X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
1999-07-22
2001-06-26
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Source
Electron tube
Reexamination Certificate
active
06252935
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to an x-ray radiator of the type having an x-ray tube containing a cathode and an anode in a vacuum housing, an electron beam emanating from the cathode during operation of the x-ray tube which strikes the anode in a focal spot from which x-rays emanate during operation of the x-ray tube, and having a deflection arrangement that deflects the electron beam such that the position of the focal spot on the anode changes as a function of the time.
2. Description of the Prior Art
Such an x-ray radiator of this type is disclosed in U.S. Pat. No. 4,637,040 as a component of a computed tomography (CT) apparatus. The deflection of the electron beam thereby ensues such that the focal spot assumes one of two discrete positions in alternation.
German OS 41 25 926 discloses a deflection means containing a deflection coil that generate a magnetic field for the deflection of the electron beam. The deflector coil is supplied with a current whose time curve corresponds to the desired deflection motion of the focal spot, or of the electron beam. In the case of this x-ray radiator, the speed with which the position of the focal spot can be changed is limited by the inductance of the deflection coil, which acts as a low-pass filter. The result is that the frequency with which the position of the focal spot can be changed is also limited.
U.S. Pat. Nos. 4,819,260 and 4,827,494, and 5,065,420 and 4,803,711 disclose keeping the position of the focal spot of an x-ray tube constant with suitable drive of a deflection means, also with the possibility of adjusting the focal spot to a limited extent. Details as to how the displacement of the focal spot can ensue at high speed, however, cannot be derived from these publications.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an x-ray radiator of the type initially described wherein the change in position of the focal spot can ensue at high speed, and thus at high frequency.
This object is inventively achieved in accordance with the invention in an x-ray radiator having an x-ray tube containing a cathode and an anode in a vacuum housing, an electron beam emanating from the cathode during operation of the x-ray tube which strikes the anode in a focal spot from which x-rays emanate during operation of the x-ray tube, a deflection arrangement that deflects the electron beam dependent on a control signal such that the position of the focal spot on the anode corresponds to a reference position changing as function of the time, a detector that acquires the actual position of the focal spot and supplies actual value signal corresponding to the actual position of the focal spot, a reference signal generator which generates a reference signal corresponding to the reference position of the focal spot, and a control unit to which the actual value signal and the reference signal are supplied and that generates the control signal.
In the inventive x-ray radiator, thus, a control unit is provided that generates a control signal on the basis of an actual value signal, acquired with the detector, and a reference signal, this control signal assuring that the position of the focal spot can change at high speed, and thus with high frequency. Differing from the prior art, the deflection arrangement is not driven with a control signal that corresponds to the time curve of the position of the focal spot, but instead is driven with a signal whose time curve can substantially deviate from the desired time curve of the position of the focal spot but ultimately assures that the focal spot assumes a reference position at every point in time that is prescribed by the reference signal.
In a preferred embodiment, the deflection arrangement includes a deflection coil supplied with a deflection signal that generates a magnetic field for the deflection of the electron beam. Such a deflection coil can be advantageously located outside the vacuum housing of the x-ray tube, which is constructed of non-magnetic material, in the area of the deflection coil, so that degradation of the vacuum of the x-ray tube is avoided and easy replacement is assured as needed, these features being problematical, for example, given deflection electrodes arranged inside the x-ray tube.
When the deflection arrangement includes a deflection coil, in one version of the invention that the deflection arrangement, dependent on the control signal, supplies energy to the deflection coil in a first operating condition and withdraws energy therefrom in a second operating condition. Due to the presence of the control unit, thus, it is not necessary to supply the deflection coil with a signal whose time curve ultimately corresponds to the desired time curve of the position of the focal spot; it is sufficient to supply energy to or withdraw energy from the deflection coil, and therefore the deflection arrangement need not contain a costly amplifier but merely switching units.
Since the deflection of the electron beam that can be achieved with a specific voltage is dependent on the energy of the electrons, and thus on the tube voltage, in a preferred embodiment of the invention the deflection arrangement sets the voltage of the deflection signal dependent on the tube voltage of the x-ray tube. This ensures that the dissipated power arising in the deflection arrangement is no higher than required for the deflection of the electron beam given the momentarily existing tube voltage.
The voltage of the deflection signal preferably lies below 100 V, so that the deflection arrangement can be constructed of economical components.
The deflection arrangement thus modifies the amplitude of the current flux through the deflection coil by pulse-width modulation, and one embodiment of the invention, the deflection arrangement, modifies the frequency of the current flowing through the deflection coil dependent on the running difference determined by comparison of the actual value signal to the reference signal.
The detector preferably acquires the position of the focal spot electro-optically. In this context, the detector preferably has a diaphragm through which a spatially limited x-ray beam emanating from the focal spot passes so as to be incident on a radiation detector that supplies an output signal corresponding to the position of the striking, location of the x-ray beam on the radiation detector. Suitable radiation detectors are commercially obtainable.
The reference signal is preferably a periodical signal. This signal can contain a temporarily constant signal component that allows the neutral position of the focal spot to be set, i.e. that position that the focal spot assumes given the absence of the periodic signal component.
REFERENCES:
patent: 4637040 (1987-01-01), Sohval et al.
patent: 4748650 (1988-05-01), Ammann
patent: 4803711 (1989-02-01), Tsujii et al.
patent: 4819260 (1989-04-01), Haberrecker
patent: 4827494 (1989-05-01), Koenigsberg
patent: 5065420 (1991-11-01), Levene
patent: 5550889 (1996-08-01), Gard et al.
patent: 6091799 (2000-07-01), Schmidt
patent: 6111934 (2000-08-01), Foerst et al.
patent: 6128367 (2000-10-01), Foerst et al.
patent: 41 25 926 (1992-08-01), None
Styrnol Siegfried
Winkelmann Helmut
Bruce David V.
Ho Allen C.
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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