Preheating filament of x-ray tube

Abstract

To makes an emission current more accurate, stable or constant in an X-ray tube.SOLUTION: A method of operating a filament of an X-ray tube having a filament functioning as a cathode and an anode, includes the steps of passing a current through the filament to preheat the filament prior to applying a high voltage between the filament and the anode, increasing the effective value of the current and applying a high voltage, and emitting X-rays in response to the increase in the effective value of the current through the filament and the application of the high voltage.SELECTED DRAWING: Figure 8

Description

[Technical field]

[0001] The present disclosure relates to thermionic emission x-ray tubes, and more particularly to preheating the filament of thermionic emission x-ray tubes. [Background technology]

[0002] Figure 1 shows a typical thermionic emission X-ray tube. In use, a constant current I0 is passed through the X-ray tube filament C, and a voltage Uh is adjusted to heat the filament C, which acts as a cathode. To emit X-rays, a high voltage Ua is applied between the filament C and a metal target A, which acts as an anode. Electrons emitted from the filament C are accelerated by the electric field of the high voltage Ua inside the vacuum tube and collide with the metal target A. This collision produces X-rays.

[0003] Figure 2 shows a typical calibration curve for an X-ray tube. Each calibration curve, corresponding to a value of the high voltage Ua (40 kV, 50 kV, 60 kV, 70 kV, 100 kV, 125 kV, or 150 kV as shown in this example), is measured by applying a constant current I0 (plotted on the X-axis) to the filament and measuring the tube emission current |I0-I1| (plotted on the Y-axis) after a quasi-steady state is reached. A filament calibration table is then created for the selected high voltage Ua and the target value of the tube emission current |I0-I1|. The filament configuration table provides the filament current I0 that produces the target value of the tube emission current |I0-I1| under the high voltage Ua. The tube emission current |I0-I1| is proportional to the amount of X-rays emitted from the tube and directed to the patient or object.

[0004] FIG. 3 shows a typical time response of an X-ray tube. The filament current (A) is maintained at a standby value until a time Prep, when the filament is preheated by increasing the current to I0 according to the calibration table. When X-ray emission is desired, the high voltage Ua is applied. When X-ray emission is no longer desired, the high voltage Ua is turned off. The filament current (A) may be returned to the standby value immediately (as shown) or after, for example, 1000 ms. The typical response of an X-ray tube shown in FIG. 3 shows that the tube emission current |I0-I1| reaches a peak immediately after the high voltage Ua is applied and then reaches a quasi-steady value (i.e., the target value) used in the calibration table. The inventors have discovered that the reason for this peak is that the filament heats up at the beginning of electron emission until it reaches a steady temperature. Summary of the Invention [Problem to be solved by the invention]

[0005] In view of the above, there is a general need in the art for an apparatus and method for achieving a more accurate (i.e. closer to the target value of the tube emission current), stable or constant tube emission current when a high voltage Ua is applied. In particular, a more accurate, stable or constant tube emission current is preferably achieved in a manner that does not cause emission delay. [Means for solving the problem]

[0006] The present application discloses an apparatus and method for achieving a more accurate, stable or constant tube emission current in a thermionic emission X-ray tube. The more accurate, stable or constant tube emission current results from modifying the filament of the thermionic emission X-ray tube to preheat before the high voltage Ua is applied, relative to the heating of the filament while the high voltage Ua is applied (i.e., while the filament is emitting electrons). Specifically, the filament is preheated before the high voltage Ua is applied by passing a current of a first effective value that generates an initial temperature in the filament. The first effective value may be selected to be lower than the second effective value (e.g., a target filament current) by an amount that reduces an initial surge or overshoot of the emission current initially expected when the high voltage Ua is applied. The preheat time may be constant and is long enough to reach a substantially steady-state temperature of the filament. When the high voltage Ua is applied, the current through the filament is increased (e.g., increased stepwise to a higher second effective value). The inventors have discovered that the combination of increasing the current and the thermal response caused by applying a high voltage Ua can result in an essentially stable or constant temperature of the filament, which translates into a more accurate (i.e. closer to the target value of the tube emission current) stable or constant tube emission current.

[0007] Thus, the techniques of the present disclosure may eliminate the undesirable overshoot behavior of the tube emission current |I0-I1| seen in Figure 3, and generally undesirable behavior (e.g., undershoot behavior, etc.). Reducing or eliminating the undesirable behavior of the tube emission current |I0-I1| allows for more accurate, stable or constant short pulses of x-ray radiation without unnecessary delays. By providing more accurate, stable or constant radiation pulses, the present disclosure may further reduce or eliminate the risk of the radiation target (e.g., patient or object) receiving unnecessary or excessive radiation.

[0008] Furthermore, by providing more accurate, stable, or consistent pulses of radiation, the techniques disclosed herein may improve the reliability and lifespan of x-ray tubes.

[0009] In one embodiment, the current through the filament during preheating is constant (ie, its value is a first effective value).

[0010] In one embodiment, the current through the filament during preheat is pulse width modulated or amplitude modulated (i.e., the first effective value is a weighted average between a low pulse value and a high pulse value determined by the pulse duration or pulse amplitude).

[0011] Preferably, the current flowing through the filament during electron emission is a constant current (ie, its value is the second effective value).

[0012] Preferably, when or after the high voltage Ua is switched off, the current through the filament is first reduced to a first effective value so that the filament reaches a suitable preheating temperature for the continued application of the high voltage Ua. If the high voltage Ua is not continuously applied for a time longer than the preheating time, the current may subsequently be reduced to a lowest standby value. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic diagram showing a thermionic emission X-ray tube. [Diagram 2] 1 is a graph showing a number of calibration curves for an X-ray tube obtained for different values ​​of the cathode-anode high voltage. [Diagram 3] FIG. 1 is a graph showing an example of a preheating process for a conventional X-ray tube that does not use the present invention, showing the measured filament current and the measured tube emission current as a function of time when the filament is preheated with a current of the same value as that when the filament emits electrons (when a high voltage is applied between the cathode and anode). [Figure 4]1 is a flowchart of a method for determining a first effective value of current during filament preheating, the first effective value of current being expressed as a percentage of a second effective value of current during filament electron emission, which can be used to achieve a more accurate, stable, or constant tube emission current. [Diagram 5] 5 is a graph of the percentage determined according to the method shown in FIG. 4 as a function of the tube emission current |I0-I1| in the quasi-steady state. [Figure 6] FIG. 2 is a schematic diagram of a closed loop control for driving a filament. [Figure 7] 1 is a flow chart of a method of operating a filament in a preheat and release stage. [Figure 8] 7 is a graph showing filament current and tube emission current as a function of time when the method of FIG. 6 is used. [Figure 9] 1 is a graph showing the filament current and the tube emission current measured as a function of time when the current flowing through the filament during preheating is pulse-width modulated. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] In the method of FIG. 4, a nominal high voltage value Ua of 100 kV is used to determine a first effective value of the current during preheating of the filament, expressed as a percentage of a second effective value of the current during electron emission of the filament. However, the method can generally be repeated for any applicable high voltage value. After using a nominal high voltage value Ua of 100 kV, the percentage calculated by the method was determined to be valid at least in the high voltage range between 80 kV and 120 kV.

[0015] In the method of FIG. 4, the maximum tube current (which is achieved at the maximum filament current I0) is used during filament preheating and filament electron emission, and the percentage is calculated. However, the method can generally be repeated for any applicable tube emission current value, for example, as shown in FIG. 5. After measurements at the maximum tube current have been made, other percentages corresponding to other values ​​of tube current can be calculated by linear extrapolation. It has been determined that linear extrapolation allows accurate calculations for calculating percentages at other values ​​of tube current, as shown in FIG. 5.

[0016] The "peak mA" and "stable mA" measurements for the method of FIG. 4 are shown in FIG. 3 by two "sample" values ​​on the line showing tube emission current as a function of time.

[0017] 6 to 8 show the operation of a filament according to an embodiment of the present invention. The user sets the exposure parameters in terms of a nominal high voltage value Ua (kV) and a desired tube emission current |I0-I1| (mA). The calibration table, having a number of columns, each corresponding to one high voltage value Ua, and a number of rows, each corresponding to one target value of the tube emission current |I0-I1|, lists the values ​​of the filament current I0 that must be used to meet these conditions in the quasi-steady state (see FIG. 2 for a graphical representation of the calibration table). The table NVRAM, having at least one column and a number of rows, each corresponding to one target value of the tube emission current |I0-I1|, records as a table the current reduction rate to be used in the preheating phase to avoid overshoot. The filament current to be used in the preheating phase is calculated using the value of the filament current I0 obtained from the calibration table and the reduction rate obtained from the table NVRAM. The filament current is supplied with a predetermined variation as a function of time, which is shown by the top solid line in FIG. 8 (i.e., a stepped solid line).

[0018] Preferably, the faster (or generally the more accurate) the tube emission current reaches a quasi-steady state, the better. In some applications, particularly medical applications, it is desirable to reach a quasi-steady state in a few microseconds. Therefore, the increases in the filament current I0 and the high voltage Ua must be timed to reach a quasi-steady state within that time interval. For example, if the filament current I0 and the high voltage Ua are being increased in steps, it is preferable that these increases occur substantially simultaneously.

[0019] Comparing Figures 3 and 8, it can be seen that by adopting a process of passing an appropriate current when transitioning from the preheating stage to the emission stage, the tube emission current |I0-I1| becomes more accurate, stable, or constant, compared to the case where such a process is not performed (i.e., no constant current is used).

[0020] 9 shows that the reduction rate of the filament current I0 during the preheating phase can alternatively be reached by using pulse width modulation or amplitude modulation, for example modulation between the standby current value and the value of the filament current I0 obtained from a calibration table. Thus, the effective value of the filament current during preheating of the filament (i.e. the weighted average of the low current pulse value and the high current pulse value) is the value that determines the response of the filament. Thus, in the method shown in FIG. 7, the step "Set the filament supply to heat reduction" is not limited to applying a constant current of a low value.

[0021] Although specific values ​​of high voltage (Ua), tube emission current (|I0-I1|), and filament current (I0) are disclosed herein, other values ​​may be used depending on the x-ray tube used, the x-ray energy, and the device used to power the x-ray tube.

[0022] Although a particular percentage of the second effective value of the current during electron emission of the filament is disclosed herein to calculate the first effective value of the current during preheating of the filament, other values ​​may be used, such as values ​​experimentally determined by the method shown in Figure 4. However, it should be noted that the present invention is not limited to using values ​​obtained by the method shown in Figure 4. Also, the present invention is not limited to expressing the first effective value of the current during preheating of the filament as a percentage of the second effective value of the current during electron emission.

Claims

1. 1. A method of operating an x-ray tube having a filament functioning as a cathode and an anode, comprising the steps of: applying a current through the filament to preheat the filament before applying a high voltage between the filament and the anode; applying the high voltage while increasing the effective value of the current; and emitting x-rays in response to increasing the effective value of the current through the filament and applying the high voltage; The method according to claim 1, further comprising:

2. 2. The method of claim 1 , The method of claim 1, wherein the step of increasing the effective value of the current and applying the high voltage comprises simultaneously increasing the effective value of the current and the voltage abruptly.

3. Apparatus for carrying out the method according to claim 1 or 2.

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