Abstract:
A medical examination installation has an MR system and an X-ray system that has an X-ray radiator with an X-ray tube and a solid-state X-ray image detector for producing X-ray exposures. The X-ray system has sensors for the acquisition of the location dependency of the stray field of the MR system in the three spatial axes, and coils for compensation of the stray field, and a computer that uses the output signal of the sensors to calculate a current for the coils which cause the stray field to be reduced in the region of the electron beams of the X-ray tube.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to a medical examination installation with an MR system and an X-ray system that has an X-ray radiator with an X-ray tube and a solid-state X-ray image detector for producing X-ray exposures. 
     2. Description of the Prior Art 
     Real-time monitoring of interventional medical procedures is necessary, in order to ensure that the procedure is proceeding as intended, and as well as to ensure that any medical instruments which are being employed are precisely positioned at the correct location in the patient. 
     Magnetic resonance (MR) imaging is a proven diagnostic method that enables tomograms and three-dimensional (3D) reconstructions to be produced. The examination time, however, is relatively long and lies on the order of magnitude of several minutes. For specific examinations, it is meaningful for shortening the exposure time and/or for planning the further execution of the MR examinations to prepare an X-ray exposure before and/or during the MR examination. The quality of the diagnosis is additionally enhanced as a result. 
     Although such MR systems can fundamentally make the 3D location information required therefor available, there are situations wherein it is desirable to have better access to the patient during the intervention than that afforded by the gantry of an MR system with superconductive magnet or even by a C-shaped magnet apparatus (open). When the patient is moved out of the gantry of the inner magnet region of the MR apparatus for the time of the intervention, for example, an open surgical intervention or the introduction of a biopsy needle can be enabled or simplified. Moreover, monitoring of the patient is improved in this way, for example the delivery of respiratory gasses, infusion tubes as well as a general monitoring of the condition of the patient. 
     However, organs can dislocate in the intervention due to the pressure of an interventional or surgical tool such as, for example, a biopsy needle or a catheter, so that the current organ position can deviate from the position at the earlier point in time of an MR image acquisition. 
     For these reasons, it is advantageous when an additional X-ray system—optimally with real-time image acquisition in the fluoroscopic mode or during transillumination—is integrated in an MR apparatus so that a relationship of the local information between the acquired X-ray images to the MR images is possible. An intervention with enhanced certainty thus is possible without delay and with the involvement of the images of both modalities. It is especially advantageous when the X-ray system can make images with 3D information available that can be correlated with the MR images. 
     PCT Application WO 96/00520 discloses a medical examination installation with an MR system and an independent X-ray system wherein an Independent X-ray device is provided in addition to an Independent MR device. The X-ray device has a voltage supply as well as a C-arm with an X-ray source and the X-ray detector, which form an X-ray unit. A patient lying on a patient support is transported back and forth between the MR device and the X-ray device. The X-ray detector can be a large-area solid-state image converter. 
     Conventional X-ray real-time image systems with X-ray tubes can be employed only conditionally at MR systems because the stray magnetic field of the MR apparatus does not allow a disturbance-free operation of the X-ray tubes, even though the magnetically deflectable electrons exhibit high speeds over short oath distances. 
     In the aforementioned POT Application WO 96/00520, for example, it is taught to align the electron path in the X-ray tube according to the magnetic field lines in the proximity of the MR magnet. This alignment of the electron oath functions only at a fixed distance of the MR apparatus from the X-ray tube because the angle of the magnetic field lines changes with the distance from the MR apparatus. Additionally, the tilt of the axis of the X-ray tube relative to the axis of the MR apparatus reduces the usable emission angle of the X-ray tube, and thus the field of view. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a medical diagnostics installation such that an online X-ray system is possible directly at the MR apparatus without influencing the electron oath in the X-ray tube with the magnetic field lines of the MR magnet even given changes in the position or attitude of the X-ray tube. 
     This object is inventively achieved in a medical examination installation having sensors which identify the magnetic, location-dependent stray field of the MR system in the three spatial axes, a computer determines the coil currents, and coils operated by the computer which compensate the stray field. 
     It has proven advantageous for the X-ray system to have magnetic field sensors for acquiring the magnetic, location-dependent stray field of the MR system in the three spatial axes. 
     The magnetic stray field can already be reduced when the X-ray system has a magnetic shielding for the X-ray tube within which the sensors for acquiring the location dependency of the remaining magnetic stray field and within which the coils are arranged. 
     It has proven advantageous to employ three coil pairs arranged such that their axes respectively reside perpendicularly relative to one another, the coil pairs being arranged in the three spatial axes. 
     Alternatively, the sensors for the acquisition of the location dependency of the stray field of the MR system in the three spatial axes can be location sensors that determine the position of the X-ray tube in view of the MR system and calculate the magnetic, location-dependent stray field of the MR system at the location of the X-ray tube on the basis of stored magnetic field profiles. 
     A compact structure derives when the X-ray system is directly attached to the MR apparatus, and the X-ray radiator and the solid-state image detector can be mounted to a C-arm attached to the MR system. The X-ray system alternatively can be mounted to stands directly next to the MR apparatus. 
     As an alternative, the X-ray radiator and the solid-state X-ray image detector can be secured independently of one another, with location sensors for determining position and angle attached to the X-ray radiator and the solid-state X-ray image detector. The X-ray radiator and the solid-state X-ray image detector are aligned relative to one another and readjusted by motor drives and electronic controls. The position and alignment of X-ray radiator and solid-state X-ray image detector are monitored by the location sensors, so that a so-called “electronic C-arm is achieved. The measurement sensor mechanism with path sensors and rotational angle sensors assures that the current, exact position of the X-ray system in relation to the MR apparatus, particularly relative to the position of the patient support thereof and of the individual components relative to one another is known, so that the components can be reliably and precisely moved on the desired paths. 
     Spatial information of tomosynthesis images can be linked with the content of stored MR images according to the image fusion technique when the X-ray system is fashioned such that, for producing exposures from a number of projections for tomosynthesis tomograms, X-ray radiator and/or solid-state X-ray image detector are moved on a plane parallel thereto, and when the workstation is configured such that the tomosynthesis tomograms and MR images are superimposed. 
     It has proven advantageous when the solid-state X-ray image detector is arranged to be displaceable in the patient support. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a first embodiment of a medical examination installation of the invention with various patient positions. 
     FIG. 2 is a block circuit diagram for the examination installation according to a second embodiment. 
     FIG. 3 illustrates the inventive X-ray radiator used in the installation of FIG.  1 . 
     FIG. 4 illustrates a third embodiment of a medical examination installation of the invention with various patient positions. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 schematically shows an MR apparatus  1  with a patient support  2  on which a patient  3  is situated. The patient support  2  is arranged to be movable in the inside of the MR apparatus  1  in the length thereof in the direction of the double arrow  4 . An X-ray device is provided having a C-arm  5  that can be attached to the MR apparatus  1 . The C-arm  5  has a digital X-ray image detector  6  and an X-ray radiator  7  mounted at its opposite ends for producing X-ray exposures. The digital X-ray image detector  6  can, for example, be a flat solid-state X-ray image detector that is based on an a-Si panel with image points arranged in a matrix. For producing digital X-ray exposures, the patient support  2  can be displaced from the MR apparatus  1  into the X-ray device without repositioning the patient. 
     Since a-Si panels operate independently of magnetic fields, they can be operated close to the MR device  1 , so that they can be secured to the MR apparatus  1  together with the X-ray radiator  7 . 
     Good access to the patient is assured as a result of the fastening of the panel and the X-ray radiator  7  to the C-arm  5  at the MR apparatus  1 . 
     The MR apparatus  1  can be an open system with lateral access to the patient, a closed system—as shown—or a system having two parts axially arranged at a distance from one another, referred to as an interventional MR system, between which access to the patient is possible. It is thereby possible, for example, to prepare a lung exposure by application of the X-radiation between these parts. 
     FIG. 2 shows a block circuit diagram of applicable to all embodiments, and an alternative arrangement of an MR apparatus  1  with an X-ray device. For preparation of digital X-ray exposures, the digital X-ray image detector  6  is arranged in the patient support  2  so as to be displaceable in the direction of the arrows  12 . The X-ray radiator  7  is height-adjustably seated at a ceiling mount  8 . The ceiling mount  8  is displaceably attached to the ceiling  13  of the examination room. 
     The digital X-ray image detector  6 , however, alternatively can be secured to the wall with a mount or, as shown in FIG. 4, can be secured to the floor of the examination room in freely movable fashion by a mount  33  so that it is height-adjustable and/or displaceable parallel to the examination plane. 
     The illustrated medical examination installation also has a voltage generator  9 , a detector electronics  10  for detector control and image editing, X-ray electronics  11  for controlling the voltage generator  9 , an MR electronics  14  for control, pre-amplification, generation and modulation of the RF signals and for gradient control, an MR image computer  15  and an MR system computer  16 . Further, a workstation  17  is provided as a common control panel for the playback of the MR image and of the X-ray image having a display monitor for MR and X-ray examinations. 
     FIG. 3 shows the X-ray radiator  7  of the X-ray system with the X-ray tube  18 , a housing  19 , a cathode  20  and a rotating anode dish  22  attached to an anode motor  21 . In a known way, the electron beam  23  emanates from the cathode  20 , producing an X-ray beam  24  after striking the rotating anode dish  22 . 
     The X-ray tube  18  is surrounded by a magnetic shielding  25  that has two openings  26  for the lead-through of the high-voltage connections and a beam exit window  27  for unimpeded passage of the X-ray beam  24  therethrough. Magnetic field sensors  28  for acquiring the remaining, magnetic, location-dependent stray field of the MR system in the three spatial axes are arranged in the proximity of the X-ray tube  18  and the X-ray beam  24 . 
     Instead of the magnetic field sensors  28 , location sensors can be utilized that identify the position of the X-ray tube  18  relative to the MR system  1  and calculate the magnetic, location-dependent stray field of the MR system at the location of the X-ray tube  18  on the basis of stored magnetic field profiles. 
     For compensating the remaining stray field, three coil pairs  29  through  31  arranged in the three spatial axes are arranged within the shielding  25 , the current in the coil pairs being adjustable such that the magnetic field they generate cancels out the remaining stray field in the respective spatial axis in the region of the electron beam of the X-ray tube  18 . The coil pair  29  serves for compensation in the Y-direction, the divided coil  30  serves this purpose in the X-direction, and the coil pair  31  (only the upper coil is shown) serves for compensation of the residual field in the Z-direction. 
     The magnetic field sensors  28  acquire the magnetic, location-dependent stray field of the MR system remaining within the magnetic shielding  25  in the three spatial axes, and the values are supplied to a computer  32  shown in FIG.  2 . The computer  32  determines the current for the coil pairs  29  through  31  therefrom. Subsequently, the magnetic field sensors  28  again check whether a residual magnetic field is still present and, if necessary the current values for the coil pairs  29  through  31  are reset until the magnetic fields in all three spatial axes have been cancelled. 
     Instead of the illustrated X-ray devices, one having an “electronic C-arm” can be employed, whereby the X-ray radiator  7  and the solid-state X-ray image detector  6  are mounted independently of one another. Location sensors attached to the X-ray radiator  7  and to the solid-state X-ray image detector  6  serve for determining position and angle. The X-ray radiator  7  and the solid-state X-ray image detector  6  can be aligned to one another and readjusted by motor drives and electronic controls. The measurement sensor mechanism with path sensors and rotational angle sensors thereby monitors the current, exact position of the X-ray system with the X-ray radiator  7  and solid-state X-ray image detector  6  in relationship to the MR apparatus  1  and, in particular, relative to the position of its patient support. As a result, X-ray radiator  7  and the solid-state X-ray image detector  6  can be aligned to one another and can be reliably and precisely moved on the desired paths. 
     When not being used, the “electronic C-arm” can simply be parked at the MR apparatus and can offer more flexibility, particularly given interventional operations, since no mechanical coupling of the two components of tube and X-ray image detector is required. 
     Regardless of whether it is a mechanical or an electronic C-arm, the C-arm can be swivelled freely in space around the patient in up to three directions that preferably reside perpendicularly relative to one another. Thus, the optimum X-ray image projection can be set for the particular intervention. 
     It is important, however, that the exact position of the projection geometry of the X-ray device  5  through  8  is known in relationship to the position of the image acquisition of the MR apparatus  1 . For example, a selected X-ray projection plane, established by the focal point of the X-ray tube and two arbitrary image points of the a-Si panel, or where a straight line or a point are respectively located in the X-ray image, can be made visible in the presentation of the stored MR images. 
     An X-ray image detector on a basis of a-Si panel form a flat surface of the X-ray image detector and, compared to RBV-FS systems, therefore have no distortions of the image geometry. For this reason, such image pickup systems are extremely well-suited for X-ray tomogram applications. A development of classic tomography is tomosynthesis wherein tomograms with a nearly arbitrary slice position and slice height can be subsequently reconstructed from a series of acquired, individual projection images. The spatial information of the tomosynthesis images can be superimposed with the content of the stored MR images by the image fusion technique. As a result, errors can be avoided in the interventional operation and the recovery chances are enhanced because of the increased precision of the intervention despite a faster operating speed. 
     At least the X-ray radiator  7  must be moved for the acquisition of the multiple projections, for example 4 through 30, required for the tomosynthesis. To that end, the X-ray radiator  7  is advantageously moved on a plane that lies parallel to the sensor plane because the magnification scale of the individual projections then does not differ. The radiator displacement can be very simply realized with the described “electronic C-arm” where there is no fixed mechanical coupling between X-ray source and X-ray image detector. Since the position of the target area of the examination (region of interest, ROI) is usually known, the image receiver can also be moved opposite the radiator movement. An enlarged image field around the ROI is thus obtained. 
     The direct spatial connection of the MR image acquisition with the X-ray image acquisition given full spatial use possibility enables an improved therapeutic care of patients to be interventionally treated. 
     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.