Switching Device for an X-Ray Generator Comprising a Time Switch

The invention relates to a switching device for an X-ray generator comprising a time switch which is incorporated in the primary circuit of a transformer which supplies the current for an X-ray tube, the actuation time being longer than the exposure time.

A time switch therein serves to interrupt the primary circuit at the end of an exposure time. The actuation time (i.e. the time during which the time switch contact in the primary circuit is closed) must then exceed the exposure time (i.e. the time which is adjusted as the exposure time by the user of the X-ray equipment or during which the X-ray tube emits the desired radiation) only because the electrical voltage across the X-ray tube can follow the voltage across the primary circuit with a given delay due to the unavoidable stray inductances, cable capacitances, etc. Therefore, to correct this problem in known X-ray generators, the actuation time is set to exceed the adjusted exposure time by a fixed amount (for example, 1 ms). This means that when the user adjusts an exposure time of, for example, 1 ms, the actuation time is actually 2 ms, whereas in the case of an adjusted exposure time of 10 ms, the actuation time is really 11 ms.

However, it was found in practice that also in that case the effective exposure time (i.e. the time during which the X-ray tube emits the desired radiation) does not correspond to the exposure time adjusted by the equipment user, particularly in the case of very short exposure times (a few milliseconds). Therefore, an object of the present invention is to design a switching device for an X-ray generator of the kind set forth such that the effective exposure time at least approximates the adjusted exposure time, also especially in the range of very short exposure times.

According to the invention, this object is achieved in that use is made of means by which the time switch actuation time is prolonged beyond the adjusted exposure time in dependence on the current through the X-ray tube.

As already stated, the exposure time is always shorter than the actuation time because the secondary voltage and the secondary current can follow the variation of current and voltage on the primary side only with a given delay. Investigations have revealed that this switch-on delay is larger as the current to be switched is larger. Because this switch-on delay is not compensated for by an equivalent switch-off delay, at least not in X-ray generators whose primary circuit is switched on and off by thyristors, a current-dependent reduction of the exposure time arises, this reduction being larger as the current to be switched is larger. It will be obvious that by using a constant (i.e. fixed) prolongation of the actuation time such as is effected in the known X-ray generators, (e.g. 1 ms), the effective exposure time can be made to correspond to the adjusted exposure time only for one given value of tube current. In the case of smaller tube currents, the effective exposure time becomes too long, while it becomes too short in the case of larger tube currents. By using the technique of current-dependent prolongation of the actuation time according to the invention, it can be achieved that the effective exposure time corresponds to the adjusted exposure time, and independent of the amplitude of the switched X-ray tube current.

In X-ray generators comprising a time switch whose actuation time is determined by a resistor which is active in the time circuit, the actuation time can be prolonged in dependence on the exposure current according to a further preferred embodiment of the invention in that the time circuit incorporates, in addition to a resistor which can be adjusted by means of the adjusting member for the exposure time, a resistor which can be adjusted by means of the adjusting member or the adjusting members for the tube current. In an X-ray generator provided with buttons for selecting fixed current values, an auxiliary resistor which is each time necessary can be switched on, for example, by the corresponding current button.

In X-ray generators with isowatt operation, in which the current varies in dependence on an adjusted power and an adjusted tube voltage, the actuation time is prolonged in dependence on the exposure current according to a further preferred embodiment of the invention in that this auxiliary resistor is adjustable in dependence on the exposure voltage and the power consumption. To this end, the resistor can be adjusted by the adjusting member for the power and also by the adjusting member for the tube voltage.

Investigations have revealed that the correspondence between effective exposure time and adjusted exposure time can be achieved roughly independent of the exposure current if, according to a further embodiment of the invention, the actuation time can be prolonged by a constant (fixed) amount and by an amount which is proportional to the tube current. In this case, the actuation time T.sub.s = T.sub.a + T.sub.c + T.sub.i (I/I.sub.O), in which T.sub.a is the adjusted exposure time, T.sub.c is the constant actuation time prolongation, T.sub.i is a time constant, I is the adjusted or measured tube current, and I.sub.O is a fixed current value. The constants T.sub.c, T.sub.i and I.sub.O can be proportioned such that the effective exposure time each time corresponds to the adjusted exposure time, independent of the adjusted tube current. A further embodiment of the invention which is suitable for this purpose is characterized in that it comprises a first timer member (i.e. a nominal timer member) which supplies an output pulse at the end of the adjusted exposure timer, and a second timer member (i.e. a correction-time member) which supplies an output pulse at the end of a period of time which is proportional to the tube current, one of the aforesaid timer members being controlled by the output pulse of the other timer member. In order to obtain the constant (fixed) actuation time prolongation T.sub.c, a separate timer member can be incorporated, but alternatively one of the two timer members can be constructed such that the output pulse thereof is delayed by this constant amount.

A further embodiment according to the invention is characterized in that the timer member comprises a generator which generates an increasing signal after having been started, an output pulse being generated when this increasing signal reaches the value of a signal proportional to the tube current.

In a correction timer member of such a construction the actuation time can be prolonged by a constant amount according to a further preferred embodiment of the invention in that a direct voltage is superimposed on the increasing signal.

In principle, the sequence of operation or connection of the nominal timer and the correction timer can be chosen at random, i.e. the correction timer member can be started by the output pulse of the nominal timer member and vice versa. However, if the correction timer member is started first (by the switch-on pulse), it may occur that the value of the increasing signal reaches the value of the signal proportional to the tube current before the tube current has reached its stable final value. The resultant error can be corrected in a further embodiment according to the invention in that the increasing signal is derived from a capacitor which can be charged via a constant resistance and a direct voltage source which can be switched on. The variation of the voltage across this capacitor is not entirely linear so that the said error can be corrected by suitable proportioning.

An advantage of a switching device comprising a correction timer member which precedes the nominal timer member is that the exposure time can be measured also in operation with an automatic exposure device. Accordingly, in a further preferred embodiment according to the invention, the output pulse of the correction timer member starts a time indicator which is stopped by the output pulse of the nominal timer member or by the switch-off pulse of the automatic exposure device.

Some preferred embodiments according to the invention will be described in detail hereinafter with reference to the drawing, in which

FIG. 1 shows a time circuit of an X-ray generator according to the invention,

FIG. 2 shows a high-voltage circuit of an X-ray generator according to the invention, including a timer member which is controlled by the current in the high-voltage circuit,

FIG. 3 shows a diagram so as to illustrate the operation of the circuit shown in FIG. 2,

FIG. 4 shows a further preferred embodiment according to the invention, and

FIG. 5 shows a diagram so as to illustrate the operation of the circuit shown in FIG. 4.

The timer circuit which is diagrammatically shown in FIG. 1 forms part of a conventional time switch, the other components of which (switching circuit for switching off the primary voltage etc.) can be of a conventional construction and which, therefore, are not shown in the drawing, nor is the X-ray generator itself shown. The timer circuit comprises a time control switch 1 which is started by a switch-on pulse which is supplied via a line 2, the said time control switch supplying a switch-off command via a line 3 at the end of an adjustable actuation time. The time control switch is a conventional circuit and may be similar to the time control circuits described in U.S. Pat. No. 2,668,909. The overall timer circuit includes not only time control switch 1, but also the resistors 6 and 7a-7f and switches 8a-8f connected to terminals 4 and 5. The actuation time of the time switch is determined by the resistance active between the terminals 4 and 5. The resistance between the terminals 4 and 5 consists of a series connection of a resistor 6 and one of a series of resistors 7a to 7f which are connected in series with the resistor 6 by the switches 8a to 8f, one of which is each time closed. The value of the resistor 6 can be adjusted by way of the adjusting members for the exposure time. The correction of the actuation time in dependence on the exposure current is effected by switching on one of the resistors 7a to 7f. The switches 8a to 8c are coupled to an external adjusting member by means of which the user adjusts the X-ray tube current (for example, a keyboard for adjusting the individual current values) so that each time one of the resistors 7a to 7c which is required for a current overall resistance is connected in series with the resistor 6.

In so-termed isowatt operation, in which the exposure current is generated automatically and independently of the adjusted power and the adjusted voltage, one of the resistors 7d to 7f is connected in series with the adjustable resistor 6 by way of one of the switches 8d to 8f. The switch contacts 8d to 8f are connected to a conventional power selector device which is not shown, the value of the resistors 7d to 7f being adjusted by means of the adjusting member for the exposure voltage such that as the adjusted tube voltage increases, the value of the resistors 7d to 7f decreases (because in isowatt operation the tube current automatically also decreases as the voltage increases).

In X-ray generators in which during so-termed dual-control operation the tube current is not directly adjusted but is indirectly determined by adjustment of, for example, the tube voltage and the desired mAs product, the prolongation of the actuation time could in principle also be effected by the addition of a resistor in the time circuit of the time switch. However, because the tube current is dependent on the exposure voltage, the mAs product and the adjusted focal spot, the value of this auxiliary resistor could be changed in dependence on the voltage, the mAs product and the adjusted focussing spot. The variation of a resistor in dependence on the said three exposure parameters, however, is comparatively expensive. Therefore, FIGS. 2 and 4 show an embodiment in which the prolongation of the actuation time is not effected by the adjusting members for the exposure parameters which influence the tube current, but in dependence on the measured tube current.

In accordance with FIG. 2, a measuring resistor 10, grounded on one side, for measuring the tube current, is included in the blocks 11 and 12 which diagrammatically represent the high-voltage circuit of an X-ray tube 13. The voltage drop across the measuring resistor 10 influences a time switching circuit 14 of the time switch. The time switching circuit 14 comprises a timer member 141 a time delay unit (referred to hereinafter as "nominal timer member") which may be similar to control block 1 of FIG. 1. Time member 141 receives a switch-on pulse via the line 2 and supplies a pulse on an output line after the expiration of a period of time (for example, 2 ms) which can be adjusted by the user. This is the adjusted exposure time, Ta. This pulse controls a pulse generator 142, for example, a monostable flipflop, which generates an output pulse after a constant period of time which is independent of the exposure time, i.e. the constant actuation time prolongation, T.sub.c. This constant period is proportioned so that for very small tube currents the effective exposure time corresponds to the adjusted exposure time. The pulse generator 142 can be dispensed with if the actuation time of the nominal timer member 141 is prolonged in advance by this constant amount. The output pulse of the pulse generator 142 is applied to a conventional ramp generator 143 which supplies a linearly increasing output signal u upon the appearance of this pulse signal u is applied to an input of a comparison stage 144. Another input of the comparison stage 144 receives a voltage which is generated by the tube current flowing in the measuring resistor 10. As soon as the voltage u reaches the value of the voltage across the measuring resistor 10, the comparison stage 144, consisting, for example, of a differential amplifier which is followed by a threshold value switch, supplies a switch-off command. The elements 143 and 144 together constitute a second time delay unit (correction time member).

FIG. 3 clearly shows that the increasing voltage u becomes equal to the voltage across the measuring resistor 10 at a time which is determined by the amplitude of the tube current (I.sub.1, I.sub.2, I.sub.3).

In a switching device as shown in FIG. 4, the sequence in which the elements 141 to 144 are arranged differs from that of FIG. 2. The switch-on pulse is applied, via the line 2, to the linear ramp generator 143 which generates an increasing signal which is applied to an input of the comparison stage 144, another input of which receives the voltage across the measuring resistor 10 (see FIG. 2). At the output of the comparison stage 144 a pulse is generated as soon as the increasing voltage u reaches the value of the voltage across the measuring resistor 10 in the high-voltage circuit of the X-ray tube. The shift in time between the input pulse on the line 2 and the output pulse at the output of the comparison circuit 144, therefore, is dependent on the tube current. The output pulse of the comparison stage 144 starts the timer switching member 141 which generates a switch-off pulse at the end of the exposure time adjusted by the user (possibly taking into account the constant actuation time prolongation effected via the line 3).

A drawback of an arrangement of the components of the time switching circuit of this kind with respect to the arrangement shown in FIG. 2 is that in the case of very large tube currents the increasing voltage already reaches the voltage drop generated across the measuring resistor 10 by these tube currents at an instant at which the current has not yet reached its final value. The resultant error can be corrected by a suitable mode of increase of the increasing voltage u. An increase curve in accordance with the formula k (1 - e - t/T) was found to be advantageous. According to this formula, the voltage changes across a capacitor which is connected to a constant direct voltage via a constant resistor. It was also found to be advantageous (as shown in FIG. 5) to make the increasing voltage start from a lower value than the voltage derived from the tube current. On the one hand, the actuation time is then prolonged by a constant amount a (FIG. 5), and on the other hand this results in a larger spacing between the increasing voltage and the voltage proportional to the tube current, so that the risk of premature pulse generation by interference voltage is reduced.

However, an advantage of the arrangement shown in FIG. 4 is that the effective exposure time can be measured in the case of operation with automatic exposure devices. For this purpose a time indicator 15 is started at the end of the correction time by the same pulse by means of which the nominal timer member 141 is started when use is made of the time switch. The time indication is stopped by the switch-off pulse of the automatic exposure device not shown. The use of an automatic exposure device in this manner is described in our copending U.S. patent application Ser. No. 451,535, filed Mar. 15, 1974. The time indicator displays the effective exposure time in this case. An exposure effected by means of an automatic exposure device can thus be repeated by means of the time switch. This will also produce the same blackening, the other circumstances being the same, if the value measured by time indicator during the first exposure is adjusted on the time selector of the time switch for the subsequent exposure (s).