Abstract:
An apparatus is provided for generating X-ray radiation in an outer magnetic field, which may be generated by a magnetic field device. The apparatus includes a cathode configured to generate an electron beam and an anode configured to retard the electrons of the electron beam and generate an X-ray beam. The apparatus further includes a device configured to generate an electric field orientated from the anode in the direction of the cathode and substantially collinear to the outer magnetic field, wherein the cathode, as an electron emitter, includes a cold cathode that passively provides free electrons by field emission.

Description:
[0001]    The present patent document is a §371 nationalization of PCT Application Serial Number PCT/EP2016/050862, filed Jan. 18, 2016, designating the United States, which is hereby incorporated by reference, and this patent document also claims the benefit of DE 10 2015 201 375.8, filed Jan. 27, 2015, which is also hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The disclosure relates to an apparatus for generating x-ray radiation in an external magnetic field generable by a magnetic field device. 
       BACKGROUND 
       [0003]    An apparatus for generating x-ray radiation includes a cathode for generating an electron beam and an anode for decelerating the electrons of the electron beam and for generating an x-ray beam. Moreover, the apparatus includes a device for generating an electric field that is directed from the cathode in the direction of the anode. 
         [0004]    In such an apparatus, the x-ray radiation arises from energetic transitions in the electron shells of atoms or molecules and from the change in velocity of the charged particles per se. In the apparatus, the electrons emitted by the cathode are initially accelerated by the applied electric field and are then incident on the anode, in which they are strongly decelerated. X-ray radiation and heat arise in the process, wherein electrons are ejected from the shells of the atoms as a result of electron and photon interactions. The holes in the shells are filled by other electrons, with, inter alia, the characteristic x-ray radiation arising. Overlaid thereon is the so-called bremsstrahlung, which is caused by the pure change in velocity of the electrons as a consequence of the interaction with the anode. 
         [0005]    By way of example, x-ray radiation may be used to shine through the human body, with predominantly bones, but also internal organs, becoming visible. In the field of medical diagnostics, there is a desire to combine x-ray imaging with other imaging methods based on magnetic fields. By way of example, an apparatus for x-ray imaging may be combined with a magnetic resonance imaging (MRI) scanner. Magnetic fields may likewise arise for guiding the catheter in angiography, an imaging medical method which images blood and lymph vessels. 
         [0006]    Medical apparatuses for generating x-rays may use hot cathodes. If hot cathodes are exposed to a strong magnetic induction, caused by a magnetic field device such as the MRI or the angiography system, the obtainable electron current is reduced. Likewise, the focusing of the electron beam emitted by the hot cathode is impaired by the optics that are characterized by electric fields. Hence, a substantially smaller electric current density (abbreviated to current density) arises at the anode in comparison with an x-ray apparatus without an external magnetic field. However, a certain, predetermined current density is required for generating the x-ray beam with an intensity that is sufficient for the medical application. It is possible to compensate the reduced current density by increasing the heating temperature of the hot cathode. However, such an increase in the heating temperature impairs the service life of the hot cathode and hence of the x-ray tube. 
       SUMMARY AND DESCRIPTION 
       [0007]    The scope of the present disclosure is defined solely by the appended claims and is not affected to any degree by the statements within this summary. The present embodiments may obviate one or more of the drawbacks or limitations in the related art. 
         [0008]    It is an object of the present disclosure to specify an apparatus for generating x-ray radiation operable in an external magnetic field and which may generate a high electric current, without there being a risk of destroying the cathode or a reduction in the service life of the cathode and without the image quality being impaired. 
         [0009]    In order to achieve the aforementioned object, an apparatus for generating x-ray radiation in an external magnetic field generable by a magnetic field device is proposed. The apparatus includes a cathode for generating an electron beam, an anode for decelerating the electrons of the electron beam and for generating an x-ray beam, and a device for generating an electric field that is directed from the anode in the direction of the cathode and that is substantially collinear with the external magnetic field. The cathode as an electron emitter includes a cold cathode that passively provides free electrons by field emission. 
         [0010]    A substantially collinear electric field refers to an electric field which need not be parallel to the magnetic field at all points. The electrons follow the magnetic field (in the case of a sufficient strength), and so the requirements in respect of the electric field in relation to the alignment thereof are reduced under these conditions. In the conventional case, the electric field needs to be formed in such a way that there is a focusing of the electron beam onto the anode. 
         [0011]    Such an arrangement facilitates the generation of a high electron current (e.g., an electron beam with a large number of electrons) by using a cold cathode, without there being a risk of the cathode being ripped apart or destroyed. Because there is no focusing of the electron beam by electric fields under the aforementioned conditions, the emission current reduction, (e.g., in the case of a hot cathode), cannot be compensated by a larger filament without increasing the focal spot. In this conventional case, a beam spot area would increase in correspondence with a projected filament size, as a result of which requirements in respect of the beam spot dimension cannot be observed. By using a cold cathode, a material-specific current density remains largely uninfluenced. 
         [0012]    The beam spot dimension describes the region of the electron beam impinging on the anode, which is influenced by the size and form of the cathode and the profile of the two fields. The beam spot may be punctiform, as a result of which the generation of the x-ray radiation will come close to that of the punctiform x-ray source. 
         [0013]    In accordance with an expedient configuration, the electron emitter has a linear embodiment. The linear electron emitter may refer to an electron emitter that extends along one direction over its entire length, e.g., a straight and not coiled electron emitter. 
         [0014]    Expediently, the electron emitter has a convex surface in the cross section in relation to an axial direction of extent, wherein the convex surface extends exclusively in the direction of the anode and represents the electron emitter. This is accompanied by a reduction in the emitting surface of the electron emitter in comparison with a filament of a hot cathode. This is accompanied by an electron current in the direction of the anode that is uninfluenced by the external magnetic field because it is provided that only electrons in the direction of the anode may emerge from the electron emitter. In particular, a reduction in the emitting surface is also avoided in comparison with a filament of a hot cathode because only the front side of the electron emitter contributes to the electron current. 
         [0015]    In the cross section in relation to an axial direction of extent, the electron emitter may have the form of a semi-cylinder. In principle, the convex surface may also be realized by other cross-sectional forms of the electron emitter. The form of a semi-cylinder facilitates a convex surface that exclusively extends in the direction of the anode. In particular, this form renders possible an enhanced field on the area of the semi-cylinder, in particular over the entire linear profile thereof, as result of which the electron emergence is simplified. 
         [0016]    It is furthermore expedient if the cathode includes a substrate on which the electron emitter is arranged. The substrate may include a semiconductor material. The substrate may also include a metal. The electron emitter and the substrate are connected to one another in an electrically conductive manner. 
         [0017]    In a further expedient configuration, the axial direction of extent extends parallel or at an angle to a first direction, which extends perpendicular to a third direction of the electric field and a second direction transverse to the electric field, wherein an impact area of the anode lies in a plane that extends parallel to the second direction and at an acute angle to the first direction. Depending on the selected dimension of the acute angle, the dimension of the punctiform property of the x-ray beam emerging from the anode may be measured. The punctiform property is satisfied to a greater extent, when a smaller dimension of the acute angle is selected. 
         [0018]    In accordance with a further expedient configuration, the cathode includes a substance or substances based on carbon. In particular, the cathode may have an irregular surface in order to simplify the emergence of electrons on account of a field enhancement. The surface may have a film of carbon nanoflakes as field emitting elements. The carbon nanoflakes may have rounded-off or sharp edges. 
         [0019]    It is known that the electrons leave the surface of the electron emitter on account of an electric field prevalent there and substantially collinear to the external magnetic field. The electric field may be generated by applying an electric voltage between the cathode and the anode. To this end, a voltage source for providing a first voltage between the cathode and the anode may be provided or interconnected. Alternatively, a further electrode may be arranged between the anode and the cathode, with a voltage source being provided for providing a second voltage between the cathode and the further electrode, the second DC voltage being less than the first DC voltage. A further electrode lying between the anode and the cathode is also known by the name of “puller electrode”. The electrons leave the surface of the electron emitter with such a low energy that they follow the field lines of the magnetic field. The voltages may be pulsed in order to switch the beam on and off, for example, with up to 30 frames per second in the case of angiography. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    Below, the disclosure is explained in more detail on the basis of exemplary embodiment in the drawings. In the drawings: 
           [0021]      FIG. 1  depicts a schematic illustration of an apparatus according to an embodiment for generating x-ray radiation in an external magnetic field. 
           [0022]      FIG. 2  depicts a perspective illustration of a cathode, as is used in an apparatus in accordance with  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0023]      FIG. 1  depicts a schematic illustration of an apparatus  1  for generating x-ray radiation  32 . The apparatus  1  includes a cathode  10  and an anode  20  that is rotatable about an axis of rotation  21  (a so-called rotating anode). The anode  20  may also be embodied as a stationary anode. By way of a DC voltage source  40 , which is interconnected between the cathode  10  and the anode  20 , an electric voltage at a given level is applied between said cathode and said anode. As a result, an electric field that is directed from the anode in the direction of the cathode arises. The apparatus  1  is arranged in an external magnetic field  50  that is generated by a magnetic field device not illustrated in any more detail. The magnetic field lines of the magnetic field  50  and the electric field lines of the electric field, which is generated between the anode  20  and the cathode  10 , extend largely collinearly. This means that the field lines of the electric field correspond to the field lines of the magnetic field  50 . 
         [0024]    The arrangement of the apparatus  1  in space is defined in the present description by a coordinate system with a first direction (e.g., x-direction), a second direction (e.g., y-direction) and a third direction (e.g., z-direction). The three directions or axes are at right angles to one another in each case, e.g., the three directions or axes form a Cartesian coordinate system. In accordance therewith, the field lines of the electric field and of the magnetic field run parallel to the x-direction, while the cathode  10  and the anode  20  extend in the xy-plane. 
         [0025]      FIG. 2  depicts a magnified illustration of the cathode  10  used in the apparatus  1  in accordance with  FIG. 1  in a perspective view. In order to elucidate the arrangement of the cathode  10  in the apparatus  1 ,  FIG. 1  presents a corresponding coordinate system. 
         [0026]    The cathode  10  includes a substrate  11  and an electron emitter  12  with a respective length  15 . By way of example, the substrate  11  includes a semiconductor material or a metal. The electron emitter  12  has a cross section  13  having a convex surface in relation to an axial direction of extent (e.g., an extent along the x-direction or alternatively at an angle to the x-direction and lying in the xz-plane), with the convex surface extending exclusively in the direction of the anode  20  when the cathode  10  is arranged in the apparatus  1 . In the embodiment illustrated in  FIG. 2 , the electron emitter has the form of a semi-cylinder in cross section. The reference sign 14 characterizes the surface of the electron emitter  12  from which the electrons emerge from the electron emitter on account of the prevalent electric field. 
         [0027]    In the exemplary embodiment in accordance with  FIG. 2 , the electron emitter  12  and the substrate  11  have the same length  15 . In principle, this is not required; the length of the substrate  11  may be greater than the length  15  of the electron emitter  12 . 
         [0028]    The electron emitter  12  includes a substance or substances based on carbon. In particular, the electron emitter  12  may have an irregular surface. Hence, the electron emitter  12  is embodied as a cold cathode. The surface  14  of the electron emitter  12  may include carbon nanoflakes. The carbon nanoflakes may have been applied to the surface  14  of the electron emitter  12  by a chemical vapor deposition (CVD) process. The carbon nanoflakes emerge from a layer made of carbon material initially applied to the substrate  11 . An electron emitter with carbon nanoflakes has a better electrical conductivity on account of its graphite structure. Moreover, an increased region for the emission of the electrons is provided. Moreover, the effect of field enhancements may be used on account of the irregular surface, as a result of which the electrons easily emerge from the material of the electron emitter. 
         [0029]    As an example of a suitable material for the electron emitter, use may be made of the material described in U.S. Pat. No. 6,819,034 B1 for providing a cold cathode for the use in a computer system. 
         [0030]    Referring back to  FIG. 1 , the cathode  10  described in  FIG. 2  is arranged in the apparatus  1  in such a way that the linear electron emitter  12  extends in the direction of the x-direction of the coordinate system. Alternatively, it may also extend at an angle in relation to the x-direction, but lies in the xz-plane. Here, the electron emitter  12  is aligned relative to the anode  20  in such a way that it is arranged in a manner covering the z-direction in relation to an impact region  22  of the anode  20 . The impact region  22  of the anode  20  lies in a plane extending in the direction of the y-axis and at an acute angle  23  in relation to the xy-plane of the coordinate system. The dimension of the acute angle  23  sets the size of the apparent surface from which the x-ray beam  32  emerges from the anode  20 . The flatter the dimension of the angle  23 , the smaller the dimension of the extent of the impact of the electron beam  30  in the z-direction if the impact of the electron beam  30  in the x-direction on the yz-plane is considered. 
         [0031]    On account of the linear form of the electron emitter  12 , the impact region  22  of the anode  20  in the xy-plane is likewise only irradiated in linear form, as a result of which it is possible, overall, to provide an x-ray beam  32  extending in the x-direction from the yz-plane, the beam spot  31  of which is comparatively small and comes close to a punctiform property. 
         [0032]    The electrons leave the surface  14  of the electron emitter  12  with such a low energy that they follow the field lines of the external magnetic field  50 . Here, the apparatus  1  is aligned in such a way that the path from the cathode  10  to the anode  20 , and hence the intended beam direction, lies collinearly in relation to the magnetic field direction of the external magnetic field  50 . As a result, a transverse movement of the electron—except for a rotation with a very small cyclotron radius about the main propagation direction in the z-direction—is practically eliminated. As a consequence, a beam spot  31  forms on the impact surface  22  of the anode  20 , said beam spot corresponding to the projection of the emitting area of the magnetic field  50  and hence likewise being linear in accordance with the form of the electron emitter  12 . 
         [0033]    As a result, it is possible to present a small projected area corresponding to the requirements of the focal spot size in the case of an apparatus  1  for generating x-ray radiation in an external magnetic field  50 . This is promoted by the convex form of the surface  14  of the electron emitter  12 , which helps the field emission at a given extraction voltage. 
         [0034]    The apparatus  1  renders it possible to generate a high electron current without there being a risk of a labile current-carrying conductor (filament) ripping. The reduction in the emitting area and hence also in the undisturbed electron current as a result of the magnetic field, as occurs in the case of a cathode with a coiled filament, does not occur in the proposed apparatus because, in any case, only the front side, (e.g., the surface  14 ), contributes to the electron current in the employed cold cathode. Hence, a material-specific current density remains largely uninfluenced. 
         [0035]    Because the focusing of the electron beam  30  through the electric field no longer occurs and is no longer required, it is possible to avoid the disadvantages that occur when using a hot cathode in a magnetic field. 
         [0036]    As a result, it is therefore possible to provide an apparatus  1  having a long lifetime and in which the required current density for generating the x-ray beam is achievable without impairing the service life of the component. This is rendered possible by using a cold cathode for the purposes of generating a sufficiently high current density. 
         [0037]    Although the disclosure has been illustrated and described in detail by the exemplary embodiments, the disclosure is not restricted by the disclosed examples and the person skilled in the art may derive other variations from this without departing from the scope of protection of the disclosure. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description. 
         [0038]    It is to be understood that the elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present disclosure. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that these dependent claims may, alternatively, be made to depend in the alternative from any preceding or following claim, whether independent or dependent, and that such new combinations are to be understood as forming a part of the present specification.