Radiation therapy system and method for monitoring subject during radiation therapy

Abstract

Embodiments of the present disclosure relate to radiotherapy systems and methods for monitoring a subject during radiotherapy. A radiotherapy system comprising: a treatment area (4) configured such that a subject (5) or a portion of a subject (5) can be placed within the treatment area; a static computed tomography system (1) comprising detector elements and an x-ray source (15) distributed around the treatment area (4); and a radiation therapy device (2) configured to create a pulsed therapy beam (21) that creates aiming at the treatment region (4) with a pulsed beam pattern comprising a pulse duration (22) of individual pulses and an inter-pulse time (23) between the pulses; wherein the computed tomography system (1) is configured to acquire scan data from the object (5) or a portion of the object (5) during the inter-pulse time (23).

Description

Technical Field

[0001] The invention relates to a radiation therapy system, a method for monitoring an object or a part of an object during radiation therapy and a corresponding computer program. Background Art

[0002] Independent of the use of the grammatical term, individuals with both male and female identities are included in the term.

[0003] In radiotherapy, treatment is usually planned based on computed tomography (CT). CT scans used for patient simulation usually represent the starting point for organ and tumor segmentation and dose calculation. In the prior art, technological advances have paved the way for improvements in radiotherapy, such as improved accuracy and precision, shortened treatment time (e.g., hypo-fractionated and / or even flash therapy) and patient-individualized treatment concepts. In this case, it is desirable to provide an adaptive treatment workflow based on the current patient anatomy, which may change during treatment. Such adaptive treatment workflows are a prerequisite for achieving high-precision radiotherapy with improved tumor control and reduced negative effects on patients, such as reduced toxicity. In order to improve the accuracy of radiotherapy, it is desirable to be able to verify, track, control and / or selectively manipulate dose delivery during treatment. For this purpose, the ability to image online, i.e., imaging the patient's treatment anatomy with high temporal resolution (e.g., less than 0.5 seconds of temporal resolution) and high soft tissue contrast in the treatment isocenter during or before and after treatment, may be very useful, even indispensable.

[0004] CT imaging is generally capable of providing imaging with sufficient image contrast. However, to the best of the inventor's knowledge, online imaging of radiotherapy via CT imaging has not yet been achieved. There are several disadvantages in achieving online imaging via CT. The typical feature of a computed tomography system is a rapidly rotating x-ray source and detector to provide high temporal resolution images with different anatomical representations. On the other hand, the radiation equipment is also usually rotated during treatment, but at a slower speed (e.g., on a time scale of more than 8 seconds). Using a CT system simultaneously with a radiotherapy device usually results in vibration caused by a CT gantry, which can have a negative impact on the accuracy of the radiotherapy device. In addition, due to the rotation of the treatment device, the rotating CT x-ray source and detector may hinder the treatment device. In addition, the CT measurement itself may be negatively affected by the radiation caused by the radiation device. In particular, the x-rays from the radiotherapy may render at least part of the CT data useless, making it impossible to reconstruct a meaningful image. Summary of the invention

[0005] It is therefore an object of the present invention to provide a method of improving radiotherapy imaging.For example, it is desirable to provide an imaging that can be performed during or temporally close to a radiotherapy treatment and that can provide good imaging contrast of the treatment area.

[0006] This object is achieved or exceeded by a system according to claim 1, a method according to claim 10 and a computer program according to claim 15. Further advantages and features are given by the dependent claims, the description and the drawings.

[0007] According to a first aspect of the present invention, there is provided a radiotherapy system. The radiotherapy system comprises a treatment area, the treatment area being configured such that an object or a part of an object can be placed in the treatment area,

[0008] A static computed tomography system, including an x-ray source and detector elements distributed around a treatment region, and a radiotherapy device, is configured to create a pulsed treatment beam that is aimed at the treatment region with a pulsed beam pattern that includes pulse durations of individual pulses and inter-pulse times between pulses. The computed tomography system is configured to acquire scan data from an object or a portion of the object during the inter-pulse time.

[0009] The radiotherapy device can be configured to rotate during the radiotherapy. For example, the radiotherapy device can be configured to rotate to another rotational position during the inter-pulse time. The radiotherapy device can be configured not to rotate during the pulse duration. Alternatively, the radiotherapy device can, for example, be configured to rotate continuously during both the pulse duration and the inter-pulse time. The radiotherapy device can be a conventional radiotherapy device known in the prior art.

[0010] A static computed tomography system is particularly a computed tomography system in which the x-ray source and the detector element do not rotate during a computed tomography scan. Both the x-ray source and the detector element are distributed around the treatment area. The computed tomography system is particularly configured to selectively activate the x-ray sources distributed around the treatment area to generate an x-ray projection that passes through an object or a portion of an object in the treatment area. Correspondingly, the x-rays can then be detected by the detector element. For example, the x-ray source can be distributed annularly around the treatment area. The axial direction can be defined as orthogonal to the area spanned by the arrangement of the distributed x-ray source. The detector element can be distributed cylindrically around the treatment area. Therefore, the distribution of the detector element can extend in the axial direction. Preferably, in the axial direction, the detector element is offset relative to the x-ray source. The advantage of using a static computed tomography system is that the x-ray source or detector does not have to be rotated during the CT scan. Therefore, advantageously, vibrations caused by rotation that may negatively affect radiation therapy can be avoided. In the absence of a rotating component, the overall design of the system will be easier and / or more flexible. When the CT system does not require a rotating component, both the radiation device and the CT system can generally be arranged more easily. In particular, since no rapid rotation needs to be initiated, CT systems can be built lighter and therefore in a more space-saving manner. Thus, a radiotherapy device can, for example, be provided with a particularly compact and easy-to-use design. Further advantageously, a high temporal resolution can be achieved, for example, via rapid switching between x-ray sources, without the need for high mechanical stresses due to rotation, with lower technical challenges for data transmission (e.g. no slip rings are required) and power supply (no need for power supply via rotating parts). Furthermore, in the absence of rotation, the amount of power required can be reduced.

[0011] The computer tomography system is configured to acquire scan data from an object or a portion of an object during the inter-pulse time. For example, the scan data may be three-dimensional scan data. For example, the object may be a human or an animal. For example, the object may be a patient. A portion of the object may be an anatomical portion of the object, such as a limb or an organ. Acquiring scan data during the inter-pulse time particularly means acquiring scan data when there is no radiotherapy pulse currently. Advantageously, acquiring scan data during the inter-pulse time can therefore avoid interference of x-rays from radiotherapy with CT measurements. Since the computer tomography system is a static computer tomography system, this configuration is particularly possible. The rotation system also cannot be controlled, since it is at least very difficult and may be impossible to control the rotation on the time scale of a typical radiotherapy pulse. For example, the rotation cannot usually be stopped on the typical time scale of a radiotherapy pulse. For example, the pulse duration may typically be in the range of a few microseconds, such as 1 microsecond to 30 microseconds, preferably 10 microseconds to 20 microseconds. For example, the inter-pulse time may generally be in the general range of milliseconds, such as 0.1 milliseconds to 20 milliseconds, preferably 0.5 milliseconds to 15 milliseconds, more preferably 1 millisecond to 10 milliseconds.

[0012] The computer tomography system can be configured to acquire data only at a specific inter-pulse time. For example, a radiotherapy device can be configured to be applied in treatment units, each treatment unit including a pulse pattern, providing a pulse duration with a first inter-pulse time and a longer second inter-pulse time between the treatment units. The computer tomography system can, for example, be configured to acquire scan data during the second inter-pulse time. Advantageously, there can therefore be a more continuous time to acquire data. Therefore, the image quality during this time may be better. Additionally and / or alternatively, the computer tomography system can be configured to acquire scan data during the first inter-pulse time. The scan data from the first inter-pulse time can even be monitored during treatment. Therefore, for example, if necessary, the treatment can even be adjusted during a single treatment unit. For example, the system can be configured to acquire high-quality images during the second inter-pulse time and acquire lower-quality images during the first inter-pulse time. This combination can allow good image quality from the pause time between treatment units and can even monitor treatment during the treatment unit. Advantageously, the system of the present invention enables adaptation to new clinical workflows and / or allows highly conformal treatment. For example, radiosurgery applications can be conceived with such a system.

[0013] The radiation therapy system may include a control device configured to control the radiation therapy device and / or the computed tomography system. The control device may be, for example, a computer device such as a medical control station, a personal computer, an online server, a tablet computer or a smart phone.

[0014] According to an embodiment, the computed tomography system is configured to reconstruct image data, in particular 3D image data, of an object or a portion of an object based on scan data acquired during the inter-pulse time. Thus, the system can be configured to reconstruct image data using the scan data. The computed tomography system can be configured to provide real-time imaging, preferably 3D real-time imaging, during radiotherapy. For example, real-time imaging can be provided by substantially immediately reconstructing and updating the acquired scan data. Additionally or alternatively, the computed tomography system can be configured to transmit the acquired scan data to another device. For example, the other device can be a device configured for image reconstruction.

[0015] According to an embodiment, the computer therapy system is configured to activate the x-ray source only during the inter-pulse time and to suspend acquisition during the pulse duration. Therefore, the computer therapy system can be configured to switch the x-ray source or not activate the x-ray source during the pulse duration. In other words, the computer tomography system can be configured to stop CT scan data acquisition whenever the radiotherapy device turns on the treatment pulse. Once the treatment beam of the radiotherapy device is turned off again, that is, during the inter-pulse time, the computer tomography system can continue to acquire data for the next CT projection. Advantageously, compared with third-generation computer tomography systems (i.e., computer tomography systems with rotating x-ray sources and detectors), static computer tomography systems are easier to activate the x-ray source specifically during the inter-pulse time. This is because static computer tomography systems do not have to physically rotate heavy components for CT scanning. In particular, it is advantageous that, in addition to the start and stop of data acquisition, time synchronization is no longer required between the CT data acquisition of the computer tomography system and the treatment beam of the radiotherapy device. For rotation-based systems, it is usually necessary to consider and optimize the correlation between the rotation time, the detector integration time of the CT system, and the pulse duration of the radiotherapy device and the inter-pulse time. In general, for rotation-based systems, it is generally necessary to optimize the time synchronization of such systems for each scanning mode of a CT system and each treatment mode of a radiotherapy device. With a static CT system, the timing control can be less complex, since it can be reduced to a simple on / off switch of the x-ray source of the CT system and the acquisition of scan data can simply be restarted from the position (projection) where it stopped, without generating any sampling gaps or any complex scanning process that might otherwise be required to avoid these gaps. Further advantageously, not activating the x-ray source during the pulse duration can reduce the imaging radiation dose on the object. For example, assuming an inter-pulse time of 1 millisecond and a pulse duration of 250μs, the imaging radiation dose can be reduced by approximately 20%.

[0016] According to an embodiment, the radiotherapy system is configured to discard scan data acquired during the pulse duration. This may be an alternative to not acquiring scan data during the pulse duration. Although the beneficial effect of reducing radiation dose may not be achieved using this alternative, this variation still has the advantage of ensuring that the acquired CT scan data that is not discarded is not interfered with by the treatment beam.

[0017] According to an embodiment, the radiotherapy system is configured to interleave individual treatment pulses with groups of computed tomography projections such that each group includes at least one computed tomography projection, preferably a plurality of computed tomography projections, wherein the computed tomography projections include activating at least one of the x-ray sources to generate at least one x-ray beam, and detecting at least one x-ray beam projected through a treatment region by at least one of the detector elements. In particular, the beam is projected through an object or a portion of an object. The at least one x-ray beam created may, for example, be a fan beam. The fan may be configured such that an object and / or a portion of an object and / or a treatment region is within the fan beam. Preferably, the groups of computed tomography projections may be configured such that the duration of all projections of a group substantially corresponds to the duration of the inter-pulse time. Preferably, the groups of computed tomography projections may be configured such that the first projection of a group substantially starts at the beginning of the inter-pulse time. Depending on the duration of the inter-pulse time and the requirements of the CT scan, such as the desired or optimal duration of a projection, there may be one or more projections in a group. Thus, interleaving the treatment pulses with the groups may mean that during the pulse duration, i.e. when the treatment beam is on, the acquisition of the static computed tomography system is paused and once the treatment beam is turned off, the data acquisition of the static computed tomography system continues with the next projection. The time interval between projections within a group may be constant throughout the group. Advantageously, this embodiment may efficiently use the available time during the inter-pulse time while also ensuring that no (mostly useless) data is acquired during the pulse duration. Optionally, the computed tomography system may be configured so that projections are acquired one after another in a virtual rotational direction of the static computed tomography system. Thus, the acquisition may correspond to an acquisition with a rotational CT, but omitting the acquisition during the pulse duration. A full CT scan may include projections from multiple groups. It is conceivable that a new full scan is started within a group. Thus, some projections of a group may belong to another full scan outside of other projections of the group, or some projections may be used for the reconstruction of another image outside of other projections of the group, respectively.

[0018] According to an embodiment, the x-ray source includes a cathode based on field emission nanotubes. Advantageously, cathodes based on field emission nanotubes can provide very good flexibility to quickly activate and deactivate. For example, there is no large afterburning effect, which may be the case with thermal-based cathodes. In addition, the technology of field emission nanotube-based cathodes has been advantageously developed in recent years, making it now possible to build static computed tomography systems with distributed x-ray sources (e.g., in the form of an array of x-ray sources) with a power of about 10-15kW. It can be expected that in the next few years, this technology can even reach higher power values, potentially corresponding to the higher power values ​​of third-generation conventional rotational computed tomography systems.

[0019] According to an embodiment, the radiotherapy device comprises a medical linear accelerator. The radiotherapy device can be configured to generate high-energy photons, in particular high-energy photons in the x-ray range, and / or electrons that generate therapeutic beams. Preferably, the radiotherapy device can include an electron and photon linear accelerator, in particular a megavolt (MV) electron and photon linear accelerator. Such devices may also be referred to as linear accelerators. For example, the radiotherapy device can accelerate electrons to collide with heavy metal materials to generate high-energy x-ray photons. The radiotherapy device can be configured to generate an x-ray beam having a shape corresponding to a target (such as a patient's tumor).

[0020] According to an embodiment, the computer tomography system is configured to acquire scan data by continuously activating the x-ray sources in the order in which they are arranged around the treatment area. Thus, the activation of the x-ray sources can be configured to allow for the acquisition of scan data corresponding to the acquisition of the rotating CT system. Therefore, conventional methods for image reconstruction may be particularly easily applicable to this embodiment. The x-ray sources can be equally spaced around the treatment area. The CT projections used for data acquisition can be placed equidistantly in azimuth at fixed time intervals.

[0021] According to an embodiment, the computed tomography system is configured to acquire scan data by activating the x-ray sources in an order such that every two successively activated x-ray sources are spaced apart by more than 90° on a circle around the treatment area. Advantageously, the x-ray sources are spaced apart so that better correction for movement can be allowed, since different angles can be monitored within a small time frame.

[0022] According to another aspect of the invention, there is provided a method for monitoring an object or a portion of an object during radiation therapy involving a pulsed treatment beam aimed at a treatment area with a pulsed beam pattern including pulse durations of individual pulses and inter-pulse times between pulses. The method comprises the following steps:

[0023] (a) acquiring scan data from the subject or a portion of the subject during the inter-pulse time using a computed tomography system comprising an x-ray source and detector elements distributed around a treatment region;

[0024] (b) Reconstructing image data, in particular 3D image data, of the object or a part of the object based on the scan data acquired during the inter-pulse time.

[0025] All features and advantages of the system may apply to the method and vice versa. The applied computed tomography system may be a computed tomography system as described herein. The radiation therapy may be a radiation therapy with pulsed treatment beams as is common in the prior art. Advantageously, the method may even enable monitoring of the object or a portion of the object during the radiation therapy. Thus, a closer monitoring may be enabled with this method.

[0026] According to an embodiment, the method further comprises:

[0027] - in step (a), performing a first computed tomography scan by acquiring scan data from the object or a portion of the object during the inter-pulse time;

[0028] - In step (b), image data are reconstructed based on the scan data from the first scan.

[0029] According to this embodiment, the method further comprises the following additional steps:

[0030] -(c) after the first computed tomography scan, performing at least one second computed tomography scan, wherein the at least one second computed tomography scan comprises a smaller number of computed tomography projections than the first computed tomography scan; and -(d) updating the image data based on the scan data from the at least one second computed tomography scan. The updated image data may in particular be the image data reconstructed in step b. Steps (c) and (d) may be applied multiple times. Thus, the updated image data may be image data that has already been updated in a previous step. The at least one second computed tomography scan may be a low-resolution scan, such that the resolution of the second computed tomography scan is lower than the resolution of the first computed tomography scan. Advantageously, the second scan may allow a fast updating of the data from the first scan, thus allowing a higher temporal resolution of the update, while still having the high-resolution information of the first scan. Thus, a low-resolution update of the at least one second scan may be performed and / or combined with higher-resolution data from the first scan. Thus, since a very high level of temporal resolution may be achieved, fast-moving objects may be imaged and / or monitored. Furthermore, artifacts caused by patient movement may be better avoided. Additionally and / or alternatively, the at least one second scan may advantageously allow a low-dose image update. Thus, the radiation dose of the subject may be reduced. Advantageously, the scan data acquisition of the at least one second scan may be combined with a compressed sensing method. Compressed sensing is a technique well known in the art. It is based on acquiring a reduced amount of data, such that data with no or little information is omitted in order to obtain compressed approximate data.

[0031] Scan data from at least one second computed tomography scan, in particular scan data from a plurality of second computed tomography scans, can be used to track an object, such as a tumor. The movement of the object can be calculated from the image data obtained from the first scan and the second scan. For example, a marker can be used to track the object, wherein the marker is estimated based on the projection of at least one second computed tomography scan. For calculating the tracking, methods existing in the prior art can be applied, such as the method described by Ross I Berbeco et al. in "Integrated radiotherapy imaging system (IRIS): design considerations of tumour tracking with linac gantry-mounted diagnostic x-ray systems with flat-panel detectors", Phys. Med. Biol. (Journal of Medical and Biological Physics) 49 (2004) 243-255. Other methods that can be applied are described in "Real-Time Tumor-Tracking Radiation Therapy for Lung Carcinoma by the Aid of Insertion of a Gold Marker Using Bronchofiberscopy", Toshiyuki Harada et al., Cancer, 2002 Oct 15;95(8):1720-7. doi:10.1002 / cncr.10856. PMID:12365020 and "Online image guided tumour tracking with scanned proton beams: a comprehensive simulation study", Ye Zhang et al., Journal of Medical and Biological Physics 59 (2014) 7793-7817.

[0032] According to an embodiment, the computed tomography projections of the at least one second computed tomography scan are directed to a region of interest, which region of interest is a sub-region of the region covered by the computed tomography projections of the first computed tomography scan. Thus, the sampling density may be optimized for the region of interest. Advantageously, a local update may thus be achieved with a high temporal resolution.

[0033] According to an embodiment, a smaller number of computed tomography projections from at least one second computed tomography scan is performed by activating a subset of x-ray sources of the computed tomography system; wherein the subset of x-ray sources is selected such that the distances between the nearest neighbors of the x-ray sources are equidistant across the subset. Advantageously, a uniform distribution of the scan data can thus be obtained.

[0034] According to an alternative embodiment, the method further comprises:

[0035] - in step (a), performing an initial computed tomography scan by acquiring scan data from the object or a portion of the object during an inter-pulse time with a small number of computed tomography projections;

[0036] - in step (b), reconstructing coarse image data based on the scan data from the first scan in order to determine an estimate of the position of the region of interest;

[0037] - (c) performing, after the initial computed tomography scan, at least one further computed tomography scan, wherein the at least one further computed tomography scan comprises further computed tomography projections optimized to image the region of interest;

[0038] - (d) reconstructing image data based on scan data from at least one further computed tomography scan.

[0039] Thus, advantageously, a rough estimate of the position of the region of interest can be determined. For example, the region of interest can be a region where a tumor is located. The small number of computed tomography projections of the initial scan can be, for example, 2 to 4 projections, preferably 2 to 3 projections, more preferably 2 projections. The two projections can be orthogonal to each other. Although a low number of projections may not be sufficient to obtain diagnostic information, a low number of projections can still allow the position of the region of interest to be estimated. Additional computed tomography projections can be suitable for efficiently imaging the region of interest. Adjusting the additional computed tomography projections can include adjusting the angles of the additional computed tomography projections. For example, the angles of the additional computed tomography projections can be adjusted so that the projections include projections corresponding to the main axis of symmetry of the object to be examined in the region of interest. For example, the object can be a tumor. Optionally, in addition to the projections corresponding to the main axis of symmetry of the object, at least one additional projection can be provided. At least one additional projection can be selected so that the resolution increases along the direction of interest. In some cases, it may be advantageous to increase the resolution along a specific direction. The specific direction can be particularly related to radiation therapy.

[0040] According to another aspect of the present invention, a computer program is provided. The described computer program comprises instructions, which, when executed by a control unit of a computer tomography system, causes the computer tomography system to perform a method according to one of the embodiments described herein. All features and advantages of the system and method may apply to the computer program, and vice versa.

[0041] According to another aspect of the present 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 according to one of the embodiments described herein. All features and advantages of the system, method and computer program may apply to the computer-readable storage medium, and vice versa.

[0042] Unless stated otherwise, the embodiments described herein may be combined with each other. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The drawings illustrate various exemplary embodiments and methods of various aspects of the invention.

[0044] Figure 1 A radiation therapy system according to an embodiment of the present invention is shown;

[0045] Figure 2 A flow chart showing a method for monitoring a subject or a portion of a subject during radiation therapy according to an embodiment of the present invention;

[0046] Figure 3shows an interleaved pattern of treatment beams and computed tomography projections according to an embodiment of the present invention;

[0047] Figure 4 shows a pattern of treatment beams and computed tomography projections which are not interleaved, but rather the computed tomography projections are applied sequentially;

[0048] Figure 5 shows a cross section of a computed tomography system with a pulse scheme according to an embodiment of the present invention;

[0049] Figure 6 shows a cross section of a computed tomography system with a pulse scheme according to another embodiment of the present invention;

[0050] Figure 7 shows a cross section of a computed tomography system with a pulse scheme according to another embodiment of the present invention;

[0051] Figure 8 A flow chart showing a method for monitoring a subject or a portion of a subject during radiation therapy according to another embodiment of the present invention;

[0052] Fig. 9 A flow chart showing a method for monitoring a subject or a portion of a subject during radiation therapy according to another embodiment of the present invention;

[0053] Fig.10 A computed tomography pulse scheme according to an embodiment of the present invention is shown; and

[0054] Fig.11 A computed tomography pulse scheme according to an embodiment of the invention is shown.

[0055] In the drawings, similar elements are denoted by the same reference numerals. DETAILED DESCRIPTION

[0056] Figure 1A radiotherapy system according to an embodiment of the present invention is shown. The radiotherapy system includes a treatment area 4, which is configured so that an object 5 or a portion of the object 5 can be placed in the treatment area 4. The radiotherapy system also includes a static computed tomography system 1, including an x-ray source and a detector element distributed around the treatment area 4. Preferably, the x-ray source may include a cathode based on field emission nanotubes. The radiotherapy system also includes a radiotherapy device 2, which is configured to create a pulsed treatment beam 21, which is aimed at the treatment area 4 with a pulsed beam pattern, which includes the pulse duration of individual pulses and the inter-pulse time 23 between pulses. The radiotherapy device 2 includes a medical linear accelerator. Preferably, the radiotherapy device 2 may include an electron and photon linear accelerator, in particular a megavolt electron and photon linear accelerator. The computed tomography system 1 is configured to acquire scan data from the object 5 or a portion of the object 5 during the inter-pulse time 23. Figure 2 A corresponding method which the computed tomography system 1 is configured to apply is shown in FIG.

[0057] Figure 2 A flow chart of a method for monitoring an object 5 or a part of an object 5 during radiation therapy according to an embodiment of the invention is shown. The radiation therapy involves a pulsed treatment beam 21 aimed at a treatment region 4, having a pulsed beam pattern comprising pulse durations of individual pulses and inter-pulse times 23 between pulses. The method itself comprises a first step 101 of acquiring scan data from the object 5 or a part of the object 5 during the inter-pulse times 23 with a computed tomography system 1 comprising x-ray sources and detector elements distributed around the treatment region 4. In a further step 102, image data of the object 5 or a part of the object 5 are reconstructed based on the scan data acquired during the inter-pulse times 23. The image data may in particular be 3D image data.

[0058] Preferably, the radiotherapy system 1 may be configured to interleave individual treatment pulses with a plurality of groups of computed tomography projections 11, such that each group comprises at least one computed tomography projection 11, preferably a plurality of computed tomography projections 11. The computed tomography projections 11 comprise activating at least one of the x-ray sources to generate at least one x-ray beam, and detecting by at least one of the detector elements the at least one x-ray beam projected through the treatment region 4. Figure 3The corresponding staggered pattern of the treatment beam 21 and the computed tomography projection 11 is shown. In this representation, the horizontal axis is the time axis. It can be seen that in this embodiment, the computer therapy system 1 is configured to activate the x-ray source only during the inter-pulse time 23 and to pause the acquisition during the pulse duration 22. In this embodiment, the inter-pulse time 23 is of the order of about 1 millisecond and the pulse duration is of the order of about 15 microseconds. In addition, in this embodiment, the projection duration 12 of the computed tomography projection 11 is in the order of about 250 microseconds. Here, there are three computed tomography projections 11 within each inter-pulse time 23. Due to the fact that the components of the static computed tomography system 1 do not rotate physically, in order to avoid scattered radiation from the treatment beam 21 on the (multiple) detectors of the static computed tomography system 1, staggering the projections 11 and the treatment beam 21 is very directly implementable. For example, the projections 11 used for imaging can be placed radially equidistant at fixed time intervals. For a rotating computed tomography system, this is generally not possible to achieve in the same direct way. The computed tomography system 1 is configured to reconstruct image data of the object 5 or a portion of the object 5 based on the scan data acquired during the inter-pulse time 23 .

[0059] Figure 4 A pattern of treatment beams 21 and computed tomography projections 11 is shown, which are not interleaved, but rather the computed tomography projections 11 are applied consecutively. Projections 11 overlapping the treatment beam 21 generally cannot be used for reconstruction, since they are corrupted by scattering from the treatment beam 21. In this example, approximately 4 projections out of 12 projections 11 are thus affected by the treatment beam 21. Rotating computed tomography systems may produce such patterns. For rotating computed tomography systems, synchronization of the computed tomography data generally cannot be as Figure 3. In the case of a fixed projection integration timing of a rotational computed tomography system, the continuous real-time acquisition would therefore destroy a part of the projections 11, since they would be destroyed due to scattering from the treatment beam 21. In this example, for an inter-pulse time 23 of approximately 1 millisecond, roughly every fourth projection 11 would need to be "ignored", since otherwise the projection 11 would have scattered radiation of the MV treatment beam 21. Therefore, for a rotational computed tomography system, sample gaps occur that do not have to occur when using a static computed tomography system 1. Since the image quality of a CT image strongly depends on the number of projections 11 acquired per half revolution, the sampling gaps in the projection direction reduce the achievable image quality. Assuming a fixed integration time, in this case 250 microseconds, this effect generally increases with increasing rotation time. For faster rotation times, the number of projections 11 acquired per half revolution is lower, so the sampling gaps will have a relatively greater impact on the final image quality. The same applies if the pulse rate of the treatment beam 21 increases. Even so, the number of sampling gaps increases on a rotational computed tomography-based system. This disadvantage can be avoided by using a static computed tomography system 1 as an imaging device on a combined radiation therapy system.

[0060] Figures 5 to 7 Each shows a cross section of a computed tomography system 1 according to an embodiment of the invention, each cross section comprising a detector arrangement 13 and an array 14 of x-ray sources 15 of the computed tomography system 1 . Figure 5 A pulse scheme of a computed tomography system 1 according to an embodiment of the present invention is shown. Here, the computed tomography system 1 is configured to acquire scan data by continuously activating the x-ray sources 15 in the order in which the x-ray sources 15 are arranged around the treatment area 4. The data acquisition of this pulse scheme is structurally similar to the data acquisition of a rotational computed tomography system. This pulse scheme can be referred to as a standard rotation with dense sampling. Figure 6 A pulse scheme of a computed tomography system 1 according to another embodiment of the invention is shown. In this embodiment, a smaller number of computed tomography projections 11 are performed by activating subgroups of x-ray sources 15 of the computed tomography system 1. Here, the subgroups of x-ray sources 15 are selected such that the distances between the nearest neighbors of the x-ray sources 15 are equidistant across the subgroups. Figure 5 Compared to the pulse scheme shown in , some of the x-ray sources 15 are not activated here. This pulse scheme can be called sparse equidistant sampling. Figure 6A pulse scheme of a computed tomography system 1 according to another embodiment of the present invention is shown. In this embodiment, the computed tomography system 1 is configured to acquire scan data by activating the x-ray sources 15 in a sequence such that every two successively activated x-ray sources 15 are spaced apart by more than 90° on a circle around the treatment area 4.

[0061] Figure 8 A flow chart of a method for monitoring an object 5 or a part of an object 5 during radiation therapy according to another embodiment of the present invention is shown. The radiation therapy involves a pulsed treatment beam 21 aimed at a treatment region 4, having a pulsed beam pattern including pulse durations of individual pulses and inter-pulse times 23 between pulses. The method itself is performed using a computed tomography system 1, which includes detector elements and an x-ray source 15 distributed around the treatment region 4. The method comprises a first step 201, performing a first computed tomography scan with the computed tomography system 1 and acquiring scan data from the object 5 or a part of the object 5 during the inter-pulse times 23. In a further step 202, image data of the object 5 or a part of the object 5 are reconstructed based on the scan data acquired during the inter-pulse times 23 of the first scan. The image data may in particular be 3D image data. In a further step 203, which is performed after the step 201 including the first computed tomography scan, at least one second computed tomography scan is performed. The at least one second computed tomography scan includes a smaller number of computed tomography projections 11 than the first computed tomography scan. For example, as Figure 6 As shown in , a smaller number of computed tomography projections 11 can be performed. Therefore, a subset of x-ray sources 15 of the computed tomography system 1 can be activated. Optionally, the subset of x-ray sources 15 can be selected so that the distances between the nearest neighbors of the x-ray sources 15 are equidistant across the subset. However, other arrangements of CT pulse schemes are also possible. For example, the computed tomography projections 11 of at least one second computed tomography scan can be directed to a region of interest 41, which is a sub-region of the region covered by the computed tomography projections 11 of the first computed tomography scan. Therefore, the region of interest 41 (i.e., the sub-region) can be scanned with the second scan, while the first scan is directed to a region with a larger area. In another step 204, the image data is updated based on the scan data from the at least one second computed tomography scan. Therefore, advantageously, due to the smaller number of projections 11, the second scan can be faster than the first scan, so a greater temporal resolution can be achieved. On the other hand, the first scan can also allow for more detailed information and / or information from a larger area.

[0062] Fig. 9A flow chart of a method for monitoring an object 5 or a portion of an object 5 during radiation therapy according to another embodiment of the present invention is shown. The radiation therapy involves a pulsed treatment beam 21 aimed at a treatment area 4, having a pulsed beam pattern including pulse durations of individual pulses and inter-pulse times 23 between pulses. The method itself is performed by using a computed tomography system 1 including detector elements distributed around the treatment area 4 and an x-ray source 15. The method comprises a first step 301 of performing an initial computed tomography scan by acquiring scan data from the object 5 or a portion of the object 5 with the computed tomography system 1 during the inter-pulse times 23 having a small number of computed tomography projections 11. For example, the small number of projections 11 may consist of two projections 11. An example of this situation is Fig.10 , wherein two computed tomography projections 11 originating from two different x-ray sources 15 are directed towards a treatment zone 4, which comprises a region of interest 41 being part of an object 5. The projections 11 are detected by a detector arrangement 13 of the computed tomography system 1 comprising detector elements. Thus, the region of interest 41 may be defined by an object, for example a tumor of the object 5. In a further step 302, image data of the object 5 or a part of the object 5 are reconstructed based on the scan data acquired during the inter-pulse time 23 in order to (roughly) determine an estimate of the position of the region of interest 41. The image data may in particular be 3D image data. In a further step 303, which is performed after the step 301 comprising the initial computed tomography scan, at least one further computed tomography scan is performed. The at least one further computed tomography scan comprises further computed tomography projections 11, which are optimized to image the region of interest 41. Fig.11 An example of a corresponding CT pulse scheme is shown. Based on the estimated position of the region of interest 41, further computed tomography projections 11 originating from different x-ray sources 15 are arranged to image the region of interest 41. Preferably, the further computed tomography projections 11 may correspond to a main symmetry axis of the object defining the region of interest 41. Fig.11 As shown in the example of , besides the computed tomography projections 11 corresponding to the main symmetry axis of the object, additional projections 11 may also be used. In a further step 304, image data are reconstructed based on scan data from at least one further computed tomography scan.

Claims

1. A radiotherapy system comprising a treatment area (4) configured so that an object (5) or a part of an object (5) can be placed in the treatment area, A static computed tomography system (1) comprising detector elements and an x-ray source (15) distributed around the treatment area (4), and a radiotherapy device (2) configured to create a pulsed therapy beam (21) aimed at the treatment area (4) with a pulsed beam pattern including pulse durations (22) of individual pulses and inter-pulse times (23) between pulses; wherein the computed tomography system (1) is configured to acquire scan data from the object (5) or a portion of the object (5) during the inter-pulse time (23); The radiotherapy system is configured to interleave individual treatment pulses with groups (11) of computed tomography projections such that each group comprises at least one computed tomography projection (11).

2. The radiation therapy system according to claim 1, The computed tomography system (1) is configured to reconstruct image data, in particular 3D image data, of the object (5) or a part of the object (5) based on the scan data acquired during the inter-pulse time (23).

3. The radiotherapy system according to claim 1 or 2, The computational therapy system is configured to activate the x-ray source (15) only during the inter-pulse time (23) and to suspend acquisition during the pulse duration (22).

4. The radiotherapy system according to claim 1 or 2, The radiotherapy system is configured to discard scan data acquired during the pulse duration (22).

5. The radiotherapy system according to claim 1 or 2, wherein the radiotherapy system is configured to interleave the individual treatment pulses with the groups (11) of computed tomography projections such that each group comprises a plurality of computed tomography projections (11), The computed tomography projection (11) comprises activating at least one of the x-ray sources (15) to generate at least one x-ray beam, and detecting the at least one x-ray beam projected through the treatment area (4) by at least one of the detector elements.

6. The radiotherapy system according to claim 1 or 2, The x-ray source (15) comprises a field emission nanotube based cathode.

7. The radiotherapy system according to claim 1 or 2, The radiotherapy device (2) comprises a medical linear accelerator.

8. The radiotherapy system according to claim 1 or 2, The computed tomography system (1) is configured to acquire scan data by successively activating the x-ray sources (15) in the order in which the x-ray sources (15) are arranged around the treatment area (4).

9. The radiotherapy system according to claim 1 or 2, The computed tomography system (1) is configured to acquire scan data by activating the x-ray sources (15) in such a sequence that every two successively activated x-ray sources (15) are spaced apart by more than 90° on a circle around the treatment area (4).

10. A computer-implemented method for monitoring an object (5) or a portion of an object (5) during a pulsed treatment beam (21) aimed at a treatment area (4) with a pulsed beam pattern, the pulsed beam pattern comprising pulse durations (22) of individual pulses and inter-pulse times (23) between pulses, The method comprises the following steps: (a) acquiring scan data from the object (5) or a portion of the object (5) during the inter-pulse time (23) between pulses with a computed tomography system (1) comprising detector elements and an x-ray source (15) distributed around the treatment area (4); wherein the pulses are interleaved with groups (11) of computed tomography projections such that each group comprises at least one computed tomography projection (11); (b) reconstructing image data, in particular 3D image data, of the object (5) or a part of the object (5) based on the scan data acquired during the inter-pulse time (23).

11. The method according to claim 10, Therein, a computed tomography system (1) according to any one of claims 1 to 9 is used.

12. The method according to claim 10 or 11, further comprising: - in step (a), performing a first computed tomography scan by acquiring scan data from the object (5) or a portion of the object (5) during the inter-pulse time (23); - in step (b), reconstructing image data based on said scan data from said first scan; - (c) performing at least one second computed tomography scan after said first computed tomography scan, wherein said at least one second computed tomography scan comprises a smaller number of computed tomography projections (11) than said first computed tomography scan; and - (d) updating said image data based on said scan data from said at least one second computed tomography scan.

13. The method according to claim 12, wherein the computed tomography projections (11) of the at least one second computed tomography scan are directed toward a region of interest (41), which region of interest is a sub-region of the region covered by the computed tomography projections (11) of the first computed tomography scan.

14. The method according to claim 10 or 11, further comprising: - in step (a), performing an initial computed tomography scan by acquiring scan data from the object (5) or a portion of the object (5) using a small number of computed tomography projections (11) during the inter-pulse time (23); - in step (b), reconstructing coarse image data based on said scan data from said first scan in order to determine an estimate of the position of a region of interest (41); - (c) performing at least one further computed tomography scan after the initial computed tomography scan, wherein the at least one further computed tomography scan comprises further computed tomography projections (11) optimized to image the region of interest (41); (d) reconstructing image data based on scan data from the at least one further computed tomography scan.

15. A computer program comprising instructions which, when executed by a control unit of a computed tomography system (1), causes the computed tomography system (1) to perform the method according to any one of claims 10 to 14.

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