG. 9 to assist orientation relative to the perspectively illustrated magnetic resonance machine in FIG.  1 . 
     The x-shielding coil also includes the further two substantially identically constructed coil sections, one of these coil sections being arranged in the first region  23 , and the other of these coil sections being arranged in the second region  27 . FIG. 10 shows for this purpose a horizontal cross section through the first region  23  of the column  22 . The coil section  66   s,  arranged in the region  23 , of the x-shielding coil has vertical conductor sections  81   s  and  82   s  that are connected in a fashion similar to the conductor sections  81  and  82  of the associated coil section  66  of the x-gradient coil. During operation of the gradient coil system, the conductor sections  81   s  have current flowing therein in a direction opposite to the direction of current flow in associated conductor sections  81 , and the current flow in the conductor sections  82   s  is opposite to the direction in the associated conductor sections  82 . Also illustrated in FIG. 10 is an electrically conductive region  24 , for example made from a metal, in which currents flowing in the coil section  66  produce eddy currents that are undesired. The coil section  66   s  of the x-shielding coil is constructed so that the previously mentioned eddy currents are minimized in the region  24 , in particular with regard to their effect on the imaging volume  35 . 
     FIG. 11 shows a perspective view of a magnetic resonance apparatus having four columns  125  as a further exemplary embodiment of the invention. This magnetic resonance apparatus has an upper element  110  and a lower element  115  that are interconnected via a connector element that substantially comprises four pillars  125 . The magnetic resonance apparatus of FIG. 11 also has a bearing device  130  that can be used to position in an imaging volume  135  of the magnetic resonance apparatus a region to be imaged of an object under examination mounted on the support device  130 . The imaging volume  135  extends around a center between the upper and lower elements  110  and  115 . 
     The magnetic resonance apparatus of FIG. 11 has a basic field magnet for producing a static basic magnetic field B 0  which proceeds in the horizontal direction and is as homogeneous as possible inside the imaging volume  135 . Parts of the basic field magnet are arranged at least in the upper and lower elements  110  and  115  of the magnetic resonance apparatus. 
     The upper element  110  in this case has an upper disk-shaped region  111 , and the lower element  115  comprises a lower disk-shaped region  116 , coil sections of a gradient coil system being arranged in the disk-shaped regions  111  and  116 . A y-gradient coil is constructed to have two disk-shaped coil sections in a way corresponding to the description relating to FIG.  2 . Furthermore, the gradient coil system has an x-gradient coil formed essentially by two disk-shaped coil sections, which are constructed and arranged in accordance with the description relating to FIG. 4, and four further coil sections that are constructed in a way similar to the description relating to FIG. 5, respectively arranged in the columns  125 . 
     The gradient coil system also has a z-gradient coil that has, inter alia, conductor sections that are arranged like rings inside the upper element  110 , the lower element  115  and two columns  125  positioned identically in the z-direction. One of the conductor sections  180  is indicated in FIG. 11 by a dashed line. The advantages described in conjunction with FIG. 6 result for such a z-gradient coil by comparison with a z-gradient coil that is formed by two disk-shaped coil sections arranged in the upper and in the lower regions  111  and  116 . 
     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 hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.