FIELD OF THE INVENTION

The invention relates to X-ray tubes. Furthermore, the invention relates to a medical imaging system having an X-ray tube arranged therein and a method of controlling a beam of charged particles in an X-ray tube. BACKGROUND OF THE INVENTION

Many analytical professions have come to rely more and more on high quality image material. Experts in the medical field but also in the fields of material sciences or engineering are able to adduce a remarkable number of clues from high resolution images. Industry reciprocates these needs by providing highly dedicated imaging equipment such as high performance X-ray imaging systems or computer tomography CT systems.

As a demand for high resolution images is ever increasing, particularly in the medical field, industry has provided X-ray imaging equipment capable of producing high flux X-ray radiation.

High flux X-ray radiation is produced by high-performance X-ray tubes. Basic components of X-ray tubes are a cathode and an anode spaced apart by a cathode- anode distance and arranged in a housing. The cathode is also sometimes referred to as electron emitter. Electrons emitted through thermionic emission by the cathode are accelerated by a high tube voltage towards a target on or close to the anode. The higher the tube voltage, the higher the velocity of the electrons and, as a consequence, the higher the frequency of the radiated X-ray radiation. High frequency X-ray radiation may be useful in the production of high resolution pictures, as such radiation may penetrate also thick objects in order to provide information about their inner structures and as furthermore the high frequency allows definition of more tightly packed impact points on a detector screen of a detector resulting in pictures with higher pixel resolution. Furthermore, it may be preferable to have a high flux of X-ray radiation e.g. in order to reduce illumination periods to avoid blurring when imaging moving objects. High-flux radiation may be produced using a high flow electron beam of the X-ray tube.

However, nearly 99 % of the incident electron beam power at the anode or at the target is usually converted into heat which needs to be dissipated from the anode to prevent damage or extensive wear. Addressing the heat dissipation issued led to the design of X-ray tubes having an extended cathode-anode distances (about 10 cm). Those extended cathode-anode distances however may render hitherto used electrostatic focusing devices for controlling the electron beam unsuitable. Advanced X-ray tube designs have been suggested having more sophisticated focusing devices put into place most notably magnetic coils also called magnetic lens systems. Magnetic lens systems allow controlling the electron beam and thus its focal spot size, shape and position. There may be however an issue with X-ray tubes having extended cathode-anode distances. First there may be a higher chance the electron beam hitting the tube housing. This may cause damage on the enclosing housing. The operation at high beam currents (ca. 1 Ampere) may increase the risk of tube damage through electron beam irradiation of the tube housing. The risk of damages may be particularly increased at the occurrence of electric arcs. Electric arcs are discharges of high luminosity current. Those high luminosity and thus high temperature discharges may be formed when the tube voltages jumps a cathode-housing distance or the cathode-anode distance.

Second, the focusing conditions may be determined by the coil currents of the magnetic focusing system and may need to be adjusted depending on the tube voltage. In an event of arcing, the focusing may not adapt to the falling voltage as it usually the case in simpler tube designs using electrostatic focusing. As a result, the electron beam may irradiate on parts of the tube housing, when the focusing conditions are not adapted in the event of tube voltage failure.

Previous attempts to address the tube damage issue in the event of electric breakdown phenomena such as electric arcing have been unsatisfactory for the following reasons.

As mentioned earlier the electron beam is based on thermionic emission at the cathode. The thermal response times are typically in the range of tens of milliseconds, given by the thermal properties of the emitter. Therefore the thermionic emitter may continue to emit electrons, even when the heating current is switched off instantly. During an arc an electric field retains, since the usual time of the voltage breakdown is in the range of several milliseconds, depending on the capacitance and load resistance of the supplying voltage cables.

There may therefore be a need in the art for better controlling the electron beam in an X-ray tube. More particularly, there may be a need to control the electron beam in the event of electric breakdown or voltage failure in the tube or magnetic lens system.

There may also be a need to keep maintenance costs in the expensive imaging systems and outage times at a minimum and to make the systems more robust against the occurrence of electric breakdown phenomena such as electric arcs.

SUMMARY OF THE INVENTION

The present invention addresses the above need by providing a medical imaging system comprising X-ray tube cathode and an anode. An accelerator voltage source provides voltage suitable for establishing a beam of charged particles such as an electron beam between the cathode and the anode. A switchable grid is arranged between the cathode and the anode. There is further provided a switch for switching the grid between an ON state and an OFF state. When the grid is switched into the ON state the grid blocks the electron beam between the cathode and the anode. In other words, in ON state the electrons may be prevented from reaching for example the target , tube housing or any other pre-determined point of impact.

There is further arranged a sensor suitable for sensing a critical condition or parameters in relation to the critical condition. The sensor is in communication with a control unit. The control unit is operative in switching the switch to effect the blocking of the beam upon the sensor sensing the critical condition. The present invention may be seen as based on the following idea: In an

X-ray tube, an additional grid is provided at a location between the cathode and the anode. The grid may be placed close to the cathode. Furthermore, one or more sensors are provided which may sense a critical condition within the X-ray tube. Such critical condition may be a condition for which fast switching-off of the electron beam may help to prevent damages to the X-ray tube. For example, a critical condition may be an abrupt voltage drop of the accelerator voltage between cathode and anode. In unipolar tubes in which the housing is on a same potential as the anode, this may correspond to a voltage drop between the housing and the cathode. Alternatively, a critical condition may be an arcing occurring within the tube. Upon sensing such critical condition, a control unit is adapted to actuate a switch thereby applying a suitable negative voltage to the grid in order to block the electron beam coming from the cathode. Accordingly, upon detecting any failure within the X-ray system, the electron beam will be prevented from uncontrollably impacting e.g. on the housing thereby damaging same. Preferably, the switching of the grid will be performed as fast as possible, e.g. in a time span of less than lOOμs, preferably less than 10 μs, in order to minimize any negative influence of an uncontrolled electron beam.

By having the switchable grid arranged in communication with the sensor suitable for sensing the critical condition allows for a more reliable control of the electron beam in the event of the critical condition being sensed. A group of sensors may be provided, each sensor in the group being suitable to sense different ones of a number critical conditions that are known to cause electric breakdown phenomena such as electric arcing in the imaging system or in the X-ray tube.

According to one exemplary embodiment of the present invention the system further comprises a grid voltage supply for supplying a blocking voltage. The blocking of the electron beam is effected by applying a negative blocking voltage through the switch and to the switchable grid. The blocking voltage may be chosen such that the electron emission from the cathode or emitter is effectively suppressed. Therein, the blocking voltage may be applied such that the grid is on a more negative potential than the cathode thereby repelling electron emitted at the cathode. The magnitude of the blocking voltage may be chosen depending on a position or geometry of the grid. According to a further exemplary embodiment of the present invention, a voltage ratio of the blocking voltage and the accelerator voltage corresponds to a distance ratio of the grid-cathode distance and an acceleration distance on which electrons emitted at the cathode are accelerated on their way to the target. Therein, the acceleration distance may correspond to an anode-cathode distance.

Having such distance-voltage-relationship, the grid may be arranged at a position where it does substantially not influence the electrical field between the cathode and the anode during an OFF state of the grid. In such OFF state, the grid is set on a positive potential with respect to the cathode which positive potential essentially corresponds to the potential between cathode and anode at the position of the grid. The positive potential may be obtained by applying a passing voltage +U _P between the cathode and the grid. Upon sensing a critical condition, the grid may be set on a negative potential with respect to the cathode by simply switching the applied voltage from + U _p to - Up = Ub (blocking voltage). Such negative potential on the grid generates a repelling electrical field at the emitter surface which suppresses electron emission from the emitter thus preventing electrons from traversing the grid. According to a further exemplary embodiment of the present invention, the voltage supply is adapted such that, for a non-blocking state of the grid, a voltage U _p is applied to the grid such that an electrical field generated between the cathode and the anode is substantially not influenced by the grid. In other words, when the grid shall let the electron beam passed therethrough, a voltage U _p may be applied to the grid such that the electrical potential of the grid essentially corresponds to the potential of the electrical field between the cathode and the anode at the location of the grid.

According to another exemplary embodiment of the present invention the switch is arranged as a fast switching voltage driver. The switching voltage driver allows for further cutting down on response time when switching the grid to block the electron beam in order to either prevent electric arcing to occur to minimize damage to the X-ray tube in the event the electric arc occurs. Switching periods may be reduced down to less than lOOμs preferably less than lOμs. Examples of high voltage drivers can be found in the applicant's copending international patent application WO 2008/099328 A2, which is incorporated herein by reference. According to a further exemplary embodiment of the present invention, there is arranged a magnetic lens for focusing the beam on a target and wherein the critical condition comprises a failure of or in the magnetic lens. By inclusion of conditions within the magnetic lens into the set of critical conditions sensed by the sensor there may be provided an even higher degree of control of the electron beam. The occurrence of a rampant electric beam impacting other parts of or in the X-ray tube than the pre-determined impact points can thus be minimized.

According to another exemplary embodiment of the present invention the switchable grid is arranged in a manner so that a grid-cathode distance is adjustable. Adjusting of the distance can be effected either manually or automatically for example by means of the control unit. The adjustability of the grid-cathode distance may allow to better address the trade-off between having the grid exposed to heat emissions from the cathode on the one hand side and on having the grid positioned as closely possible to the cathode on the other hand side. The close proximity allows for comparatively small blocking voltages when the grid is in the ON state. The close proximity also allows for small pass voltages when the grid is in the OFF state resulting in minimal impact on the electron beam when in OFF state.

The adjusting of the grid-cathode distance can be made for example responsive to the heat emitted from the cathode and sensed by the sensor. This may allow operation of the imaging system at different levels of cathode voltages corresponding to heat emission at different temperatures. The adjustability of the grid-cathode distance also allows operating the medical imaging with different types of switchable grids having different heat resistances. A maintenance overhead for the medical imaging system can be thus kept to a minimum as a larger selection of spare grids are available. Outages times of the highly expensive medical imaging system can also be minimized as one is freed from the constraint of having to rely on grids having a specific heat resistance.

According to another aspect of the present invention there is provided a method of controlling a beam of charged particles such as an electron beam in an X-ray tube. The method comprises the steps of: sensing a critical condition in relation to the X- ray tube and, in response to sensing the critical condition, switching on/off a switchable grid to effect the blocking of the beam.

According to another aspect of the present invention there is provided a program element configured and arranged to control a beam of charged particles according to the above described method of controlling the electron beam.

According to another aspect of the present invention, there is provided a computer-readable medium having stored thereon the above described program element. Finally, features of the invention may be expressed in other words as follows: For X-ray tubes described, a grid is placed in front of the emitter. When arcing has been sensed, (or any other tube parameters are irregular) an appropriate voltage is applied to the grid such that the grid forms a potential barrier for the emitted electrons and blocks the beam. By using fast switching voltage drivers ('grid-switch'), the grid voltage can be applied to the grid within microseconds and is therefore far quicker than other methods, which control the beam during tube voltage failure. In particular, the fast switchable grid may ensure that the electron beam is blocked during the transient of tube-voltage breakdown.

It has to be noted that aspects and embodiments of the present invention have been described with reference to different subject-matters. In particular, some embodiments have been described with reference to the method type claims whereas other embodiments have been described with reference to apparatus 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. BRIEF DESCRIPTION OF THE DRAWINGS Features and advantages of the present invention will be further described with respect to specific embodiments as shown in the accompanying figures but to which the invention shall not be limited.

Fig. 1 shows an X-ray imaging system having an X-ray tube according to the present invention. Fig. 2 is a cross-sectional view of an X-ray tube according to one exemplary embodiment of the present invention, and Fig. 3 shows a graph representing relationships between an accelerator voltage and a blocking voltage in an X-ray tube according to one embodiment of the present invention.

Features shown in the drawings are schematic and are not to scale.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows the basic components of an exemplary X-ray imaging system 100 in a form of a computer tomography system as used in medical facilities. The X-ray imaging system comprises an examination table 130 suitable for positioning an object, for example a patient, of which projection images are to be taken.

The X-ray imaging system 100 further comprises a rotatable gantry 105 suitable for rotation around the examination table 130, the examination table 130 being substantially arranged in the centre of said gantry 105. The X-ray imaging system 100 further comprises an X-ray tube 120 and a detector 110. The X-ray tube 120 and the detector 110 are diametrically arranged on the gantry 105. During an image acquisition phase the gantry 105 rotates around the examination table 130 while the X-ray tube 120 emits X-rays. The emitted X-rays interact with the object deposited on the examination table 130 and the interacting X-rays are then incident on the detector 110. The incident X-rays define a pattern of points of intensities which are digitally transformed into a corresponding pattern of pixels. The pattern of pixels is then available as the projection image of the examined object. The digital projection image can then be stored and/or post-processed, such as volume-rendered, by suitable software to be viewable on a monitor.

Fig. 2 shows a simplified cross-sectional view of an X-ray tube 120 according to an embodiment of the present invention.

The X-ray tube 120 comprises a cathode 210 and an anode 230 which at the same time acts as a target. The cathode 210 and the anode 230 are enclosed in a housing 122. The housing 122 is on a same electrical potential as the anode

230. Electrons emitted at the cathode being on negative potential are accelerated along an acceleration distance AC towards the grounded anode 230. Therein, the acceleration distance AC which may be defined between an electron emitting surface of the cathode 210 and a portion of the housing 122 as indicated in fig.2. This portion of the housing

122 may correspond to a beginning of a cylindrical portion of the housing 122. Beyond this portion of the housing 122, there is a field free space, i.e. the electron beam is not further accelerated on its path as there is essentially no accelerating electrical field.

The X-ray tube 120 further comprises a grid 200 arranged between the cathode 210 and the anode 230. The grid is set a distance apart from the cathode 210, called the grid-cathode distance GC which may be defined between an electron emitting surface of the cathode 210 and an electron repelling surface of the grid 200.

In operation, the X-ray tube 120 emits X-rays. In producing the X-rays a cathode voltage Uc is applied to the cathode. The cathode voltage Uc causes the cathode 210 to heat up where upon the cathode 210 releases electrons by thermionic emission. There is further applied an accelerator voltage Ua between the cathode

210 and the anode 230 or the housing 122 being on the same potential as the anode 230. The accelerator voltage Ua causes the emitted electrons to accelerate towards the anode 230 thus establishing an electron beam 215. The electron beam is incident with a high energy on the anode target 230 at a predetermined focal point. Upon incident on the target, the electrons are decelerated. The drop in kinetic energy causes X-rays 121 to be released as "Bremsstrahlung". The X-rays may exit the housing 122 through a window

123 and may then be available for the actual image acquisition.

In order to control or guide the beam of electrons there is further arranged a magnetic lens 220. The electron beam is guided by magnetic fields produced by the magnetic lens. The magnetic fields result from applying a magnetic lens voltage Um generating an electrical current in coils arranged in the magnetic lens 220. Preferably, the magnetic lens is arranged adjacent to a field free portion of the electron beam path. The guiding magnetic fields depend on the applied magnetic lens voltage and the magnetic lens voltage Um is regulated in relation to the applied accelerator voltage Ua.

A control unit 250 is in communication with a sensor 240 or a plurality of sensors 240 arranged at suitable positions in or around the X-ray tube 120. The sensor 240 is arranged to sense parameters that are known to be related to a critical condition in the X-ray tube 120. The positions for the sensor 240 are suitable if the positions allow sensing such parameters. A critical condition is a condition that can be expected to lead to damages in the X-ray tube 120 or in the X-ray imaging system 100. An example of a critical condition is a electric breakdown resulting in an occurrence of an electric arc in the X-ray tube 120. Electric arcs are known to be hard to control as they may be incident not only on the anode 230 or the target but also on points of the in generally metallic X-ray tube housing 122. The extremely high temperatures incurred by the electric arc may lead to the X-ray tube housing 122 being damaged to such a degree that the whole X-ray tube 120 needs to be repaired or even replaced.

In the X-ray tube 120 a grid 200 is arranged between the cathode 210 and the anode 230. The grid 200 may be arranged as a plate having an aperture. The aperture is aligned with the cathode 210 and corresponds to a cross section of the electron beam 215 at the position of the grid.

The grid 200 has an ON and an OFF state. When in the OFF state, the electrons of the electron beam 215 can pass the grid 200 whereas the electrons are blocked when the grid is in an ON state.

The switching between the ON and OFF states is effected by switching the grid 200 between a negative blocking Voltage Ub and a positive pass voltage Up having the same voltage value at the opposite polarity. In effect, the grid 200 allows gating the electrons of the electron beam 215 in simple manner.

The grid 200 is switchable by means of a switch 260. The switch 260 is arranged as a fast switching voltage driver to implement highly responsive switching times in the range of microseconds. Examples of high voltage drivers can be found in the applicant's international patent application WO 2008/099328 A2.

The switch 260 is operated by the control unit 250 in dependence on the parameters sensed by the sensor 240.

Having the grid 200 arranged switchably and switching the grid 200 by means of the high speed switch 260 in dependence on the parameters associated with the critical condition allows for better control of rampant electron beams and/or electric arcs. It also allows keeping the damage incurred by the occurrence of an electric arc at a minimum.

Parameters associated with the critical condition are for example the accelerator voltage Ua and/or the magnetic lens voltage Um. Alternatively, the corresponding currents may be sensed by the sensor 240. Another option is to sense heat build-up at the focal spot, the target or the anode. In any case, the sensor will need to be arranged as a transducer suitable for sensing the parameter.

According to one alternative, the parameters are to taken to be rates of changes in the magnetic lens voltage Um and/or the accelerator voltage Ua or the corresponding currents. According to this alternative the control unit 250 monitors the sensed voltages as a sequence of voltages values. The control unit 250 applies an electronic differentiator such as a filter to the sequence to obtain the rate of changes of the voltages and/or the currents.

The operation of the control unit 250 is now explained in more detail. The control unit 250 senses in step SlO the parameters associated with a critical condition. If the sensed parameters or a derived parameter for example a weighted sum or average of the sensed parameters exceeds a threshold value the control unit 250 issues a switch command to the switch 260 in order to switch in step S20 the grid 200 in to the ON state. The switching is affected by applying the negative blocking voltage Ub to the grid 200.

The grid 200 should be arranged in such a manner such as not to appreciably influence the electron beam when the grid 200 is in the OFF state. This may be addressed by having the grid 200 arranged as close to the cathode 210 as the heat resistance of the grid 200 permits. The close proximity allows having the switch 260 float on the accelerator voltage Ua as can be seen in Fig. 3.

The ratio between the accelerator voltage Ua and the blocking voltage Ub corresponds to the ratio between the grid-cathode distance GC and the anode-cathode distance AC. The relationship between the voltages and the distances can also be seen in Fig. 3. The X-ray tube 120 according to the present invention allows switching the grid into the ON state by the application of the relatively low negative blocking voltage Ub in relation to the comparatively high accelerator voltage Ua. Correspondingly, it is possible to switch back into the OFF state by applying the pass voltage Up by simply reversing the polarity. As the blocking voltage Ub has a low voltage value, so does the pass voltage Up in the OFF state. Accordingly, while an effect of the grid 200 being in OFF state may be negligible such that it does not disturb the electrical field between the cathode and the anode, the actually switched voltage may be as small as a few kilo volts, preferably less than 10 kV, compared to the accelerator voltage of e.g. more than 100 kV.

The X-ray tube 120 according to the present invention allows adjusting in step S30 the grid cathode distance GC in order to have such a favourable low voltage ratio and it allows on the other hand choosing a grid cathode distance GC to better protect the gate 200 from the heat dissipated by the cathode 210.

The control unit 250 is arranged to communicate with heat sensors (not shown) arranged at the grid 200. Responsive to the heat sensed by the heat sensors the control unit 250 adjusts or readjusts the grid-cathode GC distance. This automatic and dynamic adjusting of the grid-cathode distance GC allows operation of the X-ray tube 120 at different cathode voltages Uc. It further allows operating the X-ray tube 120 with different grids made of different materials having different heat resistances.

Summarizing, features of specific embodiments of the invention may be expressed in other words as follows: In an X-ray tube with a rotating anode, a grid is placed in front of the cathode which is electrically insulated from the cathode. The grid voltage is supplied by a electronic circuit, which consists of a voltage driver and the switching circuit permitting to switch the grid voltage in short times between - U _b and + Ub = Up. The grid switch voltage driver floats at the tube voltage potential, such that a voltage of a few kV is needed. During normal operation the grid is positively biased with respect to the cathode, allowing the electron beam to pass towards the anode. When arcing occurs, a tube voltage sensor triggers the grid switch driver to switch the voltage from + U _b to - U _b , hereby blocking the electron beam. The fast gating of the electron beam can also be triggered by other signals, which sense irregular operation parameters of the tube assembly, e.g. failure of the magnetic focusing system or overheating in the focal spot. It should be noted that the term "comprising" does not exclude other elements or steps and that the indefinite article "a" or "an" does not exclude the plural. Also elements described in association with different embodiments may be combined.lt should also be noted that reference signs in the claims shall not be construed as limiting the scope of the claims.

LIST OF REFERENCE NUMERALS

100 x-ray imaging system

105 gantry

110 detector

120 x-ray tube

121 X-ray radiation

122 x-ray tube housing

123 window

130 examination table

200 grid

210 cathode

215 electron beam

220 magnetic lens

230 anode

240 sensor

250 control unit

260 switch

Uc cathode voltage

Ua accelerator voltage

Um magnetic lens voltage

Ub blocking voltage

Up pass voltage

GC grid-cathode distance

AC acceleration distance

SlO sensing

S20 switching

S30 adjusting