Patent Application
20140316249
The invention pertains to a miniature X-ray source for use in a brachytherapy system, in particular an image-guided brachytherapy system, and a method for generating a beam of X-ray radiation in a magnetic field, in particular in/near a magnetic field of an operative MR scanner.
In high dose rate (HDR) brachytherapy of tumors it is known to apply miniaturized X-ray sources, to be inserted into the tumor by brachytherapy catheters. These X-ray sources show a number of specific advantages over conventionally used radioactive isotopes, an easier adjustment of radiation emission strength and radiation dose, the option of a continuous adjustment of an applied photon energy and much less effort regarding logistics being among those. A solid state brachytherapy applicator comprising an x-ray emitting surface is known per se from the European patent application EP 1 520 603.
It is desirable to combine the advantages of the miniaturized X-ray sources with image-guiding techniques, to allow for a physician to easily control a position and alignment of each of applied X-ray sources. The widely used MR scanners would provide a suitable imaging for that purpose.
X-ray sources make use of a sudden deceleration of electrons at an anode electrode (anode) after having gained energy from a potential difference between the anode and a cathode electrode (cathode) they had been emitted from. In the presence of a static magnetic field (B0 field) of an MR scanner, the electrons, while being accelerated from the cathode to the anode, may experience a Lorentz force that acts perpendicular to a direction of motion of the electrons.
As long as the motion direction of the electrons is parallel to the magnetic field, the Lorentz force is zero and the X-ray source will work correctly. As soon as an angle between the direction of the magnetic field and the direction of motion of the electrons becomes non-zero, the electrons will be deflected and will in general begin to follow a helical path, and intended trajectories of the electrons will be considerably affected so that the X-ray source might no longer work properly because the electrons will not hit the anode any more.
Aligning the X-ray sources parallel to the magnetic field is not a viable option in practice due to physiological motions and limited accuracy of applicator placement.
It is therefore an object of the invention to provide a brachytherapy system with an improved miniature X-ray source that can be operated in a wide range of operating directions in the presence of a strong magnetic field, such as, for instance, the static magnetic field of an MR scanner.
The phrase “strong magnetic field”, as used in this application, shall be understood particularly as a magnetic field strength in relation to an accelerating voltage that results in a radius of an electron helical trajectory which is smaller than a gap between a cathode and an anode of the X-ray source.
In one aspect of the present invention, the object is achieved by providing the brachytherapy system with a miniature X-ray source with at least one anode and at least one cathode, wherein in an operative state, an electric field between the anode and the cathode is essentially spherically symmetric in at least a continuous solid angle of more than π/2 sr about a center of the cathode.
The phrase “center of the cathode”, as used in this application, shall be understood particularly as a geometric center of all parts of the cathode that emit electrons in the operative state.
The phrase “essentially spherically symmetric”, as used in this application, shall be understood particularly such that, for a majority of more than 80% of equally spaced locations between the anode and the cathode, the electric field at any of these locations does not comprise any component that is perpendicular to a radial direction related to the center of the cathode, and that is larger than 30%, preferably larger than 15%, of an absolute value of the electric field strength at the location.
The phrase “a continuous solid angle”, as used in this application, shall be understood particularly such that the solid angle, in terms of common spherical coordinates, can be described after the well-known projection towards a center of a unit sphere by the double integral
∫∫sinθ dθ dφ,
and the integration is to be carried out over continuous intervals of the variables θ and φ.
The invention is based on the concept that, when the miniature X-ray source is disposed in an outer magnetic field and has a spherically symmetric electric field between the anode and the cathode, there will always be a path from the cathode to the anode where lines of the electric field and lines of the magnetic field are in parallel, so that electrons that are being emitted by the cathode in a direction of the electric field in the operative state of the X-ray source are not affected by the outer magnetic field. The continuous solid angle of at least π/2 sr (steradian), which for example is represented by a complete octant of the unit sphere, provides for a potentially wide range of operation of the X-ray source in terms of relative direction between the X-ray source and the orientation of the magnetic field.
The fact that a direction of the electron emission coincides with the direction of the magnetic field (or is antiparallel thereto) can be exploited to omit a specific space from being radiated by the X-ray source.
In a case where the outer magnetic field is essentially homogenous in a vicinity of the X-ray source, in the operative state of the X-ray source electrons are being emitted by the cathode and accelerated towards the anode in directions that are mainly directed both parallel and anti-parallel with regard to a direction of the outer magnetic field.
In another aspect of the present invention, the continuous solid angle which the electric field is essentially spherically symmetric in, is larger than 2π sr about a center of the cathode. With the complete space about the center of the cathode being 4π sr, a region of 2π sr provides for range of operation that may be wide enough to allow for an emission of electrons from the cathode and a successive acceleration to the anode in almost every configuration of the X-ray source relative to the outer magnetic field.
In a further aspect of the invention, the anode essentially completely encompasses the cathode. The phrase “to essentially completely encompass”, as used in this application, shall be understood particularly such that the anode covers a solid angle of at least 80%, preferably of at least 90% of the complete solid angle of 4π sr about the center of the cathode. This configuration may provide for a maximum range of operation of the X-ray source in the presence of a strong magnetic field.
In a preferred embodiment, the anode may completely enclose the cathode except for an opening in the anode that is provided to accept an electrical cable feedthrough for supplying an electrical potential to the cathode.
In another aspect of the invention, the anode of the X-ray source may comprise at least two anode elements that in the operative state of the X-ray source form the anode. One element of the at least two elements of the anode is electrically insulated from a balance of the anode, and the electrically insulated element of the anode covers a continuous solid angle of more than (4π/3) sr about the center of the cathode. In this configuration, electric field lines between the anode and the cathode may deviate from a spherical symmetry along a line from an interface region between the insulated anode element and the balance of the anode, to the cathode. Nevertheless, in a major part of space between the cathode and the balance of the anode the electric field is spherically symmetric. As a result, electrons are emitted by the cathode and accelerated to the balance of the anode, so that X-ray radiation would emerge from one side of the X-ray source only.
In another aspect of the invention, the cathode comprises a heatable filament provided for emitting electrons, allowing for a simple emission of electrons from the cathode. The filament could either be directly heated by passing a current through it, or indirectly, by providing a heating cartridge to the filament.
A rise in temperature of the cathode and/or a provision of a cooling liquid in order to hold a cathode temperature below an acceptable limit during operation of the X-ray source could be omitted when the cathode comprises a tip provided for emitting electrons by a field emission process.
A more homogeneous emission of electrons from the cathode can be obtained by furnishing an outer surface of the cathode with a plurality of field emission tips, as a portion of the outer surface of the cathode that emits electrons in the operative state that will eventually hit the anode, will comprise several field emission tips of the plurality of field emission tips.
The plurality of field emission tips may essentially be evenly distributed over the outer surface of the cathode. The phrase “evenly distributed”, as used in this application, shall be understood particularly such that a standard deviation of spacing of adjacent field emission tips is less than 30%, preferably less than 20% of a mean value of the adjacent field emission tips spacing. An even distribution of the field emission tips provides the advantage that an X-ray emission pattern of the X-ray source shows only slight variations when the outer magnetic field direction changes.
The X-ray source may further comprise a shielding unit provided for absorbing X-ray radiation generated by the X-ray source in the operative state. The shielding unit can provide a certain degree of protection of regions which are intended to be left out from X-ray radiation exposure.
Preferably, the shielding unit may comprise one or more plate-like elements made from metal, for instance from lead. In a region of electron accelerating voltages up to 50 kV, a material thickness of the plate-like elements of a few tenth of a millimeter would be sufficient for effective radiation protection.
The shielding unit may cover a total solid angle of at least π/2 sr about the center of the cathode, and can thus provide an effective protection of a large portion of space to one side of the X-ray source.
It is another object of the invention to provide an image-guided brachytherapy system that can be operated in a wide range of operating directions in the presence of a strong magnetic field. This is attained by combining an embodiment of the X-ray sources of the invention with images from an MR scanner for guiding a position and/or direction of the X-ray source of the brachytherapy system. The image-guided brachytherapy system allows for real-time control of the X-ray source position and orientation, so that applicator fixation devices may become unnecessary.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention.
In the drawings:
a illustrates an alternative embodiment of a miniature X-ray source in a side view (left) and in a top view (right),
b illustrates another embodiment of a miniature X-ray source in a side view,
c illustrates a further embodiment of a miniature X-ray source in a side view,
Except for a slight disturbance in a vicinity of the feedthrough, the electric field 18 at almost every location in the space between the cathode 14 and the anode 12 essentially has a radial component only, and, therefore, the electric field 18 is essentially spherically symmetric in a solid angle of nearly 4π sr about the center 16 of the cathode 14.
The anode 12 is formed by a metal layer 20 designed as a metal foil or a metal coating on the inside of a heat-conducting substrate 22. In principle, it may alternatively be made from solid metal.
Electrons that are being emitted from the cathode 14 by a process to be explained subsequently will be accelerated in the electric field 18, with trajectories 24 that follow the electric field 18 lines, until they hit the anode 12, the sudden deceleration of the electrons generating X-ray radiation (“Bremsstrahlung”) with a maximum photon energy that is equivalent to the energy gained by an electron in the full accelerating voltage V.
As illustrated in
a-3c show alternative embodiments of miniature X-ray sources. Components that are essentially like are assigned same reference signs or numerals. For differentiation, letters a, b or c are appended as an upper index to the reference numerals. As for function and features of the essentially like components, reference is made to a description of the embodiment shown in
a shows a miniature X-ray source 10a comprising a cathode 14a and an anode 12a essentially formed by spheres, the cathode sphere and the anode sphere being arranged concentrically as described in
The cathode 14a comprises a heatable filament 38a with two contacting ends that is electrically contacted to the cathode 14a at the one contacting end and to one of the two electric lines 28a at the other contacting end. In an operative state of the X-ray source 10a, the filament 38a is directly heated by an applied heating voltage Vh from a second source 40a provided in the control unit 84b, to emit electrons by thermionic emission. Furthermore, the miniature X-ray source 10a includes a shielding unit 42a that comprises a plastic-encased lead foil of 0.2 mm thickness and is provided for absorbing X-ray radiation generated by the X-ray source 10a in the operative state. The lead foil is shaped as a circular hollow cylinder that is cut in a direction parallel to a cylinder axis 44a to create a partial cylinder surface 48a (top view in
In
c shows the concentrical arrangement of the spheres forming the cathode 14c and the anode 12c, respectively, of the embodiment of
As described earlier, electrons are emitted from the cathodes 14 of the X-ray sources 10 in directions both parallel and anti-parallel to the external magnetic field B. In a cross-sectional side view, an intensity of an X-ray radiation beam 82 that is generated when an electron hits the anode 12 shows a double-lobe pattern 70, with the X-ray source 10 located near an apex 72 of the double-lobe pattern 70 and an axis 74 of symmetry of the double-lobe pattern 70 coinciding with a direction 76 of a trajectory 24 of the electron during deceleration (
With an applied accelerating voltage V of 50 kV, the electrons have a velocity v in a mildly relativistic region with a value for β (=v/c, c: speed of light) of about 0.42, when hitting the anode 12. The emitted X-ray radiation intensity has a maximum intensity at an angle 78 of about 44°, the angle 78 being formed between the emitted radiation and the direction 76 of the trajectory 24 of the electron during deceleration, and is lower in the direction of the axis 74 of symmetry, accounting for the recess in the radiation pattern. For an accelerating voltage V of 100 kV, the value for β is about 0.55 and the maximum X-ray radiation is emitted at an angle 78 of 35°.
As shown, the X-ray radiation intensity is quite low in a direction that is perpendicular to the direction 76 of the electron trajectory 24 that extends parallel and anti-parallel to the external magnetic field B. In other words, the direction of the external magnetic field B and to some degree the accelerating voltage V are design parameters that an operator could make use of to define regions of low exposure and regions of high exposure to X-ray radiation from the X-ray source 10. The regions of high exposure to X-ray radiation may preferably be regions within the tumor 66, whereas regions of low exposure are, of course, most suited for placing objects like organs 80 that preferably should not be exposed to any radiation at all.
The external magnetic field B described in
In an alternative configuration of the brachytherapy system which is shown in
In another embodiment of a brachytherapy system, the external magnetic field described in
While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. 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. 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. 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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 11188036.5 | Nov 2011 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2012/056190 | 11/6/2012 | WO | 00 | 5/5/2014 |
| Number | Date | Country | |
|---|---|---|---|
| 61556422 | Nov 2011 | US |
