EP0682466B1 - X-ray installation - Google Patents

Description

The invention relates to an x-ray system with an x-ray tube and an x-ray generator for operating the X-ray tube, which is heated by a heating current Contains cathode, with effective in a recording mode Means for raising the heating current to a boost value with the tube voltage switched off during a boost time and with means also effective in the recording mode Reduction of the heating current to a value suitable for the subsequent X-ray exposure and to switch on the tube voltage.

Such a system is e.g. known from EP-A-0 025 688.

If an x-ray with such an x-ray system - e.g. after previous screening - made it is desirable to take the x-ray like this to execute as quickly as possible. With x-ray tubes with a heatable cathode, however, the cathode (or the filament contained therein) only to a temperature brought in for X-rays required tube current can emit.

To shorten the time to start recording, it is known, the cathode - with the tube voltage switched off - to supply a heating current that is significantly greater than the heating current for the subsequent X-ray (with the tube voltage switched on) is necessary. The boost time depends on the tube current, which in the following Recording should flow. The greater this tube current the longer the boost time.

It is already known that the X-ray generator for Boost times required for a specific tube type store it and store it on an x-ray to call. This boost time table contained in the memory is manufactured by the X-ray tube manufacturer in one complicated measurement procedure determined, for everyone X-ray tube type separately. The given boost times are typical values, i.e. it can happen that the cathode temperature is higher at the end of the boost time or is lower than that for the respective tube current required temperature. Therefore, after the boost time the heating current is reduced to the value that it at X-ray should have. Then if after another Time interval of 200 to 300 ms the tube voltage is switched on the cathode temperature is stationary Value reached that required for the recording Value corresponds.

The object of the present invention is an x-ray system to create, in which the preparation time, i.e. the period until an X-ray begins can be further shortened.

This task is carried out on the basis of an X-ray system type mentioned solved in that the X-ray generator is designed for a special mode in which switched on tube voltage the heating current to the Boost value is raised that means for measuring the im Special mode flowing tube current are provided that Means for storing the time course of the measured Tube current or a value derived therefrom are provided and that means for deriving the boost time from the time history stored in the memory are provided.

It is essential to the invention that the boost times in a special mode of the X-ray generator can be determined, in which the tube voltage is switched on and the heating current is raised to its boost value. Grows in this mode the tube current continuously up to a maximum value after which the tube voltage is switched off and the heating current is lowered or also switched off. Which the time course resulting until the device is switched off is measured and saved. If on a subsequent X-ray, which is carried out in the recording mode certain tube current is specified, one can from the stored history over time how long it - with heating current increased to the boost value - takes until the cathode temperature has reached a value at which just the desired tube current is being emitted. This Period corresponds to the period within which stored tube current curve the relevant tube current value is reached; it is in record mode as Boost time specified.

The invention allows the exact in a simple manner Determination of the required boost times, individually for the respective x-ray tube. So that is Boost time just as big as it needs to be, so in the end the boost time exactly for the emission of the desired Tube current required temperature is reached. That's why it is no longer necessary to set the boost time to follow the second interval in which the heating current to the value required for the respective tube current is lowered. This shortens the preparation time considerably. It is also advantageous that the Special operating mode with the X-ray generator in larger time intervals can be repeated. Thereby signs of aging are taken into account that a Influence on the characteristics of the respective X-ray tube to have. When changing the X-ray tube, there is no change of the boost time memory required, and it can too X-ray tubes are used, their temperature behavior is unknown.

In general, the tube current doesn't just depend on that Heating current, but also from that on the x-ray tube applied tube voltage. There are a number of ways how to make that to a certain combination Boost time belonging to tube current and tube voltage can determine. One possibility would be to temporal course of the tube current in the special operating mode repeat for a variety of tube voltages, so that there would be a bevy of curves that the temporal course of the tube current with the tube voltage would represent as parameters. If then in normal operating mode specified a certain tube voltage would have to be in the same operating mode for the same tube voltage measured time course of the tube current used to determine the tube voltage will. This would be relatively expensive because in the special operating mode a variety of temporal tube currents should be measured and stored.

It is sufficient, however, the time course of the tube current only to be recorded with a single tube voltage, if provided according to a preferred development of the invention is that a second memory is provided in that for different tube voltages and tube currents stationary heating current values are stored and that the Means for deriving the boost time on the first memory and access the second memory.

Fig. 1

ein Blockschaltbild eines Röntgengenerators einer erfindungsgemäßen Röntgenanlage in schematischer Darstellung,

Fig. 2

Teil A den zeitlichen Verlauf von Heizstrom und Röhrenspannung im Aufnahme-Modus,

Fig. 2

Teil B den zeitlichen Verlauf von Heizstrom und Röhrenspannung im Sonder-Modus,

Fig. 3

Teil A Kennlinien, die die Abhängigkeit des Röhrenstroms vom Heizstrom mit der Röhrenspannung als Parameter im stationären Zustand darstellen,

Fig. 3

Teil B den zeitlichen Verlauf des Röhrenstromes im Sonder-Modus und eines daraus ableitbaren Heizstromwertes.

Fig. 4

ein Fluß-Diagramm für den Sonder-Modus,

Fig. 5

ein Fluß-Diagramm für den Aufnahme-Modus.

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

Fig. 1

2 shows a block diagram of an x-ray generator of an x-ray system according to the invention in a schematic illustration,

Fig. 2

Part A shows the time course of heating current and tube voltage in recording mode,

Fig. 2

Part B the time course of heating current and tube voltage in special mode,

Fig. 3

Part A Characteristic curves depicting the dependence of the tube current on the heating current with the tube voltage as parameters in the steady state,

Fig. 3

Part B shows the time course of the tube current in special mode and a heating current value that can be derived from it.

Fig. 4

a flow diagram for the special mode,

Fig. 5

a flow chart for the recording mode.

The X-ray generator shown schematically in FIG. 1 for feeding an X-ray tube 1 comprises a first one High voltage generator 2 for generating a positive High voltage for the anode of the x-ray tube and one second high voltage generator 3 for generating a negative high voltage for the cathode of the x-ray tube. The two high voltage generators 2 and 3 are over one Resistor 4 connected in series, one end of which is grounded is. The resistor 4 is used to measure the Anode of the X-ray tube 1 flowing tube current.

The high-voltage generators 2 and 3, ie the time profile of the tube voltage U generated by them, can be controlled by a control unit 5, which can contain a suitably programmed microprocessor. The voltage drop across the resistor 4, ie a value proportional to the tube current, is fed to the control unit via an analog-digital converter 6. The control unit also specifies the heating current for the cathode of the X-ray tube 1, which is generated by a heating current control circuit 7. The control unit works with a first memory 8, in which dynamic data are stored, and with a second memory 9, in which static or stationary data are stored, and links them in a manner yet to be explained with the values of for an X-ray exposure Tube current I _r and tube voltage U.

Fig. 2, part A, shows for the recording mode the time course of the heating current I _h and dashed the time course of the tube voltage U. It can be seen that before the time t = 0, the heating current is set to a constant quiescent current value, while the Tube voltage U is not yet present. This quiescent current value is chosen such that no appreciable tube current would flow if a tube voltage were switched on. A typical value for the quiescent current is 2 A.

At time t = 0, the heating current I _{h is raised} to a boost value. This boost value is usually much larger than the tube current flowing during an X-ray exposure, and it preferably corresponds to the maximum permissible value - for example 11 A. The heating current is kept at this value until the boost time has expired, ie until the time t = t _B. At the time t = t _B , the tube voltage U is switched on for the X-ray exposure. At the same time, at time t = t _B, the heating current is reduced to a value between 3 A and 7 A, ie to a value which is greater than the quiescent current and less than the boost value. Only at the time t = t _B can a tube current flow through the X-ray tube and X-rays arise, ie the actual X-ray recording only begins at the time t = t _B. After a predetermined recording time or a recording time determined by an automatic exposure device, the tube voltage and the heating current are switched off, ie the X-ray recording is ended.

1. Am Ende der Boostzeit (t=t_B) muß die Kathode durch den Heizstrom auf die Temperatur aufgeheizt sein, bei der nach dem Einschalten der Röhrenspannung U sich der gewünschte Röhrenstrom I_r einstellt.

2. Der während der Röntgenaufnahme fließende Heizstrom muß gerade so groß sein, daß das zur Zeit t=t_B erreichte Temperaturniveau während der gesamten Röntgenaufnahme beibehalten wird, so daß der Röhrenstrom konstant bzw. statisch oder stationär bleibt.

So that the desired tube current flows at time t = t _B and remains constant during the entire X-ray exposure, two prerequisites must be fulfilled:

1. At the end of the boost time (t = t _B ), the cathode must be heated by the heating current to the temperature at which the desired tube current I _{r is} set after the tube voltage U is switched on.

2. The heating current flowing during the X-ray exposure must be just large enough that the temperature level reached at time t = t _{B is} maintained throughout the X-ray exposure so that the tube current remains constant or static or stationary.

3, part A, a stationary characteristic field is shown, which for different voltages U ₁ . . . U ₄ indicates the tube current I _r , which occurs at a certain static or stationary heating current. From this diagram it can easily be seen which heating current I _{h has to be set} in the stationary case for a specific combination of tube current I _r and tube voltage U. This family of curves, ie the heating current as a function of the tube current or the tube voltage, is stored in the second memory 9. How to determine such a characteristic field individually for the respective X-ray tube is described in DE-PS 27 03 420, among others.

The following describes how the required boost time for this - and other combinations - of I _r , U can be determined simply and precisely. For this purpose, the X-ray generator is operated in special mode. Fig. 2, part B, shows the time course of heating current I _h and tube current U during the special mode. Here, too, the heating current is kept at its quiescent current value until time t = 0, in order to be increased to its boost value at time t = 0, which is the same size as in the recording mode. In contrast to the recording mode, however, a voltage U _ref is already applied to the X-ray tube at time t = 0, so that a tube current can flow as soon as the cathode is hot enough. 3, part B, shows - as a solid curve - the time profile of the tube current I _r (although with a different time scale than FIG. 2b). It can be seen that the tube current initially increases slowly and then faster and faster because the resistance of the cathode or the filament contained therein increases the hotter the cathode becomes, so that the cathode power supplied increases continuously. When the tube current has reached a maximum value, the tube voltage U = U _{ref is} switched off and the heating current I _h is likewise switched off or reduced.

From the time profile of the tube current I _r , the boost time can be read directly, which is required to reach a temperature in a subsequent X-ray exposure with the tube voltage U = U _ref at the end of the respective boost time, which when the tube voltage U = U _{ref is switched on} just lets the desired tube current flow. For this reason, the time profile of the tube current is measured and digitized during the special mode by digitizing the voltage across the resistor 4 by the analog-digital converter 6, so that a measurement value of the tube current is provided for measuring time intervals of, for example, 3 ms Available. The course measured in this way is stored in the first memory 8.

The flow chart according to FIG. 4 explains the time sequence of the steps carried out by the control unit during the special mode. First, according to block 50, the heating current is set to a quiescent current value or a _standby value I _stb . The voltage on the tube is switched off.

Thereafter (block 51), the heating current to the boost value I set _b and adjusted, the tube voltage at the value U = U _ref. A tube current then begins to flow, as shown in FIG. 3b. The tube current is measured, digitized every 3 ms and stored in the first memory 8 (block 52). In the next step (block 53) it is checked whether the measured tube current is less than a maximum value I _max at which the X-ray tube is not yet thermally overloaded. If the current I _{r is} even smaller, a new measurement and a new query etc. are carried out until the maximum value is reached. This is usually the case after 200 to 300 ms. The heating current is then reduced again to the quiescent current I _stb and the tube _{voltage is} switched off (block 54).

a) Es wird im Sonder-Modus nicht nur für eine einzige Röhrenspannung der zeitliche Verlauf des Röhrenstroms gemessen, sondern für eine Anzahl von Spannungen. Wenn bei einer nachfolgenden Röntgenaufnahme eine dieser Spannungen eingestellt wird, könnte die Boostzeit aus dem zeitlichen Verlauf abgeleitet werden, der dieser Spannung zugeordnet ist. Dies setzt jedoch eine mehrfache Wiederholung der Meß- und Speicherprozedur im Sonder-Modus voraus. Es ist jedoch auch möglich, mit dem zeitlichen Verlauf des Röhrenstromes für nur eine einzige Spannung U=U_ref auszukommen. Dabei sollte U_ref zweckmäßigerweise so gewählt sein, daß der größtmögliche Röhrenstrom (I_r=I_max) erreicht werden kann, ohne daß die Röntgenröhre thermisch überlastet wird. Ein geeigneter Wert ist z.B. 70 kV.

b) Eine erste Möglichkeit mit der Messung des Röhrenstroms bei einer Röhrenspannung auszukommen, ist in den Fig. 3a und 3b schematisch erläutert, wobei angenommen ist, daß bei einer nachfolgenden Röntgenaufnahme eine Röhrenspannung U₄ anliegt und ein Röhrenstrom I_r2 fließen soll. In einem ersten Schritt wird dabei aus dem Speicher 9 der Heizstrom I_h2 (vergl. die strichpunktierte Linie in Fig. 3, Teil A) ermittelt, der der vorgegebenen Kombination U₄, I_r2 zugeordnet ist. In einem zweiten Schritt wird dann ebenfalls aus dem Speicher 9 der Röhrenstrom I_r ermittelt, der bei dem Heizstrom I_h2 fließen würde, wenn die Spannung U_ref=U₃ an der Röntgenröhre anliegen würde. Als dritter Schritt wird in dem ersten Speicher 8 die zu diesem Wert des Röhrenstroms gehörende Boostzeit t_B ermittelt.

c) Man kann jedoch auch mit nur zwei Schritten auskommen, wenn man zuvor, beispielsweise beim Einlesen der Meßwerte des Röhrenstroms I_r oder danach, ein einziges Mal die in Fig. 3b mit einer ausgezogenen Linie dargestellte Kurve für den zeitlichen Verlauf des Röhrenstromes in eine Kurve für den äquivalenten stationären Heizstromwert transformiert wird (der äquivalente stationäre Heizstrom würde im stationären Fall bei U=U_ref gerade den jeweiligen Röhrenstrom fließen lassen). Diese Kurve ist in Fig. 3b gestrichelt angedeutet und mit I_cor bezeichnet. In der Fig. 3, Teil A, ist angedeutet, wie man für einen Wert I_r1 den zugehörigen Wert I_h1 aus der durch eine ausgezogene Linie dargestellten Kurve für U₃ (=U_ref) ermitteln kann. Dazu wird lediglich aus dem Speicher 9 der zu dem gemessenen Wert von I_r1 und der Spannung U_ref gehörende Heizstromwert I_h1 (vergl. Fig. 3a) aus dem Speicher 9 entnommen und der Meßzeit für den Wert I_r1 zugeordnet. Wiederholt man das für alle Meßwerte von I_r, ergibt sich die Kurve I_cor (um die Zeichnung zu vereinfachen, gelten für die Kurven I_h und I_r unterschiedliche Skalen auf der Ordinatenachse).

As already mentioned, the tube current I _r depends not only on the heating current I _h , but also on the tube voltage. If a tube voltage that deviates from the voltage U = U _{ref present} in the special mode is switched on in a subsequent X-ray recording in the recording mode, then the boost time cannot be derived directly from the curve stored for U = U _ref . There are a number of options for taking this additional time dependence of the tube current into account:

a) In the special mode, the time profile of the tube current is measured not only for a single tube voltage, but for a number of voltages. If one of these voltages is set in a subsequent X-ray exposure, the boost time could be derived from the time course which is associated with this voltage. However, this requires a repeated repetition of the measurement and storage procedure in special mode. However, it is also possible to get by with the time profile of the tube current for only a single voltage U = U _ref . U _{ref should} expediently be chosen so that the largest possible tube current (I _r = I _max ) can be achieved without the X-ray tube being thermally overloaded. A suitable value is, for example, 70 kV.

b) A first possibility to get by with the measurement of the tube current at a tube voltage is schematically illustrated in FIGS. 3a and 3b, it being assumed that a tube voltage U ₄ is present in a subsequent X-ray exposure and a tube current I _{r2 should} flow. In a first step, the heating current I _h2 (cf. the dash-dotted line in FIG. 3, part A) is determined from the memory 9 and is assigned to the predetermined combination U ₄ , I _r2 . In a second step, the tube current I _r is then also determined from the memory 9, which would flow at the heating current I _h2 if the voltage U _ref = U _{3 were applied} to the X-ray tube. As a third step, the boost time t _B belonging to this value of the tube current is determined in the first memory 8.

c) However, you can get by with only two steps if you have previously, for example when reading in the measured values of the tube current I _r or after, the curve shown in Fig. 3b with a solid line for the time profile of the tube current in a Curve for the equivalent stationary heating current value is transformed (the equivalent stationary heating current would just let the respective tube current flow at U = U _ref in the stationary case). This curve is indicated in dashed lines in Fig. 3b and designated I _cor . 3, part A, indicates how the associated value I _h1 can be determined for a value I _r1 from the curve for U ₃ (= U _ref ) represented by a solid line. For this purpose, only the heating current value I _h1 (see FIG. 3a) belonging to the measured value of I _r1 and the voltage U _ref (see FIG. 3a) is taken from the memory 9 and assigned to the measuring time for the value I _r1 . If this is repeated for all measured values of I _r , the curve I _cor results (to simplify the drawing, different scales on the ordinate axis apply to curves I _h and I _r ).

After the curve I _cor (FIG. 3b) has been determined in this way once or after each special mode, only the stationary heating current value I _h belonging to the predefined values of tube current I _r and tube voltage U _{h is} determined (from the memory 9 or one of the curves in FIG. 3a), and in a second step (from the memory 8 or FIG. 3b) the value of the boost time belonging to the respective value I _h on the curve I _{cor is} determined.

One could repeat this - similar to the stationary characteristic curves shown in FIG. 3, part A - for different tube currents I _r and tube voltages and would then receive a family of curves in FIG. 3, part B, for different combinations of tube current I _r and tube voltage U represent the associated boost time. If these curves were saved, the boost time could be obtained directly in the recording mode - ie without the intermediate step via curve I _cor - but this would only increase the storage effort without simplifying the process. Before each X-ray exposure, the value of the heating current must be determined anyway from the stationary characteristics of FIG. 3a or the memory 9, which must flow during the subsequent exposure, so that the tube current I _r results. It is therefore more appropriate to derive the required boost time from the x-ray image to x-ray image from the characteristic curves stored in the memories 8 and 9.

According to the block diagram in FIG. 5, the following sequence then results for an x-ray exposure: The values of tube current and tube voltage desired for the x-ray exposure are specified (block 55). The stationary heating current required for the X-ray exposure is determined from these values, with the aid of the values stored in the memory 9 (block 56). The boost time t _{B associated} with this heating current value is then determined from curve I _cor in FIG. 3, part B, or in memory 8. The heating current is then raised to the boost value during the time period t _B , with no voltage being applied to the X-ray tube (block 57). After the boost time t _B has elapsed, the heating current is reduced to the value determined in block 56 and the desired tube voltage U is switched on (block 58). The desired tube current I _r then flows.

In certain examination methods, an X-ray exposure is preceded by fluoroscopy, in which the tube current I _{r has} a small but no longer negligible value. If the filament were then heated in the recording mode during the full boost time determined in the manner described above, the temperature would be somewhat too high. This can be prevented by reducing this boost time by the value of that boost time which is assigned to the heating current I _h flowing in the fluoroscopic mode.

Claims (2)

1. An X-ray system, comprising an X-ray tube and an X-ray generator for operating the X-ray tube (1) which comprises a cathode which can be heated by a filament current (I_h), comprising means (5, 57) which are operative in an exposure mode so as to boost the filament current to a boost value (I_b), during a boost time while the tube voltage is switched off, and means (5, 58) which are also operative in the exposure mode so as to decrease the filament current to a value which is suitable for the subsequent X-ray exposure and to switch on the tube voltage (U), characterized in that the X-ray generator has a special mode in which the filament current is boosted to the boost value (I_b) while the tube voltage (U) is switched on, that means (4, 6) are provided for measuring the tube current flowing in the special mode, that means (8) are provided for storing the temporal variation of the measured tube current, or a value (I_cor) derived therefrom, and that means (5, 57) are provided for deriving the boost time from the temporal variation stored in the memory (8).
2. An X-ray system as claimed in Claim 1, characterized in that there is provided a second memory (9) in which the stationary filament current values (I_a) are stored for various tube voltages (U) and tube currents (I_r) and that the means (5, 57) for deriving the boost time access the first memory (8) and the second memory (9).

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