Reducing x-ray doses applied to subject during respiratory-related CT scans

Abstract

Embodiments of the present disclosure relate to reducing an X-ray dose applied to a subject during a breath-related computed tomography scan. A computer-implemented method for reducing an X-ray dose (12) applied to a subject (5) during breath-related computed tomography, the method comprising the steps of: (a) determining a start of a data redundant breathing phase during a beam-on period of the scan; (b) during the data redundancy breathing phase, applying redundancy modulation to the applied X-ray dose such that the X-ray dose during the data redundancy breathing phase is reduced; (c) determining the end of the data redundancy breathing stage; and (d) at the end of the data redundancy breathing phase, continuing the scanning without performing redundancy modulation of the X-ray dose.

Description

Technical Field

[0001] The present invention relates to a computer implemented method and a corresponding computer program computer readable storage medium for reducing the X-ray dose applied to a subject during a breathing related computed tomography scan, and a computed tomography scanning system. Background Art

[0002] Respiration-correlated computed tomography (CT) imaging, also known as four-dimensional (4D) CT imaging, is often used as a basis for radiotherapy treatment planning. 4D CT imaging is a method for measuring anatomical motion caused by the patient's breathing. During CT acquisition, the patient's breathing is monitored and set with respect to the start time of the CT scan. Therefore, the reconstructed CT slices can be correlated to the respiratory phase of the patient's respiratory cycle. By selecting corresponding slices associated with different phases of the respiratory cycle, CT images can be generated at multiple respiratory phases.

[0003] 4D CT image quality has traditionally been severely affected by image artifacts. This stems from the retrospective analysis and correlation of the respiratory cycle with the CT scan. For example, due to the irregularity of the patient's respiratory cycle, the acquired data coverage may not be sufficient to reconstruct image slices under a specific respiratory state. This may even lead to the necessity of repeating the CT scan and thus applying additional X-ray doses. To solve this problem, the data sufficiency condition (DSC) is proposed. DSC requires that for each bed position of the desired scanning range, projection data is acquired at least throughout the patient's entire respiratory cycle. R. Werner, T. Sentker, F. Madesta, T. Gauer, and C. Hofmann describe the corresponding method in "Intelligent 4D CT sequence scanning (i4DCT): Concept and performance Evaluation", Med. Phys. 46 (8), August 2019, pp. 3462-3474. Intelligent 4D CT sequence scanning (i4DCT) is a method for breathing-related 4D CT scanning, in which the start and end of the data acquisition process (beam-on and beam-off events) at a given bed position are triggered by the patient's breathing. Each beam-on period covers the complete respiratory cycle from end of inspiration to end of inspiration to ensure that DSC is met at each bed position. With this adaptation, it is possible to ensure that all relevant respiratory states of the patient's respiratory cycle are recorded at each bed position.

[0004] Although i4DCT thus introduces considerable advantages, further improvements in 4D CT imaging are still desired. In particular, it would be advantageous to reduce the applied X-ray dose. Summary of the invention

[0005] It is therefore an object of the present invention to provide a method for improving 4D CT imaging, preferably a method for reducing the applied X-ray dose.

[0006] This object is met or exceeded by a method according to claim 1, a computer program according to claim 13, a computer readable storage medium according to claim 14 and a computed tomography scanning system according to claim 15. Further advantages and features result from the dependent claims, the description and the drawings.

[0007] According to a first aspect of the present invention, there is provided a computer-implemented method for reducing the X-ray dose applied to a subject during a breathing-related computed tomography (CT) scan. The method comprises the following steps:

[0008] (a) determining the start of a data redundancy breathing phase during a beam-on period of a scan;

[0009] (b) during the data redundancy breathing phase, applying redundancy modulation to the applied X-ray dose so as to reduce the X-ray dose during the data redundancy breathing phase;

[0010] (c) determining the end of the data redundancy breathing phase;

[0011] (d) At the end of the data redundancy breathing phase, scanning is continued without redundant modulation of the X-ray dose.

[0012] Advantageously, the X-ray radiation dose applied to the subject may be reduced via this method by taking into account the data redundant breathing phase and adjusting the applied X-ray dose accordingly.

[0013] The subject may be a human or an animal. For example, the subject may be a patient. The subject may be a patient undergoing radiation therapy, particularly radiation therapy monitored via respiratory-related CT scans.

[0014] The term "X-ray dose" should be understood broadly in the context of the present invention. It generally describes the amount of X-ray radiation dose absorbed by and / or sent to a subject or a part of a subject. Typically, the dose can be absorbed by the tissue of the subject. It can be a measure of the radiation energy deposited by the incident radiation in the subject's tissue. The dose can be measured in SI units "Gray" (Gy) or other units (such as the unit "rad"). Both the overall dose (absorbed by the subject as a whole) and the local dose (absorbed by a part of the subject, such as the scanned organ) may be related. The X-ray dose can be controlled by controlling the output of at least one X-ray source of the CT system. For example, controlling the X-ray source can include controlling the intensity of the emitted X-ray radiation, the direction of the emitted X-ray radiation (particularly relative to the position of the subject) and / or blocking the radiation path of the emitted X-ray radiation.

[0015] The beam-on period of the scan is a period in which at least one X-ray beam is turned on. Typically, during a CT scan, the subject is moved to different bed positions and CT data is collected at different bed positions. The bed position describes the position of the subject in the direction of the longitudinal axis of the gantry of the CT system. The position of the subject is usually adjusted by moving a movable patient table. The bed position may also be referred to as the z position. The time of data acquisition usually corresponds to the beam-on period, i.e., the time when the X-ray beam is turned on for data acquisition. Preferably, the beam-on period of the CT scan can cover a complete respiratory cycle, respectively. A complete respiratory cycle can include the entire respiratory process from one respiratory state to the next periodic repetition of the same respiratory state. For example, a complete respiratory cycle can be from the end of inspiration to the end of inspiration. The end of inspiration can be defined as the respiratory state at the time of maximum inspiration, especially before the subject starts to exhale again. Preferably, in the context of the present invention, the respiratory cycle can be considered to be at the end of inspiration state, especially at the beginning and end of two consecutive end of inspiration states.

[0016] The breathing-correlated computed tomography scan may be based on covering a complete breathing cycle at each bed position. Preferably, the breathing cycle may be covered from end of inspiration to end of inspiration. This may be a point in the breathing cycle that is particularly suitable for monitoring the breathing cycle and adjusting the CT scan accordingly. The start and end of the data acquisition process at a given bed position (i.e., beam-on and beam-off events) may be triggered by a real-time analysis of the patient's breathing. Preferably, between consecutive beam-on periods, there may be at least one breathing cycle during which the X-ray beam is turned off. In other words, during consecutive beam-on periods, there may be beam-off periods of duration of at least one breathing cycle. The breathing-correlated computed tomography scan may be based in particular on intelligent 4D CT sequence scanning (i4DCT).

[0017] The term "breathing phase" is to be understood broadly in the context of the present invention. It generally describes a phase, i.e. a portion, of a breathing cycle. For example, a breathing phase may be a portion of a breathing cycle during which a subject inhales, or may be a portion during which a subject exhales, or a portion during which a subject pauses both inhaling and exhaling (e.g. between an expiratory phase and an inspiratory phase). However, any other kind of breathing phase may also be determined, e.g. depending on the context.

[0018] The data redundant breathing phase is in particular a breathing phase during which no additional CT data can be acquired during the breathing cycle and / or at a bed position containing any additional important information related to the data acquired during the rest of the breathing cycle at the current bed position. The start and end of the data redundant breathing phase can be determined by monitoring the breathing of the subject. Determining the start and / or end of the data redundant breathing phase can be based on a reference breathing cycle. The reference breathing cycle can be specific to the subject currently being examined by the CT scan. The reference breathing cycle can be determined based on data acquired before the start of the CT scan. Additionally or alternatively, the reference breathing cycle can be determined based on data acquired during the CT scan. The reference breathing cycle can be updated during the CT scan. The update can be based on monitoring the breathing of the subject during the scan. Updating the reference breathing cycle can be beneficial to counteracting drift effects of monitoring or changes in the breathing pattern of the subject. Redundant modulation can be applied by reducing the X-ray output of at least one X-ray source of the CT system. Additionally and / or alternatively, redundant modulation can be applied by changing the X-ray output direction of at least one X-ray source of the CT system. Additionally and / or alternatively, redundant modulation may be applied by blocking at least a portion of the X-ray output of at least one X-ray source of the CT system. The term "redundant modulation" is generally understood in the context of the present invention in a broad sense and is particularly defined as providing a reduction in the applied X-ray dose. Optionally, the reduction in X-ray dose may include reducing the X-ray dose to zero or near zero.

[0019] The inventors have found that for many patients, at least during some cycles, the respiratory cycle includes a respiratory phase of up to several seconds in which no significant respiratory movement occurs. This is usually the case in the end-of-expiratory state. The inventors have noticed that data acquired during such apnea does not provide significant additional information, or only provides very little additional information. Since respiratory-related computed tomography scans are intended to observe data of several respiratory states (e.g., respiratory states at maximum inspiration, minimum inspiration, and between these endpoints), data from such apnea is usually not actually required to monitor all required respiratory states. Therefore, instead of applying data sufficiency conditions by acquiring the entire respiratory cycle at full dose, the X-ray dose can be reduced in the redundant part of the respiratory cycle. Modulating the X-ray dose to be reduced can therefore advantageously prevent the subject from being exposed to unnecessary ionizing radiation. Therefore, the X-ray dose can be reduced when data redundancy allows. Therefore, the dose can be reduced without significantly limiting the full-stage selectivity of retrospective 4D CT reconstruction.

[0020] According to an embodiment, the respiratory phase is determined to be data redundant when the estimated respiratory phase does not provide significant additional respiratory state during the current respiratory cycle and / or at the current bed position. For example, it can be provided that the respiratory phase is determined to be data redundant when an apnea after an exhalation phase is detected.

[0021] The term "respiratory state" should be understood broadly in the context of the present invention. It may generally describe any state of a subject's breathing. For example, maximum inspiration or minimum inspiration during a breathing cycle may be examples of a respiratory state. A respiratory state may be specifically related to or defined by the extension and contraction of a subject's lungs. A respiratory state may be defined by a surrogate parameter. A surrogate parameter may be related to the subject's respiratory state. For example, a rise in the subject's chest may be used to define a respiratory state.

[0022] The term "significant additional respiratory state" may be understood as meaning that the additional respiratory state is very close to at least one already measured respiratory state during the respiratory cycle and / or at the current bed position. In particular, the already measured respiratory state may be a previous respiratory state. The term "very close" may be defined by a threshold difference. Additionally or alternatively, the term "very close" may be defined by a typical measurement accuracy and / or measurement signal noise.

[0023] According to an embodiment, the breathing phase is determined to be data redundant when the change in breathing state over time is estimated to be below a threshold and / or when the variance of the breathing state is estimated to remain within a predetermined range. Advantageously, the redundant modulation can thus be applied as soon as the breathing state remains substantially constant.

[0024] According to an embodiment, the respiratory signal is monitored to determine the start and end of the data redundant respiratory phase. The respiratory signal can be monitored via a substitute signal. The substitute signal can be a signal that defines a substitute parameter. For example, the substitute signal can be a signal related to the respiratory state of the subject. For example, the substitute signal can include information about the rise of the subject's chest. Therefore, although breathing generally includes the expansion and contraction of the lungs (i.e., a three-dimensional object, and therefore a change in three-dimensional space), the rise of the chest (a change in one direction) can still be useful for defining the respiratory state, because there is usually a correlation between the rise of the chest and the entire movement of the lungs / chest. The respiratory signal can be monitored by a strain gauge. The strain gauge can be connected to the subject's chest. For example, the strain gauge can at least partially surround the subject's chest. Therefore, the CT system can include a strain gauge configured to determine the respiratory state of the subject. The strain gauge can include a belt or a part of a belt, which is configured to be wrapped around the subject's chest. Therefore, the respiratory signal can be monitored by a belt wrapped around the subject's chest and including a strain gauge. Additionally or alternatively, the respiratory signal can be monitored via at least one optical marker (such as an infrared marker) and a corresponding camera (such as an infrared camera). In the context of the present invention, the term "light" should be understood in a broad sense. It may include any kind of electromagnetic signal that can be detected. In particular, the term light includes infrared light. The optical marker may be configured to emit a light signal, in particular an infrared signal. Alternatively, the optical marker may include a light reflector, in particular an infrared light reflector. The optical marker may be placed on the subject's chest. The camera may be placed at the end of the table. The camera may be configured to receive a light signal from the optical marker. The computed tomography scanning system may include the optical marker and / or the camera. Optionally, the computed tomography scanning system may include a light emitter configured to emit light so that the light is reflected by the light reflector.

[0025] In the context of the present invention, the breathing signal can be represented, for example, as ζ i , where i represents a time point. i It may particularly indicate a rise in the subject's chest. i It can also be called the respiratory state or respiratory signal at time point i. The derivative of the respiratory signal at time point i can be expressed as The derivative of the respiratory signal can also be called the respiratory rate. Based on the monitored respiratory signal, the reference respiratory signal ζ can be determined ref. In particular, the reference breathing signal is specific to the subject. Therefore, the reference breathing signal can be referred to as a subject-specific reference signal. The reference breathing signal can be determined before starting a breathing-related computed tomography scan. The reference breathing signal can be updated based on the breathing signal monitored during the breathing-related computed tomography scan. Via the reference breathing signal, a reference breathing cycle can be defined. The reference breathing cycle can be defined similarly to the definition used to generally define a breathing cycle, for example as described herein.

[0026] According to an embodiment, a necessary condition for determining the start of a data redundant breathing phase is that the signal length since the last end of the inhalation state has a defined minimum length relative to the reference exhalation duration. Preferably, if the condition is met or has been met at any time point during at least one previous or current respiratory measurement sample during the current respiratory cycle, it can be determined that the condition is met. The exhalation duration is particularly the duration from the end of the inhalation state to the minimum respiratory state or to the respiratory state that is higher than the minimum respiratory state by a defined absolute amount or relative amount. For example, the defined amount can be 5%. The amount can be based on the temporal resolution of the breathing-related CT scan. For example, the breathing-related CT scan can be configured to take images with a specific percentage of the respiratory state amplitude as a step length, such as a step length of 5% or 10% of the amplitude. For example, a reconstruction box (bin) can be provided for sorting CT data based on the respiratory state. The reconstruction box can be defined according to the percentage value of the current respiratory state relative to the maximum respiratory state. For example, a reconstruction box can be used for up to 90% to 100%, one for 80% to 90%, etc. In the context of the present invention, the respiratory state amplitude can be regarded as the difference between the minimum respiratory state and the maximum respiratory state. Advantageously, the amount can therefore be selected so that it corresponds to the time resolution. The minimum breathing state can be defined as the breathing state at maximum exhalation. The reference exhalation duration can be based on a subject-specific reference breathing signal and / or an average exhalation duration. Therefore, the reference exhalation duration can be the exhalation duration of a reference breathing cycle determined from the reference breathing signal. The average exhalation duration can be determined via respiratory monitoring and by determining the average time spent by the subject during the monitoring period. Advantageously, the signal length since the last inhalation end state is therefore monitored. Via the conditions according to this embodiment, it can therefore be ensured that redundant modulation does not begin before the end of the subject's typical exhalation phase. The relationship with the reference exhalation can be used to define the degree of certainty of such a condition relative to not losing basic data. The relationship can be defined by a pre-factor. The pre-factor can depend on the estimated variance or standard deviation of the subject's exhalation duration. The pre-factor can be preset. The pre-factor can be dynamically updated based on the monitored respiratory signal, in particular based on the determined variance or standard deviation. In the context of the present invention, variance or standard deviation should be broadly understood and include any equivalent measure of data variation. Advantageously, via the prefactor it can be ensured that the risk of losing essential data is reduced by adapting the conditions accordingly using the prefactor.

[0027] For example, the necessary conditions for determining the start of the data redundancy breathing phase may be defined by the following or equivalent terms:

[0028]

[0029] where i 100％is the sample index representing the last state of maximal inspiration (i.e., the beginning of the current respiratory cycle), H(x) is the Heaviside step function, is the sample index corresponding to the 5% exhalation state in the reference breathing cycle (i.e., 5% above the minimum breathing state). 1.1 is the pre-factor as described above. This specific pre-factor 1.1 can be replaced by any other suitable pre-factor. For example, in the above term, 1.1 can be replaced by the more general pre-factor k1. The 5% exhalation state can be replaced by any other percentage. Alternatively, the percentage can be 0%. For example, the 5% in the above term can generally be replaced by x%, where x is a free variable that defines the percentage value.

[0030] According to an embodiment, a necessary condition for determining the start of a data redundant breathing phase is that the current breathing cycle has experienced a minimum fraction of the reference breathing amplitude during exhalation since the last end of the inhalation state. Preferably, if the condition is met or has been met at any time point during at least one previous or current respiratory measurement sample during the current breathing cycle, it can be determined that the condition is met. The fraction of the reference breathing amplitude can be defined by a scaling factor. The scaling factor can be based on an estimated variance or standard deviation of the subject's breathing amplitude. For example, the subject may occasionally inhale deeper gas than the average. The setting of the scaling factor can be determined based on the expected drift of the measurement result. The scaling factor can be preset. The scaling factor can be dynamically updated based on the monitored respiratory signal, particularly based on a determined variance or standard deviation. This condition can help ensure that redundant modulation is not applied before the subject experiences substantially complete or at least mostly complete exhalation.

[0031] For example, the necessary conditions for determining the start of the data redundancy breathing phase may be defined by the following or equivalent terms:

[0032]

[0033] Where median(ζ 100％ ) is the median of all previous samples representing the measurement at the end of inspiration, and Δζ ref is the peak-to-peak amplitude of the reference respiratory cycle. 0.8 is the scaling factor as described above. This particular scaling factor 0.8 may be replaced by any other suitable scaling factor. For example, in the above term, 0.8 may be replaced by a more general pre-factor k2.

[0034] According to an embodiment, a necessary condition for determining the start of a data-redundant breathing phase is based on a low current time-dependent change in the breathing state. Preferably, it can be determined that the condition will be met if the condition is met or has been met at any point in time during at least one previous or current breathing measurement sample during the current breathing cycle. Therefore, it can be assumed that a low current change in the breathing state can indicate the start of a data-redundant breathing phase.

[0035] According to an embodiment, the phase space is defined by a coordinate system, wherein a first axis is the respiratory state and a second axis is the time-dependent change of the respiratory state, wherein a current polar angle of a point in the phase space at the current respiratory state and the current time-dependent derivative of the respiratory state is defined relative to a centroid of a reference respiratory cycle in the phase space, wherein a necessary condition for determining the start of a data redundant respiratory phase is that the current polar angle is greater than or equal to a reference angle based on the reference respiratory cycle. Preferably, it may be determined that the condition will be met if the condition is met or has been met at any point in time during at least one previous or current respiratory measurement sample during the current respiratory cycle.

[0036] The time-dependent change of the respiratory state may also be referred to as the derivative of the respiratory state or respiratory rate. Via this condition, both a low respiratory rate and a low respiratory state (i.e. close to a minimum value) may be monitored. The reference angle is in particular an angle in a reference respiratory cycle in a minimum respiratory state or a respiratory state that is higher than the minimum respiratory state by a defined absolute or relative amount. The amount may be equivalently defined as the amount under the other conditions mentioned above.

[0037] For example, the necessary conditions for determining the start of the data redundancy breathing phase may be defined by the following or equivalent terms:

[0038]

[0039] where the phase is defined as the point Polar angle relative to the center of mass of the reference breathing cycle Therefore, the phase of at least one measurement must be greater than or equal to the phase of the 5% exhalation state in the reference breathing cycle. The 5% exhalation state can be replaced by any other percentage. Alternatively, the percentage can be 0%. For example, the 5% in the above terms can usually be replaced by y%, y being a free variable that defines the percentage value.

[0040] Preferably, multiple necessary conditions may apply. Thus, the data redundant breathing phase may be determined to start only when all of these multiple necessary conditions are met during one breathing cycle and / or at one bed position.

[0041] According to an embodiment, when the current breathing state exceeds a minimum threshold and / or the change in the current breathing state exceeds the minimum threshold, it is estimated that the end of the data redundant breathing phase is reached. For example, at the end of the data redundant breathing phase, the X-ray tube current can be increased again. The minimum threshold can depend on the minimum recorded breathing state of the current breathing cycle. For example, an absolute or relative margin can be added to the minimum recorded breathing state to determine the minimum threshold. According to an embodiment, when the current breathing state exceeds a defined margin above the minimum breathing state recorded in the current breathing cycle and / or in the current bed position and / or since the last end of the inhalation state, it is estimated that the end of the data redundant breathing phase is reached. For example, the margin can be a percentage value, such as 5%. The margin can be based on the temporal resolution of the breathing-related CT scan explained above with respect to the corresponding amount. The CT system can be configured so that as long as the end condition is met, the start of the data redundant breathing phase can not be triggered. Therefore, for example, in some cases, this can completely prevent the method of triggering redundant modulation, such as when there is no pause at the end of exhalation.

[0042] According to an embodiment, when redundant modulation is applied, the X-ray dose is modulated by reducing the X-ray tube current. The reduced X-ray tube current may require a longer acquisition time to achieve good quality. Therefore, for the portion of data acquired during the redundant modulation, the reconstruction may use data acquired over a larger amount of time. In other words, the reconstruction bins within the redundant modulation period may need to cover a larger time frame. Therefore, the temporal resolution is reduced. However, since the respiratory state is expected to be substantially constant during this time, it can be expected that no motion artifacts will occur due to only little respiratory motion in this time frame.

[0043] According to an embodiment, the X-ray dose is modulated such that the X-ray dose is zero when redundant modulation is applied. For example, the X-ray source may be turned off or blocked. Advantageously, this embodiment may even further reduce the applied X-ray dose. The reconstruction box located within this period may be shifted towards exhalation or inspiration.

[0044] Unless otherwise indicated, the embodiments described herein may be combined with each other. For example, several or all of the necessary conditions may be applied simultaneously, and / or the application of at least one of the necessary conditions or several of the necessary conditions may be combined with an embodiment for estimating the end of a data redundancy breathing phase.

[0045] According to another aspect of the invention, a computer program is provided, the computer program comprising instructions which, when the program is executed by a control unit of a computer tomography system, cause the computer tomography system to perform a method as described herein. All features and advantages of the method may apply to the computer program, and vice versa.

[0046] According to another aspect of the invention, a computer-readable storage medium, in particular a non-transitory storage medium, is provided. The computer-readable storage medium comprises instructions which, when executed by a control unit of a computer tomography system, cause the computer tomography system to perform a method as described herein. All features and advantages of the method and the computer program may apply to the computer-readable storage medium, and vice versa.

[0047] According to another aspect of the invention, a computer tomography system is provided. The computer tomography system comprises a monitoring sensor for monitoring a breathing signal of a subject, wherein the system is configured to perform a method as described herein. All features and advantages of the method, the computer program and the computer-readable storage medium may be applicable to the computer tomography system, and vice versa. For example, the monitoring sensor may be a strain gauge and / or a belt with a strain gauge, in particular as described herein. Additionally or alternatively, the monitoring sensor may be a camera working with an optical marker as described herein. For example, the computer tomography system may comprise a processing unit configured to control and initiate the method steps. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The drawings illustrate various exemplary embodiments and methods of various aspects of the invention.

[0049] In the accompanying drawings, similar elements are indicated by the same reference numerals. In the accompanying drawings:

[0050] Figure 1 A flow chart showing a method according to an embodiment of the present invention is shown;

[0051] Figure 2 A computer tomography system according to an embodiment of the present invention is shown;

[0052] Figure 3 is a graph of an exemplary respiratory signal, i.e., chest elevation (in cm) over time during one respiratory cycle;

[0053] Figure 4 is a graph of an exemplary respiratory signal, namely a graph of chest elevation (in cm) versus respiratory rate;

[0054] Figure 5 is a graph of an exemplary respiratory signal, i.e., chest elevation (in cm) over time during one respiratory cycle;

[0055] Figure 6 is a graph of X-ray dose over time;

[0056] Figure 7 Two examples of respiratory signals plotted over multiple respiratory cycles are shown. DETAILED DESCRIPTION

[0057] Figure 1 A flow chart of a computer-implemented method for reducing an X-ray dose applied to a subject during a breathing-related computed tomography scan according to an embodiment of the present invention is shown. In a first step 101, during a beam-on period of a scan, the beginning of a data-redundant breathing phase is determined. To this end, a breathing signal is monitored, which can be used to determine the beginning of a data-redundant breathing phase. For example, the breathing signal can be monitored via an infrared marker 2 placed on the chest of the subject 5 and a corresponding infrared camera 1. However, the breathing signal can also be monitored in a different manner, such as via a strain gauge placed around the chest of the subject 5. The breathing signal can be a signal monitored in any manner for the purpose of a breathing-related computed tomography scan. For example, when it is estimated that the breathing phase does not provide a significant additional breathing state during the current breathing cycle and / or at the current bed position, it can be determined that the breathing phase is data-redundant. This can be achieved via conditions such as the change in the breathing state over time being estimated to be below a threshold and / or the variance of the breathing state being estimated to remain within a predetermined range. Figure 3 and Figure 4 An example of a condition based on which a data redundancy breathing phase can be determined is given in . In a further step 102, during the data redundancy breathing phase, redundant modulation is applied to the applied X-ray dose so that the X-ray dose during the data redundancy breathing phase is reduced. When redundant modulation is applied, the X-ray dose can be modulated by reducing the X-ray tube current. Optionally, the X-ray tube current can be reduced to zero, or the X-ray beam can be blocked so that the X-ray dose applied to the subject 5 is zero when redundant modulation is applied. In a further step 103, the end of the data redundancy breathing phase is determined. For this purpose, the monitored breathing signal can also be used to determine the end of the data redundancy breathing phase. In a further step 104, at the end of the determined data redundancy breathing phase, scanning is continued without redundant modulation of the X-ray dose, i.e., by applying a normal X-ray dose.

[0058] Figure 2 A computer tomography system according to an embodiment of the present invention is shown. The system is configured to perform Figure 1 The computer tomography system comprises a monitoring sensor 1 for monitoring a breathing signal of a subject 5. In this embodiment, the monitoring sensor 1 is an infrared camera 1 which works together with an infrared marker 2 on the chest of the subject 5.

[0059] Figures 3 to 5 An exemplary breathing cycle is shown, as well as a timing diagram for determining the start of a data redundancy breathing phase ( Figure 3 and Figure 4 ) and end( Figure 5) is applied. The respiratory signal, c, is plotted on the vertical axis of the graph. The absolute values ​​given here are arbitrary values ​​based on the monitoring settings. What is relevant are the correlation values ​​between different respiratory states. Figure 3 and Figure 5 The horizontal axis shows time in seconds. Figure 4 The horizontal axis of shows the respiratory rate in cm / s. Typically, the respiratory signal (also referred to as the respiratory state or breathing signal) corresponding to the rise of the chest of the subject 5 is represented by ζ i , where i represents the time point. The derivative of the respiratory signal at time point i (also known as the respiratory rate) is expressed as Respiratory signal i ( Figure 3 and Figure 5 ) and breathing rate ( Figure 4 ) is shown as a continuous line. Based on the monitored respiratory signal, the reference respiratory signal is represented as ζ ref . Reference breathing signal ζ ref Shown as a dashed line.

[0060] In this embodiment, there are three necessary conditions for determining the start of the data redundancy breathing phase. These conditions are the time criterion (t start ,See Figure 3 ), amplitude criterion (ζ start ,See Figure 3 ) and phase standard ( See Figure 4 In addition, there is the ending criterion (ζ end ,See Figure 5 ), which determines the end of the data redundancy breathing phase.

[0061] The time criteria are defined by:

[0062]

[0063] where i 100％ is the sample index representing the last state of maximal inspiration (i.e., the beginning of the current respiratory cycle), H(x) is the Heaviside step function, is the sample index corresponding to the 5% exhalation state (i.e., 5% higher than the minimum breathing state) in the reference breathing cycle. Figure 3 The name used in start Once time (tracked on the horizontal axis) crosses this line, the temporal criterion is met.

[0064] The amplitude criterion is defined by:

[0065]

[0066] Where median(ζ 100％ ) is the median of all previous samples representing the measurement at the end of inspiration, and Δζ ref is the peak-to-peak amplitude of the reference respiratory cycle. Figure 3 The name used in the start Once the respiration signal ζ drops below this line, the amplitude criterion is met.

[0067] The phase criterion is defined by:

[0068]

[0069] where the phase is defined as the point Polar angle relative to the center of mass of the reference breathing cycle Therefore, the phase of at least one measurement must be greater than or equal to the phase of the reference breathing cycle at 5% exhalation. Figure 4 The name used in The angle line is marked with the origin of the angle line at the centroid of the reference breathing cycle (dashed line). Once the point is moved counterclockwise If the angle passes through this angle line, the phase criterion is met.

[0070] If each of the three criteria is met at any point in time during at least one previous (or current) measurement sample during the current respiratory cycle, the redundant modulation is applied. The duration of the redundant modulation is marked as 11. In this example, the key criterion, i.e. the last criterion met, is the time criterion (ζ start ). However, this may vary depending on the nature of the breathing cycle of the individual subject 5.

[0071] Once the end criterion is met, the redundant modulation stops. That is, once the breathing signal exceeds a certain margin above the minimum breathing signal of the current cycle, the data redundant breathing phase and thus the redundant modulation ends. Figure 5 As can be seen from the figure, the minimum breathing signal and therefore the end criterion (ζ end ) changes with time during the exhalation phase and is essentially constant during the data redundant breathing phase. When inspiration begins (i.e. the breathing signal exhibits a slope), the end criterion also remains constant. The slope of the breathing signal triggers the end criterion ζ end , and thus triggers the end of redundant modulation.

[0072] Figure 6 A rough representation of the amount of X-ray dose applied before, during and after the data redundancy breathing phase 12 is shown. First, a normal, i.e., higher, X-ray dose is applied. Then, the amplitude criterion (ζ start), Phase criterion and time criteria (t start ). Once all three criteria are met (t start is the last one), redundant modulation is applied by reducing the X-ray dose. Once the end criterion (ζ end ), the redundant modulation stops and the original X-ray dose is applied again.

[0073] Figure 7 Two examples of respiratory signals plotted over multiple respiratory cycles are shown. The vertical axis represents the respiratory signal ζ, and the horizontal axis represents time t (in seconds). In the above example, a long respiratory pause occurs after each exhalation. Correspondingly, there is a long data redundancy phase (marked with mark 11), during which redundant modulation is applied. Therefore, in the above example, a considerable amount of X-ray dose can be reduced. Up to 50% of the X-ray dose applied to the patient can be saved without reducing the data integrity of the reconstructed complete 4D-CT data set. In the following example, only a few short respiratory pauses occur after each exhalation. Correspondingly, there are only a few short data redundancy phases (marked with mark 11), during which redundant modulation is applied. In the following example, only a small amount of X-ray dose may be saved.

Claims

1. A computer-implemented method for reducing an X-ray dose (12) applied to a subject (5) during a breathing-related computed tomography scan, the method comprising the steps of: (a) determining the start of a data redundancy breathing phase during a beam-on period of the scan; (b) applying redundancy modulation to the applied X-ray dose (12) during the data redundancy breathing phase so as to reduce the X-ray dose during the data redundancy breathing phase; (c) determining the end of the data redundancy breathing phase; (d) at the end of the data redundancy breathing phase, continuing the scanning without performing the redundant modulation of the X-ray dose.

2. The method according to claim 1, Wherein the breathing phase is determined to be data redundant when the estimated breathing phase does not provide significant additional breathing state during the current breathing cycle and / or at the current bed position.

3. The method according to any one of the preceding claims, Therein, the breathing phase is determined to be data redundant when a change in the breathing state over time is estimated to be below a threshold and / or when a variance of the breathing state is estimated to remain within a predetermined range.

4. The method according to any one of the preceding claims, A necessary condition for determining the start of the data-redundant breathing phase is that the signal length since the last end of the inhalation state has a defined minimum length relative to a reference exhalation duration.

5. The method according to any one of the preceding claims, A necessary condition for determining the start of the data redundant breathing phase is that the current breathing cycle has experienced a minimum fraction of the reference breathing amplitude during exhalation since the last end of the inhalation state.

6. The method according to any one of the preceding claims, A necessary condition for determining the start of the data-redundant breathing phase is that the current time-dependent change in the breathing state is low.

7. The method according to any one of the preceding claims, where the phase space is defined by a coordinate system where the first axis is the respiratory state and the second axis is the time-dependent change of the respiratory state, wherein the current polar angle of a point in phase space at the current breathing state and the current time-dependent derivative of the breathing state are defined relative to the centroid of a reference breathing cycle in phase space, The necessary condition for determining the start of the data redundancy breathing phase is that the current polar angle is greater than or equal to a reference angle based on the reference breathing cycle.

8. The method according to any one of the preceding claims, When the current breathing state exceeds a minimum threshold and / or when a change in the current breathing state exceeds a minimum threshold, it is estimated that the end of the data redundant breathing phase has been reached.

9. The method according to any one of the preceding claims, Therein, the end of the data redundant breathing phase is estimated to be reached when the current breathing state exceeds a defined margin above the minimum breathing state recorded in the current breathing cycle and / or in the current bed position and / or since the last end of the inspiratory state.

10. The method according to any one of the preceding claims, Wherein when the redundant modulation is applied, the X-ray dose is modulated by reducing the X-ray tube current.

11. The method according to any one of the preceding claims, Wherein the X-ray dose is modulated such that when the redundant modulation is applied, the X-ray dose is zero.

12. The method according to any one of the preceding claims, The breathing signal is monitored to determine the beginning and end of the data redundant breathing phase.

13. A computer program comprising instructions which, when the program is executed by a control unit of a computed tomography system, cause the computed tomography system to perform the method of any of the preceding claims.

14. A computer-readable storage medium comprising instructions, in particular a non-transitory storage medium, which, when executed by a control unit of a computed tomography system, causes the computed tomography system to perform the method according to any one of claims 1 to 12.

15. A computer tomography system, comprising a monitoring sensor for monitoring a breathing signal of a subject (5), The system is configured to perform the method according to any one of claims 1 to 12.

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