US3502877A

US3502877A - Grid-controlled x-ray tube control system

Info

Publication number: US3502877A
Application number: US651835A
Authority: US
Inventors: Walter E Splain
Original assignee: Picker Corp
Current assignee: Philips Nuclear Medicine Inc
Priority date: 1967-07-07
Filing date: 1967-07-07
Publication date: 1970-03-24
Anticipated expiration: 1987-03-24
Also published as: GB1228252A; NL6806011A; FR1571598A; DE1764610A1

Description

March 24, 1970 w. EQSPL'AIN 3,502,877

GRID-CONTROLLED X-RAY TUBE CONTROL SYSTEM Filid July '7, 1967 2 Sheets-Sheet 1 TUBE [I6 24 CURRENT 1 8 GRID ems CONTROL I6 RF GRID ems EXPOSURE CONTROL 29 CONTROLLER DRIVER 3 WALTER fi iw BY WM QR v/ g/a dfiuv M ATTORNEYS March 24, 1970 w. E. SPLAIN 3,502,877

GRID-CONTROLLED X-RAY TUBE CONTROL SYSTEM Filed July 7, 1967 2 Sheets-Sheet 2 I la 33 24V 00 aoF TRBOC 2; L. 30E 63 5 39/" -32 Ci I 30H A 2 K; 33 Q06 34A 340 348 g :U y 48 as 47 49 INVENTOR.

WALTER E. SPLAIN BY Q/ I M AT TO RNE YS United States Patent 3,502,877 GRID-CONTROLLED X-RAY TUBE CONTROL SYSTEM Walter E. Splain, North Olmsted, Ohio, assignor, by mesne assignments, to Picker Corporation, White Plains,

N.Y., a corporation of New York Filed July 7, 1967, Ser. No. 651,835

Int. Cl. H05g 1/08 U.S. Cl. 250-93 26 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Field of the invention The present invention is directed to X-ray systems with electron stream control devices.

Description of the prior art In a grid-controlled X-ray tube, a control grid is selectively biased to control formation and intensity of an X- ray beam to be provided by the X-ray tube. In some known grid-controlled X-ray tubes, a focusing cup is built around the filament. It is used as the grid. Slight changes in the grid bias make a large difference in the X-ray beam. As the X-ray tube bias goes more negative, the focal spot gets smaller until it eventually pinche off. It is important that the apparatus passes through this region of pinch-off very rapidly because this period rep resents severe loading on the X-ray tube as electrons are concentrated. It is also desired to turn the X-ray tube on quickly for the same reason, i.e., to keep the concentration of the electron beam small.

In addition to rapid turn-on and turn-off, it is desired that the Xray tube may be turned on for any required period so that the operator is able to make any length of exposure or exposures desired. In radiography, for example, it is desired that the exposure period be as short as, for example, 100 microseconds. In fiuoroscopy, it is desired that the X-ray tube be able to be turned on and off repeatedly and that it is able to remain on for extended periods. In addition, it is desired that the entire apparatus be fail safe, so that in the event of a power failure or a component break-down, loss of control of the X-ray tube does not occur.

Some prior grid-controlled X-ray tube systems have had the capacity for rapid turn-on and turn-off. However, these prior systems have not always been able to provide long exposure periods and certainly not indefinite exposure periods. In one prior system, a switching tube is placed in series with the secondary of a high tension transformer of a conventional X-ray tube energizing circuit. The switching tube is able to withstand 125 kilovolts. Control of the switching tube is through an external power pulse, which is driven through a pulse transformer into the grid of the tube. Upon the application of such a power pulse, the voltage across the switching tube collapses and then appears across the X-ray tube. Because these prior systems use a pulse transformer, they are limited in duration as to how long the X-ray tube can be left on. A pulse transformer that can be used for a millisecond is practicable, although quite large. However,

a pulse transformer with a capability of use for 100 milliseconds would be impractical because of its size and cost. A transformer for use for more than 100 milliseconds is too large to even be considered. Thus, these systems are not capable of operation for 100 or 200 milliampere seconds which are, in fiuoroscopy, quite common exposures. In addition, even in those systems where longer periods are obtained by continuously pulsing the pulse transformer, the short recovery periods necessary for continuous operation all but eliminate the duty cycle of the pulse transformer.

Finally, a fault occurring in many prior systems results in complete loss of control of energization of the X-ray tube. In many of the prior systems, a component break-down, such as a filament failure with the loss of an auxiliary power supply, has left the control apparatus incapable of turning the X-ray tube off.

Summary of the invention In the present system, the X-ray tube system including its control grid is operated at a potential level which is, for example, kilovolts below ground potential. The control circuitry including an exposure timer is operated at ground potential. A very high frequency carrier is used to bridge this potential gap to effect control of energization and de-energization of the X-ray tube. The carrier is keyed for the desired X-ray tube exposures and then transmitted across the potential gap of 75 kilovolts by a system including an air core transformer. The carrier is a radio frequency (RF) signal of, for example, 10 megacycles. The keyed carrier signal is then rectified or demodulated, and the resulting control signal used to control operation of switching means connected in series with the X-ray tube.

The present system has the advantage that the coupling across the voltage gap is relatively free of capacitance. In addition, there is no kilo-voltage build-up or decay that could inadvertently cause excitation of the grid-controlled system.

The keyed megacycle carrier signal provides for extremely fast turn-on and turn-off as the speed of switching is limited only by the frequency of the carrier. Any length of exposure period can be provided as the carrier can be transmitted indefinitely across the potential gap to provide indefinite energization or de-energization of the X- ray tube. Pulsing is not required, but can be accomplished by selective keying of the carrier if desired.

In the present system, the anode of the X-ray tube is at a positive voltage and the cathode is operating at the negative level of the kilovoltage required for the exposure, for example, 75 kilovolts. The grid of the X-ray tube i biased, for example, 4,000 volts negative relative to the cathode, to turn the X-ray tube off and is then biased to cathode potential to turn the X-ray tube on. In turning the X-ray tube on, the system has the capability of driving the control grid positive by a constant potential source in the grid circuit. A unidirectional currentconducting device is provided to hold the grid at zero bias as the grid voltage tends to go positive.

This assures that the control grid is always at zero bias during the pulse. In addition, it enhances turn-on and turn-off because the control system is capable of driving the control grid to a voltage which is higher than the terminal voltage. Maintaining the control grid voltage at zero bias in this manner eliminates slight variations in the grid voltage which would change the geometry in the X-ray tube electrically and result in changes in the focal spot size.

Thus, in the present system, when the X-ray tube is biased to an on condition, the X-ray beam. does not reduce in intensity or charge concentration. The focal spot gets smaller as the X-ray tube is biased off and until the beam is pinched off.

The switching means, which control energization of the X-ray tube, is preferably connected to the cathode element of the X-ray tube. An exposure control circuit is connected to the switching means so that when the switching means is turned on, the X-ray tube in energized. Resistive circuit means is connected across the switching means so that when the switching means is turned off, the voltage rise across the switching means is limited to a predetermined value. This predetermined voltage is applied between the cathode and the control grid of the X-ray tube to bias the control grid negatively by said predetermined voltage relative to the cathode of the X-ray tube.

If the switching means should fail, it fails in an open circuit and the X-ray tube is not energized. If current should try to flow in the X-ray tube for any reason, and the switching means is cut off, then the element of the switching means connected to the cathode of the X-ray tube tends to go more positive. The voltage cannot rise beyond the predetermined voltage as the circuit means limits the voltage. This predetermined voltage is a reverse bias on the control grid of the X-ray tube and holds the X-ray tube off during the fault condition.

The present invention will be better understood by those skilled in the art from the following specification and the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a schematic block diagram of the X-ray apparatus of the present invention;

FIGURE 2 is a detailed circuit diagram of the radio frequency exposure controller of the X-ray apparatus of the present invention;

FIGURE 3 is a detailed circuit diagram showing the details of a grid bias control driver and a tube current and grid bias control circuit of the X-ray apparatus of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGURE 1, a grid-controlled X-ray tube 11 having anode and

cathode elements

12, 13 and a control grid element 14 is shown. A suitable X-ray tube is marketed by Machlett Laboratories, Inc., Springdale, Conn., under the trademark Dynamax 50. The anode and

cathode elements

12, 13 are connected to a voltage source E via anode and

cathode supply conductors

15, 16 respectively.

A tube current and grid bias control circuit 21 is interposed in the cathode supply conductor 16. The tube current and grid bias control circuit 21 is connected to the control grid 14 via a conductor 23 and controls both the X-ray tube current and the control grid bias in response to a control signal on a control conductor 24 from a grid bias control driver 25. The grid bias control drive 25 provides the control signal to the tube current and grid bias control circuit 21 in response to a radio frequency (RF) signal from an RF exposure controller 26 via a conductor 27 upon closure of an exposure control switch 28 by an exposure timer control 29.

Referring to FIGURE 2, the radio frequency exposure controller referred to hereinafter as the RF exposure controller, includes a megacycle oscillator 30, which supplies a 10 megacycle sine wave through an amplifier 31 to a primary winding 32 of an air core, radio frequency signal transformer 33. A transistor switching or clamping circuit 34 is connected to the output of the oscillator 30 and supplies the 10 megacycle sine wave to the amplifier 31 in response to a positive exposure control pulse supplied to its input conductor 35 from the exposure control switch 28.

The oscillator 30 includes a lO-rnegacyle crystal 36 and a transistor 37 connected in a conventional oscillator circuit which provides a 10 megacycle signal via a conductor 38 and a capacitor 39 to the base of a commonbase configuration transistor 40. A resistor 41 and a capacitor 42 are connected in parallel between the emitter of the transistor 37 and the emitter of the transistor 40. The collector of the transistor 37 is connected to one side of the crystal 36 and, through a resistor 30A and a current-limiting resistor 30B, to a source of +24 volts DC (not shown). The other side of the crystal is connected through a capacitor 30C to the base of the transistor 37, which is also connected to the +24 volt supply through a resistor 30D and to the base of the transistor 40 through a resistor 30E. A resistor 30F is connected across the

resistors

30D, 30E. The base of the transistor 40 is connected to the base of a transistor 43 through a resistor 34A. A Zener diode 34B and a capacitor 34C are connected in parallel between the base of the transistor 40 and ground. A capacitor 306 connected between the +24 volt supply and ground smooths ripples from the DC suppply voltage. A capacitor 30H is connected to ground from the end of the resistor 30B remote from the 24 volt power source. The 10 megacycle signal is developed on the collector of the transistor 40.

In the transistor switching circuit 34, the transistor 43 is normally biased to a saturated state and clamps the collector of the transistor 40 and the 10 megacycle signal produced thereon to ground via a

conductor

45, 34 and a ground level conductor 46. A positive exposure control pulse introduced on the conductor through a resistor 47 biases the base of a transistor 48, which is a driving transistor for the transistor 43. The collector of the transistor 48 being connected to the base of the transistor 43. The base of the transistor 48 is connected to ground through a resistor 48A, and its emitter is connected directly to ground. The positive voltage signal supplied to the base of the transistor 48 turns the transistor 48 on, thereby reducing the base-emitter voltage of the transistor 43 to zero and turning the transistor 43 off. The transistor 43 in an off condition releases its clamp on the collector of the transistor and a conductor connected to it. The 10 megacycle RF signal then appears across a resistor 49 in the collector circuit of the transistor 40, and is applied via the conductor 45 to the base of a transistor 50 in the amplifier circuit 31. A capacitor 52 and a resistor 53 are connected in parallel between the emitter of the transistor 50 and the ground conductor 46. The collector of the transistor 50 is connected through a resistor 50A to the end of the resistor 30B remote from the power supply. A capacitor 54 is connected between the collector of the transistor 50 and the base of a transistor 55. A resistor 56 is in turn connected between the base of the transistor and the conductor 46. The transistor 50 and the transistor 55 together comprise a radio frequency amplifier for raising the ten megacycle potential from a small signal level to a meaningful power level. The collector of the transistor 55 is connected to the 24 volt power source through a tuned circuit comprising an adjustable capacitor 62 and the primary winding 32 of the transformer 33. The tuned circuit is tuned to the frequency of the RF carrier (10 rnegacycles), so that a 10 megacycle RF signal is impressed across the primary winding 32 of the transformer 33.

Referring to FIGURE 3, a secondary winding 63 of the radio frequency signal transformer 33 together with a variable capacitor 64 connected across secondary winding conductors 27, 27' form a tuned circuit for the radio frequency control signal provided by the RF exposure contoller 26. Faraday shielding 65, 65 is provided on the secondary side of the transformer 33 to control the amount of extraneous 10 megacycle radiation in the area and to reduce capacitance effects between the primary winding 32 and the secondary winding 63, The radio frequency control signal is rectified by

diodes

67, 67 and thereafter appears as a DC exposure control signal across

conductors

68, 68. The appearance of a DC control signal across

conductors

68, 68 signals the desired initiation of energization of the X-ray tube 11.

A conductor 70 is connected from the grid bias control circuit 25 to the cathode kilovoltage supply conductor 16, which, for example, is normally at 75,000 volts below ground potential. This connection places the grid bias control driver circuit 25 approximately 75,000 volts below ground potential. The RF exposure controller 26, as coupled to the grid bias control driver 25 via the radio frequency signal transformer 33, effectively bridges the kilovoltage gap between the grid bias control driver circuit 25 and the input signal exposure control pulse provided by switch 28 which is at a maximum of a few volts potential. Thus, the exposure control signal is transmitted across the kilovoltage gap by using it to key or modulate the very high frequency carrier signal provided by the oscillator circuit 30, transmitting the carrier signal to the grid bias control driver circuit 25 through the air core transformer 33, and then demodulating the signal to provide an exposure control signal to the driver circuit which is operating at 75,000 volts.

The DC exposure control signal appearing on the conductor 68 is supplied to the base of a transistor 72 which is connected as a driver for a switching transistor 73. The signal is applied across a resistor 74 connected be tween the conductors 68 and 68'. In the absence of a positive exposure control signal on the conductor 68, the collector of the transistor 72 is generally more positive than its base by virtue of the bias provided across

Zener diodes

75, 76 which are in turn connected in series with a resistor 75A across the output terminals of a full wave bridge rectifier 77. A capacitor 75B is connected across the diode 75. The input terminals of the bridge rectifier 77 are supplied from a secondary winding 78 of a supply transformer 79 having its primary winding 80 con nected to a 120 volt power supply through an isolation transformer 81. A smoothing capacitor 77A is connetced across the rectifier output terminals. The isolation transformer 81 has a turns ratio of 1 to l and is insulated so that it can withstand the -75 kilovolts at which the grid bias control driver circuit 25 operates.

The collector of the transistor 73 is connected to a negative output terminal of a bridge rectifier 85 which is also connected to the conductor 70. Input terminals of the rectifier 85 are connected across a secondary winding 86 of the transformer 79. A resistor 77A connects the positive output terminal of the rectifier 77 to the negative output terminal of the rectifier 85.

The positive voltage signal on the collector of the transistor 72 normally maintains the transistor 72 in an off or non-conducting condition. A positive exposure control signal appearing between the

conductors

68, 68 forward bias the transistor 72, The emitter of the transistor 72 is connected directly to the base of the transistor 73 and through a resistor 74A to the conductor 68'. Thus the transistor 72 is connected in an emitter-follower relation with the transistor 73, and forward biases the baseemitter junction of the transistor 73 when the transistor 72 conducts. This turns on the transistor 73 to effectively connect conductor 70 to the conductor 68. The

resistors

74, 74A connected to the bases of the

transistors

72, 73 take away any stored base charge when the

transistors

72, 73 are turned off.

The conductor 70 is connected to the cathode elements of a pair of high current,

vacuum tubes

91, 92, which are connected in parallel and will hereinafter be referred to as switching tubes. The conductor 68' is connected via conductors 95, 95' and

resistors

103, 104 to the control grids of the switching

tubes

91, 92 respectively. The

resistors

103, 104 aid in preventing parasitic oscillations in the grid circuits of the switching

tubes

91, 92.

The switching

tubes

91, 92 connected in parallel are in series in the kilovoltage supply conductor 16 via the conductor 70. The switching

tubes

91, 92 are pentodes in order to keep the voltage drop in the tubes low when they are saturated. Suitable pentode tubes are those designated as 6DQ5, because of their rather high standofl? voltages and their high current capacities. The plateto-cathode circuits of the switching

tubes

91, 92 can Withstand differences of from 8,000 to 10,000 volts, and each has peak current conducting capabilities of above an ampere. Together the switching

tubes

91, 92 provide a total shunt current capacity of well over two amperes. The plate-to-cathode circuits of the

swtiching tubes

91, 92 are connected in series with the X-ray tube 11 and function as switching tubes to control the current supplied to the X-ray tube.

The anodes of the switching tubes 90, 91 are con nected to a positive output terminal of a bridge rectifier 121 through a current-limiting resistor 123 and a diode 124 connected in series. A negative output terminal of the rectifier 121 is connected to the conductor 70. Input terminals of the rectifier 121 are connected across a secondary winding 120 of the transformer 79 to supply approximately 3,500 volts output from the rectifier 121. A resistor 121A and a smoothing capacitor 121B are connected in parallel across the output terminals of the rectifier 121.

The anodes of the switching

tubes

91, 92 are also connected by the conductor 16 to the cathode 13 of the X-ray tube 11. This makes the grid 14 of the X-ray tube 11 approximately 3,500 volts negative with respect to the cathode 13, when the switching

tubes

91, 92 are cut off. This potential difference is sufiicient to maintain the X-ray tube cut off.

The screen grids of the

tubes

91, 92 are connected through 100 ohm

resistors

99, 100 respectively, to the cathode of a Zener diode 82 and also to the positive output terminal of the bridge rectifier via a conductor 84 and a current-limting resistor 102. The Zener diode 82 has a breakdown voltage of volts. The anode of Zener diode 82 is connected to a negative output terminal of the bridge rectifier 85 via the conductor 70 so that the approximately 400 volts supplied by the rectifier 85 provides effectively a 100 volt constant potential source at the cathode of Zener diode 82 to bias the screen grids. A capacitor connected between the

conductors

70, 84 acts as a filter capacitor for the rectifier 85. The suppressor grids of the

tubes

91, 92 are connected to the cathodes of the tubes.

The control grids of the switching

tubes

91, 92 are also connected to the anode of a Zener diode: 93 through the

resistors

103, 104 respectively. When no exposure control signal to the grid bias control driver circuit 25 appears across the transformer 33 and the

transistors

72, 73 are off, the control grids are biased by the bridge rectifier 77 at a negative volts and held there by the clamping action of the Zener diode 93. This negative bias maintains the tubes 90, 91 cut off until the transistor 73 conducts and connects the conductors 68', 70 together to remove the negative bias from the control grids of the

tubes

91, 92. When the transistor 73 is turned on in response to an exposure control signal, the control grids of

tubes

91, 92 are effectively connected directly to the cathodes of the

tubes

91, 92 to remove any negative bias and turn on the switching

tubes

91, 92.

A protective arc gap 96 is connected across the switching

tubes

91, 92 via

conductors

16, 70. The are gap 96 fires when the voltage across the switching

tubes

91, 92 exceeds 4,000 volts. An arc gap 97 is connected across the

conductors

70, 23 and fires when the voltage across it exceeds 500 volts. The conductor 23 connects the grid 14 of the X-ray tube 11 to the arc gap 97 via a 1,000 ohm current-limiting resistor 106. A diode 107 has its anode connected to the grid 14 and its cathode connected to the cathode 13 of the X-ray tube 11. The resistor 106 and the diode 107 are actually located within the X-ray tube 11. The diode 107 serves to prevent the grid 14 from going positive with respect to the cathode 13 when the

tubes

91, 92 conduct.

If a fault occurs in the system that might cause the X-ray tube 11 to flash over and be destroyed, the

spark gaps

96, 97 fire to divert current from the X-ray tube. This occurs if the grid-to-cathode voltage exceeds 4,500 volts. In the absence of the spark gaps, the entire switching means might be destroyed.

The control grid 14 of the X-ray tube 11 is connected via the conductor 23,

resistors

106 and 108 and the conductor 84 to the positive output terminal of the bridge 85. When the switching

tubes

91, 92 begin to conduct, the control grid 14 tends to be driven positive by the positive output of the rectifier 85, and the X-ray tube is turned on. In other words, when the switching

tubes

91, 92 are turned on to turn on the X-ray tube 11, most of the negative bias on the control grid 14 is removed so that it may go substantially to zero bias. There is still some voltage drop across the switching

tubes

91, 92 so that the grid bias on the X-ray tube is not entirely removed. This is overcome by the positive potential provided by the bridge 85, which causes the control grid 14 to tend to swing positively and effectively ties it to the cathode 13 through the diode 107. The control grid 14 will be prevented from swinging positively past zero bias by the action of the diode 107.

In an X-ray system of the type described, there is generally a large cable distributed capacitance, which appears when the X-ray tube is suddenly turned off. The cable capacitance appears as a capacitor between the grid and cathode of the X-ray tube 11. The cable capacitance tends to prevent the grid bias from rapidly going to the negative voltage necessary to maintain the X-ray tube turned off. Thus, the cable capacitance allows only an exponential build-up of the grid bias voltage and results in a relatively slow turn-off of the X-ray tube.

Additional energy is supplied to the system to effectively charge this cable capacitance. The additional energy is supplied by the full Wave bridge rectifier 121. The resistor 123 and diode 124 couple this power supply to the cathode so that the 3,500 volts power supply can supply energy when required and not load down the main supply E. The additional power provided by the rectifier 121 in charging the cable capacitance is an aid to recovery of the system and facilitates a fast turn-off of the X-ray tube.

A high value resistor 130 connects the cathode of the X-ray tube 11 to a ground point and functions as a bleeder circuit. A high value resistor 131 and a Zener diode 132 are connected in series with each other and from the anode of the X-ray tube 11 and hence from the positive high voltage supply to a ground point. The resistor 131 and the Zener diode 132 function as a bleeder circuit for the anode circuit of the X-ray tube.

As an example, the

bleeder resistors

130, 131 have very high resistance values of the order of 60 megohms each. The bleeder circuit provided by the

resistors

130, 131 function to discharge external capacities in the system that may have been supplied for energy storage purposes as in asynchronous cine. The juncture of the resistor 131 and the Zener diode 132 provides a convenient metering point 134 for the high voltage on the anode of the X-ray tube. A milliammeter (not shown) may be connected to the point 134. The Zener diode 132 prevents the voltage at the metering point 134 from rising when the metering point 134 is not being used.

Capacitors

140, 141 are respectively connected to the conductors 15, 94 to com pensate for inductances in the high voltage system and to provide current to the X-ray tube 11 during extremely short exposure times.

The bleeder resistor 130 connected to the cathode 13 facilitates the development of the X-ray tube bias voltage, and enhances the ability of the system to operate under faulty conditions. When the system is turned on, the switching

tubes

91, 92 in their off condition present an open circuit to the voltage present in the conductor 16. The X-ray tube in its off condition also prevents current flow to the conductor 15. The current can then only be present through

resistors

135, 136, 137 and a Zener diode 138. The resistor 130 completes a circuit for the current from the

conductors

70 and 16. The diode 124 acts as a disconnect diode in this situation preventing any current through the resistor 130 flowing back into the rectifier 121. The current present in the resistor 130 develops a bias voltage across that resistor for the X-ray tube 11, which turns it off. The current from the rectifier 121 flowing through the

resistors

135, 136, 137 and the diode 138 aids in developing that bias voltage, although either source of bias voltage is, in itself, sufiicient to maintain the X-ray tube cut off. The current through the

resistors

135, 136, 137 and the diode 138 is diverted and flows through the switching

tubes

91, 92 when they are turned on.

The present grid-controlled X-ray tube system is thus capable of operation in the event that either or both of the auxiliary power supplies 85, 121 fail and the main power supply remains on. The system is capable of turnoff or even turn-on in the absence of the auxiliary power supplies 85, 121. The function of the auxiliary power supplies is essentially to enhance the performance of the system to provide minimum turn-off and turn-on times.

The auxiliary power supply enhances the performance of the system to provide extremely fast turn-off and turn-on times. For example, in the present system using a 10 megacycle carrier in the RF exposure controller 26, a switching time of 0.1 millisecond is readily obtainable and it is possible to switch in as little as .02 millisecond. Thus, extremely short exposures are possible.

In the event the power supply 121 fails, sufficient anodeto-cathode potential is provided by the voltage drop across the resistors 135, 156, 137 and the diode 138 to turn on the switching

tubes

91, 92 in the presence of a control signal from the grid control driver 25, although the turn-on and turn-off times will be lengthened somewhat. The same is true in the event of failure of the power supply 85 that supplies a positive potential to the grid 14 of the X-ray tube. If the power supply 77 fails, no signal can be supplied to the control grids of the switching

tubes

91, 92 to turn them on, and the X-ray tube 11 will remain in a cut-off state. Thus, the system is essentially fail-safe.

Additionally, the system is capable of making long exposures of any duration without modification of the system. The use of the RF carrier to bridge the DC voltage gap enables the system to turn the X-ray tube on for any length of time. Thus, almost any X-ray exposure period can be provided from as little as 0.5 millisecond to continuous operation such as is used in cine.

Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and scope of the invention as hereinafter claimed.

What is claimed is:

1. An X-ray apparatus comprising:

(a) an X-ray tube having anode and cathode elements and a control grid element;

(b) power supply means connected to said anode and cathode elements and providing an energizing potential across said elements with the anode element being at a first predetermined potential and the cathode element being at a second predetermined potential;

(c) current switching means connected in circuit with said cathode element, said anode element and said supply means, said switching means having first and second states for respectively permitting and cutting off current flow between said anode and cathode elements;

((1) exposure control circuit means connected to said switching means for selectively switching said switching means from one of said states to the other;

(e) control grid circuit means connected to the control grid element of the X-ray tube and to said switching means to reverse bias said control grid element when said switching means is in said second state and to forward bias said control grid element when said switching means is switched to its first state by said exposure control means; and,

(f) voltage sensitive means in said control grid circuit means for maintaining said control grid element in said reverse bias condition when said switching means is in said second state.

2. The X-ray apparatus of claim 1, further including clamping means in said control grid circuit means for maintaining said control grid element at said second predetermined potential when said switching means is in said first state.

3. The X-ray apparatus of claim 1, wherein said voltage sensitive means is a voltage breakdown device.

4. The X-ray apparatus of claim 2 wherein said clamping means is a unidirectional current conducting device connected between said cathode and control grid elements.

5. The X-ray apparatus of claim 2, wherein said control grid circuit means includes a potential source connected between said cathode and control grid elements and providing a control grid bias on the control grid element to drive the control grid element bias toward said first predetermined potential relative to said second predetermined potential.

6. The X-ray apparatus of claim 5 wherein said clamping means comprises a unidirectional current conducting device connected from said control grid element to said cathode element so that said control grid element is tied to said cathode element and is at the potential of said cathode element as said control grid element is driven from its reverse bias condition to said second predetermined potential.

7. The X-ray apparatus of claim 1, wherein said switching means includes at least one heavy current electronic switching device.

8. The X-ray apparatus of claim 7, wherein said switching device has an anode, a cathode and a control element, said anode being connected to said X-ray tube cathode element, said cathode being connected to said power supply means, and said control element being connected to said exposure control circuit means.

9. The X-ray apparatus of claim 1, including auxiliary power supply means connected to said control grid and cathode elements to supply energy for charging an intrinsic cable capacitance of the apparatus when switching said switching means from one state to another.

10. The X-ray apparatus of claim 9 including a unidirectional current device connected in said auxiliary power supply circuit between said control grid and said cathode elements and being polarized to allow current flow to said cathode element.

11. An X-ray apparatus comprising:

(a) an X-ray tube having anode and cathode elements and a control grid element;

(b) power supply means connected across said anode and cathode elements for supplying energizing potential thereto;

(c) X-ray tube current and grid bias control means connected to said elements for selectively energizing and de-enerigzing said X-ray tube;

(d) exposure control means for providing a radio frequency control signal for the duration of a desired X-ray exposure; and,

(e) coupling means for connecting said exposure con trol means to said X-ray tube current and grid control circuit so that said X-ray tube is energized in response to said control signal.

12. The X-ray apparatus of claim 11, wherein said coupling means includes an air core transformer.

13. The X-ray apparatus of claim 12, wherein said coupling means further includes a transistor switching circuit having control elements connected to an output of said air core transformer and switching elements controlling energization and de-energization of the X-ray tube by means of said tube current and grid bias control means.

14. The X-ray apparatus of claim 13, wherein said control signal provided by said exposure control circuit is a high frequency signal.

15. The X-ray apparatus of claim 14 wherein said high frequency signal is generally of a frequency of 10 megacycles.

16. The apparatus of claim 12, wherein separate tuned circuits are formed with primary and secondary windings of said transformer and are tuned to the frequency of said control signal from said exposure control circuit.

17. The X-ray apparatus of claim 13, wherein said coupling means also includes means for providing a direct current signal to said tube current and grid bias control means in response to said radio frequency control signal from said exposure control means.

18. The X-ray apparatus of claim 11, wherein said exposure control means comprises a radio frequency oscillator having an output, and means for selectively providing said radio frequency signal from said output to said coupling means.

19. The X-ray apparatus of claim 18, wherein said means for selectively providing said radio frequency signal includes means for selectively grounding said output of said oscillator.

20. The X-ray apparatus of claim 11, wherein said X-ray tube current and grid bias control means includes current switching means connected in circuit with said cathode element and said supply means, said switching means having first and second states for respectively permitting and cutting off current flow between said anode and cathode elements.

21. The X-ray apparatus of claim 20, wherein said switching means includes at least one heavy current electronic switching device.

22. The X-ray apparatus of claim 21, wherein said switching device has an anode, a cathode and a control element, said anode being connected to said X-ray tube cathode element, said cathode being connected to said power supply means, and said control element being connected to said coupling means.

23. The X-ray apparatus of claim 20, wherein said X-ray tube current and grid bias control means also includes means connected to the control grid element of the Xray tube and to said switching means to reverse bias said control grid element when said switching means is in said second state and to forward bias said control grid element when said switching means is switched to its first state by said exposure control mean.

24. The X-ray apparatus of claim 23, further including clamping means in said control grid circuit means for maintaining said control grid element at said second predetermined potential when said switching means is in said first state.

25. The X-ray apparatus of claim 23, wherein said voltage sensitive means is a voltage breakdown device.

26. The X-ray apparatus of claim 24 wherein said clamping means is a unidirectional current conducting device connected between said cathode and control grid elements.

References Cited UNITED STATES PATENTS 3,103,591 9/1963 Rogers et al. 250-93 ARCHIE R. BORCHELT, Primary Examiner S. C. SHEAR, Assistant Examiner U.S. Cl. X.R. 250--95