Abstract
The present invention relates to a system and a method for high-voltage switching for a computed tomography apparatus. The system comprises an oscillating circuit with a non-linear inductor and a capacitor. The inductor and the capacitor are connected in series, and the capacitor is connected to a high-voltage line of a high-voltage power supply. The inductor comprises an inductance that decreases with increasing current through the inductor, such that the inductance of the inductor significantly chances during a resonant operation of the oscillating circuit, thereby providing essentially a square voltage applied to the capacitor. The square voltage modulates the high-voltage of the high-voltage generator thus switching high-voltage levels applied to an electrode of a computed tomography system.
Claims
1. A system for high-voltage switching for a computed tomography apparatus, the system comprising: a high-voltage generator; an inductor having a non-linear inductance; and a capacitor; wherein a first connection terminal of the capacitor is communicationally connected to the high-voltage generator; wherein a second connection terminal of the capacitor is communicationally connected to a first connection terminal of the inductor for employing a resonant operation of a current through the inductor; wherein the inductor is configured for providing a reduction of the non-linear inductance with an increasing current through the inductor; and wherein the inductor is configured for providing the reduction of the non-linear inductance at a predefined current level of the current through the inductor below a maximum value of the current of the resonant operation.
2. The system according to claim 1, wherein the inductor comprises a magnetic core that is configured for magnetically saturating at the predefined current level.
3. The system according to claim 1, wherein the inductor is configured for providing a first inductance in case the current through the inductor is below the predefined current level and a second inductance in case the current through the inductor is above the predefined current level , and wherein the second inductance is smaller than the first inductance by a factor of at least 100.
4. The system according to claim 1, wherein the system is configured for providing in the resonant operation a voltage applied at the capacitor that is essentially constant at a first voltage level or at a second voltage level in case the current through the inductor is below the predefined current level, and wherein the voltage applied at the capacitor rapidly changes from the first voltage level to the second voltage level or from the second voltage level to the first voltage level in case the current through the inductor is above the predefined current level.
5. The system according to claim 1, wherein the inductor is configured to provide a non-linear inductance, wherein a first dependency of the inductance from the current in a first direction of the current though the inductor is different from a second dependency of the inductance from the current in a second direction of the current through the inductor, wherein the first direction is opposite to the second direction, and/or wherein the system is configured for providing a voltage applied to the capacitor that is essentially an asymmetric square voltage.
6. The system according to claim 5, wherein the inductor comprises a first inductor having a non-linear inductance, a second inductor having a non-linear inductance and connected in series to the first inductor (126), and a diode configured to act as rectifier and/or connected in parallel to the first inductor or to the second inductor.
7. The system according to claim 5, wherein the system comprises a biasing device configured to expose the inductor to an external magnetic field or wherein the system comprises a biasing circuit configured to cause a DC bias current through the inductor.
8. The system according to claim 1, wherein the system comprises an adjustment mechanism configured to adjust a resonance frequency of the resonant operation.
9. The system according to claim 1, wherein the inductor comprises a first inductor having a non-linear inductance; and a second inductor having a non-linear inductance and connected in series to the first inductor (126); wherein the system comprises a first control inductor inductively coupled to the first inductor; and a second control inductor inductively coupled to the second inductor; wherein the system is configured to provide a first control current in the first control inductor and a second control current in the second control inductor, and wherein the first control current has a same amperage and an opposite direction to the second control current.
10. The system according to claim 1, wherein the system comprises a driving mechanism configured to excite the resonant operation.
11. The system according to claim 10, wherein the driving mechanism comprises switching of the high-voltage generator, or wherein the driving mechanism comprises an amplifier inductively coupled to the inductor or capacitively coupled to the capacitor.
12. The system according to claim 1, further comprising a smoothing inductor, wherein the first connection terminal of the capacitor is connected to a high-voltage output of the high-voltage generator via the smoothing inductor.
13. (canceled)
14. A method for high-voltage switching for a computed tomography apparatus, the method comprising: providing the system according to claim 1; driving a current through the inductor thereby exciting a resonant operation and switching a high-voltage applied to an electrode of an X-ray tube of the system.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0050] FIG. 1 shows a schematic set-up of a system for high-voltage switching for a computed tomography apparatus according to a first embodiment of the invention.
[0051] FIG. 2A shows a graph of a square voltage applied at the capacitor over the time.
[0052] FIG. 2B shows a graph of an asymmetric square voltage applied at the capacitor over the time.
[0053] FIG. 3 shows a graph of the inductance of a non-linear inductor over the current through the inductor.
[0054] FIG. 4 shows a schematic set-up of a system for high-voltage switching for a computed tomography apparatus according to a second embodiment of the invention.
[0055] FIG. 5 shows a schematic set-up of a system for high-voltage switching for a computed tomography apparatus according to a third embodiment of the invention.
[0056] FIG. 6 shows a schematic set-up of a system for high-voltage switching for a computed tomography apparatus according to a fourth embodiment of the invention.
[0057] FIG. 7 shows a schematic set-up of a system for high-voltage switching for a computed tomography apparatus according to a fifth embodiment of the invention.
[0058] FIG. 8 shows a schematic set-up of a computed tomography apparatus according the invention.
[0059] FIG. 9 shows a block diagram of the method for high-voltage switching for a computed tomography apparatus according to the invention.
DETAILED DESCRIPTION OF EMBODIMENTS
[0060] FIG. 1 shows a schematic set-up of a system 100 for high-voltage switching for a computed tomography apparatus 200 according to a first embodiment of the invention. The components from left to right are the X-ray tube 210 with the electrode 220. A capacitor 195 to ground represents all the capacitances in the generator, the cables etc. A high-voltage generator 110 with a high-voltage outlet 111 is depicted on the right side of the image. The system comprises a LC series circuit shown in the figure, comprising a capacitor 130 and an inductor 120. The capacitor 130 has a first connection terminal 131 and a second connection terminal 132. The inductor 120 has a first connection terminal 121 and a second connection terminal 122. In the resonant operation of the system, a current 140 flows through the LC circuit, in particular through the inductor 120, which is a non-linear inductor. The inductance is highly non-linear. This means that it can have a magnetic core that saturates at or below 500 mA of current through the inductor 120. The relative permeability of a material of the magnetic core may be in the order of 10000 or more. This means, the time at the current amplitude below the saturation level of the magnetic core can be very long in comparison to the time for the high current, where the current is above the saturation level of the magnetic core. The times of low current below the predefined current level 145 are the constant voltage regions of the voltage 150 at the capacitor 130, and at high current, the voltage changes rapidly from one constant voltage region 151 to the other constant voltage region 152. In a simple case, the oscillation of the current 140 may be started using voltage swings by the high-voltage generator 110.
[0061] FIG. 2A shows a graph of a square voltage applied at the capacitor 130 over the time. The voltage applied at the capacitor 130 switches between a first voltage level 151 and a second voltage level 152. The switching takes place at times where the current 140 through the inductor is above the predefined current level 145.
[0062] FIG. 2B shows a graph of an asymmetric square voltage applied at the capacitor over the time. The voltage applied at the capacitor 130 switches between a first voltage level 151 and a second voltage level 152. In this figure, the time of the voltage 150 being on the second voltage level 152 is about twice as long as the time of the voltage 150 being on the first voltage level 151. Thus, a duty cycle of the system 100 can be adjusted, if the times for switching the voltage 150 are manipulated by, for example, an asymmetric behavior of the non-linear inductor 120.
[0063] FIG. 3 shows a graph of the inductance of a non-linear inductor 120 over the current 140 through the inductor 120. The inductance L is at a level of a first inductance 123 for the current 140 being smaller than the predefined current level 145. In case the current 140 is greater than the predefined current level 145, the inductance L of the inductor 120 is rapidly decreased and is at a level of a second inductance 124. The ratio of the first inductance 123 to the second inductance 124 can be greater than 100, or even greater than 10000 in preferred embodiments.
[0064] FIG. 4 shows a schematic set-up of a system 100 for high-voltage switching for a computed tomography apparatus 200 according to a second embodiment of the invention. As the embodiment of the invention shown in FIG. 1 provides only symmetrical voltage swings, which may be not optimal in terms of signal to noise ratio, one possible solution to this problem is shown in FIG. 4. Compared to FIG. 1, in this embodiment of the invention, the inductor 120 is subdivided into a first inductor 126 and a second inductor 127, which are connected in series to each other. A diode 161 is connected in parallel to the first inductor 126, and configured for short-circuiting the first inductor 126 in only one current direction of the current 140. The non-linear inductor is split into two sections and at least one of the sections is bridged by at least one diode. This has the effect that in one direction of current flow, the inductance is larger and the time at constant voltage longer. A voltage-over-time dependency derived from this embodiment is shown in FIG. 2B.
[0065] FIG. 5 shows a schematic set-up of a system 100 for high-voltage switching for a computed tomography apparatus 200 according to a third embodiment of the invention. As it is very inconvenient to excite the oscillation using the high-voltage generator, an amplifier dedicated for generating the oscillating voltage can be added. In this embodiment of the invention, the amplifier 181 is inductively coupled to the resonator, but other coupling modes (capacitive, resistive at various feed point or inductive but using a dedicated transformer ...) may be used, too. The dedicated amplifier 181 can be used in all the embodiments of the invention. In this embodiment of the invention, an additional driving mechanism 180 comprising an amplifier 181 is shown. The amplifier 181 can drive an alternating current through an inductor of the driving mechanism 180, which can be inductively coupled to the at least one of the inductors of the inductor 120. Thus, the current 140 of the resonant operation can be excited and driven through the inductor 120. However, in embodiments of the invention, this amplifier 181 inductively coupled to the inductor 120 can also be used as biasing device 162 for influencing a saturation level of a magnetic core of the inductor 120. Thus, the amplifier 181 can be used to adjust the frequency of the resonant operation to the exact desired value.
[0066] FIG. 6 shows a schematic set-up of a system 100 for high-voltage switching for a computed tomography apparatus 200 according to a fourth embodiment of the invention. As the computed tomography apparatus 200 may have several rotation speeds, the frequency of the resonant operation needs also a coarse adjustment method to change the frequency over a factor of two, for example. This figure shows an embodiment, where the capacitance in the capacitor 130 of the resonant circuit can be adjusted by suitable switches. Other locations with switched or otherwise changed capacitances and inductances are also possible. The switching may have more stages than shown in the drawing allowing for a more precise frequency adjustment. There may be no need for a broader range of adjustment than about two, as for a slower rotation of the X-ray tube, it is always possible to have more than one voltage swing per view. However, technically, it is possible to increase the frequency swing. In this embodiment, the capacitor 130 is divided into two sub-capacitors connected in parallel. One of the parallel branches comprises a switch 170 connected in series to the respective capacitor. Thus, by opening and closing of the switch, the capacitance of the capacitor 130 can be switched between two values. In case both of the sub-capacitors have the same capacitance, the capacitance of the capacitor 130 can be doubled by closing the switch 170.
[0067] FIG. 7 shows a schematic set-up of a system 100 for high-voltage switching for a computed tomography apparatus 200 according to a fifth embodiment of the invention. In this embodiment, a different approach for steering the resonant operation of the system 100 is depicted. A high-power amplifier 181 is used to modify the saturation level of the inductor 120 in the resonance path. This means, the inductance of the inductor 120 is modified and hence the times for which the voltage is constant. In the figure, the fields of the resonance current in the inductor 120 and the steering current in the first control inductor 171 and the second control inductor 172 of the adjustment mechanism 170 are co-linear and a decoupling is achieved by splitting the inductor 120 in two and driving each one of the first inductor 126 and the second inductor 126 with a current in opposite directions. However, the decoupling can be better, if the magnetic material forms a toroidal structure and the main and steering windings are shaped in a way to magnetize the core material in orthogonal direction. Naturally, the amplifier 181 needs to modulate its current through the kVp cycle to reach the desired effect. In FIG. 7, also an additional smoothing inductor 190 after the high-voltage generator 110 is shown. This smoothing inductor 190 makes the voltage curves more predictable and decreases the capacitance seen by the resonator, hence reduces its designed power handling capability.
[0068] FIG. 8 shows a schematic set-up of a computed tomography apparatus 200 according the invention. The computed tomography apparatus 200 comprises an X-ray tube 210 with an electrode 220, and the system 100 according to any of the preceding embodiments of the invention. The computed tomography apparatus 200 can further comprise a processing unit 230 configured for controlling the high-voltage switching of the system 100.
[0069] FIG. 9 shows a block diagram of the method for high-voltage switching for a computed tomography apparatus 200 according to the invention. The method comprises a first step of providing a computed tomography apparatus 200, and a second step of driving a current 140 through the inductor 120 thereby exciting a resonant operation and switching a high-voltage applied to an electrode 220 of an X-ray tube 210 of the computed tomography apparatus 200.
[0070] 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.
[0071] 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. 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.
[0072] LIST OF REFERENCE SIGNS: [0073] 100 system [0074] 110 high-voltage generator [0075] 111 high-voltage output [0076] 120 inductor [0077] 122 second connection terminal of inductor [0078] 123 first inductance [0079] 124 second inductance [0080] 126 first inductor [0081] 127 second inductor [0082] 130 capacitor [0083] 131 first connection terminal of capacitor [0084] 132 second connection terminal of capacitor [0085] 140 current through inductor [0086] 145 predefined current level [0087] 150 voltage at capacitor [0088] 151 first voltage level [0089] 152 second voltage level [0090] 161 diode [0091] 162 biasing device [0092] 170 adjustment mechanism [0093] 171 first control inductor [0094] 172 second control inductor [0095] 180 driving mechanism [0096] 181 amplifier [0097] 190 smoothing inductor [0098] 195 intrinsic capacitance [0099] 200 computed tomography apparatus [0100] 210 X-ray tube [0101] 220 electrode [0102] 230 processing unit
