An RF source

Abstract

A radiofrequency, RF, source 200 for a radiotherapy apparatus comprising at least two magnetrons 202, 204 coupled into a high-Q cavity 206. The high-Q cavity may have a Q factor of at least 15000, may

Description

The invention relates to a cavity for use with an RF source, for example for use in a radiotherapy device. 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. A radiotherapy device typically comprises a gantry which supports a beam generation system, or other source of radiation, which is rotatable around a patient. For example, for a linear accelerator (linac) device, the beam generation system may comprise a source of radio frequency (RF) energy, a source of electrons, an accelerating waveguide, beam shaping apparatus, etc. A linac uses oscillating high-power RF electric fields to accelerate electrons along a waveguide. Narrowband sources of RF are typically preferable to wideband sources for effective acceleration of electrons within the linac. Typically, a narrowband high-power source of RF such as a magnetron is used. Suitable magnetron models are commercially available and are manufactured by, for example, Teledyne, Thales, CPI, andTMD.The magnetron produces RF fields ata resonant frequency dependent upon its design, which is tailored to the linac of the radiotherapy machine. However, magnetrons constitute a very expensive component of the radiotherapy device. Typically, magnetrons of the power levels required for radiotherapy are costly and prone to failure. Statement of Invention An invention is set out in the claims. Figures Embodiments of the invention will now be described, by way of example, with reference to the drawings of which: Fig. 1 depicts a radiotherapy device or apparatus; Fig. 2 shows an RF source according to a first implementation; Fig. 3 shows an RF source according to a second implementation; Fig. 4a and 4b show RF sources according to variants of a third implementation; Figs. 5a to 5d show examples of a cavity for an RF source. Overview In overview, an RF source includes multiple magnetrons, which can be of a comparatively low power, for example of a commercially available type, coupled into a high Q-factor cavity. The use of low power magnetrons reduces the cost significantly and provides availability from multiple manufacturers. Additionally, a high-Q cavity can passively phase match inputs from the multiple magnetron sources, overcoming known problems in existing magnetrons of phase and frequency drift in operation. The multiple magnetrons can thus be combined to produce a high-power output despite loss outside the desired band from the high-Q cavity. The high-Q cavity may be a separate RF feature of the system or can be constituted as the accelerating waveguide itself provided the waveguide is designed with a sufficiently high Qto pull the multiple magnetrons into frequency and phase. Yet further, whereas in known arrangements loss of one high power magnetron may result in machine downtime, the loss of one of a plurality of magnetron sources results in some power loss, but the machine can continue to operate at a lower dose rate until repair can be carried out. It will be appreciated that the disclosure herein describes magnetrons as an RF input, but other suitable types of RF input may be used in place of the magnetrons in other examples. Detailed Description Fig. 1 depicts a radiotherapy device suitable for delivering, and configured to deliver, a beam of radiation to a patient during radiotherapy treatment. The device and its constituent components will be described generally for the purpose of providing useful accompanying information for the present invention. The device depicted in Fig. 1 is in accordance with the present disclosure and is suitable for use with the disclosed systems and apparatuses. While the device in Fig. 1 is an MR-linac with combined magnetic resonance (MR) and radiotherapy capabilities, the implementations of the present disclosure may be any radiotherapy device, for example a linear accelerator (linac)-based device. The device 100 depicted in Fig. 1 is an MR-linac. The device 100 comprises both MR imaging apparatus 112 and radiotherapy (RT) apparatus which may comprise a linac device. The MR imaging apparatus 112 is shown in cross-section in the diagram. In operation, the MR scanner produces MR images of the patient, and the linac device produces and shapes a beam of radiation and directs it toward a target region within a patient's body in accordance with a radiotherapy treatment plan. The depicted device does not have the usual 'housing' which would cover the MR imaging apparatus 112 and RT apparatus in a commercial setting such as a hospital. The MR-linac device depicted in Fig. 1 comprises a source of radiofrequency waves 102, a waveguide 104, a source of electrons 106, a source of radiation 106, a collimator 108 such as a multi-leaf collimator configured to collimate and shape the beam, MR imaging apparatus 112, and a patient support surface 114. In use, the device would also comprise a housing (not shown) which, together with the ring-shaped gantry, defines a bore. The moveable support surface 114 can be used to move a patient, or other subject, into the bore when an MR scan and / or when radiotherapy is to commence. The MR imaging apparatus 112, RT apparatus, and a subject support surface actuator are communicatively coupled to a controller or processor. The controller is also communicatively coupled to a memory device comprising computer-executable instructions which may be executed by the controller. The RT apparatus comprises a source of radiation and a radiation detector (not shown). Typically, the radiation detector is positioned diametrically opposed to the radiation source. 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 also be described as radiation detecting means and may form part of a portal imaging system. The radiation source may comprise a beam generation system. For a linac, the beam generation system may comprise a source of RF energy 102, an electron gun 106, and a waveguide 104. The radiation source is attached to the rotatable gantry 116 so as to rotate with the gantry 116. In this way, the radiation source 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 can continue to be rotated past 360 degrees. The gantry may be ring-shaped. In other words, the gantry may be a ring-gantry. 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 waveguide 104 via circulator 118 and is configured to pulse radiofrequency waves into the 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. A source of electrons 106, such as an electron gun, is also coupled to the waveguide 104 and is configured to inject electrons into the waveguide 104. In the electron gun 106, electrons are thermionically emitted from a cathode filament as the filament is heated. The temperature of the filament controls the number of electrons injected. The injection of electrons into the waveguide 104 is synchronised with the pumping of the radiofrequency waves into the waveguide 104. The design and operation of the radiofrequency wave source 102, electron source and the waveguide 104 is such that the radiofrequency waves accelerate the electrons to very high energies as the electrons propagate through the waveguide 104. The design of the waveguide 104 depends on whether the linac accelerates the electrons using a standing wave or travelling wave, though the waveguide typically comprises a series of cells or cavities, each cavity connected by a hole or 'iris' through which the electron beam may pass. The cavities are coupled in order that a suitable electric field pattern is produced which accelerates electrons propagating through the waveguide 104. As the electrons are accelerated in the waveguide 104, the electron beam path is controlled by a suitable arrangement of steering magnets, or steering coils, which surround the 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 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 waveguide 104 and in the flight tube. The vacuum system also ensures UHV conditions in the electron gun. Electrons can be accelerated to speeds approaching the speed of light in the evacuated waveguide 104. The source of radiation is configured to direct a beam 110 of therapeutic radiation toward a patient positioned on the patient support surface 114. The source of radiation may comprise 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 a 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 beam-shaping apparatus, for example by using a multileaf collimator 108, before it passes into the patient as part of radiotherapy treatment. In some implementations, the source of radiation is configured to emit either an X-ray beam or an electron particle beam. Such implementations allow the device 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 linac. 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. The subject or patient support surface 114 is configured to move between a first position substantially outside the bore, and a second position substantially inside the bore. In the first position, a patient or subject can mount the patient support surface. The support surface 114, and patient, can then be moved inside the bore, to the second position, in order for the patient to be imaged by the MR imaging apparatus 112 and / or imaged or treated using the RT apparatus. The movement of the patient support surface is effected and controlled by a subject support surface actuator, which may be described as an actuation mechanism. The actuation mechanism is configured to move the subject support surface in a direction parallel to, and defined by, the central axis of the bore. The terms subject and patient are used interchangeably herein such that the subject support surface can also be described as a patient support surface. The subject support surface may also be referred to as a moveable or adjustable couch or table. The radiotherapy apparatus / device depicted in Fig. 1 also comprises MR imaging apparatus 112. The MR imaging apparatus 112 is configured to obtain images of a subject positioned, i.e. located, on the subject support surface 114. The MR imaging apparatus 112 may also be referred to as the MR imager. The MR imaging apparatus 112 may be a conventional MR imaging apparatus operating in a known manner to obtain MR data, for example MR images. The skilled person will appreciate that such a MR imaging apparatus 112 may comprise a primary magnet, one or more gradient coils, one or more receive coils, and an RF pulse applicator. The operation of the MR imaging apparatus is controlled by the controller. The controller is a computer, processor, or other processing apparatus. The controller may be formed by several discrete processors; for example, the controller may comprise an MR imaging apparatus processor, which controls the MR imaging apparatus 110; 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 subject support surface. The controller is communicatively coupled to a memory, e.g. a computer readable medium. The linac device also comprises several other components and systems as will be understood by the skilled person. For example, in order to ensure the linac does not leak radiation, appropriate shielding is also provided. A first implementation according to the present approach is shown in Fig. 2. Fig. 2 shows a crosssection through a linac. As detailed above, the linac comprises an electron gun 210, a waveguide 208, and a flight tube 260. The flight tube is an optional component in a linac and may not be present, depending on the particular implementation. The electron gun 210 is configured to inject electrons into the waveguide 208. In this example, the electron beam may be focused by a first arrangement of focusing magnets 240 and a second arrangement of focusing magnets 245. The beam is 'steered', i.e. directed, by a first arrangement of steering magnets 260 and a second arrangement of steering magnets 265. While the linac is in use, the electron gun 210, waveguide 208 and flight tube 260 are kept under high vacuum conditions by a vacuum system or suitable vacuum apparatus. The beam is accelerated through the waveguide 208 toward a target. The target is located just before the transition from the waveguide 208 (or flight tube 260) and the treatment head 212. When the electrons are incident on the target, X-rays are produced. The resulting beam of therapeutic radiation is collimated inside the treatment head 212. The treatment head 212 may comprise other components such as dosimeters and beam-shaping apparatus. According to the present disclosure, an RF source designated generally 200 includes first and second magnetrons 202, 204 coupled into a high-Q cavity 206. Because the cavity 206 has a high Q-factor (meaning the ratio of the stored energy in relation to its losses is high, or, equivalently, the frequency to bandwidth ratio is high), the cavity outputs phase-matched (or phase-locked) and frequency-matched components of the magnetron inputs. Thus, an appropriate RF input to the linac / waveguide 208 is provided, which allows the waveguide / linac to accelerate electrons from the electron gun 210 toward the target and treatment head 212. Magnetrons 202, 204 each provide an input into the high-Q cavity 206, and the high-Q. combining cavity is configured such that all non-resonant components of these inputs are discarded. The RF source 200, comprising the magnetrons 202, 204 and high-Qcavity 206, can provide an output at the correct operating frequency despite individual phase and frequency drift in the respective low power magnetrons 202, 204. In some examples, the high-Q cavity 206 is an elliptical type cavity or pillbox type cavity (an example of which is depicted in Fig. 5a). The high-Q cavity should be designed to match the magnetron(s) used, and in one example, for a 3GHz system, the Q factor of the cavity should exceed 15000 for a 4 MW pulsed power system. In some examples, the accelerating waveguide structure of the linear accelerator itself, being a cavitybased structure, may have a Q-factor that is sufficiently high to provide the frequency and phase "pulling" effect of the high-Q cavity 206 and thus a separate high-Q cavity may not be needed or desired. Such a cavity-based linear accelerator is shown in Fig. 5b and further examples of a high Q factor linear accelerator are described herein in relation to Fig. 4a. The high-Q cavity need not be limited to normal conducting technology, and in some examples a superconducting RF cavity may be used, which would enable a Q factor of greater than lxlO11. An example structure, a superconducting Ichiro type cavity, is shown in Fig. 5c. In some examples, an RF bunch compressor structure may be used as the high Q cavity, which would also provide RF pulse compression and / or an aspect of energy storage. An example of such a structure is shown in Fig. 5d. It will be appreciated that any of the cavity types depicted in Figs. 5a to 5d may be used within any of the implementations described herein. In some examples, a series of cavities of the same type or of differing types may form an overall cavity with high Q factor. As will be appreciated by those skilled in the art, other cavity structures may be used in accordance with the approach disclosed herein. Those skilled in the art will also recognise that a specific minimum Q factor representing a "high" Q factor may be calculated for a specific system. A second implementation is shown in Fig 3. In this case, an RF source generally designated as 300 comprises first and second pairs of magnetrons 302, 304 and 306, 308. The first and second pairs of magnetrons 302,304 and 306, 308 each combine output at a respective combiner 310, 312 comprising a component such as a magic tee, and / or other hybrid combiner such as a 3dB combiner, and / or a power splitter being operated in reverse. The respective combiners 310,312 output to a high-Q cavity 314 which in turn outputs to the waveguide / linac 208 for operation in conjunction with electron gun 210 and treatment head 212 as previously described. As a result, the RF source 300 comprising the magnetrons 302, 304 and 306, 308, the combiners 310, 312 and the high-Q cavity 314 provide yet higher power input. Due to the high-Q cavity 314, the output of the RF source 300 is phase and frequency matched at the desired operating frequency as discussed above. It will be appreciated that other configurations of the number of magnetrons or RF inputs and the number of combiner components are possible. For example, three pairs of magnetrons may be used with three combiners that together provide three inputs into the high-Q cavity 314, which in turn provides a single output to the linear accelerator. In general, any number N of RF inputs or magnetrons may be used with a corresponding number N-l of combiners, and parameters thereof (such as path length values) can be determined by using a simplified circuit modelling approach to determine how to match the sources and combiners to the linac in a load-matched, or impedance-matched, system. In practice, working parameters of the system, such as the Q factor required, can be found by reworking Adler's approximation (R. Adler "A study of locking phenomena in oscillators" Proc IRE, vol 3, June 1946) with regards to the time constant required for phase locking to occur, whereby if one assumes that the frequency Af bandwidth is small (for example for an S-band system say in the region of a few 100 KHz) for system stability with or without an automatic frequency control (AFC) system the number N of RF inputs that can be effectively pulled by the high Q cavity can be determined from: Kf = I f^2Q^NXPs Where in the above equation Q L is the Q of the high Q cavity, fo is the resonant frequency of the RF source / Linac, Ps is the individual power (in Watts) of the RF power source, N is the number of sources that may be employed, and PL is the power of the high Q cavity (stored). Those skilled in the art will recognise that other methods and approximations may be used to estimate or calculate working parameters of the system. According to a third implementation shown in Fig. 4a, the RF source 400 incudes magnetrons 402,404 with outputs directly into the waveguide / linac 406a which operates again in conjunction with the electron gun 210 and the treatment head 212. In this implementation the design of the linac interior resonant space is matched to the desired operational parameters for output of electrons at the desired energy. Such implementations may or may not make use of automatic frequency control, and in some examples, an automatic frequency control system may not be required of the high-Q cavity effect of the linac interior resonant space has a sufficiently strong "pull" or effect on the magnetrons. A variance of the third implementation is shown in Fig. 4b in which magnetrons 402, 404 feed via a hybrid or other combiner at 408 into the waveguide / linac 406b and the further pair of magnetrons 410, 412 feed into a second combiner 414 which also feeds into the waveguide / linac 406b. As with the implementation of Fig. 4a, the linac 406b is configured to act as the high-Q resonance cavity. For each of the implementations described herein, it will be seen that the RF source can be driven or operated in a usual fashion, relying on the high-Q cavity operation to filter out all but the operational frequency, which is inherently phase matched by virtue of the operation of the Q-cavity. In the event that one or more of the magnetrons fails, so long as at least one magnetron remains operational, it will be seen that the system will continue to operate, albeit at a lower power. Because of the use of multiple lower-power magnetrons, the power output is acceptable even if there is limited failure of individual components, and despite the discarded bands from the high-Q cavity. Previously cavities have only been used for combining phase stable devices such that high-Q cavities have not been required. Disclosed herein is an RF source comprising at least two magnetrons coupled into a high-Q cavity. Due to the high-Q of the cavity, the phases of each of the inputs from the at least two magnetrons are matched at the resonant frequency of the high Q cavity. In some implementations, the resonant frequency of the high-Q cavity is selected to be suitable to power the waveguide of a linear accelerator for use in a radiotherapy device. This arrangement overcomes known problems in existing magnetrons, in which the phase and frequency may drift in operation. Also, by providing multiple magnetrons in this manner, each individual magnetron can be driven or operated at a lower power than would otherwise be necessary to provide power to the linear accelerator. Thus, lower power magnetrons may be used and a less strict tolerance for frequency drift is required. This significantly reduces the cost associated with the RF source of the radiotherapy device. Disclosed herein is a method of operating an RF source comprising operating at least two magnetrons coupled into a high-Q cavity. The RF source may form part of a radiotherapy device comprising a linac. It will be noted that any appropriate type of magnetron or high-Q. cavity can be implemented as long as the desired operational frequency is output by the cavity. Additionally, it will be noted that, with the exception of implementations in which a pulse compressor is used as the high-Q cavity which by definition has a pulsed output, the arrangement can be used in either a pulsed or continuous-wave configuration as appropriate. Furthermore, even if continuous-wave magnetron sources are used, they can be operated in a pulse mode because of the lower power requirements to provide acceptable operation. In implementations in which a pulse compressor such as that of Fig. 5d is used as a high-Q cavity, either continuous or pulsed magnetron inputs may be used, and the resulting output from the high-Q cavity will be equivalent to a higher power pulsed RF source. It will further be seen that any number of magnetrons and / or combiners can be used and, for example, a combination of external high-Q cavities and use of the linac also as a high-Q cavity can provide yet further preciseness of operational frequency. The above implementations have been described by way of example only, and the described implementations and arrangements are to be considered in all respects only as illustrative and not restrictive. It will be appreciated that variations of the described implementations and arrangements may be made without departing from the scope of the invention.

Claims

18 12241. A radiofrequency, RF, source for a radiotherapy apparatus comprising at least two magnetrons coupled into a high-Q cavity, wherein the high-Q cavity has a Q factor of at least 15000.

2. The RF source of claim 1, wherein the high-Q cavity comprises a linear accelerator or a waveguide of a linear accelerator.

3. The RF source of claim 1 or claim 2, wherein the high-Q cavity comprises at least one of: an elliptical cavity, a pillbox cavity, an Ichiro cavity, and a pulse compressor cavity.

4. The RF source of any of the preceding claims, further comprising a combiner and wherein the at least two magnetrons are coupled into the high-Q cavity via the combiner.

5. The RF source of any of the preceding claims, wherein the high-Q cavity comprises a superconducting cavity.

6. The RF source of any of the preceding claims, wherein the high-Q cavity is arranged to producea phase-locked and frequency-matched output based on inputs from the at least two magnetrons.

7. The RF source of any of the preceding claims, wherein the at least two magnetrons areconfigured to operate in a continuous wave mode.

8. The RF source of any of claims 1 to 6, wherein the at least two magnetrons are configured to operate in a pulsed mode.

9. A radiotherapy apparatus comprising an RF source as claimed in any preceding claim, wherein the RF source is configured to provide RF power to a linear accelerator.

10. A method of operating a radiofrequency, RF, source for a radiotherapy apparatus, the RF source comprising at least two magnetrons coupled into a high-Q cavity, the method comprising operating the at least two magnetrons coupled into the high-Q cavity, wherein the high-Q cavity has a Q factor of at least 15000.

11. The method of claim 10, wherein the high-Q cavity comprises a linear accelerator or a waveguide of a linear accelerator.

12. The method of claim 10 or claim 11, wherein the high-Q cavity comprises at least one of: an elliptical cavity, a pillbox cavity, an Ichiro cavity, and a pulse compressor cavity.

13. The method of any of claims 10 to 12, wherein the RF source comprises a combiner and the method further comprises coupling the at least two magnetrons into the high-Q cavity via the combiner.

14. The method of any of claims 10 to 13, wherein the high-Q cavity comprises a superconductingcavity.

15. The method of any of claims 10 to 14, wherein the high-Q cavity is arranged to produce aphase-locked and frequency-matched output based on inputs from the at least two magnetrons.

16. The method of any of claims 10 to 15, wherein the at least two magnetrons are configured tooperate in a continuous wave mode.

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