X-Ray Production

Question 1

How are electrons released at the cathode?

Answer 1

A current is passed through a filament at the cathode, and the resistance generates heat. This heat excites and ejects electrons via thermionic emission.

Question 2

What is the filament at the cathode made from in an X-ray tube?

Answer 2

Usually tungsten

Question 3

What is the anode in an X-ray tube made from?

Answer 3

Usually tungsten, due to its high atomic number (Z), therefore high number of electrons

Molybdenum makes up the body due to its high specific heat capacity (aids in heat dissipation)

Question 4

Why does the anode spin?

Answer 4

It aids in heat dissipation, since a lot of heat is generated in x-ray production. Heat is applied across a larger area, leading to a smaller temperature rises.

It also causes the target area to vary, which prevents wear and increases its lifespan.

Question 5

What is the role of the oil in the x-ray tube shield/housing?

Answer 5

It acts as a coolant, which helps dissipate heat produced in x-ray production.

It also acts as an electrical insulator between the live components in the insert and the housing. Since the resistance of a material decreases as heat increases, the cooling effect of the oil helps it maintain its role as an insulator.

Question 6

Why is tungsten used for the anode?

Answer 6

• Tungsten has a high melting point, so it can withstand the heat generated in x-ray production
• Tungsten has a high atomic number (Z=74), so is an efficient producer of x-rays

Question 7

What are the 2 prominent methods of x-ray production?

Answer 7

Bremsstrahlung radiation and characteristic radiation (Brem. being most prominent)

Question 8

Why is there a vacuum inside the x-ray tube?

Answer 8

To prevent interference when the electrons are accelerating across the potential difference between the cathode (-) and anode (+)

Question 9

Which parameter controls the number of electrons produced via thermionic emission?

Answer 9

mAs (controls the current passing through the tungsten filament, with mAs referring to the tube current)

Question 10

Which parameter controls the energy of the x-ray photons produced?

Answer 10

kVp = kilovoltage peak (the peak potential difference across the electrodes, greater difference = greater acceleration)

Question 11

What is the sheld/housing of an x-ray tube made from, and why?

Answer 11

It is often made from glass since it is a good electrical insulator, which prevents shocks from the live electrode insert within. It is also round to prevent the build-up of static on any corners.

Question 12

What is the ‘focal spot’?

Answer 12

Focal spot is the area of the anode surface which receives the beam of electrons from the cathode. It is the apparent source of x-rays.

Question 13

What feature of the cathode aids in emitting a uniform beam of electrons?

Answer 13

The focussing cup

Question 14

What proportions of the electron beam are converted to heat and x-ray photons?

Answer 14

Around 99.5% heat 0.5-1% x-ray photons

Question 15

What is the ‘actual/real focal spot’?

Answer 15

The width of the initial electron beam interacting with the anode

Question 16

What is ‘apparent/effective focal spot’?

Answer 16

The width of the x-ray photon beam emitted from the anode

Question 17

What is the ‘line focus principle’?

Answer 17

The line focus principle is used to decrease the size of the effective/apparent focal spot, increasing image sharpness. This is done by decreasing the angle of the anode heel.

Question 18

What is the ‘anode heel effect’?

Answer 18

This is where photons produced on the anode-side of the beam are more readily attenuated by the heel of the anode. This causes the x-ray beam to be lacking on the anode’s side.

Question 19

When is the anode heel effect most apparent?

Answer 19

• Wider collimations (collimator diaphragms usually absorb the beams furthest from the central beam, therefore the ‘outlier’ photons are absorbed on both anode and cathode side, producing a beam of uniform intensity)
• Low kVp (producing lower energy photons which are more readily absorbed by the anode’s heel)

Question 20

What are penumbra, and how do they affect image sharpness?

Answer 20

Penumbra are formed from the overlap of divergent beams from the source focal point. The area they overlap is named the penumbra, with a larger penumbra causing greater image unsharpness.

Question 21

How can image sharpness be increased?

Answer 21

• Decreasing the object-film distance (OFD), which reduces penumbra size at the film (geometric unsharpness)
• Decreasing apparent/effective focal spot, which decreases initial angles and therefore smaller chance of divergent beams overlapping (geometric unsharpness). Achieved through lowering anode-heel angle.
• Ensure the patient is still (holding their breath, comfortable) during exposure to reduce movement unsharpness
• Increase focus to film distance (FFD)

Question 22

What effects does increasing focus to film distance (FFD) have on the resultant image?

Answer 22

• Decreases magnification of the object
• Decreases geometric unsharpness (fewer overlapping beams at the object and smaller penumbra)
• Decreases image contrast due to beam intensity decreasing (Inverse square law)

Inverse square also reduces the beam’s intensity, which reduces patient dose

Question 23

State 2 ways of decreasing image magnification

Answer 23

• Decreasing object to film distance (OFD)

- Increasing focus to film distance (FFD)

Question 24

How are x-ray photons produced in characteristic radiation?

Answer 24

• An incident electron interacts with an orbital electron
• It transfers its energy, if it’s more than the orbital electron’s binding energy then it is ejected
• A cascade event occurs, whereby an electron from the orbital above ‘falls down’ to replace the lost electron
• The difference between the 2 binding energies is released as an x-ray photon

NB this can happen many times as more electrons replace orbital electrons

Question 25

How are x-ray photons produced in Bremsstrahlung radiation?

Answer 25

• Incident electron travels past the nucleus of a tungsten atom in the anode
• Electrostatic interactions between the electron and nucleus cause the electron to decelerate
• The kinetic energy lost from the incident electron is released as an x-ray photon

NB this is the dominant interaction in x-ray photon production

Question 26

What factors affect an orbital electron’s binding energy?

Answer 26

• Its distance from the nucleus (farther away = weaker electrostatic attraction and lower binding energy)
• The atomic number (Z) of the atom. Higher Z = more protons and greater electrostatic attraction between electrons and nucleus = increased binding energy

Question 27

Why do electrons from the cathode need to focussed?

Answer 27

Due to electrons within the beam being repelled, creating a larger actual focal point. This would subsequently increase the apparent/effective focal point and introduce geometric image unsharpeness.

Question 28

What 2 factors affect the energy (keV) of produced x-ray photons?

Answer 28

• The kinetic energy of the incident electrons from the cathode (controlled by the kVp)
• The atomic number (Z) of the target material at the anode. This affects the binding energies of its orbital electrons, which affects x-ray photons produced via characteristic radiation. The nucleus size affects how much the incident electrons are slowed in Bremsstrahlung radiation, therefore how much kinetic energy is lost as an x-ray photon.

Question 29

What 2 kinds of filament would you find within the focussing cup?

Answer 29

Broad and fine filaments. The broad filament generates a wider actual focal spot, which allows easier heat dissipation at the anode and is used in higher kEv emissions.
The fine filament generates a narrow focus and is used when trying in cases of low geometric unsharpness is desirable.