Physics Test 2 part 3

Question 1

Major causes of xray tube failure

Answer 1

1.Filament evaporation
2.Spark over and rupture of the glass envelope
3.Excessive anode temperature
4. Rapid increase in anode temperature
5. Bearing damage

Question 2

Filament evaporation

Answer 2

most common cause of x-ray tube failure.
As current passes through the filament causing thermionic emission, some tungsten atoms are evaporated in the process.

Question 3

Spark over and rupture of the glass envelope

Answer 3

This is caused by the tungsten coating on the inside of the glass envelope acting as a conductor of electricity.

Question 4

Excessive anode temperature

Answer 4

This causes localized melting or pitting of the anode that results in variable and reduced radiation output.

Question 5

Rapid increase in anode temperature

Answer 5

If an exposure generating a large number of heat units (high kVp & mAs) is made when the anode is cold, the rapid increase in anode temperature can cause thermal stress (rapid and uneven expansion). Can cause anode to crack.

Question 6

Bearing damage

Answer 6

The anode in a rotating anode tube is attached to a stator that rotates on a series of bearings (similar to the wheel bearings on a car). They get worn over time.

Question 7

How can you extend tube life?

Answer 7

1. Warm up the anode following manufacturer’s specifications.
2. Do not hold the rotor switch down for prolonged periods of time.
3. Use lower mA stations when possible.
4. Use lower speed on the rotor when possible.
5. Do not make repeated exposures near the tube loading limit.
6. Do not rotate the tube housing rapidly from one position to another.
7. Do not use the x-ray tube when you can hear loud rotor bearings.

Question 8

Space charge effect

Answer 8

This effect can occur when high mA values (above 1000) are employed or when the rotor or prep button is held down for prolonged periods of time before the exposure is made.
If electrons are allowed to remain in the space charge cloud for a prolonged period of time, the mutual repulsion of the electrons will begin to repel some of the electrons back into the filament.

Question 9

Anode Heel Effect

Answer 9

This is an uneven distribution of x-rays from the cathode side to the anode side of the x-ray field.
This results in an increase in radiation toward the cathode end of the field and a decrease toward the anode end.

Question 10

what 3 factors effect the degree of heel effect variation?

Answer 10

1.Anode angle
2. SID
3. IR size

Question 11

Anode angle

Answer 11

the smaller the anode angle, the greater the variation between the cathode and anode side of the x-ray field.

Question 12

SID

Answer 12

the shorter the SID, the greater the variation in x-ray quantity between the cathode and anode sides of the field.

Question 13

IR size

Answer 13

the heel effect variation is more pronounced with larger IR sizes.

Question 14

How can the heel effect be eliminated?

Answer 14

Heel effect variation can be eliminated through the use of a wedge filter.

Question 15

What should be done if the heel effect cannot be removed?

Answer 15

the patient must be positioned properly.
This means that the thicker portion of the body part should be under the cathode end of the field and the thinner portion under the anode side.

Question 16

What are some recent advancements in xray tube design?

Answer 16

1.Some manufacturers are replacing standard molybdenum disks with one made of titanium and zirconium.
This alloy is less prone to thermal stress.
2.Some manufacturers are incorporating angled slits in the anode disks at various intervals to allow for expansion during rapid heating.
3. Biangular or double angle anodes
The disks are manufactured with two separate focal tracks, each at a different angle of tilt.
This allows better selection of focal spot size with better heat dissipation.
4. Metal envelopes instead of glass.

Question 17

What are advantages of metal envelopes?

Answer 17

metal envelopes will not be bothered by tungsten deposits caused by filament evaporation.
b) no danger of cracking glass
c) more substantial support of structures inside of the tube which will allow the use of more massive anode disks (2000 grams) versus the conventional disks in current glass tubes (700 grams).