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MINIATURE X-RAY TUBE WITH VOLTAGE
The applicants hereby claim the benefit of prior European Application No. 00850058.9, filed Mar. 31, 2000. The entire contents of this European application are incorporated by reference.
FIELD OF THE INVENTION
The present invention relates to a miniature x-ray tube. In particular, the miniature x-ray tube according to the invention is useful for prevention of restenosis and for treating diseases, such as cancer, in a living body.
BACKGROUND OF THE INVENTION
In treating stenosis in coronary arteries, a restenosis occurs in 30-60% of the cases. It is known that a treatment with beta- or gamma- (x-ray) radiation will decrease the occurrence of restenosis substantially. Methods to apply this radiation to the treated stenosis are presently subject to intensive research.
Another example of an application of the present invention is treatment of cancer tumours where it is desired to deliver radiation locally.
The use of radioactive pellets or balloons etc. to introduce radioactive isotopes is known. The radioactive isotopes are introduced via a catheter, a needle or similar to the treated area. Such methods have some drawbacks, such as limited half-life of the isotope as well as the fact that the devices used emit radiation continuously. Such devices sometimes require substantial efforts to control radiation in the environment outside the patient and also exhibit problems with dose control.
The importance of controlling the radiation distribution along the vessel and of ensuring that only tissue that has been treated by coronary angioplasty will receive radiation while as little as possible radiation is applied to healthy tissue has been recognised. Thus, Novoste Corp. has introduced an array of isotope elements, enabling the radiated length of the vessel to be predetermined stepwise.
One known way to overcome some of the above drawbacks is the use of a miniature electrical x-ray tube including a cold cathode. Such a tube may be switched on and off due to its electrical activation. An example of such an x-ray tube is described in the U.S. Pat. No. 5,854,822.
However, the conventional miniature electrical x-ray tubes exhibit a problem in that the delivery of radiation has a very limited spatial extension. These radiation sources can in essence be regarded as approximately "point like" radiation sources.
Another problem present with a conventional miniature electrical x-ray tube is the dissipated heat. The temperature increase with respect to the body temperature should not be high enough to produce a local temperature exceeding approximately 41° C.
One way to handle the dissipated heat is to provide cooling by flushing a saline solution onto the tube. Cooling by flushing exhibits the problem of enlarging the geometry since the saline must be delivered to the source of heat and therefore must be directed by some means that inevitably will occupy space. Also, a flow through a catheter has to be established and maintained to cool the x-ray tube which is generally awkward.
Another way to reduce the dissipated heat is the use of a pulsed source wherein the electrically activated tube is
turned on intermittently. However, a pulsed source exhibits the drawback that the treatment time will be prolonged correspondingly, since the received dose must be held constant. This is costly and increases the discomfort for the 5 patient.
Yet another way to reduce the dissipated heat is to apply a sufficiently low current to the conventional x-ray tube. In consequence, the treatment time has to be correspondingly increased in order to apply the appropriate dose of x-ray
1° radiation. This, of course, is disadvantageous in that the longer treatment time is inconvenient to the patient and calls for raised costs in the hospital.
Yet another problem experienced with the conventional techniques including miniature electrical x-ray tubes is the
15 erosion of the electrode material. As the target is bombarded by high-energy particles, the impacts will tear away atoms from the surface. If these atoms are ionized they may be transported away from the target to be deposited on the cathode or on other parts of the interior of the x-ray tube.
Therefore, there is a need for an improved miniature electrical x-ray tube.
SUMMARY OF THE INVENTION
25 It is an object of the present invention to provide an improved miniature electrical x-ray tube.
This object is obtained with an x-ray tube according to claim 1.
With the x-ray tube of the invention, the emitted x-ray 30 dose is alternatingly emitted from more than one cathode. Thus, for each point in time there is always a cathode that is not active, thereby allowing its temperature to be cooled. At the same time, there is another cathode emitting radiation, thereby providing a substantially continuous 35 radiation. Therefore, the maximum working temperature of each cathode will be lower than the temperature of a conventional cathode for the same radiated x-ray dose and, consequently, the problem of dissipated heat is significantly reduced.
40 At the same time, the x-ray tube of the present invention provides considerably reduced treatment time as compared to a conventional pulsed x-ray tube for the same radiation dose.
In addition to these advantages, the x-ray tube of the 45 invention provides a tube with at least two sources of radiation, making it possible to cover a larger area with radiation during the treatment. This feature is enhanced in multi-cell embodiments of the invention, as will be described further below.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic cross-sectional view of a first embodiment of an x-ray tube according to the present invention in a first state of operation. 55 FIG. 2 is a schematic cross-sectional view of a first embodiment of an x-ray tube according to the present invention in a second state of operation.
FIG. 3 is a schematic cross-sectional view of a second g0 embodiment of an x-ray tube according to the present invention.
FIG. 4 is a schematic cross-sectional view of an electrode of a third embodiment of an x-ray tube according to the present invention. 65 FIG. 5 is a side view of the electrode of FIG. 4.
FIG. 6 is a schematic cross-sectional view of a pair of electrodes of a fourth embodiment of the present invention.
FIG. 7 is a schematic cross-sectional view of an electrode of a fifth embodiment of the present invention.
FIG. 8 is a schematic cross-sectional view of an x-ray tube in a state of operation (and wherein some sectional lines have been omitted for clarity). 5
FIG. 9 is a schematic cross-sectional view of an electrode of a sixth embodiment of an x-ray tube according to the present invention.
FIG. 10 is a schematic cross-sectional view of a prior art x-ray tube.
FIG. 11 is a schematic cross-sectional view of an x-ray tube according to the invention having no end walls.
FIG. 12 is a voltage vs. time diagram showing an example of a supply voltage. 15
FIG. 13 is another embodiment of an x-ray tube according to the present invention.
FIG. 14 is a current vs. time diagram for an embodiment of an x-ray system of the present invention, wherein the electrode potential switching is controlled by measuring the 20 power consumption at each electrode.
FIG. 15 is a schematic view of a device for inserting an x-ray tube according to the present invention into a body.
DETAILED DESCRIPTION OF EMBODIMENTS 25
An example of a conventional, prior art miniature x-ray tube is illustrated in the schematic cross-sectional view of FIG. 10. The tube has an enclosure consisting of a hollow cylindrical tube 102 of an x-ray transparent material, and 3Q end walls 103, 104 (although other arrangements, such as end walls integrated with the tubular enclosure are also known). The enclosure is hermetically sealed, and a vacuum is provided inside the tube. A cathode 106, adapted to emit electrons, and an anode 108, the latter adapted to emit x-ray 3J radiation, penetrates through respectively opposite ends of the tube. When the cathode is connected to the negative potential of a high voltage source 113 and the anode is connected to the corresponding positive potential of the voltage source, electrons 110 will be emitted from the 4Q cathode to impact into the anode. As the electrons hit the anode, x-ray radiation 111 is emitted from the anode. The x-ray transparent enclosure allows the x-ray radiation to be delivered to a patient.
According to the present invention, instead of having a 45 dedicated cathode and a dedicated anode, the miniature X-ray tube is provided with at least one pair of electrodes wherein each electrode is adapted to alternately serve as cathode and anode, respectively. More specifically, at a first point in time one of the electrodes of the pair acts as the 50 cathode while the other electrode acts as the anode. Then, at a second point in time, the electrode that previously acted as the cathode will now become the anode, and vice versa. Since the heat is essentially dissipated at the anode, and the position of the anode in this way is altering between two 55 physical positions during treatment, the heat dissipation is reduced at each anode position and, in consequence, the temperature at each anode is reduced due to the repeated cooling intervals provided between the active intervals.
The invention shall now be described in detail by way of 60 embodiments. Of course, the described embodiment should not be viewed as limiting for the scope of the invention.
A first and basic embodiment of the present invention is schematically illustrated with reference to FIG. 1 and 2, wherein is shown a single cell x-ray tube 1. The single cell 65 x-ray tube is defined by an enclosure 2 of a material that is penetrable by x-rays, such as glass, silicon carbide, A1203,
quartz, diamond, boron nitride, pyrolytic boron nitride etc., a first insulating end wall 3 at one end side of the tube, and a second insulating end wall 4 at the other end side. Although any suitable shape of the enclosure could be used, such as bulb like, spherical or hollow with quadrangular cross section, it is preferred to form the enclosure as a tube, i.e. as a hollow cylinder with a circular cross section or with a similar cross sectional shape, such as hexagonal.
The parts are joined by vacuum tight sealing. This may be achieved by using vacuum grade epoxies, using vacuum brazing with appropriate alloys (typically a Ag/Cu alloy) or by using glass frit. The final assembly must obviously be carried out in vacuum.
The end walls, which typically are made of a similar material as the enclosure, could in fact be integrated with the tubular enclosure, or even omitted in a case of a tube having an inner diameter of the same order as the diameter of the electrodes. An example of a miniature x-ray tube according to the invention having no dedicated end walls is shown in FIG. 11, wherein a tubular enclosure 82 holds two electrodes 86, 88.
Again referring to FIG. 1, a vacuum 5 is established in the tube.
A first electrode 6 penetrates through the first end wall 3. In the embodiment shown, the electrode 6 consists of a conductor section 6A for attachment to an external power source 13 via a conductor 14, and an internal section 6B inside the x-ray tube. The conductor section 6 A is made from a suitable conducting material, such as copper. The internal section 6B is made from a material suitable for emitting x-ray energy when hit with electrons, such as tungsten, iridium or gold.
As shown, the internal section 6B could be formed with a tip 7. The tip-formed shape provides a useful emitting area when the electrode acts as an emitter, as will be described below, although other shapes are useful as well, such as planar or hemispherical with a smooth or roughened surface.
The power source 13 is a switching power supply, i.e. a power source provided with a switching unit for providing an alternating voltage potential.
A second electrode 8, generally similar or identical in shape to the first electrode 6, penetrates through the second side end wall 4. Thus, in the embodiment shown the electrode 8 has two sections corresponding to the sections 6A, 6B of the other electrode 6. The second electrode 8 is connected to the switching power supply 13 via a conductor 12.
Of course, it is not necessary that each electrode consists of two sections. It could be made in one piece, or several sections as well,
In use, the switching power supply 13 applies an alternating voltage across the electrodes.
More specifically, during a first interval the first electrode 6 is provided with a negative electrical potential with respect to the other electrode 8, thereby making the first electrode 6 a cathode, and consequently the second electrode 8 is provided with a positive electrical potential with respect to the first electrode 6, thereby making the second electrode 8 an anode.
During a second interval following the first, as illustrated in FIG. 2 showing the x-ray tube of FIG. 1 in a state of reversed electrical potential, the potentials are switched by the switching power supply 13. Thus, the first electrode 6 is provided with a positive electrical potential, thereby making it an anode, while the second electrode 8 is provided with a negative electrical potential, thereby making it a cathode.