Production of X-Radiation Flashcards
Properties of X-Rays - 6 points
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
X-Rays originate from …
outer space, radioisotopes, X-Ray tubes, linear accelerators
1st stage in the production of an X-Ray beam using an X-Ray tube
the filament is heated, producing electrons by thermionic emission, the tungsten anode begins to rotate helping to disperse the heat
2nd stage in the production of an X-Ray beam using an X-Ray tube
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
Light beam diaphragm
controls the dimensions of the X-Ray beam
Name the 2 mechanisms of X-Ray production
Following the rapid deceleration of fast moving electrons - ‘Bremsstrahlung’ - producing a continuous spectrum
Following electron transitions between inner shells - producing a line spectrum
Describe rapid deceleration
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
The closer the electron passes to the nucleus …
the greater the deceleration of the interaction and the more energy lost as photons of X-radiation and heat
The total effect of millions of electrons interacting with the tungsten atoms in the target is …
the production of a range of X-radiation energies, termed a spectrum
Discuss the lower energy photons
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)
Describe the transition of electrons between shells
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
If there is a kV exposure then the energy of the incoming electron is …
equal - if a kV exposure is used then the incoming electron has energy of up to 70 keV
An electron filling the vacancy achieves …
stability
Describe electrons further away from the nucleus in terms of potential energy and binding energy
they have more potential energy but less binding energy,
The energy that the emitted photon has is …
equal to the difference in binding energies of the 2 electron shells involved
What are the binding energies of shells K, L and M in tungsten
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
What are would be the energies of the photon transmitted if an electron moved from
M to K,
M to L,
L to K
M to K = 67.5 keV
M to L = 8.5 keV
L to K = 59 keV
A spectrum arising from a combination of Bremsstrahlung and electron transitions would look like …
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
Name the 5 possible factors which could affect the spectrum of a beam of X-Ray photons
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)
Describe filtration
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
Due to photon beam filtration X-Ray tubes are required to have …
additional filters inserted at their exit port to comply with current regulations and to reduce the radiation dose to the patient
The total filtration present in a diagnostic X-Ray tube is …
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)
Additional filtration causes the X-Ray spectra to …
be reduced
Describe the spectrum of a different anode (target) material
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
Define beam intensity
the radiation energy passing through a unit area per unit time (has units Joules/metre squared/seconds)
Define beam quality
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
How is an X-Ray beam’s quality specified
by stating its half-value thickness (HVT) in mm of a stated material, typically Aluminium up to 150 kV and copper above 150 kV
Describe the Half-Value Thickness
that thickness of a stated material which will reduce the intensity of the X-Ray beam by 50%
The HVT increases as …
the kV used to produce the beam increases
as the average photon energy of the beam increases
The HVT decreases as …
the kV used to produce the beam decreases
as the average photon energy of the beam decreases
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
The intensity would kV = increase mA = increase SID/SDD = decrease filtration = decrease Z of target material = increase (opposite applies if factors were decreasing)
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
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)
