Computed tomography

Abstract

A computed tomography apparatus, in particular for examining the human head, operates using a plurality of fixedly arranged X-ray tubes (3), which are components of a plurality of emitter-detector elements (11, 12, 13, 14) that together form an emitter-detector ring (10), which can be opened by displacement of at least one of the emitter-detector elements (11, 12, 13, 14). The X-ray tubes (3) each comprise at least one cathode (5) arranged to emit electrons and an anode array (9) with at least one associated anode (6), where each cathode (5) has an alignment angle (α) with respect to the geometrical central axis (MA) of the computed tomography apparatus, where the focal point (BF) of the anode (6) is - ,BF,BF + ) has a surface normal (FN) that includes an anode angle (β) with the central axis (MA), where the focal point (BF - ,BF,BF + X-ray radiation (RS) emitted from the cathode (5, 25) is directed at an X-ray detector (4) at a central emission angle (γ), which is axially offset relative to the X-ray tube (3). The quotient formed by the sum of the alignment angle (α) and the emission angle (γ) and the anode angle (β) is at least 2 / 9 and at most 2, wherein each cathode (5, 25) interacts with the electrode array (21) of the X-ray tube (3) to form a focal spot (BF) at one of at least three selectable locations on the anode array (9). - ,BF,BF + ) is configured to generate

Description

[Technical field]

[0001] The present invention relates to a computed tomography apparatus whose x-ray emitter-detector array does not rotate during operation of the computed tomography apparatus.

[0002] Such a computed tomography apparatus is known, for example, from WO 2018 / 086744. This computed tomography apparatus is particularly suitable for computed tomography X-ray imaging of the human head and comprises a non-rotatable gantry with a number of X-ray emitters and X-ray detectors arranged stationarily around a geometric central axis of the computed tomography apparatus. Here, the X-ray emitters and the associated detectors are offset from one another in the direction of the central axis. The gantry is displaceable as a whole in the longitudinal direction of the computed tomography apparatus, i.e. in the direction of the central axis.

[0003] The X-ray technology device described in Russian Patent No. 2164081 comprises an electron source arranged in the center of the device, which emits an electron beam directed towards an anode provided as an X-ray source annularly surrounding the object to be examined.

[0004] A further computed tomography apparatus is described in DE 10237546. This apparatus is an X-ray computed tomography apparatus with a filter, where the filter is adapted to the direction-dependent intensity distribution of the fan-shaped X-ray radiation. Thus, in particular the heel effect occurring with the tilted anode of the X-ray tube has to be addressed.

[0005] The object of the present invention is to provide a computed tomography apparatus with a stationary gantry which is further developed compared to the prior art and which is distinguished by the properties of a suitable X-ray technology modality and at the same time by a particularly good space utilization.

[0006] This problem is solved according to the invention by a computed tomography apparatus having the features of claim 1. In a basic concept known per se, this computed tomography apparatus comprises a number of X-ray tubes arranged in fixed angular positions, i.e. non-rotatably, around a geometrical central axis of the computed tomography apparatus, the X-ray tubes being components of emitter-detector elements which together form an emitter-detector ring, which can be opened by the displacement of at least one of the emitter-detector elements.

[0007] Each X-ray tube comprises at least one cathode, typically a plurality of cathodes, arranged to emit electrons and an associated anode, where each cathode has an alignment angle α determined by the central emission direction of the electrons to be measured relative to the geometric central axis of the emitter-detector ring. The electrons emitted from the cathode impinge on the surface of the anode in a manner essentially known, forming a focal spot. A tangential surface centered at the focal spot has a surface normal that includes the anode angle β with the central axis of the emitter-detector ring and thus with the central axis of the entire computed tomography apparatus. The X-ray radiation emitted from the focal spot is directed to the X-ray detector at a central emission angle γ for measurement relative to a radial line that runs through the focal spot and is perpendicular to the central axis, where the emission angle γ should be measured between the central ray and the radial line of the X-ray bundle emitted from the focal spot. The X-ray detector is offset axially relative to the X-ray tube and relative to the geometric central axis of the computed tomography apparatus.

[0008] According to claim 1, the angles α, β, and γ have the following relationship: 2 / 9≦(α+γ) / β≦2 is given.

[0009] This relationship applies to all X-ray fluxes respectively emitted from the focal spot. Here, each cathode acting as an electron emitter is configured to interact with an electrode array present in the same X-ray tube to generate a focal spot in one of at least three selectable positions on the anode array of the X-ray tube. The various positions of the focal spots adjustable with one and the same electron emitter are here arranged in particular adjacent to one another in the circumferential direction of the emitter-detector ring, i.e. distributed in different angular positions around the central axis of the computed tomography apparatus. In particular, the focal spots producible with a single electron emitter are arranged on the circumferential surface of the emitter-detector ring at equal distances. The angular interval between the individual focal spots is, for example, a few degrees or, in extreme cases, only a fraction of a second, in both cases the individual focal spots being distinguishable from one another. The switching between the various discrete focal spot positions, which is performed with the aid of the drive control of the electrode array, is also referred to as beam toggling. Thanks to this beam toggling, the total number of possible focal spot positions corresponds to a multiple of the number of electron emitters of the computed tomography apparatus. Beam toggling can be performed in the same manner for at least one emitter-detector element that is displaceable for the purpose of opening the emitter-detector ring, and for the remaining emitter-detector elements.

[0010] The present invention is based on the following considerations.

[0011] The electrons emitted from the cathode of the X-ray tube integrated in the computed tomography apparatus can be emitted with a main propagation direction essentially parallel to the central axis of the computed tomography apparatus, in which case means for influencing the electron emission may be provided, in particular in the form of focusing electrodes. The electrons impinge on the anode of the X-ray tube, forming a focal spot on the anode surface. Typically, X-ray tubes are designed in such a way that the electrons impinge on the anode surface at an angle other than 90°. For example, in the case of a substantially linear electron source, an elongated focal spot shape similarly results.

[0012] The elongated shape of the focal spot can be optically shortened by forming the X-ray bundle emitted obliquely from the anode surface by means of a diaphragm. However, in this case, it must be taken into account that the heel effect weakens a part of the X-ray bundle near the surface of the anode. This effect occurs more the further the emitted X-ray radiation is from the plane normal to the central axis of the computed tomography device in which the focal spot is located. However, emission from said plane is necessary if the X-ray source and the associated detector are not in a common plane. In order to reduce the heel effect, it is possible to mount the X-ray tube as a whole at an angle to the central plane of the computed tomography device, i.e. to select an alignment angle α that is greater than zero. Of course, this also increases the required construction space in the radial direction with respect to the central axis of the computed tomography device.

[0013] According to the invention, the competing stated objectives are taken into account by the fact that the quotient formed by the sum of the X-ray tube alignment angle α and the emission angle γ as the numerator and the anode angle β as the denominator is at least 2 / 9 and at most 2. In particular, configurations are feasible in which the aforementioned quotient is at least 2 / 5 and at most 8 / 5, for example at least 1 and at most 1.6. In all cases the quotient is dimensionless.

[0014] The means by which the emitter-detector ring, which does not necessarily have a circular basic shape, can be opened offers practical advantages both in the preparation phase for an X-ray examination and also in activities, in particular medical interventional procedures, which are carried out after a first X-ray technique and before a further X-ray technique (i.e. in this example a computed tomography examination). In particular, the position of the patient on the patient table can be kept unchanged even if the generation of a computed tomography recording is interrupted in order to carry out an interventional procedure in which the closed emitter-detector array is in the way. In such a case, the movable part of the emitter-detector ring can be removed from the working area, the remaining fixed part of the emitter-detector ring being left in its original position, which is necessary for the computed tomography imaging. Thus, in a new computed tomography examination, there is no need to readjust the position of the patient or the position of the emitter-detector ring, unless a change in the volume to be examined is desired.

[0015] Regardless of its shape, the emitter-detector ring is preferably constructed from an odd number of emitter-detector elements. By using an odd number of emitter-detector elements, it is possible to ensure that the junction between two emitter-detector elements is not diametrically opposed to a further junction of this kind. If the focal point is in the edge region of the emitter-detector element, i.e. in the vicinity of the junction between two segments of the emitter-detector ring, the X-rays emitted from this focal point will abut on two X-ray detectors, for example photon-counting detectors, arranged adjacent to each other in the circumferential direction. The use of line detectors is also conceivable. In this connection, reference is made to WO 2019 / 057339.

[0016] The alignment angle α of the cathode, which can be adjusted by the angular position of the entire X-ray tube, independent of the shape (e.g. linear, curved) of the individual emitter-detector elements, may be greater than zero, in particular greater than 5°. This means that the electron beam emitted from the cathode is directed at least slightly radially outward, i.e. away from the central axis of the computed tomography apparatus. Preferably, the alignment angle does not exceed 30°.

[0017] The anode angle β is, for example, at least 10° and at most 60°. For an anode angle β of 10° and any preferably positive alignment angle α, the surface normal of the plane lying on the anode at the focal point, i.e. the tangential plane, subtends an angle of 10° with the central axis of the computed tomography apparatus. The central emission angle γ associated with the emission of X-rays from the focal point is, for example, at least 5° and at most 30°.

[0018] For example, if an alignment angle α of 12°, an anode angle β of 20° and a radiation angle γ of 10° are selected, the aforementioned quotient is 1.1. Similarly, a configuration is feasible in which, for example, an alignment angle α=30°, an anode angle β=30° and a radiation angle γ=18°. In this case, a quotient (α+γ) / β of 1.6 is obtained. For example, a configuration with an alignment angle α of 0°, an anode angle β of 30° and a radiation angle γ of 12° results in a quotient less than 1, in particular two-fifths. The same value of (α+γ) / β is also obtained for a configuration with an alignment angle α of 6°, an anode angle β of 45° and a radiation angle γ of 12°.

[0019] According to a first possible configuration of the computed tomography apparatus, the emitter-detector ring formed by the emitter-detector elements describes a segmented circle, in which at least one emitter-detector element configured as a segment is displaceable, in particular swingable, relative to the remaining emitter-detector elements. If there are several swingable segments, these can be, for example, rigidly connected to one another and can also be swingable as a whole from the remaining emitter-detector ring. Alternatively, it is possible to support several swingable segments of the wing-door type in the remaining emitter-detector ring.

[0020] According to a further possible configuration, the radiator-detector ring formed by the radiator-detector elements describes a polygon, in particular a rectangle, in which there may be a plurality of mutually rigidly connected radiator-detector elements that are pivotable relative to the remaining radiator-detector ring. In any configuration, if all radiator-detector elements are of uniform design, the actuation of the individual radiator-detector elements can be performed in a uniform manner, regardless of whether these elements are elements that are provided to be opened or not.

[0021] Drive control means which can also be used in the present invention are described, for example, in WO 2019 / 042587. When manufacturing a cathode configured to emit electrons, it is possible to use, for example, any of the solutions described in documents WO 2018 / 086737 and WO 2018 / 141485. Overall, the X-ray tube of the computed tomography apparatus may be configured to generate a series of X-ray pulses which differ from one another with respect to various parameters, in particular the duration and X-ray dose of the individual pulses, as well as the frequency of the X-ray radiation.

[0022] The anode is preferably a stationary anode, i.e. an anode that does not rotate in the anode housing. This anode may be configured as a liquid-cooled anode, i.e. an anode through which a coolant flows, or as an anode without cooling channels. The latter case, non-cooled anodes, will be discussed shortly.

[0023] For example, this anode can be used as described in DE 102017008810 A1. For the design of a cathode usable as an electron emitter in a computed tomography apparatus, the measures taken in WO 2019 / 057338 A1, which claims priority to the aforementioned DE 102017008810 A1, can be chosen.

[0024] In a preferred configuration, the cathode is configured to field emit electrons. In particular, the cathode comprises a nanorod, such as a carbon nanotube. Examples of possible cathode materials are given in WO 2018 / 086737.

[0025] According to a possible development, each emitter-detector element comprises at least one electron emitter of a first type as well as at least one electron emitter of a second type, where the different types of emitters within an emitter-detector element differ from each other in terms of their material and / or geometric shape.

[0026] Depending on the dimensions of the emitter-detector array, the computed tomography apparatus may be suitable for example for the examination of the human head, the breast or the whole body.

[0027] The detectors of the computed tomography apparatus are, for example, constructed as semiconductor detectors. In particular, with regard to high sensitivity, photon counting detectors are advantageous as components of the computed tomography apparatus. In this regard, reference is made, for example, to DE 10 2014 215 548 A1.

[0028] In the following, several exemplary embodiments of the invention will be explained in more detail with the aid of the drawings. [Brief description of the drawings]

[0029] [Figure 1] FIG. 1 is a schematic diagram of a computed tomography apparatus. [Diagram 2] FIG. 2 is a simplified front view of the computed tomography apparatus according to FIG. 1 with an open emitter-detector ring; [Diagram 3] FIG. 3 shows the arrangement according to FIG. 2 with a closed emitter-detector ring. [Figure 4] FIG. 4 shows a modified design of the computed tomography apparatus similar to that of FIG. 2 and FIG. 3; [Diagram 5]FIG. 4 shows a modified design of the computed tomography apparatus similar to that of FIG. 2 and FIG. 3; [Figure 6] FIG. 4 is a further diagram showing the configuration of a computed tomography apparatus with a rectangular emitter-detector ring similar to FIG. 2 and to FIG. 3; [Figure 7] FIG. 4 is a further diagram showing the configuration of a computed tomography apparatus with a rectangular emitter-detector ring similar to FIG. 2 and to FIG. 3; [Figure 8] FIG. 2 is a schematic diagram of a detail of a computed tomography apparatus having a circular segmented emitter-detector ring. [Figure 9] FIG. 2 is a schematic diagram of a detail of a computed tomography apparatus having a circular segmented emitter-detector ring.

[0030] The following description relates to all embodiments unless otherwise stated. Parts which correspond to one another or act essentially in the same way are designated by the same reference numbers in all figures.

[0031] The computed tomography apparatus generally designated 1 is specially designed for the examination of the human head, i.e. configured as a head CT. With regard to the basic structure of the computed tomography apparatus 1, reference is made to the document WO 2018 / 086744 cited at the outset.

[0032] The computed tomography apparatus 1 is shown in a roughly schematic cross-section in Fig. 1, in which the central axis of the computed tomography apparatus 1, designated MA, lies in the image plane. This central axis MA gives the z-direction of a Cartesian coordinate system. The x- and y-axes of the coordinate system extend in a plane perpendicular to the image plane. The gantry of the computed tomography apparatus, designated as a whole by the reference 2, has the basic shape of a ring lying in the xy-plane. This means that the plane in which the gantry 2 lies is aligned perpendicular to the central axis MA.

[0033] In this embodiment, the X-ray tube 3 has an elliptical, non-circular cross-section, as is evident from Figure 1. Alternatively, it is possible to use X-ray tubes whose cross-sectional shape is, for example, circular or polygonal, such as a square, hexagonal or octagonal.

[0034] The gantry 2 contains a number of X-ray tubes 3, which are distributed in fixed angular positions around the central axis MA during operation of the computed tomography apparatus 1. The associated X-ray detectors 4, in particular semiconductor detectors, are likewise arranged in an annular space around the central axis MA. The entire array of X-ray tubes 3 and X-ray detectors 4 is not rigid but can only be displaced in the z-direction, i.e. in the longitudinal direction of the central axis MA. For this purpose, the gantry 2 is mounted on a movable frame 7. The plane in which the X-ray tubes 3 lie is shifted axially, i.e. in the z-direction, relative to the plane in which the X-ray detectors 4 lie.

[0035] Each X-ray tube 3 has a number of cathodes 5 as electron emitters. The associated anodes are designated with the reference 6 and are assigned to an anode array 9 of the corresponding X-ray tube 3. Devices for influencing the electron beam, designated with the reference ES, in particular focusing electrodes, are not shown in FIG. 1 for reasons of clarity. The electron beam ES subtends an angle with the central axis MA with respect to the beam centre, which angle is referred to as the alignment angle α of the cathodes 5. In the present embodiment, the alignment angle α is greater than zero. This means that the electrons emitted from the cathodes 5 have a radially outward motion component with respect to the central axis MA.

[0036] Electrons emitted from the cathode 5 impinge on the anode 6 at a focal point, generally designated BF. In this embodiment, the anode 6 is non-rotatable, i.e. configured as a static anode. Alternatively, a turntable anode may be used. In such a case, the electron beam ES would preferably be aligned parallel to the axis of rotation of the turntable anode. The alignment angle α would therefore correspond to the angle of inclination of the axis of rotation of the turntable anode relative to the central axis MA.

[0037] The surface normal FN of the tangential surface TE, which is located on the anode 6 and lies in the focal point BF, contains an anode angle β with the central axis MA, which is at least 10° and does not exceed 45° in all the embodiments shown in the drawings.

[0038] X-ray radiation RS emits from the focal point BF, only the central beam of the X-ray bundle is depicted in Fig. 1. The alignment of the X-ray radiation RS to be used for imaging is determined by a diaphragm in a manner known per se. For the exit of the X-ray radiation RS the X-ray tube 3 has an X-ray window 8.

[0039] The X-ray radiation RS leaves the X-ray tube 3 for measurement relative to a radial line RL at a central radiation angle γ. The radial line RL is aligned perpendicular to the central axis MA and intersects with the central axis MA as well as the center of the focal point BF. Thus, a radiation angle γ of 0 degrees would mean that the X-ray radiation RS is aligned exactly radially, i.e. perpendicular to the central axis MA. Based on offsetting the X-ray detector 4 relative to the X-ray tube 3 in the axial direction with respect to the central axis MA, the radiation angle γ must be greater than zero. In this embodiment, the radiation angle γ is at least 5° but not more than 30°.

[0040] The sum of the emission angle γ and the alignment angle α is at least one-twelfth and at most two times the anode angle β.

[0041] The cathode 5 is configured in the present example as a field emission cathode, whereby several cathodes 5 in each individual X-ray tube 3 are respectively assigned to a common anode 6. In this case, the individual X-ray tubes 3 can each have several cathodes 5, 25 of uniform or different design as electron emitters.

[0042] Each X-ray tube 3 is part of an emitter-detector element 11, 12, 13, 14, to which an X-ray detector 4 is also assigned. The emitter-detector elements 11, 12, 13, 14 together represent the gantry 2 and form a non-rotating emitter-detector ring 10, which in the embodiment according to Figs. 1 to 5 and Figs. 8 and 9 has a circular shape. In contrast, in the embodiment according to Figs. 6 and 7, the emitter-detector ring 10 has a square basic shape, which does not affect its substantial basic function, neither in terms of the X-ray technology method nor in terms of handling. No housing is provided surrounding the gantry 2. It should be understood that in all cases the term "fixed" describing the arrangement of the emitter-detector ring 10 implies that no rotation of the emitter-detector unit around the central axis MA of the gantry 2 occurs when obtaining an X-ray technology exposure. Rather, by means of X-ray tubes 3 and associated X-ray detectors 4 distributed over the entire circumference of the gantry 2 , a fan-shaped bundle of X-ray radiation RS can be generated, each emanating from a focal point BF on the anode 6 of the X-ray tube 3 .

[0043] 1 to 5, the emitter-detector ring 10 is built up from a total of three emitter-detector elements 11, 12, 13, where the emitter-detector element 11 is attached to the frame 7 and is therefore regarded as the fixed emitter-detector element, whereas the emitter-detector elements 12, 13 make it possible to open the emitter-detector ring 10, for example to make it slidable laterally above a patient table 15.

[0044] 1 to 3, only one hinge 16 is provided for opening the emitter-detector ring 10, which is arranged between the emitter-detector elements, i.e. between the segments 11, 13. The segments 12, 13, which extend over a total of about 240° around the circumference of the emitter-detector ring 10, are in this case rigidly connected to one another and pivot together when the ring 10 is opened or closed.

[0045] The embodiment according to Figures 4 and 5 differs from the construction according to Figures 1 to 3 in that two hinges 16, 17 are provided, to which the segment 13 or the segment 12 is pivotally connected, so that the emitter-detector element 12, 13 can be opened like a double door.

[0046] 4 and 5 has a similarity to the construction according to figures 1 to 3 in that the emitter-detector elements 12, 13, 14, which in this case together form a U-shape, can be pivoted together. Due to the flat shape of all segments, including also the fixed segment 11, the examination area is particularly accessible when the emitter-detector ring 10 is open.

[0047] 8 and 9 show details of the components of the X-ray tube 3 and further X-ray technology methods applicable to all the embodiments described. The emitter array of the X-ray tube 3 is indicated with the reference numeral 18.

[0048] In each X-ray tube 3 there is an emitter assembly 19 for generating an electron beam ES which abuts on the anode array 9 and thus generates a focal point BF. The focal point BF does not necessarily have to have an approximately point-like shape. Rather, in a manner known per se, for example, an elongated focal point BF can also be generated, in which case the position of the focal point BF is to be understood in each case as the position of its centre point.

[0049] According to Fig. 8, the emitter assembly 19 includes different cathodes 5, 25 for generating X-ray radiation of different doses and / or wavelengths. In each case, electrons are extracted from the cathodes 5, 25 using an extraction grid 20, where the electron beam ES can be deflected in a predetermined manner using an electrode array 21 including multiple electrodes 22, 23. The multiple cathodes 5, 25 are arranged together on a circuit board 24.

[0050] The entire anode array 9, which interacts with the emitter assembly 19 of the X-ray tube 3, extends around the circumference of the emitter-detector ring 10 over an angle α' resulting from the number of emitter-detector elements 11, 12, 13, 14, i.e. the number of segments of the emitter-detector ring 10; in this case, in the arrangement depicted in FIG. 8, there are five segments of equal extent, i.e. 120°.

[0051] The angle α', which indicates the extension of the anode array 9 in the circumferential direction of the emitter-detector ring 10, is in the case of FIG. 8 somewhat smaller than 120°. The angle β', which indicates the extension of the X-ray detectors 4 on the circumferential surface of the emitter-detector ring, is moreover significantly closer to 72°. In other words, on the circumferential surface of the emitter-detector ring 10, the gaps formed between the individual X-ray detectors 4 are significantly narrower than the gaps formed between the emitter arrays 18. The multiple possible focal positions extend within the X-ray tube 3 over an angle γ', which is smaller than the angle α'.

[0052] The electrode array 21 selectively directs the electron beam ES to a focal point BF or to a focal point BF offset relative thereto in the circumferential direction of the emitter-detector ring 10. + ,B.F. - 8 and 9, the focal point BF + is deflected clockwise relative to the focal point BF, - The deflection of the electron beam ES means an offset of the focal point BF, also called beam toggling, and the focal point BF on the circumference of the emitter-detector ring 10 is deflected counterclockwise. - ,BF,BF + This allows for a particularly densely graduated arrangement of the focal positions, where a total number of several hundred focal positions can be achieved, which corresponds to a multiple of the number of electron emitters 5, 25, with the advantage being given to a low-weight construction of the computed tomography apparatus 1 and at the same time high-quality imaging. [Explanation of symbols]

[0053] 1. Computed tomography equipment 2 Gantry 3 X-ray tube 4. X-ray detector 5 Cathode, electron emitter 6 Anode 7 Frame 8 X-ray window 9 Anode Array 10 Emitter-detector ring 11 Emitter-detector element 12 Emitter-detector element 13 Emitter-detector element 14 Emitter-detector element 15 Patient Table 16 Hinge 17 Hinge 18 Emitter Array 19 Emitter Assembly 20 Extraction Grid 21 Electrode Array 22 electrodes 23 electrodes 24 Circuit Board 25 Second type cathode, electron emitter α Alignment Angle β Anode angle γ radiation angle α' is the angle at which the anode array of emitter-detector elements extends β' is the angle at which the detector of the emitter-detector element extends γ' is the angular range in which the possible foci of the anode array lie BF focus (general) BF + ,B.F. - The focal spot produced by the electron emitter (at the central position and at two offset positions around the circumference of the emitter-detector ring) ES Electron Beam FN surface normal MA center axis RL Radial Line RS X-ray radiation TE tangent surface

Claims

1. A computed tomography apparatus comprising a plurality of X-ray tubes (3) arranged at fixed angular positions around a geometric central axis (MA), the plurality of X-ray tubes (3) being components of a plurality of emitter-detector elements (11, 12, 13, 14) forming together an emitter-detector ring (10), the emitter-detector ring (10) being openable by displacement of at least one of the plurality of emitter-detector elements (11, 12, 13, 14), Each of said X-ray tubes (3) comprises at least one cathode (5, 25) arranged to emit electrons and an anode array (9) with at least one associated anode (6), Each of said cathodes (5, 25) has an alignment angle (α) with respect to said geometric central axis (MA) relative to a direction of electron emission; A tangential plane (TE) placed at the focus of the anode (6) has a surface normal (FN) that includes an anode angle (β) with the geometric central axis (MA); X-ray radiation (RS) emanating from said focal spot is directed to an X-ray detector (4) for measurement with respect to a radial line (RL) extending through said focal spot at a central radiation angle (γ), said X-ray detector (4) being arranged offset longitudinally of said geometric central axis (MA) with respect to said X-ray tube (3); the quotient formed by the sum of the alignment angle (α) and the central emission angle (γ) and the anode angle (β) is at least 2 / 9 and at most 2; each of the cathodes (5, 25) is configured to interact with an electrode array (21) of the X-ray tube (3) to generate a focal spot at one of at least three selectable positions on the anode array (9); Computed tomography scanner.

2. 2. The computed tomography apparatus of claim 1, wherein the alignment angle ([alpha]) is greater than zero and is at most 30[deg.].

3. 3. The computed tomography apparatus according to claim 1, wherein the anode angle ([beta]) is at least 10[deg.] and at most 60[deg.].

4. 4. The computed tomography apparatus according to claim 1, wherein the central radiation angle (.gamma.) is at least 5.degree. and at most 30.degree.

5. 5. The computed tomography apparatus according to claim 1, wherein the anode (6) is configured as a stationary anode.

6. 6. The computed tomography apparatus according to claim 5, wherein the anode (6) is configured as an uncooled anode.

7. 6. The computed tomography apparatus according to claim 5, wherein the anode (6) is configured as a liquid-cooled anode.

8. 8. The computed tomography apparatus according to claim 1, wherein a quotient formed by the sum of the alignment angle (α) and the central radiation angle (γ) as the numerator and the anode angle (β) as the denominator is at least 2 / 5 and at most 8 / 5.

9. 9. The computed tomography apparatus according to claim 1, wherein the cathode (5) is adapted for field emission of electrons and comprises a nanorod, in particular a carbon nanotube.

10. 10. The computed tomography apparatus of claim 9, wherein each of the emitter-detector elements (11, 12, 13, 14) comprises at least one emitter (5) of a first type and at least one emitter (25) of a second type, and the at least one emitter (5) of the first type and the at least one emitter (25) of the second type within the emitter-detector element (11, 12, 13, 14) differ from each other with respect to their material and / or geometric shape.

11. 11. The computed tomography apparatus according to claim 9 or 10, wherein each of the emitter-detector elements (11, 12, 13, 14) is configured to switch between various X-ray frequencies and / or X-ray doses, and each of the focal points is equally selectable as a source of all adjustable X-ray frequencies and X-ray doses.

12. 12. The computed tomography apparatus according to claim 1, wherein the emitter-detector ring (10) formed by the emitter-detector elements (11, 12, 13, 14) describes a segmented circle, and at least one emitter-detector element (11, 12, 13, 14) configured as a segment is displaceable, in particular oscillating, relative to the remaining emitter-detector elements (11, 12, 13, 14).

13. 13. The computed tomography apparatus according to claim 12, wherein each of the emitter-detector elements (11, 12, 13, 14) to be opened is individually pivotable.

14. 12. The computed tomography apparatus according to claim 1, wherein the emitter-detector ring (10) formed by the emitter-detector elements (11, 12, 13, 14) describes a polygon, and a number of the emitter-detector elements (12, 13, 14) rigidly connected to one another are swingable relative to the rest of the emitter-detector ring (10).

15. 15. The computed tomography apparatus according to claim 1, wherein all the emitter-detector elements (11, 12, 13, 14) have a uniform shape.

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