US3214589A

US3214589A - Protection system for rotating anode x-ray tubes including means for measuring the anode rotational speed

Info

Publication number: US3214589A
Application number: US239249A
Authority: US
Inventors: Walter E Teague
Original assignee: Picker X Ray Corp
Current assignee: Picker X Ray Corp
Priority date: 1962-11-21
Filing date: 1962-11-21
Publication date: 1965-10-26
Anticipated expiration: 1982-10-26

Description

Oct. 26, 1965 w. E. TEAGUE 3,214,589

PROTECTION SYSTEM FOR ROTATING ANODE X-RAY TUBES INCLUDING MEANS FOR MEASURING THE ANODE ROTATIONAL SPEED Filed Nov. 21, 1962 FIGI INVENTOR.

ATTORN YS WALTER E. TEAGUE BY% I M J l 23' W United States Patent ice 3,214,580 PROTECTION SYSTEM FOR ROTATING ANODE X-RAY TUBES INCLUDING MEANS FOR MEAS- URING THE ANODE ROTATIONAL SPEED Walter E. Teague, Silver Spring, Md, assignor to Picker X-Ray Corporation, Waite Manufacturing Division, Inc., Cleveland, Ohio, a corporation of Ohio Filed Nov. 21, 1962, Ser. No. 239,249 7 Claims. (Cl. 25093) The invention relates to a protection system for and method of energizing a rotating anode X-ray tube, and more particularly to a system and method that permits application of the plate voltage to the X-ray tube only when both the cathode filament is glowing and the anode is rotating at a proper speed.

X-rays are produced when a high velocity stream of electrons encounters a solid material. Of the special tubes designed to produce X-rays, the evacuated thermonic tube of the Coolidge type is widely used today. In op eration of tubes of this type, electrons are boiled off the hot filament of a cathode in a high vacuum and accelerated towards an anode of a solid material, such as tungsten, by an electric field produced by a voltage, known as the plate voltage, applied between the cathode and anode. When the electrons strike the solid anode, X-rays are produced. The penetrating ability, or hardness, of the X-rays depends on the velocity of the electrons striking the anode, and their velocity in turn depends on the magnitude of the plate voltage.

The medical profession now widely employs X-rays to, for example, determine abnormalities in a patients skeletal structure. been preferred in recent years because of their greater penetrating ability, reducing the exposure time of the patient to X-rays and also reducing the radiation dosage received by the patient. When such hard X-rays are produced, the high velocity stream of electrons bombarding the anode heats the anode, and tends to melt and destroy it. To prevent overheating of the anode, rotating anode X-ray tubes have been devised and are used today. Such X-ray tubes commonly have a frustoconically-shaped target surface on the periphery of a disc anode, and the anode is mounted within the tube envelope for rotation. 'Io rotate the anode, an electric motor is provided. The rotor of the motor is mounted within the tube envelope, and the anode is mounted on the rotor. The stator of the motor surrounds the tube envelope and rotor. To produce X-rays with such a tube, first electric power is applied to the cathode filament, to heat it to the proper temperature, and to the stator of the electric motor to rotate the rotor and attached anode of the X-ray tube. When the rotor and anode have reached the desired speed of rotation, typically from 2,000 to 4,000 revolutions per minute, and when the filament has reached the proper temperature and is glowing, boiling electrons off its surface, plate voltage is applied to the X-ray tube causing electrons from the cathode to bombard the anode, producing X-rays. Should electric power not be applied to the cathode, or should the cathode not have reached the proper temperature, application of the plate voltage will tend to pull ions rather than electrons from the cathode filament, ionizing the X-ray tube and rendering it useless. Should power not be applied to the electric motor, or should the anode not be rotating at the proper speed, bombardment of the anode with electrons will tend to overheat and destroy the anode target surface, also rendering theX-ray tube useless.

Protection systems have been devised to permit application of the plate voltage to a rotating anode X-ray tube only when the anode is rotating at the proper speed. One

For this purpose, harder X-rays have 3,214,589 Patented Oct. 26, 1965 such system in use today, disclosed in US. Patent No. 3,043,957 to E. B. Graves, indirectly determines the anode speed by detecting the electromotive force generated in the winding of the motor when the anode is rotating and when the energizing circuit is broken. When this electromotive force is of the proper magnitude, which indirectly indicates the proper anode speed, the protection system permits application of the plate voltage to the X-ray tube regardless of the condition of the cathode filament. However, although the electromotive force may be of the proper magnitude, yet the anode may not be rotating at the proper speed, as occasionally occurs when the tube is tilted and the frictional resistance to rotation of the rotor and anode increases. In this state, application of plate voltage by the protection system will tend to over heat and destroy the anode target surface, rendering the tube useless.

The protection system and method of the present invention directly determines both the anode: speed and the condition of the cathode filament, and permits applica tion of the plate voltage to the X-ray tube only when the anode has attained the proper speed and the filament has attained the proper operating temperature.

The protection system for a rotating anode X-ray tube of the present invention comprises means for energizing a cathode filament to glow and thereby emit radiation comprising electrons and light energy, said light energy being commonly classified according to frequency as infrared, visible or ultraviolet electromagnetic radiation or any combination thereof, means responsive to rotation of the anode for chopping the beam of radiation from the cathode filament into bursts, and means for sensing the bursts of light energy radiation and for permitting application of the plate voltage to the anode only when the bursts of light energy radiation are sensed, and preferably only when the frequency of the bursts of light energy radiation has reached a predetermined value. The rotating anode X-ray tube of the present invention includes means for producing a beam of light energy radiation from the glowing cathode filament, and means responsive to rotation of the anode for chopping the beam of light energy radiation into bursts.

The method for energizing a rotating anode X-ray tube of the present invention comprises the steps of energizing the cathode filament causing it to glow and thereby emit radiation comprising electrons and light energy, rotating the anode, sensing light energy radiation from the cathode filament, sensing the speed of rotation of the anode, and applying plate voltage to the anode only when both the sensed radiation of the cathode filament indicates that it is energized and the sensed speed of rotation of the anode indicates that it is rotating at the proper speed.

The accompanying drawings illustrate a preferred embodiment of the protection system of the present invention.

In the drawings:

FIG. 1 is a longitudinal sectional view of the rotating anode X-ray tube and a portion of its housing,

FIG. 2 is a schematic diagram of the control circuit, and

FIG. 3 is a longitudinal sectional view on a reduced scale of a modification of a portion of the structure shown in FIG. 1.

As shown in FIG. 1, the rotating anode X-ray tube 1 comprises an evacuated and sealed generally cylindrical glass envelope 2 in which is mounted a cathode assembly 3 and an anode assembly 4. The cathode assembly 3 generally is shaped like a crank with two cylindrical ends 6 and 7 separated by a fiat body 8. The longitudinal axis of end 6 of the cathode assembly is positioned coaxial with the axis of the X-ray tube envelope, and passes through one end of the tube envelope, the outer side of this end of the cathode assembly being sealed to the glass envelope. The axis of the other end 7 of the cathode assembly is parallel to the longitudinal axis of the X-ray tube and is offset therefrom by the flat body 8 of the cathode assembly. The outer end portion 9 of end 7 of the cathode is generally cup-shaped, and in this cup is mounted a cathode filament 11. The cup-shaped end portion 9 directs a beam of radiation outwardly from the cathode. Electrical leads 12 of the filament pass through a seal in end 6 of the cathode assembly, along body 8, and through end 7 to the cathode filament. These leads may be connected to a source of electric power to energize the cathode filament causing it to glow and emit radiation when desired.

The anode assembly 4 includes a cylindrical rotor 13 mounted for rotation in the glass envelope with its axis coaxial with the axis of the X-ray tube and end 6 of the cathode assembly. A threaded axle or stud 14 of reduced diameter projects from the end of the cylindrical rotor adjacent the cathode assembly. A disc-type anode 16 has an opening 17 at its center to receive stud 14 of the rotor, and a nut 18 is threaded onto the stud and forces the anode against the shoulder at the base of the stud, clamping the anode to the rotor and mechanically locking the anode assembly together. The disc anode has a frustoconical target face 19, of tungsten for example, adjacent its outer edge, positioned in front of and opposite the cathode filament 11 to receive electrons from the cathode filament.

The X-ray tube is placed within a lead housing 21 containing a transparent insulating and cooling oil bath 22, and supported by appropriate supports (not shown) to direct X-rays through a window 23 of the housing. A stator winding 24 is positioned about an elongated end of the tube envelope 2 extending from the main body. In the elongated end of the tube envelope the rotor 13 is supported for rotation. When electrical energy is applied to the stator winding from an appropriate source of power, the field produced by the stator causes rotor 13 to revolve, spinning anode 16. Such an arrangement is well known in the art, and is an arrangement commonly employed with rotating anode X-ray tubes.

An outwardly projecting anode terminal 26 is sealed to the elongated end of an envelope 2, and this terminal is electrically connected through rotor 13 to anode 16. Connections from a source of plate voltage are made to anode terminal 26 and through electrical connections 12 to the cathode assembly. Commonly, this is termed applying the plate voltage to the anode, since the cathode assembly often is grounded. Upon completing these connections, plate voltage is applied between the cathode assembly and anode 16 causing electrons to flow from the cathode filament when it is glowing to bombard the frustoconical face 19 of the anode, producing X-rays. By virtue of the angular relationship between the cathode filament and the frustoconical face of the anode, these X-rays are emitted from the anode in a direction generally towards window 23, and pass through this window to be used as desired. This operation of a rotating anode X-ray tube is well known in the art, and is an operation commonly employed with rotating anode X-ray tubes,

An opening or passageway 31 is provided through anode 16 radially inwardly of frustoconical face 19 and in the path of radiation from the cathode filament. When the anode is rotating, opening 31 will pass in front of the cathode filament and permit a beam of light energy radiation from the glowing cathode filament, that is, a cathode filament capable of emitting radiation comprising both electrons and light energy to pass through the anode and through the transparent tube envelope 2. A reflector 32 is mounted in the path of the light energy radiation passing through opening 31 on stator winding 24 to reflect this beam of radiation outwardly of the envelope and through a transparent window 33 in the housing 21. When the anode is rotated by rotor 13 due to energization of the stator winding 24, opening 31 will pass between cathode filament 11 and reflector 32 once each rotation, permitting a burst of light energy radiation to pass from the glowing cathode filament along an optical line of sight to reflector 32 which reflects this burst of radiation outwardly through the transparent window 33. A depression 34 of suitable size is provided in anode 16 diametrically opposite opening 31 to dynamically balance the anode for rotation.

A spaced grid of lead strips 35, mounted in a bracket 36, is attached to housing 21 on the outer side of window 33 to lie parallel to the direction of the bursts of radiation reflected from reflector 32 through window 33. A photocell 37 is attached to the housing and is positioned behind this grid to receive bursts of radiation reflected from reflector 32 and produce a pulsating electric output signal indicative of the bursts of radiation. Other devices electrically responsive to radiation may be used in place of the photocell. The grid of lead strips being so mounted permits substantially all of the bursts of radiation reflected from reflector 32 to pass to photocell 37, While preventing stray X-rays emitted by anode 16 from passing to the environment surrounding housing 21, since the lead strips are skewed with respect to the stray X-rays thus shielding the environment from stray X-rays.

A schematic diagram of the circuitry including photocell 37 for sensing the bursts of radiation is illustrated in FIGURE 2. The terminals of a plug 40 connect the circuitry to a source of electric power, such as the ordinary 60 cycle, volt electric power readily available from any ordinary wall outlet. An ordinary filament transformer 41 is connected across the terminals of plug 40 to supply filament voltage to the filaments of the vacuum tubes incorporated in the circuitry. The terminals of plug 40 are connected to

terminals

42 and 43 of a full wave rectifier 44. Thus, a 60 cycle alternating potential of 110 volts will exist between

terminals

42 and 43 of full wave rectifier 44. Across the other pair of

terminals

46 and 47 of the rectifier is connected a voltage divider comprising resistor 48 and potentiometer 49. Across

terminals

46 and 47 of the voltage divider will appear a full wave rectified 110 volts 60 cycle potential. Resistor 48 is of a much higher value than potentiometer 49, and thus most of the potential existing between

terminals

46 and 47 will exist between the terminals of resistor 48. The cathode of photocell 37 is connected to the slide contact 51 of potentiometer 49 and the anode is connected to terminal 46 of the rectifier. By varying the position of slide contact 51, the potential across the photocell may be adjusted. A Winding 52 of a tuned reed relay 53 and thyratron 54 are connected in series across resistor 48. The control grid of thyratron 54 is connected to the cathode of photocell 37. The suppressor grid of thyratron 54 is connected to the cathode of the thyratron. Due to the potential applied to photocell 37 there will normally be a slight current flow through photocell 37, termed the dark current. This current flow through potentiometer 49 produces a biasing potential between the control grid and cathode of thyratron 54, tending to cause it to conduct. By adjusting slide contact 51 of potentiometer 49, this biasing potential is set to a value which is just below the value required to cause thyratron 54 to conduct.

When a burst of reflected light energy radiation from the cathode filament strikes photocell 37 there will be a large current flow through the photocell causing a large potential to appear between the control grid and cathode of thyratron 54, which will cause thyratron 54 to conduct until the radiation impinging on photocell 7 ceases, at which time conduction also will cease.

Anodes of rotating anode X-ray tubes usually are rotated at speeds of from 2,000 to 4,000 revolutions per minute. Thus, from 2,000 to 4,000 bursts of radiation will strike photocell 37 per minute, causing from 2,000 to 4,000 pulses per minute, or from 33 to 67 pulses per second, to pass through the winding 52 of tuned reed relay 53 and thyratron 54 during normal operation of the X-ray tube. The contacts of the tuned reed relay 53 are set to close at the desired frequency of rotation of the anode. Thus, if the X-ray tube is designed for a speed of rotation of the anode of 3,500 revolutions per minute, the contacts of the tuned reed relay will be set to close at 3,500 pulses per minute. However, the setting of the tuned reed relay could be so low that any pulse would close it and plate voltage would be applied when any burst of radiation was sensed. When the setting of the tuned reed relay is reached by the anode, the contacts of the tuned reed relay 53 close, closing the circuitry of a timer 56. The timer is set for the desired duration of X-ray exposure. When the tuned reed relay closes, timer 56 completes the circuit to coil 57 of relay 58 causing contact 59 to close and apply plate voltage from a source of power 61 through transformer 62 to X-ray tube ll. Power is applied to coil 57 for the desired duration of the burst of X-rays as determined by the setting of timer 56, and when this desired duration of exposure is reached, timer 56 ceases to apply power to coil 57, and contact 59 opens removing plate voltage from X-ray tube 1.

By this arrangement, both the cathode filament 11 must be energized and glowing to emit radiation, and anode 16 must be rotating at the proper speed before the tuned reed relay 53 will close and apply plate voltage to the X-ray tube.

While an opening through the anode is preferred, other means responsive to the rotation of the anode for chopping the radiation from the cathode filament into bursts may be substituted. For example, a polished facet may be provided on the anode 16 to reflect radiation from the cathode filament to a photocell positioned to receive this radiation. Alternatively, as shown in FIG. 3, stud 14 of rotor 13' could be extended a substantial distance past nut 18 and an opening 63 provided through this stud to permit radiation from the cathode filament to pass through this opening in the stud to an appropriately positioned photocell 37' or other means for detecting the radiation.

In this embodiment, no lead strips are necessary in front of the photocell since few stray X-rays will be present on this side of the anode. This construction both simplifies the necessary modifications to the X-ray tube structure and positions the photocell opposite X-ray window 23 out of the operators way. As another alternative, rather than providing a depression 34 in the anode, another opening through the anode opposite the first opening may be provided. In the last two alternatives, two bursts of light would be received by the photocell for each rotation of the anode, rather than a single burst of light for each rotation of the anode as in the preferred embodiment. Accordingly, the setting of tuned reed relay 53 would have to be doubled. The full-wave rectifier and its connections to a source of electric power may be replaced by any other source of direct current potential as commonly would be available in an X-ray generator apparatus.

While the preferred embodiment of the invention has been described it is to be understood that various modifi cations may be made in the details of construction with out departing from the scope of the invention as set forth in the appended claims.

I claim:

1. In a protection system for a rotating anode X-ray tube which includes a cathode filament, an anode, means for rotating the anode, means for energizing the cathode filament to cause the filament to glow and thereby emit radiation comprising electrons and light energy and means for applying plate voltage to the anode; means directly rotatable with the anode for chopping the light energy radiation from the cathode filament into bursts of light energy radiation of a frequency proportional to the speed of rotation of the anode, and means for sensing the bursts of light energy radiation to measure the speed of anode rotation and means for applying the plate voltage to the anode only when said sensing means senses a predetermined minimum frequency of said bursts of light energy radiation whereby said X-ray tube is operated only when a predetermined minimum speed is exceeded.

2. The device of claim 1 wherein said means for chopping the light energy radiation into bursts of light comprises a hole in the anode aligned with said cathode filament.

3. The device of claim 1 wherein said anode is mounted on a stud and wherein said means for chopping the light energy radiation into bursts of light comprises a hole in the stud.

4. In a protection system for a rotating anode X-ray tube which includes a cathode filament, an anode, means for rotating the anode, means for energizing the cathode filament to cause the filament to glow and thereby emit radiation comprising electrons and light energy, and means for applying plate voltage to the anode; means rotatable with the anode for chopping the light energy radiation from the cathode filament into bursts; means for sensing the burst of light energy radiation and for applying a plate voltage to the anode, said last means comprising means responsive to the bursts of light energy radiation from the cathode filament for producing a pulsating electrical output signal; and means responsive to the frequency of the pulsating output signal for applying the plate voltage to the anode only when the frequency of the output signal is at least equal to a predetermined minimum value, whereby application of plate voltage to the anode is permitted only when both the cathode filament is glowing and the anode is rotating at least at a predetermined minimum speed.

5. A protection system for a rotating anode X-ray tube which includes a cathode filament, an anode, means for rotating the anode, means for energizing the cathode filament to cause the filament to glow and thereby emit radiation comprising electrons and light energy and means for applying plate voltage to the anode, comprising means responsive to rotation of the anode for chopping the light energy radiation from the cathode filament into bursts; means for sensing the bursts of light energy radiation and for applying the plate voltage to the anode, including means responsive to the bursts of light energy radiation from the cathode filament for producing a pulsating electrical output signal; means responsive to the frequency of the pulsating output signal for applying the plate voltage to the anode only when the frequency of the output signal is at least equal to a predetermined minimum value, whereby application of plate voltage to the anode is permitted only when both the cathode filament is glowing and the anode is rotating at least at a predetermined minimum speed; said means responsive to rotation of the anode for chopping the light energy radiation from the cathode filament into bursts comprising a passageway through the anode positioned, as the anode rotates, to be brought into the optical line of sight between said cathode filament and said means for producing an output signal during a revolution of the anode, thereby permitting light energy radiation to pass from the cathode filament through the opening and to the means for producing an output signal during a revolution of the anode.

6. A protection system for a rotating anode X-ray tube which includes a cathode filament, an anode, means for rotating the anode, means for energizing the cathode filament to cause the filament to glow and thereby emit radiation comprising electrons and light energy and means for applying plate voltage to the anode, comprising a passageway through the anode positioned to be brought in front of the cathode filament during rotation of the anode for chopping light energy radiation from the filament into bursts, means for directing the bursts of light energy radiation laterally with respect to the longitudinal axis of the X-ray tube, a photocell, means mounting the photocell in the path of the directed bursts of light energy radiation, whereby the photocell receives the bursts of light energy radiation and produces a pulsating electrical output signal therefrom, and frequency sensitive circuit means connected to the photocell for applying the plate voltage to the anode only when the frequency of the pulsating electrical output signal is at least equal to a predetermined minimum value, whereby application of plate voltage to the anode is permitted only when both the cathode filament is glowing and the anode is rotating at least a minimum speed.

7. A protection system for a rotating anode X-ray tube which includes a cathode filament, an anode, means for rotating the anode, means for energizing the cathode filament to cause the filament to glow and thereby emit radiation comprising electrons and light energy and means for applying plate voltage to the anode; comprising means responsive to rotation of the anode for chopping light energy radiation from the cathode filament into bursts; means for sensing the bursts of light energy radiation and for applying plate voltage to the anode, said lastmentioned means comprising means responsive to the bursts of light energy radiation from the cathode filament for producing a pulsating electrical output signal; means responsive to the frequency of the pulsating output signal for applying the plate voltage to the anode only when the frequency of the output signal is at least equal to a predetermined minimum value, whereby application of plate voltage to the anode is permitted only when both the cathode filament is glowing and the anode is rotating at least at a predetermined minimum speed; and a stud extending outwardly from said anode; said means responsive to the rotation of the anode for chopping the light energy radiation from the cathode filament into bursts comprising a passageway through said stud positioned, as the anode rotates, to be brought into the line of sight between said cathode filament and said means for producing an output signal during a revolution of the anode, thereby permitting light energy radiation to pass from the cathode filament through the opening and to the means to producing an output signal during a revolution of the anode.

References Cited by the Examiner UNITED STATES PATENTS 2,594,564 4/52 Kehrli 313- X 3,043,957 7/62 Graves 25093 3,062,960 11/62 Laser 250103 3,149,257 9/64 Wintermute 25099 FOREIGN PATENTS 684,655 12/ 39 Germany.

RALPH G. NILSON, Primary Examiner.

Claims

1. IN A PROTECTION SYSTEM FOR A ROTATING ANODE X-RAY TUBE WHICH INCLUDES A CATHODE FILAMENT, AN ANODE, MEANS FOR ROTATING THE ANODE, MEANS FOR ENERGIZING THE CATHODE FILAMENT TO CAUSE THE FILAMENT TO GLOW AND THEREBY EMIT RADIATION COMPRISING ELECTRONS AND LIGHT ENERGY AND MEANS FOR APPLYING PLATE VOLTAGE TO THE ANODE; MEANS DIRECTLY ROTATABLE WITH THE ANODE FOR CHOPPING THE LIGHT ENERGY RADIATION FROM THE CATHODE FILAMENT INTO BURSTS OF LIGHT ENERGY RADIATION OF A FREQUENCY PROPORTIONAL TO THE SPEED OF ROTATION OF THE ANODE, AND MEANS FOR SENSING THE BURSTS OF LIGHT ENERGY RADIATION TO MEASURE THE SPEED OF ANODE