EP0234603B1 - X-ray generator with control of dosing power - Google Patents

Description

The invention relates to an X-ray generator with actuators at least for the tube current and the tube voltage, a controller which acts on the actuators as a function of the actual value and the target value of the dose rate, at least one measuring device for measuring the actual value of the dose rate, and with one by the user operable selection device which determines the type of examination and the target value of the dose rate.

Such an X-ray generator is known from DE-OS 26 53 252. With this, on the one hand slice pictures can be taken and on the other hand bucky pictures in which the recording time does not exceed an upper and a lower limit value. The actuators for the tube current and the tube voltage have different time constants.

Many types of examination require different time constants. In cinema mode and in pulsed fluoroscopy mode, for example, relatively large time constants are required in order to avoid an uneasy picture impression. In contrast, in series recording mode with up to 15 frames per second, a much faster dose rate control is required.

Furthermore, different types of examination can have different actuating functions, i.e. a different assignment of the respective values of tube current and tube voltage. One would have to provide a separate controller for each type of examination if one wanted to achieve dose rate control for the various types of examination using the known X-ray generator.

It is an object of the invention to design an X-ray generator of the type mentioned in such a way that a dose rate control suitable for different types of examination is achieved with little effort.

This object is achieved in that the controller is a sampling controller formed by a microprocessor system with an adjustable sampling frequency, that a memory is provided in which the sampling frequency and the actuating function are stored for the various types of examination, and that the sampling controller according to the actuating function and after calculates the manipulated variables for the next sampling interval in a program stored in a further memory and controls the actuators accordingly.

The sampling frequency of the sampling controller and thus the control speed can be determined by means of a programmable counter contained in the microprocessor system, which is loaded from the memory with a value corresponding to the sampling interval and assigned to the respective type of examination. The same applies to the control functions. The implementation of these control functions and the control behavior (proportional behavior or integral behavior) of the dose rate control in the respective type of examination are determined by the program, according to which the manipulated variables for the next sampling interval are determined in the sampling controller.

As a rule, the dose rate in the different types of examination is not determined with the same measuring organ. The dose rate in a slice image can be determined, for example, with an ionization chamber, while it is determined in operation with a cinema camera by means of a photo multiplier and is derived from the video signal in fluoroscopic mode. In order to ensure the dose rate control independently of the specific structure of the measuring device used in each case, a further development of the invention provides that several measuring devices are provided, one of which is effective for each type of examination, that matching amplifiers are provided which output the measuring device at a predetermined dose rate bring to a predetermined level, and that a switching device is provided which couples the measuring device intended for the respective examination with the sampling controller.

Another development of the invention provides that the scanning frequency has a first value in a first section of an X-ray image and a second value in a subsequent section, and that the first value is significantly greater than the second value. As a result, a favorable transient response can be achieved if the first section is ended with a high sampling frequency as soon as, for example, the tube voltage has reached a predeterminable fraction of its starting value. In the first phase, simpler and shorter control algorithms must be used because of the increased sampling frequency, which ensure that an evaluable dose rate signal is available sufficiently quickly as long as the tube voltage does not yet deviate significantly from its starting value.

• Fig. I ein Blockschaltbild eines Röntgengenerators nach der Erfindung,
• Fig. 2 ein Blockschaltbild eines Teils des Röntgengenerators und
• Fig. 3a und 3b Stellfunktionen für verschiedene Untersuchungsarten.

The invention is explained below with reference to the drawing. Show it:

• I is a block diagram of an X-ray generator according to the invention,
• Fig. 2 is a block diagram of part of the X-ray generator and
• 3a and 3b actuating functions for different types of examination.

Fig. I shows a schematic representation X-ray system with an X-ray generator I and an X-ray tube 2, the radiation of which passes through an examination area symbolized by the body 13 and generates an X-ray silhouette either on a film positioned in a cassette holder 4 or on the output screen of an X-ray image intensifier 5. When using an X-ray film, the dose or the dose rate is measured by means of an ionization chamber 3 and fed to a unit 30 for dose evaluation. The output image of the image intensifier 5 can be recorded by means of a cinema camera 7, a sheet or roll film camera 8 or a television camera 9 or a television chain 90 with monitor 91 connected to it. An adjustable iris diaphragm 11 is located in the optical beam path.

The X-ray generator I contains i.a. Actuators with which the tube voltage, the tube current and the focal spot size can be set. The tube voltage actuator 100 can contain, for example, a medium frequency converter with a high voltage generator and a rectifier combination. A grid control unit or an electronically controlled heating circuit can serve as the tube current actuator 110, and the focal spot actuator 120 can also be designed such that only the focal spot size can be switched between two values.

The actuators 100, 110 and 120 receive their actuating signals via lines 143, 141 and 142 from a sampling controller 140. This receives its setpoints and actuating functions from a central control unit and its actual values either from the ionization chamber 3, the photomultiplier 12 or the television chain 90 Matching amplifiers 171, 172 and 173, which bring the output signals of these measuring operations to a normalized level and via the switching device 170, which is controlled via line 175 in such a way that the output signal of one of the three matching amplifiers 171. .173 is fed.

The central control unit 160 is coupled via a bidirectional connection 162 to a selection device in the form of an operating panel 180. The user selects the type of examination, for example by pressing a key, whereupon the associated sampling frequency and the actuating function are called up in a memory of the central control unit 160 and loaded into a memory of the sampling controller 140 via the connection 159.

The structure of the sampling controller 140 and the central control unit 160 results from FIG. 2. Both contain a microprocessor 147 and 165 as well as read-only and read / write memories 148 and 166 and input / output units 149 and 164, 167. Both units contain one Programmable counters 150 and 169, respectively. The sampling controller 140 also contains an analog-to-digital converter 146 for converting the analog signals representing the actual values on line 144 into digital values and digital-to-analog converters 145 which produce the digital actuating signals generated by the microprocessor 147 convert into analog signals, which reach the associated actuators via lines 141, 142 and 143.

In the memory 166 of the central control unit 160, the optimal actuating function, the required sampling frequency and further parameters, e.g. the frame rate, the minimum and the maximum recording time (with cinema recording technology) etc. are saved. These programs can be called up from the control panel 180 via the bidirectional connection 162. Before the start of the recording, they are transmitted via the input / output interface 167 of the central control unit 160 and the connection 159 to the input / output interface 149 of the sampling controller 140. The recording time is set by means of the programmable counter 169, which is started by the recording start signal and generates a signal when the recording expires, which is transmitted via the u.a. Line 161 connected to high-voltage actuator 100 switches off the tube voltage and thus the X-ray radiation, and this acts on microprocessor 147 in sampling controller 140 via interrupt line 158. According to a program stored in the memory 148, which is called up depending on the respective type of examination, the latter calculates the actuating signals for the next sampling interval within a sampling interval.

The function of the X-ray generator will be explained below for two different types of examination.

A) Cinema recordings

In this type of examination, a large number of x-ray images are generated with the cinema camera, generally between 50 and 300 images per second, x-ray images being generated. It is necessary to regulate the dose per image. Dose deviations occurring due to disturbance variables do not have to be corrected within a picture, but after a few ten pictures, since otherwise a restless picture impression (flickering) would result. The dose is therefore a scanning signal that is available after each image. The sampling frequency thus corresponds to the frame rate.

It is assumed that after the i-th picture of a sequence of cinema pictures a signal D _; to represent the dose received during this image. However, the dose required to properly darken an image may be D _s and D _; differ. The Required control signals for the tube voltage actuator 100 and the tube current actuator 110 are then calculated as follows:

berechnet.AI) First there is a tube voltage U _x according to the relationship

calculated.

U _{i is} the tube voltage set during the last image and a is the exponent with which the dose rate changes when the tube voltage changes. The voltage U _x calculated in this way represents the value that would be required in the case of a control with proportional behavior, in which the dose or the dose rate is regulated exclusively by changing the tube voltage.

berechnet. h ist dabei ein Faktor, der das Proportionalverhalten der Regelung bestimmt, während der Faktor k das Integralverhalten der Regelung definiert. Der Wert l_i-l ist dabei der Röhrenstrom während des vorletzten Bildes. A.4) Anschließend wird der beim nächsten Röntgenbild einzustellende Röhrenstrom nach der Beziehung

berechnet.The tube current I _x associated with U _x is then determined as a function of the actuating function loaded for this type of examination. An actuating function suitable for cinema operation is shown in FIG. 3a. The curve entered in the tube current / tube voltage diagram indicates the manner in which tube voltage and tube current are to be changed in order to achieve a change in the dose rate. The curve begins with the smallest possible tube voltage and the smallest possible tube current with a horizontal section, ie only the voltage is changed in this area to change the dose rate. This section is followed by a second section in which tube current and tube voltage are changed in the same direction in order to change the dose rate. This second section is followed by a third, again horizontally running section, which is determined by the maximum tube current. This third section is followed by a fourth section, which is predetermined by the resilience of the focal spot of the X-ray tube and which has a hyperbolic shape. In this section, to change a dose rate, tube voltage and tube current are changed in opposite directions to one another so that their product remains constant. The actuating function shown in Fig. 3a is stored in that the values of tube current and tube voltage are stored at the start and end points of the individual sections. For intermediate manipulated variables (eg heating current) that have non-linear dependencies on tube current and tube voltage, the description of the characteristic curve using the start and end points may not be sufficient. In this case, a large number of points are stored on the characteristic curve. A.3) After the tube current I _x belonging to the tube voltage U _x is determined in this way on the basis of the actuating function according to FIG. 3a, a tube current change AI _{x is determined} according to the formula

calculated. h is a factor that determines the proportional behavior of the control, while the factor k defines the integral behavior of the control. The value l _il is the tube current during the penultimate picture. A.4) Then the tube current to be set for the next X-ray image is based on the relationship

calculated.

berechnet, wobei C = A l_x/l_x gilt. Die so berechneten Werte von Röhrenstrom und Röhrenspannung werden den Stellgliedern 100 und 110 über die Leitungen 143 und 141 als Stellsignale vorgegeben. Die sich danach ergebende Dosis wird wiederum gemessen, wonach erneut die Stellsignale berechnet werden usw.A.5) The tube voltage U _{i + l} to be set in the next x-ray image becomes according to the relationship

calculated, where C = A l _x / l _x applies. The values of tube current and tube voltage calculated in this way are given to the actuators 100 and 110 via lines 143 and 141 as control signals. The resulting dose is measured again, after which the control signals are calculated again, etc.

The regulation in the case of pulsed fluoroscopy, in which the actual value is derived from the signals of the television chain 90, can be carried out in a similar manner. However, a different sampling frequency and a different actuating function can be specified.

B) Continuous shooting with the roll film camera 8

In this type of examination with image frequencies of up to 15 frames per second, each image should be correctly exposed, whereby the recording time should not exceed a lower and an upper limit frequency. In order not to exceed the limit of the exposure time for no single exposure, the imaging dose rate must be regulated. An adapted sampling frequency results from the shortest recording time. If it is 10 to 20 ms, the sampling frequency should be around I kHz.

• B.I) Es wird zunächst eine Spannung U_x nach der Beziehung
  
  berechnet.
• B.2) Im zweiten Schritt wird mit Hilfe des so berechneten Wertes U_x ein Wert A U_x gemäß der Formel
  
  berechnet. Dabei ist m ein das Proportionalverhalten der Regelung bestimmender Faktor, n ein Faktor, der das Integralverhalten der Regelung bestimmt und U_j-_I der Wert der Röhrenspannung im vorletzten Abtastintervall. B.3) Anschließend wird die nächste Stellgröße für die Röhrenspannung nach der Beziehung
  
  berechnet.
• B.4) In einem weiteren Schritt wird der zugehörige Röhrenstrom l_i,₁ anhand der Stellfunktion ermittelt, die in diesem Fall gemäß Gleichung B.3) durch eine Gerade definiert wird, deren Eckpunkte gespeichert sind.

For this purpose, an internal divider of the clock frequency of the sampling controller is set accordingly and switched to an interrupt input of the central unit as soon as the tube voltage has reached its start value specified at the beginning. In each sampling interval triggered thereby, the dose rate or the dose accumulated up to that point is read in as an actual value via the analog-digital converter 146 and the difference to the guide value from the memory is determined. The manipulated variable is calculated as follows:

• BI) First there is a voltage U _x according to the relationship
  
  calculated.
• B.2) In the second step, the value U _x calculated in this way becomes a value AU _x according to the formula
  
  calculated. Here m is a factor determining the proportional behavior of the control, n is a factor determining the integral behavior of the control and U _j - _{I is} the value of the tube voltage in the penultimate sampling interval. B.3) Then the next manipulated variable for the tube voltage according to the relationship
  
  calculated.
• B.4) In a further step, the associated tube current l _i , _{1 is} determined on the basis of the actuating function, which in this case is defined according to equation B.3) by a straight line whose corner points are stored.

The end of admission takes place when the target dose required for admission has been reached.

The control of slice images is carried out in a similar way. However, since the recording time is fixed in these cases, the end of the recording is done in this case with the aid of a programmable counter.

In the types of examination described above, in which the individual image must already be correctly exposed, there is a particularly favorable transient response if the sampling frequency is at a value at the start of an image in the period from switching on until approximately 95% of the starting value of the tube voltage has been reached is set, which is much higher, e.g. 5 kHz. With this increased sampling frequency, simpler and shorter control algorithms must be used, which differ from the above-mentioned ones in that on the one hand it should be detected whether there is an evaluable dose rate and on the other hand a sufficiently rapid tube voltage change should occur if the measured dose rate is significantly too high. This prevents overexposure of the X-ray image. After the settling process, the sampling frequency is set to the above-mentioned value (I kHz).

Claims (3)

1. An X-ray generator which includes adjustment means for at least the tube current and the tube voltage, a controller which acts on the adjustment means in dependence of the actual value and the reference value of the dose rate, at least one measurement device for measuring the actual value of the dose rate, and a selection device which can be activated by the operator and which defines the type of examination as well as the reference value of the dose rate, characterized in that the controller (140) is a sampling controller which is formed by a microprocessor system and which has an adjustable sampling frequency, there being provided a memory (148, 166) in which the respective sampling frequency and the respective adjustment function are stored for the various types of examination, the sampling controller calculating, in accordance with the adjustment function and a program stored in a further memory (148, 166), the adjustment values for the next sampling interval and controlling the adjustment means accordingly.

2. An X-ray generator as claimed in Claim 1, characterized in that there are provided a plurality of measurement devices (3, 12, 90), a respective one of which is active during each type of examination, there being provided matching amplifiers (171, 172, 173) which amplify the output signal of the measurement device to a predetermined level for a predetermined dose rate, there also being provided a switching device (170) which couples the measurement device associated with the relevant examination to the sampling controller.

3. An X-ray generator as claimed in any one of the preceding Claims, characterized in that the sampling frequency has a first value during a first part of an X-ray exposure and a second value during a subsequent part, the first value being substantially larger than the second value.

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