Abstract:
A device for mounting and driving a rotatable part of a gantry of a computed tomography system with respect to a stationary part of the gantry around a system axis of the stationary part, has a magnetic bearing that magnetically supports the rotatable part relative to the stationary part, and an electromagnetic drive that drives the rotatable part of the gantry relative to the stationary part.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention concerns a device for supporting and driving a rotatable part of a gantry of a computed tomography apparatus—which gantry has a system axis—relative to a stationary of the gantry of the computed tomography apparatus. The invention also concerns a computed tomography apparatus that embodies such a device. 
     2. Description of the Prior Art 
     Computed tomography apparatuses of the third generation have a gantry with a stationary part and a part that can be rotated relative to the stationary part. The rotatable part has the shape of a drum on which are arranged the components of the computed tomography apparatus (such as the x-ray source, the x-ray detector, a control system, etc.), which components revolve around a patient arranged along the system axis in a measurement field during operation of the computed tomography apparatus. In a system with only one x-ray source and one x-ray detector, a completely populated drum has a mass of approximately 800 kg to 900 kg and rotates with a rotation speed of up to 240 R/min during operation. 
     Roller bearings in different forms are presently used to support the drum relative to the stationary part of the gantry. However, the roller bearings are subject to a certain wear and must be regularly serviced (re-oiled, for example). 
     For example, the drive may be a motor interacting with a worm gear. Moreover, the noise level emanating from the roller bearings increases with increasing rotation speed such that it can be perceived as disruptive, and additional measures can be necessary to reduce it. The driving of the drum normally takes place via a drive belt. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a device and a computed tomography apparatus of the aforementioned type such that the bearing and the drive of a rotatable part of a gantry relative to a stationary part of the gantry of the computed tomography apparatus is improved. 
     According to the invention, this object is achieved by a device to support and drive a rotatable part of a gantry—the gantry having a system axis—of a computed tomography apparatus relative to a stationary part of the gantry of the computed tomography apparatus, having a magnetic bearing that magnetically supports the rotatable part of the gantry relative to the stationary part of the gantry, and having an electromagnetic drive that electromagnetically drives the rotatable part of the gantry relative to the stationary part of the gantry. 
     Due to the magnetic support of the rotatable part of the gantry relative to the stationary part of the gantry, mechanical contact no longer occurs between bearing parts. The magnetic bearing is friction-free and wear-free, such that a re-oiling or a re-greasing as with roller bearings is not necessary. With the magnetic bearing, the development of noise during operation is also markedly less than with the use of a roller bearing. 
     The electromagnetic drive that drives the rotatable part of the gantry relative to the stationary part of the gantry, and the magnetic bearing that supports the rotatable part of the gantry relative to the stationary part of the gantry, are advantageously combined into one unit (in particular a structural unit). 
     According to one variant of the invention, the magnetic bearing is formed by at least one radial magnetic bearing and at least one axial magnetic bearing. 
     According to a further variant of the invention, the magnetic bearing includes at least one permanent magnet, at least one electromagnet having a coil and/or at least one element made of a ferromagnetic material, and the electromagnetic drive includes at least one electromagnet having a coil. The radial and axial bearings can be realized as purely passive magnetic bearings with corresponding arrangement of permanent magnets and elements made of a ferromagnetic material on the rotatable part and the stationary part of the gantry relative to one another, making use of either the repulsive or attractive forces occurring between permanent magnets or the attractive forces between permanent magnets and ferromagnetic materials. However, to stabilize the magnetic bearings the radial and axial bearings preferably also have electromagnets that include coils in order to be able to modify the magnetic field (and therefore the currently acting forces in the respective magnetic bearings) by variation of the currents flowing through the coils of the electromagnets. For this purpose, a corresponding regulation is required that ensures that the respectively necessary bearing forces are provided. 
     The electromagnets (normally having multiple coils, or at least one coil) of the electromagnetic device are necessary to generate a rotating electromagnetic field to drive the rotatable part relative to the stationary part of the gantry, and for this must be controlled accordingly. The electromagnetic drive can also have permanent magnets and/or elements made of a ferromagnetic material. 
     According to one embodiment of the invention, the radial bearing and/or the electromagnetic drive have a first, radially outward, annular radial arrangement of permanent magnets, electromagnets and/or elements made of a ferromagnetic material that is associated with the stationary part of the gantry; and a second, radially inward, annular radial arrangement of permanent magnets, electromagnets and/or elements made of a ferromagnetic material that is associated with the rotatable part of the gantry. An annular radial bearing gap is located between the first and second annular radial arrangements. 
     According to a further embodiment of the invention, the at least one axial bearing has at least one first annular axial arrangement of permanent magnets, electromagnets and/or elements made of a ferromagnetic material that is associated with the stationary part of the gantry, and at least one second, annular axial arrangement of permanent magnets, electromagnets and/or elements made of a ferromagnetic material that is associated with the rotatable part of the gantry. The first and second axial arrangements are axially offset relative to one another in the direction of the tilt axis, with an annular axial bearing gap therebetween. 
     In a further embodiment, the device or the structural unit composed of the magnetic bearing and the electromagnetic drive has a measurement unit to determine the change of the width of the annular radial bearing gap and/or of the annular axial bearing gap. In order to be able to realize a functional and interference-free magnetic bearing, the width of the bearing gaps must be or be kept essentially constant. The width of a bearing gap is essentially the control variable in the regulation of the bearing forces. The width is preferably defined without contact with the measurement unit. The width is normally determined at at least two points of the annular bearing gap that are advantageously offset relative to one another by approximately 90°. Moreover, depending on design the width of a bearing gap does not always need to be the same over the entire bearing gap. Rather, the bearing gap can have a profile so that different widths of the bearing gap result at different points of the bearing gap. In such a case, the width of the bearing gap at a defined location of the bearing gap is respectively determined and used for regulation. 
     The measurement can be formed by at least one Hall sensor and/or one sensor operating inductively or capacitively. 
     According to a variant of the invention, if electromagnets are used for the magnetic bearing, the determination of the change of the width of the radial bearing gap and/or of the axial bearing gap takes place based on the effect of the inductivity of the coil of one or more electromagnets. For example, a position change of the inward radial arrangement of permanent magnets, electromagnets and/or elements made of a ferromagnetic material of the rotatable part of the gantry relative to the outward radial arrangement affects the inductivities of the electromagnets of the outward radial arrangement. Current and voltage values thereby change at the electromagnets of the outward radial arrangement, which changes are evaluated to determine the width of the respective bearing gap. 
     In an embodiment of the invention the radial bearing is arranged in a first plane, and the electromagnetic drive is arranged in a second plane that is offset relative to the first plane in the direction of the tilt axis. 
     In another embodiment of the invention the electromagnetic drive is integrated into the radial bearing. 
     According to a first variant of this embodiment, the first, radially outward, annular radial arrangement has electromagnets and the second, radially inward, radial arrangement has permanent magnets and/or elements made of a ferromagnetic material, and a group of electromagnets of the first radial arrangement is controlled such that a rotating electromagnetic field is generated to electromagnetically drive the rotatable part of the gantry. 
     According to a second variant of the integration embodiment, the first, radially outward, annular radial arrangement has permanent magnets and/or elements made of a ferromagnetic material and the second, radially inward, radial arrangement has electromagnets, and a group of electromagnets of the second radial arrangement is controlled such that a rotating electromagnetic field is generated to electromagnetically drive the rotatable part of the gantry. 
     According to a further embodiment of the invention, the first, radially outward annular radial arrangement and the second, radially inward annular arrangement have electromagnets, and a group of electromagnets of the first and/or the second radial arrangement is controlled such that a rotating electromagnetic field is generated to electromagnetically drive the rotatable part of the gantry. 
     According to another embodiment of the invention, the electromagnets of the first and/or the second radial arrangement that are provided for electromagnetic driving and/or for magnetic bearing are respectively grouped in segments. The electromagnets can respectively be grouped in two or more segments. For example, the electromagnets provided for electromagnetic driving can be grouped into three segments that respectively cover approximately 60° of the radial bearing; which segments are respectively separated from one another by a segment (likewise covering a 60° angle) in which are grouped electromagnets provided for magnetic bearing. Given a four-segment arrangement, the electromagnets provided for electromagnetic driving and the electromagnets provided for magnetic bearing alternately cover a respective 45° angle of the radial bearing, such that four segments are present for the driving and four segments are present for the bearing. Additional segmentations are likewise possible. 
     In an additional embodiment of the invention the device or the structural unit has a roller bearing as an auxiliary support bearing. In the case of a power failure, the roller bearing takes over the support function so that damage to the device is avoided. 
     Alternatively or additionally, an uninterruptible power supply (UPS) can be associated with the device. 
     The above object also is achieved in accordance with the present invention by a computed tomography apparatus having at least one device as described in the preceding. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates a computed tomography apparatus in accordance with the present invention. 
         FIG. 2  schematically illustrates a stationary part and the rotatable part of the gantry of the computed tomography apparatus of  FIG. 1 , as well as bearing and drive unit. 
         FIG. 3  is a section through a first embodiment of a bearing and drive unit according to the present invention. 
         FIG. 4  is a view in the direction of arrows IV in the section of  FIG. 3 , in a first embodiment. 
         FIG. 5  is a view in the direction of arrows IV in the section of  FIG. 3 , in a second embodiment. 
         FIG. 6  is a section through a further embodiment of a bearing and drive unit according to the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Identical or functionally identical elements in the figures are provided with the same reference characters throughout. The representations in the figures are schematic and not necessarily true to scale. The computed tomography apparatus  1  is discussed in the following and without limitation as to generality insofar as is necessary to understand the invention. 
     The computed tomography apparatus  1  shown in  FIG. 1  has a gantry  2  with a stationary part  3  and with a part  4  that can rotate around a system axis S. In the illustrated exemplary embodiment of the invention, the rotatable part  4  has an x-ray system that includes an x-ray source  6  and an x-ray detector  7  that are arranged opposite one another on the rotatable part  4 . During operation of the computed tomography apparatus  1 , x-ray radiation  8  emanates from the x-ray source  6  in the direction of the x-ray detector  7 , penetrates a measurement subject and is detected by the x-ray detector  7  in the form of measurement data or, respectively, measurement signals. 
     Furthermore, the computed tomography apparatus  1  has a patient bed  9  to support a patient P to be examined. The patient bed  9  has a bed base  10  on which a patient support plate  11  is provided to actually support the patient P. The patient support plate  11  is adjustable relative to the bed base  10  in the direction of the system axis S such that it—together with the patient P—can be introduced into the opening  12  of the gantry  2  to acquire 2D x-ray projections of said patient P, for example in a spiral scan. The computational processing of the 2D x-ray projections that are acquired with the x-ray system and/or the reconstruction of slice images, 3D images or a 3D data set based on the measurement data or the measurement signals of the 2D x-ray projections takes place with an image computer  13  of the computed tomography apparatus  1 , which slice images or 3D images can be presented at a display device  14 . 
     So that the rotatable part  7  (that is provided with the x-ray system, among other things) can rotate relative to the stationary part  3  of the gantry, a rotatable bearing as well as driving of the rotatable part  4  are necessary. In the present exemplary embodiment of the invention, a bearing and drive unit  15  (schematically illustrated in  FIG. 2 ) is provided that includes a magnetic bearing and an electromagnetic drive to effect a support and rotation of the rotatable part  4  relative to the stationary part  3  of the gantry  2 . The magnetic bearing and the electromagnetic drive can in principle be formed by permanent magnets, electromagnets and/or elements made of a ferromagnetic material. 
       FIG. 3  shows a sectional view of a first embodiment of such a bearing and drive unit  14 . In the exemplary embodiment of the invention, the bearing and drive unit  15  has a first annular support element  16  arranged at the stationary part  3  and a second annular support element  17  arranged at the rotatable part  4 . Components that produce the magnetic bearing and electromagnetic drive are arranged on the annular support elements  16 ,  17 . The components in the present exemplary embodiment of the invention are two magnetic axial bearings  18 ,  19  and one magnetic radial bearing  20 , with the electromagnetic drive integrated into the latter. 
       FIG. 4  shows a view of a section through the radial bearing  20  in the direction of the arrows IV from  FIG. 2 . In the exemplary embodiment of the invention that is shown in  FIG. 4 , the radial bearing  20  and electromagnetic drive that is integrated into the radial bearing  20  have a first, radially outward, annular radial arrangement  21  of electromagnets  22  which respectively comprise a coil (not explicitly shown). Furthermore, the radial bearing  20  and the electromagnetic drive that is integrated into the radial bearing  20  have a second, radially inward, annular radial arrangement  23  of elements  24  made of a ferromagnetic material. In the exemplary embodiment of the invention that is shown in  FIG. 4 , 50% of the electromagnets  22  serve for the magnetic bearing and 50% of the electromagnets  22  serve for the electromagnetic driving of the rotatable part  4  relative to the stationary part  3 . Every other electromagnet  22  of the radially outward, annular radial arrangement  21  is provided to generate a rotating electromagnetic field, and these electromagnets  22  are controlled by a control unit (not shown) such that—by interaction with the elements  24  made of a ferromagnetic material—the rotatable part  4  is rotated relative to the stationary part  4 . The remaining electromagnets  22  interact with elements  24  made of a ferromagnetic material to produce the magnetic bearing of the rotatable part  4  relative to the stationary part  3 . 
     Instead of only the elements  24  made of a ferromagnetic material, the second radial arrangement be formed only by permanent magnets, or by both. 
     According to an alternative embodiment of the radial bearing  20  integrated with the electromagnetic drive, the radially outward, annular radial arrangement  21  can have elements made of a ferromagnetic material and/or permanent magnets and the radially inward, annular radial arrangement  23  can have electromagnets, wherein—as described above—every other electromagnet is provided to generate a rotating electromagnetic field in order—by interaction with the elements made of a ferromagnetic material and/or the permanent magnets—to set the rotatable part  4  into rotation relative to the stationary part  3 . The remaining electromagnets, by interaction with the elements made of a ferromagnetic material and/or the permanent magnets, in turn serve for the magnetic bearing of the rotatable part  4  relative to the stationary part  3 . In these variants, the electrical energy required for the generation of the rotating field and the control and regulation signals (insofar as a corresponding control and regulation unit is not arranged on the rotatable part  4 ) must be transmitted to the rotatable part  4  via slip rings, for example. 
     In an alternative embodiment of the radial bearing  20  integrated with the electromagnetic drive, both the radially outward, annular radial arrangement  21  and the radially inward, annular radial arrangement  23  have electromagnets, with at least a portion of the electromagnets of the first and second radial arrangement producing the magnetic bearing, and a group of electromagnets of the first and/or second radial arrangement is controlled such that a rotating electromagnetic field is generated to electromagnetically drive the rotatable part  4  of the gantry  2 . In this case as well, the electrical energy required for the generation of the rotating field (and, if necessary, the control and regulation signals) must be transmitted to the rotatable part  4  via slip rings, for example. 
     The electromagnets  22  provided for the electromagnetic driving and the electromagnets  22  of the radial bearing  20  that are provided for the magnetic bearing can also be respectively grouped into segments.  FIG. 5  shows such a segmentation in a further development of the exemplary embodiment of the invention that is shown in  FIG. 4 . In the exemplary embodiment of the invention that is shown in  FIG. 5 , the electromagnets  22  provided for driving are arranged in three segments  25  that respectively cover a 60° angle, wherein a 60° angle is likewise located between each segment  25 . The electromagnets  22  that are arranged in these intervening segments  45  are provided for the magnetic bearing. Other segment arrangements with other angles are likewise possible. Such a segmentation is also possible for the previously described embodiments of the radial bearing integrated with the electromagnetic drive. 
     In principle, the axial bearings  18  and  19  are designed like the radial bearing  20  but have no means for electromagnetic driving. 
     The axial bearing  18  has a first annular axial arrangement  26  of electromagnets  30  (and possibly of permanent magnets and/or of elements made of a ferromagnetic material) which are attached to the first annular support element  16  and are associated with the stationary part  3  of the gantry  2 , and a second annular, axial arrangement  27  of electromagnets  30  (and possibly of permanent magnets and/or of elements made of a ferromagnetic material) which are attached to the second annular support element  17  and are associated with the rotatable part  4  of the gantry  2 . 
     The design of the axial bearing  19  corresponds to the design of the axial bearing  18 . The axial bearing  19  thus also has a first annular axial arrangement  28  of electromagnets  30  (and possibly of permanent magnets and/or of elements made of a ferromagnetic material) which are attached to the first annular support element  16  and are associated with the stationary part  3  of the gantry  2 , and a second annular, axial arrangement  29  of electromagnets  30  (and possibly of permanent magnets and/or of elements made of a ferromagnetic material) which are attached to the second annular support element  17  and are associated with the rotatable part  4  of the gantry  2 . 
     In order to be able to ensure an operation of the magnetic bearings without interference, the width of the radial bearing gap  31  and the widths of the axial bearing gaps  32  and  33  must always be determined. In the exemplary embodiment of the invention, a measurement system is provided for this in the form of Hall sensors. The width of the bearing gap does not need to be determined directly, but can be calculated from the radial or axial position of the annular support element  17  or of the rotatable part  4 . If the width of a bearing gap deviates from its desired width, the width must be adjusted again to the desired width via a corresponding regulation of the coil currents of the electromagnets that are relevant to this. In the exemplary embodiment of the invention, two Hall sensors  34  (of which only one is shown in  FIG. 3 ) arranged radially offset from one another by approximately 90° are provided to determine the width of the radial bearing gap  31 . Based on the defined and known design of the bearing and drive unit  15 , the width of the radial bearing gap  31  can be determined from the measurement values of the Hall sensors  34  by a control and regulation unit and be used to control and regulate the coil currents of the electromagnets relevant to the radial bearing. 
     The widths of the axial bearing gaps  32  and  33  are determined in a corresponding manner. A first arrangement of Hall sensors  35  and a second arrangement of Hall sensors  36  are shown in  FIG. 3 . At least two first arrangements of Hall sensors  35  and/or at least two second arrangements of Hall sensors  36 , radially offset relative to one another by approximately 90° relative to one another, are preferably present to acquire measurement values. The width of the axial bearing gap  32  and the width of the axial bearing gap  33  can be determined by a control and regulation unit from the measurement values of the Hall sensors  35  and/or of the Hall sensors  36  and be used for control and regulation of the coil currents of the electromagnets relevant to the axial bearing. 
     Alternatively, the determination of the widths of the bearing gaps  31  through  33  can be implemented with additional sensors with evaluation only of the changes of current and voltage values of electromagnets. The inductivities of the electromagnets relevant to the magnetic bearing are influenced as a result of a position change of the annular support element  17  or of the rotatable part  4  relative to the annular support element  16  or the stationary part  3 , thereby causing the current and voltage values at the relevant electromagnets to change. The widths of the bearing gaps can respectively be determined by the evaluation of these current and voltage values and be used for the control and regulation of the coil currents of the relative electromagnets. 
     Hall sensors can moreover also be used for rotation monitoring, wherein a Hall sensor can be used in order to detect the rotation position of the annular support element  17  relative to the annular support element  16 . One or two additional Hall sensors can be provided in order to detect the rotation direction. These sensors are also preferably integrated into the bearing and drive unit  15 . FIG  6  shows a section representation of a second exemplary embodiment of a bearing and drive unit  115  which differs from the bearing and drive unit  15  in that the radial bearing  200  and the electromagnetic drive unit  300  are separate from one another and are arranged offset relative to one another in the direction of the axis  5 . Moreover, the bearing and drive unit  115  has a support bearing  400 . The remaining components of the bearing and drive unit  115  correspond in design and function, as necessary with dimensions adapted to the structural shape of said bearing and drive unit  215 , to the components of the bearing and drive unit  15 , which is why these are provided with the same reference characters. 
     In the exemplary embodiment of the invention that is shown in  FIG. 6 , the radial bearing  200  is arranged in a first plane E 1  and—like the radial bearing  20  integrated with the electromagnetic drive—has a radially outward, annular radial arrangement  21  and a radially inward, annular radial arrangement  23  of electromagnets, and possibly of permanent magnets and/or elements made of a ferromagnetic material. 
     The electromagnetic drive unit  300  is arranged in a plane E 2 , offset relative to the plane E 1  in the direction of the system axis  5 , and likewise has a radially outward, annular radial arrangement  321  and a radially inward, annular radial arrangement  323  of electromagnets, and possibly of permanent magnets and/or elements made of a ferromagnetic material. 
     In contrast to the radial bearing  20  integrated with the electromagnetic drive, all electromagnets  22  (and if applicable permanent magnets and/or elements made of a ferromagnetic material) of the radial bearing  200  are provided for the magnetic bearing. Furthermore, all electromagnets (and if applicable permanent magnets and/or elements made of a ferromagnetic material) of the electromagnetic drive unit  300  are provided for electromagnetic driving. 
     As noted, in addition to this the bearing and drive unit  115  also has a support bearing  400  in the form of a conventional roller bearing. The support bearing  400  takes over the support function in the bearing and drive unit  115  if the magnetic field generated by the electromagnets becomes too weak (for example in the event of a power failure). Damage to the magnet bearing of the bearing and drive unit  115  is also avoided in this way. In order to fulfill this purpose, the width of the column  120  between the support bearing  400  arranged at the support el  16  and the support el  17  is smaller than the radial bearing gap  121  and smaller than the drive gap  123 . In normal operation, the support bearing  400  attached to the annular support element  16  does not contact the annular support element  17 . Only in the event of an error does the support bearing  400  contact the annular support element  17  and take over the support function, whereby a damage to the radial bearing  200  and the electromagnetic drive unit  300  is avoided. 
     Additionally or alternatively, the device or the computed tomography apparatus can have an uninterruptible power supply  124  as this is indicated in  FIG. 2 . As is shown for the bearing and drive unit  15 , a support bearing is not necessary since the power and voltage supply is ensured by the uninterruptible power supply  124  in the event of a power outage. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted heron all changes and modifications as reasonably and properly come within the scope of their contribution to the art.