Method, computer program, computer tomography system and use of a radiation filter for performing kilovolt peak switching computed tomography

Abstract

A method for performing peak kilo voltage switching computed tomography with a computed tomography system comprising an X-ray source (2), the method comprising the steps of: (a) receiving a user input for a peak kilo voltage switching computed tomography scan, the user input comprising a desired radiation dose setting and a desired speed setting for the scan; (b) setting computed tomography scan parameters based on the received user input; (c) automatically determining whether a radiation filter (6) is required to achieve the desired radiation dose setting and the desired speed setting, and if so, placing the radiation filter (6) in front of the X-ray source (2) to achieve the desired radiation dose with the set scan parameters; (d) performing a peak kilo voltage switching computed tomography scan with the set scan parameters.

Description

Technical Field

[0001] This invention relates to a method and corresponding computer program for performing kilovolt peak-switched computed tomography, a computed tomography system, and the use of a radiation filter. Background Technology

[0002] Spectral computed tomography (CT) imaging is a variation of computed tomography that uses different X-ray energy spectra to distinguish different materials. This technique takes advantage of the fact that X-ray attenuation tends to have different energy dependencies for different materials. When two different energy spectra are used, spectral computed tomography is also known as dual-energy computed tomography.

[0003] A relatively simple and inexpensive method for energy-dispersive CT imaging is rapid kVp switching (kVp-S; "kVp" stands for kilovolt peak), also referred to as fast kVp switching. The kilovolt peak refers to the maximum voltage applied to the X-ray source of a CT system to generate X-rays. This voltage is also known as the tube voltage or tube accelerating voltage. To generate X-rays in CT, electrons are typically emitted from a heated cathode and accelerated toward the anode by an applied voltage (typically in the kilovolt range, and therefore in the kilovolt peak range). The electron current of the electron beam corresponds to the emission current, also referred to as the tube current, tube emission current, or anode current. When the electrons stop at the anode, X-rays with an X-ray energy spectrum are produced, which has a maximum photon energy proportional to the applied kilovolt peak and also depends on the anode material. Fast kVp-S CT is based on using a single X-ray source and rapidly alternating (i.e., switching) the tube accelerating voltage between two different kVp levels.

[0004] However, kVp-S poses a challenge for rapid low-dose imaging. Rapid imaging requires short switching cycles to acquire sufficient projections during short scanner rotation times. To achieve low-dose imaging, the tube emission current is set small because the radiation dose is directly dependent on the tube emission current. Larger emission currents typically result in larger amounts of radiation. Small emission currents generate long voltage transition times from the high-tube accelerating voltage to the low-tube accelerating voltage. This is because the fall-off transition time is primarily defined by the generator's output capacitance and the emission current that discharges that capacitor. During the transition time, the voltage is between the first and second kV peak values. Increased rise and fall times of the voltage can reduce spectral separation. Good material differentiation typically requires a strong difference between the photon energies of the applied X-ray spectrum, and therefore a strong difference between the photon energies of the applied voltage, corresponding to strong spectral separation. For rapid switching and low-dose applications (i.e., requiring low emission currents and thus resulting in slower voltage drops), the transition time can become so long that spectral performance, which depends on good spectral separation between kV peak values, can be significantly compromised. Furthermore, especially for low-dose applications, it can even reach the lower limit, where kVp switching can no longer be performed at the desired rate because the transition time to low kVp levels is longer than the available time.

[0005] US Patent Application US2012 / 0069953 A1 discloses a k-edge notch filter with a k-edge falling between the average energies of the low and high kVp energy spectra to increase the separation of the average energy between the low and high kVp projections.

[0006] US Patent Application US2023 / 0320687 A1 discloses the operation of a kVp-s switching spectral CT imaging device, which aims to simulate the user-side operating workflow of a conventional non-spectral CT scanner by converting user-defined settings (including the desired peak tube voltage and desired tube current of the non-spectral CT scanner) into a set of spectral CT operating parameters estimated to result in the application of the same dose of X-ray radiation.

[0007] US Patent Application US2020 / 0397387 A1 discloses determining a spectral scanning protocol for acquiring CT images using a CT scanner, comprising the steps of: defining a conventional scanning protocol having scanning limitations for acquiring conventional CT images; and determining a spectral scanning protocol similar to the conventional scanning protocol such that the scanning limitations of the conventional scanning protocol are met. Purpose of the invention

[0008] Therefore, the object of this invention is to provide a better method for solving the above-mentioned problems. In particular, it is desirable to find a solution that achieves faster kVp switching while still allowing low-dose imaging. Summary of the Invention

[0009] To better achieve this objective, uses are provided for the method according to claim 1, the computer program according to claim 11, the computed tomography system according to claim 12, and the radiation filter according to claim 15. Advantageous embodiments are set forth in the dependent claims. Any features, advantages, or alternative embodiments described herein with respect to the claimed method are also applicable to other categories of claims, and vice versa.

[0010] According to a first aspect, a method is provided for performing kVp-S CT using a computed tomography (CT) system including an X-ray source. The method includes the following steps: (a) Receive user input for a kilovolt peak-switched computed tomography scan, the user input including the desired radiation dose setting and desired velocity setting for the scan; (b) Setting computed tomography (CT) scan parameters based on received user input, the CT scan parameters including kilovolt peak switching level, switching speed and transmission current; (c) Automatically determine whether the set scan parameters allow the necessary transition time between the kilovolt peak switching levels, and if not, determine that a radiation filter is needed to achieve the desired radiation dose setting and the desired velocity setting, and place the radiation filter in front of the X-ray source to achieve the desired radiation dose using the set scan parameters, and increase the emission current of the scan parameters to compensate for the attenuation effect of the radiation filter. (d) Perform kilovolt peak-switched computed tomography scans using the set scan parameters.

[0011] In particular, the method steps can be executed in a given order.

[0012] Advantageously, using the method according to the invention, rapid kVp-S CT scans can be performed for low-dose applications, even in some cases where the tube voltage transition time would otherwise become too long to meet the timing and dose constraints for rapid low-dose imaging. This is achieved in particular through the flexible selection and placement of the radiation filter. Thus, the radiation filter can be selected and / or changed according to the established scanning protocol. In the context of the invention, "radiation filter" can also be simply referred to as "filter".

[0013] A CT system may have components according to known CT systems. Specifically, a CT system may include a gantry having an X-ray source and an X-ray detector. In the context of this invention, the X-ray detector may also be referred to as a "detector". In the context of this invention, the X-ray source may also be referred to as a "source". The gantry may be a CT gantry commonly known in the prior art. The gantry may be, in particular, a short ring tunnel configured such that a subject or object (e.g., a patient or part of a patient) can be placed within the area enclosed by the gantry.

[0014] The X-ray energy spectrum for kVp-S CT is generated by an X-ray source. The X-ray source can be any X-ray source suitable for kVp-S CT. In particular, X-ray sources known from the prior art can be used. The X-ray source may include a cathode and an anode to generate X-rays. The X-ray source is specifically configured to rapidly switch the tube voltage. Preferably, the X-ray source is configured to provide a substantially stable kilovolt peak between voltage switches, thereby providing a voltage plateau for kVp-S. For the purposes of energy spectrum CT, kVp-S can be achieved by providing voltage switching between multiple kVp plateaus, particularly between two plateaus. For example, the first kilovolt peak may be in the range of 110 kVp to 1000 kVp, preferably in the range of 120 kVp to 200 kVp, and more preferably in the range of 130 kVp to 150 kVp. For example, the second kV peak value can be in the range of 30 kVp to 100 kVp, preferably in the range of 50 kVp to 95 kVp, and more preferably in the range of 60 kVp to 90 kVp. For example, the upper kVp and lower kVp can be set according to the material to be scanned, especially according to the attenuation properties of the material.

[0015] The method includes the step of receiving user input for a kilovolt peak-switched computed tomography scan. For example, the user input can be received via a user interface configured to provide such input or from another user input function. The user input includes a desired radiation dose setting and a desired speed setting for the scan. The radiation dose can, for example, depend on the object or subject to be examined. For example, the subject can be an animal or a human, particularly a patient. For example, the object can be an inanimate object or a part of an animal or human. Generally, limiting the radiation dose is particularly important for living objects (e.g., animals or humans). However, in some cases, limiting the radiation dose may also be relevant to inanimate objects, for example, to prevent damage to sensitive objects. For example, the thickness of the object or subject can be used to determine the radiation dose. For example, larger body parts and / or obese patients may require a higher radiation dose so that sufficiently significant radiated portions can still penetrate the patient and be recorded by the detector. On the other hand, for example, children, small body parts, and / or thin patients may require a lower radiation dose. The desired radiation dose can depend on patient safety regulations. The speed setting can depend on the expected movement of the subject or object being examined.

[0016] The computed tomography (CT) scan parameters are automatically set based on user input. These parameters can be set by the control processing circuitry of the CT system. The scan parameters may include typical scan parameters known from the prior art. The algorithm used to set the scan parameters may be based on known algorithms from the prior art. For example, scan parameters may include one or a combination of the following: detector configuration, kVp-S settings (such as multiple kVp values ​​and / or switching speeds), emission current, patient positioning, scan range, selected reconstruction algorithm, slice thickness, etc. In particular, a relatively high emission current can be set. This high emission current can be achieved due to the possibility of flexibly providing filtering via radiation filters and thus adjusting the dose applied to the subject or patient. The scan parameters may define the scan protocol to be performed or a portion of the scan protocol. Setting the scan parameters may take into account the possibility of determining and placing radiation filters. Therefore, scan parameters can be set such that they are designed to work with corresponding radiation filters. The scan parameters may include an indication of the radiation filter to be used. The radiation filter may be a radiation filter from a set of available radiation filters.

[0017] Based on the set scan parameters and desired radiation dose settings, it is automatically determined whether a radiation filter is needed to achieve the desired radiation dose setting. This determination can be performed by the same control processing circuitry of the computed tomography system that sets the scan parameters. Alternatively, the determination of the radiation filter can be performed by a separate processing circuitry. For example, if setting the scan parameters includes an indication of the radiation filter to be used, the determination of the radiation filter can include checking the indication and, if an indication to use a radiation filter exists, determining that the indicated radiation filter should be used. Optionally, the determination can be performed simultaneously with setting the scan parameters. For example, an iterative process can be applied to determine which scan parameters work best with which available radiation parameters. Therefore, the two steps of setting parameters and determining whether a radiation filter is needed are to be understood broadly. Thus, the computed tomography system (e.g., in the form of control processing circuitry) can be configured to determine which scan mode and with or without a filter are estimated to be optimal for user input. Advantageously, the computed tomography system can therefore apply and / or change the radiation filter according to the scan protocol to be performed.

[0018] Radiation filters are designed to filter X-rays within the energy spectrum range generated by an X-ray source. Radiation filters may include a single filter material or may include multiple filter materials.

[0019] If it is determined that a radiation filter, or a specific radiation filter among multiple available radiation filters, is needed and / or beneficial, the radiation filter is placed between the X-ray source and the subject to be imaged to reduce the radiation dose under the set scan parameters. Continuously, kilovolt peak-switching computed tomography scans with the set scan parameters are performed. Advantageously, using a radiation filter, a low dose can be applied to the object or subject to be examined while still being able to apply a relatively high emission current. This allows for faster switching of kVp, particularly faster switching from high kVp values ​​to low kVp values. Therefore, the X-ray source can operate at a relatively high dose while the actual applied dose is low. Since spectral performance typically depends on the difference between different X-ray spectra (i.e., spectra generated by different kVp values), a fast switching time can be achieved, allowing for better separation and depending on the emission current.

[0020] According to embodiments, radiation filters are adapted to improve energy spectrum imaging performance. Image radiation filters can be selected to enhance energy spectrum imaging performance. For example, radiation filters can be configured to improve spectral separation between high and low kVp. In particular, filter materials with suitable k-edges can shift the X-ray energy spectrum to reduce energy and / or prevent beam hardening. Beam hardening can shift the energy spectrum towards higher energies. Some filter materials (such as rare-earth materials like erbium or holmium) can have a k-edge filtering effect within the relevant spectral energy range. This effect can shift the X-ray energy spectrum towards lower energies and / or limit hardening effects. This can be advantageously used, for example, when higher kVp values ​​tend to produce a larger X-ray dose than lower kVp values. Therefore, lower kVp typically tends to produce a weaker output. For example, a typical output at lower kVp can be about 1 / 7 of the output at higher kVp. However, for optimal material decomposition, both energy spectra are still required: a low kVp spectrum and a high kVp spectrum. Therefore, low-kVp beams are often more problematic for material decomposition. Thus, for example, radiation filters that primarily filter higher X-ray energies (especially those higher than the levels corresponding to lower kVp) can be used. By filtering the high energies, the low-kVp X-ray spectrum can be less affected by the filter or even cleaner due to filtering, while the high-kVp X-ray spectrum can have enough energy to remain useful even if some high-energy photons are filtered out.

[0021] According to an embodiment, the method further includes a step between steps (c) and (d): automatically determining whether, given the effect of the set scanning parameters and the obtained transition time on the energy spectrum imaging performance, placing the radiation filter (6) and increasing the emission current of the scanning parameters to compensate for the attenuation effect of the radiation filter will improve the energy spectrum imaging performance, and if so, placing the radiation filter (6) in front of the X-ray source (2) and increasing the emission current accordingly to achieve improved energy spectrum imaging performance, while achieving the desired radiation dose setting and the desired velocity setting.

[0022] The performance of spectral imaging can be determined using a performance relationship between spectral imaging performance and radiation dose, retrieving and / or determining this relationship, which depends on the tube emission current and radiation filter settings of the X-ray source, wherein determining whether a radiation filter is needed is also based on the performance relationship. The radiation dose can be normalized relative to the system's maximum X-ray tube power. The performance relationship can be based on simulation calculations. Optionally, the simulation can take into account the object or subject to be examined, particularly the material and / or geometry of the object or subject. For example, the simulation calculation can search for maximum spectral performance, particularly the maximum SNR. The tube emission current can be a free parameter selected to maximize the spectral imaging performance. Similarly, radiation filters, such as those in a group of available radiation filters, can be selected to maximize the spectral imaging performance. For example, the performance relationship can be a function of the spectral imaging performance over radiation dose. The calculated radiation dose can be determined based on the emission current and radiation filters. The performance relationship can include multiple functions, particularly one function for each available radiation filter. For example, functions for different filters can be compared at a specific radiation dose, and a filter that provides optimal spectral imaging performance at that radiation dose, as determined by the function, can be selected. Therefore, the maximum spectral performance for a given allowable radiation dose and available radiation filters can be determined relatively easily via performance relationships. The radiation dose can be, for example, a singular value or a range of values. The radiation dose can specifically be as defined by user input.

[0023] According to embodiments, the performance relationship also depends on the duty cycle of the time periods for the low tube voltage interval and the high tube voltage interval for kilovolt peak switching. The duty cycle can define the time course of the tube voltage. Advantageously, by taking the duty cycle into account, a more accurate estimate of the energy spectrum imaging performance is possible. The duty cycle can be adapted to change the dose level for a given scan rate. For example, the duty cycle can be adapted as one of the scan parameters. For example, to achieve a higher radiation dose, the duration of high kVp can be increased at the expense of a low kVp duration. On the other hand, to achieve a lower radiation dose, the duration of low kVp can be increased at the expense of a high kVp duration. This allows for fine-tuning of the radiation dose to achieve a specific desired radiation dose at a given scan rate. Generally, changes in the duty cycle can be detrimental to energy spectrum performance. Therefore, having too short a duration of high kVp or too long a duration of low kVp can degrade energy spectrum performance. Advantageously, determining the combination of radiation filters (in particular, selecting from a set of radiation filters) can allow for avoiding larger variations in the duty cycle and achieving larger differences in radiation dose via the selection of filters. Therefore, for example, this embodiment can combine the advantages of improved performance and the ability to fine-tune the radiation dose via duty cycle by flexibly selecting filters for greater changes in radiation dose.

[0024] According to an embodiment, the performance relationship is based on the relationship between signal-to-noise ratio (SNR) and radiation dose. Spectral performance can be defined via SNR. SNR can be normalized, for example, relative to a reference performance and / or a reference SNR. Advantageously, SNR can provide an effective means of defining and classifying spectral performance. SNR can be calculated based on system simulation.

[0025] According to an embodiment, the minimum tube emission current of the X-ray source required for kilovolt peak-switching computed tomography at a desired scan rate setting is retrieved and / or determined, wherein a radiation filter is determined to be needed if the desired radiation dose setting cannot be achieved above the minimum tube emission current without a filter. For example, the minimum tube emission current can be determined based on performance relationships or other means. Advantageously, using this embodiment, a lower dose at a specific high scan rate can be achieved by using a radiation filter. Optionally, it may be feasible to determine only whether a filter is needed to approximately achieve a specific combination of scan rate and radiation dose.

[0026] According to an embodiment, if placing a radiation filter improves the expected spectral imaging performance for a desired radiation dose setting, then a radiation filter is determined to be needed. According to this embodiment, the step of determining whether a radiation filter is needed can also be used to improve spectral imaging performance. For example, performance relationships can be used to determine the expected spectral imaging performance. However, other methods are also feasible. For example, a database of scan rates and radiation doses with one or more assigned radiation filters can be used. For each set of scan rate values ​​and radiation doses, a specific optimal filter can be assigned, or in some cases, no filter may be assigned. For example, each available filter can be associated with a range of scan rates and radiation doses, where it is advantageous to apply that filter.

[0027] According to embodiments, the radiation filter is selected from a plurality of radiation filters. For example, there may be a set of different radiation filters from which the radiation filters are selected. The radiation filters may differ, particularly in their filter properties. Filter properties may include the filtering energy range and / or the filter transmittance. For example, different radiation filters may differ in their materials. For example, different radiation filters may differ in their thickness. There may be an option to select a combined filter from a plurality of radiation filters. A combined filter is in particular a radiation filter consisting of two or more radiation filters placed one after another to achieve a combined filtering effect of the two filters. Therefore, a combined filter may be selected, which is a combination of a plurality of radiation filters from a plurality of radiation filters.

[0028] According to an embodiment, when it is determined that a radiation filter is needed, a radiation filter is selected from a plurality of radiation filters, which provide the maximum energy spectrum imaging performance of the plurality of radiation filters for setting the desired radiation dose. The maximum energy spectrum imaging performance can be determined, for example, based on performance relationships or based on other means.

[0029] According to embodiments, the radiation filter has a K-edge of at least 20 keV, preferably at least 35 keV, more preferably at least 45 keV, and even more preferably at least 55 keV. Preferably, the K-edge should not be too high. For example, the K-edge can preferably be selected to be below 100 keV, more preferably below 90 keV. Photoelectric absorption based on these K-shell electrons can occur when X-ray photons have energies corresponding to the binding energies of the K-shell electrons (i.e., electrons with principal quantum number n=1) of the atoms of the material. Therefore, for this reason, X-ray photons with energies just above the binding energies of the K-shell electrons are more likely to be absorbed than photons with energies just below the binding energies of the K-shell electrons. In X-ray absorption spectroscopy, this manifests as a sudden increase in X-ray absorption at the energy of the K-shell electrons (i.e., at the K-edge). The energy of the K-edge depends on the material of the filter and can be adjusted by selecting a suitable material for the filter. By using a filter with a higher K-edge, the filter has a greater filtering effect on photons with higher energies. This could be advantageous for achieving better spectral imaging performance. Typically, applying a higher kilovolt peak results in a higher radiation dose than applying a lower kilovolt peak. Therefore, filtering more higher-energy photons and fewer lower-energy photons can be beneficial so as not to weaken the lower-energy spectrum in any way. Furthermore, by filtering the higher energies, the energy separation between the two applied spectra can even be improved, because while the lower-energy spectrum typically shifts to lower energies, the higher-energy spectrum can still contain a sufficient number of higher-energy X-ray photons. Thus, spectral imaging performance can be further improved.

[0030] According to an embodiment, the radiation filter comprises a material with an atomic number of at least 50, preferably at least 60. An atomic number of about 50 may correspond to a K-edge of about 30 keV. An atomic number of about 60 may correspond to a K-edge of about 44 keV. Therefore, the filter material having at least one atomic number according to this embodiment is suitable for achieving suitable filtering effects and / or improving energy dispersive spectroscopy imaging performance.

[0031] According to embodiments, the radiation filter comprises rare-earth materials and / or tungsten. For example, the rare-earth material may be selected as erbium, holmium, or gadolinium. These materials have relatively high atomic numbers z, which results in relatively high K-edge absorption. Therefore, these materials can be suitable for providing appropriate filtering effects and improving energy dispersive spectroscopy imaging performance.

[0032] According to another aspect of the invention, a computer program comprising instructions that, when executed by the control circuitry of a computed tomography (CT) system, cause the CT system to perform the methods described herein. The instructions may include any one of the steps of the methods described herein and variations thereof. The computer program may be stored on a data storage medium, particularly a non-transient data storage medium.

[0033] According to another aspect of the invention, a computer-readable data storage medium, particularly a non-transient data storage medium, includes instructions that, when executed by control circuitry of a computed tomography (CT) system, cause the CT system to perform the methods described herein. The data storage medium may include the computer program described herein. The data storage medium can be any computer-readable storage medium. For example, the data storage medium may be a solid-state storage medium or an optical storage medium. The storage medium may be a hard disk drive, solid-state drive, compact disk, DVD, flash memory, online server, cloud, etc.

[0034] According to another aspect of the present invention, a computed tomography system is provided. The system includes:

[0035] A gantry with an X-ray source, especially a rotatable gantry;

[0036] A control circuit configured to control a computed tomography scan performed by the computed tomography system;

[0037] User input functionality, optionally including a user interface, is used to obtain user input to select settings for a kilovolt peak-switching computed tomography scan, the settings including a desired radiation dose setting and a desired velocity setting for the scan, wherein the control circuitry is configured to automatically set scan parameters based on the user input, the scan parameters including a kilovolt peak-switching level, a switching velocity, and an emission current; the user input specifically includes settings selected by the user.

[0038] At least one radiation filter, wherein the computed tomography system is configured to automatically determine whether the set scan parameters allow the necessary transition time between kilovolt peak switching levels, and if not, to determine that a radiation filter is needed to achieve the desired radiation dose setting, and if so, to place the at least one radiation filter in front of the X-ray source and increase the emission current of the scan parameters to compensate for the attenuation effect of the radiation filter, thereby achieving the desired radiation dose setting.

[0039] The computed tomography system is optionally configured to perform the methods described herein.

[0040] The system may include computer programs and / or computer-readable data storage media as described herein. For example, control circuitry may be part of a control station for the system. The control station may be a local control station or a remote control station. The control station need not be a single control station, but portions thereof may optionally be distributed in different locations. User input functionality may be configured to obtain user input from a built-in user interface or from another device that is not part of the claimed system. For example, user input functionality and / or the user interface may include a user menu with corresponding options for selection or setting. If the system happens to include a radiation filter, the system may be configured to determine, based on user input, whether a radiation filter is needed to implement the set user input and / or whether the radiation filter is beneficial for improving spectral imaging performance. Alternatively, the system may include multiple radiation filters. Radiation filters or multiple radiation filters may be adapted to improve spectral imaging performance, particularly for set radiation dose settings.

[0041] According to an embodiment, the computed tomography system includes a set of radiation filters and is configured to select a radiation filter from the set of radiation filters according to defined filter suitability criteria if it is determined that a radiation filter is needed. The suitability criteria may, for example, relate to one or more of the following: spectral imaging performance, the achievement of a specific scan rate combined with the achievement of a specific radiation dose (e.g., a maximum radiation dose, a radiation dose range, or a specific number of radiation doses). The suitability criteria may particularly include selecting a radiation filter that provides the maximum spectral imaging performance of a plurality of radiation filters for a given radiation dose setting. The set of radiation filters may include radiation filters comprising different materials. The properties of the radiation filters may vary, for example, due to different atomic numbers of the materials and / or due to different K-edges of the radiation filters and / or due to different thicknesses of the radiation filters.

[0042] According to embodiments, the radiation filter or a group of radiation filters has a K-edge of at least 20 keV, preferably at least 35 keV, more preferably at least 45 keV, and even more preferably at least 55 keV. For example, the radiation filter or a group of radiation filters may each comprise tungsten and / or rare earth materials, such as erbium, holmium, and / or gadolinium.

[0043] Features and advantages of different embodiments can be combined. Features and advantages of one aspect of the invention (e.g., the method) can be applied to other aspects, such as computer programs, storage media, and computed tomography systems, and vice versa. Attached Figure Description

[0044] The invention will now be described with reference to embodiments in the accompanying drawings, in which:

[0045] Figure 1 A flowchart is shown for a method of performing kilovolt peak-switching computed tomography according to an embodiment of the present invention;

[0046] Figure 2 An exemplary simplified excerpt illustrates the process of switching the tube voltage in a kilovolt peak-to-voltage computed tomography scan.

[0047] Figure 3 The performance relationship between energy spectrum imaging performance and radiation dose according to an embodiment of the present invention is shown;

[0048] Figure 4 It shows that according to different Figure 2 An exemplary simplified excerpt of the process of switching the applied tube voltage during computed tomography scan at another duty cycle of kilovolt peak.

[0049] Figure 5 It shows that according to different Figure 4 of Figure 2 An exemplary simplified excerpt of the process of switching the applied tube voltage during computed tomography scan at another duty cycle of the duty cycle shown.

[0050] Figure 6 This is an exemplary simplified excerpt illustrating the process of applying tube voltage during a kilovolt peak-switched computed tomography scan, wherein, with Figure 2 Compared to the example in the previous example, the emission current is reduced;

[0051] Figure 7 A schematic cross-sectional view is shown from the side of a computed tomography system according to an embodiment of the present invention. Figure Labels 1 rack 2 X-ray sources 3 X-ray detectors 4 Control Station 5 User Interface 6. Radiation Filter 7 patient stations 10-tube voltage 11 Measurement interval of X-ray detector Detailed Implementation

[0052] Throughout the accompanying drawings, the same or corresponding features / elements of various embodiments are indicated by the same reference numerals.

[0053] Figure 1A flowchart illustrating a method for performing kilovolt peak-switched computed tomography (CT) according to an embodiment of the present invention is shown. In a first step 101, user input for a kilovolt peak-switched CT scan is obtained. The user input includes a desired radiation dose setting and a desired velocity setting for the scan. For example, the user input can be obtained through a user input function (e.g., user interface 5). User interface 5 may include menus that allow the user to select and / or set the radiation dose setting and velocity setting. In another step 102, CT scan parameters are automatically set based on the user input. The setting of the scan parameters can generally be performed similarly to that known in the prior art. In another step 103, based on the set scan parameters and the desired radiation dose setting, it is automatically determined whether a radiation filter 6 is needed to achieve the desired radiation dose setting. The radiation filter 6 may be adapted to improve spectral imaging performance, particularly by improving the spectral separation between the higher and lower kilovolt peak X-ray spectra of the kilovolt peak switching. Optionally, the need for the radiation filter 6 may also be determined if placing the filter 6 improves the expected spectral imaging performance for the desired radiation dose setting. Optionally, a radiation filter 6 can be selected from a plurality of radiation filters 6, the radiation filter being best suited to achieve the desired radiation dose at the desired rate and / or to provide the maximum energy spectrum imaging performance of the plurality of radiation filters 6 for the desired radiation dose setting. In another step 104, if it is determined that a radiation filter 6 is needed and / or once the most suitable radiation filter 6 has been selected, the corresponding radiation filter 6 is placed in front of the X-ray source 2 to reduce the radiation dose under the set scan parameters. The radiation filter 6 can be placed in front of the X-ray source 2, for example, automatically. For example, the system may include a mechanism for automatically selecting and setting the filter 6. In another step, a kilovolt peak-switched computed tomography scan is performed using the set scan parameters and the placed filter 6.

[0054] Figure 2The process of applying tube voltage switching for kilovolt peaks is illustrated. The vertical axis corresponds to the applied tube voltage 10, and the horizontal axis corresponds to time. The dashed line indicates the measurement interval 11 of the X-ray detector 3 on which the X-ray detector 3 integrates. It can be seen that the tube voltage 10 switches between a higher voltage plateau corresponding to a higher kilovolt peak and a lower voltage plateau corresponding to a lower kilovolt peak. Between voltage plateaus, there are transition phases where the voltage changes from one plateau to another. The total switching rate generally corresponds to the scan rate and the length of the measurement interval. The applied tube voltage 10 is used to accelerate electrons toward the anode of the X-ray source 2 and to determine the velocity of the electrons when they strike the anode. X-ray energy spectra are generated via the interaction of electrons with the anode. The X-ray energy spectra depend on the energy of the electrons, and therefore on the applied voltage 10, where the maximum energy of each X-ray energy spectrum corresponds to the currently applied voltage 10. The energy spectrum imaging performance depends on the energy spectrum separation between the two energy spectrum imaging performances (i.e., corresponding to the two voltage plateaus), and may also depend on the intensity of each plateau acquired during the measurement interval 11.

[0055] Figure 3 The performance relationship between spectral imaging performance and radiation dose according to an embodiment of the invention is illustrated. The spectral imaging performance without filter 6 (solid line) and with radiation filter 6 placed in front of the X-ray source 2 of the computed tomography system (dashed line) are shown. The performance relationship is based on the signal-to-noise ratio (SNR) on the vertical axis in relation to the radiation dose on the horizontal axis. The spectral imaging performance is given as a percentage of SNR relative to a detector-based spectral CT scanner using the same dose. The radiation dose is specified as a percentage of the maximum X-ray tube power of the CT system. The plotted SNR is an estimated performance constrained by a specific example system, and realistic performance SNR values ​​are shown. In this embodiment, performance reaches its maximum at approximately 45% dose. If the requested radiation dose is higher than this optimal value, the protocol can, for example, change the duty cycle of kVp-s towards a longer high-voltage period to achieve the requested dose level.

[0056] exist Figure 4 An example of this change in duty cycle is shown, where, with Figure 2 In contrast, the duty cycle is changed to make the high-voltage period longer. Specifically, the high-voltage period is longer than the low-voltage period. Since applying a higher voltage increases the dose of generated X-ray radiation, the radiation dose can be increased without changing the overall switching rate of the kVp switching. Therefore, the scan rate can remain approximately the same.

[0057] Refer again Figure 3As can be seen, this increase reduces performance; specifically, for radiation dose values ​​above approximately 45%, the SNR decreases. This decrease in SNR is due to the corresponding high voltage plateau (which, according to...) Figure 4 (with a longer duty cycle) and a low voltage plateau (which, according to) Figure 4 Imbalance in acquisition time (within the shorter duty cycle) leads to a decrease in SNR. On the other hand, performance also degrades if the required radiation dose is below approximately 45% of the optimal value. To achieve a lower radiation dose, the duty cycle in kVp-S can be shifted towards longer periods targeting lower high-voltage periods.

[0058] exist Figure 5 An example of this change in duty cycle is shown, where, with Figure 2 In contrast, the duty cycle is changed to make the low-voltage period longer. Specifically, the low-voltage period is longer than the high-voltage period. Since applying a lower voltage reduces the dose of the generated X-ray radiation, the radiation dose can be reduced without changing the overall switching rate of the kVp switching. Therefore, the scan rate can remain approximately the same.

[0059] Refer again Figure 3 As can be seen, reducing the radiation dose also reduces the spectral SNR. At a certain dose level, the duty cycle reaches its limit and cannot be changed further. If this occurs, the protocol can typically use a lower tube current to further reduce the dose. However, this is also limited. A lower tube current will increase the transition time, particularly the transition time from a high voltage plateau to a lower voltage plateau. This increase in transition time is because the fall-off transition time is primarily defined by the generator's output capacitance and the emission current that discharges that capacitor. Therefore, a lower emission current will result in a longer transition time. Increasing the transition time can significantly degrade spectral performance because the relative time the voltage is actually at the plateau will be shorter, and for a significant portion of the time, the voltage will be in transition, resulting in less spectral separation across the measurement interval. This is evident in... Figure 6 As shown, the ramp-down time is depicted as quite long, occurring over a significant portion of the measurement interval. At some point, the transition between voltage plateaus may become so long that the available time for acquisition is no longer sufficient to reach low kVp levels. Figure 3 In the example shown, this point is reached at approximately 18% of the radiation dose. The performance graph stops below this point. Below this point, the system cannot perform a scan with the desired dose level without applying filter 6. The effect of applying the corresponding radiation filter 6 is via... Figure 3The dashed line in the diagram illustrates the spectral imaging performance (SNR) of a system with the applied radiation filter 6. The radiation filter 6 reduces the tube output flux before irradiating the subject or examinee, resulting in a typically lower radiation dose. This can be achieved without reducing the tube emission current because the radiation filter 6 is applied after X-ray generation. Therefore, the transition time can still be relatively short. Compared to the curve (solid line) corresponding to the case without filter 6, the curve (dashed line) shifts towards lower radiation dose values. The performance shape is similar, with the best performance in the middle portion of the curve and reduced performance for both lower and higher dose levels. The maximum SNR is even increased compared to the curve without filter 6 (below approximately 31% in this example), due to the improved spectral separation achievable with the radiation filter 6. Furthermore, lower radiation dose levels (i.e., below approximately 18% in this example) are now accessible. The system can now perform low-dose imaging. Based on the requested dose level, the CT scanner can determine the optimal scanning mode with or without filter 6. Scans can be performed using the optimal settings. For example, the emission current can be increased while still maintaining a relatively low radiation dose due to the applied radiation filter 6. Therefore, the algorithm can be configured to optimize spectral imaging performance based on multiple free or fixed variables, such as emission current, radiation dose, duty cycle, scan rate, etc. Specifically, the radiation dose and / or scan rate can be fixed parameters. Specifically, the tube emission current and duty cycle can be free parameters. The duty cycle can be defined, for example, by the relative time of a higher and lower voltage applied in kVp-s. For example, the ratio of the time of the higher and lower voltages can be a free parameter within the boundaries of the desired scan rate. If multiple different radiation filters 6 are available (e.g., including different materials and / or different thicknesses), multiple different curves for different radiation filters 6 can be possible, giving even a wider range of possible combinations of scan rate and radiation dose, and achieving better spectral performance over a wider range of radiation dose settings. Multiple different curves for different radiation filters 6 can be specifically shifted to different radiation doses and / or have maximum performance at different radiation doses.

[0060] Figure 7A schematic cross-sectional view of a side view of a computed tomography system according to an embodiment of the present invention is shown. The computed tomography system includes a patient table 7 and a rotatable gantry 1 having an X-ray source 2 and an X-ray detector 3. The gantry 1 is rotatable such that the X-ray source 2 and the X-ray detector 3 rotate about an area above the patient table where a patient can be placed. The control circuitry of the computed tomography system is configured to control computed tomography scans performed by the computed tomography system. The control circuitry may be part of a control station 4. User input functions obtain user input settings regarding kilovolt peak switching computed tomography scans, for example, from a built-in user interface 5 or from another device not part of the claimed system. Settings include desired radiation dose settings and desired velocity settings for the scan. The control circuitry is configured to automatically set scan parameters based on user input. The system also includes a radiation filter 6, and the system is configured to automatically determine whether the radiation filter 6 is needed to achieve the desired radiation dose setting. If needed, the system is configured to place at least one radiation filter 6 in front of the X-ray source 2, depending on the set scan parameters and desired radiation dose settings. For example, the system may be configured to perform actions such as regarding Figure 1 The described method. Optionally, the computed tomography system may include a set of radiation filters 6 and may be configured to select a filter 6 from this set of radiation filters 6 according to defined filter suitability criteria. Suitability criteria may include selecting the filter 6 that provides the maximum spectral imaging performance of the plurality of filters 6 for a set radiation dose setting.

[0061] The above discussion is merely illustrative of the system and should not be construed as limiting the appended claims to any particular embodiment or group of embodiments. Therefore, the specification and drawings should be considered illustrative and not intended to limit the scope of the appended claims. Other embodiments of the disclosed invention can be understood and implemented by those skilled in the art upon studying the drawings, disclosure, and appended claims in practice of the claimed invention. In the claims, the word "comprising" does not exclude other elements or steps, and the words "a" or "an" do not exclude multiple. A single processor or other unit can perform the functions of several items recited in the claims. Although specific measures are recited in dissimilar dependent claims, this does not indicate that combinations of these measures cannot be advantageously used. Computer programs can be stored / distributed on suitable media, such as optical storage media or solid-state media provided with or as part of other hardware, but computer programs can also be distributed in other forms, such as via the Internet or other wired or wireless telecommunications systems. Any reference numerals in the claims should not be construed as limiting the scope.

Claims

1. A method for performing kilovolt peak-switching computed tomography using a computed tomography system including an X-ray source (2), the method comprising the following steps: (a) Receive user input for a kilovolt peak-switched computed tomography scan, the user input including the desired radiation dose setting and desired velocity setting for the scan; (b) Setting computed tomography (CT) scan parameters based on received user input, the CT scan parameters including kilovolt peak switching level, switching speed and transmission current; The method is characterized by further comprising the following steps: (c) Automatically determine whether the set scanning parameters allow the necessary transition time between the kilovolt peak switching levels, and if not, determine that a radiation filter (6) is needed to achieve the desired radiation dose setting and the desired velocity setting, and place the radiation filter (6) in front of the X-ray source (2) to achieve the desired radiation dose using the set scanning parameters, and increase the emission current of the scanning parameters to compensate for the attenuation effect of the radiation filter; (d) Perform kilovolt peak-switched computed tomography scans using the set scan parameters.

2. The method according to claim 1, further comprising the following steps between steps (c) and (d): (c2) Automatically determine whether, given the effect of the set scanning parameters and the obtained transition time on the energy spectrum imaging performance, placing the radiation filter (6) and increasing the emission current of the scanning parameters to compensate for the attenuation effect of the radiation filter will improve the energy spectrum imaging performance; and if so, place the radiation filter (6) in front of the X-ray source (2) and increase the emission current accordingly to achieve improved energy spectrum imaging performance, while achieving the desired radiation dose setting and the desired velocity setting.

3. The method according to claim 2, wherein The performance improvement is based on the signal-to-noise ratio related to radiation dose.

4. The method according to any one of the preceding claims, wherein, The radiation filter (6) has a K-edge of at least 20 keV, preferably at least 35 keV, and more preferably at least 45 keV.

5. A computer program comprising instructions that, when executed by a control circuit of a computed tomography system, cause the computed tomography system to perform the method according to any one of the preceding claims.

6. A computed tomography system, the system comprising: The frame (1) has an X-ray source (2); A control circuit configured to control a computed tomography scan performed by the computed tomography system; a user input function for obtaining a user input to select settings regarding a kilovolt peak switching computed tomography scan, the settings including a desired radiation dose setting and a desired speed setting of the scan, wherein the control circuit is configured to automatically set scan parameters including a kilovolt peak switching level, a switching speed and an emission current in dependence of the user input; at least one radiation filter (6), characterized in that the computed tomography system is configured to automatically determine whether the set scan parameters allow for a necessary transition time between kilovolt peak switching levels and, if not, to determine that a radiation filter (6) is required to achieve the desired radiation dose setting and, if required, to place the at least one radiation filter (6) in front of the X-ray source (2) and to increase the emission current of the scan parameters to compensate for an attenuation effect of the radiation filter to achieve the desired radiation dose setting.

7. The computed tomography system of claim 6, wherein, the computed tomography system comprises a set of radiation filters (6) and is configured to select a radiation filter (6) of the set of radiation filters (6) in dependence of defined filter suitability criteria, if it is determined that a radiation filter (6) is required, wherein the suitability criteria in particular include selecting a radiation filter (6) that provides a maximum spectral imaging performance of a plurality of radiation filters (6) for a set radiation dose setting.

8. The computed tomography system of claim 7, wherein the radiation filter (6) or the radiation filter (6) of the set of radiation filters (6) has a K-edge of at least 20 keV, preferably a K-edge of at least 35 keV, more preferably a K-edge of at least 45 keV.

Images