Determination of x-ray tube current

Abstract

The invention relates to a computer-implemented method of determining tube current during X-ray kVp switching. The method comprises generating (110) a calibration tube voltage pulse in a calibration phase, wherein 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 plateau and a second tube current at the second kVp plateau during the calibration tube voltage pulse; determining (130) a calibration factor from a ratio between the first tube current and the second tube current; measuring (140) a third tube current at steady state at the first kVp plateau in a kVp switching phase; and determining (150) a fourth tube current at the second kVp plateau in 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. The invention further relates to a high voltage generator (30) configured to perform the computer-implemented method, a computer program element and a computer readable medium, and an X-ray system.

Description

Technical Field

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

[0002] Computed tomography (CT) scanners typically consist of an X-ray tube mounted on a rotatable gantry opposite one or more rows of detectors. The X-ray tube rotates around an examination area located between the X-ray tube and the one or more rows of detectors, emitting broadband radiation that passes through the examination area. Electrical power is supplied to the X-ray tube using a high-voltage generator.

[0003] The one or more rows of detectors detect radiation passing through the inspection area and generate projection data indicating it. The reconstructor reconstructs the projection data to generate volumetric image data, which can be displayed, captured, archived, transmitted to another device, etc.

[0004] The detector array comprises detector pixels that convert detected X-ray photons into electrical signals indicating them. For each rotation of the rotating gantry, the detector pixels detect and convert X-ray photons over multiple integration cycles, each corresponding to a different angular position range. The duration of each integration cycle depends on the rotational speed of the gantry and the number of integration cycles per scan rotation. Using an integrating detector array, at the beginning of each integration cycle, the integrator of the detector pixels is reset, and then the integrator receives and integrates the electrical signal over the entire integration cycle. The integrated signal forms projection data for that integration cycle.

[0005] An X-ray tube typically consists of an anode and a cathode with a filament. A filament current is applied to the filament, heating it and causing it to emit electrons (thermo-ionization), creating a space charge (or cloud of negative charge) at a short distance from the filament. A peak tube voltage (kilovolt peak kVp) is applied between the cathode and anode, causing the 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., with a grid or quadrupole) can be used to control the size and direction of the electron beam. The interaction of electrons with the anode material produces heat and radiation, including X-rays, which pass through the tube window into the detection area and reach the detector.

[0006] The surface region on the anode that receives the electron beam is called the focal spot. The size of the focal spot is a factor affecting the imaging quality of volumetric image data. For example, the focal spot size affects spatial resolution; a smaller focal spot size produces greater spatial resolution than a larger one, for example, due to blurring caused by a smaller focal spot size according to geometric magnification. The size and / or location of the focal spot can depend on the X-ray tube voltage, beam focusing voltage, and tube current.

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

[0008] CT scanners configured for spectral (multi- / dual-energy) imaging utilize spectral features in the detected radiation to provide further information, such as atomic or elemental composition. Generally, spectral CT scanners are configured to detect X-ray radiation at different wavelengths (not just the entire spectrum) and generate projection data for each of the different energy bands (not just the entire spectrum). In one case, this is achieved through 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 kVp, such as switching between a first 80 kVp for odd-numbered data acquisition cycles and a second 140 kVp for even-numbered data acquisition cycles, and vice versa. The X-ray spectrum generated at lower energies (such as, but not limited to, 80 kVp) differs from the X-ray spectrum generated at higher energies (such as, but not limited to, 140 kVp).

[0009] Using fast kVp switching for spectral imaging in high-voltage generators may be a promising and cost-effective alternative to spectral imaging of, for example, multi-layer energy-identifying scintillators or direct-conversion detectors.

[0010] The published paper MULLER KET AL: "Internal 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) explores 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, tube voltage and current change very quickly, sometimes on the order of tens to hundreds of microseconds. Furthermore, due to effects such as component aging, system temperature, and filament cooling, the system's emission current characteristics may change over time during and after the imaging session. To optimize control tube and / or generator parameters during rapid kVp switching spectral imaging, it is crucial to be able to respond to instantaneous changes in X-ray tube emission current, for example, directly after the tube voltage transition. However, providing timely and sufficiently accurate emission current measurements during such rapid kVp switching cycles can be very challenging, or even impossible. Therefore, it is necessary to address this issue. Summary of the Invention

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

[0013] According to a first aspect of the invention, a computer-implemented method for determining tube current during X-ray kVp switching is provided.

[0014] The method includes

[0015] - Generate a calibration tube voltage pulse during the calibration phase, wherein the calibration tube voltage pulse includes a first kVp plateau and a second kVp plateau, wherein the second kVp plateau has a different kVp level than the first kVp plateau;

[0016] - During the calibration tube voltage pulse, measure the first tube current at the first kVp plateau and the second tube current at the second kVp plateau;

[0017] - Determine the calibration factor based on the ratio between the current of the first transistor and the current of the second transistor;

[0018] - Measure the third tube current during the kVp switching process at the steady state of the first kVp plateau; and

[0019] - Determine the fourth tube current at the second kVp plateau during kVp switching, wherein the fourth tube current is determined based on the calibration factor and the third tube current at the first kVp plateau.

[0020] When reliable current measurement data may (not yet) be available, the method can accurately determine the tube current on a timescale during rapid kVp switching. For example, at a point in time before the switching period, the voltage and current stabilize on a plateau. During rapid switching, current measurements may be affected by the charging / discharging current of the high-voltage capacity in the system, thus requiring time to reach a steady state for sufficiently accurate measurements. For example, lookup tables from previous calibration runs may also be inaccurate. The method provides a solution for determining a calibration factor based on the ratio of the tube current at the first kVp plateau and the second kVp plateau, respectively. Due to the thermal inertia of the X-ray tube filament, the filament temperature can be considered constant during rapid switching between voltage plateaus. Therefore, the relative tube current change caused by the tube voltage transition between two fixed voltage plateaus may also remain constant during rapid switching, even if the initial current at the start of the transition changes, for example, due to drift. A calibration factor based on the ratio of the currents on the two fixed voltage plateaus can be used to accurately estimate the tube current at one plateau if the current at the other plateau is known. For example, by knowing the high emission state of the system, it is possible to determine the low emission state.

[0021] In the context of this invention, the kVp switching phase comprises multiple kVp switching cycles, wherein each kVp switching cycle of the X-ray tube includes two or more different kVp energy levels, also referred to as kVp plateaus. The two or more different kVp plateaus are distinct because the plateau 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 strikes the anode (determining the energy of the generated X-rays). In an example comprising only two kVp levels, the kVp switching phase includes switching the tube voltage between a first kVp plateau and a second kVp plateau. This enables X-ray spectral imaging by utilizing corresponding measurement intervals (integration periods) for X-ray detection at different kVp energy levels. 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 distinct X-ray spectra. As a non-limiting example, the generator and X-ray tube maintain the cathode-to-anode voltage at a “low” voltage, for example, 80 kV, for a period of time, then ramp the cathode-to-anode voltage from the “low” voltage to a “high” voltage, for example, 140 kV, for a period of time, and then maintain the cathode-to-anode voltage at the “high” voltage for a period of time, thus forming a cycle that generates dual-energy X-rays and provides spectral imaging. During the kVp switching, each of the time periods can be on the order of approximately tens to hundreds of microseconds up to a few milliseconds. For example, the voltage transition time between kVp energy levels can be less than 500 μs, preferably 300 μs or less, or even as short as tens of microseconds. The 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 the two calibration phases as during the kVp switching phase. One kVp platform is higher than the other, making it possible to generate dual-energy X-rays. For example, a first kVp plateau may correspond to a "low" kVp level, such as, but not limited to, 80 kV, and a second kVp plateau may correspond to a "high" kVp level, such as, but not limited to, 140 kV. Alternatively, a first kVp plateau may correspond to a "high" kVp level, such as, but not limited to, 140 kV, and a second kVp plateau may correspond to a "low" kVp level, such as, but not limited to, 80 kV. Although a high kVp level is exemplified as 140 kV throughout this application, it may also be higher or lower, such as, for example, 120 kV, 130 kV, 150 kV, or 160 kV. Similarly, although a low kVp level is exemplified as 80 kV throughout this application, it may also be higher or lower, such as, for example, 60 kV, 70 kV, 90 kV, or 100 kV.

[0022] 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.

[0023] According to embodiments of the invention, the calibration phase occurs during the stabilization phase of the X-ray tube prior to imaging, and the kVp switching phase occurs during the spectral imaging phase. It is advantageous to determine the ratio and calibration factor during the stabilization phase of the X-ray tube prior to imaging because this phase occurs within a time frame where one or more accurate calibration pulses can be provided, under relevant system and environmental conditions. During the same operation of the system, the stabilization phase occurs prior to the spectral imaging phase. As an example, the stabilization phase can be on the order of tens of milliseconds, such as, but not limited to, the range between 10-60 ms, preferably 20-40 ms. An accurate calibration pulse can have a combined duration of a ramp and a high kVp plateau on the order of milliseconds, such as, but not limited to, the range between 1-5 ms, preferably 1-3 ms. Thus, an accurate calibration factor can be determined just before imaging begins. During the spectral imaging phase with repeated kVp switching, including X-ray detection at different kVp plateaus, there may not be time to perform such a calibration pulse within any kVp switching cycle. Due to varying conditions and parameters, calibration from different operations and / or times of day may be suboptimal.

[0024] According to embodiments of the invention, the method further includes adjusting the calibration factor during the kVp switching phase based on additional measurements of the tube current at the steady state at the first kVp plateau and the steady state at the second kVp plateau. This is advantageous because it allows monitoring of the calibration factor during the kVp switching phase and, if necessary, further fine adjustments to the calibration factor to correct for drift, variations due to the switching mode, etc. By comparing, for example, an instantaneously calculated value of the tube current at the first kVp plateau with a measured value at the steady state on the first kVp plateau based on measurements at the second kVp plateau and the calibration factor, the calibration factor can be iteratively adjusted and controlled without additional calibration tube voltage pulses.

[0025] According to embodiments of the present invention, the method further includes closed-loop control of the third tube current at the first kVp plateau and / or the fourth tube current at the second kVp plateau by adjusting the filament current. Closed-loop control of the tube current (i.e., adjusting the actual tube current by adjusting the filament current in the feedback loop based on the determined tube current) facilitates rapid and accurate X-ray dose adjustment. For example, it allows for rapid compensation of filament cooling effects. Accurate dose adjustment may be related to image quality, patient safety, etc.

[0026] According to embodiments of the invention, the method further includes adjusting the 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, contrary to a predetermined current and / or tube current measured in a steady state, focal spot control can respond very rapidly to changes in the tube current. For example, immediately following a voltage transition during a rapid kVp switching cycle.

[0027] According to a second aspect of the invention, a computer program element is provided, configured to, when executed by a processor, cause a high-voltage generator including the processor to perform the method according to a first aspect of the invention. In one example, the computer program element is made available for downloading from a server, for example, via the Internet.

[0028] According to another aspect of the invention, a (non-transitory) computer-readable medium is provided on which the above-described computer program elements are stored.

[0029] According to a third aspect of the invention, a high-voltage generator is provided, which includes a processor and is configured to perform a computer-implemented method according to a first aspect of the invention.

[0030] According to a fourth aspect of the present invention, an X-ray system is provided, comprising: an X-ray tube including an anode, a cathode, a filament, and an electron beam optics configured to adjust the size and / or position of an electron beam focal spot on the anode; and a high-voltage generator according to a third aspect of the present invention.

[0031] The high-voltage generator is configured to apply a kVp switching cycle, including at least two kVp plateaus, between the anode and cathode of an X-ray tube, and can be configured to apply a filament current to the X-ray tube filament. The high-voltage generator includes a processor.

[0032] According to an embodiment of the present invention, the X-ray system is a computed tomography system. The present invention is particularly advantageous for spectral computed tomography scans with rapid kVp switching.

[0033] These and other aspects of the invention will become apparent from the embodiments described below. Attached Figure Description

[0034] Figure 1 A method for determining the emission current according to an embodiment of the present invention is illustrated schematically.

[0035] Figure 2 A method for updating calibration factors according to an embodiment of the present invention is illustrated schematically.

[0036] Figure 3 A method including closed-loop control according to an embodiment of the present invention is illustrated schematically.

[0037] Figure 4 A method for adjusting the position / size of the focal spot according to an embodiment of the present invention is illustrated schematically.

[0038] Figure 5 An X-ray system according to an embodiment of the present invention is illustrated schematically.

[0039] Figure 6 A timing diagram according to an embodiment of the present invention is shown schematically. Detailed Implementation

[0040] exist Figure 1 A method according to an embodiment of the present invention is illustrated. As shown, the method includes the step of generating a calibration X-ray tube voltage pulse 110 during a calibration phase. The calibration tube voltage pulse includes 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, for example, 140 kV, and the second plateau may be a low plateau, such as 80 kV. After reaching a stable state of 80 kV for X-ray tube emission current control, for example during the stabilization time of the system, a kVp step to, for example, the first kVp plateau of 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).

[0041] During the calibration tube voltage pulse, the 120kV tube current (also known as the emitter current) is measured. The first tube current is measured at the first kVp plateau, and the second tube current is measured at the second kVp plateau. In this example, the second emitter current is measured when the emitter current stabilizes at the second kVp plateau of 80kV. The first emitter current is measured during a period of one or several milliseconds when the voltage remains at the first kVp plateau of 140kV and the emitter current has stabilized at that level.

[0042] By measuring both the first and second emission currents, a calibration factor of 130 can be determined based on the ratio between the currents. Therefore, the first and second emission currents (i.e., the corresponding emission currents at 140kV and 80kV in this example) are used to determine the ratio and calibration factor applicable under the current conditions.

[0043] During the kVp switching phase, such as during spectral imaging, the third tube current is measured at 140 kV. The third tube current is measured at the first kVp plateau (i.e., at 140 kV in this example). The third tube current is measured at the same tube voltage as the first tube current, and the third tube current can be similar to the first tube current. However, it may not be exactly the same because the system's emission current characteristics may change over time. Based on the third tube current and the determined calibration factor, the fourth tube current is determined at the second kVp plateau at 150 kV. 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 rapidly switches from 140 kV to 80 kV can be known by the system before it can be measured and can be used for system control.

[0044] Figure 1 The calibration phases shown in steps 110, 120, and 130 can occur during the stabilization phase of the X-ray tube before actual spectral imaging. Figure 1 The kVp switching phases in steps 140 and 150 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 at a timeframe in which one or a few accurate calibration pulses on the order of milliseconds can be generated.

[0045] Note that in this and the following examples, the first kVp plateau is selected as a high-voltage plateau, and the second kVp plateau is selected as a low-voltage plateau. However, it should be understood that the reverse is also possible, i.e., the first kVp plateau is a low-voltage plateau, and the second kVp plateau is a high-voltage plateau. In this case, the first and third tube currents correspond to low kVp, such as 80 kV, and the second and fourth tube currents correspond to high kVp, such as 140 kV.

[0046] Figure 2 A method according to an embodiment of the invention is illustrated. In this case, during the kVp switching phase, the calibration factor 220 can be further adjusted based on additional measurements 210 of the tube current at the steady state of the first kVp plateau and the steady state of the second kVp plateau during the kVp switching phase. Although the steady state time of the emitter current at each plateau in the kVp switching phase is shorter than during the accurate calibration pulse, such measurements can be used, for example, for monitoring and / or fine-tuning the calibration. The comparison between the determined expected emitter current and the actual emitter current measured during the steady state can be used to update and fine-tune the calibration factor. Since the determined or calculated emitter current may be known at the beginning of the steady state and the measured actual emitter current may be known near the end of the steady state, the combination of the two provides improved monitoring, and / or as Figure 3 As shown, the control of the transmit current occurs for most of each switching cycle.

[0047] Figure 3 A method according to an embodiment of the present invention is illustrated. Figure 3 In the example, the method includes 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 adjusting the filament current. Control and regulation of the X-ray tube emission current, such as that used for X-ray dose adjustment, can be achieved by adjusting the X-ray tube filament current. Rapid adjustment may be crucial, for example, to compensate for effects such as, but not limited to, filament cooling, and / or to control the dose for on-the-fly imaging applications. For example, emission current control can use an 80kV emission current as a reference current I_80ref. By determining a virtual I_80ref that is already in a steady state at 140kV based on a measured current at 140kV and a calibration factor, blind spots in emission current control during kVp switching can be avoided. That is, a 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 80kV steady-state stage. Once the actual measured I_80ref is available, the combined emitter current feedback value, including both actual and virtual I_80ref data, can be calculated and used for closed-loop control. Alternatively or additionally, the virtual and actual emitter currents at 140kV can also be used as inputs to emitter current control in a similar manner.

[0048] Figure 4 A method according to an embodiment of the present invention is illustrated. 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 (see also...) can be used. Figure 5The focal spot position and / or size can be controlled relative to the emitter current at one or each point on the kVp platform during kVp switching. In other words, the emitter current at 80kV can form an input to control the focal spot position and / or size at 80kV. Similarly, the emitter current at 140kV can form an input to control the focal spot position and / or size at 140kV. By using a determined “virtual” emitter current for focal spot control—that is, by using a calculated emitter current instead of the measured emitter current in steady state—the focal spot position and / or size can be controlled at the corresponding platform in a timely manner before the measured emitter current is available. For example, the emitter current at 80kV is determined during kVp switching based on a calibration factor and the measured emitter current at 140kV. Using this information, once the voltage switches from 140kV to 80kV, focal spot control can be adapted to the determined emitter current at 80kV before the (next) measured emitter current in steady state at 80kV becomes available. Once at the 80kV plateau, focal spot control can be fine-tuned using a combination of the established current and the measured current from the steady-state at 80kV. Similarly, by determining the emitter current at 140kV based on the current at 80kV and a calibration factor before measurements at the 140kV steady-state are available, timely control of the focal spot can be achieved. Knowing in advance the determined emitter current for the next kVp plateau also allows for adjustment of focal spot control before or during voltage transitions, for example, to allow time for electron beam optics to be adjusted, such as, but not limited to, the time required to charge the magnetic focusing coil.

[0049] Figure 5An X-ray system 10 according to an embodiment of the present invention is shown. 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 an electron beam optics 24. The anode 23 may be a rotating anode. When a filament current or heating current is applied to the filament 22, heating of the filament causes the filament 22 to emit electrons via thermionic emission. The heating current or filament current may 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 may be applied from the high-voltage generator 30 and controlled via the processor 40. The cathode 21, filament 22, and anode 23 are in a vacuum environment. Due to the tube voltage, electrons emitted from the cathode filament 22 are accelerated to strike the anode 23 at the focal spot, thereby 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 therefore depends on the peak tube voltage. The high-voltage generator 30 is configured to apply a kVp switching cycle, including at least two kVp plateaus, between the anode 23 and cathode 21 of the X-ray tube 20. In this way, at least two different spectra of X-rays can be generated, such as during the cycle. Electron beam optics 24, located inside and / or outside the vacuum environment, adjust the size and position of the electron beam between the filament 22 and the anode 23. Therefore, electron beam optics 24 can adjust the focal spot size and / or shape and / or position. Electron beam optics 24 can be controlled by the high-voltage generator 30 via processor 40. Alternatively, electron beam optics 24 can be controlled by a separate voltage unit. Electron beam optics 24 may include, for example, quadrupoles or other devices for magneto-electron beam focusing and positioning, and / or grid electrodes for electrostatic focusing and positioning.

[0050] Note that, although, for example, the high-voltage generator 30 and the processor 40 are in Figure 5 The processor 40 is shown as a separate block, but multiple blocks can be part of the same physical unit. As a non-limiting example, the processor 40 may be integrated with the high-voltage generator 30 and may 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 to which the high-voltage generator 30 may communicate.

[0051] Figure 6 An example timing diagram for tube voltage and emitter current feedback (superimposed with charge / discharge current) according to an embodiment of the present invention is shown. The timing is schematically shown during the stabilization phase prior to imaging and during the kVp switching phase for spectral imaging. Specifically, nine time points or markers (letters AI) further illustrate examples of the method:

[0052] [A]. This mark indicates the start of exposure with an open emitter current control loop (such as, but not limited to, via feedforward control) until the tube voltage stabilizes.

[0053] [B]. With the tube voltage stabilized, emitter current control can be enabled. Although in Figure 6 The image is shown as a simplified horizontal line, but the emission current may not have reached a steady state at that point.

[0054] [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.

[0055] [D]. The beginning of a voltage pre-pulse, such as a high-charge pulse, is followed by a steady phase (steady state) at a high voltage level V_high.

[0056] [E]. Here, both the voltage corresponding to the high voltage V_high and the emission current I_high reach a steady-state plateau. This 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 remain constant. Due to the high thermal time constant, the filament temperature is considered constant during the pulse.

[0057] [F]. Once the pre-pulse is complete and the voltage returns to V_low, the emission current can be accurately measured again during, for example, a 2ms time period. Before enabling emission current control, the actual emission current value can be compared to 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 did not change during the pre-pulse. A calibration factor can be calculated based on the ratio between I_high and I_low.

[0058] [G].kVp switching phase begins. Typically, the kVp switching cycle includes shorter high and low kVp plateaus compared to the pre-pulse.

[0059] [H], [I] During the kVp switching phase, the nearest I_high can be determined based on the calibration factor and the nearest I_low, and vice versa. At high and / or low plateaus, the emission current can be determined before the most recent measurement results are available. Although the steady-state time of the emission current at each plateau in the kVp switching phase is shorter than during the accurate pre-pulse period, such measurements can be used, for example, for time-varying monitoring and / or fine-tuning calibration. The 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 beginning of the steady-state period, and the measured actual emission current may be known near the end of the steady-state period, the combination of the two can provide improved monitoring. Furthermore, as Figure 3 and Figure 4 As shown, the determined emission current can be used for closed-loop control of the filament current and / or adapted to focal spot control in a timely manner.

[0060] It should be noted that the above embodiments are illustrative rather than limiting of the invention, and those skilled in the art will be able to devise many alternative embodiments without departing from the scope of the appended claims. Any reference numerals placed between parentheses in the claims should not be construed as limiting the claims. The word "comprising" does not exclude the presence of elements or steps other than those listed in the claims. The words "a" or "an" preceding an element do not exclude the presence of a plurality of such elements. The invention can be implemented by hardware comprising several different elements and / or by a suitably programmed processor. In device claims enumerating several means, several of these means may be embodied by a single item of hardware. Measures recited in mutually different dependent claims may be advantageously combined.

Claims

1. A computer-implemented method of determining tube current during X-ray kVp switching, the method comprising: - generating a calibration tube voltage pulse during a calibration phase, wherein the calibration tube voltage pulse comprises 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 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; - determining a calibration factor from a ratio between the first tube current and the second tube current; - measuring a third tube current at steady state of the first kVp plateau during a kVp switching phase; - determining 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 of claim 1, wherein, The calibration phase occurs during a stabilization phase of an X-ray tube prior to imaging, and wherein the kVp switching phase occurs during a spectral imaging phase.

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

4. The method of claim 1 or 2, wherein, The method further comprises closed loop control of the third tube current at the first kVp plateau and / or the fourth tube current at the second kVp plateau by adapting a filament current.

5. The method of claim 1 or 2, wherein, The method further comprises adapting control of an X-ray tube focal spot position and / or size based on the determined X-ray tube current.

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

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

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

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

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

10. A computer program element, which, when executed by a computer, is configured to perform the method according to any of claims 1-5.

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