US2946892A - Arrangement for controlling and correcting the location of the focal spot produced by a cathode-ray on the target of a roentgen-tube - Google Patents

Description

July 26, 1960 E. BAS-TAYMAZ 2 4 9 ARRANGEMENT FOR CONTROLLING AND CORRECTING THE LOCATION OF THE FOCAL SPOT PRODUCED BY A CATHODE-RAY ON THE TARGET OF A ROEN'IGEN-TUBE Filed Oct. 2, 1958 2 Sheets-Sheet "1 July 26, 1960 E. BAS-TAYMAZ 2, 9

- ARRANGEMENT FOR CONTROLLING AND CORRECTING THE LOCATION OF THE FOCAL SPOT PRODUCED BY A CATHODE-RAY ON THE TARGET OF A ROENTGEN-TUBE Filed Oct. 2, 1958 I 2 Sheets-Sheet 2 United States Patent ARRANGEMENT FOR CONTROLLING AND COR- RECTING THE LOCATION OF THE FOCAL SPOT PRODUCED BY A CATHODE-RAY ON THE TAR- GET UP A ROENTGEN-TUBE Enis Bas-Taymaz, Zurich, Switzerland, assignor to Gesellschaft Zur Forderung der Forsehung an der Eidg. Techn. Hochschule, Zurich, Switzerland Filed Oct. 2, 1958, Ser. No. 764,809 W Claims priority, application Switzerland Jan. '22, 1958 1 Claim. (Cl. 250-99) I In the conventional method of dental roentgenography, the X-ray film is introduced into the mouth of the patient and exposed to a source of X-rays arranged outside the mouth to thereby irradiate the teeth, the film usually having to be held in place by the patient. In this way, three to four teeth can be taken on one film. This method is particularly time-consuming and inconvenient, if many radiographs have to be taken consecutively, for instance, when the whole denture has to be radiographed. Such a series of radiographs further involves the drawback that portions of the skull lying behind the film within the cone of X-rays will be exposed to them and thus become repeatedy irradiated. This is the casealso .with other dental radiographs, say, with those of the jaw, and may have a very adverse effect also with other intracavernous radiographs.

This invention now relates to an improved arrangement for controlling and correcting the location of the focal spot produced by an electron beam on the target of a roentgen-tube, in which for obtaining X-radiation within an angular space exceeding 180 degrees, the target presents an inverted V-edge or a point, upon which the focal spot of the cathode ray shall be centred.

In accordance with the invention, the intensity of the secondary electron emission by the target can be determined in various directions and the direction of the cathode ray be influenced until the intensity distribution of the secondary electron emission coincides with the desired distribution of the X-radiation.

A suitable arrangement for carrying this method into practical operation has an X-ray tube which includes a cathode, a focussing electrode, an anode and a target, the latter possessing, for obtaining a roentgen radiation within an angular space exceeding 180 degrees, an inverted V-edge or a point upon which the focal spot of the cathode ray shall be centered. The arrangement according to the invention is characterized in that, within the 'X-ray tube, measuring electrodes are symmetrically ar- "with the accompanying drawing in which several forms 2,946,892 Patented July as, 1960 partially in vertical section the electrically active parts of the arrangement according to a first form of embodiment with inverted V-shaped target and electrostatic centring device for the cathode beam; r

Fig. 4 is a similar representation of a'second form of embodiment with conically shaped target; I

Fig. 5 shows partly in view and partly in vertical section a portion of a third form of embodiment with pyramidal target and electromagnetic centring device for. the

drical protective casing'll consisting of an electrically and thermally insulating material, which can .be introduced into a cavity. Theprotective casing 11 is introduced into a human mouth for taking a radiograph of the whole denture in one single picture. The protective casing 11 includes a suitably shaped portion of an X-ray tube which partiallyalsdextends into another casing 12 which accommodates further elements of the roentgen arrangement.

On the'protective casing 11 is arranged a curved holder 13 fora roentgen film 14, having in its central section an aperture through which to pass the protective casing 11. The form of the holder 13 is such as to be capable of holding the film 14 on the outside of the human head in the region of the denture. Thcfholder 13 likewise consists of electrically andthermally insulating material. I

Fig. 3 shows the inner structure of the roentgen tube diagrammatically. For producing a sharply'focussed electron or cathode beam 20, the roentgen tube includes in known manner a cathode 21, a Wehnelt cylinder 22 and an anode 23. The'cathode 21 and the Wehnelt cylinder 22 are connected to the negative pole, and the anode 23 is connected to the positive pole of an electric source 24 of high voltage. The cathode my 20 passes through-an opening 25 in the anode 23 and is directed upon-a target 26 which is located at the end of the roentgen tube to be introduced intothe body cavity and likewise 'connected to the positive pole of the high-voltage source24. Where the, cathode beam falls on the target, the so-called focal spot is produced, from which, as is well-known, emanate the X-rays to be generated. In order to obtain X-radiation within an angular space A (Fig.2) exceeding 180 degrees,-.thetarget 26 according to Fig. 3 hasimpingernent faces which intersect along an inverted V,-edge 27 upon which the focal spot-of the cathode 20 shall be angles to the inverted 'V-edge 27, there are provided of embodiment are illustrated by way of example and i between anode 23 andtarget 26 two electrostatic Ideflector plates 28, 29between which passes the cathode'beam.

I One plate 28 is connected to the positive pole of the high 60 voltage source 24, whereas the. other'plate" 29 i's connected to a potentiometer 30 by means ofwhich the potential at'the plate 29 may bevariedwith respect to that 'at the plate 28 bothgin the positive and negative directions, as known per se. i a The aforedescribed elements of the roentgen tube, except the high-voltage source 24, are arranged within an evacuated glass envelope which is substantially cylindrically shaped and projects into the casing 11. In 'a roentgen tube of this type, when in service, it is difiicult to establish whether the focal spot of the cathode ray 20 is exactly centred upon the inverted V-edge 27 of the Fig. 3 shows diagrammatically partially in view-and target'zfi or falls 'p edominantly'or entirely upon one 3 or other of the bevelled faces of the target. In the latter case, an uneven distribution of the intensity of the X- radiation will result within the angular space A.

Now according to Fig. 3 there are provided two additional measuring electrodes 31, 32 which are similar to one another and also arranged within the glass envelope, symmetrically with respect to the symmetrical plane passing through the inverted V-edge 27, and between the target 26 and deflector plates 28, 29 used for centring the cathode ray 20. The electrodes 31, 32 are connected to the positive pole of the high-voltage source 24 through identical electrical measuring instruments 35 and 36 respectively.

The modus .operandi and action of the described'roent- .gen arrangement and the method of controlling and cor recting the location of the focal spot on the target are .as follows:

By the impingement of the electron of the cathode ray upon the target 26, not only roentgen rays but also secondary electrons are produced. In the event of the cathode ray 20 falling centrically upon the inverted V- edge of the target, the intensity distribution of the secondary electron-emission coincides with the desired distribution of X-radiation. Then both emissions have the same symmetry as the target. By means of the measuring electrodes 31, 32, the secondary electrons will be intercepted in the directions concerned herein. In the current paths existing between the electrodes 31, 32 and the positive pole of the high-voltage source, there will flow electric currents the intensity of which is proportional to the secondary electron emission upon the electrodes 31, 32. By means of the measuring instruments 35, 36, the current intensities may be ascertained and compared with each other.

If the focal spot lies exactly symmetrical on the inverted'V-edge 27, both measuring instruments 35, 36 will indicate the same intensity. But if the spot is displaced, say, to the right, the secondary electron emission towards the electrode 31 is greater than towards the electrode 32, and the measuring instrument will indicate a higher intensity than the measuring instrument 36. If such is the case, the deflection plate 29 should be adjusted to a higher potential by means of the potentiometer 30, whereby the cathode ray 20 is deflected to the left accordingly. Correction of the location of the spot is carried out so long until both measuring instruments 35, 36 will indicate the same intensity, when the focal spot will be exactly centred upon the inverted V-edge 27. The procedure is quite similar if the spot has deviated to the left and thus the intensity through the instrument 36 will be greater than through the other instrument 35. In this case, the deflection plate 29 should be brought to a lower potential by means of the potentiometer 30 to direct the cathode beam accordingly to the right, until the intensity at both measuring instruments 35, 36 will bethe same.

With the aid of said instruments 35, 36 it can be determined, whether the focal spot is properly centred upon the edge27. If so, one obtains an X-radiation whose intensity in the horizontal covers an angular space A (Fig. 2) of more than 180 degrees, and which shows substantially equal distribution in the vertical over an angularvspace B (Fig. 1). It is thus possible to obtain a picture of .the whole human denture with but one single radiograph. The electrodes 23, 29 are of such design as to let therethrough the X-rays almost unweakened.

The embodiment according to Fig. 4 differs from that described first, for one thing in that the target 127 presents a conical impingement face of rotation symmetry, and thus has, instead of an inverted V-edge, a point 127 upon which the focal spot of the cathode ray 20 shall be centred in order to ensure uniform intensity distribution of the X-radiation in all directions within an angular .space of more than 180 degrees. For centring the cath- 4 ode means 20 upon the point 127, there are provided two pairs of electrostatic deflection plates 28, 23 and 38, 39 disposed at right angles to each other. The plates 28, 38 are connected to the positive pole of the high-voltage source 24, while the other plates 29, 39 are connected to a potentiometer 30 and 4.0 respectively, with the aid of which the potential of the electrode .29 or 39 .may be regulated with respect to that of the electrodes 28 and 38 in a positive or negative way. The deflection plates 28, 29 and the target 127 have three pinlike measuring electrodes 131, 132, 133 similar to one another and arranged .therebetween symmetrically-in relation to each other and to the rotation symmetry axis of the target 126.

If the focal spot of the cathode ray 20 is not centred onto the point 127 of the target 126, the intensity of the secondary electron emissio'nto the three measuring electrodes 131, 132, 133 will differ so that also varying currents will set up through the measuring instruments 35, 36, 37 similar to one another. If by means of the potentiometers 30 and 41) the cathode ray is directed in such a Way as to cause the spot to fall exactly onto the point 127 of the target, then all three measuring instruments 35, 36, 37 will indicate the same intensity. By means of said instruments 35, 36, 37 it is therefore' po'ssible to state, whether the focal spotis properly centred onto the point 127 of the target 126.

It is understood that, instead of only three, there could also be more measuring electrodes, say, preferably four symmetrically arranged to the rotation symmetry axis of the target 126.

The measuring instruments 35, 36, 37 need not in every case be needle instruments, as for the indication their place could also be taken, for instance, by some system involving the use of neon indicator lamps or the like.

Obviously, in the aforedescribed forms, instead of the electrostatic deflection means, also magnetic ones may be provided for centring the spot of the cathode beam.

A form of embodiment having a magnetic deflection means is illustrated in Figs. 5 and 6. Therein four electromagnets 41, 42, 43, 44 are disposed at right angles to each other and radially to the cylindricalglass envelope 45 of the roentgen tube. For the sake of simplicity, the means for mounting the magnets, say, a ring of ferromagnetic material forming at the same time the magnetic ground, are not shown. The target 226 of the roentgen tube is a pyramid with rectangular base and possesses therefore two planes of symmetry at right angles to each other, for instance vertically and horizontally. The target 226 is coaxial with the glass envelope 45. To the inner Wall of the envelope 45 are attached four identical measuring electrodes 231, 232, 233, 234- in the form of thin metallic layers, symmetrical to said planes of symmetry of the target 226 and opposite the sloping pyramidalfaces of the target. For example, the layers may be made of colloid graphite according to the well-known procedures of tube technique. Another way of construction is seen in the depositing by evaporation under vacuum of a suitable metal .or semiconductor, for instance by means of a mask to allow for the required insulation space "between the various electrodes. Another interesting way of construction consists in the chemical application of asemiconducting tin-oxide layer or in the depositing by evaporation under vacuum. Melted in the glass envelopeare connection wires 46 and 47 leading respectively to the measuring electrodes and target.

The arrangement described so far according to Fig. 5 and partly Fig. 6 can be employed as follows: The target 226 is connected direct, and the measuring electrodes 231-234 each are connected through an electric measuring instrument to the positive pole of the high-voltage source, similar to the previous forms of embodiment. The coils of the axially opposite electromagnets 41, 42 and 43, 44 lie in a DC. circuit which includes means (not shown) adapted to regulate the intensity and polarity of the current traversing the coils, the arrangement being such as to allow of the cathode ray 20 being directed onto the point of the pyramidal target 226. If the focal spot of the cathode ray 20 is properly centred onto the point 127, the secondary electron currents to the measuring electrodes opposite each other and through the respective measuring instruments are alike. Since the target 226 is a pyramid with an elongated rectangular base and thus no regular pyramid, no secondary electron currents exactly alike to all electrodes 231-234 will result, but only to the electrodes which are oppositely disposed in pairs. In every case the properly centred location of the spot of the cathode ray will be attained, if the secondary electron currents to the electrodes are balanced.

Fig. 6 further shows, how the directing of thecathode or the centring of the spot onto the point 127 of the target 226 may take place automatically. For the sake of simplicity only those means are shown adapted to direct the cathode beam horizontally and to make the secondary electron currents to the electrodes 231, 232 equal to each other. The other means connected tothe electrodes 233', 234 and to the magnets 43, 44 are entirely similar.-

The two measuring electrodes 231 and 232 disposed opposite each other are connected to the control grids of a double triode 49 whose cathodes are tied together and connected both to the positive pole of the high-voltage source 24 and to the negative pole of an additional D.C. source 50 of low voltage. The cathode and the control grid of each triode system of the tube 49 have inserted therebetween a comparatively high- ohmic resistance 51 and 52 respectively, through which the positive pole of the high-voltage source 24 is connected to the electrodes 231, 232. Each of the electromagnets 41-44 comprises two coils wound in opposite directions. The coils a of the magnets 41, 42, acting in sympathy with each other magnetically, are series-connected between the anode of the lefthand triode system and the positive pole of the voltage source 50. The coils b of the magnets 41, 42 which likewise act in sympathy with each other magnetically, are series-connected between the anode of the righthand triode system and the positive pole of the voltage source 50. Preferably, the coils a and b have identical numbers of turns, and the resistances 51, 52 are equal to each other.

The action of the described wiring is as follows:-

The secondary electrons intercepted by the measuring electrodes 231, 232 flow 01f through the resistances 51, 52 to the positive pole of the high-voltage source 24. In said resistances voltage drops will occur which are proportional to the secondary electron currents and act upon the double triode 49. With the focal spot of the cathode beam 20 centred upon the point of the target 226, the secondary electron currents through the resistances 51, 52 or the voltages set up at these resistances are equal to each other. Then in both halves of the double triode 49 anode currents of identical magnitude will flow so that the magnetic actions of the two coils a and b of each of the magnets 41, 42 mutually nullify each other. Consequently within the space of the cathode beam 20 there will be no magnetic field as derived from the magnets 41, 42 so that the cathode beam undergoes no deflection towards one or the other of the electrodes 231, 232.

If, however. the focal spot of the cathode beam 20 lies not centrally upon the point of the target 226, but, for instance, rather on the side of the measuring electrode 231, then the secondary electron current flowing 01f to this electrode will be higher than that flowing to the electrode 232. Therefore, at the control grid of the lefthand triode system a higher negative voltage arises with respect to the cathode than at the righthand triode system. Hence, in the triode system on the left a lower anodic current will flow than in the triode system on the right, for which reason the magnetic actions of the coils a and b of the electromagnets 41, 42 can no longer cancel each other. As a result, in the range of the cathode beam 20 there will be a magnetic field intensity capable of deviating the cathode beam towards the electrode 232 untilthevolta'g'e drops at the resistancesSl and 52 are equal, except for a through the resistances 51, '52 are practically'alike, the V focal spot of the cathode beam 20 consequently lying cen trically upon the point of the target 226, except for a practically negligible regulation error; f r I t The action of the means adapted to deviate the cathode beam 20 in vertical direction is entirely similar. To feed the double triode ,(not shown) for the verticalcontrol, the same voltage source 50 may be employed as for feed ing the double triode 49. f

In a modified form'of embodiment. (not shown), the wiring accordingjtoFiggo may be such that all coils of the magnets 41, 42 acting magnetically in sympathy are series-connected to each other and put in between the two anodes of the double triode 49. Thereby the anodes are connected to the positive pole of the voltage source 7 50 through identical resistances. .If at the'grids of the double triode 49 identical control voltages prevail, that is' to sayif the cathode beam of the X-ray tube is properly centred, equal-voltages set themselvesat the anodes of the double triode so that no current traverses the coils of the magnets and hence also'no magnetic influencing of the cathode beam in horizontal direction will take place. Should, however, the focal spot of the cathode beam havev deviated in horizontal direction from the point of the target 226, the voltages at thegrids and also the V 'Moreover, it is also possible to have the electron tube amplifiers replaced by transistor amplifiers or by magnetic amplifiers. Q 7 q In a modification (notshown) of the arrangement illustrated in Figs. 5 and 6', the triodes could be omitted and the coils a or b of the electromagnets 41, 42 be inserted direct between the electrode 231 or 232 and the positive pole of the high-voltage source 24. In a similar way, the coils a and b of the magnets 43, 44 are then inserted be 'tween the electrode 233 or 234 and the positivepoleiof the high-voltagepsource 24. The secondary electron currents will then flow directly through the coils of the electromagnets-serving to direct the cathode beam, without being amplified previously. In order to obtain an adequate regulation sensitivity, the coils a and b of the electromagnets should be given a'sufliciently high number of turns.

The electric wiring shown in Fig. 6 may be used as slightly modified even if the X-ray tube includes an elecglass envelope 45 of the roentgen tube, are now used I simultaneously as electrostatic deflection plates for directing the cathode beam. Also in this case, the target 226 has the form of a pyramid with rectangular base, and the electrodes 231-234 are disposed opposite the bevelled pyramidal faces. The positive pole of the highvoltage source 24 for feeding the roentgen tube is connected via resistances 61, 62, 63, 64 with the electrodes 231, 232, 233 and 234 respectively, and with the target 7 226 via a resistance 66. If the electrodes 231-234- and the target 226 are arranged entirely centrally and symmetrically in relation 'to one. another, the resistances 6,1, 62 or 63, 64 are chosen of the same. size, two by two. All resistance values are such' that the electrodes 231- 234 are loaded, through the secondary electron currents distributed over the resistances 61-64, up to a sufficiently high negative voltage with respect to the positive pole of the high-voltage source 24, in order'to behble td'cause electrostatic deviation of the cathode beam. Moreover, the resistances 61, 6 2 or 63, 64 are so dimensioned two by two that, with the focal spot centrally located upon the point of the target 226,;all fourelectrodes will be loaded up to the same voltage 'so' that between the single electrodes no potent-ial'difference will exist. Preferably, the resistance'66 is dimensioned in such 'a way that, through the primary electron current of. the X- ray tube flowing off over the targetfthe' latter reaches a same potential with respect to the positive pole of the high-voltage source 24, similar to theelectrodes, 231-234.

The action of the described arrangement is as follows-:

If the focal spot is centred eriactly upon the point of the target 226, the electrodes 231-234fare all at the same potential so that the cathode beam will not be subjected to deviating electrostatic field forces. Centring and symmetry fields possibly present between the'four electrodes 231-234 and the target 226 may be removed by suitably correcting the resistances 61-64. If thereupon the focal spot no longer lies centrally upori the point of the target 226, but "displaced, say, towards the electrode 233, the secondary electron current to the electrode 233 will be greater, and that to the electrode 234 smaller. This cause a voltage differential at the said two electrodes so that the cathode beam will be deflected towards the electrode 234, until the voltages at the two electrodes 233 and 234 are equal, except for a comparatively slight magnitude which can be regulated. Thus also the central location of the focal spot upon the point or the target may be re-established automatically, except for a practically negligible regulating error.

It is possible and, if necessary, preferable to have the resistances 61-64 arranged within the envelope of the X-ray tube. The resistances then connect the electrodes 231-234 with another electrode joined to thepositive pole ofthe high-voltage source 24, say, of the anode ofthe tube. In this case the resistances 61-64 may be formed by layers of an electrically conducting material applied to the'inner wall of the glass envelope 45.

According to a modified embodiment (not shown), the target of the X-ray tube could also be a pyramid whose base has a number of corners differing from four. With a pyramid, say, with triangular base, three measuring electrodes should be provided. With hexagonal base of the pyrar id, also h ee mea u n e ect od s. ill whereas, say, w h octago al base fou me surin le trodes will. be req If the basis of the pyramid forming the target is a regular polygon, the measuring electrodes, instead ofbeing opposite the "side faces of the pyramid, may also be disposed opposite the edges thereof, as exemplified in Pig. 8 with reference to a pyramidal target 326 with square base.

When in the foregoing disclosure and appended claims mentioned is made of an inverted V-edge or a point, such designation is not to he. understood strictly geometrically but rather technically, that is to say, both'the inverted V-edge and the point may be given a certain radius of curvature 'which, in the manufacture of the target on a practical scale, 'cannot be kept as small as desired. But the radius of curvature should not exceed the diameter ofthe focal spot, it a good stray of X-radiation' within an angular space exceeding degrees shall be obtained.

An X-ray generating arrangement comprising, in combination, an X-i'ay tube having in an evacuated envelope a cathoda'a focussing electrode, an anode and a target, said target being provided with a protruding impingement portion facing said cathode and upon which it"is desired to direct the focal'spot of a cathode-ray beam emitted by said cathode forproducing X-ray radiations within an lar s'pace exceeding IBD'degrees, measuring electrodes arranged about said target'ivithin said envelope 'and connected to the positive pole of a high voltagesource feeding said X rayt'ubeQand electromagnetic deflection coils for" deflecting said cathode-ray beam and connectedin e qi p i ir ui of a ar c r lq tu e e a id i lidge o'n'said tube being controlled accordance with the magnitude of secondary electron currents impinging on said measuring electrodes and emitted by the impact of the cathode-ray beam on said target wherein'deflec'tionof said cathode-ray beam is such that the focal spot said cathode ray beam is automatically centered upon said protruding impingement portion of said target. K A

References Cited in the file of this patent UNITED STATES PATENTS

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