Method and system for determining a breathing phase

Abstract

Method for determining a respiratory phase, comprising the following steps: - Receiving (REC) raw tomographic data based on a spiral scan of an examination area (17) of a patient (3), wherein the examination area (17) comprises at least a part of the torso and / or abdomen of the patient (3), - Reconstructing (PIC) pairs of slice images based on the raw tomographic data, wherein the slice image pairs each comprise two two-dimensional slice images with a first time interval (dt_1) at an identical position along a predetermined axis, wherein adjacent slice image pairs have a second time interval (dt_2), and wherein the positions of adjacent slice image pairs along the predetermined axis have a spatial distance corresponding to the second time interval (dt_2), - First determination (DET-1) of differences (dH) between reference positions (20,21) of the examination area (17) in two slice images of one of the slice image pairs as well as a second determination (DET-2) of at least one respiratory phase based on the differences (dH).,

Description

Radiation therapy with high-energy X-rays, electrons, protons, or heavy ions is regularly used to treat tumors. In this type of radiation therapy, a target volume within the patient is irradiated, with the tumor being located at least partially within this volume. The target volume should be defined as precisely as possible so that the desired effect of the radiation therapy is achieved primarily within this target volume. To plan the radiation therapy and define the target volume, it is standard practice to acquire time-resolved tomographic planning data of the patient prior to the radiation treatment. Typically, raw tomographic data is acquired using a computed tomography (CT) scanner, and the planning data is then reconstructed from this raw data. The planning data comprises several planning images with a defined temporal resolution.These planning data allow us to deduce how the patient's anatomy and the tumor's location change due to the patient's breathing. The tumor's location changes with the patient's respiratory cycle, which includes different phases of respiration. Since radiation therapy is usually continuous, it is important to consider these changes in anatomy caused by the patient's breathing when planning the radiation treatment and defining the target volume. To correlate changes in anatomy with the course of the respiratory cycle and thus the different phases of the patient's breathing, the patient's respiration is recorded during the tomographic scan using a respiratory surrogate. A respiratory surrogate is a measuring system designed to record the patient's respiratory cycle. It does not record the patient's respiratory cycle based on images of the patient, but rather measures it through the direct physical interaction of a physical component of the measuring system with the patient and / or the movement caused by breathing. For example, a respiratory surrogate can be implemented using a chest strap or a transmitter attached to the patient's chest. Alternatively, a respiratory surrogate can be implemented using a device that measures the temperature of the patient's breath.Attaching the breathing surrogate to the patient represents an additional step and is potentially prone to error. Furthermore, the breathing surrogate represents an additional cost factor. From publication WO 2006 / 085253 A2, a method is known to determine the course of the respiratory cycle by using three-dimensional image data at different times without a respiratory surrogate. The object of the present invention is therefore to reliably, accurately, and cost-effectively determine a patient's respiratory phase. This object is achieved by a method and a system according to the claims. The inventive solution to the problem is described below with regard to the claimed devices as well as the claimed method. Features, advantages, or alternative embodiments mentioned herein are also transferable to the other claimed items and vice versa. In other words, the claims (which, for example, relate to a device) can also be further developed with the features described or claimed in connection with a method. The corresponding functional features of the method are thereby realized by corresponding material modules. The proposed method for determining a respiratory phase is based on receiving raw tomographic data from a spiral scan of a patient's examination area, where the examination area comprises at least a portion of the patient's torso and / or abdomen. Then, paired tomographic images are reconstructed based on the raw data. Each pair comprises two two-dimensional images with a first temporal interval at an identical position along a predefined axis. This position refers to the position of the examination area. This type of reconstruction allows for the determination of differences between reference positions of the examination area in each pair of images, and the determination of at least one respiratory phase based on these differences.The differences correspond to changes in the anatomy of the area under investigation, with these changes occurring during the first time interval. Certain differences are characteristic of specific respiratory phases. Thus, a specific difference can be assigned to a particular respiratory phase. This allows for the reliable, accurate, and cost-effective determination of various respiratory phases of the patient. In particular, the respiratory phases can be determined without a respiratory surrogate. The proposed method can also be used to determine a respiratory cycle as well as a multitude of respiratory phases. Furthermore, the invention enables the determination of the various respiratory phases based on a spiral image, which is particularly fast compared to a stepwise image. According to the invention, adjacent pairs of slice images have a second temporal interval, wherein the positions of adjacent slice image pairs along the predetermined axis have a spatial distance corresponding to this second temporal interval. By selecting the second temporal interval, the spatial sampling of a breathing cycle and thus the accuracy of determining the breathing phase can be influenced. Furthermore, different slice image pairs have different positions along the predetermined axis. The invention thus makes it possible to establish a relationship between the breathing phase and time as well as between the breathing phase and position. According to another aspect of the invention, the second time interval is shorter than the first. A breathing cycle is thus sampled at a high frequency compared to the differences between the reference positions. This allows breathing cycles and reference phases to be determined accurately, reliably, and quickly. According to another aspect of the invention, the predetermined axis is given by the system axis of a CT device that is relevant for recording the raw tomographic data. According to another aspect of the invention, the predetermined axis is defined by the longitudinal axis of the patient. Furthermore, the relevant system axis of the CT scanner and the longitudinal axis of the patient can be parallel to each other or coincide with each other. According to a further aspect of the invention, the reference positions determine the spatial positions of the thorax and / or the abdominal wall in the tomographic images. The reference positions thus indicate which anatomical region of the examination area is to be used to determine the differences. According to a further aspect of the invention, the differences are determined by segmenting the investigation area in the layer images. Segmentation allows for the selection of sub-areas of the investigation area which are to be used to determine the differences. According to a further aspect of the invention, the differences are determined based on a plurality of points in the layer images, wherein the points correspond to the reference positions of the investigation area. The points can, in particular, be individual pixels or related groups of pixels. According to a further aspect of the invention, the reference value within a layer image is determined by calculating the average value of the points within that layer image. This aspect of the invention can be implemented with particularly low computing power. According to a further aspect of the invention, the differences are determined based on corresponding points, where points correspond to each other if they mark the same anatomical region of the examination area. Advantageously, the positions of corresponding points are subtracted from each other. This aspect of the invention allows for a particularly precise determination of the differences and thus of a breathing phase. According to a further aspect of the invention, the differences with respect to time and / or positions along the predefined axis are plotted and displayed. This establishes a direct correlation between the differences and thus the changes in the patient's anatomy over time and / or positions along the predefined axis. Since the changes in anatomy correspond to the respiratory phases, this also establishes a correlation between the respiratory phases and time and / or positions along the predefined axes. According to a further aspect, the raw tomographic data are correlated with at least one respiratory phase, whereby at least one planning image is reconstructed for each respiratory phase based on the raw tomographic data. This allows the invention to be used for improved radiation treatment planning. The treatment planning is improved by the particularly accurate and reliable correlation between the raw tomographic data and the respiratory phases. The respiratory phases are determined according to the invention based on the raw tomographic data. Furthermore, all previously described steps can be executed automatically. In the context of this application, "automatic" means that the respective step is carried out independently by the claimed system, and essentially no operator interaction is required for that step. The operator may, at most, confirm calculated results or perform intermediate steps. For example, the operator may set a seed point for segmentation. In further embodiments of the invention with "fully automatic" steps, no operator interaction whatsoever is necessary to perform these steps. In particular, all steps of the claimed method can be performed "fully automatically."Regardless of whether the individual steps are performed "automatically" or "fully automatically," the method according to the invention can be part of a workflow that additionally requires interaction from an operator. This interaction with the operator can consist of the operator manually selecting a recording protocol and / or a clinical question, for example, from a menu presented on a screen. The invention further relates to a system for determining a respiratory phase, comprising an interface for receiving the raw tomographic data and a processor. The system is configured to execute the previously described method and its aspects, in that the interface and the processor are configured to perform the corresponding method steps. In particular, the processor can be programmed to execute steps of the described method. According to a further aspect, the reconstruction steps are performed by a reconstruction unit separate from the processor. The determination steps are then still performed by the processor. Furthermore, the system for determining a respiratory phase can also include a CT scanner designed for acquiring the raw tomographic data. The invention also relates to a computer program product comprising a computer program and a computer-readable medium. A largely software-based implementation has the advantage that existing systems or computers can be easily retrofitted via a software update to operate according to the invention. In addition to the computer program, such a computer program product may optionally include additional components such as documentation and / or additional components, as well as hardware components such as hardware keys (dongles, etc.) for using the software. The invention will now be described and explained in more detail with reference to the exemplary embodiments shown in the figures. Figure 1 shows a flowchart of a procedure for determining a respiratory phase, Figure 2 shows two longitudinal sections of the examination area, Figure 3 shows a pair of tomographic images of the examination area, Figure 4 shows a differential respiratory curve with several respiratory phases, Figure 5 shows a system for determining a respiratory phase, and Figure 6 shows a system for determining a respiratory phase using a CT scanner. Fig. 1 shows a flowchart of a method for determining a respiratory phase. The embodiment shown here includes acquiring IMG tomographic raw data with a CT scanner 1 based on a spiral scan of an examination area 17 of a patient 3. The tomographic raw data are four-dimensional in that they have three spatial dimensions and one temporal dimension. The tomographic raw data are based on a multitude of X-ray projections, with each individual X-ray projection being spatially two-dimensional. Since the X-ray projections are acquired at different times and from different projection angles, they form a four-dimensional dataset. However, a sufficiently high sampling rate of the examination area 17 allows identical sub-areas of the examination area 17 to be reconstructed as cross-sectional images at different times.The tomographic images can be displayed as two-dimensional images. Since the tomographic images have a finite slice thickness, they are also spatially three-dimensional. Furthermore, the raw tomographic data may have been pre-processed before storage or transmission; for example, they may have been filtered. Therefore, the raw tomographic data can also be referred to as a 4D CT dataset. Since the changes in anatomy caused by the patient's respiration are periodic, planning images of the examination area 17 can be reconstructed, provided that the corresponding respiratory phase is known for a given planning image. In order to use reconstructed planning images, the individual respiratory phases must first be determined and correlated with the raw tomographic data. The procedure described below allows for the simple and reliable determination of the respiratory phases and thus also improves radiation treatment planning. To ensure that the raw tomographic data has a temporal resolution sufficient to reconstruct the same sub-area of ​​the examination area 17 at different times, the spiral scan must be performed with a low pitch. In spiral mode, the pitch is defined as the ratio of the patient table's movement 6 per rotation of the X-ray source 8 to the beam collimation. Beam collimation is determined by the extent of the X-rays 2 along the system axis 5. If the X-ray detector 9 is fully illuminated by the X-rays 2, beam collimation is determined by the extent of the detection area of ​​the X-ray detector 9 along the system axis 5. In various embodiments of the invention, the pitch can be a maximum of 0.5, 0.25, or 0.1. Preferably, the raw tomographic data is acquired with a particularly low pitch of a maximum of 0.1, as this results in a particularly high temporal resolution.For example, the rotation time of the recording unit 22 is 0.5 seconds at a pitch of 0.09. The raw tomographic data is then received (REC), in particular via an interface 16. Subsequently, image pairs are reconstructed (PIC) based on the raw tomographic data. Each image pair comprises two two-dimensional slices with an initial time interval dt_1 at an identical position along the specified axis. The initial time interval dt_1 must be large enough to allow for a significant change in the anatomy due to the patient's respiratory movement 3. For example, the initial time interval dt_1 can be equal to the rotation time of the acquisition unit 22 and be a maximum of 2 seconds, 1 second, or 0.5 seconds. Furthermore, it is advantageous if the initial time interval dt_1 is identical for all image pairs. The tomographic images can be reconstructed using conventional reconstruction algorithms, such as a Feldkamp algorithm or iterative reconstruction. Preferably, the tomographic images of a tomographic image pair have an identical slice thickness. It is particularly advantageous if the tomographic images of all tomographic image pairs have an identical slice thickness. For example, the slice thickness can be a maximum of 5 millimeters, a maximum of 2.5 millimeters, or a maximum of 1 millimeter. A smaller slice thickness increases the spatial resolution with which the respiratory phase can be determined. The tomographic images are preferably reconstructed such that the planes of the tomographic images are oriented perpendicular to the predetermined axis. In the example described in more detail here, the predetermined axis is defined by the system axis 5 of the CT scanner 1, which is responsible for acquiring the raw tomographic data.Furthermore, the specified axis can be defined by the longitudinal axis of patient 3. Preferably, the reconstruction of the tomographic images is performed as a complete reconstruction. The tomographic images are then based on raw tomographic data originating from within an angular interval of at least 180° + α of the X-ray source 8, where α is the opening angle of the X-ray beams 2 in the plane of rotation of the X-ray source 8. The X-ray beams 2 can be fan-shaped, conical, or pyramidal. The angular interval encompasses a plurality of projection angles. In particular, the tomographic images can be based on raw tomographic data originating from within an angular interval of exactly 180° + α of the X-ray source 8. In this case, the complete reconstruction is performed with the highest possible temporal resolution, so that the changes in anatomy caused by the patient's respiration 3 can be determined as accurately as possible. In a spiral scan, the position of the X-ray source 8 along the predefined axis in the reconstructed volume is a continuously differentiable function of time t. In contrast, with tomographic raw data based on a sequential scan, the corresponding position of the X-ray source 8 is not continuously differentiable. Therefore, the time-dependent position along a predefined axis is referred to below as the z-position s_z(t), where time t is a variable. Due to the high sampling rate, however, several time points can be assigned to a single z-position s_z(t). Thus, the two slice images of a slice image pair are assigned the same, but different, first z-positions s_z1. This relationship is also illustrated in Fig. 2, which shows two longitudinal sections of the examination area 17. These longitudinal sections are based on the tomographic raw data. The two in Fig.The two highlighted areas at the first z-position s_z1 correspond to the two layer images shown in Fig. 3. The two layer images of the first layer image pair shown in Fig. 3 are also referred to as the first layer image 18 and the second layer image 19. Preferably, adjacent pairs of layer images have a second time interval dt_2, wherein the z-positions s_z(t) of adjacent layer image pairs have a spatial distance corresponding to the second time interval dt_2. It is particularly advantageous if all second time intervals dt_2 are identical. A first layer image pair is adjacent to a second layer image pair if the z-position s_z(t) of the second layer image pair has the smallest distance to the first layer image pair compared to other layer image pairs. Due to the pitch, there is a direct relationship between the time and the z-position s_z(t) that can be assigned to a layer image. Therefore, the adjacent second layer image pair also has the smallest second time interval dt_2 compared to the first layer image pair compared to other layer image pairs. The second time interval dt_2 can refer to the interval between the first slice images of adjacent slice image pairs, the interval between the second slice images of adjacent slice image pairs, or an average of these intervals. For example, the first slice image 18 of a first slice image pair has the first z-position s_z1 = s_z(t1) and the first time t1. Then the second slice image 19 of the first slice image pair also has the first z-position s_z1 = s_z(t2) = s_z(t1 + dt_1) and a second time t2 = t1 + dt_1. The first layer image of an adjacent second layer image pair has the second z-position s_z2=s_z(t3)=s_z(t1+dt_2) and a third time t3=t1+dt_2, and the second layer image of the adjacent second layer image pair has the second position s_z2=s_z(t4)=s_z(t2+dt_2) and the fourth time t4=t2+dt_2. In this process, dt_1 and dt_2 are generally unequal. Preferably, the second time interval dt_2 is smaller than the first time interval dt_1. This is because a sufficiently large first time interval dt_1 allows the change in anatomy between the two tomographic images of a tomographic pair to become visible. Furthermore, a sufficiently small time interval dt_2 allows for the determination of as many differences dH as possible. This enables a particularly precise determination of the patient's respiratory cycle 3 and the individual respiratory phases. Furthermore, a precise determination of the respiratory cycle and the respiratory phases allows for a particularly accurate correlation between the raw tomographic data and the respiratory phases. This, in turn, allows for the reconstruction of particularly accurate and reliable planning images. For example, the second time interval dt_2 is at most 0.5, 0.25, or 0.1 of the first time interval dt_1. Then, the first determination (DET-1) of differences dH between reference positions 20, 21 of the examination area 17 is performed in two slice images of the slice image pair, followed by a second determination (DET-2) of a respiratory phase based on the differences dH. The first determination (DET-1) of a difference dH between reference positions is also shown in Fig. 3 using a slice image pair. The second determination (DET-2) is shown in Fig. 4 using a differential respiratory curve 25 with several respiratory cycles. The reference positions of the examination area 17 preferably relate to spatial positions of the thorax and / or abdominal wall in the tomographic images. A first reference position 20 is determined in the first tomographic image 18 of a first pair of tomographic images, and a second reference position 21 is determined in the second tomographic image 19 of a first pair of tomographic images. The differences dH of the reference positions can be determined, in particular, as differences dH of positions along a predefined axis. In the example shown in Fig. 3, the differences dH of the reference positions are determined along the vertical axis of the tomographic images. Furthermore, the differences dH can be determined by segmenting the examination area 17 in the tomographic images. For segmentation, for example, a region-oriented segmentation algorithm or an edge-based segmentation algorithm is used. In the example shown in Fig. 3, the difference dH is averaged over the length of the torso L in the layer images. The difference dH can be determined based on a plurality of points in the layer images, where the points correspond to the reference positions of the investigation area 17. These points can, in particular, mark the contour of a sub-area of ​​the investigation area 17. The points can be determined automatically, semi-automatically, or even manually in various embodiments. A point can be either a single pixel or a contiguous group of pixels. Furthermore, a reference value within a layer image can be determined by calculating the mean value of the points within that layer image. The mean value refers to the mean position of the points, particularly along a predefined axis. In the example shown in Fig.In the example shown, the mean value of the points along the vertical axis of the slice images can be determined. Averaging reduces the information content of the image values. Averaging applies, for example, to the calculation of an arithmetic mean, a geometric mean, a harmonic mean, a quadratic mean, or a median. A difference dH can also be determined by first averaging the points of individual slice images and then relating the mean values ​​to each other, in particular by subtraction. Furthermore, it is also possible to identify corresponding points in the slice images of a slice image pair, whereby points correspond to each other if they designate the same anatomical region of the examination area 17. An anatomical region could, in particular, be a specific sub-area of ​​the chest or abdominal wall of the patient 3. Then the positions of the corresponding points can be related to each other, in particular by subtraction. As shown in Fig. 4, the differences dH can be plotted and displayed against time t. The differences dH are plotted in centimeters against time t in seconds. Output is typically via an output unit, for example, a screen 11. The difference dH between a first reference position 20 and a second reference position 21 can be assigned to the first time t1, the second time t2, or an average of t1 and t2. It is important that the assignment is performed in the same way for all differences dH to ensure a physically meaningful relationship between the differences dH and time t. Since there is also a direct relationship between the differences dH and the z-position s_z(t), the differences dH can also be plotted against the z-positions s_z(t).Assigning the differences dH results in a differential breathing curve 25. To make this differential breathing curve 25 appear smooth and / or differentiable, the differences dH can be connected. In particular, running means of the plotted differences dH can be determined, or a function can be fitted to the plotted differences dH. Specifically, a polynomial function can be fitted to the plotted differences dH. To illustrate the relationship between the respiratory phase determined for the tomographic raw data at a specific z-position s_z(t), the differential respiratory curve 25 is superimposed with a longitudinal section of the examination area 17 in Fig. 4. The differential respiratory curve 25 can be considered a derivative of the patient's 3 original respiratory curve, which in this embodiment is determined by the change in the height of the patient's 3 torso. Consequently, a change in the differential respiratory curve 25 from a positive to a negative value means that the slope of the original respiratory curve changes from a positive to a negative value, and thus the respiratory phase of maximum inhalation is present at this z-position s_z(t). Therefore, the times of the zero crossings of the falling edges in the differential respiratory curve 25 correspond to the times of maximum inhalation.These time points are highlighted by filled circles in Fig. 4. Furthermore, the time points of the zero crossings of the rising edges in the differential breathing curve 25 correspond to the time points of minimum inhalation. Preferably, at least the breathing phases of inhalation 26 and exhalation 27 are determined. In Fig. 4, the breathing phase of inhalation 26 and the breathing phase of exhalation 27 are shown as examples for one breathing cycle. It is advantageous to subdivide the individual breathing cycles into further breathing phases. For example, each breathing cycle within the differential breathing curve 25 can be subdivided into N = 5, 10, 15, or 20 breathing cycles. The subdivision can be based, in particular, on the time of maximum inhalation and / or minimum inhalation. In particular, the breathing cycles can be subdivided such that adjacent breathing phases have the same time interval between them. Furthermore, the raw tomographic data can be correlated with the respiratory phases, whereby at least one planning image is reconstructed for each respiratory phase based on the raw tomographic data. This correlation can be performed using either a phase-based or an amplitude-based method. In a phase-based method, the raw tomographic data for reconstruction are selected such that the patient's respiratory cycles are scanned at equidistant intervals. The respiratory phases are then determined to be temporally equidistant. The planning data are thus reconstructed so that adjacent planning images have the same temporal interval between them. In an amplitude-based method, the raw tomographic data for reconstruction are selected such that the amplitudes of the patient's respiratory cycles are scanned at equidistant intervals.Adjacent planning images may then have different time intervals between them. Fig. 5 shows a system for determining a respiratory phase. The system comprises an interface 16 for receiving raw tomographic data based on a spiral scan of an examination area 17 of a patient 3, wherein the examination area 17 comprises at least a part of the torso and / or abdomen of the patient 3. Furthermore, the system comprises a processor 15, wherein the processor 15 is designed to perform at least the following steps: - Reconstructing PIC of slice image pairs based on raw tomographic data, wherein a slice image pair comprises two two-dimensional slice images with a first time interval dt_1 at an identical position along a predetermined axis; - Firstly determining DET-1 of differences dH between reference positions of the examination area 17 in each of the two slice images of a slice image pair; and secondly determining DET-2 at least one respiratory phase based on the differences dH. Furthermore, the system for determining a respiratory phase can have a reconstruction unit 14, which is specifically designed to perform the PIC reconstruction step. This allows the processor 15 to omit the PIC reconstruction step. The system can also have multiple processors 15, which are designed to perform the steps of the proposed method, in particular the determination steps. In further embodiments of the invention, the processor 15 can also be configured to perform the other previously described method steps. Furthermore, raw tomographic data can be stored on a server 23, so that the raw tomographic data can be transmitted via a network 24 to a client-based system for determining a respiratory phase. In the example shown here, the client is implemented by the computer 12.The client then stores a computer program containing program sections for executing the method for determining a breathing phase. In a further embodiment of the invention, the system for determining a breathing phase comprises both the client and the server 23. Fig. 6 shows a system for reconstructing planning images using a CT scanner 1. The CT scanner 1 shown here has an acquisition unit 22 comprising an X-ray source 8 in the form of an X-ray tube and an X-ray detector 9 in the form of a multi-line detector. The acquisition unit 22 rotates around a system axis 5 during the acquisition of tomographic raw data, and the X-ray source 8 emits X-rays 2 during the acquisition. In the example shown here, a patient 3 lies on a patient table 6 during the acquisition of the tomographic raw data. The patient table 6 is connected to a table base 4 in such a way that it supports the patient table 6 with the patient 3. The patient table 6 is designed to move the patient 3 along an acquisition direction through the opening 10 of the acquisition unit 22.The acquisition direction is generally determined by the system axis 5, around which the acquisition unit 22 rotates when acquiring the raw tomographic data. In a spiral scan, the patient table 6 is continuously moved through the opening 10 while the acquisition unit 22 rotates around the patient 3 and acquires the raw tomographic data. Thus, the X-rays 2 describe a spiral on the surface of the patient 3. In the example shown here, interface 16 is configured as part of a computer 12. Interface 16 is a commonly known hardware or software interface, such as the PCI bus, USB, or FireWire hardware interfaces. The computer 12 is connected to an output unit in the form of a screen 11 and an input unit 7. The screen 11 is designed to display various information, particularly cross-sectional images. The input unit 7 can be used to start a computer program with program sections for determining a breathing phase or to select parameters for executing the procedure for determining a breathing phase. The input unit 7 could be, for example, a keyboard, a mouse, a touchscreen, or a microphone for voice input. Furthermore, the computer 12 of the system shown here includes a reconstruction unit 14. The system also has a processor 15. The processor 15 can interact with a computer-readable medium 13, in particular to execute a procedure for determining a breathing phase using a computer program with program code. The computer program can also be stored on the computer-readable medium 13 in a retrievable manner. The computer-readable medium 13 can be a CD, DVD, Blu-ray Disc, memory stick, or hard drive. The reconstruction unit 14 can have components in the form of hardware and / or software. For example, the reconstruction unit 14 can be a so-called FPGA (acronym for "Field Programmable Gate Array") or comprise an arithmetic logic unit. The processor 15 can be a microprocessor and have multiple cores.A processor 15 can be programmed to perform specific steps. A processor 15 is then designed to execute specific instructions, which are implemented in software.

Claims

Method for determining a respiratory phase, comprising the following steps: - Receiving (REC) raw tomographic data based on a spiral scan of an examination area (17) of a patient (3), wherein the examination area (17) comprises at least a part of the torso and / or abdomen of the patient (3), - Reconstructing (PIC) pairs of slice images based on the raw tomographic data, wherein the slice image pairs each comprise two two-dimensional slice images with a first time interval (dt_1) at an identical position along a predetermined axis, wherein adjacent slice image pairs have a second time interval (dt_2), and wherein the positions of adjacent slice image pairs along the predetermined axis have a spatial distance corresponding to the second time interval (dt_2), - First determination (DET-1) of differences (dH) between reference positions (20,21) of the examination area (17) in two slice images of one of the slice image pairs as well as a second determination (DET-2) of at least one respiratory phase based on the differences (dH)., Method according to claim 1, wherein the second time interval (dt_2) is less than the first time interval (dt_1). Method according to one of the preceding claims, wherein the predetermined axis is given by the system axis (5) of a CT device (1) that is relevant for recording the tomographic raw data. Method according to one of the preceding claims, wherein the predetermined axis is given by the longitudinal axis of the patient (3). Method according to one of the preceding claims, wherein the reference positions (20, 21) determine the spatial positions of the chest and / or abdominal wall in the layer images. Method according to one of the preceding claims, wherein the differences (dH) are determined by segmenting the investigation area (17) in the layer images. Method according to one of the preceding claims, wherein the differences (dH) are determined based on a plurality of points in the layer images, the points corresponding to the reference positions (20,21) of the investigation area (17). Method according to claim 7, wherein a reference quantity is determined within one of the layer images by determining an average of the points within that layer image. Method according to claim 7, wherein the differences (dH) are determined based on corresponding points, wherein points correspond to each other if they mark the same anatomical area of ​​the examination area (17). Method according to one of the preceding claims, wherein the differences (dH) with respect to time (t) and / or with respect to the positions are plotted and output along the specified axis. Method according to one of the preceding claims, wherein the tomographic raw data are correlated with the at least one respiratory phase, and wherein at least one planning image is reconstructed for each respiratory phase based on the tomographic raw data. System for determining a respiratory phase, comprising the following modules: - Interface (16) for receiving (REC) raw tomographic data based on a spiral scan of an examination area (17) of a patient (3), wherein the examination area (17) comprises at least a part of the torso and / or abdomen of the patient (3), - Processor (15) configured to perform the following steps: - Reconstructing (PIC) slice image pairs based on the raw tomographic data, wherein a slice image pair comprises two two-dimensional slice images with a first time interval (dt_1) at an identical position along a predetermined axis, wherein adjacent slice image pairs have a second time interval (dt_2), and wherein the positions of adjacent slice image pairs along the predetermined axis have a spatial distance corresponding to the second time interval (dt_2), - First determination (DET-1) of differences (dH) between reference positions (20,21) of the examination area (17) in each of two slice images of a slice image pair as well as a second determination (DET-2) of at least one respiratory phase based on the differences (dH)., System according to claim 12, wherein the interface (16) and the processor (15) are configured to execute a method according to any one of claims 1 to 11. System according to claim 12 or 13, further comprising a CT device (1) designed for recording (IMG) the tomographic raw data. Computer program product comprising a computer program which can be directly loaded into a memory of a computer (12), comprising program sections to execute all steps of the method according to any one of claims 1 to 11 when the program sections are executed by the computer (12). Computer-readable medium (13) on which program sections readable and executable by a computer (12) are stored to execute all steps of the method according to any one of claims 1 to 11 when the program sections are executed by the computer (12).

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