Topic 1: ionising radiation dose + exposure Flashcards
Characteristics of EM waves
- Amplitude
- Wavelength
- Frequency
- Speed
Speed of EM in vacuum
- Constant = c
- π = π π₯ π
Describe the wavelength + frequency of an X-ray
- Wavelength = nanometers
- Frequency = Hertz
Describe gamma rays
- AKA photons = bundle of radiation
- Difference between light + gamma photons = energy + frequency
- π = π π± f
What is the unit of photon energy?
- Electronvolt
- 1 Joule = π. ππππππ Γ ππ^18 eV
Describe particulate radiation
- Electron is the only 1 that needs to be considered in diagnostic radiology
What is the rest mass/energy of an electron?
- Rest mass = 9.109 x 10^-31kg
- Rest energy = 511 keV
What type of energy is ionizing?
- Frequency higher than UV region
- E.g. UV + gamma + X-rays
What type of energy is non-ionizing?
- Frequency below UV region
- E.g. visible + IR + radio
What are the 3 interactions of photons with matter?
1) Photoelectric absorption
2) Compton scattering
3) Pair production
Describe photoelectric absorption
- Photon absorbed = transfers all energy > inner orbital e-
- e- ejected from atom + photon disappear = loses all energy + has no mass
- Atom left with vacant inner e- orbit = fill with outer electron = releases small amount of energy = X-ray photon
- X-ray = AKA characteristic photon = because its energy is characteristic of absorbing
- X-ray = fixed energy = because orbital e- = fixed energy corresponding to orbital
When is photoelectric absorption more likely?
- Incident photon = low energy
- The lower the energy = more likely to be absorbed via photoelectric
- Energy < 25 keV
Explain the probability of photoelectric absorption
- Odds of photon absorption = depend of element
- Number of protons in nucleus is important
- More likely = larger atomic number = Z
- E.g. heavy metals = lead = good absorber of X-ray
Describe the attenuation of X-ray
- Air = negligible
- Bone = significant = high density + Z of Ca
- Soft tissue = similar to H2O
- Fat tissue = less important than H2O
- Lungs = weak = low density
What is the effect of kVp on the photoelectric effect?
LOW:
- Best for imaging bones = absorb X-rays strongly = appear bright
- Not good for soft tissues = absorb little = creating high contrast BUT missing details in areas like lungs
- E.g. hand X-rays = bones stand out clearly.
HIGH:
- Better for imaging both bones + lungs together = more X-rays can pass through bones while still showing soft tissue details
- E.g. chest X-rays = need to see both ribs + lungs
Describe the Compton effect
- Interaction of photon with outer e- not bound tightly
- Photon collides with e- = gives it energy
- If collision head on = photon direction reversed + loses max energy to e-
- If collision glancing = energy given small
- e- is ejected + scattered proton is passed through
- Single photon = can have several collisions = lose energy everytime = eventually absorbed by photoelectric effect
When is Compton effect more likely?
- Energy > 200 keV - 2 Mev
When can both photoelectric + Compton occur?
60 keV - 90 keV
What factors affect the Compton effect?
- Unlike the photoelectric effect Compton = not depend on what element the material is made of + Z
- Cares about the total number of e- present in the material = thicker + denser have more e- = more absorption of X-rays
- Therefore materials with higher e- density = will experience more Compton scattering
What are the differences in sensitivities to anatomy between photoelectric + Compton?
- Most soft tissue = similar densities = Compton effect insensitive compared to photoelectric effect
How do the photoelectric + Compton effects help in X-ray imaging?
- Differenes in density + thickness = contribute to contrast
- Photoelectric absorption = dominant role = contrast affected more by chemical composition + photon energy
Describe pair production
- Less important than photoelectric + Compton
- Only in very high-energy photons
- Photon with at least 1.02 MeV = absorbed by nucleus = produces e- + positron
- If photon has more than 1.02 MeV = excess increases velocity of e- + positron
What happens to the positron after it is emitted?
- Doesnβt live very long
- If it meets e- = combines = produce 2 photons of 0.51 MeV
- Photons = annihilation radiation
What is the use of positron emitters?
- Basis of the technique of positron emission tomography = PET
- Uses synchronous detection of 2 0.51 MeV gamma photons to pinpoint emission
