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Radiology Physics > Ch. 5/Ch. 7 X-Ray Tube > Flashcards

Ch. 5/Ch. 7 X-Ray Tube Flashcards

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1
Q

What are the four conditions necessary for the production of X-ray photons?

A
  • Need a source of electrons (filament)
  • Appropriate target material (anode disk made of Tungsten)
  • High voltage - generate power
  • Vacuum tube - X-ray tube
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2
Q

What are the two main sides of an X-ray tube?

A
  • Cathode side
  • Anode side
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3
Q

What is the purpose of the glass or metal envelope in an X-ray tube?

A

To encase both electrodes and maintain a vacuum

The envelope protects the internal components and ensures the proper functioning of the X-ray tube.

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4
Q

What is thermionic emission in the context of X-ray production?

A

The release of electrons from the filament due to heat

This process is initiated by heating the filament, allowing electrons to escape and form a thermionic cloud.

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5
Q

What happens when high voltage is applied to the thermionic cloud?

A

Electrons are accelerated towards the anode target

The kinetic energy of these electrons results in the production of X-rays and heat upon colliding with the anode.

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6
Q

What is the primary function of the cathode assembly in an X-ray tube?

A

To produce the thermionic cloud and focus the stream of electrons to the anode

The cathode assembly includes the filament, focusing cup, and associated wiring.

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7
Q

Fill in the blank: The anode disk in an X-ray tube is typically made of _______.

A

Tungsten

Tungsten is used due to its high atomic number and melting point, making it ideal for X-ray production.

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8
Q

True or False: The X-ray tube is sometimes referred to as a diode.

A

True

The term ‘diode’ refers to the presence of two electrodes in the tube system.

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9
Q

What materials are typically used for the housing of an X-ray tube?

A

Metal protective housing, lead lined

The lead lining is essential for protecting against leaking radiation.

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10
Q

What are the parts of the cathode assembly in an X-ray tube?

A
  • Filament
  • Focusing Cup
  • Associated wiring

Each part plays a role in the generation and focusing of electrons towards the anode.

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11
Q

What is a filament in radiography?

A

A small coil of wire made of Thoriated Tungsten, typically 0.1-0.2 mm thick, 1-2 mm wide, and 7-15 mm long.

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12
Q

What is the purpose of using small filaments?

A

To achieve more image detail for extremities using low mA (25-200 mA).

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13
Q

What is the purpose of using large filaments?

A

To achieve less image detail for the axial skeleton using higher mA (225-1400 mA).

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14
Q

What is the function of a filament?

A

It is coiled to create enough resistance in the electrical current to cause a ‘back up’ and then a release of electrons, known as ‘Thermionic Emission’.

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15
Q

Why is tungsten used for filaments?

A

Tungsten is chosen for its high melting point and difficulty to vaporize.

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16
Q

What problems can arise from vaporized tungsten?

A
  1. Not all electrons reach the anode; some float around or turn to gas.
  2. When tungsten cools and hardens, it deposits on the glass envelope, causing the ‘Minoring effect’.
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17
Q

What is arcing in relation to filaments?

A

Arcing occurs when current jumps to tungsten on the window, making a snapping noise and potentially puncturing the tube.

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18
Q

What is the melting point of tungsten?

A

Tungsten’s melting point is 3310°C.

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19
Q

What is thoriated tungsten?

A

It is tungsten with 1-2% thorium added to increase the efficiency of thermionic emission and extend the life of the tube.

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20
Q

What are other metal options for filaments?

A

Rhenium (melting point 3170°C) and Molybdenum (melting point 2620°C).

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21
Q

What happens when 10% of the filament’s diameter has vaporized?

A

There is a risk for breakage due to the filament becoming very brittle and thin.

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22
Q

What causes filament breakage?

A
  1. Too cold of a filament.
  2. Excessive vaporization leading to brittleness.
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23
Q

What is the effect of added unwanted filtration?

A

It absorbs some of the primary beam protons at the window, reducing the intensity of the primary beam.

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24
Q

What causes tube failure in radiography?

A

Issues such as arcing, added unwanted filtration, and filament breakage can all lead to tube failure.

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25
Q

What should be done with the exposure button during an X-ray procedure?

A

It should be depressed in one motion, except for pediatric patients or difficult patients.

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26
Q

What is the role of the stator in an X-ray tube?

A

It activates the rotor to start spinning the anode.

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27
Q

What happens after the thermionic cloud is formed?

A

The high voltage circuit is activated to send the thermionic cloud over to the anode, producing X-rays.

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28
Q

What is the average tube use for an X-ray machine?

A

10,000-20,000 exposures or 8-9 hours of ‘beam on’ time.

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29
Q

What is the Focusing Cup made of?

A

Nickel.

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30
Q

What is the main function of the Focusing Cup?

A

To narrow the thermionic cloud as it travels over to the anode.

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31
Q

What is the thermionic cloud also referred to as?

A

Space charge.

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32
Q

What does the Space Charge Effect describe?

A

The phenomenon where more electrons build up in the area of the filament, opposing the emission of additional electrons.

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33
Q

What is the purpose of the anode assembly in an X-ray tube?

A

It serves as a target area for incident electrons from the cathode filament.

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34
Q

What are the three functions of the anode assembly?

A
  • Serves as a target area for incident electrons
  • Becomes the source of X-ray photons
  • Serves as a primary thermal conductor.
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35
Q

What percentage of the kinetic energy of the incident electrons is converted into heat?

A

99%

Only 1% of the energy is converted into x-ray photons.

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36
Q

What are the two types of anodes used in X-ray equipment?

A
  • Stationary
  • Rotating
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37
Q

What is the primary material used in the construction of a stationary anode?

A

Rhenium-alloyed tungsten

It is embedded into a 45° angle end of a copper rod and does not turn during the exposure, making it only good for low volume x-ray equipment

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38
Q

What is the size range of rotating anodes?

A

5-13 cm in diameter

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39
Q

What material is the whole disk of a rotating anode made from?

A

Molybdenum

40
Q

Why is the focal track/ target track (area) made of rhenium-alloyed tungsten?

A

To give strength and flexibility

41
Q

What additional material can be used to support the rotating anode?

A

Graphite

42
Q

Why is tungsten the material of choice for anodes?

A
  • High atomic number (74)
  • High melting point (1000-2000°C)
  • Good heat conductor
43
Q

What is the target area on the anode where the electron thermionic cloud could impact called?

A

Focal track

44
Q

What are other terms that refer to the precise point where the electron impacts on the focal track?

A
  • Target
  • Focus
  • Focal Point
  • Focal Spot
45
Q

Where are x-ray photons created?

A

At the focal spot

46
Q

What is measured from the focal spot?

A

SID (Source to Image Distance)

47
Q

What is the approximate distance from the anode to the bottom of the collimator?

A

About 12 inches

48
Q

What is the effect of frequent ‘overloading’ of the tube?

A

Pitting caused by high exposure technique

‘Overloading’ refers to excessive radiation exposure settings which can damage the anode.

49
Q

What can cause a cracked anode?

A

Not warming up the tube prior to use

Warming up the tube is essential to prevent thermal shock/ cracking to the anode.

50
Q

What is the average KVP for the standard warm-up procedure?

A

65-75

51
Q

What is the average mA used in the standard warm-up procedure?

A

200-300

52
Q

What is the duration of the exposure during the standard warm-up procedure?

A

One 1-second exposure
Two 2-second exposures

53
Q

What is the purpose of the rotor in an x-ray tube?

A

To rotate the anode for heat dissipation

54
Q

What is the composition of the rotor in an x-ray tube?

A

Hollow copper cylinder attached to the anode with a molybdenum shaft

The materials are chosen for their thermal conductivity and strength.

55
Q

What are the common RPM ranges for high-speed rotors?

A

common rotors are 3200-3600 RPM and high speed rotor are 10,000-12,000 RPM

Higher RPMs improve heat dissipation and efficiency of the x-ray tube.

56
Q

What happens at the end of the exposure in relation to the rotor?

A

A reverse current runs through the rotor/stator to quickly slow it down

This process helps prevent damage to the rotor by allowing it to decelerate safely.

57
Q

What is the function of the stator in an x-ray tube?

A

To turn the rotor using an induction motor

The stator is located outside the vacuum tube and is essential for initiating rotor movement.

58
Q

What causes the rotor to not turn if the stator stops working?

A

Bad induction motor

If the stator fails, it can lead to overheating of the anode and potential tube failure.

59
Q

What noise indicates bad ball bearings in the rotor?

A

Grinding noise during exposure

Flattened balls in the bearings can cause uneven rotation and overheating.

60
Q

Fill in the blank: The stator is the only part located ______ of the vacuum tube.

A

outside

Its external placement allows for effective operation without compromising the vacuum integrity.

61
Q

True or False: The rotor rotates at the same rate as the anode disk.

A

True

The rotor is mechanically connected to the anode disk, ensuring synchronized rotation.

62
Q

What is the function of the envelope for the diode?

A

Encapsulates the diode (cathode and anode)

Made of heat-resistant glass (like Pyrex) or metal with a glass window

63
Q

What is the purpose of the window area of the tube?

A

Made of thinner glass to absorb less X-ray photons

Allows for the exit of primary beam while filtering out unnecessary photons

64
Q

What is the significance of the vacuum in the envelope?

A

Removes all air molecules to allow electrons to travel from cathode to anode

Achieved through a vacuum pump

65
Q

What is the function of the protective housing?

A

Helps to control leakage radiation

Leakage radiation is considered secondary radiation, not scatter radiation

66
Q

What is leakage radiation?

A

Radiation exiting the protective housing

The protective housing is lined with lead to minimize leakage

67
Q

What is the maximum allowable leakage radiation?

A

1 mGya per 1 hour at 1 meter (3.3 ft)
(1 mGy subscript a)
## Footnote

Leakage tested using an ionization chamber

68
Q

What is the purpose of the special dielectric oil?

A

Protects against electrical shock and acts as a thermal cushion

Located between the X-ray tube and protective housing

69
Q

What are the additional functions of the protective housing?

A

Isolates high voltage wires, helps cool the tube, protects from rough handling

Essential for safety and performance of the X-ray system

70
Q

What are tube rating charts used for?

A

Help the tech prevent thermal damage

Each X-ray tube has its own rating chart

71
Q

What information does a tube rating chart provide?

A

Max technique that can be used with the tube

Includes plotting KVp, mA, and time on the chart

72
Q

How many types of charts are there for tube rating and cooling?

A

Three types of charts

tube rating chart, anode cooling chart, housing cooling chart

73
Q

What does the anode cooling chart calculate?

A

The time it takes for the anode to cool down before another exposure can be made.

74
Q

What is the formula to calculate heat units (Hu)?

A

Hu = MA x time x KVp x rectification unit.

75
Q

What is the rectification unit for single-phase equipment?

A

1Ø= 1.0

76
Q

What is the rectification unit for three-phase equipment?

A

3Ø=1.35.

77
Q

What is the rectification unit for high-frequency equipment?

A

1.40.

78
Q

Calculate the heat units generated by an exposure of 80 KVp, 200 mA, and 0.2 seconds on a single-phase rectified unit.

A

3200 HU.

79
Q

What does the housing cooling chart measure?

A

The time it takes the protective housing to cool down before additional exposures can be made.

80
Q

Which cools first, the anode or the housing?

A

The anode.

81
Q

List one recommendation for extending tube life.

A
  • Warm up anode by manufacturer’s recommendations
  • Do not hold rotor switch unnecessarily
  • Use lower mA stations when possible
  • Use a lower speed rotor when possible
  • Do not make repeated exposures
  • Do not rotate tube housing rapidly
  • Do not use tube when you can hear loud bearings.
82
Q

What does Actual Focal Spot (AFS) describe?

A

The physical area on the focal track that the electrons are impacting.

83
Q

What is the Effective Focal Spot (EFS)?

A

The area of the focal spot that is being projected out of the tube towards the object (patient).

84
Q

What does Spatial Resolution (SR) relate to?

A

It relates to the Effective Focal Spot (EFS).

85
Q

Fill in the blank: The formula for calculating heat units is Hu = MA x time x KVp x _______.

A

[rectification unit].

86
Q

What principle is used to reduce the effective focal spot for better spatial resolution?

A

Line Focus Principle

This principle helps manage heat production while maintaining effective focal spot size.

87
Q

How does filament size affect the effective focal spot (EFS)?

A

Small filament = Smaller AFS = Smaller EFS; Large filament = Larger AFS = Larger EFS

AFS refers to the actual focal spot size.

88
Q

What is the relationship between anode angle and effective focal spot?

A

As the anode angle increases, AFS and EFS increase

Most target angles range from 7-11°, with 10° being most common.

89
Q

What is a disadvantage of having a very small anode angle?

A

Very small useful primary beam

This can affect imaging quality.

90
Q

What effect does the line focus principle have on primary beam intensity?

A

Anode Heel Effect

It causes intensity variation from anode side to cathode side.

91
Q

What causes the variation in primary beam intensity in the Anode Heel Effect?

A

Some of the primary beam gets absorbed by the heel of the anode

This leads to more photons on the cathode side and fewer on the anode side.

92
Q

What is the maximum percentage variation of photons from the anode to the cathode side?

A

Up to 45% total variation

This can be significant in imaging.

93
Q

What is the impact of the Anode Heel Effect on imaging the thorax or abdomen?

A

Can be used to advantage based on patient thickness and composition

Helps optimize image quality in these body types.

94
Q

What is Off Focus Radiation?

A

X-rays produced outside the actual focal spot

These occur when electrons from the thermionic cloud stray outside the target area.

95
Q

What percentage of the primary beam can Off Focus Radiation make up?

A

25-30%

These photons are typically low energy, increasing Entrance Skin Dose.

96
Q

What should the tube housing and collimator shutters do regarding Off Focus Radiation?

A

Should absorb a majority of it

This minimizes unnecessary radiation exposure.

Radiology Physics (6 decks)

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