EP0234603A2 - X-ray generator with control of dosing power - Google Patents

Abstract

The invention relates to an X-ray generator with a regulator for the dose rate. In order to be able to use this controller universally for various types of examination, a sampling controller formed by a microprocessor system is used as the controller. There are memories in which the sampling frequency and the actuating functions are stored for the various types of examination as well as the programs according to which the sampling controller calculates the manipulated variables for the various types of examination.

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. 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.

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. On the other hand, in continuous shooting mode with up to 15 frames per second, a much faster dose rate control is required.

Furthermore, different types of examination can require different actuating functions, ie 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 with 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, in that a memory is provided in which the sampling frequency and the actuating function are stored for the various types of examination, and in that the sampling controller corresponds to the actuating function and after another The stored program calculates the manipulated variables for the next sampling interval and controls the actuators accordingly.

The sampling frequency of the sampling regulator 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 photomultiplier and is derived from the video signal in fluoroscopic mode. In order to regulate the dose rate regardless of the specific structure of the To ensure 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 bring the output signal of the measuring device to a predetermined level at a predetermined dose rate, and that a switchover device is provided which couples the measuring device intended for the respective examination to 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 larger 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. l 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:

• 1 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.

1 shows a schematic representation of an x-ray system with an x-ray generator 1 and an x-ray tube 2, the radiation of which passes through an examination area symbolized by the body l3 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 connected to it with a monitor 91. An adjustable iris diaphragm 11 is located in the optical beam path.

The X-ray generator 1 contains, among other things. Actuators with which the tube voltage, the tube current and the focal spot size can be set. The tube voltage actuator l00 can contain, for example, a medium frequency converter with a high voltage generator and a rectifier combination. A lattice control unit or an electronically controlled heating circuit can serve as the tube current actuator ll0, 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 l00, ll0 and l20 receive their control signals via lines l43, l4l and l42 from a sampling controller l40. This receives its setpoints and control functions from a central control unit and its actual values either from the ionization chamber 3, the photomultiplier l2 or the television chain 90 via matching amplifiers l7l, l72 and l73, which provide the output signals bring these measuring processes to a normalized level and via the switching device l70, which is controlled via line l75 in such a way that the output signal of one of the three matching amplifiers l7l ... l73 is fed to the sampling controller l40 via line l44.

The central control unit l60 is coupled via a bidirectional connection l62 to a selection device in the form of a control panel l80. The user selects the type of examination, for example, by pressing a button, 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 l40 and the central control unit l60 results from FIG. 2. Both contain a microprocessor l47 or l65 as well as read-only and read / write memories l48 or l66 and input / output units l49 or l64, l67. Both units contain a programmable counter l50 or l69. The sampling controller l40 also contains an analog-digital converter l46 for converting the analog signals representing the actual values on the line l44 into digital values and digital-analog converter l45, which convert the digital control signals generated by the microprocessor l47 into analog signals, which are transmitted via the Carry lines l4l, l42 and l43 to the associated actuators.

The optimal actuating function, the required sampling frequency and further parameters, for example the image frequency, the minimum and the maximum recording time (in the case of cinema recording technology) etc., are stored in the memory l66 of the central control unit l60 for each type of examination. These programs can be bidirectional Connection l62 can be called up from the control panel l80. Before the start of the recording, they are transmitted via the input / output interface l67 of the central control unit l60 and the connection l59 to the input / output interface l49 of the scanning controller l40. The recording time is set by means of the programmable counter l69, which is started by the recording start signal and generates a signal at the end of the recording, which switches off the tube voltage and thus the X-ray radiation via the line l6l connected to the high-voltage actuator l00, among other things, and that via the Interrupt line l58 acts on microprocessor l47 in sampling controller l40. According to a program stored in the memory l48, 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 there is a signal D _{i which is} intended to represent the dose received during this picture. The for however, the correct density of an image required may be D _s and differ from D _i . The required control signals for the tube voltage actuator l00 and the tube current actuator ll0 are then calculated as follows:

Al) First there is a tube voltage U _x according to the relationship

U _x = U _i * (D _s / D _i ) ^{l / a}

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.

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 section, again horizontal, 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 (e.g. 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.

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 Δ I _{x is determined} according to the formula

Δ I _x = h (I _x -I _i ) + k (I _i -I _il )
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 I _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

I _{i + l} = I _i + ΔI _x

calculated.

A.5) The tube voltage U _{i + l} to be set in the next x-ray image becomes according to the relationship

U _i + (U _x -U _i ) (lC)

calculated, where C = Δ I _x / I _x applies. The values of tube current and tube voltage calculated in this way are given to the actuators l00 and ll0 as control signals via lines l43 and l4l. 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 this is 10 to 20 ms, the sampling frequency should be about 1 kHz.

• B.l) Es wird zunächst eine Spannung U_x nach der Beziehung
  
  U_x = U_i(D_s/D_i)^l/(a+l)
  
  berechnet.
• B.2) Im zweiten Schritt wird mit Hilfe des so berechneten Wertes U_x ein Wert Δ U_x gemäß der Formel
  
  Δ U_x = m (U_x-U_i) + n (U_i-U_i-l)
  
  berechnet. Dabei ist m ein das Proportionalverhalten der Regelung bestimmender Faktor, n ein Faktor, der das Integralverhalten der Regelung bestimmt und U_i-l der Wert der Röhrenspannung im vorletzten Abtastintervall.
• B.3) Anschließend wird die nächste Stellgröße für die Röhrenspannung nach der Beziehung
  
  U_i+l = U_i + Δ U_x
  
  berechnet.
• B.4) In einem weiteren Schritt wird der zugehörige Röhren­strom I_i+l 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 reference value from the memory is determined. The manipulated variable is calculated as follows:

• Bl) First there is a voltage U _x according to the relationship
  
  U _x = U _i (D _s / D _i ) ^{l / (a + l)}
  
  calculated.
• B.2) In the second step, the value U _x calculated in this way becomes a value Δ U _x according to the formula
  
  Δ U _x = m (U _x -U _i ) + n (U _i -U _il )
  
  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 _{il 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
  
  U _{i + l} = U _i + Δ U _x
  
  calculated.
• B.4) In a further step, the associated tube current I _{i + l is} determined using 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 value is set to 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 which is much higher, e.g. 5 kHz. With this increased sampling frequency, simpler and shortened control algorithms must be used, which differ from the above-mentioned in that, on the one hand, it should be detected whether an evaluable dose rate is present and, on the other hand, a sufficiently rapid change in tube voltage should occur if the measured dose rate is significantly too high. This prevents overexposure of the X-ray image. After the transient process has ended, the sampling frequency is set to the above-mentioned value (1 kHz).

Claims (3)

1. X-ray generator with actuators for at least 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 that can be actuated by the user, the type of examination and the setpoint of the dose rate setting device,
characterized in that the controller (l40) is a sampling controller formed by a microprocessor system, that a memory (l48, l66) is provided in which the sampling frequency and the actuating function are stored for the various types of examination, and in that the sampling controller corresponding to the actuating function and after a program stored in a further memory (l48, l66), the manipulated variables for the next sampling interval are calculated and the actuators are controlled accordingly. 2. X-ray generator according to claim 1,
characterized in that a plurality of measuring devices (3, l2, 90) are provided, one of which is effective for each type of examination, that matching amplifiers (l7l, l72, l73) are provided which reduce the output signal of the measuring device to a predetermined level at a predetermined dose rate bring, and that a switching device (l70) is provided which couples the measuring device intended for the respective examination with the sampling controller. 3. X-ray generator according to one of the preceding claims,
characterized in that the sampling frequency has a first value in a first section of an X-ray image and a second value in a subsequent section, and in that the first value is substantially greater than the second value.

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