Determination of X-ray tube current

Abstract

The invention relates to a computer-implemented method of determining a tube current during X-ray kVp switching. The method comprises generating (110) a calibration tube voltage pulse in a calibration phase, where the calibration tube voltage pulse comprises a first kVp plateau and a second kVp plateau different from the first kVp plateau; measuring (120) a first tube current at the first kVp platform and a second tube current at the second kVp platform during the calibration tube voltage pulse; determining (130) a calibration factor as a function of a ratio between the first tube current and the second tube current; measuring (140) a third tube current in a steady state of the first kVp platform during a kVp switching phase; and determining (150) a fourth tube current at a second kVp platform during the kVp switching phase, where the fourth tube current is determined based on the calibration factor and the third tube current at the first kVp platform. The invention also relates to a high voltage generator (30), a computer program element and a computer-readable medium configured to carry out the computer-implemented method and an X-ray system.

Description

Field of the Invention

[0001] The present invention relates to a computer-implemented method for determining tube current during X-ray kVp switching. The present invention also relates to a high-voltage generator configured to perform the computer-implemented method. The present invention also relates to a computer program element and a computer-readable medium. The present invention also relates to an X-ray system. Background Art

[0002] A computed tomography (CT) scanner typically includes an X-ray tube mounted on a rotatable gantry opposite a row or multiple rows of detectors. The X-ray tube rotates around an examination area located between the X-ray tube and the row or multiple rows of detectors and emits broadband radiation that passes through the examination area. Electric power is supplied to the X-ray tube using a high-voltage generator.

[0003] The row or multiple rows of detectors detect the radiation passing through the examination area and generate projection data indicative thereof. A reconstructor reconstructs the projection data to generate volumetric image data, which can be displayed, photographed, archived, transmitted to another device, etc.

[0004] The detector array includes detector pixels that convert detected X-ray photons into electrical signals indicative thereof. For each rotation of the rotatable gantry, the detector pixels detect and convert X-ray photons over a plurality of integration periods, each integration period corresponding to a different angular position range. The duration of the integration period depends on the rotation speed of the rotatable gantry and the number of integration periods per scan rotation. Using an integrating detector array, at the start of each integration period, the integrators of the detector pixels are reset, and then the integrators receive and integrate the electrical signals over the entire integration period. The integrated signal forms the projection data for that integration period.

[0005] An X-ray tube typically includes an anode and a cathode having a filament. A filament current is applied to the filament, which heats the filament such that the filament emits electrons (thermionic emission), creating a space charge (or cloud of negative charge) at a short distance from the filament. A peak tube voltage (kilovolt peak voltage kVp) is applied between the cathode and the anode, causing an electron beam to accelerate from the cathode and strike the anode. The X-ray tube current or emission current represents the number of electrons flowing from the cathode to the anode per second. Electrostatic or magnetic focusing (e.g., having a grid or quadrupole) can be employed to control the size of the electron beam and steer the electron beam. The interaction of the electrons with the anode material generates heat and radiation, including X-rays, which enter the detection area through the tube window and reach the detectors.

[0006] The surface area of the anode that receives the electron beam is called the focal spot. The size of the focal spot is a factor that affects the imaging quality of volumetric image data. For example, the focal spot size affects the spatial resolution, and a smaller focal spot size produces a greater spatial resolution than a larger focal spot size. For example, due to geometric magnification, a smaller focal spot blurs less. The size and / or position of the focal spot can depend on the X-ray tube voltage, the beam focusing voltage, and the tube current.

[0007] Voxels of volumetric image data are displayed using gray values corresponding to relative radiation density. The gray values reflect the attenuation characteristics of the scanned object and represent anatomical structures. The detected radiation also includes spectral information because the absorption of photons by the matter of the object and / or body depends on the energy of the photons passing through the matter. Such spectral information provides additional information, such as information indicating the atoms, elements, or material composition of the matter. However, the projection data does not reflect the spectral characteristics because the projection data is proportional to the energy flux integrated over the energy spectrum (e.g., 40 keV to 120 keV), and the volumetric image data does not reflect the energy-related information.

[0008] A CT scanner configured for spectral (multi / double-energy) imaging utilizes the spectral characteristics in the detected radiation to provide further information, such as atomic or elemental composition information. Generally, a spectral CT scanner is configured to detect X-ray radiation in different energy bands (rather than just the entire spectrum) and generate projection data for each of the different energy bands (rather than just the entire spectrum). In one case, this is achieved by so-called kVp switching. For example, in a dual-energy configuration, the X-ray tube voltage (i.e., kVp) can be switched back and forth between two kVps, e.g., between a first 80 kVp for odd data acquisition cycles and a second 140 kVp for even data acquisition cycles, and vice versa. The X-ray spectrum generated at low energy (such as but not limited to 80 kvp) is different from the X-ray spectrum generated at high energy (such as but not limited to 140 kvp).

[0009] Spectral imaging using fast kVp switching in a high-voltage generator may be a promising cost-effective alternative to spectral imaging using, for example, multi-layer energy-discriminating scintillators or direct-conversion detectors.

[0010] The published document MULLER KET AL: "Interventional dual - energy imaging - Feasibility of rapid kV - switching on a C - arm CT system", MEDICAL PHYSICS, AIP, MELVILLE, NY, US, vol. 43, no. 10, 20 September 2016 (2016 - 09 - 20) explored the feasibility of implementing rapid kV - switching technology on a clinically available angiography system to acquire dual - energy C - arm CT images.

[0011] During rapid kVp - switching, the tube voltage and tube current change very rapidly, sometimes on the order of tens to hundreds of microseconds. Additionally, due to effects such as component aging, system temperature, and filament cooling, the emission current characteristics of the system may change over time during an imaging session and during the imaging session. To optimize the control of tube and / or generator parameters during rapid kVp - switching spectral imaging, it is important to be able to respond to the immediate change in the X - ray tube emission current, for example, directly after the tube voltage conversion. However, during such rapid kVp - switching cycles, providing emission current measurements that are both timely and sufficiently accurate can be very challenging, or even impossible. Therefore, it is necessary to solve this problem. Summary of the Invention

[0012] An object of the present invention is to provide improved determination of the X - ray tube emission current during rapid kVp - switching.

[0013] The present invention is defined by the independent claims. Advantageous embodiments are defined in the appended claims.

[0014] According to a first aspect of the present invention, there is provided a computer - implemented method for determining tube current during X - ray kVp - switching.

[0015] The method includes

[0016] - generating a calibration tube voltage pulse during a calibration phase, wherein the calibration tube voltage pulse includes a first kVp plateau and a second kVp plateau, and wherein the second kVp plateau has a kVp level different from that of the first kVp plateau;

[0017] - measuring a first tube current at the first kVp plateau and a second tube current at the second kVp plateau during the calibration tube voltage pulse;

[0018] - determining a calibration factor based on the ratio between the first tube current and the second tube current;

[0019] - Measure a third tube current at a steady state of the first kVp platform during kVp switching; and

[0020] - Determine a fourth tube current at a second kVp platform during kVp switching, wherein the fourth tube current is determined based on the calibration factor and the third tube current at the first kVp platform.

[0021] When reliable measurement data of current may (not yet) be available, the method can accurately determine the tube current during rapid kVp switching on a time scale. For example, at a time point before the switching period, the voltage and current are stable on the platform. During rapid switching, the current measurement result may be affected by the charging / discharging current of the high-voltage capacitance in the system, so it takes time to reach a steady state for a sufficiently accurate measurement. For example, the look-up table from a previous calibration run may also not be accurate enough. The method provides a solution for determining the calibration factor by respectively based on the ratio of the tube currents at the first kVp platform and the second kVp platform. Due to the thermal inertia of the X-ray tube filament, during rapid switching between voltage platforms, the filament temperature can be regarded as constant. Therefore, the relative change in tube current caused by the tube voltage conversion between two fixed voltage platforms may also remain constant during rapid switching, even if the initial current at the start of the conversion changes, for example due to drift. The calibration factor based on the ratio of the currents on two fixed voltage platforms can be used to accurately estimate the tube current at one platform if the current at the other platform is known. For example, by knowing the high emission state of the system, it is possible to determine the low emission state.

[0022] In the context of the present invention, the kVp switching phase comprises a plurality of kVp switching cycles, wherein each kVp switching cycle of the X-ray tube comprises two or more different kVp energy levels, also referred to as kVp platforms. The two or more different kVp platforms are different because the platform kVp levels are different. A kVp level (peak tube voltage) is applied across the anode and the cathode of the X-ray tube, and an electron beam is accelerated from the cathode and impacts the anode (determining the energy of the generated X-rays). In an example that includes only two kVp levels, the kVp switching phase comprises switching the tube voltage between a first kVp platform and a second kVp platform. Using corresponding measurement intervals (integration periods) detected by X-rays at different kVp energy levels, this can achieve X-ray spectral imaging. During the kVp switching cycle, the tube voltage switches between at least a "high" kVp energy level and a "low" kVp energy level such that the X-ray tube generates at least two different X-ray spectra. As a non-limiting example, the generator and the X-ray tube hold the cathode-to-anode voltage at a "low" voltage, such as 80 kV for a period of time, then ramp the cathode-to-anode voltage from the "low" voltage to a "high" voltage, such as 140 kV for a period of time, and then hold the cathode-to-anode voltage at the "high" voltage for a period of time, thereby forming a cycle that generates dual-energy X-rays and provides spectral imaging. During kVp switching, each of the time periods can be on the order of tens to hundreds of microseconds up to a few milliseconds. For example, the voltage transition time between kVp energy levels can be shorter than 500 μs, preferably 300 μs or shorter, or as short as tens of microseconds. The time period during which the voltage stabilizes at each energy level can be less than 2 ms, preferably less than 1 ms, more preferably less than 600 μs, or even shorter. Each of the corresponding at least two kVp platforms is at the same or substantially the same voltage level during two calibration phases as during the kVp switching phase. One of the kVp platforms is higher and the other is lower such that dual-energy X-rays can be generated. For example, the first kVp platform can correspond to a "low" kVp level, such as but not limited to 80 kV, and the second kVp platform can correspond to a "high" kVp level, such as but not limited to 140 kV. Alternatively, the first kVp platform can correspond to a "high" kVp level, such as but not limited to 140 kV, and the second kVp platform can correspond to a "low" kVp level, such as but not limited to 80 kV. Although the high kVp level is exemplified as 140 kV throughout this application, it can also be higher or lower, such as for example 120 kV or 130 kV or 150 kV or 160 kV. Similarly, although the low kVp level is exemplified as 80 kV throughout this application, it can also be higher or lower, such as for example 60 kV or 70 kV or 90 kV or 100 kV.

[0023] In the context of the claimed invention, a computer-implemented method refers to a method involving the use of a processor, which may include a computer, a computer network, and / or another programmable device, such as a single-core and / or multi-core processing unit, a graphics processing unit, an accelerated processing unit, a digital signal processor, a field programmable gate array, and / or an application specific integrated circuit, etc.

[0024] According to an embodiment of the present invention, the calibration phase occurs during the stabilization phase of the X-ray tube before imaging, and the kVp switching phase occurs during the spectral imaging phase. It is advantageous to determine the ratio and calibration factor during the pre-imaging stabilization phase of the X-ray tube because this phase occurs under relevant system and environmental conditions and within a time frame that can provide one or more accurate calibration pulses. During the same operation of the system, the stabilization phase occurs before the spectral imaging phase. As an example, the stabilization phase can be on the order of approximately dozens of milliseconds, such as but not limited to a range between 10 - 60 ms, preferably 20 - 40 ms. An accurate calibration pulse can have a combined time of a ramp and a high kVp plateau on the order of approximately milliseconds, such as but not limited to a range between 1 - 5 ms, preferably 1 - 3 ms. Thereby, an accurate calibration factor can be determined just before the start of imaging. During the spectral imaging phase with repeated kVp switching, including X-ray detection at different kVp platforms, there may not be time to perform such calibration pulses within any kVp switching cycle. Due to varying conditions and parameters, calibration from different runs and / or times of day may be suboptimal.

[0025] According to an embodiment of the present invention, the method further includes adjusting the calibration factor based on additional measurement results of the tube current at the stable state at the first kVp platform and the stable state at the second kVp platform during the kVp switching phase. This is advantageous because it allows monitoring of the calibration factor during the kVp switching phase and, if necessary, making further fine adjustments to the calibration factor to correct for drifts, changes due to switching patterns, etc. By comparing, for example, the instant calculated value of the tube current at the first kVp platform with the measured value of the stable state on the first kVp platform based on the measured value and the calibration factor at the second kVp platform, the calibration factor can be iteratively adjusted and controlled without additional calibration tube voltage pulses.

[0026] According to an embodiment of the present invention, the method further includes closed-loop control of the third tube current at the first kVp platform and / or the fourth tube current at the second kVp platform by adjusting the filament current. Closed-loop control of the tube current (i.e., adjusting the actual tube current by adjusting the filament current based on the determined tube current in a feedback loop) is beneficial for fast and accurate X-ray dose regulation. For example, quickly compensating for filament cooling effects, etc. Accurate dose regulation may be related to image quality, patient safety, etc.

[0027] According to an embodiment of the present invention, the method further includes adapting control of the X-ray tube focal spot position and / or size based on the determined X-ray tube current. By adapting the X-ray tube focal spot and / or size to the determined X-ray tube current, as opposed to a predetermined current and / or tube current measured in a steady state, very fast reaction to changes in the tube current can be achieved with focal spot control. For example, immediately following a voltage transition during a rapid kVp switching cycle.

[0028] According to a second aspect of the present invention, there is provided a computer program element configured to, when executed by a processor, cause a high-voltage generator including the processor to perform the method according to the first aspect of the present invention. In one example, the computer program element is made available for download, for example, from a server via the Internet.

[0029] According to another aspect of the present invention, there is provided a (non-transitory) computer-readable medium having stored thereon the above computer program element.

[0030] According to a third aspect of the present invention, there is provided a high-voltage generator including a processor and configured to perform a computer-implemented method according to the first aspect of the present invention.

[0031] According to a fourth aspect of the present invention, there is provided an X-ray system including: an X-ray tube including an anode, a cathode, a filament, and an electron beam optical device configured to adjust the electron beam focal spot size and / or position on the anode; and a high-voltage generator according to the third aspect of the present invention.

[0032] The high-voltage generator is configured to apply a kVp switching cycle including at least two kVp platforms between the anode and the cathode of the X-ray tube and may be configured to apply a filament current to the X-ray tube filament. The high-voltage generator includes a processor.

[0033] According to an embodiment of the present invention, the X-ray system is a computed tomography system. The present invention can be particularly advantageous for spectral computed tomography with rapid kVp switching.

[0034] These and other aspects of the present invention will be apparent with reference to the embodiments described below. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 A method for determining an emission current according to an embodiment of the present invention is schematically illustrated.

[0036] Figure 2 A method including an update of a calibration factor according to an embodiment of the present invention is schematically illustrated.

[0037] Figure 3 Schematically shows a method including closed-loop control according to an embodiment of the present invention.

[0038] Figure 4 Schematically shows a method including adjusting the focal spot position / size according to an embodiment of the present invention.

[0039] Figure 5 Schematically shows an X-ray system according to an embodiment of the present invention.

[0040] Figure 6 Schematically shows a timing diagram according to an embodiment of the present invention. Detailed implementation manners

[0041] In Figure 1 is shown a method according to an embodiment of the present invention. As shown, the method includes the step of generating 110 calibration X-ray tube voltage pulses during a calibration phase. The calibration tube voltage pulses include a first kVp plateau and a second kVp plateau different from the first kVp plateau. As an example, the first kVp plateau may be a high plateau, such as 140 kV for example, and the second plateau may be a low plateau, such as 80 kV. After the X-ray tube emission current control reaches a steady state at 80 kV, for example during the stabilization time of the system, a kVp step to the first kVp plateau of, for example, 140 kV is initiated and the voltage is held at the first plateau for, for example, one millisecond or several milliseconds. The calibration pulse is completed by ramping down to the second plateau at 80 kV before the end of the calibration phase (and, if applicable, the stabilization time of the system).

[0042] During the calibration tube voltage pulses, the tube current (also referred to as the emission current) is measured 120. A first tube current is measured at the first kVp plateau and a second tube current is measured at the second kVp plateau. In this example, the second emission current is measured when the emission current stabilizes at the second kVp plateau of 80 kV. The first emission current is measured during the one millisecond or several milliseconds when the voltage is held at the first kVp plateau of 140 kV and the emission current has stabilized at this level.

[0043] In the case of measuring both the first emission current and the second emission current, a calibration factor can be determined 130 based on the ratio between the currents. Thus, the first emission current and the second emission current (i.e., the corresponding emission currents at 140 kV and 80 kV in this example) are used to determine the applicable ratio and calibration factor in the current situation.

[0044] During the kVp switching phase, such as during spectral imaging, a third tube current is measured 140. The third tube current is measured at the first kVp plateau (i.e., in this example, at 140 kV). The third tube current is measured at the same tube voltage as the first tube current, and the third tube current may be similar to the first tube current. However, since the emission current characteristics of the system may change over time, it may not be exactly the same. Based on the third tube current and the determined calibration factor, a fourth tube current at the second kVp plateau is determined 150. In this example, the tube current at 80 kV is determined based on the tube current at 140 kV and the calibration factor. In other words, the next tube current that will follow when the voltage is rapidly switched from 140 kV to 80 kV can be known to the system before it can be measured and can be used for system control.

[0045] Figure 1 The calibration phase shown in steps 110, 120, and 130 of can occur during the stabilization phase of the X-ray tube before actual spectral imaging occurs. Including Figure 1 The kVp switching phase including steps 140 and 150 in can occur during the spectral imaging phase. It is advantageous to determine the ratio and calibration factor during the pre-imaging stabilization phase of the X-ray tube because this phase occurs under relevant system and environmental conditions and within a time frame where one or a few accurate calibration pulses can be generated on the order of milliseconds.

[0046] Note that in this and the following examples, the first kVp plateau is selected as the high voltage plateau, and the second kVp plateau is selected as the low voltage plateau. However, it is to be understood that the reverse is also possible, i.e., the first kVp plateau is the low voltage plateau and the second kVp plateau is the high voltage plateau. In this case, the first tube current and the third tube current correspond to a low kVp, such as 80 kV, and the second tube current and the fourth tube current correspond to a high kVp, such as 140 kV.

[0047] Figure 2 shows a method according to an embodiment of the present invention. In this case, during the kVp switching phase, the calibration factor can be further adjusted 220 based on additional measurements 210 of the tube current at the stable state of the first kVp plateau and the stable state of the second kVp plateau during the kVp switching phase. Although the time of the stable state of the emission current at each plateau during the kVp switching phase is shorter than during the accurate calibration pulse, such measurement results can be used for, for example, monitoring and / or fine-tuning the calibration. The comparison between the determined expected emission current and the actual emission current measured during the stable state can be used to update and fine-tune the calibration factor. Since the determined or calculated emission current may be known at the start of the steady period and the actual emission current measured is known near the end of the stable state, the combination of the two provides improved monitoring, and / or asFigure 3 Control of the emission current during most of each switching cycle, as shown.

[0048] Figure 3 A method according to an embodiment of the present invention is shown. In Figure 3 the example, the method includes a closed-loop control 310 of a third tube current at a first kVp plateau and / or a fourth tube current at a second kVp plateau by adjusting the filament current. Control and regulation of the X-ray tube emission current, such as for X-ray dose regulation, can be achieved by adjusting the X-ray tube filament current. Fast regulation may be crucial, for example, to compensate for effects such as, but not limited to, filament cooling, and / or to control the dose of the imaging application at hand. For example, the emission current control can use the emission current at 80 kV as the reference current I_80ref. By determining a virtual I_80ref that is already in a steady state at 140 kV based on the measured current at 140 kV and a calibration factor, blind spots in the emission current control during kVp switching can be avoided. That is, the virtual I_80ref can be determined and fed into the emission current control before the actual I_80ref is available. In this way, continuous or semi-continuous emission current control can be possible without waiting for each steady-state 80 kV phase. Once the actual measured I_80ref is available, a combined emission current feedback value, including actual and virtual I_80ref data, can be calculated and used for closed-loop control. Alternatively or additionally, the virtual and real emission currents at 140 kV can also be used as inputs to the emission current control in a similar manner.

[0049] Figure 4 A method according to an embodiment of the present invention is shown. In this example, the method further includes adjusting the control 410 of the X-ray tube focal spot position and / or size based on the determined X-ray tube current. As an example, electron beam optics can be used (see also Figure 5)Control the focal spot position and / or size relative to the emission current at one or each of the kVp platforms during kVp switching. In other words, the emission current at 80 kV can form an input to control the focal spot position and / or size at 80 kV. Similarly, the emission current at 140 kV can form an input to control the focal spot position and / or size at 140 kV. By using the determined "virtual" emission current for focal spot control, i.e., by using the calculated emission current instead of the measured steady-state emission current, the focal spot position and / or size can be controlled at the corresponding platform in a timely manner before the measured emission current is available. For example, during kVp switching, the emission current at 80 kV is determined based on a calibration factor and the measured emission current at 140 kV. Using this information, once the voltage is switched from 140 kV to 80 kV, the control of the focal spot can be adapted to the determined emission current at 80 kV. Before the (next) measured emission current at 80 kV in the steady state is available. Once on the 80 kV platform, a combination of the already determined current and the measured current from the steady state at 80 kV can be used to fine-tune the focal spot control. Similarly, by determining the emission current at 140 kV based on the current at 80 kV and the calibration factor before the measurement results at 140 kV in the steady state are available, timely control of the focal spot can be achieved. By knowing in advance the determined emission current for the next kVp platform, the focal spot control can also be adjusted before or during the voltage conversion, for example to allow time for the electron beam optics to adjust. Such as but not limited to the time required to charge the magnetic focusing coils.

[0050] Figure 5Shows an X-ray system 10 according to an embodiment of the present invention. The system includes an X-ray tube 20, a high-voltage generator 30, and a processor 40. The X-ray tube 20 includes a cathode 21 having a filament 22, an anode 23, and electron beam optics 24. The anode 23 can be a rotating anode. When a filament current or heating current is applied to the filament 22, heating of the filament can cause the filament 22 to emit electrons by thermionic emission. The heating current or filament current can be applied from the high-voltage generator 30 and controlled via the processor 40. The peak tube voltage between the cathode 21 and the anode 23 can be applied by the high-voltage generator 30 and controlled via the processor 40. The cathode 21, the filament 22, and the anode 23 are in a vacuum environment. Due to the tube voltage, the electrons emitted from the cathode filament 22 are accelerated to strike the anode 23 at the focal spot, generating X-ray radiation. The X-ray tube current or emission current represents the number of electrons flowing from the cathode 21 to the anode 23 per second. The spectrum of the produced X-rays depends on the energy of the electrons striking the anode 23 and thus on the peak tube voltage. The high-voltage generator 30 is configured to apply a kVp switching cycle including at least two kVp platforms between the anode 23 and the cathode 21 of the X-ray tube 20. In this way, at least two different spectra of X-rays can be generated during a cycle, for example. The electron beam optics 24 inside and / or outside the vacuum environment adjusts the size and position of the electron beam between the filament 22 and the anode 23. Therefore, the electron beam optics 24 can adjust the focal spot size and / or shape and / or position. The electron beam optics 24 can be controlled by the high-voltage generator 30 via the processor 40. Alternatively, the electron beam optics 24 is controlled by a separate voltage unit. The electron beam optics 24 can include, for example, quadrupoles or other devices for magnetic electron beam focusing and positioning and / or grid electrodes for electrostatic focusing and positioning.

[0051] Note that although, for example, the high-voltage generator 30 and the processor 40 are shown as separate blocks in Figure 5 , a plurality of blocks can be part of the same physical unit. As a non-limiting example, the processor 40 can be integrated with the high-voltage generator 30 and include, for example, a digital signal processor and / or a field programmable gate array and / or an application specific integrated circuit. In another example, the processor 40 is an external computer with which the high-voltage generator 30 can communicate.

[0052] Figure 6 Shows an example of a timing diagram for tube voltage and emission current feedback (superimposed with charge / discharge current) according to an embodiment of the present invention. The timing is schematically shown during a stable phase before imaging and during a kVp switching phase for spectral imaging. Specifically, nine time points or markers (letters A - I) further show an example of the method:

[0053] [A]. This marker indicates the start of the exposure, with an open emission current control loop (such as but not limited to via feedforward control) until the tube voltage stabilizes.

[0054] [B]. Once the tube voltage has stabilized, emission current control can be enabled. Although shown as a simplified horizontal line in Figure 6 , the emission current may not have reached a steady state at this point.

[0055] [C]. At this point, the emission current has reached a steady state. The nominal value of the emission current I_low corresponding to the low voltage V_low has been reached and can be accurately determined.

[0056] [D]. The start of a voltage pre-pulse, such as a high charging pulse, followed by a stationary phase (steady state) at the high voltage level V_high.

[0057] [E]. Here, both the voltage and the emission current I_high corresponding to the high voltage V_high reach a steady state plateau. The plateau can be, for example, 2 ms, but it can also be longer or shorter, as long as a sufficiently accurate determination of the emission current is achieved. At this point, the emission current controller can be disabled and the filament current can be kept constant. Due to the high thermal time constant, the filament temperature is considered constant during the pulse.

[0058] [F]. When the pre-pulse is complete and the voltage returns to V_low, the emission current can be accurately measured again during a period of, for example, 2 ms. Before enabling the emission current control, the actual emission current value can be compared with the nominal value to verify the assumption of a constant filament temperature. If the actual values before and after the pre-pulse are the same, the filament temperature has not changed during the pre-pulse. A calibration factor can be calculated based on the ratio between I_high and I_low.

[0059] [G]. The kVp switching phase begins. Generally, the kVp switching cycle includes shorter high and low kVp plateaus compared to the pre-pulse.

[0060] [H], [I]. During the kVp switching phase, the most recent I_high can be determined based on the calibration factor and the most recent I_low and vice versa. At the high and / or low plateau, the emission current can be determined before the most recent measurement result is available. Although the steady-state time of the emission current at each plateau during the kVp switching phase is shorter than during an accurate pre-pulse, such measurement results can be used, for example, to monitor and / or fine-tune the calibration over time. A comparison between the determined expected emission current and the actual emission current measured during the steady state ([H], [I]) can be used to update and fine-tune the calibration factor. Since the determined or calculated emission current may be known at the start of the stationary period and the measured actual emission current may be known near the end of the steady state, the combination of both can provide improved monitoring. Additionally, as Figure 3 and Figure 4 shown, the determined emission current can be used for closed-loop control of the filament current and / or to adapt the focal spot control in a timely manner.

[0061] It should be noted that the above embodiments illustrate rather than limit the invention, and those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps other than those listed in the claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention can be implemented by means of hardware comprising several distinct elements and / or by means of a suitably programmed processor. In a device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. The measures recited in mutually different dependent claims can be advantageously combined.

Claims

1. A computer-implemented method for determining tube current during X-ray kVp switching, the method comprising: - Generating (110) calibration tube voltage pulses during a calibration phase, wherein the calibration tube voltage pulses include a first kVp plateau and a second kVp plateau, wherein the second kVp plateau is at a different kVp level compared to the first kVp plateau; - Measuring (120) a first tube current at the first kVp plateau and a second tube current at the second kVp plateau during the calibration tube voltage pulses; - Determining (130) a calibration factor based on a ratio between the first tube current and the second tube current; - Measuring (140) a third tube current at a steady state of the first kVp plateau during a kVp switching phase; - Determining (150) a fourth tube current at the second kVp plateau during the kVp switching phase, wherein the fourth tube current is determined based on the calibration factor and the third tube current at the first kVp plateau.

2. The method according to claim 1, wherein The calibration phase occurs during a steady state of the X-ray tube before imaging, and wherein the kVp switching phase occurs during a spectral imaging phase.

3. The method according to claim 1 or 2, wherein The method further comprises adapting (220) the calibration factor based on additional measurements of tube current at a steady state of the first kVp plateau and tube current at a steady state of the second kVp plateau during the kVp switching phase.

4. The method according to any one of the preceding claims, wherein, The method further comprises performing closed-loop control (310) of the third tube current at the first kVp plateau and / or the fourth tube current at the second kVp plateau by adapting the filament current.

5. The method according to any one of the preceding claims, wherein, The method further comprises adapting (410) control of the X-ray tube focal spot position and / or size based on the determined X-ray tube current.

6. A high-voltage generator (30), which includes a processor (40), wherein, The high-voltage generator is configured to perform the computer-implemented method according to any one of the preceding claims.

7. A computer program unit, which when executed by the high-voltage generator according to claim 6 is configured to cause the high-voltage generator to perform the method according to any one of claims 1-5.

8. A computer-readable medium having stored thereon the computer program unit according to claim 7.

9. An X-ray system (10), comprising: An X-ray tube (20) including an anode (23), a cathode (21), a filament (22), and electron beam optics (24), the electron beam optics being configured to adjust the electron beam focal spot size and / or position on the anode; and The high-voltage generator (30) according to claim 6.

10. The system according to claim 9, wherein The system is a computed tomography system.

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