US 3333104 A
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J. BOUGLE 3,333,104 TUBE POWER VLNG PULSE MEANS July 25, 1967 PLURAL X-RAY SUPPLY HA FOR CONTROLLING THE CONDUCTION OF SAID TUBES Filed NOV. 25, 1954 N 9. w w u w m2; 1 w 1 u i I I I mm! o O. m, E 0. L L O. L O. l d a I I I 5 E? mm m 0?. 0E Q? n Q 8 mm mm ow F Q P ll N II- N N LS 111 EEEEW $553M mm E @9855 5 $555 g V1\l .w my 9 f: i moZmEEou moQmfizoo m m ll 9 V\\ on W 52 553 2? Mom 57:33 mm @0555 .|\\ll| 1065mm.
*2 i\| moEmwzmw $1.- M95? mm m zmmmhmm United States Patent C) 3,333,104 PLURAL X-RAY TUBE POWER SUPPLY HAVING PULSE MEANS FOR CONTROLLING THE CON- DUCTION OF SAID TUBES Jean Bougle, Paris, France, assignor to Compagnie Generale de Radiologie, Paris, France Filed Nov. 25, 1964, Ser. No. 413,718 Claims priority, application France, Feb. 28, 1964, 965,492, Patent 1,395,015 1 Claim. '(Cl. 250-94) ABSTRACT OF THE DISCLOSURE At least two X-ray tubes are connected across a D-C supply for being operated sequentially and at different voltages if desired. Each X-ray tube is between and in series with a pair of switching tubes. Voltage across each X-ray tube is compared with a reference voltage and the difference voltage is used in a closed loop to control the grid voltage and the voltage drop across the switching tubes to thereby maintain a predetermined voltage on the X-ray tubes.
The present invention relates to high voltage generators for X-ray tubes and is particularly concerned with a generator that is capable of supplying several X-ray tubes at different voltages simultaneously for enabling a rapid succession of radiographs or radiostereoscopic views to be taken at different angles through the subject of an X-ray examination.
It is often of interest to obtain a profile and front view picture or radiograph of an organ of the human body at a given instant in the course of an X-ray diagnosis. To do this, according to a known method, two X-ray tubes are connected in parallel with a common high voltage generator. The instantaneous power which the generator must be able to furnish is the sum of the power consumed by each tube. This has disadvantages. First, a human organ usually transmits X-rays better in one direction than in another so it is desirable to use different penetrating power on each tube, which is to say that a different voltage should be used on each tube. Secondly, a beam of X-rays that is directed toward one recording medium, such as a film cassette, is diffusely scattered by the organ into the other film in which case there is a loss of definition on each film.
A known method for overcoming these disadvantages is to supply each X-ray tube from an individual high voltage generator and to take the radiographs successively and alternately with small time intervals between them. In this method the radiographic films are contained in film changers capable of synchronously delivering film into the path of each beam at a high rate, twelve per second, for example. It is seen that with this method it is possible to obtain for each angle of incidence or beam direction, a series of radiographs, those corresponding with one direction being alternate in time with those of the other. The scattered radiation from one beam does not get onto the film related to the other beam but the method requires use of two expensive high voltage generators. Moreover, when conventional high voltage generators are used, it has been difiicult to exceed taking more than twelve radiographs per second.
An object of the present invention is to provide a high voltage generator, preferably three phase, that allows supplying one, two, or more X-ray tubes for taking as many as two hundred radiographs per second in one or more directions and that has independent regulation of the high voltage on each of the tubes.
Another object of the invention is to control the X-ray 3,333,104 Patented July 25, 1967 tubes with tetrode switching tubes that determine the start and end of an X-ray exposure and assist in maintaining the high voltage as a predetermined constant value during an exposure.
Achievement of the foregoing and other objects of the invention will appear from time to time in the course of the following detailed description of a preferred but not limitative embodiment of the invention taken with the drawing in which:
FIGURE 1 is a diagram of the essential features of a high voltage power supply and control for X-ray tubes according to the invention;
FIGURE 2 is a graph of the form and timing of the pulses delivered by the X-ray tubes shown in FIGURE 1; and,
FIGURE 3 is a graph showing the relationship between the voltage of the high voltage generator and the voltage on the X-ray tube over a conduction interval of the latter.
In FIGURE 1, two X-ray tubes 21 and 22 are adapted for being supplied from a single D-C source of high voltage to permit taking a rapid sequence of radiographs from different incidences. In this case, the DC is obtained by rectifying A-C from a three phase transformer 1 having star connected secondary windings 3 and 4 which are coupled with primary windings 2 and 2. The primary windings 2, 2' are respectively in star and delta to produce a phase shift in their secondary voltages or twelve pulses per cycle, which facilitates rectifying and filtering as is well-known.
There are two rectifier groups that are connected with the output terminals of the secondary windings 3, 4. Each group is divided at a point of symmetry 39 which is grounded. The positive, or upper group in FIGURE 1, comprises diodes numbers 5 to 10. The negative, or lower group, is numbered 11 to 16. The diodes in each group are fed in parallel from their secondary windings and each group is in series with the other. Thus, the total D-C voltage appearing between positive line 41 and negative line 42 is equal to twice the voltage between either line 41 or line 42 and the symmetry point 39. In a practical case, line 41 may be kilovolts maximum above ground, and line 42, 80 kilovolts below ground thereby attaining kilovolts to be applied across the X-ray tubes 21 and 22. A milliammeter 40 reads the current through the X-ray tubes. 3
The positive rectifier group has a damping resistor 17 in series with a capacitor 18 which resistor and capacitor are in parallel with a discharge resistor 43. The negative group has series connected capacitor 19 and resistor 20 in parallel with resistor 44. The capacitors just named function in a complex way to yield energy when the X- ray tube begins to conduct, to obtain a fast rise time, and to absorb energy at the end, to avoid overvoltage.
The invention may use any stable D-C source of high voltage from a single or polyphase supply. The three phase supply described above is well-known as exemplified by Swiss Patent 204,063 dated July 17, 1939.
In FIGURE 1, two X-ray tubes 21 and 22 are shown although additional tubes may be supplied from high voltage lines 41, 42 if one desires more sequences of X-ray pictures from different aspects. Since the circuitry associated with each X-ray tube is basically the same, the description will be focused on that of X-ray tube 21 first. This tube has in series with it a pair of tetrode high voltage switching tubes 23 and 24 which are controllable to connect the X-ray tube to the D-C line to start an exposure and to disconnect the X-ray tube at the end of an exposure. Because of the symmetry of the system, each tetrode switches one-half of the total D-C voltage. In a commercial embodiment, about 800 volts negative bias 3 is applied to the control grid of each tetrode to cut it off at 80 kilovolts on the plate.
The high voltage switching tetrodes 23 and 24 are controlled by a loop circuit that includes a reference voltage generator 27 and a comparator 28. During an X-ray exposure, the reference voltage generator 27 delivers rectangularly shaped voltage pulses to the input 50 of comparator 28.
The voltage applied to X-ray tube depends on the amplitude of these pulses and the duration of an exposure interval depends on the width of the pulses. In the part of the X-ray controls that are not shown there are the usual exposure time and voltage selector switches which the operator adjusts to a desired value to control through a switch 45, the reference voltage generator 27 which establishes the pulse height and width.
The comparator 28 has two symmetrical portions, one positive and one negative with respect to ground point 39'. When X-ray tube 21 is conducting, a voltage appears between points 48 and 48 on a voltage divider circuit comprising resistors 30, 31, 46, and 47. Part of this voltage appears across resistors 46 and 47 in opposite polarity with respect to ground point 39' and this constitutes another input to comparator 28. Thus, it is seen that the comparato'r has the properties of the well-known differential amplifier with two inputs, a reference voltage signal on.
50 and a signal across 46, 47 that depends on the voltage across X-ray tube 21. The comparator 28 compares these two signals, that is, takes the difference between them, and this difference is impressed n the control grids of the tetrodes 23 and 24 through a pair of small high voltage isolating transformer 49 and 49' respectively.
Comparator chassis 28 also has built into it a high frequency oscillator, not shown, but which feeds a pair of detector amplifiers 29 and 32 through the transformers 48 and 49. The oscillator output frequency may be about 2.5 megacycles or any frequency that is high compared to the time of the exposure intervals. This high frequency is a carrier wave which is modulated by the error signal.
Each detector amplifier 29, 32 demodulates and produces D-C bias voltage from the modulated signal and this voltage is applied to the control grids of the tetrodes 23, 24. When there is no reference voltage input from 27 the X-ray tube does not conduct and there is no error signal or modulation. The voltage on the control grids of vthe tetrodes is then maximum negative and the tetrodes are blocked.
An important characteristic of the system resides in the tetrodes behaving like a variable resistance. Before and after an exposure, tetrode 23 has nearly unlimited resistance to maintain the potential at point 48 at zero. During exposure, the voltage drop between its cathode and anode is constantly variable to maintain the potential at 48 precisely at the desired positive value. The effect is the same at point 48 for tetrode 24, except for the sign.
The available voltage of the system must be greater than the voltage which is applied to the X-ray tube 21 if the tetrodes 23 and 24 are to regulate properly. Thus the line voltage to the transformer primaries 2, 2' must be high enough to account for the voltage drop in secondaries 3 and 4, in the diodes -16 in the filters and the DC lines. The ripple of the D-C voltage appearing between lines 41 and 42 with respect to ground must not swing toward zero so much as to reduce the voltage on the X-ray tube below the desired total amount. This may be understood better by referring to FIGURE 3 which shows various voltages related to time. Above the zero axis or ground reference there is a twelve pulse ripple voltage wave 55 that represents the voltage on positive D-C line 41. Wave 56 is the counterpart for negative line 42. The square wave voltage held across the X-ray tube by the tetrodes has its maximum positive and negative amplitudes marked 57 and 58. It may be seen, for example, that there is a difference be. tween the minimum amplitude of the ripple 55 and the X-ray tube voltage 57 and that the tetrodes must be able to regulate within the range of this difference. Then, according to the invention, the voltage between 57 and 58 that is applied to the X-ray tube 21, is always smooth and no ripple is imposed on it because of the variable impedance action of the controlled tetrodes.
In one commercial design, about 0.0005 of the voltage applied to the X-ray tube appears across each resistor 46, 47. Thus when point 48 is kilovolts positive the comparator 28 gets a positive input of about 40 volts.
Means for controlling the X-ray tube current are not shown because they are not partof the invention and are known. Suflice it to say that tube current is controlled by regulating the temperature and hence the emissivity of the X-ray tube cathode.
FIGURE 1 shows that tetrodes 25 and 26 in a companion circuit are controlled in the manner of tetrodes 23 and 24 described above. Again, there is a reference voltage generator 33, a comparator 34, resistors 36 and 37, and amplifiers 35 and 38.
The invention operates as follows: all tetrodes 2326 being blocked, voltage is applied to the primaries of high voltage transformer 1. This puts a direct voltage on lines 41, 42. The timer, not shown, then may initiate an expo'sure by exciting the reference voltage generator 27 through closed switch 45. Because of the fast response rate a voltage appears almost instantaneously'on the COII'C? spond X-ray tube 21. Any error signal produced in the comparator 28 immediately varies the bias on the tetrode control grids to maintain a constant voltage on X-ray tube 21. Each X-ray tube is associated with a rapid film changer or a recording camera, neither being shown, for recording radiographs or picture taken from an image intensifier tube. The camera shutters may be 180 out of phase in a two X-ray tube system so that the aperture of one camera will be closed while the other is open. The camera may be provided with commutator switches symbolized by 45 and 45' which may control their corresponding reference voltage generators in synchronism witheach other.
A sequence of pictures of difierent aspects of an organ may be run at very high recording speeds. If the cameras run at pictures per second one of the X-ray tubes produces X-ray pulses of 2 milliseconds, for example, a width represented by the distance a-b in FIGURE 2, corresponding with an X-ray pulse rate of 100 per second; thus the pulses are separated by 10 millisecond time intervals f-k. The other X-ray tube also produces 2 millisecond X-ray pulses, as represented by the width c-d at the same rate but offset in relation to the first series of pulses by 5 milliseconds represented by the distances e-f. The hatched zones H indicate the times during which the shutter of the corresponding camera is closed.
From the foregoing description, it is evident that the following advantages have been achieved: (1) two or more X-ray tubes may be connected in parallel to the same high .voltage generator; (2) the instantaneous power which the generator must furnish corresponds with the power absorbed by a single tube only; (3) the high voltage on each X-ray tube is adjustable independently; and, (4) the effects of scattered radiation are minimized. As compared with the old practice of using two high voltage generators, there is only one in this case and it can be op-' erated at high speed'because the controls are purely electronic.
Having disclosed a preferred embodiment of the invention, many obvious modifications will now appear to those skilled in the art. Therefore, it is to be understood that the details of the construction disclosed herein are for the purpose of illustration, and not limitation, except as made necessary by the scope of the claim which follows.
It is claimed:
An X-ray systems comprising:
(a) a high voltage D-C source,
(b) a pair of X-ray tubes each having a cathode and an anode that are respectively connectable to opposite polarity sides of said source,
(c) a pair of tetrode switching tubes connected in series with each X-ray tube between its anode and cathode respectively and the source,
((1) a voltage divider circuit in parallel with each X-r-ay tube and including a resistor that is divided in parts at an intermediate point which is grounded, the said parts each producing first and second signals of opposite polarity with respect to ground and that are proportional to the voltage between the anode and cathode of the X-ray tube and ground,
(e) a pair of individual comparator means each having as one input the opposed ends, respectively, of each said resistors,
(f) a pair of adjustable reference voltage generators that produce separate series of pulses constituting a third signal voltage the amplitude of which corresponds respectively with the desired voltage and the duration of which corresponds with the desired conduction interval of each x-ray tube, the pulses being fed to another input of the individual comparators,
g) the said comparators producing error signals representative of the diflerence between the reference vo1tage pulses and the first and second signals,
(h) a pair of oscillators that are connected to receive the respective error signals and produce a high frequency carrier wave that is modulated by said error signals,
2,838,681 2,936,413 5/1960 Searcy 32118 (i) a detector amplifier means associated with each tetrode and having input terminals for receiving the respective modulated signals and output terminals connected to supply a demodulated D-C signal to the control grid of each tetrode respectively, whereby to control the voltage on the tetrodes and hence the voltage on the X-ray tubes at values corresponding with the reference voltage generator output,
(j) switch means in circuit with each reference voltage generator and adapted when opened to cause discontinuance of the reference voltage pulses, and when closed, to continue said pulses,
(k) whereby one X-ray tube is made conductive while the other is nonconductive by controlling said switch means.
References Cited UNITED STATES PATENTS 6/1958 Graves et al. 250-94 FOREIGN PATENTS 689,799 4/1953 Great Britain.
RALPH G. NILSON, Primary Examiner.
A. L. BIRCH, Assistant Examiner.