week 5 Flashcards
what is attenuation and what determines the level of attenuation?
attenuation- the reduction in intensity of an x-ray beam as it passes through an object (absorption/scattering)
is determined by x-ray energy, material passed through
what are half-value and tenth-values?
half-value: thickness of a certain material needed to reduce intensity of incident beam to 50%
tenth-value: thickness needed to reduce intensity to 10%
what’s the difference between linear attenuation coefficient and mass attenuation coefficient? why is this important to radiotherapy?
linear attenuation coefficient (μ) is the fraction of x-rays removed from the beam (0-1.0)
mass attenuation coefficient (μ/ρ) is the linear coefficient divided by the density of a material so external conditions don’t affect density and thus results
a higher contrast between attenuation factors for different matter (bone, tissue) means the image will be higher contrast and thus clearer
explain photoelectric effect
- Incident photon absorbed by orbital electron
- electron is emitted (photoelectron)
- Any extra energy is given to the electron as kinetic energy
- Photoelectric effect is followed by characteristic x-ray
(the energy of the incident photon must be bigger than the binding energy of the electrons in that specific shell in order to remove that electron)
what affects the probability of photoelectric effect occurring?
Low energy=more likely
High energy= less likely
High atomic number= more likely
Low atomic number= less likely
explain compton scattering
- incident photon collides with orbital electron
- some energy is lost to electron
- photon slows down
- orbital electron is emitted due to extra energy
- scattered at an angle
(larger incident x-ray energies= smaller scattering angle -> small scattering angle= small energy transfer from x-ray to electron)
what affects the probability of compton scattering occurring?
high atomic number=more likely
low atomic number= less likely
high energy x-ray= less likely
low energy x-ray= more likely
(but not as much as with PE ^)
explain pair production
- incident x-ray passes close to nucleus
- nucleus absorbs x-ray, has extra energy
- this energy is emitted as an electron-positron pair (e-/e+)
- positron will annihilate with an electron, producing 2 gamma rays at 180 degrees
(incident x-ray needs at least 1.02MeV for this to occur (threshold energy), extra energy is given to e as kinetic energy)
what is the formula for transmission (T)
T = Intensity of transmitted beam/intensity of orig beam
T=e^-μx
x=thickness of absorber
what affects the probability of pair production occurring and why is it not used in diagnostic imaging?
high energies= more likely
larger atomic number= more likely
what’s the difference between intensity and energy?
intensity- number of x-ray beams
energy- strength of beams
list the possible electron interactions
collisions with protons- incident electron loses kinetic energy/changes direction of motion
collisions with orbital electrons- ionisation of orbital electron(energy given)/excitation of orbital electron(not enough energy given)
eventually electron slows down and stops after losing energy with thousands of collisions
how do bremsstrahlung x-rays occur?
- incident electron interacts with nucleus and is deflected
- nucleus has excess energy
- emitted through a bremsstrahlung x-ray
(used in diagnostic and treatment, strength of beam produced depends on how close the electron passes by the nucleus)
