Xray Tube Failures

Saturday, February 21, 2015

With carefull use from x-ray technologist or radiologic technologist, x-ray tubes can provide many years of service. With inconsiderate use, x-ray tube life may be shortened substantially. The length of x-ray tube life is primarily under the control of technologist. Basically, x-ray tube life is extended by using the minimum radiographic factors of mA, kVp, and exposure time that are appropriate for each examination. Fast image receptor is also an advantage of making x-ray tube life longer.

Factors causes x-ray tube failure:


Thermal Characteristics of the x-ray tube

 

Enormous heat is generated in the  anode of the x-ray tube during x-ray exposure, this heat must be dissipated for the x-ray tube to continue to function.


x-ray tube failure

Bearing Damage

 

During an exposure lasting 1 to 3 seconds, the temperature of the anode may be sufficient to cause it to glow like an incandescent bulb. But this heat is dissipated by radiation. Between exposure, heat is dissipated, primarily through conduction but some heat during conduction is conducted through narrow molybdenum neck to the rotor bearings. And excessive heat of bearings will results in increased rotational friction and may cause an imbalance to the rotor anode assembly. Bearing damage is another cause of tube failure.
One example of x-ray tube thermal stress is during fluoroscopy, during fluoroscopy, the x-ray tube current is usually less than 5 mA, unlike in radiography a hundreds of mA is used. Under such fluoroscopic situations, the rate of heat from the rotating target attains equilibrium with the rate of heat input, and sometimes this rate is sufficiently to cause a defects in the surface of the target. However, when continuous heat delivered to the rotor assembly, the oil bath, and the x-ray tube housing, not only the bearing will fail, but also the glass enclosure can crack and failure of the tube housing.


Intermittent Change in Tube Current

 

Even in normal use of x-ray tube, the temperature of the filament is high, the tungsten atoms are slowly vaporized, and the plate in the glass or metal enclosure. This will cause an abrupt and intermittent change in tube current, which often leads to arcing and tube failure.

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