Production of X-Radiation

Question 1

Properties of X-Rays - 6 points

Answer 1

they are a type of electromagnetic radiation (photons/waves), travel in straight lines at the speed of light in a vacuum, obey inverse square law, cause excitation/ionisation - biological effect - cause stochastic/deterministic effect on body, can penetrate matter - undergoing differential absorption, have a photographic effect

Question 2

X-Rays originate from …

Answer 2

outer space, radioisotopes, X-Ray tubes, linear accelerators

Question 3

1st stage in the production of an X-Ray beam using an X-Ray tube

Answer 3

the filament is heated, producing electrons by thermionic emission, the tungsten anode begins to rotate helping to disperse the heat

Question 4

2nd stage in the production of an X-Ray beam using an X-Ray tube

Answer 4

a potential difference (the kV) is applied across the X-Ray tube, accelerating electrons across to the target, X-Rays are then produced at the target in all directions so they have to be shielded by lead lined casing

Question 5

Light beam diaphragm

Answer 5

controls the dimensions of the X-Ray beam

Question 6

Name the 2 mechanisms of X-Ray production

Answer 6

Following the rapid deceleration of fast moving electrons - ‘Bremsstrahlung’ - producing a continuous spectrum
Following electron transitions between inner shells - producing a line spectrum

Question 7

Describe rapid deceleration

Answer 7

produces a continuous spectrum of radiation ‘Bremsstrahlung’, the fast moving electrons are attracted to the positive charge of the nucleus of the target atoms and therefore the electrons undergo varying amounts of deceleration depending on how close they are to the nucleus, energy lost by the electrons if given off as photons of X-radiation and heat and the electron now has reduced kinetic energy

Question 8

The closer the electron passes to the nucleus …

Answer 8

the greater the deceleration of the interaction and the more energy lost as photons of X-radiation and heat

Question 9

The total effect of millions of electrons interacting with the tungsten atoms in the target is …

Answer 9

the production of a range of X-radiation energies, termed a spectrum

Question 10

Discuss the lower energy photons

Answer 10

they do not have the energy to emerge from the X-Ray tube, preferentially absorbed by the components of the X-Ray tube itself - (a spectrum of the useful beam would start at 20 kV)

Question 11

Describe the transition of electrons between shells

Answer 11

an incoming electron ejects inner (e.g. K) shell electron creating a ‘vacancy’, however to do this we MUST have enough energy to overcome the binding energy of the inner shell electron of a tungsten target atom in order to eject it - (ONLY happens when operating an X-Ray tube at a voltage of 70kV & over when using a TUNGSTEN target), the vacancy created is then filled by an electron from an outer shell, the loss of energy from the outer shell during the transition is given off as an X-Ray photon

Question 12

If there is a kV exposure then the energy of the incoming electron is …

Answer 12

equal - if a kV exposure is used then the incoming electron has energy of up to 70 keV

Question 13

An electron filling the vacancy achieves …

Answer 13

stability

Question 14

Describe electrons further away from the nucleus in terms of potential energy and binding energy

Answer 14

they have more potential energy but less binding energy,

Question 15

The energy that the emitted photon has is …

Answer 15

equal to the difference in binding energies of the 2 electron shells involved

Question 16

What are the binding energies of shells K, L and M in tungsten

Answer 16

K = 70 keV
L = 11 keV
M = 2.5 keV
In tungesten, its such that the difference in binding energies is such that the photon is in the X-Ray region of the spectrum

Question 17

What are would be the energies of the photon transmitted if an electron moved from
M to K,
M to L,
L to K

Answer 17

M to K = 67.5 keV
M to L = 8.5 keV
L to K = 59 keV

Question 18

A spectrum arising from a combination of Bremsstrahlung and electron transitions would look like …

Answer 18

a Bremsstrahlung spectrum beginning at 20 keV with a large increase curving with a slower decrease up to 70 keV, and 2 large short increase in electrons where electron move from L to K at 59 keV and from M to K at 67.5 keV

Question 19

Name the 5 possible factors which could affect the spectrum of a beam of X-Ray photons

Answer 19

Tube Voltage (kV)
Tube Current (mA) - if higher = higher intensity
Source to image/detector distance (SID/SDD) (cm) - if shorter SDD = higher intensity
Filtration (mm)
Target material (Z)

Question 20

Describe filtration

Answer 20

the reduction in intensity of the X-Ray beam as a result of the photon beam having to pass through 1 or more materials before it reaches the patients - used to modify quality of the X-Ray beam or in radiotherapy used to ‘shape’ the beam and to enhance the effectiveness of treatment

Question 21

Due to photon beam filtration X-Ray tubes are required to have …

Answer 21

additional filters inserted at their exit port to comply with current regulations and to reduce the radiation dose to the patient

Question 22

The total filtration present in a diagnostic X-Ray tube is …

Answer 22

that which is inherent and that which is added (Inherent filtration is equivalent to approx. 1mm of Aluminium, in diagnostic radiography, added filtration is typically a further 2mm of Aluminium)

Question 23

Additional filtration causes the X-Ray spectra to …

Answer 23

be reduced

Question 24

Describe the spectrum of a different anode (target) material

Answer 24

A higher atomic number (e.g. Tungsten) has a greater intensity with electron transitional spikes at a higher keV
A lower atomic number (e.g. Molybdenum) has a lower intensity with electron transitional spikes at a lower keV

Question 25

Define beam intensity

Answer 25

the radiation energy passing through a unit area per unit time (has units Joules/metre squared/seconds)

Question 26

Define beam quality

Answer 26

refers to the penetrating ability of the X-Ray beam, the higher the average energy of the beam, the greater its penetration and the higher its quality

Question 27

How is an X-Ray beam’s quality specified

Answer 27

by stating its half-value thickness (HVT) in mm of a stated material, typically Aluminium up to 150 kV and copper above 150 kV

Question 28

Describe the Half-Value Thickness

Answer 28

that thickness of a stated material which will reduce the intensity of the X-Ray beam by 50%

Question 29

The HVT increases as …

Answer 29

the kV used to produce the beam increases

as the average photon energy of the beam increases

Question 30

The HVT decreases as …

Answer 30

the kV used to produce the beam decreases

as the average photon energy of the beam decreases

Question 31

Discuss the effect on intensity if 
the kV increases
the mA increases
the SID/SDD increases
the filtration increases
the Z of target material increases

Answer 31

The intensity would
kV = increase
mA = increase
SID/SDD = decrease
filtration = decrease
Z of target material = increase 
(opposite applies if factors were decreasing)

Question 32

Discuss the effect on quality if 
the kV increases
the mA increases
the SID/SDD increase
the filtration increases
the Z of target material increases

Answer 32

The quality would
kV = improve 
mA = no effect
SID/SDD = no effect
filtration = improve
Z of target material = marginal effect
(opposite applies if factors were decreasing)