LECTURE 9 1. CARE AND PRECAUTION OF X-RAY TUBE 2. X-RAY TUBE AGING KAAB

Transcription

1 LECTURE 9 1. CARE AND PRECAUTION OF X-RAY TUBE 2. X-RAY TUBE AGING KAAB

2 LESSON OBJECTIVES At the end of the lesson, the student should be able to:- Discuss methods to extend x-ray tube life. Explain characteristics of x-ray tube damages with due to aging process.

3 CARE AND PRECAUTION OF X-RAY TUBE PURPOSE: TO EXTEND X-RAY TUBE LIFE

4 1. WARM UP THE ANODE FOLLOWING MANUFACTURER'S RECOMMENDATIONS.

5 2. MINIMIZE FILAMENT BOOST ( PREP ) TIME. Current applied for too long will shorten filament life and will lead to unstable operation.

6 3. USE LOWER TUBE CURRENT (MA) The high filament current required to produce high tube current (ma) will cause evaporation of the tungsten from the filament and then, it will be deposited on to the glass envelope. Whenever possible, use a lower ma station and a longer exposure time to arrive at the desired mas.

7 4. FOLLOW RATING CHARTS AND ANODE HEATING/COOLING CURVES Operation beyond published ratings will result in premature focal track wear or damage. Even moderate pitting of the focal track will result in a fall-off in radiation output, because electrons from the filament which strike on the target material produce radiation that is mostly absorbed in the surrounding target material. More severe melting, results in the liberation of gasses from the target material which causes tube instability. Excessive heat transfer from the target into the rotor body will cause bearing failure or slow rotation which will result in melts on the focal track.

8 5. LIMIT OPERATION TO 80% OF MAXIMUM SINGLE EXPOSURE RATINGS Although higher power levels are both possible and permitted, this reduction will help assure long focal track life. Also, it will minimize the reduction in radiation output associated with a roughened focal track.

9 6. DO NOT EXCEED ANODE THERMAL CAPACITY OR DISSIPATION RATE OF THE TARGET The greatest danger is to heat flow into the bearing structure. In addition, gasses may be emitted from the various metals within the tube if the temperature reached during clinical use is appreciably higher than that used during the outgassing stage of manufacture. If outgassing occurs during clinical use, the tube will become unstable.

10 7. DO NOT MAKE HIGH MA EXPOSURES ON A COLD TARGET Uneven expansion caused by thermal stress from a high power exposure can result in a cracked target. Do not assume that a thermally relieved target design provides absolute protection. Always follow the recommended warm-up procedure. The procedure may need to be repeated between patients, if the idle time is long enough, in addition to being performed at the beginning of the work day.

11 8. AVOID LONG INTERVALS BETWEEN SPOT-FILMS Most systems provide for a holdover period of up to approximately 25 seconds between spotfilms, during which the rotor is kept at high speed before the rotor brake/reboost cycle is allowed to occur. In some systems, the filament current remains at the exposure value during this period, thereby causing evaporation of filament material and resultant tube instability.

12 9. LIMIT ROTOR START/STOP OPERATIONS Rotor start/stop operations especially to/from high speed (150/180 Hz) generate considerable heat in the stator windings, which will lead to stator damage in extreme cases. Generally there should be a minimum of 30 to 40 seconds between starts. Tubes equipped with a heat exchanger will be less sensitive to this potential problem because oil circulation will help prevent hot spots from occurring around the stator windings.

13 10. DO NOT ROTATE THE TUBE HOUSING RAPIDLY FROM ONE POSITION TO ANOTHER. Sudden movement will produce tension or pressure to the x-ray tube. The gyroscopic effect may crack or otherwise damage the rotor.

14 X-RAY TUBE AGING

15 1) DEPOSITION OF FILAMENT TUNGSTEN Deposition of filament tungsten to the internal surface of the glass envelope will cause some electrons strike the glass envelope (depositions of filament tungsten creates new anode target ). Tungsten layer deposited at tube window will add extra filtration, will harden the beam (less useful x-ray will emit from tube window).

16 2. PRODUCES GAS (GASSY TUBE) By the time, glass envelope that struck by electrons produces gas (gassy tube), which cause tube failure due to x-ray tube is not vacuum anymore.

17 3. PUNCTURED TUBE A punctured tube can be diagnosed, as oil sucked into the tube insert, when the tube is turned, can easily be heard.

18 4. CARELESS Careless handling might break the glass envelope, stress fracture may occur (affect vacuum condition).

19 5. DAMAGES TO ANODE 1) Surface crazing (rough) - cause to different expansion and reduce the radiation output. 2) Anode disc may crack - sudden injection of great heat. (not apply warm up)

20 6. DAMAGES TO ROTOR AND BEARINGS Bearing damage cause to slow rotation / not rotate. Due to surface melting, it will release gas (gassy tube) because the heat applied to small area. Recommendations:- 1. minimize time (as necessary only), 2. provide braking to reduce speed after exposure (special unit), 3. interlock circuits prevent or terminate exposures if anode is not rotating.

21 7. DAMAGES TO FILAMENT Filament thin with age or time through evaporisation or mechanical damage. 1) As filament becomes thinner (evaporisation), its resistance decreases - reducing ma. 2) Intermittent (on and off) fault problem on filament circuit Recommendations:- 1. boosting filament the shortest time as possible, 2. avoid use high ma unnecessarily.

22 8. DAMAGES TO TUBE HOUSING Oil escape from tube will cause a reduction of insulation level and it is harmful to operator. Evidences of leakage:- 1. oil outside the housing, 2. a spot of oil on the floor under the tube

23 9. DAMAGES TO STATOR Broken in stator windings (which provide electromagnetic field) will cause no or intermittent rotation. As a result, it will harm to the anode target. Recommendation:- 1. provide safety circuit to terminate exposure.

24 NEXT LESSON.. X-ray Circuits, Generators, High Tension Cables