X-RAY TUBE WITH ION DEFLECTING AND COLLECTING DEVICE MADE FROM A GETTER MATERIAL

Field of invention

The present invention relates to the field of generating X-rays by means of X-ray tubes. In particular, the present invention relates to an X-ray tube comprising an ion manipulation arrangement, which is adapted to collect residual particles being present in an evacuated envelope of the X-ray tube.

The present invention further relates to an X-ray system, in particular to a medical X-ray imaging system, wherein the X-ray system comprises an X-ray tube as mentioned above. Further, the present invention relates to a method for generating X-rays, which are in particular used for medical X-ray imaging. The X-rays are generated by means of an X-ray tube as mentioned above.

Art Background

High-end and future X-ray tube generations will need to provide the possibility of a variable focal spot shape, focal spot size and focal spot position. In comparison to conventional X-ray tubes, theses tubes have a larger distance between a cathode representing an electron source and a target anode onto which the focal spot is generated.

Figure 5 schematically depicts such a high-end X-ray tube 500. The X- ray tube 500 comprises an electron source 510, which is adapted to generate an electron beam 515 projecting along a beam path and terminating at a target anode 520. The electrons are released from an electron emitter filament 511. By contrast to standard X- ray tubes, the distance between the electron source and the target anode 520 is comparatively large.

In between the electron source 510 and the target anode 520 there is arranged a field electrode 530. The field electrode 530 comprises an opening 530a. The opening 530a allows the electron beam 515 to traverse the field electrode 530.

The electron source is connected to a high voltage supply 512, which charges the electron source 510 respectively the electron emitter filament 511 with a negative high voltage (-HV) with respect to a not depicted housing of the X-ray tube 500. The target anode 520 and the field electrode 530 are both connected to ground voltage level. Therefore, the whole region extending in between the electron source 510 and the target anode 520 can be divided into two regions, a first region 531 extending between the electron source 510 and the field electrode 530 and a second region 532 extending between the field electrode 530 and the target anode 520, respectively. The first region 531 comprises an electron acceleration field. The second region 532 defines a field- free or zero-field region. In the field- free region 532 there is arranged an electron optic 536 in order to properly focus the electron beam 515 by an appropriate electric and/or magnetic multipole field. In order to achieve optimal focusing properties, it is necessary to place the electron emitter 511 on the optical axis of the electron optic system 536.

The released electrons are accelerated from the electron source 510 towards the target anode 520. Thereby, they travel through the opening 530a and enter the field-free region 532 where they are focused. When hitting the target anode 520 in the focal spot 521, nearly 40% of the primary electrons are backscattered and travel on straight lines 522 within the field- free region 532. The energy of said scattered electrons is deposited in not depicted water-cooled sidewalls.

Due to an imperfect vacuum inside the tube, atoms and molecules of the residual gas can be ionized. Such ionized particles may be influenced by the high voltage and/or by the electro-magnetic and electro -static lenses of the optical system. Some of these ions are accelerated towards the electron emitter. The optical system may focus these ions, which then impinge onto the surface of the emitter in a small spot. This could damage the emitter structure and hence reduces the lifetime or lead to an immediate failure of the emitter structure. In particular, X-ray tubes with a high voltage

acceleration region and a following electrical field-free region are characterized by this behavior.

US 4,521,900 discloses an electron beam assembly for a scanning electron beam computed tomography scanner. An electron beam production and control assembly produces its electron beam within a vacuum-sealed housing chamber, which is evacuated of internal gases, except inevitably for small amounts of residual gas. The electron beam is produced by suitable means within the chamber. Since there is residual gas within the chamber, the electrons of the beam will interact with it and thereby produce positive ions, which have the effect of neutralizing the space charge of the electron beam. In order to reduce the neutralizing effect of these ions there are provided ion-clearing electrodes. These electrodes are configured to produce a uniform electric field normal to the axis of the electron beam. The electrodes are laterally aligned with potential wells in order to remove positive ions from the region of the electron beam.

US 2003/0021377 discloses a mobile X-ray source, which includes a low-power consumption cathode element and an anode optic creating a field-free region to prolong the life of the cathode element. An electric field is applied to an anode and a cathode that are disposed on opposite sides of an evacuated tube. The anode includes a target material to produce X-rays in response to impact of electrons. The cathode includes a cathode element to produce electrons that are accelerated towards the anode in response to the electric field between the anode and the cathode. A field- free region can be positioned at the anode to resist positive ion acceleration back towards the cathode element. An anode tube can be disposed at the anode between the anode and the cathode, and electrically coupled to the anode so that the anode and the anode tube have the same electrical potential, to form the field- free region. A getter material is disposed in the evacuated tube in order to remove residual gasses in the tube after vacuum sealing. The getter can be positioned in a field free region of the tube.

US 5,509,045 discloses an X-ray tube. The X-ray tube comprises an evacuated envelope in which an anode, a cathode and a getter shield are disposed. With respect to the anode the getter shield is arranged behind the cathode. The getter shield

includes a sleeve and a cap. The cap defines an annular groove. A getter material is deposited in the groove and sintered to define a porous volume. The getter material is activated during normal exhaustion of the X-ray tube during manufacture. During operation of the X-ray tube, the waste heat is absorbed by the cap raising the getter material to its pumping temperature.

There may be a need for providing an X-ray tube, which allows for an effective removal of residual atoms and molecules. Summary of the Invention

This need may be met by the subject matter according to the independent claims. Advantageous embodiments of the present invention are described by the dependent claims.

According to a first aspect of the invention there is provided an X-ray tube comprising (a) an electron source, which is adapted to generate an electron beam projecting along a beam path, (b) a target anode, which is arranged within the beam path and which is adapted to generate an X-ray beam originating from a focal spot of the electron beam, and (c) an ion manipulation arrangement, which is adapted to deflect and to collect ions being generated from a collision of the electron beam with atoms and molecules being present within the beam path or within the space where scattered electrons occur. The ion manipulation arrangement comprises a collector electrode, which is chargeable in order to provide an electro -static attraction force for at least some of the ions and which is made at least partially from a getter material.

This aspect of the invention is based on the idea that the ion manipulation arrangement is capable of both actively attracting the ions and absorbing the ions. This has the effect, that ions being generated within the interior of an evacuated envelope of the described X-ray tube can be effectively removed from a wide region within the X- ray tube. By contrast to ion manipulation arrangements, which comprise usual electrodes only and which only convert the ion into the corresponding atom or molecule by adding or removing at least one electron to or from the ion, the described ion manipulation arrangement allows for a permanent removal of both the ions and the corresponding atoms or molecules from a vacuum chamber of the X-ray tube.

In case the X-ray tube is accommodated within an evacuated envelope, which has been permanently sealed after evacuation, the permanent removal of the ions contributes for maintaining a low residual pressure within the envelope for a long period of time. Further, the permanent removal of residual ions reduces the arcing rate.

An arcing might occur if the ions enter a high voltage region of the X-ray tube.

Further, the permanent removal of residual ions reduces the space charge compensation that influences the electron optic in an undefined manner.

Furthermore, the permanent removal of residual ions reduces an unintended ion bombardment of the electron source, in particular of an electron emitter filament of the electron source. Such a bombardment may happen if positive charged ions are attracted by the negative charged electron source and impinge onto the electron emitter filament. Since the electron emitter filament typically is a mechanically very sensitive element, such an ion bombardment may cause an accelerated deterioration of the electron source such that the life cycle of the X-ray tube may be reduced significantly.

The described X-ray tube can be understood as a modified getter pump. In this respect the electron beam, which is used for X-ray generation, is also used for ionizing atoms and molecules being present within the evacuated chamber of the X-ray tube. This means that the electron beam acts as an ionization means for residual atoms and molecules. Due to this ionization effect, the atoms and molecules can be attracted from a comparatively wide spatial region within the vacuum chamber. By contrast thereto, getter devices being used in known X-ray tubes can only absorb ions respectively atoms and molecules, when they accidentally impinge on the getter material. Therefore, the collection efficiency of the described collector electrode is significantly enhanced compared to know getter devices used for X-ray tubes. Preferably, the getter material is a metal such as titanium and/or aluminum-zirconium-alloy. Therefore, when the ions come into contact with the getter material they are immediately neutralized and can be permanently absorbed by the collecting electrode. Thereby, a complete removal from the vacuum chamber of the X- ray tube can be easily achieved.

The atoms and molecules are typically atoms and molecules of a residual gas situated within the evacuated chamber of the described X-ray tube. The residual gas is e.g. carbon monoxide, nitrogen, oxygen, water, a metal vapor and/or in particular hydrocarbons. According to an embodiment of the invention the ion manipulation arrangement comprises a further electrode. This may provide the advantage that in between the collector electrode and the further electrode a precisely defined electric field can be established. Thereby, the drift paths for the attracted ions, which drift paths end at the collector electrode, can be defined in a spatial reliable manner. This means that for each possible ion position within an interaction region of the ion manipulation arrangement, in particular within a region being defined by the spatial arrangement of the collector electrode and the further electrode, a predefined ion drift path is given.

At this point it has to be mentioned that the ion manipulation arrangement can also be provided with more than two electrodes. In general, the ion manipulation arrangement may comprise any multipole arrangement of electrodes, which is adapted to generate an attracting force to charged particles being situated in the interaction region of the ion manipulation arrangement.

According to a further embodiment of the invention the collector electrode is at a negative voltage level with respect to ground voltage and the further electrode is at a positive voltage level with respect to ground voltage. This may provide the advantage that both electrodes generate a force on charged particles being situated in the interaction region of the ion manipulation arrangement. Thereby, both forces act together in a supporting manner. For instance for negative charged ions the collector electrode exerts an attracting force onto the ions and the further electrode exerts a repelling force onto the negative charged ions.

It has to be mentioned that for generating the described dipole field both electrodes have to be connected to a supply voltage being different from ground voltage. This means that the described X-ray tube has to be provided with two isolator elements in order to electrically separate the two electrodes from a housing of the X-ray tube. In this respect it is further mentioned that also the target anode can be at ground level,

whereas the electron source has to be at a negative high voltage level in order to provide for the necessary electron acceleration voltage.

According to a further embodiment of the invention the X-ray tube further comprises a field electrode, which is arranged in between the electron source and the target anode. Thereby, there is defined (a) a first region extending between the electron source and the field electrode and (b) a second region extending between the field electrode and the target anode. The field electrode is adapted to be at substantially the same voltage level as the target anode.

In this respect it has to be mentioned that the field electrode comprises at least one small opening such that the electron beam may penetrate the field electrode without being attenuated.

In the described design of the X-ray tube the first region comprises the electric field being necessary for accelerating the electrons released from the electron emitter. The accelerated electrons leave the first region through the opening and enter the second region defining a field- free zone. Therein, the electrons may travel on a straight line towards the target anode.

The provision of a field- free second region has the advantage that within the second region there may be arranged electron beam manipulation devices such as an electron beam focusing optic and/or an electron beam deflection unit. These devices may employ an electric and/or a magnetic field for the respective electron beam manipulation. The beam focusing optic may be used for directing the electron beam onto a focal spot on the surface of the target anode, whereby the focal spot has a predetermined but variable shape and predetermined but variable dimensions. The electron beam deflection unit may be used to control the position of the focal spot. For instance the focal spot may be varied discretely between at least two predefined focal spot positions such that a multiple focal spot X-ray tube is realized.

According to a further embodiment of the invention the ion manipulation arrangement is located in the second region. This may provide the advantage that the above described ion manipulation can be carried out without being influenced by the electron acceleration field. Therefore, compared to the acceleration voltage relatively

small voltages are sufficient in order to provide a reliable deflection and collection of ions being present within the evacuated envelope of the described X-ray tube.

According to a further embodiment of the invention the ion manipulation arrangement is located close to the electron beam. In this respect the term "close" means that the electrical field of the ion manipulation arrangement at the location of the ionization is strong enough such that the produced ions are attracted to one electrode of the ion manipulation arrangement. Specifically, this ionization location is within the electron beam. This means that the ion attraction force is adapted to overcompensate thermal fluctuations of the ions, which thermal fluctuations typically scale with the absolute temperature.

The term "close" may further mean that there are no other elements of the X-ray tube arranged in between the ion manipulation arrangement and the beam path.

In case the ion manipulation arrangement is positioned within the substantial field- free second region, the electric field generated by the ion manipulation arrangement has to be stronger than a stray field of the electron acceleration field, which stray field unintentional enters the second region, which is pretended to be field- free.

If the ion manipulation arrangement comprises two electrodes, preferably these electrodes are arranged directly around the electron beam. Further, the collector electrode and the further electrode can be located close to optical electron beam focusing elements. That means that ions can be removed directly where they are produced i.e. near the electron beam and the region where scattered electrons exist. This may provide the advantage that ions will not enter the critical high voltage first region of the X-ray tube such that the arcing rate may be reduced significantly.

According to a further embodiment of the invention the X-ray tube is adapted to control the temperature of the getter material.

In this respect the term "control" has to be understood in wide meaning. Apart from controlling the temperature in a defined manner the term "control" also stands for any measure, which is adapted to impact, to modify and/or to manipulate the temperature of the getter material. This may provide the advantage that the efficiency of the used getter material can be improved by keeping the temperature of the collector electrode within predefined temperature limits.

According to a further embodiment of the invention the getter material is heated. Usually the optimal temperature range for improving the ion capturing capability of the getter material is higher than the average temperature being typically existing within X-ray tubes in use. Therefore, the efficiency of the getter electrodes can be improved significantly by heating up the getter material respectively the ion collector electrode.

According to a further embodiment of the invention the X-ray tube further comprises a heater control unit, which is adapted to actively heat up the getter material. Thereby, the active heating may be realized e.g. by means of heating wires being installed at and/or within the getter material.

The described active heating may allow for a very precise temperature control of the getter electrode. The temperature control can be accomplished either open loop or closed loop by using an appropriate temperature sensor.

According to a further embodiment of the invention the X-ray tube is designed to passively heat up the getter material. Thereby, a passive temperature raise of the getter material respectively the ion collector electrode may be realized by waste heat of the X-ray tube, in particular by waste heat originating from the target anode. The described passive heat up of the getter material may provide the advantage that the temperature raise can be accomplished without providing the ion collector electrode with additional electrical connections, which are used for powering heating wires being installed at and/or within the getter material. Therefore, by avoiding further cable connections the overall setup of the described X-ray tube will be comparatively simple such that the manufacturing costs for the described X-ray tube can be kept relatively low. According to a further embodiment of the invention the X-ray tube is designed to heat up the getter material by means of scattered electrons originating predominately from the focal spot on the target anode. Thereby, the scattered electrons are generated in the focal spot onto the surface of the target anode and, if the target anode is located in the essentially field- free second region, travel on substantially straight lines. When hitting a surface of the getter material respectively the ion collector

electrode, the electrons release their energy in the bulk material. During operation of the X-tray tube this causes an automatically heating up of the ion collecting electrodes.

The height of the temperature raise depends on the thermal contact of the ion collector electrode to any cooled surface of the X-ray tube. Therefore, a consciously designed small thermal contact will cause a comparatively large temperature raise. In other words, by providing an appropriate thermal conductivity between the ion collecting electrode and the cooled surface, an appropriate temperature range of the ion collecting electrodes can be selected.

The passive heat up of the ion collecting electrodes by means of scattered electrons has the advantage that the heating of the getter material immediately starts when the X-ray tube is put in operation

According to a further embodiment of the invention the X-ray tube further comprises a shielding element, which covers at least a predefined portion of the collector electrode from being hit by the scattered electrons. This may provide the advantage that the ion collector electrode respectively the getter material can be protected from an overheating such that the temperature will not exceed an upper temperature limit. Above such an upper temperature limit the ion absorbing efficiency of the getter material would be reduced.

The shielding of the ion collector may be realized by means of a simple protrusion, which provides for a shadowing effect of the scattered electrons. The protrusion may be formed within a tube type member surrounding the electron beam path.

It has to be mentioned that the shielding element can also be designed in such a manner that the particle radiation dose of the scattered electrons is reduced. In other words, the shielding element does not completely absorb and/or reflect the electrons being directed to a portion of the collector electrode; the shielding element only reduces the radiation dose acting on the collector electrode. This may provide the advantage that a damaging or an accelerated degeneration due to an extensive electron bombardment can be effectively avoided for all portions of the ion collector electrode. According to a further embodiment of the invention the X-ray tube is designed to heat up the getter material by means of thermal radiation and/or by means

of thermal conduction originating from the target anode and terminating at the getter electrode.

Thereby, the thermal radiation respectively the thermal conduction may originate in particular from the target anode, which typically represents the hottest element within the described X-ray tube. However, also other heat sources may be used for heating up the getter material by means of thermal radiation and/or thermal conduction. As has already been pointed out above, this may also provide the advantage that the heating of the getter material automatically starts when the X-ray tube is put in operation. According to a further embodiment of the invention the X-ray tube further comprises an attenuation element, which protects at least a predefined portion of the collector electrode from being irradiated by the thermal radiation and/or from being heated up by means of thermal conduction.

The provision of an attenuation element may also protect from an overheating of the collector electrode such that the temperature does not exceed an upper temperature limit. Therefore, the attenuation element can be arranged at any suitable position within the described X-ray tube.

According to a further aspect of the invention there is provided an X-ray system, in particular a medical X-ray imaging system like a computed tomography system. The provided X-ray system comprises an X-ray tube according to any one of the above-described embodiments.

This aspect of the invention is based on the idea that the above-described

X-ray tube may be used in an advantageous manner for various X-ray systems, in particular for X-ray systems being used for medical diagnosis. It has to be mentioned that the described X-ray system may also be used for other purposes than medical imaging. For instance the described X-ray system may also be employed e.g. for security systems such as baggage inspection apparatuses.

According to a further aspect of the invention there is provided a method for generating X-rays, in particular for generating X-rays being used for medical X-ray imaging like computed tomography. The provided method comprises using an X-ray tube according to any one of the above-described embodiments of the X-ray tube.

It has to be noted that embodiments of the invention have been described with reference to different subject matters. In particular, some embodiments have been described with reference to apparatus type claims whereas other embodiments have been described with reference to method type claims. However, a person skilled in the art will gather from the above and the following description that, unless other 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, in particular between features of the apparatus type claims and features of the method type claims is considered to be disclosed with this application. The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited.

Brief Description of the Drawings

Figure 1 shows a cross sectional view of an X-ray tube, which is equipped with an ion manipulation arrangement comprising two electrodes, wherein the electrodes are arranged laterally of an electron beam path.

Figure 2a shows a cross sectional view perpendicular to the electron beam axis of the ion manipulation arrangement depicted in Figure 1.

Figure 2b shows a cross sectional view of the ion manipulation arrangement depicted in Figure 1 , wherein several ion paths towards an ion collector electrode are indicated.

Figure 3a shows a cross sectional view of an ion manipulation arrangement, which is heated by scattered electrons originating from a focal spot on a target anode.

Figure 3b shows a cross sectional view of an ion manipulation arrangement, which is partially protected by a protrusion from receiving a bombardment of scattered electrons.

Figure 4 shows a simplified schematic representation of a computed tomography (CT) system according to an embodiment of the present invention, wherein the CT system is equipped with a multiple electron beam X-ray tube.

Figure 5 shows a schematic representation of a prior art X-ray tube comprising a comparatively large distance between an electron source and a target anode and a field- free region, wherein an electron optic is arranged.

Detailed Description The illustration in the drawing is schematically. It is noted that in different figures, similar or identical elements are provided with the same reference signs or with reference signs, which are different from the corresponding reference signs only within the first digit.

Figure 1 shows a cross sectional view of an X-ray tube 100. The X-ray tube 100 comprises an electron source 110 having an electron emitter 111. Upon heating up the electron emitter 111 is being capable of releasing electrons projecting along a beam path 115. For sake of clarity beam optics like a Wehnelt cylinder are not indicated in Figure 1.

The X-ray tube 100 further comprises a high voltage supply 112, which is connected to the electron source 110. The High Voltage supply charges the electron source 110 with a negative High Voltage -HV. A field electrode 130 is arranged in between the electron source 110 and a target anode 120. According to the embodiment described here, the field electrode 130 is a tubular element having different diameters. Therefore, in the cross sectional view depicted in Figure 1, the field electrode 130 has a rather sophisticated shape partially surrounding the electron source 110.

It has to be mentioned that the invention might be realized with different types of target anodes. In particular the target anode 120 can be either a rotatable anode or a stationary anode.

The field electrode 130 and the target anode are both connected to an electrical ground level with respect to a not depicted housing of the X-ray tube 100. Therefore, the whole region extending in between the electron source 110 and the target anode 120 can be divided into two regions, a first region 131 extending between the electron source 110 and the field electrode 130 and a second region 132 extending between the field electrode 130 and the target anode 120, respectively. The first region 131 comprises an electron acceleration field. The second region 132 defines a field- free or zero-field region. In the field- free region 132 there may be arranged an electron optic (not depicted) in order to properly focus the electron beam 115 onto the target anode 120. Thereby, there is defined a focal spot 121.

The released electrons are accelerated from the electron source 110 representing a cathode to the target anode 120. Thereby, they travel through an opening 130a and enter the field-free region 132. Upon impinging onto the target anode 120 they generate an X-ray beam originating from the focal spot 121.

Due to an imperfect vacuum inside the tube, atoms and molecules of the residual gas can be ionized by the electron beam 115 and by scattered electrons being reflected from the focal spot 121. Thereby ions 150 are generated.

In order to compensate for an overheating of certain components of the X-ray tube there are provided water-cooled sidewalls 135. According to the embodiment described here, the water-cooled sidewalls 135 are formed integrally with the field electrode 130. The described X-ray tube 100 is equipped with an ion manipulation arrangement 140. The ion manipulation arrangement 140 comprises two electrodes, an ion collector electrode 141 and a further electrode 142. Both electrodes 141, 142 are arranged directly next to the electron beam path 115. In this context the term "directly" means that there are no other elements located in between each of the electrodes 141, 142 and the electron beam path 115.

The ion collector electrode 141 is connected to a negative voltage -V with respect to a not depicted housing being at ground voltage level. The further electrode 142 is connected to a positive voltage +V with respect to ground level. Thereby, there is generated an electric dipole field in between the two electrodes 141, 142, which electric field attracts positive ions 150 to the ion collector electrode 141. The corresponding ion path is indicated with reference numeral 152.

It has to be mentioned that within a first section of the ion path 152 the ions 150 may be attracted by a residual electric field extending in between the electron source 110 and the target anode 120. However, when approaching the interaction region of the ion manipulation arrangement 140, the ions 150 will experience the electrical dipole field generated by the two electrodes 141, 142. Depending on the polarity of the ion 150 this causes an ion deflection towards one of the electrode 141, 142.

It has to be mentioned that the ion manipulation arrangement 140 may also comprise more than two electrodes or only one, the collecting electrode. For instance the ion manipulation arrangement 140 may be a monopole, a quadrupole, a pentapole, a hexapole or an octopole. In general, the ion manipulation arrangement can comprise any multipole arrangement of electrodes, which multipole arrangement is suitable to generate an attracting force to charged particles being situated in the interaction region of the ion manipulation arrangement 140. The two electrodes 141, 142 are both made from a getter material, which is capable of permanently absorbing the ions 150 when they come into contact with the respective electrode. The getter material is a metal such as titanium and/or aluminum- zirconium-alloy. Therefore, when the ions 150 come into contact with the getter material they are immediately neutralized. This makes a permanent absorption of the ions 150 much more easy. The permanent removal of the ions 150 contributes for maintaining a low residual pressure within the envelope for a long period of time. As a consequence a residual arcing rate caused by atoms and molecules entering the first electron acceleration region 131 is also reduced.

As can be seen from Figure 1, the ion collector electrode 141 is electrically coupled to a heater control unit 145. The same holds for the further electrode 142. However, for sake of clarity of the drawing the corresponding cables are

omitted in Figure 1. The heater control unit 145 is capable of powering a not depicted heat wires with a heat current. The heat current may be an AC or a DC current.

The heat wires are installed within or at the electrodes 141, 142, thus allowing the getter material of the two electrodes to be heated up. By heating up the electrodes the getter efficiency respectively the getter rate of the getter material can be enhanced significantly. By actively controlling the heat current being supplied to the two electrodes 141, 142, a harmful overheating of the getter material can be avoided.

By contrast thereto, known X-ray tubes, which also comprise an electric field- free region but which do not comprise the described ion manipulation arrangement 140, have the risk of an accelerated deterioration of the emitter structure. Such a deterioration may be caused by ions being generated within the electric field- free region e.g. by a collision of residual atoms and molecules with the electron beam. Some of these ions are accelerated towards the electron emitter. An electron optical systems being arranged within the electric field- free region may focus these ions, which then impinge onto the surface of the emitter in a small spot. This could damage the emitter structure and hence reduces the lifetime or lead to an immediate failure.

It has to be mentioned that the described ion bombardment induced deterioration of the electron source is also possible within standard X-ray tubes having no electric field- free region. However, especially high-end X-ray tubes with a high voltage acceleration region and a following electrical field-free region are characterized by this behavior.

In this context the described ion manipulation arrangement 140 can be interpreted as a reliable barrier for ions 150 from entering the high voltage acceleration region 131. The described X-ray tube is applicable to any field in which an ion bombardment onto an electron emitter has to be avoided to maintain a steady state operation of the X-ray tube. Additionally, the described X-ray tube it is applicable in electron beam tubes to reduce the arcing rate that is induced by ions.

Figure 2a shows a cross sectional view perpendicular to the electron beam axis of the ion manipulation arrangement 140 depicted in Figure 1, which is now denoted with reference numeral 240. The field electrode 230, which is connected to

electric ground level, has a tubular shape. In between the field electrode 230 there are arranged the ion collector electrode 241 and the further electrode 242. Both electrodes 241, 242 comprise the shape of an arc. The ion collector electrode 241 is connected to a negative voltage -V. The further electrode 242 is connected to a positive voltage +V. Electrical isolation elements 243 are provided in order to allow for a galvanic separation between the grounded field electrode 230 and the ion collector electrode 241 and the grounded field electrode 230 and the further electrode 242, respectively.

Figure 2b shows a cross sectional view of the ion manipulation arrangement 240. The electrical dipole field extending in between the two electrodes 241 and 242 causes an electrostatic force acting on the ions 250 being generated by the not depicted electron beam and by the not depicted scattered electrons. Several ion paths 252 towards the ion collector electrode 241 are indicated.

Figure 3a shows a cross sectional view of an ion manipulation arrangement 340 according to a further embodiment of the invention. In accordance with the embodiment described above, the ion manipulation arrangement 340 is also arranged within a tubular field electrode 330, which represents a water-cooled sidewall 335. The ion manipulation arrangement 340 comprises two electrodes, a collector electrode 341 and a further electrode 342, both made at least partially from a getter material.

By contrast to the embodiment described above, the getter materials are not heated up actively. The getter materials are rather heated up in a passive way. In particular, the electrodes 341 and 342 are heated up by a particle radiation comprising electrons. According to the embodiment described here, these electrons are generated by a backscattering of electrons, which backscattered electrons are generated in the focal spot on the target anode and which backscattered electrons travel on straight lines within the electrical field- free region. These scattered electrons are indicated with the arrows 322. When hitting a surface they release their energy in the bulk material of the electrodes 341, 342.

It has to be mentioned that the electrodes 341, 342 can also be heated up by thermal radiation and/or thermal conduction in between the target anode and the electrodes 341, 342.

Exceeding a temperature limit that depends on the used getter material decreases the ion getter rate. Therefore, it is advantageous to avoid overheating.

Figure 3b shows a principle possibility to limit the energy release for a fixed geometry of the ion manipulation arrangement 340. A shielding element 337 covers a defined portion of the electrodes 341 and 342 and hence, the number of electrons impinging onto the electrons 341 and 342 is reduced. Therefore, with respect to the embodiment shown in Figure 3 a the maximum temperature of the electrodes 341 and 342 decreases.

According to the embodiment described here the shielding element is a simple protrusion being formed at the tubular field electrode 330. However, it is clear that also other types of shielding elements might be used in order to achieve the same technical effect of reducing the intensity of scattered electrons and/or thermal radiation.

It has to be mentioned that due to reaching the maximum temperature not before the end of an application load, the passive heating by scattered electrons 322 is less exact than the above described active heating with heating wires. However, the effort for realizing the heating is much less for the passive alternative, which might be accomplished also by thermal radiation and/or by thermal conduction.

Figure 4 shows a computer tomography (CT) apparatus 470, which is also called a CT scanner. The CT scanner 470 comprises a gantry 471, which is rotatable around a rotational axis 472. The gantry 471 is driven by means of a motor 473. Reference numeral 475 designates a source of radiation such as an X-ray tube, which emits polychromatic radiation 477. The X-ray tube 475 is an X-ray tube corresponding to any of the above-described embodiments.

The CT scanner 470 further comprises an aperture system 476, which forms the X-radiation being emitted from the X-ray tube 475 into a radiation beam 477. The radiation beam 477, which may by a cone-shaped or a fan-shaped beam 477, is

directed such that it penetrates a region of interest 480a. According to the exemplary embodiment described herewith, the region of interest is a head 480a of a patient 480. The patient 480 is positioned on a table 482. The patient's head 480a is arranged in a central region of the gantry 471, which central region represents the examination region of the CT scanner 470. After penetrating the region of interest 480a the radiation beam 477 impinges onto a radiation detector 485. In order to be able to suppress X-radiation being scattered by the patient's head 480a and impinging onto the X-ray detector 485 under an oblique angle there is provided a not depicted anti scatter grid. The anti scatter grid is preferably positioned directly in front of the detector 485. The X-ray detector 485 is arranged on the gantry 471 opposite to the X- ray tube 475. The detector 485 comprises a plurality of detector elements 485a wherein each detector element 485a is capable of detecting X-ray photons, which have been passed through the head 480a of the patient 480.

During scanning the region of interest 480a, the X-ray source 485, the aperture system 476 and the detector 485 are rotated together with the gantry 471 in a rotational direction indicated by an arrow 487. For rotation of the gantry 471, the motor 473 is connected to a motor control unit 490, which itself is connected to a data processing device 495. The data processing device 495 includes a reconstruction unit, which may be realized by means of hardware and/or by means of software. The reconstruction unit is adapted to reconstruct a 3D image based on a plurality of 2D images obtained under various observation angles.

Furthermore, the data processing device 495 serves also as a control unit, which communicates with the motor control unit 490 in order to coordinate the movement of the gantry 471 with the movement of the table 482. A linear displacement of the table 482 is carried out by a motor 483, which is also connected to the motor control unit 490.

During operation of the CT scanner 470 the gantry 471 rotates and in the same time the table 482 is shifted linearly parallel to the rotational axis 472 such that a helical scan of the region of interest 480a is performed. It should be noted that it is also possible to perform a circular scan, where there is no displacement in a direction parallel to the rotational axis 472, but only the rotation of the gantry 471 around the

rotational axis 472. Thereby, slices of the head 480a may be measured with high accuracy. A larger three-dimensional representation of the patient's head may be obtained by sequentially moving the table 482 in discrete steps parallel to the rotational axis 472 after at least one half gantry rotation has been performed for each discrete table position.

The detector 485 is coupled to a pre-amplifier 488, which itself is coupled to the data processing device 495. The processing device 494 is capable, based on a plurality of different X-ray projection datasets, which have been acquired at different projection angles, to reconstruct a 3D representation of the patient's head 480a. In order to observe the reconstructed 3D representation of the patient's head 480a a display 496 is provided, which is coupled to the data processing device 495. Additionally, arbitrary slices of a perspective view of the 3D representation may also be printed out by a printer 497, which is also coupled to the data processing device 495. Further, the data processing device 495 may also be coupled to a picture archiving and communications system 498 (PACS).

It should be noted that monitor 496, the printer 497 and/or other devices supplied within the CT scanner 470 might be arranged local to the computer tomography apparatus 470. Alternatively, these components may be remote from the CT scanner 470, such as elsewhere within an institution or hospital, or in an entirely different location linked to the CT scanner 470 via one ore more configurable networks, such as the Internet, virtual private networks and so forth.

It should be noted that the term "comprising" does not exclude other elements or steps and the "a" or "an" does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.

In order to recapitulate the above described embodiments of the present invention one can state:

It is described an X-ray tube 100 comprising an ion manipulation arrangement 140 having at least one ion collector electrode 141. The ion collector electrode 141 is made at least partially from a getter material. The ion manipulation

arrangement 140 is in particular beneficial for high-end X-ray-tubes including an electrical field- free region 131. The ion manipulation arrangement 140 produces an electrical field, which deflects ions 150. When impinging onto the getter electrode 141 the ions 150 are permanently collected and thus removed from the interior of an evacuated envelope of the X-ray tube 100. This avoids ion bombardment on an electron emitter 111 of the X-ray tube 100. Additionally the arcing rate caused by residual gas can be reduced significantly. A heating of the getter material may be realized with heating wires or by a defined bombardment of scattered electrons 322 onto the electrodes 341, 342 comprising the getter material.

LIST OF REFERENCE SIGNS:

100 X-ray tube

110 electron source

111 electron emitter filament

112 high voltage supply

115 electron beam / beam path

120 target anode

121 focal spot

125 X-ray beam

130 field electrode

130a opening

131 first region / electron acceleration region

132 second region / field- free region

135 water cooled sidewalls

140 ion manipulation arrangement

141 collector electrode

142 further electrode

145 heater control unit

150 ion

152 ion path to ion collector electrode 141

230 field electrode

240 ion manipulation arrangement

241 collector electrode

242 further electrode

243 electrical isolation element

250 ion

252 ion path to collector electrode 241

322 scattered electrons

335 water cooled sidewalls

337 shielding element / protrusion

330 field electrode

340 ion manipulation arrangement

341 collector electrode

342 further electrode

470 medical X-ray imaging system / computed tomography apparatus 471 gantry

472 rotational axis

473 motor

475 X-ray source / X-ray tube

476 aperture system 477 radiation beam

480 object of interest / patient

480a region of interest / head of patient

482 table

483 motor 485 X-ray detector

485a detector elements

487 rotation direction

488 Pulse discriminator unit 490 motor control unit 495 data processing device (incl. reconstruction unit)

496 monitor

497 printer

498 Picture archiving and communication system (PACS) 500 X-ray tube 510 electron source

511 electron emitter filament

512 High vo ltage supply

515 electron beam / beam path

520 target anode 521 focal spot

522 scattered electrons

530 field electrode 530a opening

531 first region / electron acceleration region

532 second region / field- free region 536 electron optic