Misc. Flashcards
Define penumbra
Width of the beam outside the flat part defined as distance between 80% and 20% dose
Penumbra depends on
Source size
Collimator edge
Beam energy (electron range)
Head scatter
When does photoelectric effect happen?
Photon energy is relatively low(KEV range)
Irradiated material has high Z(Atomic number)
When does Compton scattering happen?
Photon energy is much higher than electron binding energy (MV range)
Doesn’t depend directly on Z
Varies with electron density
When does pair production occur?
Photon with high energy(1.022MeV) interact with magnetic field of atomic energy
Irradiated material has high Z(Atomic number)
What is positron?
Electron with positive charge
Describe Positron Annihilation?
A
Draw curve of
Photoelectric effect
Compton scattering
Pair production
A
What is the difference between pair production and positron annihilation?
In pair production, photon incidence would produce pair of electron/positron
In positron annihilation, electron/positron mass are converted to two 0.511MeV photons
Define exponential attenuation
I=Ioe(^-meu)t
Linear attenuation coefficient depends on
Photon energy
Medium composition
Medium density
Difference between energy transferred and energy absorbed?
Etransferred - Eradiated = Eabsorbed
Beam Hardening
Preferential attenuation of low energy beam components
Usually in high Z materials due to photoelectric absorption of low energy components
Reduces skin doses
Beam softening
Occurs when high energy beam pass through high Z material due to pair production
How low(KV) beams are hardened?
With aluminum and/or copper filters
