Patent Application
20070086573
1. Field of the Invention
The present invention concerns an x-ray apparatus according to the preamble of the type having a cooling device for cooling the anode through which a fluid coolant flows.
2. Description of the Prior Art
X-ray tubes with a high load capacity are conventionally used in the field of x-ray computed tomography. These can be either x-ray tubes with a rotary anode as are known, for example, from DE 40 12 019 B4 or U.S. Pat. No. 5,541,975 or rotating envelope tubes as they are, for example, known from DE 103 35 664 B3. Cooling devices for dissipation of the heat generated in the anode are provided. In rotary anodes the cooling device extends in an anode shaft (fashioned hollow) and/or in the anode plate (fashioned hollow). In rotating envelope tubes a vacuum housing embodying the anode is surrounded by the cooling device. An electrically-insulating oil or a thermally insulating oil typically flows through the cooling device as a coolant. In addition to the property of electrical isolation, thermally insulating oil can be used given a working temperature of 200° C. due to its relatively high boiling point. However, insulating oil disadvantageously exhibits only a relatively low heat capacity, meaning the heat accumulating at the anode cannot be dissipated particularly efficiently with such insulating oil. Apart from this the insulating oil is highly mobile. A durable, complete sealing of the cooling device can be realized only with difficulty in practice. To overcome this disadvantage it has been proposed according to U.S. Pat. No. 5,541,975 to use liquid metal as a coolant, but this is expensive and complicated.
An object of the present invention is to provide x-ray apparatus with a cooling device that avoids the disadvantages according to the prior art. An x-ray apparatus that can be produced as simply as possible, with a particularly effective and durable cooling device, should be achieved. A further object is in the specification of a suitable coolant.
The above object is achieved in accordance with the invention by an x-ray apparatus having a cooling device wherein the coolant is water and a pressure generation device is provided to generate a water pressure of at least 1.1 bar acting in at least one region of the anode to be cooled.
The heat capacity of water is higher by approximately 2.5 times than that of insulating oil. A particularly effective heat dissipation thus can be realized by using water as the coolant. By maintaining a water pressure of at least 1.1 bar by means of the pressure generation device in a region of the anode to be cooled, the boiling point of the water is increased and thus an unwanted formation of a vapor film between the region to be cooled and the water is avoided. The “region of the anode to be cooled” means a region in which the maximum temperature is generated during operation. In the case of a rotary anode as well as a rotating envelope tube, this is thereby an annular region. A cooling device operated with water as a cooling fluid normally can be reliably sealed for a long time span. The inventive x-ray apparatus has a long lifespan and is characterized by a particularly efficient cooling.
The water pressure is advantageously more than 5 bar, preferably at least 10 bar. The water pressure is set such that the formation of an unwanted vapor film in the region to be cooled is safely and reliably avoided. It has proven to be particularly appropriate to generate a water pressure of at least 10 bar, at least in the region of the anode to be cooled. The boiling point of the water in this case is approximately 180° C.
Additives can be added to the water used as a cooling fluid. These can be additives that lower the freezing point and/or counteract a corrosion of the cooling device through which the cooling fluid flows.
In an embodiment of the invention, the cooling device is connected in a fixed manner with the anode. In this case channels or hollow spaces (voids) of the cooling device surround at least the region of the anode to be cooled. It is thereby achieved that the coolant is rotated with essentially the same angular velocity as the region of the anode to be cooled. In this case the pressure generation device can be a device for rotation of the anode. The coolant is rotated essentially simultaneously with the device for rotation of the anode and with the same angular velocity. In this case centrifugal forces that generate the desired water pressure in the region of the anode to be cooled act on the coolant. In addition, the water pressure is dependent on the radius, i.e. on the distance of the region to be cooled from the rotation axis of the anode. The following relation applies:
½ρω2R2=P,
wherein
ρ is the density of the coolant,
ω is the rotation frequency, and
P is the fluid pressure.
In this particularly simple embodiment it is not necessary to provide separate further devices for generation of a water pressure. The water pressure can be generated solely by the rotation of the cooling device connected in a fixed manner with the anode and the centrifugal forces that act upon the coolant.
In a further embodiment of the invention, the cooling device has a housing surrounding the anode. This enables a compact design of the x-ray apparatus. The housing can itself be cooled such that a heat radiated from the anode can be dissipated by the housing.
According to a further embodiment, the water flows through an intermediate space formed between the housing and the anode. In this case the housing forms, for example, the outer casing of a rotating envelope tube. The housing is advantageously connected in a fixed manner with an internal, vacuum-sealed tube housing embodying the anode. In this case the cooling fluid is rotated with the same speed as the x-ray housing. A high actuation power required to overcome the friction between the water and the tube housing in the case of high rotation speeds thus can be avoided.
According to a further embodiment of the invention, the anode can be a rotary anode. A hollow space for passage of the coolant can be provided inside the rotary anode. The hollow space can merely extend across the anode shaft, or the hollow space can extend across the anode shaft and the inside of the anode plate. This enables a particularly efficient cooling of the rotary anode.
According to a further embodiment, at least one flow conductor element is provided in the intermediate space or in the hollow space. This enables the feed of cold cooling fluid to a region of the anode to be cooled on a short path. An unwanted pre-heating of the coolant before reaching the region to be cooled thus is avoided.
The pressure generation device can has a pump. The pump can be designed such that it is driven by a drive for rotating the rotating envelope tube or the rotary anode. In this case it is ensured that the pump is always also in operation given a rotation of the anode.
According to a further embodiment, the pressure generation device has a flow deflector (baffle) arranged in the intermediate space or hollow space. The baffle can rotate or can be stationary relative to the rotation of the anode. The required water pressure can be generated in a simple manner by the relative movement between the rotating cooling fluid and the baffle. This embodiment of the pressure generation device is particularly simple and insensitive to disruptions. The baffle can be, for example, a disc provided with flow conductor loops or plates that is arranged between the tube housing and the housing. In the case of a rotatable bearing of such a disc it is possible to set a desired water pressure by the adjustment of the rotation speed of the disc relative to the rotation speed of the x-ray housing.
The above object also is achieved according to the invention, by a method including the steps of using water as a coolant in an x-ray apparatus and maintaining the water at a pressure of at least 1.1 bar in a cooling device for cooling of the anode of the x-ray apparatus is provided. The use of water as a cooling fluid enables a particularly effective cooling of the x-ray apparatus. Apart from this, seals for a cooling device operated with water as a coolant remain tight significantly longer compared to a cooling device operated with insulating oil. The inventive method enables the production of x-ray apparatuses that are cooled more effectively and are less prone to repair.
Water at a pressure of more than 5 bar (preferably at least 10 bar) is advantageously used. An increase of the boiling point is thereby achieved that enables the use of water as a coolant in conventional rotating envelope tubes or x-ray tubes with rotary anode.
As can be seen from
The function of the rotating envelope tube is as follows:
Water as a coolant is supplied via the coolant inlet 9. The patient table (i.e. in particular the housing 6, the intermediate space 7 and the x-ray housing 1) is rotated around the rotation axis A by means of the actuation device (not shown here). Cooling fluid located in the intermediate space 7 is placed under pressure, in particular in the radially outlying regions of the anode 3, as a result of centrifugal forces. The pressure is dependent on the selected angular velocity as well as the radius. It increases with increasing radius. As a consequence the water pressure is particularly high in the radially outlying regions of the anode 3 to be cooled. The water at a specific water pressure flows through the intermediate space 7, absorbs the heat generated by the anode 3 and transports it away through the coolant outlet 10. The heat can be removed from the water via a conventional heat exchanger and the cooled water can be subsequently supplied again to the coolant inlet 9. A conventional pump or an accumulator can be used to generate the pressure.
A pump, an accumulator or a disc (that can rotate relative to the housing 6) are provided for generation of the water pressure in the intermediate space 7 extending between the anode plate 2 and the housing 6. The disc can be mounted on a shaft that extends through the coolant inlet 9 into the intermediate space 7. By means of a separate drive device the disc can be moved relative to the housing 6 with a rotation speed and/or rotation direction deviating from that of the housing 6. Flow conductor structures can be provided on the top and underside of such a disc. The flow conductor structures, given a relative movement of the disc with respect to the housing 6, effect an acceleration of the water (supplied through the coolant inlet 9 into the intermediate space 7) at the radially outlying region of the anode plate 2 to be cooled. This consequently leads to the desired formation of a water pressure of at least 10 bar in the radially outlying region of the anode plate 2.
It is also possible to produce a water pressure of at least 10 bar in the radially outlying region of the anode plate 2 by a correspondingly high rotation speed of the rotating envelope tube. In this case the water pressure is effected via the centrifugal forces acting on the supplied water and via a tapering or narrowing of the intermediate space 3 provided downstream of the radially outlying regions of the anode plate 2.
The function of the x-ray tube with rotary anode is as follows:
Here as well water as coolant is supplied through the coolant inlet 9 at a pressure of at least 10 bar. The disc 16 acts as a flow conductor means and conducts the supplied cool water to the radially outlying region of the rotary anode plate 13. There the water absorbs the heat generated there and discharges it again through the annular gap (formed between the sleeve shaft 17 and the anode plate 14) via the coolant outlet 10.
As has already been mentioned in the preceding, in this case as well the pressure can be developed by an accumulator or a correspondingly-fashioned pump. Here as well it is also possible to provide the disc 16 with corresponding flow conductor structures, such that the water supplied through the coolant inlet 9 is accelerated (via a relative movement between the disc 16 and the rotary anode plate 13) in the direction toward the radially outlying region of the rotary anode plate 13. As a result, a water pressure of at least 10 bar develops, in particular in the region of the rotary anode plate 13 to be cooled, so the unwanted formation of a vapor film between the regions to be cooled and the water is safely and reliably avoided. At correspondingly high rotation speeds it is also possible to generate the pressure solely by the centrifugal forces acting on the water in the radially outlying region of the rotary anode plate 13 to be cooled. In this case it is appropriate to adapt an effective flow-through cross-section of the hollow space 15 downstream of the region to be cooled, such that the desired water pressure develops.
Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2005 049 270.3 | Oct 2005 | DE | national |
