WO2013038287A1

WO2013038287A1 - X-ray radiation with multiple photon energies

Info

Publication number: WO2013038287A1
Application number: PCT/IB2012/054420
Authority: WO
Inventors: Rolf Karl Otto Behling
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2011-09-13
Filing date: 2012-08-29
Publication date: 2013-03-21

Abstract

The present invention relates to X-ray radiation with multiple photon energies. To provide an X-ray tube capable of generating multiple-energy X-ray radiation with an improved switching capacity and a reduced design setup, a multiple-energy X-ray tube, comprises a cathode (12), an anode (14), and a switchable current controlling unit (16). The cathode comprises a first connection port (18), which is connectable with a negative terminal of a generator to be supplied with a high-voltage energy to generate a voltage potential between the cathode and the anode. The anode is an electrically floating isolated anode. The switchable current controlling unit is electrically arranged between the anode and a second connection port, which second connection port is connectable with a ground or positive terminal of the generator to provide an adjustable current transfer (30) from the anode via the switchable current controlling unit and the second connection port to the generator. Further, the switchable current controlling unit is controllably adaptable such that at least a first current transfer and a second current transfer are adjustable, wherein the second current transfer is different from the first current transfer. The first current transfer leads to a first voltage between the cathode and the anode resulting in an electron beam (86) which generates X-radiation with a first spectrum, and the second current transfer leads to a second voltage between the cathode and the anode resulting in an electron beam (86) which generates X-radiation with a second spectrum, wherein the first X-ray spectrum is different from the second X-ray spectrum.

Description

X-RAY RADIATION WITH MULTIPLE PHOTON ENERGIES

FIELD OF THE INVENTION

The present invention relates to X-ray radiation with multiple photon energies. The invention relates in particular to a multiple-energy X-ray tube, an X-ray imaging system, a method for generating multiple-energy X-ray radiation, as well as a computer program element and a computer readable medium.

BACKGROUND OF THE INVENTION

In order to provide improved, for example more detailed, object information when using X-ray imaging, multiple-energy X-ray radiation is used. As an example, an X-ray image with an increased diagnostic value can be provided by using multiple X-ray photon energies, which X-ray photon energies are also referred to as X-ray colours. Further, also in the field of material investigation, or object inspection such as screening of luggage at airports or the like, multiple X-ray energies provide enhanced information. For the generation of X-ray radiation with different X-ray energies, a single X-ray tube may be supplied with alternating tube voltages in order to provide different X-ray energy radiation. The generation of X-ray radiation with multiple spectra, e.g. dual energy X-rays, is described in

WO 2011/051860. Further, also the use of multiple X-ray tubes with multiple tube voltages and X-ray filters is known. However, multiple X-ray tubes have shown to be rather expensive and occupy valuable construction space in an X-ray examination apparatus. Further, using a single X-ray tube supplied with alternating tube voltages has shown to be of slow operation, due to the large capacity of the high-voltage circuitry, for example.

SUMMARY OF THE INVENTION

There is thus a need to provide an X-ray tube capable of generating multiple- energy X-ray radiation with an improved switching capacity and a reduced design setup.

The object of the present invention is solved by the subject matter of the independent claims, wherein further embodiments are incorporated in the dependent claims.

It should be noted that the following described aspects of the invention apply also for the multiple-energy X-ray tube, the X-ray imaging system, the method for generating multiple-energy X-ray radiation, as well as for the computer program element and the computer readable medium.

According to a first aspect of the present invention, a multiple-energy X-ray tube is provided that comprises a cathode, an anode, and a switchable current controlling unit. The cathode has a first connection port, which is connectable with a negative terminal of a generator to be supplied with a high- voltage energy to generate a voltage potential between the cathode and the anode. The anode is an electrically floating isolated anode. The switchable current controlling unit is electrically arranged between the anode and a second connection port, which is connectable with a ground or positive terminal of the generator to provide an adjustable current transfer from the anode via the switchable current controlling unit and the second connection port to the generator. The switchable current controlling unit is controllably adaptable such that at least a first current transfer and a second current transfer are adjustable, wherein the second current transfer is different from the first current transfer. The first current transfer leads to a first voltage between the cathode and the anode resulting in an electron beam which generates X-radiation with a first spectrum. The second current transfer leads to a second voltage between the cathode and the anode resulting in an electron beam which generates X-radiation with a second spectrum. The first X-ray spectrum is different from the second X-ray spectrum.

For example, the second current transfer is larger than the first current transfer, and the first X-ray spectrum is larger than the second X-ray spectrum.

The first and the second current transfers are provided in an alternating manner.

According to an example, the cathode is provided with a single high- voltage energy level.

However, the cathode may also be provided with at least two different high- voltage energy levels in an alternating manner, wherein during each respective energy level supply, at least two different current transfers are provided. Thus, for example, at least four different X-ray spectra can be provided. The high- voltage switching occurs with lower frequency than the current transfer switching.

According to a further exemplary embodiment, the X-ray tube comprises a vacuum envelope with a housing, and the switchable current controlling unit is arranged inside the housing. For example, the housing may be provided as the second connection port.

According to a further exemplary embodiment, the anode is a rotating anode.

According to an exemplary embodiment, the switchable current controlling unit comprises a plurality of light-modulated emitters. The light-modulated emitters each comprise a carbon nanotube electron emitter combined with a photodiode. The carbon nanotube electron emitters provide an electron current from the anode to the second connection port. The electron current is steerable by the photodiodes.

According to a further exemplary embodiment, the switchable current controlling unit comprises a triode system in which electron emitters are arranged to provide an electron current from the anode to the second connection port. A control grid connected to an external control voltage source is provided between the electron emitters and the second connection port.

The emitters may be light-modulated emitters, for example carbon nanotube electron emitters comprising a photodiode.

According to a second aspect of the invention, an X-ray imaging system is provided, comprising a multiple-energy X-ray tube according to one of the above mentioned examples, a high- voltage generator, an X-ray detector, and a processing device. The high- voltage generator is provided to supply the first connection port of the X-ray tube. The multiple-energy X-ray tube is provided to generate X-ray radiation with at least two different X-ray energies. The X-ray detector is provided to receive the multiple-energy X-ray radiation after radiating an object. The processing device is provided to control the switchable current controlling unit to provide the at least first and second current transfers.

According to a third aspect of the present invention, a method for generating multiple-energy X-ray radiation with an X-ray tube is provided, comprising the fo llo wing steps : a) Supplying a high- voltage tube current from a negative terminal of a generator via a first connection port to a cathode in order to emit electrons towards a target surface of an electrically floating isolated anode.

b) Controlling a current transfer from the anode to a ground or positive terminal of the generator with a switchable current controlling unit such that at least a first current transfer and a second current transfer are alternately provided, wherein the second current transfer is different from the first current transfer, and wherein the first current transfer leads to a first voltage between the cathode and the anode resulting in an electron beam which generates X-radiation with a first spectrum, and the second current transfer leads to a second voltage between the cathode and the anode resulting in an electron beam which generates X- radiation with a second spectrum. The first X-ray spectrum is different from the second X-ray spectrum.

c) Alternately generating X-ray radiation with the first spectrum, and X-ray radiation with the second spectrum. According to an aspect of the present invention, an X-ray tube with an electrically floating anode, for example an electrically floating rotating isolated anode, is proposed. The current transfer from the anode to the positive generator terminal, which positive generator terminal may be ground potential in case of single-ended anode grounded tubes, for example, is provided by a switchable or steerable current controlling unit in order to control the current transfer. Thus, by applying or providing different current transfers, different electrical potentials between the cathode and the anode can be provided, resulting in different electron beams for the generation of X-ray radiation. Since different electron beam characteristics are provided, X-ray radiation with different energy characteristics are generated. This allows, for example, supplying a constant high- voltage to the cathode, while allowing a fast switching of the high- voltage potential of the anode, and with it the photon energy of the generated X-rays. As a further advantage, the switching of the current transfer is provided with low capacitance allowing the fast switching. As a benefit, cost-savings in the high- voltage generator are achieved, since only a single high- voltage supply to the cathode has to be provided. Further, constructional space is also only used in a minimum amount by the switchable current controlling unit allowing the implementation of a multiple X-ray energy tube according to the present invention instead of single X-ray energy X-ray tubes.

These and other aspects of the invention will become apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will be described in the following with reference to the following drawings. Fig. 1 shows an example for a multiple-energy X-ray tube according to the present invention.

Fig. 2 shows an X-ray imaging system according to an exemplary embodiment of the invention.

Figs. 3, 4, 5, and 6 show further exemplary embodiments of multiple-energy X-ray tubes according to the present invention.

Fig. 7 shows an example of an electrical scheme according to the present invention.

Fig. 8 shows an exemplary embodiment of a switchable current controlling unit according to the present invention. Fig. 9 shows basic method steps for generating multiple-energy X-ray radiation according to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

As indicated above, when using a CT scanner, for example, the pulse time of high and low energy periods should be in the range of an integration period of the detector, e.g. 200 Ts, for an optimized diagnostic value of the X-ray image. To achieve a sufficiently high duty cycle and photon flux, the transition time needs to be a small fraction of this. When using a regular X-ray tube and altering the high-voltage, the capacity of the high- voltage cable makes discharging a slow process in practice. Thus, short pulsing can hardly be achieved with reasonable effort.

According to the present invention, short pulsing is provided with an X-ray tube setup where a current transfer from the anode to ground or a positive terminal is controlled such that different high- voltage potentials result in an alternating, such that different electron beams generate X-ray radiation with different spectra.

Fig. 1 shows the basic concept of an example for a multiple-energy X-ray tube 10. The X-ray tube 10 comprises a cathode 12, an anode 14, and a switchable current controlling unit 16. The cathode is provided with a first connection port 18, which is connectable with a negative terminal 20 of a generator to be supplied with a high- voltage energy to generate a voltage between the cathode 12 and the anode 14.

It must be noted that the generator, as well as the negative terminal 20 of the generator, are not part of the multiple-energy X-ray tube 10 itself.

The possibility of a connection of the first connection port 18 with the negative terminal is therefore indicated with a dotted line 22.

The anode 14 is an electrically floating isolated anode, which, for illustration purposes, is symbolically indicated with two isolating support symbols 24, which are not meant to represent any constructive embodiment of such floating and isolated support of the anode 14. The switchable current controlling unit 16 is electrically arranged between the anode 14 and a second connection port 26, which is connectable with a ground or positive terminal 28 of the generator to provide an adjustable current transfer, indicated with an arrow 30 inside the frame representing the switchable current controlling unit 16. The possibility of a connection with the ground or positive terminal 28 is indicated with a further dotted line 32, as already mentioned in connection with the electric connection of the first connection port 18 with the negative terminal 20. Further, the electrical arrangement of the current controlling unit 16 between the anode 14 and the second connection port 26 is indicated with two respective connection lines 34, 36. The switchable current controlling unit 16 is controllably adaptable such that at least a first current transfer and a second current transfer are adjustable, wherein the second current transfer is different from the first current transfer.

For example, the second current transfer is larger than the first current transfer.

The first current transfer leads to a first voltage, or first potential, between the cathode and the anode resulting in an electron beam 38 which generates X-radiation with a first spectrum.

For example, the electron beam resulting from the first potential between the cathode and the anode has a first energy which is generating the X-ray radiation with the first spectrum, or first X-ray energy spectrum.

The X-ray radiation with first spectrum is indicated with a fan-like beam 40.

The second current transfer leads to a second voltage, or second potential, between the cathode and the anode resulting in an electron beam 42 which generates X- radiation with a second spectrum.

For example, the electron beam resulting from the second potential between the cathode and the anode has a second energy which is generating the X-ray radiation with the second spectrum, or second X-ray energy spectrum.

The X-ray radiation with second spectrum is indicated with a fan-like beam, with reference numeral 44.

It must be noted that the first electron beam 38 and the second electron beam 42 are shown with a common arrow only, since the two different electron beams occur in an alternating manner. Similar is the case for the first X-ray beam, i.e. the X-ray radiation with the first X-ray energy spectrum 40, and the second X-ray beam, i.e. the second X-ray radiation with the second X-ray spectrum 44.

The first X-ray spectrum is different from than the second X-ray spectrum.

For example, the first X-ray spectrum is larger than the second X-ray spectrum.

Fig. 2 shows an X-ray imaging system 46 in form of a CT system 48, as an example. The CT system 48 comprises an X-ray source 50 and a detector 52 arranged on opposite sides on a gantry 54, such that they can be rotated together around an object, for example a patient 56, in order to provide X-ray images from different orientations. During the X-ray image acquisition procedure, the patient 56 can be moved in relation to the CT system 48. Therefore, a table 58 is provided to support the patient, which table 58 is adjustable in the length and height direction, and movable in longitudinal direction to move through the gantry 54. Further, a processing device 60, a display device 62, and an interface unit 64 are provided. The X-ray source 50 is provided as a multiple-energy X-ray tube according to the principle shown in Fig. 1, and also in accordance to one of the below mentioned exemplary embodiments according to the present invention. Further, a high- voltage generator 66 is provided to supply the first connection port of the X-ray tube (not further shown). The multiple-energy X-ray tube is provided to generate X-ray radiation with at least two different X-ray energies. The X-ray radiation is indicated with a fan beam 68. The X-ray detector 52 is provided to receive the multiple-energy X-ray radiation after radiating an object, for example the patient 56.

The processing device is provided to control the switchable current controlling unit of the X-ray tube 50 to provide the at least first and second current transfers.

According to the present invention, also other X-ray imaging modalities or systems are provided, for example C-arm systems, or other medical X-ray imaging systems, for example, also with fixedly mounted X-ray sources.

According to a further example (not shown), a luggage inspection device is provided as an X-ray imaging system according to the present invention. The luggage inspection device is provided with a multiple-energy X-ray tube according to the present invention to generate X-ray radiation with at least two different energies. The luggage inspection device may be provided for small items, such as carry-on baggage or hand baggage or suitcase inspections, as well as inspection for large-scale objects, for example for inspecting containers or other large-scale pieces of equipment.

As shown in Fig. 3, but also applicable to other embodiments of the X-ray tube according to the present invention, the X-ray tube 10 may comprise a vacuum envelope with a housing 70 and the switchable current controlling unit 16 is arranged inside the housing.

The switchable current controlling unit may also be provided inside a separate vacuum (not further shown).

As shown in Fig. 3, the switchable current controlling unit 16 can be provided with two switchable current controlling unit components 72. The adjustable current transfer 30 is indicated with two pairs of arrows 74, leading from the switchable current controlling unit components 72 to the housing 70. The connection of the switchable current controlling unit 16 with anode 14 is not further shown.

The anode 14 may be a rotating anode 76, which is indicated with a rotating arrow 78 and a stem-like support structure 80 resting on an isolator 82 shown for illustration purposes only in a simplified manner.

Further, a focal track 83 on the rotating anode 76 is indicated.

The cathode 12 is also provided with an isolating support 84, also shown in a simplified manner. The cathode 12 is provided for emitting an electron beam, indicated with arrow 86, to impinge on a target, i.e. the focal track 84, for generating X-rays (not further shown).

For example, when supplying the first connection port 18 with a high- voltage potential of -160 kV, by adjusting or controlling the current transfer 30, the anode 14 can be provided with an electrical potential lying in the range of -20 kV to -80 kV.

Fig. 4 shows an example for an embodiment of the switchable current controlling unit 16.

It must be noted that throughout the figures, similar graphic representations relate to same or similar features and are therefore not provided with reference numerals at every instant, and are in addition also not mentioned or discussed in a repeating manner for each figure. It must be understood that explanations and descriptions relating to a particular figure also apply for other figures in which the same component or same feature is indicated in the same graphical manner.

The switchable current controlling unit 16 comprises a plurality of light- modulated emitters 88. Each light-modulated emitter comprises a carbon nanotube electron emitter 90, as also shown in Fig. 8. The carbon nanotube electron emitter 90 is combined with a photodiode 92. The carbon nanotube electron emitters 90 provide an electron current 94 from the anode 14 to the second connection port 26, for example the housing 70. The electron current 94 is steerable by the photodiodes 92. The carbon nanotube electron emitters comprising the photodiode are illuminatable by a light source, symbolically indicated with wave-like arrow 96, for example a fast controlled laser beam, to steer the current 94 emitted at a specific voltage level.

The adjustable current transfer is provided form the anode via the switchable current controlling unit and via the housing as the second connection port 26 to the generator, i.e. the ground or positive terminal 28 of the generator. The switchable current controlling unit 16 connects the anode 14 to ground. The switchable current controlling unit 16 provides an auxiliary electron current.

The switchable current controlling unit may provide a controlled relationship of voltage/electron emission current of the auxiliary cathode system.

As shown in Fig. 5, according to an exemplary embodiment, the switchable current controlling unit 16 may comprise a triode system 98 in which electron emitters 100 are arranged to provide an electron current, indicated with two arrows 102, from the anode 14 to the second connection port, for example the housing 70. A control grid 104 is connected to an external control voltage source 106, which external control voltage source 106 is provided between the electron emitters 100 and the second connection port, for example the housing 70.

The grid 104 is thus provided in the area or spatial volume in which the electron current 102 occurs.

The grid 104 thus acts as a pole electrode directly influencing the auxiliary electron current from the electron emitters 100 towards the second connection port.

As a pole voltage, i.e. the voltage of the external control voltage source 106, approximately -1 kV to +1 kV with respect to the anode potential, for example -20 kV to -80 kV can be applied.

The cathode 12 can also be referred to as main cathode, and the electron beam 38, 42 can be referred to as the primary electron beam, whereas the electron current 102 can be referred to as the auxiliary electron current, as mentioned above. The electron emitters 100 can be referred to as the auxiliary cathode.

As shown in Fig. 6, the triode system 98 comprises a plurality 108 of carbon nanotube electron emitters.

The carbon nanotube electron emitters may be provided in combination with a photodiode, as mentioned in relation with Fig. 4, and also in relation with Fig. 8. Thus, the electron current can be controlled both by a fast controlled laser beam, indicated with the wave-like arrow 96, and the pole voltage 106.

According to a further embodiment (not shown), the control grid 104 is provided in combination with the plurality 108 of carbon nanotube electron emitters without the photodiodes.

According to a further embodiment (not shown), an X-ray filter is provided, which is advantageously switched in synchronization with the switching of the current transfer 30.

Fig. 7 shows electrical schematics. As can be seen, a high-voltage potential of -160 kV is provided by the negative terminal 20 of the generator to the cathode 12. For example, a current of 0.1 to 2 A is provided. The anode 14 is connected to the ground terminal 28 of the generator via the switchable current controlling unit 16. The switchable current controlling unit 16 provides an electron emission current in form of the current transfer 30, indicated with a dotted line 31, for example, leading to a potential of -20 kV to -80 kV for the anode 14. Due to the high-voltage potential thus resulting between the cathode 12 and anode 14, a primary emission current 37, i.e. the electron beam 38, 42, is provided in a range of 140 keV max. to 80 keV max. photons.

An arrow 17 arranged diagonally in a box 19 indicates the controlled relationship of voltage/electron emission current of the auxiliary cathode system.

In Fig. 8, an example for a control of an auxiliary cathode is shown. As mentioned above, according to an example, the switchable current controlling unit 16 may comprise a combination of carbon nanotube electron emitters 90 and photodiodes 92. A light source 91 illuminates the photodiodes 92 and steers the current (not further shown in Fig. 8) emitted at a specific voltage level. The anode potential is symbolically shown with a connection arrow 93.

Since the housing 70 is connected to the positive or ground terminal 28 of the generator, the voltage difference, i.e. the potential, leads to the current transfer 30.

According to a further embodiment, also shown in Fig. 8, a triode system of the carbon nanotube cathode and a control grid 110 is provided. The control grid, similar to the control grid 104, is connected to an external control voltage source, also referred to as the pole voltage 112. In case of a rotating anode this may be provided by a connection via a rotatable current feed. Thus, two control possibilities are provided: namely the control grid and the controllable electron pole voltage, and the activation or control of the emitted current via the photodiodes. It must be noted that the above described features relating to the carbon nanotube electron emitters also apply for the above mentioned embodiment without a control grid arranged in the electron path between the electron emitters and the housing, as an example.

It is further noted that the features discussed in relation with the control grid may also be applied to other types of electron emitters, and are not restricted to the use in combination with the carbon nanotube electron emitter/photodiode combination.

Fig. 9 shows basic steps of a method 200 for generating multiple-energy X-ray radiation with an X-ray tube. In a supply step 210, a high-voltage tube current 212 is supplied from a negative terminal of a generator via a first connection port to a cathode in order to emit electrons towards a target surface of an electrically floating isolated anode. In a control step 214, a current transfer 216 from the anode to a ground or positive terminal of the generator is controlled with a switchable current controlling unit such that at least a first current transfer 218 and a second current transfer 220 are alternately provided. The second current transfer 220 is different from the first current transfer 218. The first current transfer leads, indicated with box 222, to a first voltage, or potential 224, between the cathode and the anode resulting in an electron beam which generates X-radiation with a first spectrum. For example, the electron beam resulting from the first potential has a first energy 226. The second current transfer leads 228 to a second voltage, or potential 230, between the cathode and the anode resulting in an electron beam which generates X-radiation with a second spectrum. For example, the electron beam resulting from the second potential has a second energy 232. The first X-ray spectrum is different from the second X-ray spectrum. This leads to an alternately generating 234 of X-ray radiation with a first spectrum 236, or first X-ray energy spectrum, i.e. with the electron beam with the first energy, and a generating 238 of X- ray radiation with a second spectrum 240, or second X-ray energy spectrum, i.e. with the electron beam with the second energy.

For example, the second current transfer 220 is larger than the first current transfer 218 and the first X-ray spectrum is larger than the second X-ray spectrum.

The supply step 210 is also referred to as step a), the control step 214 as step b), and the alternately generation step 234, 236 as step c).

In another exemplary embodiment of the present invention, a computer program or a computer program element is provided that is characterized by being adapted to execute the method steps of the method according to one of the preceding embodiments, on an appropriate system.

The computer program element might therefore be stored on a computer unit, which might also be part of an embodiment of the present invention. This computing unit may be adapted to perform or induce a performing of the steps of the method described above. Moreover, it may be adapted to operate the components of the above-described apparatus. The computing unit can be adapted to operate automatically and/or to execute the orders of a user. A computer program may be loaded into a working memory of a data processor. The data processor may thus be equipped to carry out the method of the invention.

This exemplary embodiment of the invention covers both, a computer program that right from the beginning uses the invention and a computer program that by means of an up-date turns an existing program into a program that uses the invention.

Further on, the computer program element might be able to provide all necessary steps to fulfil the procedure of an exemplary embodiment of the method as described above.

According to a further exemplary embodiment of the present invention, a computer readable medium, such as a CD-ROM, is presented wherein the computer readable medium has a computer program element stored on it which computer program element is described by the preceding section.

A computer program may be stored and/or distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the internet or other wired or wireless telecommunication systems.

However, the computer program may also be presented over a network like the World Wide Web and can be downloaded into the working memory of a data processor from such a network. According to a further exemplary embodiment of the present invention, a medium for making a computer program element available for downloading is provided, which computer program element is arranged to perform a method according to one of the previously described embodiments of the invention.

It has to be noted that embodiments of the invention are described with reference to different subject matters. In particular, some embodiments are described with reference to method type claims whereas other embodiments are described with reference to the device type claims. However, a person skilled in the art will gather from the above and the following description that, unless otherwise notified, in addition to any combination of features belonging to one type of subject matter also any combination between features relating to different subject matters is considered being disclosed with this application.

However, all features can be combined providing synergetic effects that are more than the simple summation of the features.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing a claimed invention, from a study of the drawings, the disclosure, and the dependent claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfil the functions of several items re-cited in the claims. The mere fact that certain measures are re-cited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A multiple-energy X-ray tube (10), comprising:

a cathode (12);

an anode (14); and

a switchable current controlling unit (16);

wherein the cathode comprises a first connection port (18), which is connectable with a negative terminal (20) of a generator to be supplied with a high- voltage energy to generate a voltage potential between the cathode and the anode;

wherein the anode is an electrically floating isolated anode;

wherein the switchable current controlling unit is electrically arranged between the anode and a second connection port (26), which is connectable with a ground (28) or positive terminal of the generator to provide an adjustable current transfer (30) from the anode via the switchable current controlling unit and the second connection port to the generator;

wherein the switchable current controlling unit is controllably adaptable such that at least a first current transfer and a second current transfer are adjustable, wherein the second current transfer is different from the first current transfer; and

wherein the first current transfer leads to a first voltage between the cathode and the anode resulting in an electron beam (38) which generates X-radiation with a first spectrum (40); and the second current transfer leads to a second voltage between the cathode and the anode resulting in an electron beam (42) which generates X-radiation with a second spectrum (44); wherein the first X-ray spectrum is different from the second X-ray spectrum.

2. Multiple-energy X-ray tube according to claim 1 , wherein the X-ray tube comprises a vacuum envelope with a housing (70); and

wherein the switchable current controlling unit is arranged inside the housing.

3. Multiple-energy X-ray tube according to claim 2, wherein the housing is provided as the second connection port.

4. Multiple-energy X-ray tube according to claim 1, 2 or 3, wherein the anode is a rotating anode (76).

5. Multiple-energy X-ray tube according to one of the preceding claims, wherein the switchable current controlling unit comprises a plurality of light-modulated emitters (88);

wherein the light-modulated emitters each comprise a carbon nanotube electron emitter (90) combined with a photodiode (92);

wherein the carbon nanotube electron emitters provide an electron current (94) from the anode to the second connection port; and

wherein the electron current is steerable by the photodiodes.

6. Multiple-energy X-ray tube according to one of the preceding claims, wherein the switchable current controlling unit comprises a triode system (98) in which electron emitters (100) are arranged to provide an electron current (102) from the anode to the second connection port;

wherein a control grid (104) connected to an external control voltage source (106) is provided between the electron emitters and the second connection port.

7. Multiple-energy X-ray tube according to claim 6, wherein the triode system comprises a plurality (108) of carbon nanotube electron emitters.

8. An X-ray imaging system (46), comprising:

a multiple-energy X-ray tube (50) according to one of the preceding claims; a high-voltage generator (66);

- an X-ray detector (52); and

a processing device (60);

wherein the high- voltage generator is provided to supply the first connection port of the X-ray tube;

wherein the multiple-energy X-ray tube is provided to generate X-ray radiation (68) with at least two different X-ray energies;

wherein the X-ray detector is provided to receive the multiple-energy X-ray radiation after radiating an object; and

wherein the processing device is provided to control the switchable current controlling unit to provide the at least first and second current transfers.

9. A method (200) for generating multiple-energy X-ray radiation with an X-ray tube, comprising the following steps:

a) supplying (210) a high- voltage tube current (212) from a negative terminal of a generator via a first connection port to a cathode in order to emit electrons towards a target surface of an electrically floating isolated anode;

b) controlling (214) a current transfer (216) from the anode to a ground or positive terminal of the generator with a switchable current controlling unit such that at least a first current transfer (218) and a second current transfer (220) are alternately provided;

wherein the second current transfer is different from the first current transfer; wherein the first current transfer leads (222) to a first voltage (224) between the cathode and the anode resulting in an electron beam which generates X-radiation with a first spectrum (226); and the second current transfer leads (228) to a second voltage (230) between the cathode and the anode resulting in an electron beam which generates X-radiation with a second spectrum (232);

wherein the first X-ray spectrum is different from the second X-ray spectrum; and

c) alternately generating (234) X-ray radiation with the first spectrum (236), and generating (238) X-ray radiation with the second spectrum (240).

10. Method according to claim 9, wherein the X-ray tube comprises a vacuum envelope and wherein the first and second current transfers are provided inside the vacuum.

11. A computer program element for controlling an apparatus according to one of the claims 1 to 8, which, when being executed by a processing unit, is adapted to perform the method step of claim 9 to 10.

12. A computer readable medium having stored the program element of claim 11.