X Ray Production

Question 1

What is the main component of the Tubehead? What does it consist of?

Answer 1

X ray tube

• glass envelope containing vacuum

Cathode -ve
- filament
- focusing cup

Anode +ve
- target
Heat-dissipating block

Question 2

What occurs at the cathode?

Answer 2

Low voltage high current passed through tungsten (high mp, atomic number) filament

Filament becomes incandescent and thermonic emission occurs (cloud of electrons at cathode)

Focusing cup negatively charged to repel electrons released at filament

Question 3

Describe the cathode-anode relationship in an x-ray tube

Answer 3

High potential difference across the cathode and anode so electrons jump from cathode to anode

Electrons have very high kinetic energy when they collide with anode

Question 4

How are high voltages achieved across the X-ray tube andf low voltage across the filament?

Answer 4

Step up transformer for X-ray tube

Step down for filament

Question 5

What is an eV?

Answer 5

Electron volt

1eV = kinetic energy gained by 1 electron moving across a potential difference of 1 volt

X ray tube typically has 70keV

Question 6

What occurs at the anode? What is the focal spot?

Answer 6

Target block of tungsten bombarded by electrons from cathode

Photons (inner electrons) and heat (outer electrons) produced

Target embedded into a heat dissipating block (copper) so risk of overheating is reduced

FS area where electrons collide and x-rays produced (source)

Question 7

What is the penumbra effect?

Answer 7

Blurry radiograph due to the focal spot not being a single spot but rather a small area

Question 8

Why is the target angled?

Answer 8

Angle increases surface area for heat to dissipate

The way electrons hit the slant the apparent surface to be smaller (where electrons reflect off angulation)

Question 9

How are photons stopped from leaving the glass tube?

Answer 9

Leaded glass prevents omnidirectional photons from escaping in unwanted directions

Question 10

What safety fractures does the tubehead have surrounding the x-ray tube?

Answer 10

Leaded shielding to prevent photon release

Oil surrounding the tube to help with heat dissipation

Aluminium filter to remove lower energy non diagnostic x-rays

Question 11

What are ideal focus to skin distances?

Answer 11

<60kV 100mm

> 6okV 200mm

Question 12

What is a collimator?

Answer 12

Lead diaphragm attached to end of spacer cone

Changes x-ray beam to rectangle rather than circle

Reduces patient dose

Question 13

Give a heat producing interaction

Answer 13

Incoming electron comes into close proximity or contact with tungsten outer electron and is decelerated and deflected, releasing heat

Question 14

What are the photon producing reactions?

Answer 14

Continuous radiation

• incoming electron reaches close proximity of tungsten nucleus, is decelerated and deflected producing photons. Closer to nucleus = higher energy (more useful) max energy = 70keV (input)

Characteristic radiation

• incoming electron collides with inner shell electron and displaces it to a more peripheral shell, or removes it completely. Electrons rearrange to fill the lower shell, energy is lost as photons. Max energy = binding energy of electron shell

Question 15

Give the equation for dental x-ray beam spectrum

Answer 15

Continuous radiation + characteristic radiation - filtered photons