Xray interaction in tissue Flashcards
atom
Number of protons=atomic number (Z)
If there is an unequal number of electrons to protons, the atom is ionized
The further ‘away’ from the nucleus, the more energy the electron has
But, the ‘pull’ of the nucleus is weaker (binding energy)
5 types of attenuation
attenuation - any time an x-ray photon loses energy
anything that increases the power of xrays - amplification
Coherent Scattering (Rayleigh/Thompson/classical/characteristic)
Photoelectric Absorption (effect/event)
Compton Scattering (Compton effect)
Pair Production
Photodisintegration
Coherent Scattering (Rayleigh/Thompson/classical/characteristic)
deflected by the atom
nothing changes in the atom.
photon comes in nothing happens and then it leaves
very low energy
99% of coherent scattering never leave x tube filters it out.
Photoelectric Absorption(Photoelectric event, Photoelectric effect)
1 - incident photons interacts with an inner orbital K or L electron giving all of its energy to the electron ejecting it from orbit. the photon is absorbed.
2 - the ejected electron (photo electron) imparts the atom with an energy equal to the excess imparted by the photon.
3 - there is a vacancy in the inner orbital shell K or L which must be filled. one of the electrons from the outer orbital shell, usually the next orbit out drops to the void.
4 - as the electron drops to the void it may shed its excess energy as a secondary photon
Photoelectric absorption why do radiographers love it
Photoelectric absorption is more likely to happen in areas with a high atomic number
In fact, the probability of photoelectric absorption is atomic number to the power 3
PEA=Z3/Energy3
What does that mean?
material and their atomic number
Fat - 6.3
Lung - 7.4
Muscle and soft tissue - 7.4
Bones - 13.8
Iodine Contrast. - 56
Lead. - 82
Compton Scatter(Compton Effect)
1- photon interacts with an outer orbital electron impairing some of its energy to the electron ejecting it from orbit.
2- the ejected electron (Compton electron) leaves the atom with an energy equal to the excess imparted by the photon.
3- the photon continues on an altered path scattered with less energy long wavelength than before the collision.
Compton Scattering
Is not influenced by atomic number (Z)
Can happen at any point of the diagnostic/therapeutic range
Is slightly inversely proportional to photon energy (E)
horizontal -
Coherent scattering
Photoelectric absorption
Compton Scattering
vertical -
Energy Range of incoming xray photon
Outgoing
Likelihood
Impact on image
impact on patient
Impact on staff
Energy
Range of incoming xray photon
Outgoing
Likelihood
Impact on image
impact on patient
Impact on staff
(Coherent scattering)
0-30KeV
Scattered xray photon with same energy
Z/E^2
Not significant
Not significant
Not significant
(Photoelectric absorption)
0-100KeV
(predominant 50KeV)
Electron + characteristic xray stopped locally
Z^3/E^3
Primary contrast
Interactions deposit dose
Not significant
(Compton Scattering)
Always a likelihood
Electron (stopped locally), scattered photon
Independent of Z
Inversely proportional to E
Background haze
Big – electrons deposit dose
Dominant source of stray radiation
Z=atomic number, E=photon energy
Differential Absorption
=The difference in x-ray interactions with matter
(what is absorbed, and what is transmitted)
Pair Production
what is it
horizontal
X-ray photon energy
% probability of PEA
% probability of Compton
% probability of pair production
verticle
10KeV Diagnostic Imaging
95%
5%
0%
26KeV Diagnostic Imaging
50%
50%
0%
60KeV. Significantly more scatter, image quality is reduced due to loss of contrast
7%
93%
0%
150KeV Radiotherapeutic energy for superficial tumours
0%
100%
0%
4MV. Radiotherapeutic energy. Compton scattering in a forward direction
0%
94%
6%
10MV Radiotherapeutic energy for deeper tumours. Compton scattering in a forward direction
0%
77%
23%
24MV. Not used clinically
0%
50%
50%
