GB2489681A - Slip ring power source for radiotherapy apparatus - Google Patents

Abstract

Radiotherapeutic apparatus comprising a patient support, magnetic coils disposed around the patient support for creating a magnetic filed therewithin, a radiation source producing a beam of radiation directed toward the patient support and mounted on a rotatable source thereby to rotate the radiation source around the patient support. A slip ring for conveying electrical power to the radiation source. A substantially airtight chamber surrounding and enclosing at least the slip ring, the patient support lying outside the substantially airtight chamber. In another aspect a power supply for conveying electrical power to the radiation source comprising, in sequence, a step-up transformer, a slip ring, and a step down transformer.

Description

Radiotherapeutic Apparatus

FIELD OF THE INVENTION

The present invention relates to apparatus for the delivery of radiotherapy.

BACKGROUND ART

Radiotherapeutic apparatus is well-known, and consists of a source of radiation which emits a beam of radiation that is directed toward a patient in order to destroy or otherwise harm tumourous cells within the patient. Usually, the beam is collimated in order to limit its spatial extent to a desired region within the patient, usually the tumour or a sub- section of the tumour. The source can be a linear accelerator for high-energy (MV) x-radiation, or an isotopic source such as Co-60.

The source is often rotated around the patient in order to irradiate the desired region from a number of different directions, thereby reducing the dose applied to healthy tissue around the desired region. The shape of the defined desired region can change dynamically as the source rotates, in order to build up a complex dose distribution for tumours with more challenging shapes and/or which are located near to sensitive areas.

As the dose distribution becomes more closely tied to the exact shape of the tumour, and as the accuracy of the dose delivery improves, it has become necessary to know the current position of the patient, their internal organs, and the tumour with greater accuracy.

As a result, low-energy x-ray sources are often provided on the apparatus in addition to the high-energy therapeutic source, to allow for x-ray or CT imaging of the patient before or during treatment. Portal imagers are often provided, which detect the therapeutic beam after attenuation by the patient. Both provide a degree of information as to the patient, but are subject to the inherent limitations of x-ray imaging, in particular the poor contrast obtained in areas of soft tissue. Generally, x-ray imaging is able to provide good contrast between areas of bone, soft tissue, and air, which allows for the detection of the gross patient position but has difficulty in detecting internal movements of the patient and the sub-structure within the soft tissue.

Efforts have therefore been directed towards combining a radiotherapy source with an MRI imager. MRI provides contrast within soft tissue, and is therefore suitable.

However, there are significant practical problems in combining these two very different technologies.

SUMMARY OF THE INVENTION

One such practical problem is the delivery of power to the source. Linear accelerators have significant power demands, typically in the region of 10-14kW. Delivered via a standard 415V three-phase supply, this therefore involves current flows of up to 30A.

Isotopic sources also need power in order to operate collimators and the like, although their current demands will usually be somewhat lower.

Given that the source needs to rotate around the patient, this power will usually be delivered by way of a slip ring arrangement. This involves conducting the current via an at least part-circumferential path around (or within) the MRI coils, which will create stray magnetic fields that interfere with the MRI field(s) and degrades the image quality.

We therefore propose a radiotherapeutic apparatus comprising a patient support, magnetic coils disposed around the patient support for creating a magnetic field therewithin, a radiation source producing a beam of radiation directed toward the patient support and mounted on a rotatable source thereby to rotate the radiation source around the patient support, a slip ring for conveying electrical power to the radiation source, a substantially airtight chamber surrounding and enclosing at least the slip ring, the patient support lying outside the substantially airtight chamber.

The substantially airtight chamber can be evacuated, or filled with a gaseous dielectric medium such as Sulphur hexafluoride (SF6), Octafluorocyclobutane (R-C318) (C4F8), Perfluorobutane (R-3-1-10) (C4F10), Perfluoropropane (R-218) (C3F8), Hexafluoroethane (R-116) (C2F6), Carbon tetrafluoride (R-14) (CF4), 1,2-Dichlorotetrafluoroethane (R-1 14) (CF2CICF2CI), Dichiorodifluoromethane (R-12) (CF2CI2), Nitrogen (N2), or mixtures of one or more of these with air. A vacuum may be preferable, as SF6 and other gases are potent greenhouse gases and can react under an electrical arc to form harmful gases. On the other hand, the absence of a pressure differential may mitigate in favour of a dielectric gas.

Although substantially airtight, the chamber may have arrangements for allowing fluid communication into and out of the chamber in order to establish a vacuum, or provide or replenish the gaseous dielectric medium. By "airtight", therefore, we mean that the atmosphere within the chamber can be maintained distinct from the general atmosphere elsewhere, such as around the patient support. Equally, no vacuum is perfect and thus where the chamber is evacuated, this implies that it has an internal gas pressure that is substantially below atmospheric pressure, such as to increase the dielectric strength between the conductors of the slip ring.

To obtain the benefits of the invention, the airtight chamber only needs to enclose the slip ring. However, in practice it may be easier for it to enclose other parts of the apparatus in addition. Thus, it may also enclose a connection to a mounting structure for the radiation source, or the radiation source itself. However, this is not essential to the invention.

This allows the power to be delivered to the radiation source at an elevated voltage of (for example) 10kV or higher. This will reduce the current flowing in the slip ring accordingly, meaning that the stray magnetic field produced by the current is also reduced accordingly. By sufficient control of the power consumption of the radiation source and sufficient elevation of the voltage, the current demand can be reduced so that the stray field is sufficiently low to have no deleterious effect. A step-down transformer between the slip ring and the radiation source and/or a step-up transformer prior to the slip ring can be provided to "correct" the voltages accordingly.

The magnetic coils can be located within the substantially airtight chamber, and the radiation source can be a linear accelerator.

In another aspect, the present invention provides a radiotherapeutic apparatus characterised by the elevated voltage at which the power supply is transmitted over the slip ring. Thus, such as apparatus comprises a patient support, magnetic coils disposed around the patient support for creating a magnetic field therewithin, a radiation source producing a beam of radiation directed toward the patient support and mounted on a rotatable source thereby to rotate the radiation source around the patient support, and a power supply for conveying electrical power to the radiation source comprising, in sequence, a step-up transformer, a slip ring, and a step-down transformer.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way of example, with reference to the accompanying figures in which; Figure 1 shows a radiotherapy apparatus according to the present invention, combining an MRI and linear-accelerator; Figure 2 shows a vertical section through the apparatus of figure 1, illustrating the chamber; and Figure 3 shows a schematic arrangement of the elements making up a radiotherapy apparatus according to the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Figure 1 shows a system 2 according to an embodiment of the present invention, comprising a radiotherapy apparatus 6 and a magnetic resonance imaging (MRI) apparatus 4.

The system includes a couch 10, for supporting a patient in the apparatus. The couch 10 is movable along a horizontal, translation axis (labelled "I"), such that a patient resting on the couch is moved into the radiotherapy and MRI apparatus. In one embodiment, the couch 10 is rotatable around a central vertical axis of rotation, transverse to the translation axis, although this is not illustrated. The couch 10 may form a cantilever section that projects away from a support structure (not illustrated). In one embodiment, the couch 10 is moved along the translation axis relative to the support structure in order to form the cantilever section, i.e. the cantilever section increases in length as the couch is moved and the lift remains stationary. In another embodiment, both the support structure and the couch 10 move along the translation axis, such that the cantilever section remains substantially constant in length, as described in our earlier patent application published as WO 2009/007737, the contents of which are incorporated by reference and to which the skilled person is referred for a full understanding of the described embodiment.

As mentioned above, the system 2 also comprises an MRI apparatus 4, for producing near real-time imaging of a patient positioned on the couch 10. The MRJ apparatus includes a primary magnet 16 which acts to generate the so-called "primary" magnetic field for magnetic resonance imaging. That is, the magnetic field lines generated by operation of the magnet 16 run substantially parallel to the central translation axis I. The primary magnet 16 consists of one or more coils with an axis that runs parallel to the translation axis I. The one or more coils may be a single coil or a plurality of coaxial coils of different diameter. In one embodiment (illustrated), the one or more coils in the primary magnet 16 are spaced such that a central window 17 of the magnet 16 is free of coils. In other embodiments, the coils in the magnet 16 may simply be thin enough that they are substantially transparent to radiation of the wavelength generated by the radiotherapy apparatus. The magnet 16 may further comprise one or more active shielding coils, which generates a magnetic field outside the magnet 16 of approximately equal magnitude and opposite polarity to the external primary magnetic field. The more sensitive parts of the system 2, such as the accelerator, are positioned in this region outside the magnet 16 where the magnetic field is cancelled, at least to a first order.

The MRJ apparatus 4 further comprises two gradient coils 18, 20, which generate the so-called "gradient" magnetic field that is superposed on the primary magnetic field. These coils 18, 20 generate a gradient in the resultant magnetic field that allows spatial encoding of the protons so that their position can be determined, for example the gradient coils 18, can be controlled such that the imaging data obtained has a particular orientation. The gradient coils 18, 20 are positioned around a common central axis with the primary magnet 16, and are displaced from one another along that central axis. This displacement creates a gap, or window, between the two coils 18, 20. In an embodiment where the primary magnet 16 also comprises a central window between coils, the two windows are aligned with one another.

An RE system causes the protons to alter their alignment relative to the magnetic field. When the RF electromagnetic field is turned off the protons return to the original magnetization alignment. These alignment changes create a signal which can be detected by scanning. The RF system may include a single coil that both transmits the radio signals and receives the reflected signals, dedicated transmitting and receiving coils, or multi-element phased array coils, for example. Control circuitry controls the operation of the various coils 16, 18, 20 and the RF system, and signal-processing circuitry receives the output of the RF system, generating therefrom images of the patient supported by the couch 10.

As mentioned above, the system 2 further comprises a radiotherapy apparatus 6 which delivers doses of radiation to a patient supported by the couch 10. The majority of the radiotherapy apparatus 6, including at least a source of radiation 30 (e.g. an x-ray source and a linear accelerator) and a multi-leaf collimator (MLC) 32, is mounted on a chassis 28. The chassis 28 is continuously rotatable around the couch 10 when it is inserted into the treatment area, powered by one or more chassis motors. In the illustrated embodiment, a radiation detector is also mounted on the chassis 28 opposite the radiation source 30 and with the rotational axis of the chassis positioned between them. The radiotherapy apparatus 6 further comprises control circuitry, which may be integrated within the system 2 shown in Figure 1 or remote from it, and controls the radiation source 30, the MLC 32 and the chassis motor.

The radiation source 30 is positioned to emit a beam of radiation through the window defined by the two gradient coils 18, 20, and also through the window defined in the primary magnet 16. The radiation beam may be a cone beam or a fan beam, for

example.

Tn other embodiments, the radiotherapy apparatus 6 may comprise more than one source and more than one respective multi-leaf collimator.

In operation, a patient is placed on the couch 10 and the couch is inserted into the treatment area defined by the magnetic coils 16, 18 and the chassis 28. The control circuitry 38 controls the radiation source 30, the MLC 32 and the chassis motor to deliver radiation to the patient through the window between the coils 16, 18. The chassis motor is controlled such that the chassis 28 rotates about the patient, meaning the radiation can be delivered from different directions. The MLC 32 has a plurality of elongate leaves oriented orthogonal to the beam axis; an example is illustrated and described in our EP-A-0,314,214, the content of which is hereby incorporated by reference and to which the reader is directed in order to obtain a full understanding of the described embodiment. The leaves of the MLC 32 are controlled to take different positions blocking or allowing through some or all of the radiation beam, thereby altering the shape of the beam as it will reach the patient.

Simultaneously with rotation of the chassis 28 about the patient, the couch 10 may be moved along a translation axis into or out of the treatment area (i.e. parallel to the axis of rotation of the chassis). With this simultaneous motion a helical radiation delivery pattern is achieved, known to produce high quality dose distributions.

The MR.I apparatus 4, and specifically the signal-processing circuitry, delivers real-time (or in practice near real-time) imaging data of the patient to the control circuitry. This information allows the control circuitry to adapt the operation of the MLC 32, for example, such that the radiation delivered to the patient accurately tracks the motion of the target region, for example due to breathing.

Clearly, the radiotherapy apparatus 6 will have a significant power consumption, mainly due to the need to power the linear accelerator 30, but also the collimator 32 and the like. This needs to be transmitted to the rotating chassis 28, which would normally be achieved via a slip ring. These consist of a number of longitudinally spaced conductive circular rings 40, 42, 44 to which power is fed from a fixed connection and from which power is drawn via a brush contact that can slide (or slip) circumferentially around the ring.

The brush contacts can be mounted on the chassis 28 and thus power is transmitted from a fixed supply to the rotating chassis. This allows the chassis to rotate continuously around the couch 10. The alternative, a flexible conduit linking the chassis 28 or the radiotherapy apparatus 6 to a fixed point, requires that there be limitations on the range of angular movement of the radiotherapy apparatus 6.

A slip ring has the problem that the current drawn from a standard three-phase 415V supply could have a significant disruptive effect on the magnetic fields produced by the primary coil 16 and the gradient coils 18, 20, if it is not properly controlled. The slip rings, by their nature, extend around the couch 10 and thus have a coil form and are capable of creating a magnetic field. Their coil strength is not large, but the currents flowing in them may be substantial and thus the magnetic field created by those currents may be significant relative to the magnetic fields being created by the primary coil 16 and the gradient coils 18, 20. This could therefore adversely affect the quality of the images produced by the MRI system.

S Accordingly, a substantially airtight chamber 48 is defined, surrounding and enclosing at least the radiation source and the slip ring. The base sections of the end walls 50, 52 which partly define the chamber are visible in figure 1; these are sealed to opposite ends of an inner cylindrical liner 54 which extends through the apparatus around the couch 10. The end walls 50, 52 extend further upward as shown in figure 2 (omitted from figure 1 for clarity) to an outer cylindrical wall 56 which completes the chamber. Thus, an annular region is defined, containing the primary coils 16, the slip ring, and the radiotherapy apparatus 6, but excluding the couch 10 and the cylindrical section in which the patient lies.

As illustrated in figure 2, the chamber 48 is rotationally symmetric around the axis I, being defined by end walls 50, 52 that connect a central cylindrical liner 54 and an outer cylindrical wall 56. However, in preferred embodiments of the invention it may be convenient to provide a chamber that is non-symmetric, by using adjacent structures such as existing walls, floors and the like provided either for the building in which the apparatus is being constructed or for the apparatus itself.

This chamber 48 can then be provided with an atmosphere that is insulating and resistant to electrical arcing. A vacuum is one option, or to operate at higher voltages a gaseous dielectric medium can be provided. Suitable gaseous dielectric media include Sulphur hexafl uoride (SF6), Octafluorocyclobutane (C4F3), Perfluorobuta ne (C4F10), Perfluoropropane (C3F8), Hexafluoroethane (C2F6), Carbon tetrafluoride (CF4), 1,2-Dichlorotetrafluoroethane (CF2CICF2CI), Dichlorodifluoromethane (CF2CI2), Nitrogen (N2), or a mixture of at least one of these with air.

This permits the slip ring to operate at a substantially higher voltage than would otherwise be the case. The usual 415V three-phase supply can be fed through a step-up transformer to increase the voltage by a factor of 10, 100, or 1,000, or the like, or more.

This will decrease the current drawn through the slip ring by a corresponding factor. A step-down transformer between the slip ring and the radiotherapy apparatus 6 can convert the supply back to a 415V supply (or whatever supply voltage is preferred by the radiotherapy apparatus 6).

This reduction in the current drawn through the slip ring will reduce the stray magnetic field produced by the slip ring. The appropriate level of voltage transmitted through the slip ring can be chosen so as to reduce the current and the magnetic field to an acceptable level. This will depend on the sensitivity of the MRT system and the geometry of the installation as a whole. Usually, it is not feasible to operate slip rings at elevated voltages due to arcing problems, but the provision of a vacuum or a dielectric gas resolves this.

Figure 3 shows the schematic arrangement of the system. A treatment planning system 100 is loaded with the desired dose distribution and the various apparatus constraints and produces a treatment plan consisting of beam shapes and doses to be delivered from specific rotational directions. This is passed to a control apparatus 102 which sends instructions to the radiotherapy apparatus 104 to rotate the linear accelerator 106 to the desired position using the drive motor 108 and set the collimator(s) 110 as required.

The control apparatus 102 also instructs the MRI primary coils 112, gradient coils 114 and rf system 116 as required in order to obtain images of the patient prior to, during, and/or after treatment Thus, embodiments of the invention are able to provide a satisfactory power supply to a rotating radiotherapy apparatus without at any time allowing current to be conducted in a circular path around the longitudinal axis.

It will of course be understood that many variations may be made to the above-described embodiment without departing from the scope of the present invention.

Claims (11)

1. CLAIMS1. Radiotherapeutic apparatus comprising; a patient support; magnetic coils disposed around the patient support for creating a magneticfield therewithin;a radiation source producing a beam of radiation directed toward the patient support and mounted on a rotatable source thereby to rotate the radiation source around the patient support; a slip ring for conveying electrical power to the radiation source; a substantially airtight chamber surrounding and enclosing at least the slip ring; the patient support lying outside the substantially airtight chamber.
2. 2. Radiotherapeutic apparatus according to claim 1 in which the substantially airtight chamber is evacuated.
3. 3. Radiotherapeutic apparatus according to claim 1 in which the substantially airtight chamber is filled with a gaseous dielectric medium.
4. 4. Radiotherapeutic apparatus according to claim 2 in which the gaseous dielectric medium is one of Sulphur hexafluoride (SF6), Octafluorocyclobutane (C4F8), Perfluorobutane (C4F10), Perfluoropropane (C3F8), Hexafluoroethane (C2F6), Carbon tetrafluoride (CF4), 1,2-Dichlorotetrafl uoroethane (CF2CICF2CI), Dichlorodifluoromethane (CF2CI2), Nitrogen (N2), or a mixture of at least one of the aforesaid and air.
5. 5. Radiotherapeutic apparatus according to any one of the preceding claims in which the radiation source is located within the substantially airtight chamber.
6. 6. Radiotherapeutic apparatus according to any one of the preceding claims in which the magnetic coils are located within the substantially airtight chamber.
7. 7. Radiotherapeutic apparatus according to any one of the preceding claims in which the radiation source is a linear accelerator.
8. 8. Radiotherapeutic apparatus according to any one of the preceding claims in which the electrical power provided to the radiation source via the slip ring is at a voltage of at least 10kV.
9. 9. Radiotherapeutic apparatus according to any one of the preceding claims in which a step-down transformer is provided between the slip ring and the radiation source.
10. 10. Radiotherapeutic apparatus according to any one of the preceding claims in which a step-up transformer is provided prior to the slip ring.
11. 11. Radiotherapeutic apparatus comprising; a patient support; magnetic coils disposed around the patient support for creating a magneticfield therewithin;a radiation source producing a beam of radiation directed toward the patient support and mounted on a rotatable source thereby to rotate the radiation source around the patient support; and a power supply for conveying electrical power to the radiation source comprising, in sequence, a step-up transformer, a slip ring, and a step-down transformer.