X-ray tube voltage determination

Abstract

A computer implemented method 400 for determining a tube voltage to be used for driving an x-ray tube (210, fig. 2), the tube supported by a gantry (230, fig. 2), the gantry supporting an x-ray detect

Description

This disclosure relates to X-ray tubes, and in particular but without limitation to methods, systems, and / or computer readable media for determining a tube voltage to be used for driving an X-ray tube. Background Radiotherapy can be described as the use of ionising radiation, such as X-rays, to treat a human or animal body. Radiotherapy is commonly used to treat tumours within the body of a patient or subject. In such treatments, ionising radiation is used to irradiate, and thus destroy or damage, cells which form part of the tumour. However, in order to apply a prescribed dose to a tumour or other target region within a subject, the radiation must pass through healthy tissue, irradiating and hence potentially damaging it in the process. It is a general aim of the field to minimise the dose received by healthy tissue during radiotherapy treatments. Many different radiotherapy technigues exist, allowing radiation to be applied from different angles, at varying intensities, and for specific time periods. Before radiotherapy treatment, a radiation therapy treatment plan is created to determine how and where the radiation should be applied. Typically, such a treatment plan is created with the assistance of medical imaging technology. For example, a computed tomography (CT) scan may be taken of the patient in order to produce a three-dimensional image of the area to be treated. The three-dimensional image allows the treatment planner to observe and analyse the target region and identify surrounding tissues. In addition, the three-dimensional image can be recorded over a period of time, such as a breathing cycle, in order to provide a four-dimensional (4DCT) video that can inform the treatment plan. Imaging systems including X-ray imaging systems, CT imaging systems and cone beam computed tomography (CBCT) imaging systems involve the application of radiation, which must pass through healthy tissue and may damage it in the process. It is preferential to minimise the dose of radiation received during imaging while ensuring that repeated images do not have to be taken due to a low image quality. Summary An invention is set out in the claims. Figures Specific examples are now described, by way of example only, with reference to the drawings, in which: Figure 1 shows a radiotherapy apparatus; Figure 2 shows an imaging apparatus in a first configuration; Figure 3 shows the imaging apparatus of figure 2 in a second configuration; Figure 4 shows a flowchart of a method for determining an X-ray tube voltage; Figure 5 shows a block diagram of one implementation of a radiotherapy system; and Figure 6 shows a computer readable medium or, more generally, a computer program product. Detailed description Fig. 1 shows a radiotherapy (RT) apparatus 100 which comprises a source 102 of radiofrequency waves, RF transmission apparatus 103, an accelerating waveguide 104, a source of electrons 106, a treatment head including a collimator 108 such as a multi-leaf collimator used to shape a treatment beam 110, housing 112 (shown partially cut away), and a patient support 114. The depicted apparatus does not have the usual housing which would cover the entire RT apparatus in a commercial setting such as a hospital. In use, the apparatus would also comprise the housing which, together with a ring-shaped gantry, defines a bore. The patient support 114 is moveable and can be used to support a patient and move them, or another subject, into the bore when radiotherapy is to commence. The RT apparatus beam generation system comprises the source 102 of radiofrequency waves, the accelerating waveguide 104, and the source of electrons 106. The beam generation system is configured to produce a beam of radiation, otherwise known as the treatment beam 110, that is collimated and shaped by the collimator 108 and directed towards the bore. The beam generation system is based on a linear accelerator (linac) design. The source 102 of radiofrequency waves, such as a magnetron, is configured to produce radiofrequency waves. The source 102 of radiofrequency waves is coupled to the accelerating waveguide 104 via the RF transmission apparatus 103, which may include a circulator, and is configured to pulse radiofrequency waves into the accelerating waveguide 104. Radiofrequency waves may pass from the source 102 of radiofrequency waves through an RF input window and into an RF input connecting pipe or tube. The source of electrons 106, such as a diode or triode electron gun, is also coupled to the accelerating waveguide 104 and is configured to inject electrons into the waveguide 104. The injection of electrons into the accelerating waveguide 104 is synchronised with the pumping of the radiofrequency waves into the accelerating waveguide 104. The design and operation of the source 102 of radiofrequency waves, source of electrons 106, and the accelerating waveguide 104 is such that the radiofrequency waves accelerate the electrons to very high energies as the electrons propagate through the accelerating waveguide 104. The design of the accelerating waveguide 104 depends on whether the accelerating waveguide 104 accelerates the electrons using a standing wave or travelling wave, though the waveguide typically comprises a series of cells, each cell connected by a hole or ‘iris’ through which the electron beam may pass. The cells are coupled in order that a suitable electric field pattern is produced which accelerates electrons propagating through the accelerating waveguide 104. As the electrons are accelerated in the accelerating waveguide 104, the electron beam path may be controlled by a suitable arrangement of steering magnets, or steering coils, which surround the accelerating waveguide 104. The arrangement of steering magnets may comprise, for example, two sets of quadrupole magnets. Once the electrons have been accelerated, they may pass into a flight tube. The flight tube may be connected to the waveguide by a connecting tube. This connecting tube or connecting structure may be called a drift tube. The electrons travel toward a heavy metal target which may comprise, for example, tungsten. Whilst the electrons travel through the flight tube, an arrangement of focusing magnets act to direct and focus the beam on the target. To ensure that propagation of the electrons is not impeded as the electron beam travels toward the target, the accelerating waveguide 104 is evacuated using a vacuum system comprising a vacuum pump or an arrangement of vacuum pumps. The pump system is capable of producing ultra-high vacuum (UHV) conditions in the accelerating waveguide 104. The vacuum system also ensures UHV conditions in the source of electrons 106 and, if used, the drift tube and flight tube. Electrons can be accelerated to speeds approaching the speed of light in the evacuated waveguide. The beam generation system is configured to direct the treatment beam 110 toward a patient positioned on the patient support 114. The treatment beam 110 comprises therapeutic radiation. The beam generation system may comprise an X-ray tube comprising a heavy metal target toward which the high energy electrons exiting the waveguide are directed. When the electrons strike the target, X-rays are produced in a variety of directions. A primary collimator may block X-rays travelling in certain directions and pass only forward travelling X-rays to produce the treatment beam 110. The X-rays may be filtered and may pass through one or more ion chambers for dose measuring. The beam can be shaped in various ways by beamshaping apparatus, for example by using the collimator 108, before it passes into the patient as part of radiotherapy treatment. In some implementations, the beam generation system is configured to emit either an X-ray beam or an electron particle beam. Such implementations allow the apparatus to provide electron beam therapy, i.e. a type of external beam therapy where electrons, rather than X-rays, are directed toward the target region. It is possible to ‘swap’ between a first mode in which X-rays are emitted and a second mode in which electrons are emitted by adjusting the components of the beam generation system. In essence, it is possible to swap between the first and second mode by moving the heavy metal target in or out of the electron beam path and replacing it with a so-called ‘electron window’. The electron window is substantially transparent to electrons and allows electrons to exit the flight tube. Typically, a radiation detector is positioned diametrically opposed to the collimator 108. The radiation detector is suitable for, and configured to, produce radiation intensity data. In particular, the radiation detector is positioned and configured to detect the intensity of radiation which has passed through the subject. The radiation detector may form part of a portal imaging system. The beam generation system is attached to the rotatable gantry 116 so as to rotate with the gantry 116. In this way, the beam generation system is rotatable around the patient so that the treatment beam 110 can be applied from different angles around the gantry 116. In a preferred implementation, the gantry is continuously rotatable. In other words, the gantry can be rotated by 360 degrees around the patient, and in fact may continue to be rotated past 360 degrees. The gantry may be ring-shaped, i.e. a ring-gantry. The RT apparatus 100 of Fig. 1 may be controlled by a controller (not shown). The controller is a computer, processor, or other processing apparatus. The controller may be formed by several discrete processors; for example, an RT apparatus processor, which controls the operation of the RT apparatus; and a subject support surface processor which controls the operation and actuation of the patient support 114. The controller is communicatively coupled to a memory, e.g. a computer readable medium. Fig. 2 shows an imaging apparatus 200 in a first configuration. The imaging apparatus may be a standalone imaging apparatus or may be combined with the RT apparatus 100 of Fig. 1 - for example by being located adjacent to one another so that the patient support 114 of Fig. 1 can be translated into, or shares, a bore with the imaging apparatus 200. The imaging apparatus 200 may be a CT imaging system or a CBCT imaging system. As used herein, a CT imaging system is distinguished from a CBCT imaging system. These terms are used to refer to different imaging modalities and different associated imaging devices. CT imaging involves rotating a source of imaging radiation 360° around a patient at axially spaced positions with a fan beam directed towards a relatively narrow panel in order to acquire data which can be used to produce a 2D cross-sectional image of the patient. Either the source of imaging radiation, or the patient, can then be incrementally advanced in order to acquire another 2D image and thereby build up a 3D image of the patent or region of interest via multiple 360° rotations around the patient. Alternatively, rather than incremental advancements, the patient or source of radiation can be advanced slowly and continuously while the radiation source rotates, in order to acquire the necessary data to construct a 3D CT image via a spiral or helical delivery of imaging radiation. A CBCT imaging system uses a broader, cone-shaped beam and larger panel in order to cover a large volume of the patient with a single full or half or other angle of rotation around the patient, thereby acquiring multiple 2D projections of the object from various angles used to reconstruct a 3D image, and CBCT systems are therefore able to provide 3D images quickly and with a reduced number of gantry rotations in comparison to traditional CT systems. For example, it is possible to achieve a 3D imaging volume via a single rotation of around 200 degrees about a patient using a CBCT modality Although this disclosure describes various examples and embodiments relating to X-ray imaging, it should be understood that the systems, devices, and methods disclosed herein may in fact be used to perform other types of cross-sectional imaging using X-rays, including 2D imaging and 3D imaging. The imaging apparatus of Fig. 2 comprises an X-ray tube 210 arranged to emit X-rays towards an X-ray detector 220. The X-ray tube 210 and the X-ray detector 220 are coupled to a rotatable gantry 230, which may be a ring gantry and may be the gantry of RT apparatus 100. The X-ray tube 210 is configured to emit X-rays along an imaging beam axis 240 so as to image a patient 260 supported by a patient support 250 - which may be the patient support 114 of RT apparatus 100. The patient support may be embodied as a table or bed or any other structure capable of supporting a least a part of the patient 260 so as to facilitate maintenance of an at least approximately fixed spatial relationship between the patient 260, or at least a portion thereof, and the patient support 250. The patient support 250 may be configured to move the patient 260 into, and out from, a bore of the gantry 230. The patient support 250 may be moveable by a suitable arrangement or configuration of motors and actuators. To provide a 3D image of the patient 260, the patient support 250 may be actuated to move the patient into the bore of the gantry 230 during imaging. The rotatable gantry 230 is rotatable in one or both of directions A and B about an axis approximately perpendicular to the page upon which Fig. 2 is shown. To obtain an image, the gantry 230 rotates in direction A or B whilst the X-ray tube 210 emits X-rays and the X-ray detector 220 detects X-rays incident thereon. By using a knowledge of the radial position at which the X-ray detector 220 detected incident X-rays and the detected intensity thereof along with a reconstruction algorithm, such as a back projection algorithm, an image of an imaged portion of the patient may be created. In the configuration of Fig. 2, X-rays produced by the X-ray tube 210 travel along the imaging beam axis 240 on their way to the X-ray detector 220 and pass through the space occupied by the patient 260. The distance that such X-rays travel through that space is labelled in Fig. 2 as d1. As the patient is not a perfect cylinder centred upon the axis of rotation of the rotatable gantry, the respective distance that an X-ray produced by the X-ray tube 210 travels through the space occupied by the patient 260 on its way to the X-ray detector 220 changes with the rotational position of the rotatable gantry 230. This is illustrated in Fig.3, where the rotatable gantry 230 has rotated in direction A (clockwise) by around 90 degrees to a second position thereby changing the respective distance that an X-ray produced by the X-ray tube 210 travels through the space occupied by the patient 260 on its way to the X-ray detector 220 to become the distance labelled as d2 in Fig. 3. As rotation of the gantry 230 causes the respective distance that an X-ray produced by the X-ray tube 210 travels through the space occupied by the patient 260 on its way to the X-ray detector 220 to change with rotation, the amount of attenuation caused by the patient’s body as experienced by X-rays travelling along imaging beam axis 240 on their way to the X-ray detector 220 also changes with the rotational position of the gantry. The imaging apparatus 200 of Fig. 2 further comprises a sensor 270 configured to acquire information indicative of the volumetric space occupied by a patient. The sensor 270 may be configured to measure, determine or generate the volumetric information using any suitable technique / imaging modality. The sensor may comprise, by way of non-limiting example, one or more: optical cameras, time of flight cameras, an MR imaging apparatus, a lidar sensor, an ultrasound sensor. As one possibility the sensor 270 may perform an optical scan of the patient. For example, the sensor 270 may comprise an infrared (IR) distance sensor and / or an optical camera. An IR distance sensor can be used to send and receive infrared light that reflects from objects, and can measure the distance between the sensor and the object being measured i.e., the patient 260. The light intensity, phase shift or time-of-flight may be used to determine the distance from the sensor 270 to the patient 260 and thus the volumetric space occupied by the patient 260. When a time-of-flight camera is used to measure distances within a scene, it may be configured to send high power optical pulses receive light reflected from the patient 260. Imaging using infrared or visible light is preferential to, say, using a CT scout scan to identify the volumetric space occupied by the patient as it does not involve ionising radiation. The sensor 270 may not be able to differentiate between the patient 260 and other parts of the imaging apparatus 200 and so some post processing of the data acquired by the sensor 270 may be required - for example to remove background information representative of the location of parts of the imaging apparatus 200 (such as the patient support). In situations where the sensor 270 is arranged in a static location relative to one or more parts of the imaging apparatus 200, the acquisition by the sensor 270 of data whilst there is no patient on the patient support 250 prior to acquisition when the patient 260 is present on the patient support 250 may enable a background removal (or subtraction) operation to occur. The sensor 270 may be arranged to produce true volumetric data - such as a voxel representation of the volume around the patient support 250. Alternatively or additionally, the sensor 270 may be arranged to produce a surface representation - such as a point cloud or triangular mesh. In any event the data produced by the sensor 270, once processed either by the sensor itself, or elsewhere, results in the acquisition of information indicative of the volumetric space occupied by the patient 260. To produce good quality X-ray images but not deliver unnecessary radiation to the patient, appropriate X-ray exposure settings are required. Too much radiation will saturate the X-ray detector or produce an image quality that exceeds the clinical need, meaning the delivery of an unnecessarily high dose to the patient. Too little radiation will not provide enough dose to the X-ray detector to give sufficient image quality. If the images do not satisfy the clinical need, then a second set of images may have to be taken - leading to the patient receiving a cumulative X-ray dose that is unnecessarily high. For volumetric (CT or CBCT) imaging, the reconstruction algorithm generally requires the tube voltage (the voltage provided to the X-ray tube when driving it-typically in the region of 120KV) to be kept constant for all images. Due to hardware limitations, the tube current (the current provided to the X-ray tube when driving it - typically measured in milli Amps or milli Amps per Second) can only vary within a limited range and it is the combination of the tube current and the tube voltage that determines the amount of X-ray radiation that is created by the X-ray tube. Selection of an appropriate tube voltage to allow sufficient range of adjustment of the tube current whilst ensuring that the X-ray detector receives an appropriate dose of X-ray radiation (neither so low that the images would not satisfy the clinical need, nor significantly higher than is needed to satisfy that need) is thus a focus of this disclosure. Selection of a tube voltage to be used to drive an X-ray tube can be made based on an expected amount of attenuation of X-ray radiation as it travels from the X-ray tube 210 to the X-ray detector 220. If there is a higher degree of expected attenuation, for example if there is a larger distance for the radiation to travel through the patient 260, then a larger dose will be needed such that the clinically necessary dose reaches the X-ray detector 220. To this end, a model of X-ray attenuation may be employed. The model may include a tissue type (such as bone, fat, muscle, brain matter etc.) and / or imaged region (such as a head, chest, pelvic region etc.) / an expected attenuation amount for that tissue type or imaged region. The model may take the form of an attenuation per unit distance for a given radiation intensity (as may be produced by a given X-ray tube operating at a specific tube voltage). As the X-ray tube is calibrated, the user can know its radiation output for a given tube voltage and current and attenuation coefficients can be looked up (for example in a look-up table), and so the model may further be arranged to identify, for a given tissue type or imaged region and a given tube voltage and respective distance, the tube current required to deliver a predetermined X-ray dose (which may be specified based on a clinical need) to the X-ray detector. In some examples, the model may comprise an expected X-ray attenuation per unit distance for the patient. The attenuation per unit distance is based on a region of the patient that is to be subjected to X-rays, and the region of the patient to be X-rayed may comprise a: head, chest and / or pelvic region. Based on so determined tube currents, a selection of a tube voltage to be used for driving the X-ray tube may be made. Factors that may be taken into account when making the selection include: ensuring that the radiation dose received at the X-ray detector meets the required clinical needs (optionally at all rotational positions of the gantry), that the tube voltage is as low as possible (this being preferable from an image quality point of view), and that, for the selected tube voltage, the range of available tube currents enables tube current alteration during imaging (i.e. the tube voltage can be selected such that the tube current required to deliver a radiation meeting the clinical needs is not at its limit such that it cannot be varied during imaging if needed). Such an approach may facilitate the selection of a most suitable tube current to use for each rotational position of the gantry given the selected tube voltage - for example, so as to both meet the clinical dosage need whilst not going beyond it. Maximum (or largest) distance: if distance d1 through patient 260 in Figure 2 is greater than distance d2 in Figure 3, then assuming that the portion of the patient 260 being imaged has roughly uniform attenuation characteristics, there will be greater attenuation at the rotational position shown in Figure 2. The tube current at the rotational position shown in Figure 2 will therefore need to be higher in order for an appropriate dose of X-ray radiation to reach X-ray detector 220. As one possibility, a maximum distance through the patient 260 may be identified in order to determine the maximum expected amount of attenuation that X-rays travelling from the X-ray tube 210 to the X-ray detector 220 will experience as the gantry is rotated. As one example, when transversely imaging a chest, the maximum distance may be in the shoulder-to-shoulder direction. The maximum distance may be determined statistically, for example by an assessment of what distance is required in order to lie within a certain percentile of a range of the distances - such as within a 90th, 95th or 99th percentile of all distances that X-rays pass through the space occupied by the patient as the gantry is rotated. The model can then be employed to select a tube voltage to be used for driving the X-ray tube based on the tube currents that would be needed to provide a predetermined X-ray dose to the X-ray detector at each (or all) of a plurality of available (or given) tube voltages for that maximum attenuation distance. Once the tube voltage has been selected, a tube current to be used for driving the X-ray tube can be selected based on the selected tube voltage. As one possibility, for the given tube voltage corresponding to the selected tube voltage, the respective determined tube current is selected as a tube current to be used for driving the X-ray tube. That tube current may then be used to drive the X-ray tube at all rotational positions. As one possibility, the step of finding a maximum distance may be omitted and instead, for each of a plurality of (or all) rotational positions of the gantry, the model may be used to determine tube currents that would be respectively required for the X-ray tube, when operating at each of a plurality of (or all) available (given) tube voltages, to deliver the predetermined X-ray dose to the X-ray detector. The tube voltage to be used for driving the X-ray tube may then be selected based on the tube currents and one or more ranges of tube current that are available to drive the X-ray tube. For the given tube voltage corresponding to the selected tube voltage, and for each of the plurality of rotational positions, the respective determined tube current may be selected as a tube current to be used for driving the X-ray tube at that rotational position. In other words, the selected tube currents vary as a function of the rotational position of the gantry. Those selected tube currents may then be respectively used to drive the X-ray tube as the gantry rotates through the rotational positions. Turning to Figure 4, Figure 4 shows a flowchart of a method for determining an X-ray tube voltage according of the present disclosure. At block S410, information is received that is indicative of a volumetric space occupied by a patient supported by the patient support. Optionally, the information indicative of the volumetric space occupied by a patient is information indicative of a surface of the patient. As discussed previously, the acquiring the information indicative of the volumetric space occupied by a patient may be acquired by performing an optical scan of the patient - for example using one or more time-of-flight cameras. At block S420, for each of a plurality of rotational positions of the gantry, a respective distance through the space occupied by the patient that an X-ray produced by the X-ray tube would travel on its way to the detector is determined. At block S430, based on a model of X-ray attenuation along one or more of the respective distances and a predetermined X-ray dose required at the X-ray detector, the tube voltage to be used for driving the X-ray tube is determined. The model of X-ray attenuation may comprise an expected X-ray attenuation per unit distance for the patient. In some implementations, attenuation per unit distance is based on a region of the patient that is to be X-rayed, for example a head, chest and / or pelvic region. The attenuation per unit distance may be based on typical anatomical composition for that region of the body, and typical ratios of different types of tissues such as bone, fat, muscle, brain matter and so on. The model of X-ray attenuation may be determined by retrieving attenuation values from published material attenuation tables. A memory may store a look-up-table comprising attenuation values. The method may optionally comprise retrieving at least one attenuation value from the look-up-table in order to determine the attenuation per unit distance for the patient. The predetermined X-ray dose may be based on an image noise requirement. In other words, the predetermined X-ray dose may depend on appropriate X-ray exposure to provide an image quality that meets the clinical need and gives sufficient image quality. In some implementations, determining the tube voltage may comprise determining a largest of the respective distances. In other words, the maximum distance is determined. The model may be used to determine tube currents that would be respectively required for the X-ray tube, when operating at each of a plurality of given tube voltages, to deliver the predetermined X-ray dose to the X-ray detector along the largest of the respective distances. The tube voltage to be used for driving the X-ray tube may be determined based on the tube currents and one or more ranges of tube current that are available to drive the X-ray tube. Additionally or alternatively, determining the tube voltage may comprise, for each of the plurality of rotational positions of the gantry, using the model to determine tube currents that would be respectively required for the X-ray tube, when operating at each of a plurality of given tube voltages, to deliver the predetermined X-ray dose to the X-ray detector along the respective distance. The tube voltage then being determined based on the tube currents and one or more ranges of tube current that are available to drive the X-ray tube. Each of the one or more ranges of tube current that are available to drive the X-ray tube may be associated with a respective given tube voltage. At optional block S440, the patient may be imaged by providing the determined tube voltage to the X-ray tube. In an example implementation, the method may comprise: first determining an outline or shape of the patient based on an optical surface imaging system. Second, for a plurality of (or each available) gantry angle: calculating the distance that radiation-rays emitted by the X-ray tube pass through the patient by projecting the imaging beam axis through the patient outline. Third, for a plurality of (or every) available tube voltages: calculating the tube current as a function of gantry angle needed to provide a given dose to the detector. This may be done by calculating the attenuation of radiation caused by the patient for the given tube voltage using published material attenuation tables, and knowing the radiation output of the tube as a function of tube current at the given tube voltage. Fourth, selecting the tube voltage that best matches the available range of tube currents. Once the tube voltage has been chosen, an appropriate tube current can be selected based on the clinical need and the tube voltage. In some circumstances, it can be assumed that the exposure time per image is constant. As the radiation output is proportional to the product of exposure time and tube current, exposure time may be added as an additional or alternative degree of freedom in the selection of appropriate tube voltage. In circumstances where the assumption that the exposure time per image is constant does not hold - for example in CBCT systems that use pulsed X-rays and change the pulse length in addition to, or instead of, changing tube current in order to control exposure of the image - a selection of an appropriate range of exposure times per image may be added as an additional or alternative degree of freedom in the selection of appropriate tube voltage. Fig. 5 illustrates a block diagram of one implementation of a radiotherapy / imaging system 500. The radiotherapy / imaging system 500 comprises a computing system 510 within which a set of instructions, for causing the computing system 510 to perform any one or more of the methods discussed herein, may be executed. Whereas a radiotherapy / imaging system 500 is described, it is contemplated that the system 500 may be any of: a radiotherapy system, an imaging system, or a combined imaging and radiotherapy system. In such circumstances, any redundant parts of the system 500 could be dispensed with. The computing system 510 shall be taken to include any number or collection of machines, e.g. computing device(s), that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methods discussed herein. That is, hardware and / or software may be provided in a single computing device, or distributed across a plurality of computing devices in the computing system. In some implementations, one or more elements of the computing system may be connected (e.g., networked) to other machines, for example in a Local Area Network (LAN), an intranet, an extranet, or the Internet. One or more elements of the computing system may operate in the capacity of a server or a client machine in a client-server network environment, or as a peer machine in a peer-to-peer (or distributed) network environment. One or more elements of the computing system may be a personal computer (PC), a tablet computer, a set-top box (STB), a Personal Digital Assistant (PDA), a cellular telephone, a web appliance, a server, a network router, switch or bridge, or any machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine. The computing system 510 includes controller circuitry 511 and a memory 513 (e.g., read-only memory (ROM), flash memory, dynamic random access memory (DRAM) such as synchronous DRAM (SDRAM) or Rambus DRAM (RDRAM), etc.). The memory 513 may comprise a static memory (e.g., flash memory, static random access memory (SRAM), etc.), and / or a secondary memory (e.g., a data storage device), which communicate with each other via a bus (not shown). Controller circuitry 511 represents one or more general-purpose processors such as a microprocessor, central processing unit, accelerated processing units, or the like. More particularly, the controller circuitry 511 may comprise a complex instruction set computing (CISC) microprocessor, reduced instruction set computing (RISC) microprocessor, very long instruction word (VLIW) microprocessor, processor implementing other instruction sets, or processors implementing a combination of instruction sets. Controller circuitry 511 may also include one or more special-purpose processing devices such as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a digital signal processor (DSP), network processor, or the like. One or more processors of the controller circuitry may have a multicore design. Controller circuitry 511 is configured to execute the processing logic for performing the operations and steps discussed herein. The computing system 510 may further include a network interface circuitry 518. The computing system 510 may be communicatively coupled to an input device 520 and / or an output device 530, via input / output circuitry 517. In some implementations, the input device 520 and / or the output device 530 may be elements of the computing system 510. The input device 520 may include an alphanumeric input device (e.g., a keyboard or touchscreen), a cursor control device (e.g., a mouse or touchscreen), an audio device such as a microphone, and / or a haptic input device. The output device 530 may include an audio device such as a speaker, a video display unit (e.g., a liquid crystal display (LCD) or a cathode ray tube (CRT)), and / or a haptic output device. In some implementations, the input device 520 and the output device 530 may be provided as a single device, or as separate devices. In some implementations, the computing system 510 may comprise image processing circuitry 519. Image processing circuitry 519 may be configured to process image data 580 (e.g. images, or imaging data), such as medical images obtained from one or more imaging data sources, a treatment device 550 and / or an image acquisition device 540. Image processing circuitry 519 may be configured to process, or pre-process, image data. For example, image processing circuitry 519 may convert received image data into a particular format, size, resolution or the like. In some implementations, image processing circuitry 519 may be combined with controller circuitry 511. Image processing circuitry 519 may be configured to process volumetric data 595 that is indicative of the volumetric space occupied by a patient. In some implementations, the radiotherapy system / imaging 500 may further comprise an image acquisition device 540 and / or a treatment device 550, such as those disclosed herein in the examples of Fig. 1 and Fig. 2. The image acquisition device 540 and the treatment device 550 may be provided as a single device. In some implementations, treatment device 550 is configured to perform imaging, for example in addition to providing treatment and / or during treatment. The treatment device 550 comprises the main radiation delivery components of the radiotherapy system, such as the beam generation systems and linear accelerator components disclosed herein. Image acquisition device 540 may be configured to perform positron emission tomography (PET), computed tomography (CT), and magnetic resonance imaging (MRI). Image acquisition device 540 may be configured to output image data 580, which may be accessed by computing system 510. Treatment device 550 may be configured to output treatment data 560, which may be accessed by computing system 510. Computing system 510 may be configured to access or obtain treatment data 560, planning data 570, volumetric data 595, and / or image data 580. Treatment data 560 may be obtained from an internal data source (e.g. from memory 513) or from an external data source, such as treatment device 550 or an external database. Planning data 570 may be obtained from memory 513 and / or from an external source, such as a planning database. Planning data 570 may comprise information obtained from one or more of the image acquisition device 540 and the treatment device 550. Sensor 590 (which may be an optical image scanner) may be configured to acquire information indicative of a volumetric space occupied by a patient supported by the patient support, such as information indicative of a surface of a patient. Scanner 590 may be configured to output information indicative of a volumetric space occupied by a patient 595, which may be accessed by computing system 510. Additionally or alternatively, the output of the scanner may be provided to the computing system for processing to determine information indicative of a volumetric space occupied by a patient supported by the patient support. The various methods described above may be implemented by a computer program. The computer program may include computer code (e.g. instructions) 610 arranged to instruct a computer to perform the functions of one or more of the various methods described above. The steps of the methods described above may be performed in any suitable order. The computer program and / or the code 610 for performing such methods may be provided to an apparatus, such as a computer, on one or more computer readable media or, more generally, a computer program product 600)), depicted in Fig. 6. The computer readable media may be transitory or non-transitory. The one or more computer readable media 600 could be, for example, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, or a propagation medium for data transmission, for example for downloading the code over the Internet. Alternatively, the one or more computer readable media could take the form of one or more physical computer readable media such as semiconductor or solid state memory, magnetic tape, a removable computer diskette, a random access memory (RAM), a read-only memory (ROM), a rigid magnetic disc, and an optical disk, such as a CD-ROM, CD-R / W or DVD. The instructions 610 may also reside, completely or at least partially, within the memory 513 and / or within the controller circuitry 511 during execution thereof by the computing system 510, the memory 513 and the controller circuitry 511 also constituting computer-readable storage media. In an implementation, the modules, components and other features described herein can be implemented as discrete components or integrated in the functionality of hardware components such as ASICS, FPGAs, DSPs or similar devices. A “hardware component” is a tangible (e.g., non-transitory) physical component (e.g., a set of one or more processors) capable of performing certain operations and may be configured or arranged in a certain physical manner. A hardware component may include dedicated circuitry or logic that is permanently configured to perform certain operations. A hardware component may comprise a special-purpose processor, such as an FPGA or an ASIC. A hardware component may also include programmable logic or circuitry that is temporarily configured by software to perform certain operations. In addition, the modules and components can be implemented as firmware or functional circuitry within hardware devices. Further, the modules and components can be implemented in any combination of hardware devices and software components, or only in software (e.g., code stored or otherwise embodied in a machine-readable medium or in a transmission medium). Examples of the present disclosure may be described herein in terms of functional and / or logical block components and various processing steps. It should be appreciated that such block components may be realised by any number of hardware, software, and / or firmware components configured to perform the specified functions. For example, an example of the present disclosure may employ various integrated circuit components, e.g., memory elements, digital signal processing elements, logic elements, look-up tables, or the like, which may carry out a variety of functions under the control of one or more microprocessors or other control devices. In addition, those skilled in the art will appreciate that examples of the present disclosure may be practised in conjunction with any number of systems, and that the systems described herein are merely exemplary embodiments of the present disclosure. For the sake of brevity, conventional techniques compared to signal processing, data transmission, signalling, control and other functional aspects of the systems (and the individual operating components of the systems) may not be described in detail herein. Furthermore, the connecting lines shown in the various figures contained herein are intended to represent example functional relationships and / or physical couplings between the various elements. It should be noted that many alternative or additional functional relationships or physical connection may be present in an example of the present disclosure. As described herein, a tube voltage and / or current to be used for driving an X-ray tube supported by a rotatable gantry may be determined based on information indicative of a volumetric space occupied by a patient and a predetermined X-ray dose required at an X-ray detector also supported by the gantry. The term “apparatus” as used herein may refer to either a single apparatus or plural apparatus and should not be understood as being particularly limited to either a single discrete apparatus or a plurality of discrete apparatus unless a particular apparatus is further described as such. Those skilled in the art will recognise that a wide variety of modifications, alterations, and combinations can be made with respect to the above described examples without departing from the scope of the disclosed concepts, and that such modifications, alterations, and combinations are to be viewed as being within the ambit of the disclosed concepts. Those skilled in the art will also recognise that the scope of the invention is not limited by the examples described herein but is instead defined by the appended claims.

Claims

1. A computer-implemented method for determining a tube voltage to be used for driving an X-ray tube, the X-ray tube being supported by a gantry, the gantry supporting an X-ray detector for detecting X-rays produced by the X-ray tube and being rotatable about a patient support, the method comprising:receiving information indicative of a volumetric space occupied by a patient supported by the patient support;determining, for each of a plurality of rotational positions of the gantry, a respective distance through the space occupied by the patient that an X-ray produced by the X-ray tube would travel on its way to the X-ray detector;based on a model of X-ray attenuation along one or more of the respective distances and a predetermined X-ray dose required at the X-ray detector, determining the tube voltage to be used for driving the X-ray tube.

2. The method of claim 1, further comprising imaging the patient by providing the determined tube voltage to the X-ray tube.

3. The method of any preceding claim, wherein the model comprises an expected X-ray attenuation per unit distance for the patient.

4. The method of any preceding claim, wherein determining the tube voltage to be used for driving the X-ray tube comprises:determining a largest of the respective distances;using the model to determine tube currents that would be respectively required for the X-ray tube, when operating at each of a plurality of given tube voltages, to deliver the predetermined X-ray dose to the X-ray detector along the largest of the respective distances; andselecting the tube voltage to be used for driving the X-ray tube based on the tube currents and one or more ranges of tube current that are available to drive the X-ray tube.

5. The method of claim 4, further comprising, for the given tube voltage corresponding to the selected tube voltage, selecting the respective determined tube current as a tube current to be used for driving the X-ray tube.

6. The method of any of claims 1 to 3, wherein determining the tube voltage to be used for driving the X-ray tube comprises:for each of the plurality of rotational positions of the gantry, using the model to determine tube currents that would be respectively required for the X-ray tube, when operating at each of a plurality of given tube voltages, to deliver the predetermined X-ray dose to the X-ray detector along the respective distance; andselecting the tube voltage to be used for driving the X-ray tube based on the tube currents and one or more ranges of tube current that are available to drive the X-ray tube.

7. The method of claim 6, further comprising, for the given tube voltage corresponding to the selected tube voltage, and for each of the plurality of rotational positions, selecting the respective determined tube current as a tube current to be used for driving the X-ray tube at that rotational position.

8. The method of any of claims 4 to 7, wherein each of the one or more ranges of tube current that are available to drive the X-ray tube is associated with a respective given tube voltage.

9. The method of any preceding claim, wherein the information indicative of the volumetric space occupied by a patient is information indicative of a surface of the patient10. The method of any preceding claim, further comprising acquiring the information indicative of the volumetric space occupied by a patient by performing an optical scan of the patient.

11. The method of claim 10, wherein the optical scan is performed using at least one time-of-flight camera.

12. The method of any preceding claim wherein the predetermined X-ray dose is based on an image noise requirement13. A system comprising:an X-ray tube;an X-ray detector for detecting X-rays produced by the X-ray tube;a patient support;a gantry rotatable about the patient support and supporting the X-ray tube and the X-ray detector; andone or more processors arrange to cause performance of the method of any preceding claim.

14. The system of claim 13, further comprising an optical scanner arranged to acquire the information indicative of the volumetric space occupied by a patient.

15. A computer readable medium carrying computer-executable instructions which, when executed by one or more processors, cause the method of any of claims 1 to 12 to be carried out.27

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