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Title:
OPTICAL ELEMENT WITH MANIPULATED COATING RESISTANCE
Document Type and Number:
WIPO Patent Application WO/2014/068434
Kind Code:
A2
Abstract:
The invention relates to effective charged particle optical elements which enable a reduced outer diameter of the optical column. The electron optical lenses are formed with high-resistive coatings on the inner wall or the bore of the charged particle optical column. The coating resistance along the inner wall is manipulated to generate different voltage drop per length along the optical column.

Inventors:
ZHANG YANXIA (NL)
RIBBING CAROLINA (NL)
Application Number:
PCT/IB2013/059397
Publication Date:
May 08, 2014
Filing Date:
October 16, 2013
Export Citation:
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Assignee:
KONINKL PHILIPS NV (NL)
International Classes:
H01J35/14
Foreign References:
US7130381B22006-10-31
Attorney, Agent or Firm:
STEFFEN, Thomas et al. (AE Eindhoven, NL)
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Claims:
CLAIMS:

1. An optical element for focusing or manipulating a charged particle beam, said optical element comprising a charged particle optical column, wherein an inner wall or bore of said charged particle optical column is adapted to provide a coating resistance with at least two different values varying stepwise or continuously along a direction of said charged particle beam or along a radial direction of said charged particle optical column.

2. The optical element as defined in claim 1, wherein said varying coating resistance is provided by coating said inner wall or bore of said charged particle optical column with a high-resistive material, and wherein said charged particle optical column is adapted to apply different electrical potentials to the start and the end of said high-resistive material so as to form an electrostatic optical element.

3. The optical element as defined in claim 1, wherein said varying coating resistance is provided by first and second coating portions (29-1, 29-2) with different coating thicknesses at different locations of said inner wall or bore in said direction of said charged particle beam.

4. The optical element as defined in claim 1, wherein said varying coating resistance is provided by first and second coating portions (29-1, 29-2) with different coating materials of different resistivity at different locations of said inner wall or bore in said direction of said charged particle beam.

5. The optical element as defined in claim 1, wherein said coating resistance varies in said radial direction for a predetermined length of said inner wall or bore along said direction of said charged particle beam.

6. The optical element as defined in claim 2, wherein the thickness of said high- resistive material is less than the thickness of a bulk material (28) of said charged particle optical column.

7. The optical element as defined in claim 2, wherein said high-resistive material has lower resistivity than a bulk material (28) of said charged particle optical column. 8. The optical element as defined in claim 1, wherein said optical element is adapted to focus, defocus or shape said charged particle beam.

9. A vacuum device comprising an optical element as defined in claim 1. 10. The vacuum device as defined in claim 9, wherein said vacuum device is an electron optical system, an X-ray tube, an ion beam system or a proton beam system, and wherein said vacuum device comprises a charged particle source.

11. An electronic brachytherapy system charged particle system comprising a vacuum device as defined in claim 9.

12. A method of producing an optical element for focusing or manipulating a charged particle beam, said method comprising manipulating a coating resistance of an inner wall or bore of a charged particle optical column of said optical element so as to obtain at least two different resistance values, the resistance varying stepwise or continuously along a direction of said charged particle beam or along a radial direction of said charged particle optical column.

13. The method as defined in claim 12, wherein said manipulating comprises dip- coating, brushing, sintering, depositing, evaporating or sputtering a high resistive coating to a bulk material (28) of said charged particle optical column.

14. The method as defined in claim 12, wherein said manipulating comprises applying a high resistive coating to a bulk material (28) of said charged particle optical column by a sol-gel process, an implantation process, a doping process or a diffusion process into the surface of said bulk material (28).

The method as defined in claim 13 or 14, wherein said manipulating further forming said high resistive coating with different thicknesses or different materials by applying at least two coating processes or masking a portion (29-1, 29-2) of a surface of said bulk material in one or more of said coating processes, or by depositing said high resistive coating and subsequently removing it locally or locally thinning it down.

Description:
OPTICAL ELEMENT WITH MANIPULATED COATING RESISTANCE

FIELD OF THE INVENTION

The invention relates to the field of optical elements for focusing or manipulating charged particle beams, which may be used e.g. in a radiation application apparatus for applying radiation at a location within an object.

BACKGROUND OF THE INVENTION

An alternative to using radioactive isotopes for brachytherapy is so-called electronic brachytherapy where the isotope is replaced by a miniature x-ray tube operating at a modest voltage. The miniature x-ray tube is navigated to a desired location within a person, at which x-rays are to be applied, for example, for treating a tumor, wherein the x-ray tube is operated at a voltage of, for instance, 50 kV.

The advantages of electronic brachytherapy include: 1) the tube can be turned on and off which facilitates transport, storage, and provides the possibility of user-adjustable dose rate; 2) the acceleration voltage and tube current can be tuned, within a certain range, to user-adjustable dosimetric properties; and 3) less radiation exposure to medical staff, and no radioactive waste concerns. Since the radiation energy is relatively low to modest, the treatment does not have to be carried out in a shielded vault, but can be done in an unshielded room. The mobile nature of the electronic brachytherapy systems, as well as the limited shielding requirements makes electronic brachytherapy a logical modality to be utilized for intraoperative radiation therapy (IORT) e.g. in the treatment of early stage cancer.

In a charged particle optical system, the charged particle beam is usually focused or manipulated by electrostatic lenses, magnetic lenses or multi-poles. The bore of these optical elements is approximately 10 times of the diameter of the charged particle beam. The outer diameters of these optical elements are larger, by another factor of 3-5. In applications where the outer diameter of the charged particle optical column is limited to a few millimeters, such as, but not limited to, electronic brachytherapy, the conventional optical elements are often unpractical or unfeasible.

The US7130381 B2 discloses a miniature X-ray tube which has an extractor cup surrounding a cathode filament. This extractor cup also serves as an electrostatic lens, focusing the electron beam into the anode at the other end of the tube. Due to the limited space (especially the limited tube diameter), this extractor cup poses engineering challenges to assembly, contacting, and surface finish to avoid field emission. In addition, any misalignment or imperfection may cause non-uniformity in the current density resulting in charge-up of isolation material and electrical breakdown.

A similar system has been proposed for intra-operative radiotherapy (IORT), where only the anode part of the tube is inserted in a still open tumor cavity. The electron source, the acceleration, and the beam deflector are situated ex vivo, housed in a tube with relative larger diameter. The focused and accelerated electron beam enters the electron drift tube, travels and impinges on the anode in vivo. The drift tube is around 3.2 mm in diameter, and 10 cm in length. Because the drift tube is relatively long and has no electron lens due to the limited diameter, high demands are put in the electron accelerator and deflector - resulting in increased system complexity. In addition, to avoid beam divergence due to electron-electron repulsion in the drift tube, the tube current (and the dose rate) is limited.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an effective charged particle optical element with reduced outer diameter.

This object is achieved by an optical element as claimed in claim 1, a vacuum device as claimed in claim 9, an electronic brachytherapy apparatus as claimed in claim 11, and a production method as claimed in claim 12.

Accordingly, an inner wall or bore of the charged particle optical column is adapted to provide a coating resistance with at least two different values varying stepwise or continuously along a direction of the charged particle beam or along a radial direction of the charged particle optical column. By manipulating the coating resistance at the inner wall, an effective charged particle optical element can be provided without increasing the outer diameter of the optical element. The outer diameter can be reduced by replacing

conventionally used extractor lenses with the high resistivity coating lenses. The achieved smaller outer diameter provides opportunities for new applications for conventional electronic brachytherapy system, such as prostate cancer, lung cancer and other percutaneous interventions.

Moreover, the proposed solution presents an effective means to compensate for the electron-electron repulsion in the drift tube of conventional systems, by having a focusing high-resistive coating lens in the drift tube. The tube current, which was limited to e.g. 40 μΑ, may be increased further - meaning higher dose rate or less treatment time.

Besides focusing or de-focusing electrostatic lenses, the proposed solution can be used to create multi-pole lenses, by changing the voltage drop along the radial direction. The multi-pole lenses can be used to change current distribution in the anode cup - leading to more uniform current density in the anode cup (no hot spots).

According to a first aspect, the varying coating resistance may be provided by coating the inner wall or bore of the charged particle optical column with a high-resistive material, wherein the charged particle optical column is adapted to apply different electrical potentials to the start and the end of the high-resistive material so as to form an electrostatic optical element. Thereby, an electrostatic lens or other electrostatic optical element can be formed in a simple and flexible manner.

According to a second aspect which can be combined with the first aspect, the varying coating resistance may be provided by first and second coating portions with different coating thicknesses at different locations of the inner wall or bore in the direction of the charged particle beam. Alternatively or additionally, the first and second coating portions may be provided with different coating materials of different resistivity at different locations of the inner wall or bore in the direction of said charged particle beam. Hence, a desired variation of the coating resistance can be achieved in various ways and in a reliable manner.

According to a third aspect which can be combined with the first or second aspect, the coating resistance varies in the radial direction for a predetermined length of the inner wall or bore along the direction of said charged particle beam. Thereby, the proposed solution can be applied to provide multi-pole lenses with predetermined characteristics.

According to a fourth aspect which can be combined with any one of the first to third aspects, the thickness of the high-resistive material may be less than the thickness of a bulk material of the charged particle optical column. This ensures that the outer diameter of the optical element is kept small.

According to a fifth aspect which can be combined with any one of the first to fourth aspects, the high-resistive material may have a lower resistivity than the bulk material of the charged particle optical column. This ensures that the bulk material does not influence the characteristic of the coating resistance.

According to a sixth aspect which can be combined with any one of the first to fourth aspects, the optical element may be adapted to focus, defocus or shape the charged particle beam. The proposed solution can thus be used for various applications regardless of whether a focused or defocused charged particle beam is required. E.g., when the coating resistance varies radially, a multi-pole lens is obtained, which re-shapes the charged particle beam.

According to a seventh aspect which can be combined with any one of the first to sixth aspects, the manipulating may comprise dip-coating, brushing, spinning, sintering or (vapor) depositing (e.g. evaporating, sputtering, chemical vapor deposition (CVD)) a high resistive coating to a bulk material of the charged particle optical column. As an alternative, the manipulating may comprise applying the high resistive coating by a sol-gel process, an implantation process, an oxidation or reduction process, a doping process or a diffusion process into the surface of the bulk material. In at least some of the above coating processes, the high resistive coating may be formed with different thicknesses or different materials by applying at least two coating processes or masking a portion of a surface of the bulk material in one or more of the coating processes, or by depositing the high resistive coating and subsequently removing it locally or locally thinning it down. Hence, various options are provide for forming the proposed non-uniform coating resistance.

It shall be understood that the substance determining the optical element of claim 1, the vacuum device of claim 9, the electronic brachytherapy apparatus of claim 11, and the method of claim 12 have similar and/or identical preferred embodiments, in particular, as defined in the dependent claims.

It shall be understood that a preferred embodiment of the invention can also be any combination of the dependent claims with the respective independent claim.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

In the following drawings:

Fig. 1 shows schematically and exemplarily an embodiment of a radiation source having an optical element and arranged within a catheter,

Figs. 2a to 2c show schematically and exemplarily the working principle of an electrostatic lens with a coating of uniform resistance, and

Figs. 3a to 3c show schematically and exemplarily the working principle of an electrostatic lens with a coating of two different resistances according to an embodiment of the present invention. DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows schematically and exemplarily a radiation source 20 within a catheter 10, in which the present invention can be implemented. The radiation source 20 comprises a filament cathode 26 and an anode 24 for accelerating electrons 30 towards the anode 24. When the electrons 30 are incident on the anode 24, x-rays are generated in a known way. The radiation source 20 is an x-ray tube, wherein electrons 30 generated by the filament cathode 26 are accelerated between a first electrode 22 (including the filament cathode 26) and the anode 24 towards the anode 24. The filament cathode 26 is connected via electrical connections with an external voltage source such that the filament cathode 26 can emit electrons. The anode 24 and the further electrode 22 may be electrically connected with a bulk material 28 (i.e. electrical isolator) of a housing of the radiation source 20.

The catheter 10 may comprise built-in guiding means (not shown in Fig. 1), which can be controlled by a catheter navigation unit (also not shown in Fig. 1). The catheter 10 can, for example, be steered and navigated by the use of steering wires, in order to guide the distal end of the catheter 10 to a desired location within a treated object or person.

A radiation application apparatus in which the radiation source 20 of Fig. 1 may be provided is preferentially adapted to perform electronic brachytherapy, wherein a miniature x-ray tube operating at a modest voltage like 50 kV is used. Advantages of the electronic brachytherapy include that the tube can be turned off and that the radiation energy is relatively low, in particular, compared to standard isotopes used for radioactive

brachytherapy, and thus has a short radiation range. This implies that the treatment does not have to be carried out in a shielded vault, but can be performed in interventional x-ray facilities and operation rooms. Therefore, electronic brachytherapy is possible in various departments and outpatient settings and the treatment can be performed by, for example, an interventional radiologist. The healthy tissue of the patient and treatment personnel are spared, and cumbersome isotope logistics and regulations can be disregarded.

In the radiation source of Fig. 1, the applied voltage gives the energy of the radiation, i.e. the maximum energy of the bremsstrahlung spectrum, and thus the radiation range in tissue. An acceleration voltage of 50 kV gives a mean energy of about 25 keV. The distance to the target tissue is preferentially within the range of 0.5 to 4 cm, requiring radiation energy of about 20 to 50 keV. This means that the acceleration voltage of the radiation source may be in the range of e.g. 30 kV to 100 kV.

The radiation source 20 can be intraoperatively or percutaneously placed using, for instance, a needle, in, for instance, a tumor, or in a tumor cavity using a balloon applicator. The radiation application apparatus can be adapted for the treatment of, for example, prostate, breast, rectum, vaginal, liver, kidney, esophagus, lung, skin, head and neck cancer.

Figs. 2a to 2c show schematically and exemplarily the working principle of an electrostatic lens with a coating of uniform resistance.

A uniform coating with the same resistance is applied on a bulk material 28 at the inner surface or wall of the optical tube or column. The optical column consists of a filament cathode 26 (e.g., a hair-pin or a spiral filament), an extractor cup 22, the bulk material 28 with coating on the inner wall, an electrode 24 and an anode cup 40. The extractor cup 22 is at lower potential than the electrode 24, and they both make contact to the high-resistive coating on the bulk material 28. For the present coating with uniform resistance, the equipotential lines 50 are depicted in the cross-sectional side view of Fig. 2a, and the voltage (V) along the coating is shown in the diagram of Fig. 2b over the axial length (L) of the coated zone. Additionally, a cross-sectional side view with an electron beam trajectory 70 is illustrated in Fig. 2c. As can be gathered from Fig. 2c, the electron beam is focused by the extractor cup to some extent towards the electrode 24 and the anode cup 40.

Figs. 3a to 3c show schematically and exemplarily a working principle of an electrostatic lens with a coating of two different resistances according to an embodiment of the present invention. The contents of Figs. 3a to 3c correspond to those of Figs. 2a to 2c but with the difference of a non-uniform coating resistance of the bulk material. Here, the electrostatic lens is formed with a coating of two different resistances, namely a first coating area 29-1 on the left side of the dashed line in Fig. 3a, which has a resistance 5 times lower than that of a second coating area 29-2 on the right side of the dashed line. Thereby, the voltage drop (or the electric field) along the coating areas 29-1, 29-2 of the bulk material is different, as illustrated by the equipotential lines 50 in Fig. 3a and the voltage diagram in Fig. 3b. As a result, a positive (focusing) electrostatic lens is formed by the coating, and the electron beam is focused more into the anode cup, as can be gathered from the electron beam trajectory 70 illustrated in Fig. 3c. The collecting efficiency of the anode cup 40 is thereby increasing from 80% in Fig. 2c to 100% in Fig. 3c.

In the embodiment, the inner wall or the bore of the charged particle optical column, which are insulating, are coated with high-resistive material(s), and different electrical potentials are applied to the start and the end of the coating zone. The coating resistance along the inner wall is manipulated or modulated to generate different voltage drop (or different electric field strengths) along the optical column, which is necessary to form an electrostatic optical element. The different coating resistance can be realized either by having the same coating material but different coating thickness at different portions of the inner wall of the bulk material 28 of the optical column, or by having different materials of different resistivity but with the same thickness at different portions of the inner wall of the bulk material 28 of the optical column, or a combination of both. Depending on the change of the electric field strength along the coating, the high-resistive coating lens can be either focusing or de-focusing (diverging).

For multi-pole lenses, the coating resistance can be manipulated along the radial direction of the optical column (e.g., through use of multilayer coatings or doped or implanted profiles) for a certain length along the inner wall or bore of the charged particle optical column.

In general, the thickness of the coating should be thinner than that of the bulk material 28. The thickness may vary from a few nanometers to hundreds of micro-meters, depending on the properties of the coating materials, the required strength of the electrostatic lens, and system requirements, etc. Furthermore, the coating material should have lower resistivity than that of the bulk material 28, but its resistance should be high enough so that the leakage current through the coating is small. In addition, the coating material may be vacuum, high- vacuum (HV), or ultra-high vacuum (UHV) compatible, depending on the applications. Finally, the coating material should be relatively chemically and physically stable under charged particle bombardment.

The resistance along the coating may have at least two different values, varying either step-wise or continuously, either along the direction of a charged particle beam (i.e. axial direction of the optical column) for electron optical lenses, or along the radial direction of the optical bore for multi-pole lenses.

The high-resistive coating may be dip-coated, brushed, sintered, or deposited by chemical vapor deposition (CVD) or physical vapor deposition (PVD) to the bulk material 28. The coating may also be made via the sol-gel process, implantation, doping process, or by diffusion into the surface of the bulk material. In order to create different resistance either by coating with the same material but different thicknesses or with different materials, one or more coating processes may be needed, and masking of a part of the bulk surface in one or more of these processes may be necessary so that different materials or different quantity of the same material may be applied to the bulk surface. Alternatively, material may be deposited along the whole length of the optical axis and subsequently locally sacrificed, e.g. by drilling or etching. Exemplary bulk materials may include, but not limited to AI 2 O 3 , BN, PTFE, PTA, quartz, fused silica, kapton, diamond, mica, Si. And the high-resistive coating materials may include, but are not limited to Cr 2 0 3 , glasses, silicates, TiO x , MnO x , amorphous C, doped diamond, DLC, graphite, epitaxial metals.

In summary, an effective charged particle optical element has been described, which enables a reduced outer diameter of the optical column. The electron optical lenses are formed with high-resistive coatings on the inner wall or the bore of the charged particle optical column. The coating resistance along the inner wall is manipulated to generate different voltage drop along the optical column.

The present invention can be used to construct effective electrostatic lenses for charged beam focusing or defocusing, or to construct multi-pole for manipulate the charged particle beams. The present invention can be used in, but is not limited to, vacuum devices containing an electron beam, such as electron guns, X-ray tubes and electron optical columns, or vacuum devices containing other charged particle beams, such as ion beam systems, proton beam systems, accelerators, etc.

The present invention is particularly useful for charged particle systems where the outer diameter of the optical column is limited, such as electronic brachytherapy systems. Thereby, system complexity can be reduced to improve system performance, or size can be reduced to broaden the range of applications to prostate cancer, lung cancer and other percutaneous interventions.

Although in the above described embodiment the radiation source comprises a filament cathode 26 and an anode cup 40, in other embodiments the radiation source can also comprise another cathode and/or anode. The cathode may be, for example, a thermal filament, a field emitting cathode, a Shottky cathode, a piezo- or ferroelectric cathode, or a combination thereof. Moreover, between the cathode and the anode an intermediate dielectric can be provided. Furthermore, the anodes can be transmission type anodes, reflection type anodes or a mixture of a transmission type and a reflection type anode.

Although in the above described embodiments the radiation source 20 is an x- ray tube, in other embodiments also other radiation sources can be used. For instance, the radiation source can be a radiation source generating radiation within another wavelength range, for instance, in the soft x-ray, ultraviolet (UV), or visible wavelength range. The radiation source can also be a lasing device. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. The figures are schematical only. For instance, they are not to scale, i.e., for example, the electrodes are thinner than shown in the figures.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality.

A single unit or device may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Any reference signs in the claims should not be construed as limiting the scope.