LECTURE 91. CARE AND PRECAUTION OF X-RAY TUBE

2. X-RAY TUBE AGING

KAAB

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.

CARE AND PRECAUTION OF X-RAY TUBE

PURPOSE: TO EXTEND X-RAY TUBE LIFE

1. WARM UP THE ANODE FOLLOWING

MANUFACTURER'S RECOMMENDATIONS.

2. MINIMIZE FILAMENT BOOST (“PREP”) TIME.

Current applied for too long will shorten filament

life and will lead to unstable operation.

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.

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.

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.

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.

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.

8. AVOID LONG INTERVALS BETWEEN SPOT-FILMS

Most systems provide for a “holdover” period of

up to approximately 25 seconds between spot-

films, 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.

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.

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.

X-RAY TUBE AGING

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).

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.

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.

4. CARELESS

Careless handling might break the glass

envelope, stress fracture may occur (affect

vacuum condition).

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)

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.

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.

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

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.

NEXT LESSON..

X-ray Circuits, Generators, High Tension Cables