US 2878393 A
Description (OCR text may contain errors)
March 17, 1959 E. B. GRAVES X-RAY TUBE WITH VARIABLE FOCAL SPOT AND CONTROL MEANS THEREFOR Filed Jan. 12, 1955 INVENTOR. fowl/v0 B- GR W S qrronwtro United States Patent X-RAY TUBE WITH VARIABLE FOCAL SPOT AND a CONTROL MEANS THEREFOR Edward B. Graves, University Heights, Ohio, assignor to Picker X-Ray Corporation, Waite Manufacturing Division, Inc., Cleveland, Ohio, a corporation of Ohio Application January 12, 1955, Serial No. 481,377
4 Claims. (Cl. 250 -99) This invention relates to improvements in an X-ray tube with a variable focal spot and control means for the same.
One of the objects of the present invention is an improvement in an X-ray tube having a filament adapted to emit a stream of electrons striking the focal spot on an anode and having electrical means for energizing the tube including means controlling an energizing electrical supply to the tube filament to increase and decrease the milliamperage flowing in the tube, the improvement comprising means for controlling the size of the focal spot and a single control means for varying this focal spot size in step with the milliamperage flowing in the tube.
Another object of the present invention is the provision of an electromagnetic focusing coil embracing the stream of electrodes passing from the filament to the anode of the X-ray tube with means for supplying electrical current to the coil, the arrangement being such that the current in said focusing coil is increased or decreased in step with the adjustment of the increase and decrease respectively of the milliamperage flowing in the tube.
A further object of the present invention includes means for varying the kilovoltage supplied to the X-ray tube and means responsive to the variation of the kilovoltage for changing the size of the focal spot.
Still a further object of the present invention is the provision in combination With an X-ray tube as described in the preceding paragraphs, of a control grid adjacent the filament of the X-ray tube and positioned to control the emission of electrons from the filament, together with control means for supplying to the control grid a potential negative with respect to the filament and completely blocking electron fiow from the filament, this block being removed when it is desired to initiate an exposure, whereby the selected conditions for operation of the X-ray tube may be pre-selected with reference to milliamperage flow, kilovoltage and focal spot size, after which the exposure may be initiated simply by removing the negative bias from the control grid.
Other objects and advantages of the present invention will be apparent from the accompanying drawing and description and the essential features thereof will be set forth in the appended claims.
In the drawing, an X-ray tube is shown somewhat diagrammatically as having the usual glass wall in one end of which is embedded the filament 11, and in the other end of which there is fixed the anode 12. In the present embodiment of the invention, the anode end of the tube is grounded so that the anode 12 may be water cooled if desired but this forms no part of the present invention.
Electrical means is provided for energizing the X-ray tube and this is shown as comprising an auto'transformer 13 supplied from the line source L1, L2 and having suitable taps connected to a kilovoltage selector 14 which in turn is connected to line 15 and switch 16 to the primary 17a of the high voltage transformer 17. i The other 1 any other suitable source.
we show a transformer 39 utilized for this purpose and end of the transformer primary is connected through lines 18 and 19 back to the auto-transformer. The secondary 17b of the high voltage transformer is connected by line 20 to the filament end of the X-ray tube, and is connected by line 21 through the milliamperage stabilizer, 22, line 23, milliammeter 24 and line 25 to ground.
The energizing electrical supply for the filament 11 comprises the secondary 26b of transformer 26 which is connected by lines 27 and 28 to the filament 11. The primary 26a of this transformer is connected by line 19 to one tap on the auto-transformer 13. The other end of the primary winding 26a is connected by line 29 through milliamperage stabilizer 22, line 30, resistance 31, line 32, milliamperage stabilizer 22, line 33, variable resistance 34, resistance 35 with its various taps, the filament current selector 36 and line 37 back to another tap on the'auto-transformer. As mentioned above, one of the objects of the present invention is to vary the focal spot size as the milliamperage is changed on the X-ray tube. To this end, an electromagnetic focusing coil 38 is provided around the anode stem 10a of the X-ray tube so as to provide the known focusing effect on the stream of electrons passing from the filament 11 to the anode 12. A suitable source of energizing electricity is provided for the focusing coil and this might be a battery or In the present embodiment,
having its secondary 39b connected across lines 40 and 41 utilizing the rectifier 42 and condenser 43 to supply rectified current to the focusing coil. The line 40 is connected through the focal spot control deck of a control monitor 44 which has various taps connected to points on resistance 45 which in turn is connected through line 46 to the focusing coil.
The device shown at 47 is a milliamperage calibration deck on the monitor control which has a plurality of taps connected at spaced points on the resistance 31 previously described.
It results from the above described construction, that the X-ray tube 10 may be operated as follows: The various decks of the monitor control are shown diagrammatically connected by the means 48 so that they may all be adjusted simultaneously. The operator adjusts the monitor control means 48 to move the filament input selector 36 and the milliamperage calibration device 47 to the desired point to provide the desired milliamperage flow through the tube. At the same time, the monitor deck 44 moves the focal spot control device so that as the milliamperage flow through the X-ray tube is changed,
the current in the focusing coil 38 is changed by a proportionate change in resistance 45. This gives the X-ray the potential on the focusing coil 38 in proportion to the kilovoltage applied to the transformer 17 so as to maintain the relatively uniform focusing as selected by the initial monitor setting. To understand this modification, it should be noted that the supply to the primary 39a of the transformer 39 is by way of the isolated winding 13a on the auto-transformer which is connected by lines 49 and 50 with the transformer winding 39a. To provide the above mentioned kilovoltage correction, the secondary winding 51b of transformer 51 is included in series in line 50. The primary 51a of this transformer is connected by lines 52 and 53 to pass through the kilovoltage selector 14 so that the kilovoltage selected is automatically imposed on the winding 51a. It results from this arrangement, that after the preliminary setting of the devices shown at 36, 44 and 47, to control the milliamperage flowing in the tube and to control the focal spot size, thereafter an increase or decrease in the kilovoltage selected at 14 will cause an increase or decrease respectively in the potential applied through lines 40 and 41 to the focusing coil 38 so that the higher kilovoltage selected will tend to decrease the size of the focal spot on the anode 12. This will tend to maintain relatively uniform focusing as selected by the initial monitor setting by the device 48.
.One of the objects of the present invention is to provide a tube wherein all of the necessary factors for an exposure may be pre-set so that very short exposures under predetermined conditions may be provided. Under conditions presently common, wherein 60 cycle current is used, exposures down to 14 second are possible. By the use of higher frequencies, the exposure time might be reduced to 4 or even to of a second, During such short exposures, no time is available to adjust the desired tube conditions as to milliamperage flow and focal spot size or to adjust for changes in kilovoltage. In order to obtain this instantaneous control of exposures, we have combined with the previously described devices a trigger grid or control grid 54 which is positioned adjacent the filament 11 in such a manner that when it is properly biased, it will completely block the flow of electrons from the filament 11. In the embodiment shown, a means is shown for providing a negative bias on the grid 54 with respect to the filament 11 through the secondary winding 55b of the transformer 55. This winding is connected through line 56, rectifier 57, and line 58 to the grid 54. The other end of winding 55b has its connection through line 59 and lines 20 and 27 to thefilament 11. A condenser 60 is connected across the rectifier in the usual manner. The primary 55a of transformer55 isconnected through line 61, variable resistance 62, ,exposure switch 63 and line 64 to a suitable tap on thetransformer13. The other end of the winding 55a is connected through lines 65 and 19 to the auto-transformer.
It results from the above connection of the control grid 54, that, with switch'63 closed, the monitor may be pre-set to make the desired selection of milliamperage flow through the X-ray tube and size of focal spot while device 14 may also be pre-set for the desired kilovoltage. Meanwhile, the negative'bias on grid 54 stops all electron flow from filament 11 toward the anode 12. When it is desired to initiate an exposure, switch 63 is opened 'for the desired exposure and closed again, preferably by use of an automatic timer, many of which are well known and in common use today and the showing of switch 63 and the device 63a is only a diagrammatic ,showing'to illustrate the use of the present invention.
If desired, but not necessary to the use of this invention, the tube and transformers 17, 26 and 55,,may be submerged in a dielectric liquid or gas in a tank 66 indicated indot-dash line and grounded at 67.
What is claimed is:
' 1 In an X-raytube having, afilament adapted to emit atstream of electrons striking the focal spot on an anode and having electrical means for energizingtsaid tube, inclndingzmeans controlling an energizingelectrical. supply to said filament to increase and decrease the milliarnperage flowing in said tube, the combination therewith of means controlling said focal spot size including an electromagnetic focusing coil embracing said stream of electrodes intermediate said filament and anode, means for supplying electrical potential to said coil, means for varying the kilovoltage in said tube energizing means, and means for increasing and decreasing said potential instep with increase and decrease respectively in said kilovoltage.
2. In an X-ray tube having a filament adapted to emit a stream of electrons striking the focal spot on an anode and having electrical means for energizing said tube including means controlling an energizing electrical supply to said filament to increase and decrease the milliamperage flowing in said tube, the combination therewith of means controlling said focal spot size independent of said filament electrical supply control means and providing a focal spot of varying area, and a single control means for varying said focal spot size control means in step with said filament electrical supply control means,
said single control means being constructed andarranged to decrease and increase said focal spot size in step with adjustment of said filament electrical supply control means to increase and decrease respectively said milliamperage flowing in said tube.
3. The combination of claim 2 including an electromagnetic focusing coil embracing said stream of electrodes intermediate said filament and said anode, means for supplying electrical current to said coil, said control means having operative connections for increasing and decreasing said current in step with said adjustment of said filament electrical supply control means to increase and decrease respectivelysaid milliamperage flowing in said tube, means for varying the kilovoltage in said means for energizing said tube, and means responsive to variation of said kilovoltage for increasing and decreasing the potential of said current supply to said coil in step with increase and decrease respectively in said kilovoltage in said tube energizing means.
4. The combination of claim 2 including a control grid adjacent said filament and positioned to control emission of electrons from said filament and control means for supplying to and removing from said grid potential negative with respect to the filament and respectively completely blocking and permitting electron flow therefrom, whereby the desired milliamperage flow in saidtube and focal spot size may be selected and thereafter an exposure may be initiated simply by removing said bias from said grid.
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