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Tube life

1. Jericho neil c.baldove BSRT-2A FACTORS AFFECTING TUBE LIFE TUBE RATING CHARTS

2. FACTORS AFFECTING TUBE LIFE The enclosure maintains a vacuum inside the tube. This vacuum allows for more efficient x-ray production and a longer tube life. When just a little gas is in the enclosure , the electron flow from cathode to anode is reduced , fewer x-rays are produced, and more heat is generated .

3. Filaments are usually made of thoriated tungsten. Tungsten provides for higher thermionic emission than other metals. Its melting point is 3410   ° C; therefore it is not likely to burn out like the filament of a light bulb. Also, tungsten does not vaporize easily. If it did, the tube would become gassy quickly, and its internal parts would be coated with tungsten. The addition of 1% to 2% thorium to the tungsten filament enhances the efficiency of thermionic emission and prolongs tube life.

4. X-RAY TUBE FAILURE With careful use, 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 radiologic technologists. 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. The use of faster image receptors results in longer tube life. X-ray tube failure has several causes, most of which are related to the 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.

5. When the radiologic technologist pushes the exposure button of a radiographic imaging system, there is a short delay before an exposure is made. This allows the rotor to accelerate to its designated rpm while the filament is heated. Only then is the kVp applied to the x-ray tube. During this time, filament current is increased to provide the correct x-ray tube current. When a two- position exposure switch is used, the switch should be pushed to its final position in one motion. This minimize the time that the filament is heated and prolongs tube life.

6. Excessive heat results in reduces x-ray tube life When the temperature of the anode is excessive during a single exposure, localized surface melting and pitting of the anode can occur. These surface irregularities result in variable and reduced radiation output. If surface melting is sufficiently severe, the tungsten can be vaporized and can plate the inside of the glass enclosure. This can cause filtering of the x-ray beam and interference with electron flow from the cathode to the anode.

7. TUBE RATING CHARTS Radiologic technologists are guided in the use of x-ray tubes by x-ray tube rating charts. It is essential that technologists be able to read and understand these charts even though many of these charts are now digitally stored in the operating console. Three types of x-ray tube rating charts are particularly important: the radiographic rating chart, the anode cooling chart, and the housing cooling chart.

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