US2825816A - System for maintaining constant quantity rate and constant quality of x-radiation from an x-ray generator - Google Patents

Description

March 4, 1958 ROGERS SYSTEM FOR MAINTAINING CONSTANT QUANTITY RATE AND CONSTANT QUALITY OF X-RADIATIQN FROM AN X-RAY GENERATOR Filed Nov. 13, 1952 I NVENT OR @696: ERS

1 THOMAS H.

ATTORNEY V Unite States SYSTEM FOR NIAINTAINING CONSTANT QUAN- TITY RATE AND CQNSTANT QUALITY OF X-RADIATIGN FROIW AN X-RAY GENERATQR Application November 13, 1952, Serial No. 320,238

5 Claims. (Cl. 250-95) This invention relates to a system for maintaining constant the quantity rate of X-radiation produced by an X-ray generator. Means is also provided by this invention for maintaining constant the quality of the radiation as well as the quantity rate thereof.

Many X-ray applications require irradiation by a constant quantity rate of X-radiation. Some applications also require relatively constant quality in the radiation. For instance, X-ray therapy is an application wherein both constant quantity and constant quality of irradiation are highly desirable. A constant quantity rate is desirable because it makes it possible to give a subject a prescribed X-radiation dosage by merely timing his exposure. Constant quality is desirable because it insures the prescribed kind of irradiation.

It has long been customary to provide regulator means to insure that the current .in the filament circuit and the voltage applied across the anode and cathode of the X-ray tube in an X-ray generator are maintained constant in spite of line voltage fluctuations. It has also been possible to regulate the current through the X-ray tube so that the average current is maintained constant even with variation in effective voltage across the tube and with variation in the eifects of wall charge upon the internal bias of the tube. However, even in those cases where the average current through the X-ray tube is maintained constant and the eiiective voltage across the tube is maintained constant, considerable variations in the average X-ray output of the tube have been experienced, particularly in the case of X-ray generators of the pulsating voltage type. Such variations are attributable to wall charge effects.

No satisfactory means has been devised for controlling and keeping constant the amount of wall charge in a given tube. However, I have devised a system for maintaining constant the quantity of X-radiation produced by a given generator without regard to wall charge eifects. This system is independent of wall charge effects because it is dependent upon X-radiation output only. More specifically, it employs X-ray sensitive signal producing means as the criterion of circuit adjustment to correct quantitatively the X-radiation output of the generator. The. X-ray sensitive signal producing means employed is advantageously an ionization chamber which is positioned in the path of the X-ray beam. The ionization chamber is in turn connected to a regulator, and the regulator is coupled to the X-ray generator circuitry.

Since X-radiation output varies with the filament current, the regulator may be coupled to the filament circuit. In coupling the regulator to the filament circuit, the regu lator is adjusted to sustain a selected mean signal from the X-ray sensitive signal producing means, which mean signal is that signal produced by the desired amount of X-radiation. The regulator functions in response to fluctuations from this selected mean signal to produce whatever increase or decrease in the amount of filament current is needed to cause the X-radiation to be readjusted until it produces a signal of the selected mean value. For

' atent O 2,825,816 Patented Mar. 4, 19 58 2 instance, where an ionization chamber is employed, the regulator acts to increase or decrease filament current in response to variations in ionization current in such a manner as to cause the X-radiation to maintain a predetermined ionization current within the ionization chamher.

Since X-radiation varies approximately with the square of voltage applied across the X-ray tube, the regulator may be coupled to the voltage supply circuit. Accordingly, in another version of my invention, the regulator is coupled to the voltage supply circuit across the X-ray tube electrodes in such a manner that a change in the signal from the X-ray sensitive signal producing means will cause the regulator to produce whatever increase or decrease in the amount of tube potential is needed to cause the X-radiation to be readjusted until it produces a signal of the selected mean value.

Insuring the constancy of output from the X-ray tube will not necessarily insure a constant quality because the bias due to Wall charges in the X-ray tube changes quality as well as quantity. Accordingly, in order to provide both constant quality and constant quantity rate of X- radiation, I provide a system which includes two X-ray sensitive signal producing means, such as two ionization chambers, located in the path of the output of the X-ray generator with a filter of a suitable material, such as copper, placed between the two chambers. Thus, the X-radiation reaching one chamber is unfiltered, whereas the radiation reaching the other chamber is filtered. if one ionization chamber is connected to a regulator which regulates the filament current in order to obtain constancy of X-radiation output as previously described, the other ionization chamber is advantageously connected to a second regulator which regulates the voltage applied to the X-ray tube. Such connections tend to produce constancy in output of both filtered and unfiltered radiation. If both the filtered and unfitered output of the X-ray generator are constant quantitatively, the quality of output must also be essentially constant.

For a better understanding of my invention reference is made to the following drawings:

Fig. 1 illustrates schematically the essential components and circuitry of one version of my invention, this version having a regulator in the filament current supply circuit for the purpose of maintaining constant the rate at which X-radiation is produced by the X-ray generator.

Fig. 2 schematically illustrates the essential components and circuitry of another version of my invention wherein regulators are employed in connection with both the filament current supply circuit and the voltage supply circuit supplying potential across the X-ray tube for the purpose of maintaining constant both the quantity rate and the quality of X-radiation.

Fig. 3 is a circuit diagram of a simple regulator which may be employed with my invention.

Referring to Fig. 1, one of the possible circuits for maintaining constant the output rate of an X-ray generator is schematically represented showing only the portions of the generator which are pertinent to this invention. The X-ray tube 10 consists of a vacuum envelope containing an anode 11 and preferably a filamentary cathode 12. The tube 10 is advantageously placed within a shockproof, rayproof housing 13 in such a manner that X-radiation will be permitted egress from a window in the housing. A high positive potential is applied to the anode 11 of the X-ray tube when X-rays are being produced. The potential across the tube may be supplied from energy source 14 through transformer 15. The cathode 12, which is frequently a coil of tungsten wire, is supplied current from energy source 17 through transformer 18.

lected' mean ionization current.

in the amount'ofX-radiatiou will produce a decrease In the Fig. 1 version ofmylinventiomthe current supplied to filament 12 of the X-ray tube may be modified by the use of a regulator 19which, in response to a signal from'an X-ray sensitive means-20, cause {the filament current'to increase or decrease. The signal producing means 2011s situated inthepath-ofitheg-x-ray output-from X-ray tube and is connected to=the regulator1-9.- The regulator'is so adjustedthat a selected mean currentpror duced in the signal producing means by the desired quantityrate ot X-radiation will not produce atchange in the filament current.-- However, upon variation from this .gnean. current, the regulator will seekto counteractthe in theamount of ionization current thereby causing regulater 19 to produce an increase in the filaments current which ultirnatelycauses an increase in X-radiation output until themean value of ionization current is reestablished.

Althoughthe signal producingmember ZOof-thecircuit of Fig. 1 is described as anionization chamber, it is quite possible to substitute any .Xi-ra-y sensitive component capable of producing signal current fluctuations in repons to flu tuat o i X-ray pu o instance,

In this case, a second. iouizationchamber 23 is employed. This ionization chamber is placed'on the opposite side of filter 21, from ionization chamber 20'. Ioni zation chamber 23 thus is subjected tothe filtered'X-ray output of tube 10' while ionization chamber20 is subject to the unfiltered radiation produced by .tube 19. The signal :produced (in-ionization ."chaxnbrf 23 "is similar :to that produced i in v ionization chamber; 20 and. its ,elfect 'upon re-gulator24is similar -to-the efliect of ionization currents-from ionization chamber 20" upon-regulator 19:. More specifically, an increase in the ionization current over mean value in ionizationchamber 23 afiects regula tor 24 in such a manner, that it tends to reduce the oltage applied across tube 10, hence decreasing 'X-ray output;

to the level which'producesthemeanionization current.

Conversely, a decrease in the quantity of filtered X-radiation will produce a decrease fromthemeanof the ioniza-' tion current 'ther'eby causingan inc'reaseinvoltage across tube 10- until the quantity of X ra'diation. increases to the point, where themea'n ionization current is again produced.

Maintaining constant the uuantityrate of the filtered as well as of the unfiltered radiaticnwill result in constant quality of X-ray radiation, since the quality ofradiation is defined by the ratio of the quantities of filtered and unfiltered radiation. fIt is possible .to select quality. by

alteration or regulator responsejin either circuit toiest'ablish a difierejnt meanfionizationcnrrent..therein, .or' by varying jthethickness of (the filter employed, or by a combination of both'method's'; I

a matrix of cadmium sulphidecrystals might be employed in place of the'ionization chamber. Various types of ionization chambers may-also be employed. However, an ionization chamberemploying-anexternal. and internal electrode is preferred, inasmucheas such 'a chamber is usually more sensitive-to X-radiation. Sucha chamber may be made extremely thin with windows in its walls composed of a thin film of material highly transparent to X-rays coated with a thin layer of'conductive mate- .rial, such as the product commercially known asfaquadag. The internal electrode may alsobemade extremelythin and transparent to X-rays, or it may be con-. structed in such a manner that therX-ray beam does not have to passthrough it, In -any-event, the ionization chamber-[may be made such that the amount. of attenuation'produced by'it will be minorcompared with the .at-, teuuation produced by filter member 21. The filter 21 maybe placed'either between'the X-ray source and the signal producing means, as shown, or beyond the filters, so that the signal producing means lies, between it and the X-ray source. 7

Big. 2 illustrates schematically one ofthe circuits .of

7 my invention necessary for maintaining both constant quality of X-ray output. Again 60 a quantity rate and constant an X-raytube 10' having an anode 11' and. a filamentary cathode, 12' is enclosed within a ,shockproof, rayproof' housing l3". The potential across. the tube is supplied byenergy source-14" through transformer 15'. The ex citation of the filament is supplied by eneregy source17' through transformer 18'. Again filament current is modified by the action of a regulator 19' which responds to. signals from ionization chamberz20" which lies in the path of X-ray output from tube 10'. An increase in X-ray output produces an increase in ionization current in chamber--20 which will cause the'regulator 19' to reduce the amount of filament current supplied to cathode 12' thereby ultimately reducing the X-ray output of the tube. Conversely decreased- X-ray output Will cause the regulator to increase the filament current and hence increasethe v output of the tube.

' As previously mentioned, for regulating quantity rate.

only, it is necessary to employ only one ionization chamber androne regulator coupled thereto. ,Theiregulator may be placed either in the'voltage supply circuit as. in the case or regulator'24 of Fig. 2 or;-in the filament current supply circuit as in the case of Fig. 2; The 'ionization chamber may be placed either above or 7 below the filter 21f.

Various types of regulators may be ernployed with my invention. An electronic regulator of the saturable reactor type, suchas the one-illustrated in Fig. 3,;is.one-example of a simple regulator-which maybe used. Inorder to more clearly illustrate the operation of my invention, the

operationof this. regulator relative to its p'erfo rmance in connection with my invention will be explained: j

The regulator shown in Fig. 3 consists -of -an electrometer tube stage, an amplifier. stageland a saturable core reactor. The ionizationchamber is biased by'battery: 31 which has its positive terminal connected to the internal electrode 32 of the ionization chamber. and itsnegative termin'aliconnected to theexternal electrode 35. The sigml is impressed on the grid of electrometer tube 33v by placing resistor'34' across the mutually insulated ionization. chamber electrodes 32 ai1d'35. -Res'istorr34 is ad-, vantageously madevariable in order to allow themeanionization current which the regulator will sustain toibe varied. A by-pass. condenser36 advantageously placed in, parallel with resistance 34. .Pe'rmanent biasis supplied to the cathode 37 relative to the grid 38 by battery 39; Battery 40 supplies the heater current to cathode 37 of. electrometer' tube 33. Anode 41 is biased. positive by. battery 42 which also effects ,a negativebiasonthe cathode 43 of. amplifier tube .44 with respect to' its grid 45; The resistance 46 impresses thes'ig'nal on grid 45 of the amplifier tube 44. The battery 54suppliesheater current to the cathode "43. plate 47 of amplifier tube 44 is positively biasedby connection to batteryterminal B+. The D. C. output of amplifier 44 is,fedthr0ugh coil 48 which surrounds saturable ironcore 49; f Coil50 which also surroundscore' 49 is connectedin aseparate circuit in series with an alternatingcurrentsource 51 which corresponds to 17, 17 or 14'of Figs. 1, 2 rerun; nals 52 and '53 of the r'egulatorarethereafterconnected to the primary of a transformer such as 1 8' or l5' s'hown of the ionization chamber is placed at a more positive potential than the external electrode 35 due to their respective connections to the negative and positive terminals of the battery 31. Positive ions generated by X-radiation within the ionization chamber will flow to the negative external electrode 32 Where they will be neutralized by taking electrons from the electrode. Conversely, negative ions or electrons will flow to the positive internal electrode 35 and be neutralized. Thus, upon irradiation of the ionization chamber current will flow through resistance 34 from the internal electrode to the external electrode. When using generators of the pulsating voltage type, condenser 36 will cause the current flow through resistance 34 to be steady rather than pulsating. The voltage drop in resistance 34 produced by the flow of current therethrough will affect the D. C. potential upon the grid 38 of electrometer tube. The grid 38 of this tube 33 will normally be held in this manner by the mean ionization current at a potential such as to produce a steady flow of electrons from cathode 37 to anode 41, which is positively biased by battery 42. This flow will in turn produce a potential upon grid 45 of amplifier tube 44 dependent upon the voltage drop across resistor 46 due to the electrometer tube anode current flowing therethrough. This potential will be sutficiently positive to permit a fiow of current through amplifier tube 44 to anode 47. The anode current from amplifier tube 44 passes through coil 48 surrounding iron core 49. The magnetic flux due to the D. C. amplifier anode current of coil 43 partially saturates core 49 thereby making the A. C. inductance of coil 50 less than it would otherwise be.

It can be seen that, as the ionization current increases due to increased X-radiation, the potential drop across resistor 34 will increase, hence the potential of grid 38 will become more positive and allow more flow of current through electrometer tube 33. This increased flow through the electrometer tube will make the potential on the amplifier tube grid 45 more negative due to the increased drop across resistor 46. Hence the flow through the amplifier tube will be decreased. With the amplifier tubes anode current decreased, the magnetic saturation of the core 49 will be decreased, and hence the inductance of coil 50 increased. Thus, it may be seen that an increase in inductance in coil 50 is produced by an increase in ionization current. Hence, the current in the circuit with which the coil is in series is decreased because of the added inductance in said circuit.

The action of the regulator may be described as a feedback effect. When variation from the mean signal is severe, the feedback current from the regulator to bring it back to the mean is strong. As the variation from the mean becomes less severe, the feedback becomes less and less. There is some tendency to hunt, i. e. to overshoot the mean, but this tendency can be corrected to a large extent by the selection of circuit components having time constants which tend to damp out oscillations. Selection of the proper components having proper circuit characteristics for operation at a specified mean signal can be done by one skilled in the art. Selection by variation of resistance 34 is a preferred method.

I claim:

1. A system for maintaining constant quantity rate and constant quality of X-radiation from an X-ray generator comprising, in combination with the X-ray generator, a pair of X-ray sensitive signal producing means arranged successively with a filter between them in the path of X-ray output of said generator, said signal producing means each being coupled through separate regulator means to the filament circuit and the voltage supply circuit, respectively, in such manner as to restore to a mean the signal in both signal producing means in order to maintain both the filtered and unfiltered radiation output constant.

2. A system for maintaining constant quantity rate and constant quality of X-radiation from an X-ray generator comprising, in combination with the X-ray generator, a first X-ray sensitive signal producing means positioned in the path of the X-ray output of said generator, a first regulator to which the first signal producing means is connected and which is in turn coupled to the filament circuit, such that the first regulator acts to increase or decrease the filament current in response to variations from a first predetermined mean signal corresponding to the signal produced by the desired quantity of X-radiation in such a manner as to reestablish said first mean signal, a second X-ray sensitive signal producing means positioned in the path of the output of the X-ray generator, a second regulator to which the second signal producing means is connected and which in turn is coupled to the high voltage supply circuit, such that the second regulator acts to increase or decrease the voltage across the X-ray tube in response to variations from a second predetermined mean signal corresponding to the signal produced by the desired quantity of X-radiation in such a manner as to reestablish said second mean signal, and an X-ray filter interposed between the first and second signal producing means.

3. A system for maintaining constant quantity rate and constant quality of X-radiation from an X-ray generator as described in claim 2 characterized in that the X-ray sensitive signal producing means are ionization chambers,

4. A system for maintaining constant quantity rate and constant quality of X-radiation from an X-ray generator as described in claim 2 characterized in that the X-ray sensitive signal producing means are placed in such position that the first signal producing means is subjected to unfiltered radiation and the second signal producing means is subjected to filtered radiation.

5. A system for maintaining constant quantity rate and constant quality of X-radiation from an X-ray generator as described in claim 2 characterized in that the X-ray sensitive signal producing means are ionization chambers which are so positioned that the first signal producing means is subjected to unfiltered X-radiation and the second signal producing means is subjected to filtered X-radiation.

References Cited in the file of this patent UNITED STATES PATENTS 2,445,305 Hochgesang July 13, 1948 2,503,075 Smith Apr. 4, 1950 2,537,914 Roop Jan. 9, 1951 2,542,022 Friedman Feb. 20, 1951 2,643,343 Rainwater June 23, 1953

Images