Patent Number: 059848537
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The basic physical principle behind the radiation source is well known from the literature of modern physics. When high energy electrons are retarded by nuclei having a large atomic weight, electromagnetic radiation is emitted. The primary radiation, denoted "bremsstrahlung", has a continuous spectrum with a peak corresponding to a given fraction of the electron energy. The emitted radiation can have an energy peak from a few electron volts (eV) to several million electron volts (MeV) depending on the energy of the incident electrons. In terms of wavelength, this corresponds to a range from ultraviolet light (10-4000 .ANG.) via X-rays (0.1-100 .ANG.) to gamma radiation (&lt;0.10 .ANG.). Thus, by varying the energy of the electrons, the wavelength peak can be displaced accordingly. In addition to bremsstrahlung, which basically has a continuous spectrum, absorption or emission peaks corresponding to atomic electron transition may be embedded in the spectrum, depending on the materials contained in the transmission medium. The details of the radiation source and its function will be described with reference to FIGS. 1 and 2. Basically, the source is built up from two plates 2, 3 with a recessed region forming a microcavity 1 at one or several localities. An anode material 5 and a cathode 4 with extremely small dimensions, and having the form of a sharp tip 10 are located within this microcavity. The radius of curvature of the tip 10 of the cathode is preferably in the nanometer range. If a voltage is applied between the anode 5 and cathode 4, the electric field strength will be extremely high at the cathode. A positive voltage on the anode will cause electrons to be emitted from the cathode by the phenomenon known as field emission. Alternatively, the cathode may be heated to high temperatures, giving rise to thermal emission of electrons. This will be further discussed below with reference to FIG. 6. The electrons are accelerated by the electric field, until they are retarded by the impact at the anode. The anode 5 preferably consists of a metal having a high atomic weight, corresponding to an atomic number exceeding 50. In a preferred embodiment, the anode 5 is made of tungsten which is an endurable metal that can be deposited in the form of thin films either by physical or chemical deposition techniques. Other metals include cobalt, molybdenum and aluminium. The cathode preferably consists of a thin deposited film of a material having a low work function, i.e. the energy required for an electron to be emitted from the surface into the ambient. Materials with this property are oxides of metals from Groups I and II in the periodic table, including cesium, barium and magnesium. The anode 5 and cathode 4, may be connected to a voltage source by electrically conducting leads 6, 7, which may, at least partly, be an integral part of the plates 2, 3. This can be achieved by deposition of stripes by evaporation, sputtering or chemical vapor deposition. Alternatively, if the plates 2, 3 are semiconductors, the leads 6, 7 may be doped regions according to well-known technology. In a preferred embodiment, a third electrode 11 is also present within the microcavity 1. This electrode 11 acts as a gate, controlling the electron current emitted toward the anode 5. The gate electrode has a separate lead 12, enabling a separate voltage source to be connected. According to the well-known theory of vacuum tubes, the anode current is controlled by the gate voltage. This will directly influence the intensity of the emitted radiation which is approximately proportional to the anode current. The emitted dose is simply the time integral of this intensity. By separate and independent control of the gate and anode voltages, it is thus possible to independently control the emitted dose and energy, respectively. The leads 6, 7 and 12 must be properly isolated to avoid short circuit or current leakage. If the plate materials by themselves are not isolating themselves, passivating films may be necessary to ensure proper isolation. Furthermore, the lateral location of the leads is preferably chosen to minimize the electric field across material barriers. The voltage to the anode and cathode should preferably be in the kV range in order to obtain radiation of sufficient energy. With reference to FIG. 6, there is schematically shown an implementation wherein the thermionic emission principle is employed. Through a thin wire 601 or filament disposed in a microcavity 602, such as the one disclosed above, a current I is passed. The temperature will be so high that electrons will be emitted and accelerated by an electronic voltage imposed across the filament 601 and an anode 603, also disposed in microcavity 602. There are two principally different ways of fabricating the radiation source according to the invention. One way is to use two separate solid substrates and define the structures containing the cathode 4, the gate 11, with their leads 7 and 12, and the recess or microcavity 1 in one substrate. The anode 5 and its lead 6 are defined in the second substrates. Lithographic techniques according to well-known art are preferably used in defining these structures. Then finally the two substrates, corresponding to plates 2 and 3, are bonded together, using techniques such as solid-state bonding. If the bonding is performed in a vacuum, the microcavity 1 will remain evacuated, since the bonded seal is almost perfectly hermetic, provided that no organic materials are used. Absolute vacuum is not a necessity, but the density of gas molecules inside the microcavity must not be so high that the accelerating electrons are excessively impeded. A requirement for successful bonding is that the bonded surfaces 8 and 9 are flat and smooth with a precision corresponding to a few atomic layers. A second requirement is that all structures are able to withstand a relatively high annealing temperature, approximately 600-1000.degree. C., without damage. This first fabrication technique is basically known as bulk micromachining, in contrast to its alternative, surface micromachining. According to this, all structures are formed by depositions on one single substrate, again using lithography to define the two-dimensional pattern on the surface. The microcavity 1 is formed by first depositing a sacrificial layer which is etched away after the uppermost layers have been deposited. Closing the microcavity can be done by depositing a top layer, covering openings which are required for the etching of the sacrificial layer. Both described methods of fabrication are feasible and lead to similar device performance. Indeed, from examining a final device, it may be difficult or even impossible to conclude which fabrication procedure has been used. An important characteristic of the proposed fabrication techniques is that the manufacturing cost per unit becomes very small when the source elements are fabricated in large numbers. This is due to the fact that batch fabrication with thousands of units per batch is feasible. In FIG. 3 an embodiment is shown where the source and its leads 6, 7 are mounted inside a tubular element, such as a cannula 100, consisting of a material which is transparent to the emitted radiation. Preferably, the tubular element (or the hollow portion where the source is mounted in the case of a needle), is made from elements having a low atomic number. As shown in the cross section A--A the leads 6, 7 are connected to wires 101, 102 having isolated mantles 103, 104. In a preferred embodiment, the outer diameter of the tubular element is smaller than 2 mm. The cannula is then sufficiently small to penetrate tissue in order to reach a certain location where radiation therapy is required. FIG. 4 shows a further embodiment where the source 200 is located near the distal end of a wire 201, having high bending flexibility in order to prevent organs and tissue from perforation or penetration by mistake. Instead, the wire 201 can be guided to the tissue where radiation therapy is required by insertion through a catheter which has previously been inserted in the tissue by well-known techniques. A cross section B--B of the wire 201 shows that it consists of a tubular member 202, and power transmitting leads 203, 204. The leads 203, 204 are proximally connectable to an external power source by connecting elements 205, 206. Geometrically, the connecting elements 205, 206 have a diameter approximately equal to the diameter of the wire to allow insertion of the wire into a catheter. Referring now to FIG. 7a and b, other vehicles for the radiation source are conceivable, e.g a needle 700 with a solid distal portion 701 having a sharp tip for the easy penetration of soft and hard tissue, and a hollow portion 702, proximal to the solid tip, wherein the radiation source 703 is mounted. In still another embodiment the radiation source may be mounted in a tube 704, the distal end of which, 705, has been bevelled to render it sharp enough for penetration purposes. The open end of the tube may be plugged at 706 so that the interior of the tube housing the source will not be soiled by tissue. The power leads supplying power to the radiation source can either be electrical or fiberoptic leads, according to well-known technology. In the case of optical power transmission, it is necessary to convert the optical power into electrical voltage to provide voltage supply to the source. This may be done by providing optical energy through the fiberoptic leads and letting the light impinge onto a photodiode which converts the light into a voltage. FIG. 5 shows an electronic circuit element M capable of multiplying an input voltage 305 to its output terminals 307, 308 by a factor of approximately two. The circuit operates with two switching elements, for example diodes 301, 302, and two capacitors 303, 304. If two circuit elements as that shown in FIG. 5 are cascaded, the input voltage will be multiplied by a factor of approximately four. Even larger multiplication factors are possible by cascading more circuit elements of a similar type. The diodes 301, 302 may be replaced by other switching elements, such as transistors. Preferably, electronic circuitry M such as that shown in FIG. 5 may be integrated with one of the plates 2, 3 accommodating the source (schematically shown in FIG. 8a). Alternatively, the circuitry consists of a separate electronic chip located close to the source (schematically shown it FIG. 8b). The high voltage generation may of course alternatively be disposed outside the body, e.g in the external power supply. The method of providing a controlled dose of radiation is carried out as follows. The physician localizes the region of interest, e.g. a tumor to be treated. Depending on the site and type of tissue, various vehicles for the radiation source may be employed, e.g. a needle for penetrating through soft tissue, or a guide wire possibly in combination with a catheter, or the insertion may be made through blood vessels or other body channels, such as intestines. When the radiation source has been correctly located inside the body, the radiation source is activated and the required dose is given. The device is switched off and the source is withdrawn from the patient. This procedure may be repeated frequently until the desired clinical result has been achieved. While several embodiments of the invention have been described, it will be understood that it is capable of further modifications, and this application is intended to cover any variations, uses, or adaptations of the invention, following in general the principles of the invention and including such departures from the present disclosure as to come within knowledge or customary practice in the art to which the invention pertains, and as may be applied to the essential features hereinbefore set forth and falling within the scope of the invention or the limits of the appended claims.