US2936376A

US2936376A - Installations for making X-ray photographs

Info

Publication number: US2936376A
Application number: US693285A
Authority: US
Inventors: Gerrit D A Hoekstra
Original assignee: NL RONTGEN APP NFABRIEK NV
Current assignee: "N V NEDERLANDSCHE RONTGEN-APPARATENFABRIEK"; NL RONTGEN APP NFABRIEK NV
Priority date: 1956-10-31
Filing date: 1957-10-30
Publication date: 1960-05-10
Anticipated expiration: 1977-05-10
Also published as: FR1185535A; GB840780A; NL92458C; DE1075229B

Description

May 10, 1960 G. D. A. HOEKSTRA 2,

INSTALLATIONS FOR MAKING X-RAY PHOTOGRAPHS Filed Oct. 30, 1957 A K V 200 #4 United States Patent INSTALLATIONS FOR MAKING X-RAY PHOTOGRAPHS Gerrit D. A. Hoekstra, Delft, Netherlands, assignor to N.V. Nederlandsche Rontgen-Apparatenfabriek, Delft, Netherlands Application October 30, 1957, Serial No. 693,285 Claims priority, application Netherlands October 31, 1956 9 Claims. (Cl. 250-103) The invention relates to an installation for making X-ray photographs, in particular for purposes of medical examination.

When making medical X-ray photographs or radiographs in the usual way, the object to be exposed is placed in front of a fluorescent screen and the image appearing on this screen is photographed by means of a camera arranged to the rear of the screen. In order to obtain an exposure of high quality, it is essential that the photographic film has the required density and that the image is sharply defined and has the right degree of contrast. This quality of the image is greatly influenced by the quality, i.e. the hardness of the X-rays and the quantity thereof. The quality of the X-rays depends on the voltage of the tube (kv.) and the quantity is determined by the product of the current intensity of the tube and the time of exposure (ma-see).

In the conventional method for making X-ray photographs, for each object a distinct tube voltage is chosen,

in most cases as low a voltage as practical and with which it is expected that a good picture can still be obtained. This is generally done by measuring or weighing the patient to be radiographed and determining the desired tube voltage by means of a table. In connection with the value for the tube voltage so found (usually varying between ca. 50 kv. and 125 kv.), the tube current is then so chosen thatin the shortest possible exposure time the required film density is obtained.

In the conventional X-ray installations the tube-voltage is, therefore, adjustable but has to be kept at a constant value during the exposure. This implies that an auxiliary voltage stabilizing apparatus is required to compensate possible fluctuations of the voltage of the network feeding'the installation. X-ray installations of the transportable kind which must, therefore, be connectable to different networks, also need an apparatus for adapting the installations to the resistance of the network. Apart from the means for adjusting the tube voltage, a further complicated circuit for protecting the X-ray tube against overloading is required. All these auxiliary devices and circuits make the installation cumbersome, complicated and expensive. i

. The invention has for its main object the provision of an X-ray installation for making X-ray photographs of the kind described which as regards operation and adjustment is considerably simpler and therefore quicker to handle as compared with the known installations.

Another important object of the invention is to provide an X-ray installation of the kind described of which the auxiliary apparatus can be greatly reduced, thereby considerably reducing the weight, volume and manufacturing cost of the installation.

X-ray installations of the type as discussed above normally comprise a source of high-voltage, such as a highvoltage transformer or other high-voltage generator means to be fed by the network, which high-voltage source. is connected across the anode and hot incandescent 2,936,375 a [C6 Patented May 10, 1960 cathode of an X-ray tube at the start of an exposure by the operation of an exposure switch or the like and is again disconnected after a selected time of exposure by means of a time switch means which may be of the type that has to be pre-set according to the desired exposure time or which may be of the known automatic photo-timer switch type to be discussed below. According to the invention, the installation further comprises circuit elements which immediately after the switching on of the generator means by the actuation of the exposure switch cause the voltage across the tube to rise quickly from a selected minimum value and the current through the tube to fall from a selected maximum value.

At the start of the exposure, the tube voltage is thus relatively low, for instance 50 kv., the voltage however rising quickly, in say 0.2 sec., towards a maximum value of, for instance, -125 kv. Generally, however, this maximum value of the tube voltage will not be reached because-before that the film will already have obtained the desired film density so that the tube is disconnected by the time switch means at a lower voltage. Children and thin persons are thus exposed and photographed at a relatively low average tube voltage and a correspondingly high average tube current, comparable to the conventional exposure methods, which provides a good picture quality. When exposing heavier persons, the required greater X-ray dose is obtained in that towards the end of the longer exposure time as a result of the rising the requirements of the conventional radiographical exposure techniques, whereas insofar as these requirements are not fully met the additional dose of harder rays serves as a correcting factor.

By properly selecting and adjusting the relevant circuit elements, the tube-voltage-time curve and tube current-time curve during exposure can be made to follow such a course that when at the end of the exposure the required film density is reached, the picture at the same time has a right degree of contrast.

An especially advantageous result is obtained when, according to a preferred embodiment of the invention, a photo-timer switch arrangement is used for controlling the exposure time. Such a photo-timer switch arrangement, in itself well known in the art, comprises a photo,-

electric device which cumulatively measures the quantity with the type of film used, it is thus possible to have the exposure finished at exactly the moment the right film density is reached. t

In this way a practically automatically functioning X-ray installation is obtained. The patient need no longer be weighed or measured in order to determine the correct values of the tube voltage, current and ex posure time and to adjust the installation accordingly,'

but without any adjusting operation the patient can immediately be placed in front of the screen and the exposure switch be operated whereupon the photo-timer switch disconnects the X-ray apparatus after the correct 1 exposure time." This photo-timer switch arrangement in 3 cooperation with the described novel characteristics of the'X-ray installation thus, as it were, automaticallyse lects the desired average tube voltage for the object concerned.

'It follows from the foregoing that fluctuations of the network voltage around the nominal value' thereof cannot aversely affect the quality of the resulting picture which means that a tube voltage stabilizing apparatus can be dispensed with. Only the voltage of the heating circuit of the X-ray tube must be stabilized which can, however, be done with relatively simple means.

According to a preferred embodiment of the invention, the installation comprises circuit elements which after the switching on of the X-ray tube cause the tube current to fall sharply, while the high-voltage generator means has a high impedance which is so chosen that the tube current fall effects the desired quick tube voltage rise. When, for instance, the generator means comprises a high-voltage transformer, the latter is given a high internal resistance so as to obtain a sharply dropping current-voltage characteristic of the transformer output. Because of this high impedance or high internal resistance of the transformer, variations in the resistance of the network to which the installation is connected, will no longer have any real effect so that the otherwise necessary apparatus for adapting the installation to the network resistance can be dispensed with.

The tube-current fall can be easily realized'by decreasing the voltage across the heater of the X-ray tube in one step from a high to a low level at the moment the exposure switch is operated. The tube cathode will thereby quickly cool and the tube current 'fall correspondingly to a lower level of intensity.

As follows from the foregoing, the. high-voltage generator means or transformer can during an exposure directly be connected to the network, i.e. without the interposition of any voltage regulating or stabilizing devices. It is, furthermore, possible to adapt the highvoltage transformer to the characteristics of the X-ray tube in such a way that the focus of the latter cannot be overloaded but that, on the other hand, during exposure themaximum admissible load is approached. For the maximum power of the X-ray tube can be expressed by the product of the tube voltage and the accompanying tube current and decreases with time. By a proper selection of the current-voltage characteristic of the high-voltagegenerator means and the time constant of the relevant elements of the installation it is possible to.insure that the maximum adrnissable value of this product can at no time be-surpassed. In that case, the apparatus for protecting the tube against overloading can likewise be dispensed with. As a result, the X-ray installation according to the invention can be made small, light andeasily transportable, while, furthermore, because of'its relatively simple construction, the possibility of a fault or break-down is greatly reduced.

These and other novel features of the invention will appear more fully from the following description and claims in conjunction with the drawing which shows an embodiment of the invention.

In the drawing:

Fig. l is a schematic diagram of connections of an X-ray installation provided in accordance with the in? vention; v

Fig. 2 represents the current-voltage characteristic of the high-voltage transformer of the diagram of Fig. 1; and

Fig. 3 illustrates the tube-voltage curve and'tube-current curve as plotted against exposure time.

The X-ray installation diagrammatically shown in Fig. 1.has' a high tension transformer 1 with a primary winding 2 and a secondary winding 3 provided with a grounded center tap. The winding 3 has a highresistance giving the transformer on its secondary side a current-voltage characteristic as shown somewhat idealized in Fig. 2, i.e, at a decrease'of the secondary transformer currentfrom about 200 ma. to 40 ma., the secondary transformer voltage rises from about 50 kv. to kv. The trans former winding 3 feeds a biphase full-wave rectifier arrangement 4, the DC. terminals of which are connected to the cathode 5 and anode 6, respectively, of an X-ray tube 7.

The primary transformer winding 2 is fed from a network 8 through a main switch .9 and leads 10 comprising a pair of contacts 11 of a relay switch 12 having an actua= tion coil 13.

V The heater of cathode 5 of the tube 7 is fed by a heating current transformer 14 which on its primary side is connected by leads 15 tothe leads 10-through a variable resistance 16 for the adjustment of the heating current:

on the one hand and through a second variable resistance 17 on the other hand. The latter resistance 17 can be bridged by a contact 18 of a relay switch 19 having a coil 20.

Coil 13 of relay switch 12 is connected to the. leads. 10 through a series connection of a contact 21 of a relay,

switch 22 having a relay coil 23 and

contacts

24, 25 of:a double manually operated exposure switch 26. The mov-- able contact 24: of the switch 26 further cooperates with a. contact strip 27 in such a way that this switch has an" intermediate or pre-exposure position between its illustrated position and its exposure position in which preexposure position contact 24 only touches contact strip 27 whereas in its exposure position the movable switch contact 24 is connected both to

contacts

27 and 25; Switch 26 has a second contact arm carrying a movablecontact 28 which in the exposure position of the switchis connected with a further fixed switch contact 29.

Coil 20 of relay 19 is connected to leads 10 through V the

contacts

24, 27 of the exposure switch 26, a lead 30,

' a contact 31 of the relay switch 12 and a contact 32 of a further relay 33 on the one side and through a lead 34 on the other side. The coil 35 of this relay 33 is connected to leads 10 through lead 34, and through a further. movable contact 36 of relay switch 12 and a lead 37. Re. lay 33 has a holding contact 38 comprised in a lead 39. leading from one end of the relay coil 33 to the contact strip 27 of the exposure switch 26. V

The relay 22 is controlled by a photo-timer switch arrangement of well known type comprising a photo-electric cell 40, a condenser 41 connected across this cell to, be charged by the latter and a thyratron 42 controlled by the voltage of the condenser 41- which is connected across, the gridand cathode thereof. The output circuit of the. thyratron 42 comprises thecoil 23 of relay 22 in

serieswiththe contacts

28, 29 of the exposure switch 26.- The photo-electric cell 40 is so arranged as to receive light radiated by a portion of the picture appearing on the;

fluorescent screen S of the installation during the exposure. ofan object placed in front of this screen which picture:. is at the same time photographed by a camera arranged: together with the cell to the rear of this screen. The, construction and relative arrangement of the elements .of. the radiographical outfit comprising an X-ray tube, fluores'cent screen, camera and photo-timer arrangement is,

well known in the art and is, therefore, not shown; and. described in detail.

The working of the installation represented in Fig. 1. is as follows.

When the main switch 9 is closed, the X-ray tube-7- receives a reduced heating current through leads 15 and transformer 14, the valve of this current being determined by the setting of the variable resistances 16 and 17. The

whereby a circuit is closed from the upper one of leads... 10, through

contacts

24 and 27 of the exposure switch,

lead 30, contact 31 of relay switch 13, contact 32 of relay 38,- coil20 of relay-19 and back'through lead 34"to the" otherone leads 10. RelayJ-Q, thus excited-closes contact flishort circuiting the resistance 17, whereby the'voltage at the heating current transformer 14 is raised to its full working value as adjusted by the variable resistance 1,6; and the-tube7 is fully heatedup: 1

Thereupon the switch 26 is moved to its exposure position, its

contacts

24, 25 thus closing the circuit through cont'act21 of relay 22 and the coil 13 of relay switch 12 immediately closes-its contacts 11 and 36 and opens contact 31.- By the closing of contacts 11, the X-ray tube 7 is put under voltage. At this moment, the tube current as determined by the initial full heating current voltageis, for instance, 200 ma. and the tube voltage 50 kv.

The opening of relay switch contact 31 at the same time interrupts the circuit of relay coil 20 whereby relay contact 18 is opened without delay and the resistance 17 is again connected in the heating current circuit. The voltage across the transformer 14 thus drops to a lower level, causing a gradual cooling of the tube cathode, whereby the tube current decreases within about 0.2 see. from its top value of 200 ma. to a lower level of about 40 ma. according to curve 43 of Fig. 3. Because of the high internal resistance of the transformer 1, the tube voltage will correspondingly rise from 50 kv. to 100 kv. according to curve 44 of Fig. 2.

Before these latter values of the tube voltage and tube current are reached, however, the X-ray tube will generally be disconnected by the photo-timer switch arrange ment 22, 4042. Part of the light radiated by the fluorescent screen S during exposure is received by the photo-electric cell 40, and the condenser 41 is thus charged by the resulting current flowing through the cell circuit, the speed of charging being dependent on the quantity of light received by the cell 40. As soon as the voltage across the condenser 41 has reached a selected valve, the thyratron 42 tires, whereby relay coil 23 is excited, opening its relay contact 21. The circuit of relay switch coil 13 is thus interrupted and the high-voltage transformer 1 disconnected. By properly selecting and adjusting the elements of the photo-timer switch circuits with respect to the properties of the film used for photographing the image appearing on the fluorescent screen S this disconnection can be made to occur exactly at the moment the right film density has been obtained.

The return of the several contacts of relay switch 12 to their initial position at the end of the exposure time does not affect relay 33 since its coil 35 remains excited through its holding contact 38. Contact 32 therefore remains open and the circuit of relay coil 20 interrupted, thereby preventing contact- 18 from closing and again short-circuiting resistance 17 the moment the transformer 1 is disconnected by switch 12. Relay coil 23 keeps drawing current through the fired thyratron 42 and holds its contact 21 open, thus preventing switch 12 from re-closing its contacts 11. Only by moving the exposure switch 26 back to its initial neutral position can the circuits of relay coils 35 and 23 be interrupted, whereupon the next exposure can be made in the same manner as described.

Obviously, it is possible to replace the photo-timer switch arrangement by a normal clockwork time switch in which case this switch has to be adjusted to the proper exposure time prior to the exposure.

The form of the tube voltage-time and tube currenttime curves 43 and 44 of Fig. 3 can be so chosen that no matter after. what exposure time (i.a. at which point of the curves), the right film density is reached and the X- ray tube disconnected, the photographic picture also has a right degree of contrast; furthermore, the form of the curves can be so chosen as to prevent the possibility of overloading the X-ray tube no matter how long the exposure, since the momentary load is a function of the value of the tube voltage and the intensity of the tube current occurring at each given moment. The form of the curves 43 and 44 is determined by the time constants and characteristics of the various circuit elements involved. More particularly, theirformdepends on the temperature-time curve of the cathode of the X-ray tube and on the proper selection and adaptation of the current-voltage characteristic of the transformer 1 with respect to thistemperature-time curve. After the tube and the transformer have been selected, however, some adjustment of the curves 43 and 44 remains possible within a small range by variation of the resistance 17 and, to a lesser extent, of the resistance 16.

The described installation of Figure 1 needs no stabilizing circuit for the tube voltage. A device for stabilizingthe voltage of the heater current circuit may possibly be required but is not shown; such a device is relatively simple and can be of any appropriate known kind.

It will be obvious to those skilled in the art that various modifications of the embodiment of the invention as shown and described can be resorted to within the scope of the following claims.

What is claimed is:

l. A method of controlling an X-ray tube for making an X-ray exposure on a photographic film comprising, during exposure of said film to the X-rays, continuously increasing the anode voltage of the tube from a selected minimum value and simultaneously continuously decreasing the current flow from a selected maximum value and disconnecting the tube when the desired exposure of the film, for the subject being X-rayed, is attained.

' 2. An X-raying method for making an exposure of a photographic film comprising applying an anode voltage to an X-ray tube for the generation of X-rays to obtain an exposure on said film of the subject to be X-rayed, applying to the cathode of the tube during exposure of the film a heater voltage and decreasing the heater voltage during the film exposure so as to decrease the cathode temperature whereby current flow through the tube de creases and the anode voltage thereof increases in a continuous manner thereby similarly increasing the intensity of the X-rays, and disconnecting the X-ray tube at the end of the exposure.

3. A method as claimed in claim 2 comprising measuring the exposure caused by the X-rays as affected by the subject being X-rayed and disconnecting the tube when a predetermined exposure is obtained.

4. Apparatus 'for X-ray exposures of a photographic film comprising an X-ray tube, means coupled to said tube for applying operating voltage thereto, means coupled to said tube for continuously decreasing the current passing therethrough during exposure, means coupled to said tube for causing the anode voltage of the tube to increase continuously on said decrease of said tube current, and means coupled to said means for applying operating voltages to disconnect said tube at the end of an exposure.

5. Apparatus as claimed in claim 4 in which said means for disconnecting the tube comprises means for sensing said exposure so as to cause the tube to be automatically disconnected when thedesired exposure of the subject being X-rayed is obtained.

6. X-ray apparatus for exposure of a photographic film comprising an X-ray tube including an. anode, cathode, means to heat said cathode, a voltage source coupled to the anode and a control coupled to said means for lowering the heating of the cathode during the film exposure and thus decreasing the current flowing through the tube, and means coupled to said tube for increasing the anode volt-age when said tube current decreases.

7. Apparatus as claimed in claim 6 wherein the first said means comprises a heating circuit, a resistor in the heating circuit and short-circuiting means to by-pass said resistor, said control comprising a switch device coupled to said short-circuiting means to control the by-passing of said resistor.

8. X-ray apparatus comprising an X-ray tube including an anodeand a thermal responsive electron emission electrode a voltage source; aresistor coupling 'said'source' to said emission electrode; a higi'i impedance devi'ce for coupling said source to said anode; means for lay-passing said resistor at 'least in part, a-switch' for coupling the source to the anode via's-aid high impedance device a'nd actuating said meanswhereby the current through said tubeis decreased and the voltage at said anode increased during exposure of a photographic film, means for registering the exposure of a subject, means operatively disposed with respect to the second said means for sensing said exposure and being coupled to said source for disconnecting the tube from said source when an exposure of predetermined magnitude has been obtained.

2054,4 93 2,319,322 7 Hefel May; 1-18, 1943;- 2,379,1 25 Weisglass June 26, 1945. 2,617,047 Kuntke Nov; 4, 1952; 2,667,587 K'untke. et a1, Jan 26,=19,-5 2,681,417

Ball June 15, 1955;;