X-ray tube comprising an extended area emitter
The invention relates to an X-ray tube comprising a vacuum container, an anode and a cathode disposed within said container, wherein the cathode is an extended area emitter. The invention further relates to an X-ray system, notably for X-ray diagnostic applications, comprising an X-ray tube of the kind mentioned in the opening paragraph. The invention also relates to a method of operating an X-ray tube of the kind mentioned in the opening paragraph.
Cathodes in the form of extended area emitters are known from several applications, such as X-ray tubes, TV picture tubes or electron microscopes. Compared with a conventional thermal emitter in spiral form, an extended area emitter has the advantage that because of the large emitting area the required emission is achieved at a low emission density and hence at a relatively low temperature. As a result, artefacts caused by thermal speed of the emitted electrons remain marginal. In general, such extended area emitters are operated as thermal emitters, which are heated either directly or indirectly. The emission current depends directly on the temperature of the emitter and can be controlled very precisely. A disadvantage of bulky extended area emitters is their relatively high heat capacity. A rapid change of the emission current can therefore hardly be reached. Such rapid change is essential however for many applications, e.g. for controlled or pulsed X-ray exposures. On the other hand, a disadvantage of thin extended area emitters is their tendency to deform more or less during heating. This leads to an unwanted change of focusing and emission current and possibly to displacement and destruction of the extended area emitter itself.
It is an object of the present invention to provide an X-ray tube, an X-ray system and a method of operating an X-ray tube of the kinds mentioned in the opening
paragraphs, which provide rapid controllability of the extended area emitter of the X-ray tube without compromising the emitter's mechanical stability. In order to achieve this object an X-ray tube according to the invention comprises a vacuum container, an anode and a cathode disposed within said container, wherein the cathode is an extended area emitter, and a radiation module adapted to direct pulsed radiation onto the emitter to heat the emitter surface to cause thermionic emission of electrons. In order to achieve this object an X-ray system according to the invention comprises an X-ray tube in accordance with the invention. In order to achieve this object a method of operating an X-ray tube in accordance with the invention comprises the step of directing pulsed radiation onto the emitter to heat the emitter surface to cause thermionic emission of electrons. A basic idea of the present invention is to indirectly heat the extended area emitter cathode using temporary, short-time radiation. As a result of this treatment only a very thin surface layer of the extended area emitter is heated, e.g. in a range of some atomic layers or some nanometers, while deeper layers of the emitter remain essentially unaffected. The surface of the extended area emitter is excited for thermionic emission of electrons. At the same time, the temperature of the extended area emitter as a whole and especially its interface with a holder or mounting structure is substantially unaffected. Immediately after each pulse, the temperature of the emitter surface area exposed to the pulsed radiation adapts to the temperature of the subsurface material of the emitter, without deformation taking place. In contrast to known, indirectly heated emitters, which are controllable solely in a slow and delayed manner, according to the present invention the X-ray dose rate can be controlled in a very precise and easy way. In other words, rapid and efficient control of the emitter current is ensured, e.g. using known techniques for controlling the pulse duration of the radiation, the pulse repetition frequency of the radiation or a combination of both. Because of the pulsed radiation used for heating the extended area emitter, the resulting X-ray is "pulsed" as well. However, the resulting X-ray is "quasi-continuous", such that the resulting X-ray can still be used in all standard applications, including medical applications, without any constraints. Foπns of radiation suited for the inventive technique are those showing a low penetration depth. Therefore, in a preferred embodiment of the invention, pulsed laser energy is used. Laser systems which can be used with the present invention show short pulse
durations within a range of less than one microsecond down to a few femtoseconds, repetition rates between 0.2 kHz and 100 MHz, an adequate pulse output of a few Joules and a mean power of a few Watts. Such laser systems are commercially available. Using such laser systems, rise times of less than one microsecond can be achieved for the tube emission current. According to a preferred embodiment of the invention, laser systems with pulsed laser energy within an energy range of 1 μj to 100 μJ are used. According to another embodiment of the invention, pulsed electron radiation is used to heat the extended area emitter. Preferably electrons within an energy range of 100 eV to 100 keV are used. Outstanding results can be achieved if electrons within an energy range of 1 keV to 10 keV are used. According to yet another embodiment of the invention, ions or neutral atoms or molecules are directed onto the emitter, the radiation energy being preferably within an energy range of 100 eV to 1 MeV. The pulse repetition frequency of the pulsed radiation is, according to another embodiment of the invention, above 0.2 kHz, preferably above 2 kHz. If pulsed laser energy is used, best results are achieved using a pulse repetition frequency above 10 MHz, preferably above 100 MHz. According to another preferred embodiment of the present invention, very short radiation pulses are used. The pulse duration is preferably in a range from the reciprocal value of the twentyfold of the pulse repetition frequency to the reciprocal value of the fivefold of the pulse repetition frequency. In an embodiment of the invention the pulse duration is less than 1 μs, preferably less than 10 ns, more preferably less than 1 ns. According to an embodiment of the invention, the extended area emitter is heated extensively, i.e. using unfocussed radiation. Here the whole emitter surface can be heated very quickly. However, there is a risk of damaging the extended area emitter because of excessive heat. Alternatively, the pulsed radiation, especially the pulsed laser energy, is focussed onto a relatively small sub-area of the emitter surface. The sub-area corresponds preferably to less than 10% of the total emitting area of the extended area emitter. The radiation beam is then moved over the whole emitting surface of the extended area emitter. As a result, excessive heating is prevented. Preferably the sub-area is changed such that by the time the pulsed radiation reaches the same sub-area again, the temperature of said sub- area has reached a value which differs less than 100 K from the mean temperature of the extended area emitter. The moving of the radiation beam can be performed continuously or
erratically. Preferred types of movement are linear scanning, meander scanning or stochastic scanning. The X-ray tube preferably comprises a radiation module adapted to direct the pulsed radiation onto the emitter's surface. In a preferred embodiment this radiation module is connected to a radiation control module outside the X-ray tube, said radiation control module being adapted to control the pulse duration, pulse repetition frequency etc. of the radiation. Preferably the radiation control module is further adapted to control the focussing and/or the movement of the focussed radiation beam across the emitter surface. The radiation control module, preferably a computer, is part of an X-ray system, which, besides the X-ray tube, further comprises radiation protection elements, cooling elements, high- voltage electrical equipment etc. The computer is adapted to run a computer program comprising computer instructions adapted to perform the method according to the invention when the computer program is executed in the computer. The technical effects necessary according to the invention can thus be realized on the basis of the instructions of the computer program in accordance with the invention. Such a computer program can be stored on a carrier such as a CD-ROM or it can be available over the internet or another computer network. Prior to executing the computer program, it is loaded into the computer by reading the computer program from the carrier, for example by means of a CD- ROM player, or from the internet, and storing it in the memory of the computer. The ■ computer includes inter alia a central processor unit (CPU), a bus system, memory means, e.g. RAM or ROM, storage means, e.g. floppy disk or hard disk units and input/output units. Preferably the computer is an integral component of the X-ray system. Furthermore, in another embodiment of the invention, a radiation transport element is employed. In the case of pulsed laser energy being used to heat the extended area emitter, the radiation transport element preferably comprises optical fibres to transport the laser light from a laser source outside the X-ray tube into the vacuum container.
Embodiments of an X-ray tube and an X-ray system in accordance with the invention will be described in detail in the following description with reference to the drawing, in which: Fig. 1 is a block diagram diagrammatically showing an X-ray system and an X-ray tube according to the invention.
An X-ray system according to the invention is shown in a simplified block diagram of Fig. 1. The X-ray system mainly comprises an X-ray tube 1 according to the invention and an external laser source 2. The X-ray tube 1 comprises a high vacuum container 3, an anode 4 and a cathode 5 disposed within said container 3. The container 3 is made of glass or metal. The tube shield and insulating oil surrounding the vacuum container 3, which prevent radiation leakage, and other system elements, such as high- voltage cables leading to the cathode and anode end of the tube, are not depicted for the sake of clarity. The cathode 5 is an extended area emitter made of tungsten, which releases electrons when heated. The cathode 5 is connected to a negative high voltage. The size of the emitting area 6 is within a range of 1 mm2 to 100 mm2. The pulsed laser radiation 7 emitted by the laser source 2 passes a laser optic 8, where a focussed laser beam 9 is formed. The laser beam 9 is then fed into the vacuum container 3 through a laser light window 10 made from laser-transparent material. Laser source 2 and laser optic 8 are controlled by a computer-based radiation control module 11. The radiation control module 11 is adapted to control the laser pulse duration, laser pulse repetition frequency as well as the laser focussing and the movement of the focussed laser beam 9 across the emitting area 6. According to the heating sequence run by the radiation control module 11, the focussed laser beam 9 moves across the emitter surface 6, thereby heating the surface to cause thermionic emission of electrons 12. The emitted electrons accelerate from the cathode 5 towards the anode surface 13, where X-rays . 14 are produced mainly by "bremsstrahlung" and to a smaller extent by characteristic radiation. The anode 4 is embodied so as to be a rotating anode consisting e.g. of a thin tungsten surface 13 on copper plus graphite base. The anode 4, which is connected to a positive high voltage, is driven by an induction motor 15 situated outside the vacuum container. The motor 15 is controllable by the radiation control module 11. The high voltage across the X-ray tube 1 is achieved e.g. by applying half the voltage needed with respect to earth, e.g. +50 kV, to the anode 4, and the other half, -50 kV, to the cathode 5, for a total high voltage of 100 kV. For medical diagnostics, high mean X-ray dose rates for 10 ms to 100 s are needed. Using the non-linear characteristic curve of known emitters, such mean X-ray dose rates can be achieved by heating the emitter area 6 to temperatures corresponding to mean emission currents, which are significantly higher (preferably at least tenfold higher) than the
mean emission currents necessary. Pulse duration ton and pulse pause duration t0ff are calculated as follows: = (a-1) * ton , wherein a = Ip / Im and where Im denotes the mean emission current and Ip denotes the emission current during a with a > 1, preferably a > 10. The term "mean emission current" denotes the averaged emission current, taking into account the fact that the emission current is "pulsed" because of the pulsed radiation used for heating the emitter's surface. If the cathode 5 shows a relatively thin layout, only a very thin surface layer will be heated in order to prevent the build-up of unwanted heat within the emitter material. Therefore, the pulse duration is controlled by the radiation control module 11, such that it is shorter than the time necessary for the generated heat to penetrate the emitter material to an unacceptable depth. Assuming a penetration depth of 10 nm, using tungsten as emitter material, the pulse duration % is set to τ=100 nm*cw/λw=100ns, wherein cw denotes the specific heat capacity of tungsten and λw denotes the specific thermal diffusivity of tungsten. Taking the above into account, pulse repetition frequencies above 10 MHz and pulse durations below 10 ns are adequate. If pulsed particle radiation is used instead of laser, • penetration depths in the μm-range may be attained. With an emitting area of the order of a few square millimetres, energy losses due to heat conduction are negligible if short laser pulses with a pulse duration below 10 ns are used. The time constant for tungsten as emitter material is less than 10 μs. For the present case, a surface layer having a thickness of approximately 1 μm will be heated to temperatures between 2,000 K and 3,000 K. Starting from room temperature, the energy needed amounts to 4-70 μj in the case of tungsten as the emitter material. This corresponds to 7-70 W of power input for laser pulses with a pulse duration of 100 ns. By preheating the cathode 5 in a conventional way up to a temperature at which no significant emission occurs (approximately. 1500 K), e.g. by direct heating, power requirements can be further diminished. The preheating process is preferably controlled by the radiation control module 11. The extended area emitter used within the present invention is preferably produced as a bulk material, e.g. showing a thickness of a few tenths of a millimeter or even a few millimeters. Alternatively, a coating on a base made of another material may be used as an emitter. It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention maybe
embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. It will furthermore be evident that the word "comprising" does not exclude other elements or steps, that the words "a" or "an" does not exclude a plurality, and that a single element, such as a computer system or another unit, may fulfil the functions of several means recited in the claims. Any reference signs in the claims shall not be construed as limiting the claim concerned.