US2499546A

US2499546A - Device for making short-time exposures by means of x-rays

Info

Publication number: US2499546A
Application number: US685153A
Authority: US
Inventors: Vingerhoets Antonius Wilhelmus
Original assignee: Hartford National Bank and Trust Co
Current assignee: Hartford National Bank and Trust Co
Priority date: 1943-05-21
Filing date: 1946-07-20
Publication date: 1950-03-07
Anticipated expiration: 1967-03-07
Also published as: FR904462A; CH258429A; GB632308A

Description

Much 1950 A. w. VINGERHOETS 2,499,546

DEVICE FOR MAKING SHORT-TIIE EXPOSURES BY MEANS OF X-RAYS Filed July 20, 1946 wrnvrm -,4/vmv/us W/LHELMUS V/NGfKHOETS tube. through the X-ray tube has a high intensity,

'time X-ray exposures. determined proportion of the energy present in Patented Mar. 7, 1950 DEVICE FOR MAKING SHORT-TIME EXPOSURES BY MEAN S OF X-RAYS Antonius Wilhelmus Vingerhoets, Eindhoven, Netherlands, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as

trustee Application July 20, 1946, Serial No. 685,153 In the Netherlands May 21, 1943 Section 1, Public'Law 690, August 8, 1946 Patent expires May 21, 1963 2 Claims. (Cl. 250-98) In making short-time exposures by means of tions which occur during the load due to the loss of voltage in the valves and condensers, within the admissible limits.

When for the operation of the X-ray tube during short periods, use is made of pulsating direct current, the tube is subjected to the full load only during part of every half-wave of the rectified voltage. Besides, the results of the X-ray exposure are dependent, to an appreciable proportion, upon the course of the voltage applied to the tube when loaded, which voltage determines the contrast efiect in the photographic material. .If

the voltage is excessively high, the rays are too hard and an insufficient contrast efiect is obtained. If on the contrary, the voltage is excessively low, an appreciable proportion of the rays is absorbed by the body to be radiographed. With a pulsating direct-current voltage, the tube voltage is always too low during part of the phase, with the result that only part of the energy supplied to the tube is converted into useful X-rays so that a low efficiency is obtained.

For feeding the X-ray tube of an X-ray apparatus use is often also made of a condenser which, after being charged up to a determined voltage, is caused to discharge through the X-ray The current which, in this case, flows whilst the discharge is of very short duration. Such devices are very suitable for making short- In order to ensure that a the condenser is converted by the X-ray tube into useful X-rays, the condenser must be charged to a higher voltage than is desirable in view of a satisfactory contrast effect of the X-rays. At the start of the discharge the rays are too hard. After the voltage has fallen below a certain value, the Y hardness of the rays is too low.

With such a device it is known to connect a self-inductance coil in series with the X-ray tube,

so'that it is possible to act upon the voltage of the X-ray tube during the discharge of the con denser in such manner that the maximum value "oi'the voltage diflers'less from the average value VII ' load voltage.

of the voltage within the range in which the condenser discharge for taking a radiograph yields useful results. It is true that thus it is achieved to convert a larger proportion of the energy available in the condenser into useful X-rays, but the voltage variations at the X-ray tube in the period of time in which the exposure is efiected are still appreciable.

With a device for making short-time X-ray exposures which comprises a source of voltage which funishes a voltage impulse or a periodically greatly variable voltage and an inductance coil having an iron core, which coil is connected in series with the X-ray tube, the invention has for its object to effectuate the load on the X-ra tube at a substantially constant voltage. According to the invention, this is possible by varying the selfinduction of the choke coil during the passage of the load current. It is possible to utilize for this purpose means which have such an influence on the magnetisation of the iron core that the self-induction of the coil increases after the voltage of the supply device, after the load circuit has been closed, has attained the load voltage and decreases when this voltage falls below the The desired influence on the magnetisatlon of the iron core may be obtained by providing on the iron core of the choke coil an auxiliary winding through which a Variable current is sent. In order to obtain an increase of the self-induction,

, this current must be directed in such manner that it brings about a magnetic field which is oppositely directed with respect to the field generated by the current in the self-inductance coil. In-

' versely, the current must generate a field of the same direction if a decrease of the self-induction is desired. The current variations in the auxiliary winding may be brought about by connectin a source of direct-current voltage of the auxiliary winding via a commutator which is driven in synchronism with the alternating voltage of the supply mains. It is also possible to provide the iron core with an air gap and to make this gap larger or smaller during the operation of the tube.

In an apparatus wherein the tube current is provided by a charged condenser the desired variations of the self-induction may be brought about by connecting a condenser in series with the auxiliary winding and b connecting this combination of elements in parallel with the inductance coil. The course of the discharge current of the condenser which acts as a source of energy for the X-ray tube is in this case suitable for the automatic control of the magnetlsation of the iron core due to the passage of current through the auxiliary winding.

It is advisable to extend the control of the selfinduction over the Whole of the loading period. Provision be made to ensure that during the period of time required for raising the voltage of the X-ray tube to the operation value the self-induction has a value lower than that which is determined by the factors connected with the construction of the coil, so that the counter-Voltage set up across the coil is low. As a result, the voltage on the tube increases rapidly to the desired value, here referred to as the operation value, which is chosen in connection with the correct contrast eifect in the photographic material and with the path to be covered by the X-rays in the object.

The value of the load current is adjusted in such manner that the exposure is made in the shortest possible timein order to ensure that the unsharpness produced when moving objects are radiographed will be as slight as possible. The

' current is determined by the load capacity of the tube andin the case of a constant voltage on the tube it may be taken higher than in the case of a greatly varying voltage on the tube.

In. order that the invention may be clearly understood and readily carried into efiect, it will now be explained more fully withreference to the accompanying drawing in which Figure 1 represents some voltage curves of an apparatus connected in circuit as shown in Figure 2 and- Figure 3 represents some voltage CllIV6S,'f0l a device of which a simplified diagram of connections is shown in Figure 4 and in which the X-ray tube is supplied from a charged condenser. Fig. 5 shows a commutator arrangement for varying the self-inductance of the inductor and Fig. 6 shows a core for the inductor with a variable air-gap.

In Figure 1 the course ,of the voltage on the X-ray tube is represented by a curve I if, according to Figure 2, the supply device is constituted by a high-voltage transformer M and the current is converted by a rectifying device, for example a discharge tube l5, into a rectified alternating'curre'nt, an inductance coil I! being con- .nected in series with an X-ray tube 6. The portion a of the curve, wherein the voltage is negative, is produced due to the switching-on phenomena which occur in such a device when the load circuit is closed, which may be effected by means of a switch I8. The portion b has a substantially normal, sinusoidal course. On the iron core of the inductance coil I1 is provided an auxiliary winding l9 through which passes a variable current which is provided by a source of direct current 20 and which is supplied to the winding i 9 via a commutating device 2| as shown in Fig. 5. Alternatively, the self-induction of coil I? can be varied by varying the reluctance of the core, i. e. by employing with a variable air-gap as shown in Fig. ,6.

At the start of the load the self-induction of the coil may be given a low value by magnetising the iron core with the aid of a direct current sent through the auxiliary winding 1.9,. When switching-in the load current, the voltage on the tube then increases rapidly to the operation value. The control of the self-induction which is low in the beginning, must be large in a portion of. the voltage curve I), in which the voltage submitting the current in the auxiliary winding i9 to determined variations with the aid of a commutator 2|. If use is made of a simple change-over switch the current in the auxiliary winding may exhibit the course represented by the dotted line 0 and an appreciable improvement is already-obtained. The voltage on the tube has in this case the course indicated in the figure by d. The invention aims at obtaining a voltage which is constant over the greatest possible portion of the load and which is denoted in Figure 1 by e. In order to achieve this, th current control in the auxiliary winding becomes somewhat more complicated, but nevertheless it is still practicable. Owing to the control of the self-induction it is achieved that of the excessive energy provided by the supply device at the voltage which exceeds the operation value, a larger portion is absorbed by the impedance than if this control were not utilized. Likewise, a larger portion of the energy absorbed by the impedance is delivered to the X-ray tube than in case of the self-induction remaining constant, said portion being larger according as the self-induction is smaller. This results in that the voltage on the X-ray tube is maintained at the operation value during a longer period of time and then falls rapidly to the instantaneous value of the voltage provided by energy consumed by the X-ray tube; it appears therefrom that the ratio between the total energy available and the useful energy is considerably more favourable for the voltage shape according to curve 3 than for the voltage shape according to .curve I.

It is found that, since the current intensity in the tube is limited by the load capacity of the tube and by the maximum voltage that occurs, a higher current intensity is admissible, so that the time of the exposure may be reduced.

The explanations given above with respect to one half-wave of the voltage also apply if the loading period extends over more waves of the alternating voltage.

Figure 3 shows three curves 4, 5 and 6 which represent the course of the voltage in the case of a .device comprising a charged condenser for operating the X-ray tube.

In order to ensure that a determined portion of the energy present in the condenser is converted by the X-ray tube into useful X-rays, it is necessary to charge the condenser up to the voltage e1. At this voltage the rays are too hard to produce satisfactory contrasts in the photographic material. Only after the voltag has fallen to a value a have the X-rays lost so much of their hardness that a sharply defined radiograph is obtained. Below the value es the quantity of rays which penetrate through the object is too small.

If use is made of a choke coil in series with the X-ray tube, the course of the voltage may approximately correspond to curve ,5. The voltage increases rapidly to the value e3, at which the 'X-ray tube begins to yield useful rays. It retains a value in the useful range of the condenser discharge; the generation of active rays terminates after the value eg has been attained again.

.It may ,be seen from the figure that in both cases only part of the energy accumulated in.

the condenser is efiiciently utilized. An appreciable portion of the total charge is lost and is converted in the X-ray tube into heat unless a device is provided by which the circuit is interrupted immediately after the voltage on the tube has dropped to the value es. The measures required therefor involve a complication, due to which the simplicity of the apparatus and consequently the advantages with respect to devices wherein measures are taken to reduce the loading period are lost.

The invention renders it possible to take greater advantage of the energy accumulated in the condenser. With the device according to Figure 4 the self-induction of the impedance l3 connected in series with the X-ray tube varies automatically during the discharge of the condenser l which acts as a source of supply for the X-ray tube 8. The flow of the discharge current through the X-ray tube can be effected with the aid of a switch which may, for example, be constituted by an electron discharge tube having a control grid. The passage of current through the inductance coil 9 brings about at the terminals of the coil a potential difference which acts upon the passage of current through the auxiliary winding l I which is provided on the iron core 13 of the impedance and whose field also determines the magnetisation of the iron core which is brought about by the field of the coil 9. By connecting a condenser l2 in series with the auxiliary winding H the current flowing through the auxiliary winding acquires such a course that the production thereof causes variations of the self-induction of the iron-cored coil in a sense which is favourable for the suppression of deviations from the operation value of the tube voltage.

The difference in the voltages set up on the X-ray tube appears from Figure 3 in which the voltage curves approximately represent the average course of the voltage on the tube. The curve 5 represents the voltage in the case of the use of inductance coil in the discharge circuit of the condenser. The curve 6 represents the same voltage in case the invention is carried into effect. No useful radiation is obtained in the one case during the period of time from to to it and in the other case during the period of time from to to 281 since the voltage on the X-ray tube is less than the value 63 and consequently is too low. The period of time from to to h may be considerably shorter than that from to to ft. The loss of energy at the start of the discharge is accordingly smaller. It may also be seen from the figure that the curve 6 has dropped to the value 63 at a later moment t2 than the curve 3. Due to the greater consumption of energy during the useful load the voltage on the X-ray tube then decreases rapidly. According to curve 6 a greater portion of the available energy than according to curve 5 is consequently consumed in the range in which the voltage has a favourable value. If a determined loading period is desired, the capacity of the condenser which provides the load current may be taken lower by carrying out the invention. If, for example, a capacity of 0.6 1. is used, the self-induction may amount to approximately 1000 henries.

The device required for charging the condenser is not shown in Figure 3. It may be realized exactly according to the circuit-arrangements known for this purpose, wherein the charging current is provided by a high-voltage transformer and is supplied to the condenser via a rectifier. Instead of the condenser, use may also be made of a battery of plurality of condensers, whilst the rectifier may consist of one or more discharge tubes or of rectifying devices of other types, such as selenium cells. In order that the dimensions of the complete outfit may be kept small and that the latter may be connected to an alternating current mains which can provide only little energy, the charging current may be chosen in such manner that the absorption of energy by the condenser is distributed over a number of waves of the alternating voltage. In this case it is not necessary to interrupt the charging circuit when the discharge circuit is closed. The switching device in the discharge circuit may, however, also be coupled with an interrupter of the primary current of a highvoltage transformer, in which case the source of charging current is separated from the supply mains at the same moment when the discharge is switched-in.

What I claim is:

1. A circuit arrangement for energizing an X-ray tube for a short-time exposure comprising a charge capacitor source of high tension potential for the tube and an iron cored inductance element in series with the capacitor and the tube for supplying substantially constant voltage thereto during an X-ray exposure, the iron cored inductance element including a main winding in series with the capacitor and the tube and an auxiliary winding in series with a source of unidirectional current of variable potential connected in parallel with said main winding for varying the self-inductance of the iron cored inductance element during the discharge of said capacitor source when said tube is being energized to maintain the voltage applied to the tube substantially constant.

2. A circuit arrangement for energizing an X-ray tube for a short-time exposure comprising a charged capacitor source of high tension potential for the tube and an iron cored inductance element in series with the capacitor and the tube for supplying substantially constant voltage thereto during an X-ray exposure, the iron core inductance element including a main winding in series with the capacitor and the tube and an auxiliary winding in series with a second charged capacitor connected in parallel with the main winding for varying the self-inductance of the iron cored inductance element during the discharge of said capacitor source when said tube is being energized to maintain the voltage applied to the tube substantially constant.

ANTONIUS WILHELMUS VINGERI-IOETS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,214,871 Westendorp Sept. 17, 1940 2,222,536 Kuntke et al. Nov. 19, 1940 2,356,621 Sciaky Aug. 22, 1944 2,416,718 Shockley Mar. 4, 1947