WO2008046654A1 - Tube hood for preheating the housing of an x-ray tube and method for operating an x-ray tube - Google Patents

Abstract

The invention relates to a tube hood (1) for an X-ray tube, in particular for a microfocus tube, with a housing body (2). The invention provides that the tube hood (1) has an active heating apparatus for the housing body (2). The invention also relates to an X-ray tube with a cathode and an anode (6), which are arranged in a tube hood (1) according to the invention. The invention finally also relates to a method for operating an X-ray tube, in which the invention provides that the temperature of the tube hood (1) is maintained using a heating apparatus at a predeterminable value within a tolerance range.

Description

 PIPE HOOD FOR HEATING THE HOUSING OF AN X-RAY TUBE AND METHOD FOR OPERATING AN X-RAY TUBE

The invention relates to a tube hood for an X-ray tube with a housing and an X-ray tube with a cathode and an anode, which are arranged in a tube cap. In addition, the invention also deals with a method for operating an X-ray tube.

X-ray tubes are known which have a very small focal spot, so-called microfocus tubes. The focus size is partly in the micrometer range, for example at a diameter of 1 to 10 microns. Due to this low geometric fuzziness, it is possible to use such microfocus tubes to irradiate objects with high geometric magnification. In some applications, such as industrial computed tomography, an exact positioning of the imaging elements to each other is essential to obtain a good image quality. For this purpose, the position of the focal spot must be precisely determined. If the focal spot moves during the measurement or between the calibration of the X-ray tube and the measurement on the anode, this leads to significant losses in terms of image quality. The geometry of the X-ray hood changes depending on the

Temperature, since elongation takes place. As is known, only a small part of the kinetic energy of the electrons accelerated in the high-voltage field is converted into X-rays in X-ray tubes. Most of this energy is deposited as heat in the anode and must be dissipated via a cooling system. Although the cooling systems work well, the anode heats up because it is integrated into the hood and there is no complete thermal decoupling. Thus, when starting the X-ray tube or at a partial load operation, a different elongation of the tube hood the result. In a continuous operation of the X-ray tube is a thermally stable state after about two Hours reached. The x-ray tube is then warm. Until this stable state is reached, the entire X-ray system can only be used to a limited extent since high-magnification images do not reliably lead to a reproducible result. This follows from the fact that the migration of the focus on the anode can be within a range of up to 15 μm due to the thermally induced elongation of the tube cap. With a very small focus in the range of 1 to 2 μm, this means that the imaging geometry has to be constantly rejuvenated, otherwise the focus can completely run out of the usable area.

The object of the invention is therefore to provide a tube hood, an X-ray tube and a method for operating an X-ray tube with which the usable measuring time of an X-ray tube, in particular a microfocus tube, can be increased.

The object is achieved by a tube hood with the features of claim 1. The fact that the housing body is provided with an active heater, this can be quickly brought to the temperature it has in case of operation of the X-ray tube under full load in continuous operation. This temperature is reached significantly faster than if there is only a thermal coupling between the anode and the housing body. As a result, a fast stationary state of the tube hood, in particular with respect to the elongation, achieved and the focus on the anode remains unchanged in place, without wandering. This means that the X-ray tube can be used for measuring at a much earlier date. In addition, it is then possible for a partial load operation, to keep the temperature of the housing body constant and thus have to make any readjustment of the focus. An active heating device is understood to mean a device for specifically heating the tube hood, which is independent of the remaining, heat or regulated or controlled by the x-ray components of the x-ray tube.

An advantageous development of the invention provides that the heating device is a heating foil which is applied directly to the housing body. As a result, the required heat is introduced directly into the housing body by means that are easy to control.

Another advantageous embodiment of the invention provides that the heating device is a heat radiation source that radiates its thermal radiation directly onto the housing body. Such heat radiation sources are well known in the art and operate reliably and inexpensively.

In addition, a heating device in the form of a hot water heater is advantageous. In such a heater, the cooling water circuit of the anode of the X-ray tube can be modified integrated. Thus, the infrastructure used for cooling the anode can be used directly for tempering the housing body.

A further advantageous embodiment of the invention provides that the heating power of the heater is adjustable. This makes it possible to vary the heating power in different operating modes or at different operating times and to introduce the optimum heating power in the housing body, so that it is kept at a constant temperature and thus subject over long periods of no elongation.

It is particularly advantageous if the heating power of the heater corresponds to the heat transfer from the anode to the housing body under full load. As a result, it is possible to always introduce the minimum heating power transmitted by the coupling between the heated anode and the housing body into the housing body. It is not necessary to supplement the heat output delivered by the stationary operation of the X-ray tube by the heat coupling to the housing body by additional heating power of the heater. Thus, one obtains a constant temperature of the housing body at the lowest possible level over a long period of time.

The object is also achieved by an X-ray tube having the features of patent claim 8. Since such an X-ray tube comprises a tube hood according to the invention, the statements made above on the tube hood and the advantages thus achieved apply analogously to the entire X-ray tube.

Finally, the object is also achieved by a method having the features of patent claim 9. By keeping the temperature of the tubular hood by means of the heating device to a predeterminable value within a tolerance range, the advantages already set out above for patent claim 1 are achieved. It will be a continuous operation at an earlier date - possibly right after the

Starting up the X-ray tube - enables measurements over the entire operating time of the X-ray tube without having to readjust the focus due to temperature.

Preferably, the heater is turned on before or during startup of the X-ray tube, so that the constant operating temperature of the housing body of the tube hood is given already after the startup of the X-ray tube and the focus no longer changes due to a lack of elongation of the tube hood. This allows a longer operating time without readjusting the focus.

The advantages that result from time-varying heating performance have already been explained above with regard to the further developments of the tube hood. The same applies with respect to a constant sum of the variable heat output together with the actual heat transfer from the anode to the anode Tubular body at a constant value, which corresponds to the heat transfer within the X-ray tube under full load.

Further details and advantages of the invention are explained in more detail with reference to the embodiment shown in FIGS. Show it:

1 shows a schematic view of a tube hood with different heating devices and

Fig. 2 is a diagram of the temperature of the tube hood as a function of time.

FIG. 1 shows a schematic arrangement of various heating devices on a tubular hood 1. The tubular hood 1 has a housing body 2, in which inter alia an anode 6 is installed. The anode 6 is thermally not completely decoupled from the housing body 2, so that the heat deposited in it is transferred to the housing body 2 despite a predominant discharge through a cooling water circuit according to the laws of thermodynamics.

Directly on the housing body 2 heating foils 3 are applied. These are known in principle from the prior art, so that further details of their operation will not be discussed below. They serve to keep the housing body 2 at a constant temperature or to bring it as quickly as possible to this temperature. About the operation and in particular the procedure of the invention will be discussed in more detail with regard to the description of FIG.

Another heating device in the form of a heat radiation source 4 is shown symbolically in FIG. The heat radiation source 4 emits heat radiation 5. This is directed directly to the housing body 2 (thus obliquely in this embodiment, via the anode 6 on the housing). Body 2 radiant) and is absorbed by this. As a result, either the temperature of the housing body 2 increases or it is kept at a constant value. More about the temperature setting will be described with reference to FIG.

In addition, the tube hood 1 still has a third heating device. It is the above-mentioned hot water supply in the form of a modified cooling water circuit. For this purpose, a cooling water connection 7 is shown, the cooling water past the anode 6 and thus dissipates a portion of the heat generated therein. The cooling water circuit is via the anode 6 with the housing body 2 in thermal contact and can thus be used to control their temperature. Before and during start-up, however, hot coolant must circulate if the hot water supply is not to be used as additional heating, but as the only heating device.

The three heaters do not all have to be present. In the illustrated embodiment, it is a redundant embodiment, since even a single heater is sufficient to achieve the advantages of the invention. In addition to the three heating devices explicitly shown, all other heating devices for heating or keeping the temperature of the housing body 2 are also interactible with it. The three heaters shown are designed so that they can transmit a variable heating power to the housing body 2. For example, the heating foil 3 is designed so that the heating power can be changed by the control of the heating current. Thus, during the conditioning of the x-ray tube, which takes place almost without current and thus also almost without heating power, the housing body 2 can already be heated up to the stable state. Two curves are shown in FIG. 2, the temperature T of the housing body 2 being plotted over time t (indicated in minutes).

The lower curve shows the temporal temperature profile of the housing body 2 in a known from the prior art X-ray tube having no additional heating device. At time 0, start-up 8 of the X-ray tube is started. Startup 8 is completed after 30 minutes. This is only an exemplary value that can vary greatly with different X-ray tubes. During startup 8 of the X-ray tube, the temperature of the housing body 2 rises only very slowly - and in the illustrated embodiment, linear - on. The X-ray tube then runs under full load 12 (from approx. 30 minutes to approx. 140 minutes). The temperature of the housing body 2 rises more strongly under full load 12 and approaches asymptotically the limit value - the stationary temperature distribution T _a -, which the housing body 2 assumes during continuous operation under full load 12.

If, as an example, after about 140 minutes, the X-ray tube is no longer driven at full load 12, but only in a partial load operation 9, reduces the heat output, which is discharged from the anode 6 to the housing body 2, since the total heat output, the is produced in the anode 6, also decreases. This means that - depending on the height of the partial load - the temperature of the housing body 2 decreases. This applies analogously to any type of load change.

As a result, this results in the housing body 2 being subjected to a longitudinal expansion since it regularly consists of a metal or a metal alloy. This leads to the already described with respect to the prior art problem that the geometry of the X-ray tube is changed and the focus on the anode 6 moves. As stated above, this can be up to 15 microns. In applications requiring high accuracy in imaging geometry, for example at high magnification, it is thus necessary to re-adjust the focus. Every adjustment takes time and extra effort, so this problem should be eliminated.

This is done by means of the already described above for Figure 1 heaters. In Figure 2, the temperature curve over time is plotted in the upper curve, which results for a tube hood 1 according to the invention according to Figure 1 and a method according to the invention.

During start-up 8 of the x-ray tube, preheating 10 takes place through the heating device. Here as much heat output from the heater is transferred to the housing body 2 in order to achieve as quickly as possible the stationary temperature distribution T _{a of} the housing body 2 under full load 12 of the X-ray tube. This is achieved in the illustrated embodiment after about 30 minutes. This corresponds to the time when the X-ray tube is conditioned and started up. From this point on, the heating power of the heating device is varied so that the sum of it and the heat output currently transmitted by the X-ray tube keeps the temperature of the housing body 2 constant. In FIG. 2, the additional heat output that is currently introduced by the heating device can be recognized by the size of the difference between the two temperature curves at the respective time. Since the X-ray tube is driven in the example to about 140 minutes under full load 12, the additional heat energy required by the heater decreases continuously. The respective heater is - as stated above - regulated accordingly.

From the time at which the X-ray tube of full load 12 is switched to part-load operation 9, again more heat energy through the heater on the housing body 2 ü be transferred. Again, the measure of additional heat energy is well recognized by the difference between the two curves.

The constant held temperature of the housing body 2 is in the illustrated embodiment slightly above the stationary temperature distribution T _a , which would be given 12 without a heater at full load. Thus, a certain amount of heating is constantly provided by the heater; This also applies to the steady-state operation of the X-ray tube under full load 12. It is equally possible to choose the stationary temperature distribution T _a so that it corresponds to the temperature without heater at full load 12 of the X-ray tube. Then no additional heating power is required during steady-state full load operation of the x-ray tube.

In summary, the invention can be described as integrating a kind of "auxiliary heating" into an X-ray tube which brings the housing body 2 as quickly as possible to a constant temperature and then also maintains this temperature during operation of the X-ray tube The fastest possible attainment of the desired temperature increases the usable measuring time 11 and it is not necessary to adjust the focus of the x-ray tube since it is not subject to migration due to the elongation of the tube hood 1. This leads to a saving of time and a smaller one technical effort. LIST OF REFERENCE NUMBERS

Tubehousing

housing body

heating film

Radiant heat source

thermal radiation

anode

Cooling water connection

go up

Partial load operation

Preheat usable measuring time

full load

stationary temperature distribution

Claims

claims 1. tube hood (1) for an X-ray tube, in particular for a microfocus tube, with a housing body (2), characterized in that it comprises an active heating device for the housing body (2). 2. tube cap (1) according to claim 1, characterized in that the heating device is a heating foil (3), which is applied directly to the housing body (2). 3. tube cap (1) according to claim 1, characterized in that the heating device is a heat radiation source (4), which radiates its thermal radiation (5) directly to the housing body (2). 4. tube cap (1) according to claim 1, characterized in that the heating device is a hot water supply. 5. tube cap (1) according to claim 4, characterized in that the heating system is formed as part of the cooling water serkreises an anode (6) of the X-ray tube and thermally conductively connected to the housing body (2). 6. tube cap (1) according to one of the preceding patent claims, characterized in that the heating power of the heater is adjustable. 7. Tubular cap (1) according to one of the claims 1 to 6, characterized in that the heating power of the heating device corresponds to the heat transfer from the anode to the housing body (2) under full load (12). 8. X-ray tube, in particular microfocus tube, with a cathode and an anode (6), which are arranged in a tube hood (1), characterized in that the tube cap (1) is designed according to one of the preceding claims. 9. A method for operating an X-ray tube, in particular a microfocus tube, in which the temperature of the tube hood (1) is held by means of a heating device to a predefinable value within a tolerance range. 10. The method according to claim 9, characterized in that the heating device before or during the Startup (8) of the X-ray tube is turned on. 11. The method according to claim 9 or 10, characterized in that the heater emits a temporally variable heating power. 12. The method according to claim 11, characterized in that the sum of the variable heating power together with the current power consumption of the X-ray tube is always the same as the heat transfer from the anode to the housing body (2) under full load (12).