1. Field of the Invention
The present invention relates to X-ray image intensifier tubes notably for medical applications. These X-ray image intensifier tubes are normally used in a sequence or set of units consisting of an X-ray generator, an object to be examined which is most usually a patient, the intensifier tube itself which converts the image of the object given by X photons into an intensified light image and finally a image-taking and image-analysis system generally comprising a photography camera, a motion picture camera, a video camera and an image processing circuit.
2. Description of the Prior Art
In certain applications, notably in cardiology, two sets of instruments of this type are used, positioned at right angles to each other and working alternately. When one system works, the other one does not, for the object is not irradiated by two X-ray beams at the same time. These two sets of instruments enable the obtaining of X-ray images in two directions. When one set is in operation, the X-ray image intensifier tube of the other one must be shuttered or turned off so as not to produce any image. Indeed, the patient produces a large quantity of X-rays by scattering. These X-rays may be picked up by the X-ray image intensifier tube of the inactive set of instruments, and then this tube will produce a poor image.
Generally, the two sets of instruments work alternately at a frequency varying from 30 to 90 Hertz. Each of the generators gives an X-ray pulse having a duration that generally varies from 50 .mu.s to 8 ms. Each X-ray image intensifier tube has to be turned off or on in less than 400 .mu.s or even less if possible.
An X-ray image intensifier tube such as the one of FIG. 1 is formed by a tightly sealed casing 1 comprising an input face 2 that receives an X-ray beam 3 emerging from an object 4 to be examined. The X photons enter by the input face 2 into a primary screen 5 which, from the input face 2 onwards, comprises a scintillator 6, a conductive layer 7 and a photocathode PC. The scintillator 6 converts the X photons into light photons, and these light photons excite the photocathode PC.
The photocathode PC converts light photons into electrons. The conductive layer 7 may be made of indium oxide. The electrons are then extracted, accelerated and focused by a series of electrodes among which there are three successive electrodes G1, G2, G3 followed by an anode A. At the end of their travel, the electrons bombard a secondary screen 8 or an output screen which in turn converts electrons into light photons. An intensified image is formed on the secondary screen 8. It gives a reconstitution, in smaller form, of the image coming from the object 4 to be examined.
All the electrodes have to be supplied with DC current in a stable way. A stabilized supply is necessary (it is not shown in FIG. 1). A single supply with several outputs may be used. The magnitudes of the nominal voltages of each electrode are as follows:
photocathode PC: 0 V PA1 electrode G1: 0 V to +350 V PA1 electrode G2: +200 V to +2000 V PA1 electrode G3: +2 kV to +20 kV PA1 anode A: +30 kV
The voltage of the electrodes G1, G2, G3 is generally adjustable. This makes it possible to obtain a magnifying-glass effect on a secondary screen. The voltage of the photocathode and the anode A is generally fixed.
In the X-ray image intensifier tubes of recent design such as that of FIG. 1, the sealed casing 1 has a first metal part 11 that includes the front face 2 and forms the electrode G1. The photocathode PC is electrically isolated from this metal part 11 and an insulation beam 9 is provided. The metal part 11 is extended by a glass part 12 to close the casing 1. The other electrodes G2, G3, A go through this glass part. The oldest tubes have an entirely glass casing.
Usually, the operation of turning the X-ray image intensifier tube off is obtained by switching over the voltage of the electrode G1 and/or the electrode G2. Several methods are used at present. One of them consists in switching over the voltage of the electrode G1 to about -700 V while it is between 0 and +350 V when the tube is in operation.
This method cannot be applied to every X-ray image intensifier tube and in every mode. Furthermore, in the case of the X-ray image intensifier tubes where the electrode G1 is a part of the casing, it may be dangerous to take this electrode to a voltage very far from the ground voltage.
Another known method consists in applying a negative voltage of about -1300 V to the electrode G2. The electrode G2 is used to focus the electron beam. During the switch-over operation used to turn on the tube, the electrode G2 must recover an appropriate operating voltage (ranging between +200 V and +2000 V) with a precision of about 3 per thousand to prevent a defocusing of the tube.
The switch-over operation aimed at turning the X-ray image intensifier tube off must be done at high speed and the great difference in potential (between -1300 V and +2000 V) applied to the electrode G2 prompts disturbances, by capacitive coupling, in the voltage of the electrodes in the vicinity especially of the electrode G3. This leads to substantial deterioration in the quality of image.
During the switch-over of the electrode G2 aimed at turning the X-ray image intensifier tube on, the voltage of the electrode G3 increases in forming a peak. Then it decreases slowly to return to its nominal value. The stabilizing of the voltage of the electrode G3 comes into play only after some milliseconds while it is sought to restore the voltage of the electrode G3 to a level substantially below 1 per thousand at the end of 400 .mu.s.
Furthermore, the great difference in potential applied to the electrode G2 during the switch-over operations and the precision of restoration of the voltage at the electrode G2 during a switch-over operation aimed at turning the X-ray image intensifier tube on result in a complex switch-over circuit.
Another known method consists in switching over the electrode voltage G1 and that of the electrode G2 simultaneously. For this purpose, the voltage of the electrode G2 is lowered by about 700 to 1000 V (if it is about 2000 V when the X-ray image intensifier tube is in operation) and the electrode G1 is taken to about -700 V. This method is used to minimize the disturbances in the electrodes near the electrode G2 during a switch-over operation. However, the switching over of two high voltages with high precision of restoration leads to a complicated and expensive switch-over circuit.
The switch-over circuits commonly use either several series-connected bipolar transistors or an oscillator transformer followed by a rectifier.
A circuit with bipolar transistors is complicated to design and therefore expensive.
A circuit with a transformer is limited in terms of switched-over voltage and speed and dissipates a great deal of power. It therefore has low efficiency.
The electrode to be switched over is linked to the switch-over circuit by an armored cable so as to minimize the capacitive coupling with the other electrodes and hence the disturbances in the voltages of the other electrodes that are caused by the switching over. In the variant where two electrodes are switched over simultaneously, two armored cables are needed.
The electrodes that are close to the switched-over electrode and that have their voltage undergo disturbances through capacitive coupling, require a voltage stabilization circuit. Since these electrodes are carried to very high voltages, the stabilization circuits must be sized accordingly. It is possible to use either a large decoupling capacitor or a fast regulation circuit. The capacitor is bulky and dangerous because it stores a great deal of energy. It is well known that it increases the voltage stabilizing time.
The regulation circuit is complicated, costly and difficult to protect against transients. Furthermore, it is bulky.