X-ray or gamma ray systems or devices – Electronic circuit – X-ray source power supply
Reexamination Certificate
1998-08-17
2001-01-23
Font, Frank G. (Department: 2877)
X-ray or gamma ray systems or devices
Electronic circuit
X-ray source power supply
C378S138000, C378S109000
Reexamination Certificate
active
06178226
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a method for controlling the electron current flowing in an x-ray tube in the form of an electron beam propagating between an electron emitter and an anode, the electron emitter having a focussing electrode and being continuously heated during the operation of the x-ray tube the electron beam striking the anode in a focal spot, with the tube voltage being across the electron emitter and the anode, and the focal electrode being at a focussing electrode potential. The invention also relates to an x-ray system operating according to this method.
In contemporary x-ray tubes, continuously heated tungsten helical coil is employed as the electron emitting component almost exclusively. The tube current—i.e. the electron current emanating from the electron emitter given a defined tube voltage—is therein determined by the temperature of the helix, which is adjusted by the heating current through the tungsten helix. Because of the low heating capacity of the tungsten helix it is possible to rapidly alter the tube current while maintaining the respective size of the focal spot by altering the heating current, which is necessary for many medical recording techniques. In continuously heated low-temperature emitters which are fashioned from materials—e.g. LaB
6
—with a lower specific electron work function than tungsten and which as a rule have a significantly higher heating capacity than tungsten, alterations of the tube current at the filament are not possible with the same speed as with a tungsten helix, which is why low-temperature emitters cannot be employed everywhere. In many modern x-ray tubes—e.g. rotating bulb tubes with central emitters or x-ray tubes with oblique bombardment—round emitters with a small emission surface and a high emission current are needed to generate an electron beam with an approximately circular cross-section. The known tungsten helices are unsuitable for these tube geometries. The low-temperature emitters that are otherwise suitable, however, cannot bear rapid temperature changes such as are necessary for medical recording techniques with rapidly varying tube current. If a low-temperature emitter should be employed despite this, then the controlling of the tube current—i.e. the adjustment of the electron current—must occur in a different way than by alteration of the heating current. This can be effected by an additional electrode, for example a grid connected upstream, a Wehnelt cylinder or a focussing electrode at a different potential than that of the electron emitter. A disadvantage of such approaches, however, is that the potential distortion brought about by the additional electrode simultaneously influences the spread of the electron beam such that the abovementioned arrangement is only suitable for turning the electron current, and thus the tube current, on and off in alternation, but is not suitable for variable control without simultaneously adversely influencing other focussing, and thus the size of the focal spot dependent on the potential at the additional electrode, and thus on the tube current.
An x-ray tube having the capability of adjusting of the tube current but without any consideration of the tube voltage and/or the size of the focal spot, known from U.S. Pat. No. 5,617,464.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method and a device of the abovementioned type wherein variable current control is possible with constant focussing, i.e. with constant size of the focal spot.
This object is inventively achieved in a method for controlling the electron current flowing in an x-ray tube in the form of an electron beam between an electron emitter with focussing electrode and an anode, the emitter being continuously heated during the operation of the x-ray tube wherein the electron beam strikes the anode in a focal spot, and wherein tube voltage is across the electron emitter and the anode, and wherein the potential at the focal electrode is pulsed with a pulse frequency between a conducting-state voltage, selected dependent on the desired size of the focal spot and/or the tube voltage, and a blocking or reverse voltage interrupting the electron current to the anode, the pulse width being modulated (adjusted) to control the electron current.
The inventive method thus provides a pulse width-modulated current control for an x-ray tube. The potential at the focussing electrode is altered in pulse-like fashion with a pulse frequency between two fixed voltages, namely a conducting-state voltage —given which the field generated by the focussing electrode allows the electrons emitted at the electron emitter reach the anode—and a blocking voltage—given which the field generated by the focal electrode completely shields the electrons emitted at the electron emitter from the anode. The conducting-state voltage is inventively selected such that a defined focus is set, i.e., a focal spot of the desired size is generated on the anode. The desired size of the focal spot is thus the factor according to which the amplitude of the conducting-state voltage is selected. In addition, in the case of x-ray tubes with adjustable tube voltage the amplitude of the conducting-state voltage depends on the prevailing tube voltage, which is likewise considered in the selection of the amplitude of the conducting-state voltage.
The electron beam between the electron emitter and the anode is thus switched on and off in alternation, whereby in the on-condition a focal spot of the desired size is generated on the anode as a result of the conducting-state voltage, which is selected corresponding to the desired size of the focal spot dependent on the prevailing tube voltage, if necessary. The control of the effective chronologically averaged flowing tube current occurs by pulse width modulation, i.e., by adjusting (corresponding to the desired tube current) the duration of the time intervals during which the focussing electrode is at the conducting-state voltage. In this way, the invention allows an altering of the tube current without influencing the size of the focal spot. This holds true regardless of the type of the electron emitter which is employed, i.e. also for a continuously heated low-temperature emitter. As a consequence of the pulse width modulation, rapid modifications of the tube current such as are necessary in many medical recording techniques thus are possible without influencing the size of the focal spot.
In an embodiment of the invention the pulse frequency is greater than 1 kHz, this frequency being selected from a range between 1 kHz and 10 kHz, in particular. In the ideal case the time characteristic of the voltage at the focussing electrode corresponds to a rectangular curve. Such a curve is not exactly realizable in practice, however. In order to avoid only a gradual rise, or drop of the tube current due to excessively low edge steepnesses of the curve of the voltage at the focussing electrode, rather than the rectangular alteration desirable per se, in an embodiment of the invention the edge steepness with which the voltage at the focal electrode is altered between the blocking voltage and the conducting-state voltage, and vice versa, is selected such that the time in which the voltage at the focussing electrode is switched from the blocking voltage into the conducting-state voltage and vice versa is shorter than 100 &mgr;s, particularly shorter than 10 &mgr;m. The times in the range of 10 &mgr;m and smaller can still be achieved without great outlay.
In x-ray systems employed in medicine, for example, a detector system is disposed in the path of x-rays emitted from the x-ray tube. If the tube current, and thus the generated x-ray radiation, is pulsed in the manner described, this also affects the image recording behavior of the detector system. In order to account for this, in an embodiment of the invention the pulse frequency is selected dependent on the image recording frequency of a detector system connected to the x-ray tube downs
Hell Erich
Schardt Peter
Font Frank G.
Rodriguez Armando
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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