US2942126A - Rotating anode X-ray tube - Google Patents

Description

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June 21, 1960 K. SILBERMANN 2,942,126

ROTATING ANODE X-RAY TUBE Filed Dec. 26. 1957 2 Sheets-Sheet 1 K. SILBERMANN ROTATING ANODE X-RAY TUBE Y June 21, 1960 2 Sheets-Sheet 2 Filed Dec. 26, 1957 United States Patent O 2,942,126 t RoTATlNGANoDE X-RAY TUBE Karl Silbermann, Erlangen, Germany, assigner to Sie- Y mens- Reiniger `Werke` Aktiengesellschaft, Erlangen,

Germany Filed Dec. 26, 1957, Ser. No. 705,176

` u Claims priority, application Germany June 4, 1957 The invention relates to X-ray tubes, and more specifically to an `Xray tube of the rotating anode type.

It is known in the art that the definition of an image produced by an X-ray beam depends upon the optically effective area of the focus on the target and that this can be substantially improved by slightly inclining a s'malllineal bombardment area in relation to the central ray of the useful beam. If this angle of inclination is say 19, a rectangular bombardment area of which the side lengths are for instance in the proportions of 1:3 will appearforeshortened to a square when viewed in the direction of the central ray, thus reducing the apparent, optically effective, ,total area to one third its actual size without affecting the intensity of radiation. In practice, the angle of inclinationof the, anode surface in relation to the central ray in a rotating anode tube is between 15 and 20. These angles will hereinafter be referred to` as anode face angles because they represent the angle formed by a generatrix of the cone-shaped anodeface and a plane normal to the axis of the anode plate. For the operation of an Xray` tube a reduction of the face angle to less than 15 becomes impracticable because the target surface itself cuts oi one side of the useful beam, namely that towards the anode, and since it is desirable that the central ray should define the center of the image, the face angle will determine the maximum effective angular image iield and hence the maximum possible size of image available for a` given distance between focus and fluorescent screen. Y

A vface angle of 15 would therefore limit the availabl angular eld of the image to 30 and at the minimum practical exposure distance of 65 cms. between f ocus and screen this would theoretically produce an image size of roughly 34 x 34 cms. However, in practice this theoretical image size is not available because, onA the one hand, at very acute angles between emitted ray and anode surface the intensity of the beam falls rapidly and, on the other hand, slight superficial roughness'or irregularities on the surface of the plate after longer periods of operation cast shadows` on the image edge. In diagnostic practice face angles under 15 in rotating anode X-ray tubes have therefore not been used because, for instance in sighting skiagraphs, the image area would frequentlyA be` insuiiiciently defined throughout an `imageof the required size. Even face angles of 15 are not often encountered in rotating anode tubes; an angle of 17.5 to `ensure a uniform exposure throughout an image size of 40 x 40 cms. at a distance of 65 cms. being generally preferred.

The invention is based upon the thought that the limitation of the image size will cease to be material if a rotating anode tube is used with two separate concentric target tracks each with a different face angle, the

. smaller face angle being reserved for skiagraphs for which small sizes are suflicient. Such an arrangement imposes no restrictions upon radio-diagnostic techniques because when high-definition skiagraphs are needed it is the general rule that they are not required to be of greater size,

f. YICC? a maximum. of 13 x 18 cms. being large enough. For obtaininga highquality image the smallest possible size will always be preferred to minimize the clouding of the picture that arises from the` scattering of the rays inside fthe object. On the other hand, when taking general views high definition is less essential than a well ex posed major area taken very near the tube.

The invention therefore proposes, in a rotating anode X-ray tube` with two concentric target tracks ofY different face angles to make the targetA track having the smaller face angle at least as wide asl the target track havingthe larger face angle to equalize the loading capacities ofthe two tracks.

In this context the target track is understood to he the entire annular edge portion on the anode plate struck by electrons in the course of one revolution of the anode plate, the width of Athe target track being equal to the length of the lineal bombardment area. Loading capacity is understood to be the energy a single track can handle during` a` period of one tenth of a second, that is, in the course of a short exposure.

Apart from taking general and detail skiagraphs the X-ray tube as above described can be usefully employed for teleskiagraphs, that is, for taking skiagraphs at a major distance of the body from the X-ray tube, such as skiagraphs of the chest, of the whole of the body, and lateral views of the lumbar vertebrae, without appreciable distortion. This `diagnostic technique calls `for a consid-` erable output of the X-ray tube. T he long distance between-focus and film (2 to 3 meters) largely compensates the lack `of denition caused `by the nite size of the focal area; For high output teleskiagraphs it would therefore be possible to use an X-ray tube in which a greater output is achieved by an increase in the size of the focal area beyond the usual size. In view of the rarity of the occasions upon which such high output teleskiagraphs are' needed in the practice of `radiography when compared with the frequency of normal skiagraphs which call for small focal areas and short distances of the` object from the tube, the provision of such a special tube would be an unjustiable expense.

According to -a further feature of the invention the X-ray output of a rotating anode X-ray tube constructed according to the invention with two radially adjacent target tracks can be appreciably increased for highpower `teleskiagraphs -by providing control means which in addition to selectively bringing into oper-ation the one or the other target track permit both target tracks to be operated for the simultaneous emission of Xradiation, Since the two target tracks each have the saine loading capacity the maximum total output of the tube can be thus considerably increased and, in order of magnitude, approximately doubled. Hence the sum of the optimum permissible power outputs from each track, `say 35 kw.-l30 kw.,` when loaded for IAO of a second, will be available for such a skiagraph. The -fact that the shape ofthe focal'point area is in such a case for instance yin the yform of two adjacent squares, about 1.5 x 1.5 mm. and 0.9 x 0.9 mm. in size, will not affect the quality of the image when the distance between focus and film is considerable, for the-reasons that have already been explained.

The invention will now be described in greater detail with reference to the illustrative examples shown in the accompanying drawings in which:

Fig. 1 is an axial section through an anode plate according to the invention `for use in a rotating anode X- ray tube, two cooperating hot cathodes being likewise shown;

Fig. 2 is a fractional top plan view of the anode .plate shown in section in Fig. 1;

Fig. 3 is a section through a modified plate with a stepped edge portion;

Fig. 4 is a wiring diagram of the control circuits of an X-ray tube according to the invention for the selective operation of one or two electron beams.

Referring now to, Fig. 1', two hot cathodes 2 and 3 are allotted to a'rotating anode plate 1, electron bombardment areas 4 and 5 extending along side upon the surface of the anode plate 1 corresponding to the cath odes 2 and 3, such areas in turn determining two concentric target tracks 6 and 7 shown in Fig. 2.` The side by side disposal of the two cathodes may be realized in simple manner by departing yfrom the more conventional platev diameter of' 90 mm. in favor of one of 100 mm., a change which'at the same time increases the available loading capacity of the plate. The inside target track 6 forms an angle ofonly 9 with a radial plane 9 normal to the axis of revolution 8 of the rotating anode and, to provide an approximately similar loading capacity for a likewise square focal point, it is somewhat wider radial- 1yY than the outer target track whichxforms an angle of 17.5 with the radial plane. The provision of .a target track of somewhat smaller face angle inside an outer target trackY has the advantage that the surface of this inner track will `be better protected from distortion than if it Were located on the edge.

. A face angle of 17.5 appears to foreshorten a-rectangular bombardment area of which the side lengths are in the proportion of 123.3 to av square whenY viewed in theydirection of the centralray. For a face angle of 9 the relative proportion is 1:6.4. If the same length of edge isA provided Afor the optically leffective (square) focal points inthe case of both face angles, that is, if the rectangular lelectron impact areas are of identical width, the Ifocal p'oint at the face angle of 9 will give off twice the radiation in operatively effective radiation direction than the focal point of thelarger face angle, if being thereby assumed that the areas of the electron impact surfaces, which are in proportion of about 2:1 are bombarded ywith identical electron density. If two electron impact surfaces of identical area assumed for comparison for the two face angles, the twofocal points allotted thereto will radiate with identical intensity, but the optically eifective focal point of the face angle of 9 will be only ha-lf as large, that is, itsA illumination density will be twice as great as that of the focal point of the face angle ofk 17l/ Y(the lengths of edge being in a proportion of 1/2\/2:1).- The focal point of the smaller face angle therefore will produce considerably greater reproduction sharpnessthan the focal point of the larger. face angle.

It has been assumed in connection with the foregoing explanations that the .loading capacity of an electron impact Vor bombardment surface is solely proportional to its size. vIn actual fact, however, the loading capacity of a bombardment area increases the more elongated its shape. For instance, if a bombardment area is longitudirnally halved, the loading capacity Bof each`half is not reduced by halt" but by a substantially smaller amount. vThis has to do with the ilow of heat intoV the anodematerial bordering on the bombardment area, s'uch How ofheat beinggreater in the case of an elongated! bombardment Earea with a longer bordering line relative to the surface content than in the case of a bombardment area of the same area but with a relatively short bordering line. This has led in practice to the adoption of a rule of thumb starting that, when a rectangular focal point with `a maximum permissible loading B is longitudinally halved, the resulting focal point which is only half as wide will not tolerate only half the loading 1/2B but a loading corresponding to the value l/zBA/ Consequently the bombardment area associated with the smaller face angle may be smaller optically effective (square) focal area can be quite considerably less than half the size of the larger focal area. This additional gain inherent in the reduction of the focal area is only slightly offset by the fact that the loading capacity of the tube on the inner track is somewhat lessened by the slower speed of travel of the bombardment area on the anode surface.

An X-ray tube according tov the invention is especially suited for taking series of general skiagraphs with X-ray apparatus equipped with sighting gear, giving -a satisfactory overall exposure of ar'large size plate-at short range, by using the Youter target track, andalso for taking detail skiagraphs ofl much ner definition and a full overall exposure of even the normal 13 x 18 cms. plate, by using theinner target track. Moreover, a tube according to the invention by means Aof which general and detail skiagraphs are taken with the aid of different target tracks, is superior to known types of tube with a singletarget track in that it has a longer life, sincewthe deterioration of an X-ray tube is due to the gradual roughening of the surface of the' track landY the progressive vaporization of the hot cathode filament.

' In principle the advantages inherent in the presentinq venton can also be secured ifthe smaller face angle. is

to provide the same loading capacity as the bombard- Y i associated with the outer target track. Fig. 3V shows a plate in ,section with a stepped edge portion forming an inner electron bombardment area 11 and' an outer Selectron bombardmentarea 12. lThe stepped edge portionA is so disposed that the radiation emitted by theinner elec-4 tron bombardment area 11 will not be intercepted by the sunken plate edge associated with the, bombardment'area '12. This plate therefore permits the outer target track to have the smaller faceang'le. ff On the other hand, the plate shown in Fig. 3 isnot quite as simple to produce as' that according to Fig. 1, andthe central rays radiated from the two focal areas `are not quite so close together as in the case of Fig. l, where theymay be practically contiguous. ad vantage of the arrangement according to Fig. S'istliat the smaller focal area which gives the better definition `is on the outside track so that the greater speed of the track' -allows a higher specific loading of the plate materialthn is the case on the inside track. This may be a feature of significance if the diameter of the revolving plate. is tc be as small as possible, say only mm.

Fig. 4 is a circuit diagram for the operation of an X-ray tube of the kind exemplified by Figs. l and 2. .The hotcathodes 2 and 3 are connected by leads 13, 14, 15 with the secondaries of two heating transformersi and 17. An on-oif switch 18 and a target selector switch 19 permiteeither one or b oth the transformers .16 and 17 to be ,connected with a current source 20. To this end theselector switch 19 has three positions, in whicha contact 22 provided on a switch arm 21 is in engagement either with one of two contact points 23 and 24 or lestablishes electricalcommunication between two insulatedad; jacent cooperating contacts 25 and 2,6. y

The heating current is controlled, separately-for each of the cathodes 2 and 3, by regulating resistors 27 and 28.` Variable resistors 29 to V312Vserve to adjust the control voltages to the emission characteristics of the; two

cathodes and if desired to match the radiation intensities of the two foci associated with the cathodes` when both are simultaneously required for teleskiagraphsas hasbeen above described. From the point of View ofzimage quality it may be Ian advantage if the two components'of the combined focus have approximately equal radiation intensities. To this end the switch 19 and two switches $3 and 34 which `are coupled with the switchr19, are arranged to select dilferent tappings on resistors 29l to 32 for the simultaneous or the separate operation of the cathodes. The provision of a series and arshupntlresistor 27 and 28 formatching facilities adjustment. The: two shunt resistors 29 and 31 do not affect Ithecurrent' passing through the tube when the operational resistors Z7 and 2S respectively have been set to Zero. If in this position the series resistors 3l) and 32, respectively, are set to maximum excitation of the tube, any subsequent adjustment of the parallel resistors for setting to a second value will not affect this maximum.

When using one not cathode only, the current passing through the tube may bc adjusted with reference to an indicator scale 35. The values of the resistors 29 to 32 are for this purpose so determined that the same currents for each of the cathodes will be associated with the same division on the scale. The two tube currents in the simultaneous operation of both cathodes `are adjusted by reference to a second scale 36, if desired after the calibrating resistors 29 to 32 have been set to adjust the efective radiation intensities of both foci to equality. As is the case when adjusting to the emission characteristic of the cathodes the two regulating resistere, for given control characteristics of the two resistors 27 and 23, can produce exact equality at two operational settings only. However, when exact equality at two operational settings has been achieved, the intervening values will be sufficiently well matched for practical operation.

As has been explained, when employing both foci simultaneously for high power teleskiagraphs, virtually the sum ofthe individual outputs will be available. However, this applies only to short exposure times during which the heating effect on the plate will be substantially conned to the focal areas. For short exposure times special overload protection will therefore not be required in bifocal working, since the existing overload devices for single focus working will fulfill the purpose. However, for longer exposures, during which the heat will aiect the entire body of the tungsten plate, the simultaneous employment of maximum load on both foci will no longer be admissible. Generally, for long exposure times the full output represented by maximum loading of both tracks will hardly be required. lt will therefore be advisable to provide suitable switching means which afford a reliable safeguard against overload-ing by reducing, in dependence upon the duration of the exposure, the energy turnover associated with each focus to a lower value than that which corresponds with the sum of the two maximum emission loads.

In a simple way this can be done by the provision of a time switch 37 which controls a Variable resistor 38 in the input line of transformers 16 and 17 and introduces this resistance when exposure time exceeds a critical maximum limit. By coupling a switch 39 with the selector switch arm 2l the input resistor 3S can be prevented from coming into operation unless both target tracks are being used together.

Changes and modifications may be made within the scope and spirit of the appended claims which define what is believed new and desired to have protected by Letters Patent.

I claim:

1. X-ray tube of the rotating anode type comprising an integral anode plate having a contiguous unitary body mounted upon a rotatable stem, two electronemitting cathodes spaced from said anode plate, and two concentric annular target tracks of different face angles formed on said anode plate the concentric annular target track having the smaller face angle being at an angle of not more than 10 with a radial plane normal to the axis of revolution of the rotating anode and the other concentric annular target track being at an angle of not less than 17.5 and not more than 20 with said radial plane whereby the focal spot on the smaller face angle annular target track projects as the smaller focal spot in a direction parallel to the primary X-ray beam so that it is a fine focus X-ray source and the other focal spot functions as a relatively broad focus X-ray source, the annular target track having the smaller face angle being wider than that having the greater face angle.

2. Rotating anode X-ray tube as claimed in claim 1, wherein said anode plate has a diameter of at least about mm.

3. Rotating anode X-ray tube as claimed in claim 1, wherein said target tracks form an obtuse angle, the target track with the smaller face :angle being on the inside.

4. Rotating anode X-ray tube as claimed in claim 1, wherein said anode plate has a stepped surface carrying said target tracks upon the respective stepped portions thereof so that the radiation from the inner target track will not be shielded by the stepped-down outer target track, the outer target track having the smaller face angle.

5. Rotating anode X-ray tube as claimed in claim 1, wherein the smaller face angle is 9 and the greater face angle is 17.5.

6. Rotating anode X-ray tube as claimed in claim 1, wherein said electron-emitting cathodes are provided with an electrical circuit comprising switching means for selectively operating either one of said annular target tracks for the emission of radiation, and further switching means for simultaneously operating both target tracks for the emission of radiation.

7. A structure according to claim 6, comprising adjusting means for loading both said target tracks so that the radiation intensities from both tracks will be equal.

References Cited in the tile of this patent UNTTED STATES PATENTS 2,121,631 Gross et al .Tune 21, 1938 2,215,426 Machlett Sept. 17, 1940 2,764,706 Atlee Sept. 25, 1956 2,767,341 Atlee Oct. 16, 1956 2,836,757 Atlee May 27, 1958 FOREIGN PATENTS 877,157 France Sept. 1, 1942 594,434 Germany Apr. l0, 1934 618,988 Germany Sept. 20, 1935 875,975 Germany May 7, 1953

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