# Patent

## Title
Combined radiation therapy and magnetic resonance unit

## Abstract

The invention relates to a combined radiation therapy and magnetic resonance unit. For this purpose, in accordance with the invention a combined radiation therapy and magnetic resonance unit is provided comprising a magnetic resonance diagnosis part with an interior, which is limited in radial direction by a main magnet, and a radiation therapy part for the irradiation of an irradiation area within the interior, wherein at least parts of the radiation therapy part, which comprise a beam deflection arrangement, are arranged within the interior.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This is a continuation in part of U.S. Ser. No. 12/072,851 filed on Feb. 28, 2008, which claims priority of German application No. 10 2007 010 132.7 filed Feb. 28, 2007 and claims priority of German application No. 10 2008 007 245.1 filed Feb. 1, 2008. All of the applications are incorporated by reference herein in their entirety.


FIELD OF THE INVENTION
The invention relates to a combined radiation therapy and magnetic resonance unit.
BACKGROUND OF THE INVENTION
Generally in radiation therapy the aim is to irradiate a target within the human body in order to combat diseases, in particular cancer. For this purpose a high dose of radiation is specifically generated in an irradiation center (isocenter) of an irradiation apparatus. During irradiation the problem often arises that the irradiation target in the body can move. For instance, a tumor in the abdomen can move during breathing. On the other hand, in the period between radiation treatment planning and actual radiation treatment a tumor may have grown or have already shrunk. It was therefore proposed to check the position of the irradiation target in the body during radiation treatment by imaging, in order to control the beam or if necessary discontinue the irradiation, and thus increase the success of the therapy. This is in particular relevant for irradiation targets in the upper and lower abdomen as well as in the pelvic area, for example the prostate. To minimize the dose of radiation outside the target volume and thus protect healthy tissue, the entire radiation generation is moved around the patient. This concentrates the radiation dose in the beam in the area of the rotational axis.
Both X-ray and ultrasound systems were proposed as the imaging medium for monitoring the therapy. These, however, provide only a limited solution to the problem. In the case of ultrasound imaging the necessary penetration depth is lacking for many applications. In X-ray imaging the X-ray sensors can be disrupted or damaged by the gamma radiation of the accelerator. Furthermore, the quality of the tissue images is often unsatisfactory.
For this reason, at present mainly positioning aids and fixing devices or markings made on the skin of the patient are used to ensure that the patient is in the same position in the irradiation apparatus as decided in the radiation treatment planning and that the irradiation center of the irradiation apparatus is actually consistent with the irradiation target. These positioning aids and fixing devices are, however, expensive and in most cases they are also uncomfortable for the patient. In addition, they conceal the risk of irradiation errors because as a rule no further check of the actual position of the irradiation center is carried out during irradiation.
Magnetic resonance is a known technique which permits both particularly good soft-tissue imaging as well as spectroscopic analysis of the area being examined. As a result this technique is fundamentally suitable for monitoring radiation therapy.
In U.S. Pat. No. 6,366,798 a radiation therapy device is combined with various magnetic resonance imaging systems. In all the different versions mentioned here the magnet arrangement of the magnetic resonance imaging system is divided into two parts. In addition, in some versions key parts of the magnetic resonance imaging system rotate with the radiation source of the radiation therapy device. In each case the radiation source is outside the magnetic resonance imaging system and must be protected by means of shields from the stray field of the magnetic resonance imaging system. A division of the magnet, a rotatable magnet and shielding of the radiation source represent elaborate technical solutions and increase the cost.
In GB 2 427 479 A, U.S. Pat. No. 6,925,319 B2, GB 2 247 478 A, US 2005/0197564 A1 and US 2006/0273795 A1 further devices are described in which a radiation therapy device or an X-ray imaging system are combined with a magnetic resonance imaging system.
GB 2 393 373 A describes a linear accelerator with an integrated magnetic resonance imaging system. In one exemplary embodiment the magnetic resonance imaging system comprises means for compensation of a magnetic field in order to minimize the field strength of the magnetic field of the magnetic resonance imaging system at the location of the accelerator. In another exemplary embodiment a filter is used in order to compensate for possible heterogeneity caused in a therapy beam by the magnetic field of the magnetic resonance imaging system.
SUMMARY OF THE INVENTION
The object underlying the invention is therefore to provide a combined radiation therapy and magnetic resonance unit which with little constructional expense permits high-quality image monitoring by means of magnetic resonance during radiation therapy.
This object is achieved by the subject matter of the independent claim. Advantageous embodiments are described in the dependent claims.
In accordance with the invention a combined radiation therapy and magnetic resonance unit is provided comprising a magnetic resonance diagnosis part with an interior which is limited in radial direction by a main magnet, and a radiation therapy part for irradiation of an irradiation area within the interior, whereby at least parts of the radiation therapy part are located within the interior.
The magnetic diagnosis part permits a movement analysis of the irradiation target in real time and thus optimal monitoring and control of radiation therapy. The integration of the radiation therapy part in a magnetic resonance device and the resultant proximity of the radiation therapy part to the irradiation target permit a high radiation luminance as well as high accuracy in controlling the beam path. The use of a conventional main magnet is possible.
Advantageously, the radiation therapy part comprises an electron accelerator. Electrons are simple to generate and the accelerated electrons can easily generate a therapy beam.
In a particularly advantageous embodiment an electron beam direction runs essentially parallel to the axis of the main magnetic field in the electron accelerator. Electrons which move parallel to a magnetic field are not influenced by it in their flight path. It is therefore easier to determine their flight direction and speed compared with electrons influenced or accelerated by magnetic fields.
Advantageously, the radiation therapy part comprises a beam deflection arrangement which in particular comprises an electromagnet which deflects the electron beam inward into the interior. In this way the accelerated electrons are directed into the desired therapy beam direction.
In a particularly advantageous embodiment the beam deflection arrangement comprises a pulsed magnet. This can (briefly) generate a magnetic field as big as the main magnetic field or bigger without any particular expense. Moreover, by using particularly short pulses, disruption of the magnetic resonance device is minimized.
Expediently, the radiation therapy part comprises a target anode to produce an X-ray beam along an X-ray beam path. X-ray radiation, in particular high-energy X-ray radiation, is particularly suitable for radiation therapy and is not influenced by the electromagnetic fields prevailing in a magnetic resonance device.
In a particularly advantageous embodiment the target anode is a transmission anode. A transmission anode is particularly suitable for generating high-energy X-ray radiation.
In a further embodiment a homogenizing body is arranged in the X-ray beam path after the target anode. The homogenizing body for example weakens the beam core and homogenizes the X-ray beam distribution in the beam cross-section.
Advantageously, a collimator is arranged in the X-ray beam path after the target anode. The collimator enables the direction of the X-ray beam and the cross-section of the X-ray beam to be regulated.
In a particularly advantageous embodiment the collimator is arranged at least partially between two axially distanced partial gradient coils of the magnetic resonance device. This represents a particularly space-saving arrangement and at the same time provides for advantageous proximity to the irradiation target.
In a further advantageous embodiment the collimator comprises adjusters for changing the cross-section of the X-ray beam. As a result, the cross-section of the X-ray beam can be ideally adapted to the shape of the irradiation target.
At least partial gradient coils of the gradient coil system are advantageously shielded against the radiation therapy part. This permits independent, in particular also simultaneous, operation of the radiation therapy part, which rotates in operation, and of the magnetic resonance diagnosis part as the changing gradient fields of the gradient coils in this way do not influence the rotating radiation therapy part.
If the radiation therapy part is rotatable around the axis of the main magnetic field the further advantage results that the applied dose of radiation outside the target volume, i.e. outside the irradiation center, can be minimized. This means that adverse effects on healthy tissue during radiation therapy are reduced.



BRIEF DESCRIPTION OF THE DRAWINGS
Further advantages and details of the invention will emerge from the exemplary embodiment described below and with reference to the drawing. The examples listed do not represent a restriction of the invention. The figures show:
FIG. 1 a schematic representation of a combined radiation therapy and magnetic resonance unit in accordance with the invention,
FIGS. 2-4 schematic representations of segments of further configurations of a combined radiation therapy and magnetic resonance unit in accordance with the invention,
FIGS. 5-8 exemplary configurations of beam deflection arrangements which can be used in a combined radiation therapy and magnetic resonance unit in accordance with the invention,
FIGS. 9A-9D further examples of suitable deflection units, each producing a dipole magnetic field and each suitable for use in an arrangement such as shown in FIG. 7,
FIGS. 10A-10E examples of quadrupole magnetic field units, similar to the dipole deflection units of FIGS. 8, 9A-9D respectively,
FIG. 11 an embodiment of a beam deflection apparatus according to an embodiment of the present invention,
FIGS. 12 and 13A-13D further views of the arrangement of FIG. 11.



DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a schematic representation (not to scale) of a combined radiation therapy and magnetic resonance unit 1 with a magnetic resonance diagnosis part 3 and a radiation therapy part 5. The magnetic resonance diagnosis part 3 comprises a main magnet 10, a gradient coil system comprising two in this case symmetrical partial gradient coils 21A,21B, high-frequency coils 14, for example two parts of a body coil 14A,14B, and a patient bed 6. All these components of the magnetic resonance part are connected to a control unit 31 and an operating and display console 32.
In the example presented, both the main magnet 10 and the partial gradient coils 21A,21B are essentially shaped like a hollow cylinder and arranged coaxially around the horizontal axis 15. The inner shell of the main magnet 10 limits in radial direction (facing away vertically from the axis 15) a cylinder-shaped interior 7, in which the radiation therapy part 5, the gradient system, high-frequency coils 14 and the patient bed 6 are arranged. More precisely the radiation therapy part 5 is located in the interior 7 between the outer side of the gradient coil system 21A and 21B and the inwardly facing shell surface of the main magnet 10.
In addition to the magnet coils the main magnet 10 comprises further structural elements, such as supports, housing etc., and generates the homogenous main magnetic field necessary for the magnetic resonance examination. In the example shown the direction of the main magnetic field is parallel to the horizontal axis 15. High-frequency excitation pulses which are irradiated by means of high-frequency coils 14 are used to excite the nuclear spins of the patient. The signals emitted by the excited nuclear spins are also received by high-frequency coils 14.
The axially distanced partial gradient coils 21A,21B in each case comprise gradient coils 20, which are in each case completely enclosed by a shield 27. The gradient coil 20 comprises supports and individual gradient coils which irradiate magnetic gradient fields for selective layer excitation and for location-coding of the magnetic resonance signals in three spatial directions.
The radiation therapy part 5 is arranged on a gantry 8 and comprises an electron accelerator 9, which here is configured as a linear accelerator, a beam deflection arrangement 17, a target anode 19, a homogenizing body 22 and a collimator 23. The gantry 8 can feature a recess (broken lines), by which access to the magnetic resonance diagnosis part remains possible also from this side.
The electron accelerator 9 of the radiation therapy part 5 comprises an electron source 11, for example a tungsten cathode, which generates an electron beam 13, which is accelerated by the electron accelerator 9 preferably pulsed parallel to the main magnetic field of the main magnet 10. The electron accelerator 9 for example generates electron beam pulses with a length of 5 μs every 5 ms. If the electron accelerator 9 generates pulsed electron beams 13, it can be built more compactly, e.g. with a length of about half a meter, and still withstand the impact of the high-energy electron beams 13.
The electrons of the electron beam 13 are accelerated by electric alternating fields in cylinder-shaped hollow conductors of the electron accelerator 9. The electrons of the electron beam 13 are accelerated to energies up to a magnitude of several MeV. The electron accelerator 9 is connected to an accelerator control unit 12 to control the alternating fields and the electron source 11.
The electron beam 13 leaves the electron accelerator 9 at the end opposite the electron source and is deflected by the beam deflection arrangement 17 through 90° radially inward in the direction of axis 15. For this purpose the beam deflection arrangement 17 comprises a magnet which generates a suitable magnetic field. The magnet is configured as an electromagnet made of non-ferromagnetic materials to prevent undesired interaction with the surrounding magnetic fields. As the beam deflection arrangement 17 has to work in a strong, outer magnetic field, it has been modified compared with conventional beam deflection arrangements.
To be able to deflect the pulsed electron beam 13 in a small space, the beam deflection arrangement 17 must generate strong magnetic fields. To reduce the power loss, the magnetic field of the beam deflection arrangement 17 is a pulsed magnetic field which is synchronized with the pulsed electron beam 13. For this purpose the beam deflection arrangement 17 is connected to a beam deflection control unit 18 which is also connected to the accelerator control unit 12.
The deflected electron beam 13 hits the target anode 19 and generates an X-ray beam that emerges from the target anode in the beam elongation along an X-ray beam path. The X-ray beam is homogenized by the homogenizing body 22.
The collimator 23 is arranged in an annular slot between the distanced partial gradient coils 21A,21B in the X-ray beam after the target anode 19. The proximity to the irradiation target thus achieved improves the radiation luminance and also the effectiveness of the collimator 23.
The collimator 23 enables the direction of the X-ray beam and the cross-section of the X-ray beam to be influenced. For this purpose the collimator 23 incorporates moveable adjusters 24, which permit the X-ray beam to pass only in a certain direction, e.g. only parallel to the radial axis 26 or up to at most in one direction through an angle α away from the axis 26, and only with a certain cross-section. It is also possible to set the adjusters 24 of the collimator 23 in such a way that no X-ray beams can pass parallel to the axis 26 and only angled X-ray beams in one direction through certain angles away from the axis 26 can pass through. To control the adjusters 24 the collimator 23 is connected to a collimator control unit 25. Such collimators are adequately known. By way of example reference can be made to multi-leaf collimators. They make it possible to perform intensity modulated radiation therapy (IMRT), in which the size, shape and intensity of the X-ray beam can be optimally adapted to the irradiation target. In particular IMRT also enables the irradiation center to be positioned outside the rotational axis of the radiation therapy device.
The X-ray beam penetrates the examination subject, in this case the patient P, and the X-ray beam path runs through a diagnosis volume D of the magnetic resonance diagnosis part 3. To minimize the local dose of radiation outside the irradiation target volume, the radiation therapy part rotates around the axis of the main magnetic field. As a result, the full dose is applied only in the irradiation center B. The collimator 23 constantly adapts the cross-section of the X-ray beam to the actual outline of the irradiation target even during rotation. The gantry 8 is configured for rotation of the radiation therapy part. A gantry control unit 29 controls the movement of the radiation therapy part 5. As an example the radiation therapy part 5 is shown as radiation therapy part 5′ after rotation through 180°.
The gantry control unit 29, the collimator control unit 25, the beam deflection control unit 18, the accelerator control unit 12 and the control unit 31 are connected to each other so that the diagnosis data collected by the magnetic resonance diagnosis part, for example the three-dimensional shape of the irradiation target, the rotational position of the radiation therapy part, as well as the collimator settings with regard to cross-section and direction of the X-ray beam and the generation of pulsed beams described above can be coordinated with each other.
The patient bed 6 is preferably moveable in three spatial directions so that the target area of the irradiation can be positioned precisely in the irradiation center B. For this purpose the control unit 31 is expediently configured for controlling a movement of the patient bed.
FIGS. 2 to 4 show segments of further exemplary configurations of a combined radiation therapy and magnetic resonance unit in accordance with the invention. In the exemplary configurations shown in particular the arrangement of a respective radiation therapy part 5,105,205,305 varies from the exemplary embodiment in FIG. 1. For the sake of clarity, therefore, only the upper section of a main magnet 110,210,310 of the combined radiation therapy and magnetic resonance unit up to about one high-frequency coil 114,214,314 of the combined radiation therapy and magnetic resonance unit is shown. The rest of the configuration and its mode of operation are, unless otherwise described, essentially the same as in the example shown in FIG. 1, to which reference is hereby made.
FIG. 2 shows a main magnet 110 of the combined radiation therapy and magnetic resonance unit on whose side facing an interior 107 of the combined radiation therapy and magnetic resonance unit a gradient coil system 120 is arranged. The gradient coil system 120 comprises in particular primary coils 121 and secondary coils 122. Between primary coils 121 and secondary coils 122 a free space is located in which the radiation therapy part 105 of the combined radiation therapy and magnetic resonance unit is arranged. Such a distanced arrangement of the primary and secondary coils 121 and 122 increases the efficiency of the gradient coil system 120. In addition, high-frequency coils 114 are arranged on the side of the gradient coil system facing the interior 107.
The gradient coil system 120 or at least the primary coils 121 as shown in the example in FIG. 1 can be divided into two partial gradient coils 121A,121B and arranged in such a way that at least parts of the radiation therapy part 105 can move in an annular space between the parts in a rotation of the radiation therapy part 105 around the axis of the main magnetic field. In this case the high-frequency coils 114 are also advantageously divided correspondingly into two partial high-frequency coils 114A and 114B.
Alternatively it is conceivable for the gradient coil system 120 to be configured in such a way that together with the radiation therapy part 105 it can rotate around the axis of the main magnetic field. In this case a division of the gradient coil system 120 or of the primary coils is not absolutely appropriate. It suffices to configure the primary coil 121 in such a way that it can let the radiation therapy part 105 penetrate into the interior 107 at one point in order to emit the therapy beams onto an irradiation center B. The same applies to the high-frequency coils 114. It may be necessary here to compensate for the mechanical turning of the gradient coil system 120 by suitable activation of the gradient currents. Such an electric rotation of gradient fields is, however, a usual capability of standard magnetic resonance systems. Nevertheless, high requirements should be imposed on the accuracy and reproducibility of the rotation.
Thanks to its particularly compact design this exemplary embodiment gives the patient an exceptional amount of room in the interior 107. Advantageously, a collimator of the radiation therapy part 105 is incorporated in a particularly flat configuration in the exemplary embodiment shown in FIG. 2 in order to give the patient even more room in the interior 107 of the combined radiation therapy and magnetic resonance unit.
FIG. 3 presents a segment of a further exemplary embodiment of a combined radiation therapy and magnetic resonance unit. In this exemplary embodiment a gradient coil system 220 as in a standard magnetic resonance device is arranged on the side of a main magnet 210 facing an interior 207 of the combined radiation therapy and magnetic resonance unit. Standard components can be used for the main magnet 210 and the gradient system 220, which among other things reduces cost.
Again on the side of the gradient system 220 facing the interior 207 high-frequency coils 214 are arranged. Between the gradient system 220 and the high-frequency coils 214, however, adequate space is left in order to arrange a radiation therapy part 205 of the combined radiation therapy and magnetic resonance unit between the gradient system 220 and the high-frequency coils 214.
During irradiation of an irradiation center B the radiation therapy part 205 rotates around the main magnetic field axis of the combined radiation therapy and magnetic resonance unit. In a similar way as in the exemplary embodiment of FIG. 2 the high-frequency coils 214 can here too either be divided into two partial high-frequency coils 214A and 214B in such a way that at least parts of the radiation therapy part 205 can move in an annular gap between the partial high-frequency coils 214A and 214B. Or the high-frequency coils 214 can be rotated with the radiation therapy part 205.
FIG. 4 shows schematically a segment of a further exemplary embodiment of a combined radiation therapy and magnetic resonance unit. In this case, as in a conventional design of a magnetic resonance unit, high-frequency coils 314 are arranged within a gradient system 320 which itself is arranged within a main magnet 310. A radiation therapy part 305 is arranged on the side facing an interior 307 of the combined radiation therapy and magnetic resonance unit. As in the exemplary embodiments presented above the radiation therapy part 305 rotates during irradiation around the main magnetic field axis of the combined radiation therapy and magnetic resonance unit. In this exemplary embodiment no particular structural measures are necessary with regard to the gradient system 320 and the high-frequency coils 314 to make this rotational movement of the radiation therapy part possible.
Advantageously the inner radius of the high-frequency coils 314 is as big as possible and the radiation therapy part is as flat as possible so that the patient is not cramped in the interior 307.
The radiation therapy part 105,205,305 of the exemplary embodiments in FIGS. 2 to 4 in each case incorporates essentially the same construction as the radiotherapy part 5 from the exemplary embodiment in FIG. 1. For the sake of clarity the individual components are not shown again. The rotational movement of the radiation therapy parts 105,205,305 and/or the gradient coil system 120,220,320 and/or the high-frequency coils 114,214 is indicated in each case by a broken-line arrow.
If necessary, in the exemplary embodiments of FIGS. 2, 3 and 4 the radiation therapy part 105,205,305 and the magnetic resonance part, in particular the gradient system 120,220,320 and/or the high-frequency coils 114,214,314, are not operated at the same time but are alternated in order to exclude possible disruptive interaction, in particular between moving parts of the radiation therapy part 105,205,305 and electromagnetic alternating fields of the magnetic resonance part.
FIGS. 5 to 8 show three examples of possible configurations of beam deflection arrangements 17 which can be used in a radiation therapy part 5,105,205,305.
FIG. 5 shows a beam deflection arrangement 17′ which consists of two annular deflection coils 17A′ and 17B′. The deflection coils 17A′,17B′ are arranged in a combined radiation therapy and magnetic resonance unit essentially vertically to the main magnetic field of the main magnet of the combined radiation therapy and magnetic resonance unit. The field direction of the deflection coils 17A′,17B′ runs essentially parallel to a radial axis 26 of the combined radiation therapy and magnetic resonance unit, i.e. in the direction of exit desired for a therapy beam. Through the combination of the main magnetic field and the magnetic field of the deflection coils 17A′,17B′ an electron beam 13′ is deflected in the desired direction.
The current density in the deflection coils 17A′,17B′ must be determined among other things according to the main magnetic field strength of the combined radiation therapy and magnetic resonance unit and energy of the electron beam. For example, with an electron beam of 6 MeV and a main magnetic field of 1.5 T the deflection coils 17A′,17B′ can deflect the beam in the desired direction if the current density in the deflection coils 17A′,17B′ amounts to approx. 500 MA/m2.
FIGS. 6 to 8 show beam deflection arrangements 17′′ and 17′′′, whose configurations were calculated by means of computer simulation programs, for example based on a finite elements method or a finite differences method. In each case the underlying problem was the question as to how an electron beam which enters a main magnetic field in a parallel direction can be deflected in a direction which is vertical to the main magnetic field in order to hit a target. For this purpose, the field necessary for such deflection and how it can be produced were calculated.
FIG. 6 shows an active coil pair 17′′ as the beam deflection arrangement which is shaped in such a way that it solves the problem posed. An electron beam 13′′ arriving parallel to a horizontal axis 15, and therefore in the direction of a main magnetic field of a combined radiation therapy and magnetic resonance system, enters between the coil pair 17′′ at the position “I” and is guided in such a way that it leaves the coil pair 17′′ parallel to a radial axis 26 of the combined radiation therapy and magnetic resonance unit at the position “O”. The strength of the field strength of the coil pair 17′′ can be varied according to the energy of the electron beam.
An exemplary coil pair 17′′ generates a quadrature-axis field of approx. 0.3 T. As a result, e.g. an electron beam of 6 MeV can be deflected in a combined radiation therapy and magnetic resonance unit of 1.5 T field strength in the desired way.
FIGS. 7 and 8 present a further possible solution of the above-mentioned problem, this time using a passive beam deflection arrangement.
As shown in FIG. 7, the beam deflection arrangement 17′′′, which generates the quadrature-axis field necessary for solving the problem, comprises four deflection units 117A,B,C,D. The arrangement of the deflection units 117A,B,C,D was determined by iteration in such a way that an electron beam 13′′′ entering parallel to a main magnetic field as shown in FIG. 7 from the right into the deflection unit 117A is deflected in such a way by deflection unit 117A to deflection unit 117B that the electron beam 13′′′ is deflected upward through deflection unit 117B. From deflection unit 117B the electron beam 13′′′ is deflected by deflection unit 117C, whereby the electron beam 13′′ leaves the deflection unit 117C in such a way that it is deflected downward to deflection unit 117D, so that it leaves deflection unit 117D vertically to the main magnetic field. References such as “upward”, “downward”, “right” and “left” relate in each case to the example shown in FIG. 7.
FIG. 8 shows more precisely a deflection unit 117. The deflection unit 117 comprises several permanent magnets 118A,B,C,D,E,F,G,H, which are preferably made of rare earths, e.g. NdFeB or SmCo.
FIGS. 7 and 8 present a further possible solution of the above-mentioned problem, this time using a passive beam deflection arrangement.
As shown in FIG. 7, the beam deflection arrangement 17′′′, comprises four deflection units 117A,B,C,D. Each of these deflection units is a magnetic dipole generating a magnetic field which is substantially perpendicular to the direction of the charged particle beam as it passes through the dipole. The angle of rotation of the dipole's magnetic field about the direction of the charged particle beam is selected to provide the desired deflection of the charged particle path. It is the interaction of the dipole magnetic field and the charged particles flowing in a direction perpendicular to the dipole magnetic field which causes a force to act upon the charged particles as they travel through the dipole magnetic field and causes deflection of their path. The direction of the deflection is perpendicular to the dipole magnetic field and perpendicular to the direction of travel of the charged particle beam as it arrives at the dipole magnetic field. For this reason, each of the dipoles 117A,B,C,D are arranged with their dipole magnetic fields substantially perpendicular to the particle beam as it passes through the corresponding dipole magnetic field. The arrangement of the deflection units 117A,B,C,D may be determined by iteration or modeling in such a way that an electron beam 13′′′ entering the first deflection unit in a direction parallel to a main magnetic field, as shown in FIG. 7 from the right into the deflection unit 117A, is deflected in such a way by deflection unit 117A to deflection unit 117B that the electron beam 13′′′ is deflected upward through deflection unit 117B. From deflection unit 117B the electron beam 13′′′ is deflected by deflection unit 117C, whereby the electron beam 13′′ leaves the deflection unit 117C in such a way that it is deflected downward to deflection unit 117D, so that it leaves deflection unit 117D vertically to the main magnetic field. References such as “upward”, “downward”, “right” and “left” relate in each case to the example shown in FIG. 7. The degree and direction of deflection caused by each deflection unit will vary according to the angle of rotation about the beam path of the dipole magnetic field, and the orthogonality of the dipole magnetic field to the beam path.
The deflection effects if each deflection unit may be controlled by selecting the angle of rotation of the dipole field about the direction of the charged particle beam, and suitable selecting the strength of the dipole magnetic field.
FIG. 8 shows more precisely an example of a deflection unit 117. The deflection unit 117 comprises several permanent magnets 118A,B,C,D,E,F,G,H, which are preferably made of rare earths, e.g. NdFeB or SmCo.
For example, to deflect a 6 MeV electron beam in a main magnetic field of 1.4 T in the desired way, six permanent magnets 118A,B,C,D,E,F with dimensions of approx. 10×4×4 mm and two permanent magnets 118G,H with dimensions of 10×8×4 mm are arranged as shown. In each case three of the permanent magnets of the smaller size 118A,B,C,D,E,F are stacked on top of each other with alternating direction of the respective magnetic fields of the permanent magnets 118A,B,C,D,E,F. The two larger permanent magnets 118G,H are arranged between this stack. The arrows indicate the respective magnetic field directions in the permanent magnets 118A,B,C,D,E,F,G,H.
This configuration is particularly easy to produce and, furthermore, forms a particularly compact solution. In addition, only low static forces are exerted here, so that no particular fastening measures are needed. Furthermore, initial results show that the beam deflection arrangement 17′′′ produces only a very small stray field. However, permanent magnets are sensitive to changes in the ambient temperature or the surrounding main magnetic field.
The resulting dipole magnetic field of the deflection unit 117 of FIG. 8 will be in a direction from magnet 118G to magnet 118H, perpendicular to the path of a charged particle beam passing through the deflection unit. By arranging such deflection units with their magnetic dipoles substantially perpendicular to the charged particle beam, and at desired angles of rotation about the charged particle beam, a desired deflection path may be obtained in three dimensions, such as shown in FIG. 7, even in the presence of a background magnetic field.
Further examples of suitable deflection units, each producing a dipole magnetic field and each suitable for use in an arrangement such as shown in FIG. 7 are shown in FIGS. 9A to 9D. The arrangement of 9A has the advantage of low stray fields, which may be important when placed within an MRI system, so as not to interfere with the magnetic fields used for imaging. However, it does require the production of several different orientations of magnetized trapezium cross-section pieces. A similar arrangement, but with even lower stray field losses, is shown in FIG. 9B, where arc-shaped cross-section magnetized pieces are assembled into a cylindrical deflection unit. FIG. 9C shows a deflection unit made up of an arrangement of identical magnetized octagonal cross-section pieces. This arrangement has the advantage that only one type of magnetized piece is required, and has been found to have a tolerable stray magnetic field. The arrangement of FIG. 9D is further simplified, requiring only four identical magnetized pieces. In the illustrated example, these are of square cross-section. While simple, light and cost-effective, this arrangement has a significant stray field and a correspondingly low dipole magnetic field at the centre of the deflection unit.
In the following description, arrangements such as shown in FIG. 9C, employing magnetized pieces of octagonal cross-section, will be referred to.
While arrangements such as shown in FIG. 7 have been found to deflect the charged particle beams satisfactorily, some degradation of beam focus and beam quality has been observed. Prior to entering the first deflection unit 117A, the charged particle beam 13′′′ is travelling parallel to the background magnetic field of the main magnet 10. This helps to maintain beam focus and quality, as any divergence of charged particles from the direction of the main beam path will interact with the background magnetic field and cause a force to act on the diverging particles which acts to return them to the main beam path. However, once the particle beam has diverged from its path parallel to the background magnetic field, for example as a result of the interaction with a first deflection unit 117A, the beam focus degrades, resulting in a loss of quality of the beam. The beam becomes wider and less intense. The effect continues with further deflections and with increasing path length.
According to an aspect of the invention, a multipole magnetic field unit is introduced, along with the dipole deflection units discussed above. The multipole magnetic field unit may be a quadrupole of higher polar magnetic field unit, as will be discussed further below.
FIGS. 10A-10E show examples of quadrupole magnetic field units, similar to the dipole deflection units of FIGS. 8, 9A-9D respectively. Representative magnetic fields are also shown within the central cavity of each quadrupole. As can be seen, the magnetic field strength at the centre of the quadrupole is essentially zero, with magnetic fields in alternating directions around the periphery of the cavity.
Quadrupole magnetic field units similar to those shown in FIGS. 10A-10E are known in themselves, for use in the field of high-energy physics, for example in relation to particle beams operating with energies of several GeV. Their use in relation to linear accelerators for medical particle beam therapy, where beam energies of 6 MeV are typical, is believed to be hitherto unknown. There has previously been no issue of beam defocus and beam quality reduction in medical particle beam therapy apparatus due to the very different equipment conventionally used for beam steering. Conventionally, the particle beams were filtered, for example by bending the beam through 270°, to remove particles of unusually low or high energies. This filtering generated a lot of heat which required cooling, and the equipment was large, expensive and heavy. Use of quadrupole magnetic field units and presently proposed dispenses with these cooling requirements.
That part of a particle beam which passes through the centre of the cavity of the quadrupole passes through a region of zero magnetic field from the quadrupole, and its trajectory is unchanged by the quadrupole, although it may still be influenced by the background magnetic field. For diverging parts of the beam, caused by the loss of focus and reduction in beam quality, these will encounter one of the magnetic fields which extend in alternating directions around the periphery of the cavity. When a part of the beam encounters one of these fields, it will experience a force tending to deflect it back towards the centre of the beam, radially re-focusing the beam and compensating for dispersion introduced by the steering. In this way, the quadrupole will tend to refocus the particle beam, but will not deflect it from its trajectory. The refocusing performed by a quadrupole is not perfect, and there is a tendency for the refocused beam to have a somewhat cruciform cross-section.
In alternative embodiments, higher-polar arrangements, such as sextupole or octupole and so on may be used instead of, or in conjunction with, a quadrupole magnetic field unit. The higher-polar arrangements will be less powerful at focusing for a given size and field strength of it components, but will not deform the cross-section of the beam as much as a quadrupole.
FIG. 11 shows an embodiment of a beam deflection apparatus according to an embodiment of the present invention, using dipole deflection units 210 and a quadrupole magnetic field unit 220. In this embodiment, the dipole deflection units 210 and quadrupole magnetic field unit 220 are constructed from magnetized octagonal cross-section pieces as shown in FIGS. 9C and 10D. FIGS. 12 and 13A-13D show further views of the same arrangement. This embodiment resembles the arrangement of FIG. 7 in that it includes a sequence of four dipole deflection units 210, with a quadrupole magnetic field unit 220 placed in the middle of the sequence. Each of the dipole deflection units functions to deflect the charged particle beam 13 as in the arrangement of FIG. 7, and the quadrupole magnetic field unit acts to refocus the beam. The quadrupole magnetic field unit 220 may be replaced with a higher polar arrangement such as a sextupole or octupole of similar construction, if preferred.
The quadrupole or higher polar arrangement may be positioned in the path of the particle beam amongst the steering dipoles, for example in a DDQDD arrangement as shown in FIGS. 11-13D. Alternatively, the quadrupole or higher polar arrangement may be positioned in the path of the particle beam after all of the dipole deflection units, which may be referred to as a DDDDQ arrangement, if four dipoles are used as in the arrangement of FIG. 7. In further embodiments, more than one quadrupole or higher polar arrangement may be used, for example in a DDQDDQ arrangement.
In embodiments using only a single quadrupole or higher polar arrangement, it may be preferred to place the quadrupole or higher polar arrangement after the last dipole in the direction of the particle beam, so that the refocusing and beam quality improvement may correct the distortions introduced by the beam deflection at all dipoles. Alternatively, use of two or more quadrupole or higher polar arrangements allows partial correction of defocus and loss of beam quality before the beam spreads so far that it can no longer be corrected by a single final quadrupole or higher polar arrangement.
Two or more quadrupoles or higher polar arrangements may be used in sequence, such as in a DDDDQQ arrangement, which will allow a desired focusing and beam quality restoration effect while allowing a lower field strength in each quadrupole or higher polar arrangement. This may help to minimize distortion of the magnetic fields of the imaging system.
A computer modeling step may be performed to determine the necessary positions, orientations and magnetic field strengths of the dipoles and quadrupoles or higher polar arrangements so as to achieve the desired beam steering with an acceptable final beam quality.
While the dipoles, quadrupoles and higher polar arrangements have been described above in terms of permanent magnets, alternative embodiments of the present invention provide such structures in the form of electromagnets, preferably using coils of non-magnetic wire such as copper or aluminum to minimize distortion of the magnetic fields of the imaging system.
If required, control may be exercised over the final beam quality by variation of a parameter of the quadrupole(s) or higher polar arrangement(s), for example by repositioning of a permanent magnet quadrupole, or electrical control of electromagnet quadrupoles.
Further improvements in final beam quality may be achieved by using edge focusing.

## Claims

1. A passive beam deflection arrangement for deflecting a beam of charged particles in a main magnetic field, comprising:
a plurality of dipole deflection units,
wherein each of the dipole deflection units generates a dipole magnetic field substantially perpendicular to a path of the beam of charged particles passing through the dipole magnetic field,
wherein the beam of charged particles enters a first of the dipole deflection units parallel to an axis of the main magnetic field and is deflected by the deflection units away from the axis,
wherein the beam of charged particles leaves a final of the dipole deflection units perpendicular to the main magnetic field,
wherein the each of the deflection units comprises eight permanent magnets,
wherein six smaller size permanent magnets of the eight permanent magnets are stacked on top of each other in two stacks,
wherein each of the two stacks comprises three magnets with an alternating direction of a respective magnetic field of the magnets, and
wherein two larger size of the eight permanent magnets are arranged between the two stack.

2. The passive beam deflection arrangement as claimed in claim 1, further comprising a multipole magnetic field unit arranged in the path of the beam for focusing the beam.

3. The passive beam deflection arrangement as claimed in claim 2, wherein the multipole magnetic field unit is arranged in the path of the beam after the final dipole deflection unit.

4. The passive beam deflection arrangement as claimed in claim 2, wherein the multipole magnetic field unit is arranged in the path of the beam between two of the dipole deflection units.

5. The passive beam deflection arrangement as claimed in claim 2, wherein the multipole magnetic field unit is a quadrupole.

6. The passive beam deflection arrangement as claimed in claim 2, wherein the multipole magnetic field unit is a sextupole.

7. The passive beam deflection arrangement as claimed in claim 2, wherein the multipole magnetic field unit is an octupole.

8. The passive beam deflection arrangement as claimed in claim 1, further comprising a plurality of multipole magnetic field units arranged in the path of the beam for focusing the beam.

9. The passive beam deflection arrangement as claimed in claim 8, wherein two of the multipole magnetic field units are arranged consecutively in the path of the beam.

10. The passive beam deflection arrangement as claimed in claim 8, wherein at least one of the multipole magnetic field units is arranged in the path of the beam after the final dipole deflection unit.

11. The passive beam deflection arrangement as claimed in claim 8, wherein at least one of the multipole magnetic field units is arranged in the path of the beam between two of the dipole deflection units.

12. The passive beam deflection arrangement as claimed in claim 8, wherein at least one of the multipole magnetic field units is a quadrupole.

13. The passive beam deflection arrangement as claimed in claim 8, wherein at least one of the multipole magnetic field units is a sextupole.

14. The passive beam deflection arrangement as claimed in claim 8, wherein at least one of the multipole magnetic field units is an octupole.