X-ray diagnostic apparatus for preparing x-ray exposures including an automatic illuminating device and automatic adjustment of the exposure voltage

Abstract

An automatically illuminatable X-ray diagnostic apparatus which, in comparison with the state of the technology is considerably simplified with respect to its operation by avoiding the need for adjustment of the X-ray tube voltage. The X-ray tube voltage therein automatically adjusts itself in conformance with a preselected contrast. The X-ray tube voltage is automatically lowered by means of adjusting means for commencing an exposure within a short period as compared with the shortest exposure time, from a maximum initial value corresponding to the largest patient bulkiness, and in which the adjusting means is controllable by a comparator element having introduced at one input thereof a differential quotient of a proportional value of the dosage load measured behind the patient in the direction of the X-rays pursuant to the X-ray tube voltage at the instantaneous X-ray tube voltage as the actual value, while at the other input thereof there is provided a reference value signal corresponding to the desired contrast and which conveys a signal to the adjusting means for maintaining the X-ray tube voltage as soon as both input signals are equal. The reference value signal herein is a percentage figure of a differential quotient of the dosage load behind the patient, and corresponding to the desired contrast, pursuant to the X-ray tube voltage at a relative reference X-ray tube voltage of proportional value.

Description

FIELD OF THE INVENTION

The present invention relates to an X-ray diagnostic apparatus for the preparation of X-ray exposures, including an automatic exposure timer device for the automatic switching off of the X-ray tube upon a predetermined ray dosage being received by the X-ray film, and including automatic adjusting means for the X-ray tube voltage.

DISCUSSION OF THE PRIOR ART

In known X-ray diagnostic apparatus having automatic exposure timer devices, adjustment must be provided for the X-ray tube voltage. The switching off of the X-ray tube, in effect, the completion of an exposure, is obtained the required X-ray dosage predetermined for optimum film darkening has been imparted to the film. The X-ray tube voltage must, in that instance, be adjusted in dependence upon the object being penetrated by the rays, so as to provide an optimum picture contrast. The adjustment of the X-ray tube voltage is effected by means of tables, or based on the experience of the examining person.

Through German Pat. No. 1,227,570 there has become known automatically illuminated X-ray diagnostic apparatus having adjusting means for the X-ray tube voltage and the X-ray tube current, including an overload protective device for indicating and adjusting the limiting time in dependence upon the preadjusted X-ray tube load by means of the X-ray load nomograph or computing chart, as well as through an X-ray dosage load measuring device located behind the object being X-rayed. In this X-ray diagnostic apparatus there is provided a control arrangement, which upon an adjustment of the overload protective device, located on the basis of the preadjusted value of the existent tube load presently lies at a predetermined breakage point below the limiting load of the X-ray tube, then raises the X-ray tube voltage during the first exposure phase until it attains the limiting load limit of a permissible magnitude, when the predetermined dosage load measurement commenced with exposure initiation indicates that the required X-ray dosage for the desired film darkening cannot be attained with the preadjusted dosage load within the set time limit. In this X-ray diagnostic apparatus there is achieved an automatic correlation between the X-ray tube voltage and the bulk of the patient. Also attainable is a predetermined degree of automation of the adjustment of the X-ray tube voltage. However, the initial value of the X-ray tube voltage must always be selected in conformance with the body portion or organ which is to be X-rayed. Additionally, for the exposure of a predetermined body portion or organ there are only two different X-ray tube voltages available, so that correlation to the varied constitutions of patients is only roughly obtained. Consequently, also this X-ray diagnostic apparatus, not always can there be obtained an optimum illumination time at the lowest possible ray load on the patient and at an optimum image contrast.

SUMMARY OF THE INVENTION

It is, accordingly, an object of the present invention to provide an X-ray diagnostic apparatus of the type described, in effect, an automatically illuminatable X-ray diagnostic apparatus which, in comparison with the state of the technology is considerably simplified with respect to its operation, inasmuch as there is no longer a need for adjustment of the X-ray tube voltage. The X-ray tube voltage therein automatically adjusts itself in conformance with a preselected contrast.

The foregoing task is inventively solved in that the X-ray tube voltage is automatically lowered by means of adjusting means for commencing an exposure within a short period as compared with the shortest exposure time, from a maximum initial value corresponding to the largest patient bulkiness, and in which the adjusting means is controllable by a comparator element having introduced at one input thereof a differential quotient of a proportional value of the dosage load behind the patient pursuant to the X-ray tube voltage at the instantaneous X-ray tube voltage as the actual value, while at the other input thereof there is provided a reference value signal corresponding to the desired contrast and which conveys a signal to the adjusting means for maintaining the X-ray tube voltage as soon as both input signals are equal. The reference value signal herein is a percentage figure of a differential quotient of the dosage load behind the patient, and corresponding to the desired contrast, pursuant to the X-ray tube voltage at a relative X-ray tube voltage of proportional value. In the inventive X-ray diagnostic apparatus there is no longer a requirement for the adjustment of the X-ray tube voltage. It is merely necessary to select the desired image contrast. This selection sequence, however, is only to be relatively seldomly required. Within the scope of the invention, the desired image contrast may also be fixedly adjusted when a contrast variation is not considered to be needed. The operation of the inventive X-ray diagnostic apparatus thus becomes extremely simple, inasmuch as practically no exposure data need be selected. Consequently, a fully automatic operation of the apparatus becomes possible.

Within the framework of the invention there may be, on the one side conveyed to a differentiating element, for the formation of a value proportional to a differential quotient of the dosage quantity behind the patient in the direction of teh X-rays according to the X-ray tube voltage, a voltage of the automatic exposure timer device which is proportional to the instantaneous dosage, and, on the other side, a voltage which is proportional to the instantaneous X-ray tube voltage. The automatic exposure timer device herein is employed for the generation of switching off signal for the X-ray tube, as well as for adjustment of the X-ray tube voltage. Consequently, there is provided thereby an extremely simple construction.

A particularly simple construction is achieved when the reduction of the X-ray tube voltage for commencement of the exposure from the maximum initial value thereof is effected in a time-proportional manner. In that instance it is not required to actually determine the X-ray tube voltage. It is much more adequate that the differentiating element timewise differentiates a voltage from the automatic exposure timer device proportional to the instantaneous dosage.

A further embodiment of the invention consists of in that the X-ray tube voltage, through there is imparted the required differential quotient for the formation of the reference value, is also switched over in dependence upon the selected contrast, and wherein the maximum initial value of the X-ray tube voltage for initiating the exposure lies only a small amount above the present X-ray tube voltage effective for the differential quotient.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages of the present invention may be now ascertained from the following detailed description of an exemplary embodiment thereof, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a graphical representation of the relationship of the dosage load at various X-ray tube voltages with respect to the dosage load at a predetermined maximum X-ray tube voltage for various patient bulks;

FIG. 2 illustrates differential quotients of the dosage load behind the patient (DL) according to the X-ray tube voltage in dependence upon the X-ray tube voltage, as obtained from the curves in FIG. 1, regulated for these differential quotients at a maximum X-ray voltage (U.sub.RMax) for various patient bulks;

FIG. 3 illustrates a circuit diagram for an X-ray diagnostic apparatus according to the present invention;

FIG. 4 graphically illustrates a sequence for the X-ray tube voltage in the X-ray diagnostic apparatus of FIG. 3; and

FIG. 5 shows circuit details of the circuit diagram of FIG. 3.

DETAILED DESCRIPTION

Referring now to the drawing, in FIG. 1 the X-ray tube voltage is designated by U.sub.R. The curve 1 relates to a thin patient, whereas curve 4 relates to a bulky patient. The curves 2 and 3 correspondingly relate to intermediate values of patient bulks.

The image contrast K of an X-ray exposure remains, not-withstanding variations in the bulk D of a patient, approximately constant when the X-ray tube voltage U.sub.R is adjusted in a manner in which the quotient Q remains constant from the differential quotient of the dosage quantity behind the patient pursuant to the X-ray tube voltage at the location U.sub.1 (exposure voltage) and the differential quotient of the dosage quantity behind the patient pursuant the X-ray tube voltage at the location of a suitable X-ray tube voltage U.sub.RMax which is larger than U.sub.1.

In short form:

K .noteq. f(d), in the event U.sub.R = U.sub.1, wherein U.sub.1 is such, that ##EQU1##

The sequence or graph of the quotient Q in dependence upon the X-ray tube voltage U.sub.R is illustrated in FIG. 2. The curves 5 through 8 herein correspond to curves 1 through 4 in FIG. 1. In FIG. 2 there is shown a line 9 which defines a particular preselected contrast. From FIG. 2 it may be ascertained that in order to obtain a contrast according to line 9 for patients represented by curves 1 and 5, there is required an X-ray tube voltage of approximately 70 kv, while for patients represented by curves 4 and 8, there is required an X-ray tube voltage of approximately 95 kv. From FIG. 2 it is ascertainable that completely determinate X-ray tube voltage U.sub.1 =f(d) belongs to each patient constitution, and which there is obtained a predetermined contrast.

The invention recognizes that the image contrast, as well as the given quotient of the differential quotients, are dependent upon the patient bulk and the exposure voltage, and that this dependence of the image contrast and the given quotient from the patient bulk may be compensated for in that the exposure voltage follows the patient's bulk in a determined function U.sub.1 = f(d), and wherein this function U.sub.1 = f (d) is the same for the image contrast and the quotient Q, so that it is only necessary to maintain the X-ray tube voltage at that value which provides a preselected quotient, in order to thereby concurrently also achieve the associated image contrast.

An X-ray diagnostic apparatus which operates in accordance with this principle is illustrated in FIG. 3 of the drawings. The X-ray diagnostic apparatus according to FIG. 3 includes a rotary or three-phase high voltage transformer 10 having a split secondary side. Two high voltage rectifiers 11 and 12 supply an X-ray tube 13 through two conduits 14 and 15 which are positioned in the high voltage circuit. The conduits 14 and 15 have control arrangements or amplifers 16 and 17 associated therewith, by means of which the anode-cathode resistance of the conduits 14 and 15, and thereby the voltage reduction of the conduits 14 and 15, becomes adjustable. Through the intermediary of control arrangements or amplifiers 16 and 17, the X-ray voltage tube thus may be adjusted.

The control of the control arrangements or amplifiers 16 and 17 is effected through a function generator 18 which has an input 19, and which maintains to final value through which it controls the control arrangements or amplifiers 16 and 17 as soon as a signal is generated at its input, in effect, in the conduit 19.

The conduit 19 is, at the other side thereof, connected to a comparison circuit 20 which receives at a portion comprising a resistance 21, as a reference value the output voltage of a sample and hold circuit 22, and as an actual value the timewise differentiated output voltage of a dose rate meter 25, through a condensor 23 and resistance 24. The dose rate meter 25 is connected with a suitable ray detector 26 which is located behind the patient 27 in the direction of the X-rays and, in FIG. 3, in front of (in practice also possibly in back of) the X-ray film 28.

The output voltage of the dose rate meter 25, which has been timewise differentiated by means of condensor 23 and resistance 24, is also conveyed to the circuit 22. The differentiated output voltage of the dose rate meter 25 which, at a particular moment after commencement of the exposure, is present at the resistance 24, is sensed by the sample and hold 22, supplied and then maintained at the resistance 21 for as long until a signal appears in the conduit 19.

The output signal of the dose rate meter 25 is, in addition to the differential components 23, 24, conveyed toward an integrator 29, and the integrator result to a cut-off amplifier 30 which has a reference value input 31, and which compares the integrator result with the reference value signal at the input 31. When both of these signals coincide, in effect, meaning when the desired optimum film darkening has been attained, the cut-off amplifier 30 delivers at its output 32 a cut-off signal to the function generator 18. The switching off is effected through the closure or disconnecting of conduits 14 and 16, but may also be effected through a switching element located in the primary circuit of the high voltage transformer 10 for the switching off of the X-ray tube 13.

At the output of the dose rate meter 25 there is formed a voltage which is proportional to the measured dosage quantity in back of the patient 27 in the direction of the X-rays. The resistance 24 is small in comparison with the fraction T.sub.O/C in which T.sub.O> T.sub.M (see FIG. 4) for that time, within which the X-ray tube voltage, for commencing the exposure at a suitably high value, reaches its correct value U.sub.1, and in which C is the capacity of condensor 23. The voltage at the resistant 24 is defined by the following equation: ##EQU2## wherein U.sub.K is the voltage at the resistance 24, i is the current flowing through this resistance, and R the resistive value of this resistance. C is the capacity of the condensor 23, U.sub.DL is the output voltage proportionate to the dosage quantity DL of the dose rate meter 25, t the time, and c' is a constant.

In the exemplary embodiment according to FIG. 3 it is basic that the X-ray tube voltage drops linearly with time. For the foregoing, the following equation defines the X-ray tube voltage: ##EQU3## in which K* is a constant. In order to obtain a linear dropping off of the X-ray tube voltage, the function generator 18 is programmed in a manner so that this linear reduction is effected for as long as the voltage at the resistance 24 is larger than the voltage at the tap-off for resistance 21, in effect meaning, as long as no signal is transmitted from the comparison circuit 20 to the conduit 19.

If dt with respect to the relationship for the X-ray tube voltage U.sub.R is introduced in the relationship for the voltage U.sub.K at the resistance 24, then the following equation is obtained for the voltage U.sub.K : ##EQU4##

In this equation the differential quotient of the dosage quantity is obtained pursuant to the X-ray tube voltage. Based on the prior assumption that the X-ray tube voltage drops off linearly with time, then the voltage U.sub.K in the embodiment of FIG. 3 may be utilized for obtaining the quotient Q, as follows: ##EQU5##

The operation of the X-ray diagnostic apparatus of FIG. 3 is as follows, having reference to FIG. 4:

The X-ray tube voltage U.sub.R, after commencement of the X-ray exposure, jumps to a suitable maximum value U.sub.RMax. This is retained until time point T.sub.M, and then drops linearly.

At the resistance 24 there appears a voltage impulse in conformance with the sudden occurrence of the dosage. After the voltage impulse has reduced prusuant to T.sub.M, and the voltage U.sub.K is now proportional to the differential quotient d (DL)/d U.sub.R, then U.sub.K = (U.sub.K) at U.sub.RMAX is sensed and supplied or energized.

(U.sub.K) at U.sub.RMax then appears at the output of circuit or unit 22. Through the intermediary of resistance 21, a predetermined portion (percentage) corresponding to the desired image contrast is transmitted to the comparison circuit 20, as represented by:

Q .sup.. (U.sub.K) at U.sub.RMax mit Q < 1

At t.sub.o, or respectively, at U.sub.R = U.sub.1, the following pertains:

Q .sup.. (U.sub.K) at U.sub.RMax = (U.sub.K) at U.sub.1 ; ##EQU6##

For U.sub. 1 there results for various patient bulks, from exposure to exposure, always a different value, and namely the X-ray tube voltage which independently of the patient's bulk always constantly provides the same contrast. At the time point T.sub.O there is accordingly maintained the X-ray tube voltage at the value U.sub.1 , and the X-ray exposure is prepared at the voltage U.sub.1 up to the time point T.sub.1, at which the cut-off amplifier 30 terminates the exposure. After the time point T.sub.1, the X-ray tube voltage drops exponentially toward zero. The slope or degree of drop-off of the X-ray tube voltage for initiating the exposure is so selected, that the dosage which is produced in the time T.sub.O is also at the lowest occurring X-ray tube voltage small in comparison with the dosage between the time T.sub.O and the minimum employed exposure time T.sub.1, so that during the largest portion exposure time there provided at the X-ray tube an X-ray tube voltage corresponding to the optimum contrast.

In summation, it may be readily ascertained that in the X-ray diagnostic apparatus according to FIG. 3 there is no requirement for an adjustment of the X-ray tube voltage. Since the X-ray tube current may also be rigidly programmed pursuant to the loading nomograph of the X-ray tube, the operation and manipulation of this X-ray diagnostic apparatus is considerably simplified, inasmuch as practically no exposure data need be adjusted. The desired contrast may singly be adjusted for a plurality of exposures at the resistance 21, and the correlation with the employed X-ray film similarly fillows for a plurality of exposures at a single instance through generation of a corresponding signal at the input 31 of the cut-off amplifier 30. The X-ray diagnostic apparatus according to FIG. 3 consequently is completely automatedly exposed, and in which the adjusting sequence for the X-ray tube voltage is incorporated in the automation thereof.

The embodiment according to FIG. 3 is particularly simple since the X-ray tube voltage drops off linearly with time, and consequently no particular evaluation of the X-ray tube voltage changes need be effected through corresponding measuring elements. Within the framework or scope of the invention, the reduction or dropping off of the X-ray tube voltage may also follow in conformance with another function. If this function is fixedly programmed in the function generator 18, then also in this instance no measurement of the particular actual value of the X-ray tube voltage change is required, but merely the differentiation cannot be effected by means of the RC-element 23, 24, and a more suitable differentiator must be introduced. Within the scope of the invention it is also possible, to ascertain the dosage quantity change, as well as the X-ray tube voltage change be ascertained through corresponding measuring elements, and to form the differential quotient ##EQU7## by means of a differentiating element. It is always important for the invention that a signal be transmitted to the comparison circuit 20 which is proportional to the differential quotient of the dosage quantity pursuant to the X-ray tube voltage, and in which the signal is compared with a signal proportional to the reference value of the contrast corresponding percentage value Q of a differential quotient of the dosage quantity pursuant to the X-ray tube voltage at a reference X-ray tube voltage, and upon the balancing of the signals the X-ray tube voltage is maintained.

The construction of the function generator 18 is described in greater particularity in FIG. 5 of the drawings. From FIG. 5 it is ascertained that the function generator 18 includes a condensor 33 which is dischargeable through a discharge resistance 34, as well as through a relay contact 35. The relay contact 35 is actuatable by means of a relay 36 which is controlled by the cut-off amplifier 30. The switching-in of the discharge resistance 34 at the condensor 33 is effected through a circuit switch 37 and a relay contact 38, whose relay 39 is controlled by the signal in the conduit 19.

Prior to commencing an exposure, the exposure switch 37 is opened and the condensor 33 is charged. In order to initate an exposure, the switch 37 is closed. The condensor 33 discharges approximately linearly through resistance 34 up to the time point T.sub.O. Since the X-ray tube voltage corresponds to the condensor voltage, the X-ray tube voltage also drops linearly up to time point T.sub.O pursuant to FIG. 4. The control voltage for the conduits or triodes 14 and 15 namely corresponds with the voltage in the conduit 40, which is amplified through the control amplifiers 16 and 17.

At the time point T.sub.O, the voltage U.sub.1 is attained in the X-ray tube, and the comparison circuit 20 transmits a signal to conduit 19, which causes the relay 39 to open its contact 38 and interrupt the discharge of condensor 33. The condensor voltage thereby remains constant up to time point T.sub.1, whereby the X-ray tube voltage also maintains the value U.sub.1 up to time point T.sub.1. At time point T.sub.1 there is effected the termination of the exposure by means of the output signal of the cut-off amplifier 30, which causes the excitation of relay 36. Relay 36 closes its contact 35 and completely discharges condensor 33. The condensor voltage, and therewith the X-ray tube voltage, extend herewith from time point T.sub.1 on, pursuant to the graph in FIG. 4, until reaching zero value. Prior to initiating a new exposure, the relay 36 is again de-energized so that the contact 35 is opened. Furthermore, the exposure switch 37 is again opened prior to initiating an exposure, and for the making of a new exposure, closed again. The above-described sequences are then repeated.

While there has been shown what is considered to be the preferred embodiment of the invention, it will be obvious that modifications may be made which come within the scope of the disclosure of the specification.

Claims

What is claimed is:

1. In an X-ray diagnostic apparatus having an X-ray tube for the making of X-ray exposures of a patient; including an automatic exposure timer means for automatically switching off the X-ray tube upon an X-ray film being subject to a predetermined dosage of rays; and means for automatically adjusting the voltage of the X-ray tube, the improvement comprising; said voltage adjusting means being adapted to automatically lower the X-ray tube voltage at the initiation of each exposure during a small time interval in comparison with the shortest exposure time of said apparatus from a maximum initial value, equal for all examinations, and based on a largest patient bulk; and comparison circuit means operatively connected to said voltage adjusting means and adapted to control the latter, said comparison circuit having a first input receiving a signal as an actual value proportionate to a differential quotient of the X-ray dosage quantity measured behind the patient in the direction of the X-rays pursuant to the X-ray tube voltage, and a second input receiving a reference value signal corresponding to a desired image contrast; and means transmitting a signal from the output of said comparison circuit means to said voltage adjusting means for maintaining the X-ray tube voltage constant upon said first and second input signals being equal.

2. An apparatus as claimed in claim 1, said reference value signal at the second input of said comparison circuit means being in correspondence with the desired image contrast, a percentage of a signal proportional to the differential quotient of the dosage quantity measured behind the patient in the direction of the X-rays pursuant to an X-ray tube voltage.

3. An apparatus as claimed in claim 1, comprising differentiating means for generating a signal conforming to the formation of change in the dosage quantity measured behind the patient in the direction of the X-rays; and dose rate meter means connected to said differentiating means for measuring voltage proportional to the dosage quantity.

4. An apparatus as claimed in claim 3, said differentiating means having an output signal adapted to generate one of the input signals of said comparison circuit means.

5. An apparatus as claimed in claim 1, comprising means for fixedly programming the function of the reducing rate of the X-ray tube voltage.

6. An apparatus as claimed in claim 4, said programming means linearly reducing said X-ray tube voltage as a function of time.