Production of X-rays

Question 1

What are the main components of an x-ray tube? (5)

Answer 1

Tubehead
Collimator
Positioning arms
Control Panel
Circuitry

Question 2

Describe the 9 components of the tubehead.

Answer 2

1. Glass x-ray tube containing - filament, copper block and target
2. Step-up transformer - high kV across x-ray tube
3. Step-down transformer - low filament mA
4. Glass may be leaded with lead-free window
5. Lead shield to minimise radiation leakage
6. Oil to facilitate heat removal
7. Aluminium filter removes low energy, (soft) damaging x-ray photons
8. Collimator to restrict the size and shape of the beam
9. Beam-indicating device (BID) or spacer “cone” - may be part of the collimator

Question 3

To produce x-rays, a small electric current is passed through a filament, what is the current?

Answer 3

8-10 mA

Question 4

What does the small current used to produce x-rays need to be passed through?

Answer 4

A step-down transformer

Question 5

What happens to the filament during the production of x-rays?

Answer 5

The filament is heated by current due to the excitation of electrons within the wire.

Question 6

During the production of x-rays, the electrons from the outer shells are lost, what happens to these ‘free’ electrons?

Answer 6

They surround the filament in an electron cloud.

Question 7

During the production of x-rays, what happens between the cathode and anode?

Answer 7

A large potential difference (SI) is applied.

Question 8

What does the step-up transformer do?

Answer 8

It changes 240 volts to 50-90 kV

Question 9

What does the focusing cup do?

Answer 9

Focusing cup points electrons towards target/focal spot of anode.

Question 10

What happens to electrons as the current is flowing through the filament?

Answer 10

The -vely charged electrons are attracted to the +vely charged anode, they are pulled across by the high voltage kV between the cathode and anode, electrons have high energy and are fast moving.

Question 11

How do electrons then cause the production of x-rays?

Answer 11

Electrons hit the target and come to a sudden stop, losing energy, the energy is converted to 99% heat and 1% X-rays.

Question 12

What happens to the heat produced when electrons hit the target?

Answer 12

The heat is absorbed by the copper and dissipated into the surrounding oil.

Question 13

What happens to the x-rays produced by the electrons hitting the target?

Answer 13

The x-rays radiate in all directions, most are absorbed by the leaded glass and the target directs x-rays towards the unleaded window so a beam of radiation is emitted.

Question 14

What are the two types of x-ray spectra?

Answer 14

1. The continuous spectrum and 2. The Characteristic spectrum.

Question 15

Describe the continuous spectrum.

Answer 15

Bremsstrahlung or breaking radiation, has a wide range of photon energies.

Question 16

Describe the characteristic spectrum.

Answer 16

Depends on the material used in the anode, emitted by loss of electrons from the K and L shells.

Question 17

What are the properties of x-rays? (11)

Answer 17

1. Travel in straight lines in free space
2. X-Ray photons form a divergent beam
3. Can travel through a vacuum
4. Can penetrate matter
5. Can be absorbed
6. Can be scattered
7. Not detectable by human senses
8. Produce a latent image on film emulsion
9. Cause ionisation
10. Can cause biological damage
11. Cause certain salts to fluoresce and emit light

Question 18

Definition of Scattering

Answer 18

Random change in direction after hitting something.

Question 19

Definition of Absorbtion

Answer 19

Deposition of energy in tissues.

Question 20

Definition of Intensity

Answer 20

Number of x-ray photons in a defined area of the beam.

Question 21

Definition of Attenuation

Answer 21

Reduction in intesity of beam due to scattering and absorption.

Question 22

Definition of Ionisation

Answer 22

Removal of electron from neutral atom to give -ve (electrons) and +ive (atom) ions.

Question 23

Definition of Penetration

Answer 23

The ability of photons to pass through or into tissues/materials.

Question 24

What are the four possible interactions of the x-ray beam?

Answer 24

Completely Scattered = with no loss of energy
Absorbed = with total loss of energy
Scattered = with some absorption and loss of energy
Transmitted unchanged

Question 25

What are the four possible interactions of x-rays at atomic level?

Answer 25

Rayleigh (unmodified) scattering = pure scatter
Photoelectric effect = pure absorption
Compton effect = absorption and scatter
Pair production = pure absorption

Question 26

Explain the Photoelectric effect.

Answer 26

Incoming x-ray interacts with the K electron, the electron is ejected with much energy and the PHOTOELECTRON undergoes further interactions.
The x-ray photon has lost all energy and disappears.
The inner shell vacancy is filled by the outer shell, excess energy is emitted as light or low energy x-ray.
Free electron is captured and there is then atomic stability and a neutral state.

Question 27

Explain the Compton effect.

Answer 27

An incoming x-ray photon interacts with loosely bound, outer shell electron which is then ejected with some energy from x-ray.
The compton recoil electron can cause ionisation.
The remaining photon energy is scattered, may undergo further Compton reactions or Photoelectric reactions. They escape tissues into the clinical environment.