One type of x-ray tube is a computerized tomography (CT) x-ray tube which is used in CT scanners.
FIG. 1 shows one type of CT scanner which is described in U.S. Pat. No. 5,086,449. The CT scanner includes a stationary patient receiving region 10. A gantry 12 is mounted for rotation around the patient receiving region 10. An x-ray tube assembly 14 which produces a radiation beam through an x-ray port across the patient receiving region 10 is mounted to gantry 12 for purposes of rotation. Coolant fluid is circulated between x-ray tube assembly 14 and a cooling system 17 (including heat exchanger and pump) which is also mounted on the gantry 12. The coolant fluid flows through x-ray tube assembly 14 to remove heat created during x-ray generation. Finally, an arc or ring of radiation detectors 28 surround the patient receiving region.
During operation, typically, x-ray tube assembly 14 generates a planar beam of radiation which is then rotated around the body. Various detectors 28, located around the patient, detect the intensity of the beam. Detectors 28 are connected to a computer which, based on intensity readings, generates an image of a slice of the body. The patient is then moved longitudinally through the gantry with the x-ray tube assembly 14 generating slices so that the computer can generate a three-dimensional image of the body.
In the course of generating slices, much heat is generated by x-ray tube assembly 14 and this heat must be removed if the service life of the x-ray tube is not to be unduly reduced. As described above, it is known to cool x-ray tubes by circulating a fluid, typically oil, within the tube and externally through a cooling system to remove as much heat as possible. In addition to being used as vehicle for cooling, the fluid is also used for its dielectric properties in order to insulate the anode connection from ground (and/or the cathode connection).
Even employing this type of fluid for purposes of cooling and electrical insulation, x-ray tubes have a finite service life. There are several causes of x-ray tube failure, most of which are related to thermal characteristics of the x-ray tube. Hence, heat removal is an important concern in attempting to extend the service life of an x-ray tube.
A first type of tube failure is related to excessive anode temperature during a single exposure which may result in localized surface melting and pitting of the anode.
A second type of tube failure results from maintaining the anode at elevated temperatures for prolonged periods. If the thermal stress on an x-ray tube anode is maintained for prolonged periods, such as during fluoroscopy, the thermal capacity of the total anode system and of the x-ray tube housing is the limitation to operation.
During fluoroscopy, the rate of heat dissipation from the rotating target attains equilibrium with the rate of heat input. Although this rate is rarely sufficient to cause surface defects in the target, the tube can fail because of the continuous heat delivered to the coolant fluid, the rotor assembly, and/or the x-ray tube housing.
Coolant fluid, due to continuous heat and repeated arcing, will eventually break down. When the oil breaks down its dielectric properties as well as its ability to carry away heat (i.e. viscosity) are adversely affected. This results in less electrical insulation between the anode connection and ground connections (and/or the cathode connection) which leads to more arcing and, eventually, tube failure. Hence, proper electrical insulation (i.e., maintaining the proper dielectric property of the coolant fluid) is also an important concern in attempting to extend the service life of an x-ray tube.
A third type of failure involves the filament. Because of the high temperature of the filament, tungsten atoms are slowly vaporized and plate the inside of the glass envelope, even with normal use. This tungsten, along with that vaporized from the anode, disturbs the electrical balance of the x-ray tube, causing abrupt, intermittent changes in tube current, which often leads to arcing and tube failure.
Due to the above-described potential problems in current x-ray tube designs, manufacturers of CT x-ray tubes, which generally cost approximately $25-40,000, typically include a warranty for 40,000 slices, where a slice is a single picture taken by the CT scanner.
In a typical radiology center, one CT scanner running full time uses any where from 1-4 x-ray tubes a year which becomes very expensive. Obviously, it would be very advantageous, in terms of time and money, for a radiology center or the like to be able to extend the service life of an x-ray tube.