FIELD OF THE INVENTION

The present invention relates to an X-ray tube and an X-ray generation system.

BACKGROUND OF THE INVENTION

Some X-Ray tubes for medical CT imaging use electrostatic electron beam forming in combination with temperature limited emission. The emission current depends on the fdament temperature and electrical heating is typically used to control the emission current. The electron beam is steered by electrical grids for focal spot sizing and positioning. The control of the emission via the temperature of the fdament is a relatively slow process because the heating and cooling of the fdament requires a certain amount of time. This slow emission, via control of the current flowing through the fdament, is a problem for some applications. Although the emission is controlled via the fdament temperature, the tube voltage and the voltage of the grids also impact the emission.

Slow emission current control is a severe burden for rapid kVp switching (kVp-S) where the tube voltage is switched between consecutive acquisition intervals (e.g. 80 kV and 140 kV). The X- Ray generation is much more efficient for high voltages. The difference of the X-Ray output dose between 80 kV and 140 kV of a tube at the same fdament temperature may easily become as large as a factor of 7. For spectral imaging the flux of the low kV and high kV intervals should be roughly the same to obtain good spectral material separation. The imbalance of the flux between low and high kV can be partly compensated with longer integration periods for low tube voltages. However, it is not possible to rapidly increase the emission current to lead to an associated increase in fdament temperature and emission current, because such fdament heating is too slow.

There is a need to resolve this issue.

SUMMARY OF THE INVENTION

It would be advantageous to have an improved X-ray tube for rapid KVp switching between different high voltages. The invention is defined by the independent claims, wherein further embodiments are defined by the dependent claims.

In a first aspect, there is provided an X-ray system comprising a controller, and an X-ray tube having an anode and a cathode, the cathode comprising: an electron emitter fdament; a first pair of grids; and a second pair of grids.

The first pair of grids are located at opposite sides of the electron emitter filament. The second pair of grids are located at opposite sides of the electron emitter filament, closer to the electron emitter filament than the first pair of grids, and with longitudinal directions parallel to a longitudinal direction of the electron emitter filament. The controller is configured to control a voltage or an average voltage of the first pair of grids to be different to a voltage of the second pair of grids. The controller is configured to control the voltage or the average voltage of the first pair of grids to regulate steering and/or focusing of an electron beam between the electron emitter filament and the anode. The controller is configured to control the voltage of the second pair of grids to regulate an emission current magnitude between the electron emitter filament and the anode.

In this manner a system with a new design and control of a cathode of an X-ray tube has been developed, where an extra set of grids are provided inside of a normal pair of grids around filament that can be controlled at a different potential to the normal grids. In this way, the voltages of the grids can be optimized, with respect to a high voltage between the cathode and an anode of the X-ray tube, to rapidly switch, e.g. to increase, the electron emission from the filament. The emission current is not completely independent of the focal spot focusing voltage over the first pair of grids, but the influence is much less than in a design with only one pair of grids. The emission current is therefore mainly controlled by the voltages at the surfaces located very close to the filament, i.e. the voltages of the second pair of grids. The focal spot size and/or position is mainly controlled by the electric field within the whole cathode cup, i.e. generated by the first pair of grids. Advantageously, the grids of the first pair of grids may be larger in size than the grids of the second pair of grids. Preferably, the length of each of the electrodes in the first and second pair of grids are at least equally long as the length of the filament. The grids of the second pair of grids are lined up such that the length directions of the grids are parallel or relatively parallel to the length direction of the filament. Along the length axis of the filament, the minimum distance to a length edge/side of a grid of the second pair of grids is essentially constant. In this description and in the claims, the notion “parallel” allows for a deviation compared to perfectly parallel. If 0 mm deviation means perfectly parallel, the allowed deviation is preferably less than 2 mm, more preferably less than 0.5 mm and even more preferably less than 0.05 mm. Neither the length directions of the grids of the first pair of grids nor the grids of the second pair of grids are perpendicular to the length direction of the filament.

The improved system means that when switching between a first high voltage of the X- ray tube (for example 140 kV) to a second high voltage of the X-ray tube (for example 80 kV) the electron emission from the filament can be increased over that normally achievable during the 80 kV operation, without increasing heating current flow through the filament to increase the thermal emission (that would incur a time lag). This means that voltage switching of the grids around the filament in combination with switching of the voltage between the cathode and anode can be utilized to provide a closer match between the X-ray flux produced by the X-ray tube during both the 80 kV mode of operation and the 140 kV mode of operation, because the electron emission during the 80 kV operation can be boosted.

In an example, the controller is configured to control the voltage of the grids of the first pair of grids such that the grids are at different voltages to each other. In this way, the focal spot can be moved from a central position.

In an example, the controller is configured to control the voltage of the grids of the second pair of grids such that the grids are at the same voltage to each other. Having the grids of the second pair of grids at the same voltage is beneficial for regulating the emission current.

In an example, the controller is configured such that the voltage or the average voltage of the first pair of grids is greater than the voltage of the second pair of grids.

In an example, the controller is configured to control the voltages such that the voltage or the average voltage of the first pair of grids is two times greater than the voltage of the second pair of grids.

In an example, the controller is configured to control the voltages such that the voltage or the average voltage of the first pair of grids is ten times greater than the voltage of the second pair of grids.

In an example, the controller is configured to control the voltages such that the voltage or the average voltage of the first pair of grids is thirty times greater than the voltage of the second pair of grids.

As mentioned above, the focal spot size and/or position is mainly controlled by the electric field within the entire cathode cup, i.e. generated by the first pair of grids. Regulation of the focal spot and emission current with the respective first and second pairs of grids may therefore be improved when the voltage or the average voltage of the first pair of grids is significantly larger than the voltage of the second pair of grids.

In an example, the system further comprises: a high voltage supply; a low-medium voltage supply; and wherein in a first mode of operation: the controller is configured to control the high voltage supply to apply a first high voltage between the anode and the cathode; and wherein in a second mode of operation: the controller is configured to control the high voltage supply to apply a second high voltage between the anode and the cathode that is greater than the first high voltage; and wherein in the first and the second mode of operation: the controller is configured to control the low-medium voltage supply such that a voltage or an average voltage of the first pair of grids is different to a voltage of the second pair of grids; the controller is configured to control the low-medium voltage supply such that the voltage or the average voltage of the first pair of grids regulates steering and/or focusing of an electron beam between the electron emitter filament and the anode; and the controller is configured to control the low-medium voltage supply such that the voltage of the second pair of grids regulates an emission current magnitude between the electron emitter filament and the anode.

This enables ultra-fast dose (tube current) modulation and provides a significant improvement for fast kVp-Switching by boosting the flux for lower tube voltages

In an example, the controller is configured to control the low-medium supply to apply different voltages to the grids of the first pair of grids.

In an example, the controller is configured to control the low-medium supply to apply different voltages to the grids of the first pair of grids at a same average voltage to steer the electron beam.

In an example, the controller is configured to control the low-medium supply to apply a voltage or different voltages to the grids of the first pair of grids, wherein the voltage and/or the average voltage applied to the grids is of a different magnitude in the first mode of operation compared to the second mode of operation. In this way the focal spot size and/or position can be adaptively regulated to configurations that are optimal to the tube peak voltage in each mode of operation.

In an example, the controller is configured to control the low-medium supply to apply a same voltage to the grids of the second pair of grids, wherein the voltage is of different magnitude in the first mode of operation compared to the second mode of operation. In this way the emission current can be adaptively and rapidly regulated to configurations that are optimal to the tube peak voltage in each mode of operation.

In an example, a reduction in the magnitude of the voltage magnitude applied to the grids of the second pair of grids in the first mode of operation and/or second mode of operation for a fixed voltage or average voltage applied to the grids of the first pair of grids is configured to increase the emission current of the electron beam and increase a focal spot size.

In an example, the controller is configured to maintain a focal spot size between the first mode of operation and the second mode of operation through application of a voltage of different magnitudes between the grids of the first pair of grids and the grids of the second pair of grids in the first mode of operation and in the second mode of operation.

In an example, the controller is configured to control the low-medium supply to apply a first voltage magnitude between the grids of the first pair of grids and the grids of the second pair of grids in the first mode of operation and to control the low-medium supply to apply a second voltage magnitude between the grids of the first pair of grids and the grids of the second pair of grids in the second mode of operation, wherein the second magnitude is less than the first magnitude.

The above aspect and examples will become apparent from and be elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will be described in the following with reference to the following drawing:

Fig. 1 shows a schematic representation of an example of an X-ray tube;

Fig. 2 shows a schematic representation of an example of an X-ray generation system;

Fig. 3 shows a conventional cathode of an X-ray tube, shown at the left in 3D, and at the right in cross section;

Fig. 4 shows a new cathode of an X-ray tube, shown at the left in 3D, and at the right in cross section;

Fig. 5 shows the emission current (top) and focal spot (FS) size (bottom) in a conventional cathode design of an X-ray tube as a function of X-ray tube voltage (kV);

Fig. 6 shows the emission current (top) and focal spot (FS) size (bottom) in a conventional cathode design of an X-ray tube as a function of inner grid voltage of the cathode with a fixed outer grid voltage of the cathode and a fixed X-ray tube voltage;

Fig. 7 shows the emission current (top) and focal spot (FS) size (bottom) in a new cathode design of an X-ray tube as a function of X-ray tube voltage (kV) for various inner grid and outer grid voltages of the cathode; and

Fig. 8 shows electron-track visualizations. On the left the inner grids and outer grids of the cathode have identical grid voltages (705 V), the FS size is 1.2 mm, and the emission current is 480mA. On the right the outer grids of the cathode are at a voltage of 1500 V, and the inner grids of the cathode are at 50 V, the FS size is 1.2 mm, and the emission current is 650 mA.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows an example of an X-ray tube 10 comprising a cathode 20. The cathode 20 comprises an electron emitter filament 30, a first pair of grids 40, and a second pair of grids 50. The first pair of grids are located at opposite sides of the electron emitter filament. The second pair of grids are located at opposite sides of the electron emitter filament closer to the electron emitter filament than the first pair of grids. The cathode is configured such that a voltage or an average voltage of the first pair of grids is different to a voltage of the second pair of grids.

According to an example, the grids of the first pair of grids are configured to be at different voltages to each other. According to an example, the grids of the second pair of grids are configured to be at the same voltage to each other.

According to an example, the cathode is configured such that the voltage or the average voltage of the first pair of grids is greater than the voltage of the second pair of grids.

According to an example, the cathode is configured such that the voltage or the average voltage of the first pair of grids is two times greater than the voltage of the second pair of grids.

According to an example, the cathode is configured such that the voltage or the average voltage of the first pair of grids is ten times greater than the voltage of the second pair of grids.

According to an example, the cathode is configured such that the voltage or the average voltage of the first pair of grids is thirty times greater than the voltage of the second pair of grids.

Fig. 2 shows an example of an X-ray generation system 100. The system 100 comprises an anode 60, a cathode 20, a high voltage supply 70, a low-medium voltage supply 80, and a controller 90. The cathode comprises an electron emitter filament 30, a first pair of grids 40, and a second pair of grids 50. The first pair of grids are located at opposite sides of the electron emitter filament. The second pair of grids are located at opposite sides of the electron emitter filament closer to the filament than the first pair of grids.

In a first mode of operation: the controller is configured to control the high voltage supply to apply a first high voltage between the anode and the cathode; the controller is configured to control the low-medium voltage supply such that a voltage or an average voltage of the first pair of grids is different to a voltage of the second pair of grids; and electrons emitted from the electron emitter filament are formed into an electron beam and focused on the anode;

In a second mode of operation: the controller is configured to control the high voltage supply to apply a second high voltage between the anode and the cathode that is greater than the first high voltage; the controller is configured to control the low-medium voltage supply such that a voltage or an average voltage of the first pair of grids is different to a voltage of the second pair of grids; and electrons emitted from the electron emitter filament are formed into an electron beam and focused on the anode.

According to an example, the controller is configured to control the low-medium supply to apply different voltages to the grids of the first pair of grids.

According to an example, the controller is configured to control the low-medium supply to apply different voltages to the grids of the first pair of grids at a same average voltage to steer the electron beam. According to an example, the controller is configured to control the low-medium supply to apply a voltage of different magnitudes to the grids of the first pair of grids or an average voltage of different magnitudes to the grids of the first pair of grids.

According to an example, the controller is configured to control the low-medium supply to apply a same voltage of different magnitudes to the grids of the second pair of grids.

According to an example, a reduction in the magnitude of the voltage magnitude applied to the grids of the second pair of grids in both the first mode and second mode of operation for a fixed voltage or average voltage applied to the grids of the first pair of grids is configured to increase a current of the electron beam and increase a focal spot size.

According to an example, the controller is configured to maintain a focal spot size between the first mode of operation and the second mode of operation through application of a voltage of different magnitudes between the grids of the first pair of grids and the grids of the second pair of grids in the first mode of operation and in the second mode of operation.

According to an example, the controller is configured to control the low-medium supply to apply a first voltage magnitude between the grids of the first pair of grids and the grids of the second pair of grids in the first mode of operation and to control the low-medium supply to apply a second voltage magnitude between the grids of the first pair of grids and the grids of the second pair of grids in the second mode of operation, wherein the second magnitude is less than the first magnitude.

The new cathode of an X-ray tube and an X-ray generation system having such a cathode are now described in specific detail, where reference is made to Figs. 3-8.

It was realized that although the electron emission from a conventional cathode is controlled via the filament temperature, the tube voltage and the grid voltage also impact the emission, and that an introduction of two sets of grids of the cathode that can operate at different voltages would enable the electron emission to be increased. Thus, the electron emission at a lower switch voltage e.g. 80 kV could be increased above that presently available, whilst that at 140 kV could be kept at levels now achievable. The higher voltage electron emission could be increased, but by increasing just the lower switch voltage the resultant X-ray doses between the low voltage and high voltage of X-ray tube operation could be brought closer together.

To help explain the new cathode design, an existing typical cathode design is introduced. Fig. 3 shows a typical cathode design. The filament is embedded in a cup with steering grids on both sides. The grid voltages are used to position and size the focal spot (FS) . The common part of the voltages defines the FS size. High voltages will constrict the emitted electron beam and form a small focal spot (and vice versa). A voltage difference between the grids can be used to position the focal spot. The common voltage will also impact the emission by changing the electrical field. In Fig. 3, GridVoltage 1 is represented by “A”, and GridVoltage 2 is represented by “B”. The grids can be at different voltages indicated by the A and B. In the new cathode design, such an existing design is changed, and another grid (actually, two inner grids) are added close to the filament, see Fig. 4. The voltage of this grid is used to control the emission current. The impact on the focal spot size is then compensated with the second set of grids (outer grids). Since the steering voltages can rapidly be changed, the new design allows for ultra-fast emission current control without impacting on the focal spot size. In Fig. 4, GridVoltage 1 is represented by “40 - A”, and GridVoltage 2 is represented by “40 - B”. The outer grids 40 can be at different voltages indicated by the A and B. In Fig. 4, GridVoltage 3 is represented by “50 - C” The inner grids 50 are generally held at the same voltage to each other indicated by the C.

Continuing with Fig. 4, this shows an embodiment where the inner grids are close to the filament and can efficiently control the emission even with relatively small voltages. The outer grids are mainly used to shape the focal spot and to correct for the focal spot impact of the inner grids.

For a desired focal spot size, there is a variety of grid voltage combinations that realizes the size but generate different emission currents. For kVp-S, this capability can be used to have a moderate emission current at high tube voltages and to increase the emission current for low voltages by maintaining the desired focal spot size.

Fig. 5 shows two emission current (mA) at the top and FS size (mm) at the bottom curves as a function of the X-ray tube voltage (kV) for one filament temperature using a conventional tube design. The two curves belong to two different grid voltages (705 V, 1150 V). These grid voltages have been selected to get the same focal spot size (1.2 mm) at 80 kV and 140 kV in this example. As it can be seen the emission current goes down from about 570 mA (@ 140 kV, grid 1150V) to 480 mA (@ 80 kV, grid 705 V). In a typical kVp-S protocol, the grid voltage may be switched forth and back during the tube voltage transition.

Fig. 6 shows the emission current (top) and focal spot size (bottom) for the new design of cathode. This shows what happens if the grid voltages for the inner grids 50 (GridVoltage3 “50 - C”) is different form the outer grids 40 (Gridvoltage 1 “40 - A” and GridVoltage2 “40 - B”). It can be seen how the emission current increases from about 480 mA to 750 mA for smaller voltages on the inner grids. However, at the same time the focal spot gets larger, where it is to be noted that the voltage on the outer grids 40 has been kept constant.

In Fig. 7, it is shown how this unwanted focal spot (FS) enlargement can be compensated with higher voltages on the outer grids 40. The outer grid voltages have been selected to maintain the desired FS size of 1.2 mm at 80 kVp (bottom). The emission currents however are very different. The maximal improvement of the current relative to the conventional cathode design is a gain of about 35% (from 480 mA to 650 mA). This is a significant improvement for kVp-S and it enables ultra-fast dose modulation.

In Fig. 8, the differences can be visualized with an electron track simulation. It shows in effect the conventional setting (left) with all grids (outer and inner) at 705 V to generate a FS size of 1.2 mm - this then equates to the conventional design with only one pair of grids. The image to the right shows the improved case associated with the new cathode design with 50V on the inner grids 50 and 1500 V on the outer grids. Although the same FS size is realized on the anode, the electron cloud is much larger close to the filament indicating the increased current.

Therefore, during kVp switching with the new design it is desired to change the emission current and at the same time to maintain the focal spot size. To do this, the common part (in effect the average voltage of the pair) of the outer grids 40 and the voltage of the inner grids 50 are switched to get the required combination of FS size and current.

Therefore, one can consider there to be two scenarios with different tube behaviors (80 and 140 kV). There are three controllable parameters for the new cathode design (the voltage of the outer grids 40, the voltage difference between the outer grids and the voltage of the inner grids 50). There, are then three relevant outputs: FS position, FS size and current. For each of the two operating scenarios (80/140 kV) the 3 input parameters are set in order to get the desired outputs. This can be achieved in a similar process to that shown graphically in the figures described above for particular X-ray tube designs.

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 fulfill 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.