GB2575898A

GB2575898A - Method and apparatus for controlling a focal spot position

Info

Publication number: GB2575898A
Application number: GB1905830.4A
Authority: GB
Inventors: Erler Marco
Original assignee: Carl Zeiss Industrielle Messtechnik GmbH
Current assignee: Carl Zeiss Industrielle Messtechnik GmbH
Priority date: 2018-04-26
Filing date: 2019-04-26
Publication date: 2020-01-29
Anticipated expiration: 2039-04-26
Also published as: GB2575898B; GB201905830D0; DE102018206514A1

Abstract

An apparatus 1 for controlling a position of a focal spot 50 on a target 30 in an x-ray tube 10 comprising: at least one charge collection element (which may be an electrically conductive plate) 60 for detecting secondary electrons produced in the focal spot, the at least one charge collection element is electrically insulated from other elements of the x-ray tube, the detector is placed upstream from an electron beam with a clear line of sight to the target; a measuring device 270, connected to the at least one charge collection element, for measuring the charge of secondary electrons collected per time interval; and an evaluation device 280 connected to the measuring device, for converting a change in the charge captured per time interval into a positional change and for providing a position signal 290. In one embodiment there are multiple charge collection plates.

Description

Method and apparatus for controlling a focal spot position

The invention relates to a method and an apparatus for controlling a position of a focal spot in a monitored region around an intended focal spot position on a target in an x-ray tube, in particular in an x-ray tube of an industrial computed tomography measuring apparatus.

In the metrology of industrial computed tomography, accurate knowledge about the various geometric parameters of the measuring apparatus is of decisive importance. This also includes knowledge about the position of the focal spot of the x-ray source, which is also referred to as source spot or spot. Inaccurate knowledge of the position of the generation of the x-ray radiation, i.e., the focal spot position, directly flows into measurement deviations of a test object that is measured by means of an industrial computed tomography measuring apparatus.

Even if the focal spot position is known at the start of each measurement and preferably is the same for all measurements, a difficulty nevertheless exists in the fact that the focal spot position on the target of the x-ray source may drift or migrate, even during a measurement of a test object. Such variations or changes in the focal spot position in an x-ray source occur particularly if a new thermal equilibrium must set in during the measurement.

The prior art has disclosed various options of taking account of or compensating a focal spot drift. In one variant, projections are recorded at very large angle increments at the start of a scan of a test object using x-ray radiation. By way of example, if angle increments of 45° are used, it is necessary to capture eight projections at the eight resulting reference angles. These projections of the test object then serve as reference projections. If one of the reference angles is reached during the fine scanning of the test object, an offset between the current projection and the previously captured reference projection is determined by way of an image processing method and this offset is used for translational correction purposes. This does not represent a complete correction of a focal spot position change since this does not take account of a perspective effect of the change in the focal spot position.

A further variant provides for the measurement of a test object to be interrupted after a predetermined number of measuring steps and for a reference object to be introduced into the measurement region, in each case instead of or in addition to the test object. Then, the focal spot position is determined on the basis of this reference object. In such a case, the new geometry can be directly assigned to the projections and can be taken into account during the reconstruction. However, a measurement of a test object needs to be interrupted at regular intervals. If continuous measurements are undertaken, the individual sequences of the measurement must be carried out in overlapping fashion. Moreover, for very small objects that are measured with a great magnification, in particular, it is very disadvantageous not only to rotate said object through small angle increments but to also repeatedly drive it out of the beam path. Here, there is the risk in each case that an object movement that is of the order of the captured resolution occurs in the holder.

A further option consists in continuously also keeping a known highly accurate reference object in the image in addition to the actual test object to be measured. A disadvantage hereof is that valuable space in a height axis is lost to this end and a multiplicity of reference objects are required, graded for different magnifications.

Consequently, the invention is based on the object of specifying an improved method and an improved apparatus, which control a change in the focal spot position. The term controlling is understood to mean, in the narrower sense, the capture and monitoring of the focal spot position, for example the capture of a change in the focal spot position. In a broader sense, controlling may also comprise a quantitative ascertainment of the change in position and/or, additionally, a controlled update or return of the focal spot position into an initial focal spot position or an intended focal spot position.

The invention is achieved by an apparatus having the features of Patent Claim 1 and a method having the features of Patent Claim 9. Advantageous embodiments are evident from the dependent claims.

The invention is based on the concept of capturing a change in a position of the focal spot on the basis of secondary electrons that are produced in addition to the x-ray radiation in the focal spot during the operation of the x-ray tube and that are emitted into a field-free half space (hemisphere) above a target surface of a target. If use is made of a charge collection element that is arranged at a distance from the target surface in the x-ray tube and that, on account of its geometric design and alignment and, optionally, on account of stops and other objects between the target surface and the charge collection element, can only collect secondary electrons that were emitted from the focal spot into a capture solid angle that does not comprise the entire half space above the target surface, a change in the position of the focal spot can be deduced on the basis of changes in the charge collected by the charge collection element per time interval. Only secondary electrons that are emitted along a clear line of sight between the focal spot position and a position on the charge collection element reach the charge collection element. A clear line of sight refers to a line along which an unimpeded sight of a position on the charge collection element is possible from a position in the focal spot or, expressed differently, along which secondary electrons can reach from the focal spot to the charge collection element along a straight line, without being impeded by material obstacles. The charge collection element is arranged in such a way that the capture solid angle spanned by the clear lines of sight between the current focal spot position and the charge collection element changes in the case of a change in the focal spot position. Consequently, a change in the visible area of the charge collection element for the secondary electrons in the focal spot can be deduced from the change in a captured amount of charge per time interval.

The term “above” the target surface should be understood within the meaning of upstream of the target surface in the radiation direction of the primary electrons in each case, independently of an orientation of the target surface in space.

In particular, an apparatus for controlling a position of a focal spot on a target in an x-ray tube in a monitored region around an intended focal spot position is proposed, said apparatus comprising: at least one charge collection element for collecting some secondary electrons that were produced in the focal spot and emitted into the hemisphere above a target surface of the target, wherein the at least one charge collection element is arranged at a distance from the target in the x-ray tube and electrically insulated from other elements of the x-ray tube, wherein the at least one charge collection element is arranged relative to the target surface of the target in such a way that, in each case, a capture solid angle, spanned by clear lines of sight of the secondary electrons, is smaller than the solid angle of the hemisphere above the target surface in the intended focal spot position, wherein clear lines of sight are those straight lines which extend in a straight line from one of the generation positions of the secondary electrons in the focal spot to one of the positions on the at least one charge collection element and along which one of the secondary electrons can move in unimpeded fashion from the target to the at least one charge collection element; and a measuring device, connected to the at least one charge collection element, for measuring the charge of secondary electrons collected on the at least one charge collection element per time interval and an evaluation device connected to the measuring device, said evaluation device being suitable for converting a change in the charge captured per time interval into a positional change and for providing a position signal.

Furthermore, a method for controlling a focal spot position on a target in an x-ray tube in a monitored region around an intended focal spot position is proposed, said method comprising the steps of:

collecting secondary electrons on at least one charge collection element, said secondary electrons forming some of the secondary electrons that were produced in the focal spot on the target and emitted into a hemisphere above a target surface of the target, wherein the at least one charge collection element is arranged relative to the target surface of the target in such a way that, in each case, a capture solid angle, spanned by clear lines of sight of the secondary electrons, is smaller than the solid angle of the hemisphere above the target surface in the intended focal spot position, wherein clear lines of sight are those straight lines which extend in a straight line from one of the generation positions of the secondary electrons in the focal spot to one of the positions on the at least one charge collection element and along which one of the secondary electrons can move in unimpeded fashion from the target to the at least one charge collection element; iteratively measuring the charge collected on the at least one charge collection element per time interval;

ascertaining as to whether a change has occurred in the amount of charge collected per time interval and, should this be the case, ascertaining a position deviation of the focal spot position, which corresponds to a change in the capture solid angle of the at least one charge collection element, wherein the change in the capture solid angle corresponds in turn to the ascertained change in the charge captured per time interval, and outputting a position signal which comprises a position information item.

An advantage of the invention is that secondary electrons generated directly in the focal spot are used to determine the position of the focal spot and/or a change in the focal spot position. Particularly in the case of x-ray sources where the primary electrons are directed onto the target of the x-ray tube at a flat angle and the produced x-ray radiation likewise emerges from the target surface at a flat angle, it is possible to use a space perpendicular above the target surface in order to detect the produced secondary electrons. It was found that it was already possible to detect changes that correspond to one tenth of the diameter of the focal spot. This applies at least to focal spot diameters in the range from 10 pm to 1 mm.

What is exploited is that, upon the incidence of high-energy electrons on the target, only some of the electrons are slightly reflected at the focal spot but a significantly larger number of low-energy secondary electrons are emitted from the focal spot. These are emitted from the focal spot in all spatial directions in the hemisphere above the target surface. It was found that the overall amount of charge emitted by the target into the hemisphere above the target surface equals approximately 50% of the charge applied to the target by the primary electrons. However, only a fraction thereof is captured by the charge collection element since the capture solid angle of the charge collection element is smaller, as a rule significantly smaller, than the solid angle of the hemisphere above the target surface.

Preferably, in one embodiment, the at least one charge collection element is arranged with such a spacing from the target that the capture solid angle, spanned by the clear lines of sight which extend from the focal spot to a position on the at least one charge collection element and along which a propagation of the secondary electrons emitted from the focal spot in a straight line is possible to a position on the at least one charge collection element, is dependent on the focal spot position along a labelled or predetermined direction on the target surface. As a result of this, not only a displacement of the focal spot position from its initial position is rendered capturable but a qualitative and optionally quantified statement along at least one labelled direction is also made possible. This then also opens up the possibility of actively returning the focal spot position into the original focal spot position along this labelled direction by way of electron optical elements that act on the primary electrons.

In such embodiments, in which the position deviation can be quantified at least along a labelled spatial direction, the output position information item may specify, for example, the ascertained direction of the change in position or, possibly, the position deviation or zero. Here, the position deviation can be specified relative to a focal spot position captured in advance and, accordingly, a zero can be output as position information item if no relative change in the focal spot position has occurred. Alternatively, it is also possible to specify a deviation of the intended focal spot position as a position deviation, with zero then being output if the current focal spot position corresponds to the intended focal spot position. Other embodiments only specify the direction of the deviation if the latter has exceeded a tolerance threshold.

In a particularly preferred apparatus, provision is made for a detection area comprising a proximal delimiting edge and a distal delimiting edge to be formed, wherein the proximal delimiting edge and the distal delimiting edge are each oriented parallel to one another and to the target surface and are formed by stop edges and/or edges of other elements of the x-ray tube and/or edges of the at least one charge collection element and delimit the capture solid angle of the clear lines of sight from the focal spot at the intended focal spot position to the at least one charge collection element, wherein the proximal delimiting edge has a shorter distance from the target surface than the distal delimiting edge, wherein the proximal delimiting edge and the distal delimiting edge are oriented perpendicular to the labelled direction on the target surface. The detection area is bounded perpendicular to the labelled direction by the proximal delimiting edge and by the distal delimiting edge in such a way that an aperture angle range of the clear lines of sight is restricted in planes perpendicular to the proximal delimiting edge and the distal delimiting edge such that a change in position of the focal spot along the labelled direction on the target surface, which is oriented perpendicular to the delimiting edges, is very well capturable. Consequently, the capture solid angle is delimited perpendicular to the labelled direction by two mutually parallel edges that are parallel to the target surface in the focal spot. As a result of the delimiting edges, i.e., the proximal delimiting edge and the distal delimiting edge, having different distances from the target surface, the detection area is not oriented parallel to the target surface in the focal spot but at an angle in relation to the target surface.

The proximal delimiting edge and hence the distal delimiting edge, too, have a straightlined embodiment as a rule.

All clear lines of sight of secondary electrons detected on the charge collection element extend through the detection area or end on the detection area if an area of the charge collection element coincides with the detection area. Consequently, the detection area is always formed between charge collection element and target surface or coincides with the charge collection element. Consequently, the embodiment is dependent on an arrangement of the charge collection element above the target surface and, optionally, on stop edges arranged between the charge collection element and the target surface or edges of other objects between the charge collection element and target surface. The orientation of the detection area with a plane embodiment is set in space and consequently relative to the target surface by the proximal delimiting edge and the distal delimiting edge, which is at a distance therefrom and oriented parallel thereto. These also delimit the detection area perpendicular to the labelled (predetermined) direction .

Therefore, provision is made in some embodiments for the position signal to specify a position deviation of the focal spot position in a direction of the target surface that is oriented perpendicular to the delimiting edges of the detection area.

Particularly large changes in the charge captured per time interval arise if the detection area includes an obtuse angle with a line of sight from an intended focal spot position of the focal spot to the proximal delimiting edge, wherein the line of sight is oriented perpendicular to the proximal delimiting edge.

The flatter the angle between the line of sight on the proximal delimiting edge and the detection area, the smaller the capture solid angle spanned by the clear lines of sight of the secondary electrons between the focal spot and the charge collection element. The area of the charge collection element visible to the secondary electrons therefore becomes smaller, the flatter the angle is between the line of sight on the proximal delimiting edge and the detection area. Conversely, the capture solid angle becomes greater, the steeper the angle is between the line of sight on the proximal delimiting edge and the detection area. A movement along the labelled direction (which is also referred to as predetermined direction) on the target consequently leads to a reduction in the capture solid angle and hence to a smaller amount of charge collected on the charge collection element per time interval, and a movement in the opposite direction leads to an increase in the amount of charge collected on the charge collection element per time interval.

In a preferred embodiment, provision is made for the detection area to include an angle a preferably selected from the angle range between 1° and 10°, more preferably from the angle range between 3° and 7°, with a surface normal of the target surface. Here, the angle a is the vertical angle between the surface normal and the detection area. The smaller the vertical angle a, the more sensitive the detection in relation to the change in position of the focal spot along the labelled direction.

Particularly preferably, the at least one charge collection element is arranged vertically above the intended focal spot position. This is tantamount to the surface normal in the intended focal spot position intersecting the charge collection element and the surface normal representing a clear line of sight. Firstly, sufficient installation space, in particular, is present here in an x-ray tube and an efficient capture is possible under expedient angles for a position capture with a good resolution.

However, an embodiment in which the at least one charge collection element is not arranged and embodied vertically above the intended focal spot position and in which there is no clear line of sight along the surface normal of the target surface in the intended focal spot position is also possible.

Preferably, the charge collection element has an electrically conductive embodiment. Preferably, the charge collection element is embodied as a metal layer or metal sheet. As a result of this, it is possible to collect a large amount of charge on the charge collection element.

Preferably, the at least one charge collection element is embodied as an electrically conductive plate. In these embodiments, both the proximal delimiting edge and the distal delimiting edge can be embodied by edges of the conductive plate. However, it is likewise possible for the proximal delimiting edge or the distal delimiting edge or else both delimiting edges, i.e., the proximal delimiting edge and the distal delimiting edge, to be formed by electrically conductive stop edges or edges of other elements of the x-ray tube.

If a plate is used as the charge collection element, the detection area can be provided on at least one part of the plate if the charge collection element at the same time has the proximal delimiting edge and the distal delimiting edge. Consequently, such an embodiment represents a simple embodiment of the apparatus. A distance from the target must be large enough to avoid significant heating of the charge collection element on account of thermal radiation emitted by the focal spot on the target surface.

In another embodiment, provision is made for the detection area not to have a physical embodiment and the proximal delimiting edge to be an edge of a stop or of an other element of an x-ray tube measuring head of the x-ray tube. Such an embodiment can be advantageous for the purposes of shielding thermal radiation emitted by the focal spot on the target surface from the charge collection element and being able to increase the distance between the focal spot and the charge collection element.

In order to achieve the smallest possible disturbing influence in relation to movements perpendicular to the labelled direction on the target surface, one embodiment provides for the at least one charge collection element to have a greater extent parallel to the direction of the proximal and the distal delimiting edge than the projections of the proximal delimiting edge and of the distal delimiting edge, projected onto the target surface perpendicular to the target surface, are spaced apart from one another. Consequently, movements transverse to the labelled direction do not influence, or only minimally influence, the angles along this transverse direction, at which the charge collection element is seen by the produced secondary electrons, and at least influence these significantly less than a change in position parallel to the labelled direction.

Consequently, embodiments in which the detection area and the charge collection element have a greater extent parallel to the delimiting edges of the detection area than the projections of the delimiting edges onto the target surface, produced perpendicular to the target surface, are spaced apart from one another are advantageous.

In order to ascertain the charge collected on the at least one charge collection element, provision is made in one embodiment of the apparatus for the at least one charge collection element to be connected to the measuring device by way of a capacitor and for the measuring device to be embodied to maintain a potential across the capacitor and for the amount of charge that the measuring device must feed to the capacitor for the purposes of compensating the change in charge caused by the incident secondary electrons to be used as a measurement signal for measuring the change in charge per time interval.

In an alternative embodiment, the voltage drop across a resistor when discharging the at least one charge collection element is used as measurement signal.

In yet another embodiment of the invention, provision is made for the current caused by the incident charges to be measured as a measurement signal for the collected charge.

What is common to all embodiment is that a reliable ascertainment is possible, even in the case of small collected amounts of charge.

High dynamics when measuring the charge or current facilitates a measurement of small changes on a high basic signal and consequently a high position resolution of the focal spot position. A resolution between the minimum captured current or the minimum captured charge and a maximum captured current or a maximum captured charge that comprises at least 2¹⁰, i.e., 1024, digitization increments is preferred.

A particularly compact structure is obtained if the charge collection element is embodied as a termination element or as part of a termination element of a cutout in an x-ray tube measuring head. In the case of such a structure, the x-ray tube measuring head is preferably manufactured from a solid metallic block into which two cutouts have been worked, for example two bores whose directions of extent or bore axes intersect and meet at a flat angle. The target is arranged at this point of intersection. One of the two cutouts (bores) serves to radiate-in the primary electrons; the other of the two cutouts serves as an emergence channel for the produced x-ray radiation and is sealed by an emergence window that is as transparent as possible to x-ray radiation.

In a particularly preferred embodiment, a third cutout, for example once again in the form of a bore, is introduced into the metal block in such an x-ray tube measuring head, said third cutout preferably being oriented perpendicular to the target surface and extending up to the target surface, preferably up to the intended focal spot position on the target surface. Consequently, the at least one charge collection element can be arranged in this third cutout.

In a particularly preferred embodiment, the charge collection element is embodied as a termination element or as a constituent part of the termination element of this third cutout of the x-ray tube measuring head. Optionally, a protection apparatus as protection against touch is additionally necessary on the outer side of the x-ray tube measuring head.

In order to be able to take account of changes, not caused by changes in the focal spot position but by changes of the secondary electron emission as a whole at the target, in the charges captured per time interval by the at least one charge collection element, provision is made in one embodiment for the evaluation device to be embodied to capture a normalization signal, which is correlated to an overall number of secondary electrons produced, and to take account of the normalization signal when ascertaining a change in the charge captured per time interval. As a result, a very reliable determination of the position and control of the focal spot position can be carried out even in those operating states in which, for example, the primary electron current is not constant or in which a change in the focal spot size occurs on account of a change in the focusing of the primary electron beam. In the process, there is a change in power density and hence a change in the temperature of the target, which in turn influences the emission of secondary electrons.

In order to be able to capture changes in the focal spot position in a different direction on the target surface, said different direction being oriented in oblique, preferably perpendicular, fashion with respect to this labelled direction, provision is made in one embodiment for at least one further charge collection element to be arranged over the target surface in electrically insulated fashion in the x-ray tube and for a further detection area comprising a further proximal delimiting edge and a further distal delimiting edge, which are oriented perpendicular to a further labelled direction, to be formed, wherein the further proximal delimiting edge and the further distal delimiting edge are each oriented parallel to one another and to the target surface and are formed by further stop edges or further edges of other elements of the x-ray tube or edges of the at least one further charge collection element and delimit a further capture solid angle of further clear lines of sight from the focal spot at the intended focal spot position to the at least one further charge collection element, wherein the further proximal delimiting edge has a shorter distance from the target surface than the further distal delimiting edge, wherein the further charge collection element is connected to the measuring device or a further measuring device for measuring the charge of secondary electrons collected on the further charge collection element per time interval and the evaluation device connected to the measuring device or the further measuring device is suitable for converting a change in the charge captured per time interval on the further charge collection element into a change in position of the focal spot along the further labelled direction on the target surface and for providing a further position signal, wherein the further labelled direction is oriented transversely to the labelled direction.

In such an embodiment, the further delimiting edges are oriented in transverse, preferably perpendicular, fashion with respect to the delimiting edges.

In order to obtain an independence of the ascertainments of charge for the at least one charge collection element and the at least one further charge collection element, the at least one charge collection element and the at least one further charge collection element are preferably arranged in such a way that the set of clear lines of sight from the intended focal spot position to the at least one charge collection element is in each case disjoint from the set of the further clear lines of sight from the intended focal spot position to the at least one further charge collection element, even if there is a position change in the focal spot position around the intended focal spot position.

The invention is explained in more detail below with reference to a drawing. In the drawing:

Fig. 1 shows a schematic illustration of a first embodiment of the apparatus according to the invention;

Fig. 2A

Fig. 2B

Fig. 2C shows a schematic illustration of the geometry in a sectional plane perpendicular to the areal extent of a charge collection element of an apparatus according to the first embodiment, in which a detection plane is delimited by edges of the charge collection element;

shows a schematic illustration of the geometry in a sectional plane perpendicular to the areal extent of a charge collection element of an apparatus according to a second embodiment of the invention, in which the detection plane is delimited by an edge of the charge collection element and a stop edge;

shows a schematic illustration of the geometry in a sectional plane perpendicular to the areal extent of a charge collection element of an apparatus according to a third embodiment of the invention, in which the detection plane is delimited by two stop edges;

Fig. 3 shows a graph which specifies a functional relationship of an offset-signal ratio, also abbreviated to OSR below, of a vertical angle a between a surface normal of the target surface and a detection area;

Fig. 4 shows a graphical illustration of a functional relationship of the offset-signal ratio, OSR, of a movement of the focal spot position for different focal spot diameters;

Fig. 5 shows a schematic illustration of a fourth embodiment of the invention with an xray tube measuring head, in which the charge collection element is embodied as a termination element of a functional opening; and

Fig. 6 shows a schematic illustration of a fifth embodiment of the invention, in which the charge collection element is embodied as a constituent part of the x-ray window.

Figure 1 schematically illustrates a section of an x-ray tube measuring head 11 of an x-ray tube 10. A primary electron beam 20 is incident on a target 30. X-ray radiation 40 is produced at the location of incidence 21 of the primary electrons of the primary electron beam 20, said x-ray radiation being used, for example, for measuring a test object (not illustrated) in a computed tomography device. Some of the energy of the primary elections leads to the target being heated. Therefore, the location of the production of the x-ray radiation 40 is referred to as focal spot 50. The position at which the x-ray radiation production occurs is referred to as focal spot position 52 or else source position.

Some of the primary elections incident on the target 30 are reflected by the target. Additionally, low-energy secondary electrons are emitted from the focal spot 50 in all directions of the hemisphere above the target surface 31. These propagate in a straight line. The angle distribution depends on the energy of the energy-rich primary electrons.

In order to be able to ascertain the focal spot position 52 on the basis of the emitted secondary electrons, a charge collection element 60 is arranged in the x-ray tube 10 at a distance from the target 30. The charge collection element 60 can only be “seen” by secondary electrons, which are emitted from an intended focal spot position 55, at a restricted capture solid angle Θ. If there are changes in the focal spot position 52 on the target surface 31, there is a change in the capture solid angle Θ within which the secondary electrons see the charge collection element 60 from the modified focal spot position 52’.

In order to obtain the greatest possible change of the capture solid angle Θ in the case of a change in position of the focal spot position 52, the charge collection element 60 is arranged at an angle with respect to the target surface 31.

In the illustrated embodiment, the charge collection element 60 is embodied as a metallic plate 61. The metallic plate 61 is oriented almost perpendicular with respect to the target surface 31. The charge collection element 60, and hence plate 61, have a proximal edge 62 and a distal edge 63. The proximal edge 62 has a shorter distance 72 from the target surface 31 than the distal edge 63; see Figure 2A.

The proximal edge 62 and the distal edge 63 each extend perpendicular to the plane of the drawing. This is also the second direction of extent of the plate 61 or of the charge collection element 60. Since the target surface 31 also has an extent perpendicular to the plane of the drawing, the proximal edge 62 and the distal edge 63 are oriented parallel to one another and to the target surface 31.

Only those secondary electrons that can reach the charge collection element 60 along a straight line from the focal spot 50 are collected on the charge collection element 60. A straight line, along which a secondary electron can propagate from its emission location in the focal spot 50 on the target 30 to a position on the charge collection element 60 without being impeded by an obstacle, is referred to as a clear line of sight. A clear line of sight 101 is plotted for an intended focal spot position 55, said clear line of sight extending from the intended focal spot position 55 to the proximal edge 62 of the charge collection element 60. Additionally, a clear line of sight 102 is plotted, said clear line of sight extending from the intended focal spot position 55 to the distal edge 63 of the charge collection element 60. These are the two clear lines of sight 101, 102, which, in the plane of the drawing, delimit the angle range θ for the directions from which secondary electrons that are emitted at the intended focal spot position 55 reach the charge collection element 60. Therefore, the proximal edge 62 and the distal edge 63 are also referred to as delimiting edges 65, or as proximal delimiting edge 66 and, accordingly, as distal delimiting edge 67. Consequently, only lines of sight whose angles relative to the target surface 31 falls in the angle range Θ, which is delimited by the lines of sight 101, 102, lead to the charge collection element and represent clear lines of sight for secondary electrons. Consequently, only some of the secondary electrons that are emitted into the hemisphere above the target surface 31 are collected on the charge collection element 60. The angle range Θ is the viewing angle, within which secondary electrons emitted at the intended focal spot position 55 see the charge collection element 60.

If the focal spot position 52 changes along a labelled direction 80 to form a modified focal spot position 52’, a modified angle range Θ’ spanned by clear lines of sight 10T, 102’ also emerges, said clear lines of sight extending from the modified focal spot position 52’ to the charge collection element 60, i.e., to the proximal delimiting edge 66 and distal delimiting edge 67, respectively. The modified angle range θ’, within which secondary electrons emitted at the modified focal spot position 52’ see the charge collection element 60, is smaller than the angle range Θ, within which secondary electrons emitted at the intended focal spot position 55 see the charge collection element 60. This reduction in the size of the angle range θ when changing the focal spot position 52 on the target surface 31 along the labelled direction 80, which extends perpendicular to the proximal delimiting edge 66 and the distal delimiting edge 67, also leads to the amount of charge collected per time interval on the charge collection element 60 being reduced. Consequently, the change in the angle range θ and, thereby, the change in position of the focal spot position can be deduced from the reduction in the charge captured per time interval.

By contrast, if there is a change in the focal spot position 52 from the intended focal spot position 55 to another modified focal spot position 52” counter to the labelled direction 80”, the angle range θ increases to a modified, increased angle range Θ”, which is delimited by the clear lines of sight 101”, 102” to the proximal delimiting edge 66 and the distal delimiting edge 67.

More secondary electrons are collected on the charge collection element 60 from this other modified focal spot position 52”. An increase in the charge captured per time interval corresponds to the increased angle range Θ”. From this, it is then likewise possible to ascertain a different change in position of the focal spot position 52, which is counter to the labelled direction 80.

Consequently, a position deviation of the focal spot position on the target can be determined parallel to the labelled direction 80, which is oriented perpendicular with respect to the delimiting edges 65, on the basis of the measured collected charge.

It should be noted here that the capture solid angle Θ is a solid angle and Θ specifies the associated angle range in the plane of the drawing. Therefore, for reasons of simplification, only the angle range θ or the modified angle range Θ’ or the modified angle range Θ” in the plane of the drawing is plotted in each case in this and the following figures. The change in this angle range is decisive for a change in the charge captured per time interval if the focal spot displaces its position along the labelled direction 80.

It was found that not the actual orientation of a surface collecting the charge, i.e., of the charge collection element 60, but only the position of the delimiting edges 65, i.e., the proximal delimiting edge 66 and the distal delimiting edge 67, in relation to one another is of importance. This should be elucidated on the basis of Figures 2A to 2C. In all figures, the same technical features are denoted by identical reference signs.

Figure 2A schematically shows the situation analogous to the embodiment illustrated in Figure 1. The charge collection element 60 embodied as a plate 61 has a proximal edge 62 and a distal edge 63, the clear lines of sight 101, 102 extending thereto and delimiting the angle range θ within which secondary electrons emitted at the intended focal spot position 55 see the charge collection element 60. Consequently, the proximal edge 62 of the charge collection element 60 represents the proximal delimiting edge 66 and the distal edge 63 of the charge collection element 60 represents the distal delimiting edge 67 for a so-called detection area 90, which is the area through which the clear lines of sight, which extend from the focal spot 50 to a position on the charge collection element 60, extend.

For reasons of simplification, only the lines of sight 101, 102 that delimit the angle range θ in the plane of the drawing are shown. The labelled direction 80, along which changes in the focal spot position are capturable, also extends in the plane of the drawing, parallel to the target surface 31. In the embodiment of Figure 2A, the detection area 90 coincides with the area of the charge collection element 60.

A change in the focal spot position perpendicular to the plane of the drawing, i.e., parallel to the delimiting edges 65, however does not lead to a change, or only to a minimal change, in the capture solid angle, within which the secondary electrons see the charge collection element 60. It is particularly preferred for a distance of projections 165, a projection 166 of the proximal delimiting edge 66 and a projection 167 of the distal delimiting edge 67, on the target surface 31 to have a smaller distance 168 from one another than the extent of the charge collection element 60 perpendicular to the plane of the drawing, i.e., parallel to the delimiting edges 66, 67.

The focal spot diameter 51 is also plotted in Figure 2A.

Figure 2B shows an embodiment in which the proximal delimiting edge 66 is formed by an edge 112 of a stop 110. The distal delimiting edge 67 is an outer edge 64 of the charge collection element 60 in this embodiment. In this embodiment, too, there is a detection area 90, the size of which is identical to the one according to Figure 2A. However, only the size and position of this detection area 90 determines how much charge is collected on the charge collection element 60 per time interval. In this embodiment, the charge collection element 60 extends parallel to the target surface 31 beyond the proximal delimiting edge 66.

Finally, Figure 2C illustrates an embodiment in which, in addition to the proximal delimiting edge 66, which is formed by the edge 112 of the stop 110, the distal delimiting edge 67 is also formed by an edge122 of a further stop 120. In this embodiment, too, the size and position of the detection area 90 is identical to the detection areas 90 of the other embodiments according to Figures 2A and 2B. In this embodiment, too, the charge collection element 60 extends parallel to the target surface 31 beyond the proximal delimiting edge 66 and the distal delimiting edge 67.

Reference is once again made here to the fact that a change in the capture solid angle Θ spanned by the clear lines of sight or the directions thereof, within which secondary electrons can reach the charge collection element 60 from the focal spot, is substantially determined by a change in the angle range Θ in the plane perpendicular to the delimiting edges 65, i.e., the plane of the drawing in this case.

In the illustrated embodiments of Figures 2A to 2C, the charge collection element 60 is situated perpendicularly above the intended focal spot position 55. This means that a surface normal 35 of the target surface intersects the charge collection element 60. The surface normal 35 and the detection area 90 include a vertical angle a that is selected to be as small as possible. Preferably, the vertical angle a lies in an angle range between 1° and 10°, particularly preferably between 3° and 7°.

The line of sight 101 to the proximal delimiting edge 66 includes a flat angle β with the detection area 90.

An expedient minimum vertical angle a depends on, firstly, the maximum expected size of the focal spot, i.e., the maximum expected focal spot diameter, on manufacturing accuracy and on a distance 72 of the proximal delimiting edge 66 from the target surface 31.

In a graph 200, Figure 3 graphically plots a signal-signal change ratio or signal-offset ratio, denoted OSR below, against vertical angle a between the surface normal 35 and the detection area 90. The signal-signal change ratio OSR is defined as the ratio of the charge L(x) detected per time interval at a focal spot position 52, for example the intended focal spot position 55, divided by the difference AL of the charge detected in each unit time, which is caused by a change in position Δχ of the focal spot position 52, i.e.,

OSR = L(x)/((L(x)-L(x+ Δχ)) or expressed differently OSR =L/ AL.

Deviations in which AL>0 are considered. For AL -> 0, OSR ->0 applies; i.e., for minimal deviations in the amount of charge captured per time AL caused by changes in the focal spot position, the signal-signal change ratio OSR tends to 0.

Changes of a large signal that are as small as possible should be captured. The smaller the offset-signal ratio OSR, the more accurately deviations in the focal spot position can be captured.

The various vertical angles a are plotted on the abscissa and the signal-signal change ratio OSR is plotted on the ordinate. What emerges from the illustrated functional relationship of the graph 200 is that the vertical angle a should be chosen to be as small as possible. Nevertheless, the vertical angle a should be chosen in such a way that secondary electrons emitted by the focal spot do not reach the back side of the charge collection element 60, even in the embodiment according to Figure 2A. This restriction does not apply if a second charge collection element, for which the charge collected per time interval is evaluated separately, is on the back side of the charge collection element 60. However, this requires twice the outlay during the charge measurement and evaluation in order to be able to determine the focal spot position along the labelled (predetermined) direction 80 on the target 30.

In Figure 4, a functional relationship of the signal-signal change ratio OSR in relation to changes in the focal spot position is plotted schematically in each case for three different focal spot diameters. A functional relationship for a focal spot diameter of 1 mm is shown in a graph 151, a functional relationship for a focal spot diameter of 100 pm is shown in a graph 152 and a functional relationship for a focal spot diameter of 10 pm is shown in a graph 153. What can be gathered from the graphs 151 to 153 is that smaller deviations are already capturable for smaller focal spot diameters than for larger focal spot diameters. What applies again here is that as the signal-signal change ratio OSR becomes smaller, the more easily and the more precisely a focal spot position can be ascertained.

The charge deposited on the charge collection element 60 can be measured in different ways by means of a measuring device 270. Firstly, it is possible to measure the voltage across a large resistor, by which the charge is conducted away from the charge collection element. Alternatively, the current signal arising during the discharge can be amplified directly. An in turn alternative embodiment provides for a charge collection element to be kept at a voltage like a capacitor 260, which is discharged by the deposited charge. This embodiment is indicated in Figure 1.

The charge measurement can be implemented over relatively long time intervals by means of the measuring device 270 since a change in the focal spot position likewise only occurs, as a rule, over relatively long time periods. Therefore, even small changes in the amounts of charge collected or deposited per time interval can be reliably captured. Preferably, the deposited charge or the charge current necessary to maintain a predetermined potential is sampled and integrated at short time intervals. If a change in charge, i.e., a change in the secondary electron current, as it were, is determined per time interval by means of an evaluation device 280, the change in position of the focal spot position 52 can be ascertained therefrom. In a simple embodiment, merely the fact that the focal spot position has changed is output as position signal 290. In a preferred embodiment, the direction of the change in position is additionally specified as position signal 290. In yet a different, improved embodiment, the distance or the quantitative size of the change in position is additionally also specified. The position signal 290 can be used by a downstream control device 300 to control electron optical devices 310 that influence the primary electron beam 20 and thus displace the focal spot 50 to the intended focal spot position 55 again.

In order also to be able to take account of variations in the secondary electron emission, which are not caused by changing the focal spot position, the evaluation device 280 has a signal input 281, by means of which a normalization signal 315 can be captured. Byway of example, the normalization signal 315 is provided by a current measuring device 320, which measures a current occurring on the target 30 and produces the normalization signal 315 therefrom.

Figure 5 schematically shows an embodiment of an x-ray tube measuring head 11 of an xray tube 10, in parts in a sectional illustration. The x-ray tube measuring head 10 is manufactured from a metal block 12. Worked into the latter are a first cutout 13 and a second cutout 14, for example both in the form of a bore, the axes of which meet at a flat angle on a surface of a target 30, which fills out an even further functional cutout 18. The primary electron beam 20 is incident on the target 30 in the focal spot 50 through the first cutout 13. The x-ray radiation 40 emerges from the x-ray tube measuring head 11 through an x-ray window 15 via the second cutout 14, said x-ray window terminates the second cutout 14, and is used to measure a test object 45.

A further cutout 16, for example in the form of a bore or milled hole, is formed perpendicularly or virtually perpendicularly above the target surface 31 of the target 30. An outer face 19 of the x-ray tube measuring head 11 is angled in relation to an axis 25 of the cutout 16 and in relation to the target surface 31. Preferably, the outer face 19 has a vertical angle a in relation to the surface normal 35 of the target 30, said vertical angle being suitable for a charge collection element 60 at the position at which said outer face is penetrated by the cutout 16. Here, a termination 26 of the cutout 16 is formed as a charge collection element 60 and fastened on the metal block 12 so as to be insulated in relation to the latter, and said termination seals the cutout 16. If it is not possible to establish a suitable inclination of the outer face 19 of the x-ray tube measuring head 11 in relation to the surface normal 35 of the target surface 31 in the region of the cutout 16, the detection area 90, which is decisive for the charge measurement, can be set by way of an optional stop 110 or two optional stops 110, 120 in the region of the cutout 16.

In a further embodiment, illustrated in Figure 6, the charge collection element is formed as a constituent part of the x-ray window 15.

In addition to measuring and controlling the focal spot position 52 along a labelled spatial direction 80 which, in the embodiments illustrated previously in the figures, lies in the depicted plane of the drawing in each case, it is also possible to make additional use of a further charge collection element, which is likewise angled in relation to the target surface, to ascertain a change in position of the focal spot position 52 in a spatial direction transverse to this labelled direction 80, in particular perpendicular to the labelled direction 80. Analogously, such an embodiment has a further detection area which is restricted in terms of an angle range φ in a plane perpendicular to the plane of the drawing of the previous figures by a further proximal delimiting edge and a further distal delimiting edge. It is not possible to likewise arrange the further charge collection element perpendicularly above the intended focal spot position. However, arranging the further delimiting edges perpendicular to the delimiting edges 65 should be implemented in such a way where possible that the clear lines of sight from the focal spot position 52 to the charge collection element 60, which extend through the detection area 90, and the further clear lines of sight to the further charge collection element, which extend through the further detection area, are in each case disjoint from one another, even in the case of predetermined deviations of the focal spot position from the intended focal spot position.

The invention is distinguished by virtue of the fact that, in particular, movements of the focal spot in the magnification direction, i.e., the direction along which movements on the target surface cause a change in the magnification in the case of a CT recording, can be monitored and corrected where necessary. Then, this magnification direction is chosen as labelled direction.

Movements of the focal spot in a direction perpendicular to the magnification direction lead to a “washout” of the CT recording, which has the same effect on all imaged edges of a test object and consequently brings about a loss of resolution. This can be additionally improved if the displacement of the focal spot on the target transversely to the magnification direction is monitored by way of a second charge collection element.

List of reference signs

Apparatus

X-ray tube

X-ray tube measuring head

Metal block

First cutout

Second cutout

X-ray window

Further cutout

Functional cutout

Outer face

Primary electron beam

Location of incidence

Axis

Termination

Target

Target surface

Surface normal

X-ray radiation

T est object

Focal spot

Focal spot diameter

Focal spot position

Modified focal spot position

Other modified focal spot position Intended focal spot position Charge collection element

Plate

Proximal edge

Distal edge

Outer edge

Delimiting edges

Proximal delimiting edge

Distal delimiting edge

Distance

Labelled direction

Detection area

Clear lines of sight (Modified) clear lines of sight (Other modified) clear lines of sight

Stop

Edge

Stop

Edge

Graph (1 mm)

Graph (100 pm)

Graph (10 pm)

Projections

Projections of the proximal delimiting edge

Projection of the distal delimiting edge

Distance

Graph

Capacitor

Measuring device

Evaluation device

Signal input

Position signal

Control device

Electron optical device

Normalization signal

Current measuring device

Angle between the detection area and a surface normal of the target surface

Angle between detection area and a clear line of sight to the proximal delimiting edge

Capture solid angle

Angle range spanned by the clear lines of sight in a plane

Claims

1. Apparatus (1) for controlling a position of a focal spot (50) on a target (30) in an xray tube (10) in a monitored region around an intended focal spot position (55), comprising:

at least one charge collection element (60) for collecting some secondary electrons that were produced in the focal spot (50) and emitted into the hemisphere above a target surface of the target (30), wherein the at least one charge collection element (60) is arranged at a distance from the target (30) in the x-ray tube (10) and electrically insulated from other elements of the x-ray tube (10), wherein the at least one charge collection element (60) is arranged relative to the target surface (31) of the target (30) in such a way that, in each case, a capture solid angle (Θ), spanned by clear lines of sight (101,102; 10T, 102'; 101, 102) of the secondary electrons, is smaller than the solid angle of the hemisphere above the target surface (31) in the intended focal spot position (55), wherein clear lines of sight (101,102; 10T, 102'; 101, 102) are those straight lines which extend in a straight line from one of the generation positions of the secondary electrons in the focal spot (50) to one of the positions on the at least one charge collection element (60) and along which one of the secondary electrons can move in unimpeded fashion from the target (30) to the at least one charge collection element (60); and a measuring device (270), connected to the at least one charge collection element (60), for measuring the charge of secondary electrons collected on the at least one charge collection element (60) per time interval and an evaluation device (280) connected to the measuring device (270), said evaluation device being suitable for converting a change in the charge captured per time interval into a positional change and for providing a position signal (290).

2. Apparatus (1) according to Claim 1, characterized in that a detection area (90) comprising a proximal delimiting edge (66) and a distal delimiting edge (67) is formed, wherein the proximal delimiting edge (66) and the distal delimiting edge (67) are each oriented parallel to one another and to the target surface (31) and are formed by stop edges (112, 122) and/or edges of other elements of the x-ray tube (10) and/or edges (62, 63) of the at least one charge collection element (60) and delimit the capture solid angle (Θ) of the clear lines of sight (101, 102) from the focal spot (50) at the intended focal spot position to the at least one charge collection element (60), wherein the proximal delimiting edge (66) has a shorter distance from the target surface (31) than the distal delimiting edge (67), wherein the proximal delimiting edge (66) and the distal delimiting edge (67) are oriented perpendicular to the labelled direction (80) on the target surface (31).

3. Apparatus (1) according to Claim 2, characterized in that the detection area (90) includes an obtuse angle (β) with a clear line of sight from an intended focal spot position (55) of the focal spot (50) to the proximal delimiting edge (66), which is oriented perpendicular to the proximal delimiting edge (66).

4. Apparatus (1) according to either of Claims 2 and 3, characterized in that the detection area (90) includes an angle (a), preferably selected from the angle range between 1° and 10°, more preferably from the angle range between 3° and 7°, with a surface normal of the target surface (31).

5. Apparatus (1) according to any one of the preceding claims, characterized in that the at least one charge collection element (60) is formed as electrically conductive plate (61).

6. Apparatus (1) according to any one of the preceding claims, characterized in that the charge collection element (60) is formed as termination element (26) or as part of a termination element (26) of a cutout (16) in an x-ray tube measuring head (11).

7. Apparatus (1) according to any one of the preceding claims, characterized in that the evaluation device (280) is embodied to capture a normalization signal (315), which is correlated to an overall number of secondary electrons produced, and to take account of the normalization signal (315) when ascertaining a change in the charge captured per time interval.

8. Apparatus (1) according to any one of the preceding claims, characterized in that at least one further charge collection element is arranged over the target surface (31) in electrically insulated fashion in the x-ray tube (10) and a further detection area comprising a further proximal delimiting edge and a further distal delimiting edge, which are oriented perpendicular to a further labelled direction, is formed, wherein the further proximal delimiting edge and the further distal delimiting edge are each oriented parallel to one another and to the target surface (31) and are formed by further stop edges or further edges of other elements of the x-ray tube (10) or edges of the at least one further charge collection element and delimit a further capture solid angle of further clear lines of sight from the focal spot (50) at the intended focal spot position to the at least one further charge collection element, wherein the further proximal delimiting edge has a shorter distance from the target surface (31) than the further distal delimiting edge, wherein the further charge collection element is connected to the measuring device (270) or a further measuring device for measuring the charge of secondary electrons collected on the further charge collection element per time interval and the evaluation device (280) connected to the measuring device (270) or the further measuring device is suitable for converting a change in the charge captured per time interval on the further charge collection element into a change in position of the focal spot (50) along the further labelled direction on the target surface (31) and for providing a further position signal, wherein the further labelled direction is oriented transversely to the labelled direction.

9. Method for controlling a focal spot position (52) on a target (30) in an x-ray tube (10) in a monitored region around an intended focal spot position (55), comprising the steps of:

collecting secondary electrons on at least one charge collection element (60), said secondary electrons forming some of the secondary electrons that were produced in the focal spot (50) on the target (30) and emitted into a hemisphere above a target surface (31) of the target (30), wherein the at least one charge collection element (60) is arranged relative to the target surface (31) of the target (30) in such a way that, in each case, a capture solid angle (Θ), spanned by clear lines of sight (101,102; 10T, 102'; 101, 102) of the secondary electrons, is smaller than the solid angle of the hemisphere above the target surface (31) in the intended focal spot position (55), wherein clear lines of sight (101,102; 10T, 102'; 101, 102) are those straight lines which extend in a straight line from one of the generation positions of the secondary electrons in the focal spot (50) to one of the positions on the at least one charge collection element (60) and along which one of the secondary electrons can move in unimpeded fashion from the target (30) to the at least one charge collection element (60), iteratively measuring the charge collected on the at least one charge collection element (60) per time interval;

ascertaining as to whether a change has occurred in the amount of charge collected per time interval and, should this be the case, ascertaining a position deviation of the focal spot position (52), which corresponds to a change in the capture solid angle (Θ) of the at least one charge collection element (60), wherein the change in the capture solid angle (Θ) corresponds in turn to the ascertained change in the charge captured per time interval, and outputting a position signal (290) which comprises a position information item.