Radiation Physics 2 : Radiation risks and basic radiation protection Flashcards
Contrast Resolution
Contrast Resolution → the ability of the system to distinguish objects with a small signal difference to background
Beam quality
Beam quality = How contrast is influenced by beam energy & filtration & target material (i.e. the shape of the spectrum)
Can increase the contrast by:
- Decrease the kVp (although we know that this increases the patient dose)
- Reduce the scattering volume (e.g. compression or smaller field size)
- Change patient orientation (put object of interest closest to the film)
Unsharpness:
the ability of the system to resolve small objects- spacial resolution.
Focal Spot size
Contributes to geometric unsharpness= when you can distinguish particular shapes due to overlapping penumbras (blurring of the edges of objects)
The larger the focal spot, the larger the penumbra on the edge of an object.
Motion unsharpness
When a pt moves it creates a blurred image
Why don’t we just have a tiny focal spot (geometric unsharpness)?
Anode becomes too hot due to the high amount of energy deposited into it.
Noise
Random fluctuations in image signal
relative to 1 over/ the square root of the number of x rays
More x rays you have the lower the noise is going to be.
Reducing noise
Noise is linked to statistical fluctuations in the number of photons reaching each part of the image.
So have to increase the number of photons forming the image.
- Increase the dose (increase the mAs or number of electrons going across x ray tube)
- Stop more of the photons (detector with greater stopping efficiency)
- Use more photons per pixel (increase the pixel size)
Absorbed Dose?
The amount of energy transferred by the radiation into the absorbing matter, per unit mass
Equivalent dose
Measured in Sieverts
Accounts for type of radiation i.e. has a higher number for alpha which is far more ionising than beta or gamma
Effective dose
Also measured in Sieverts
Builds on equivalent dose and takes into account the body part that is being irradiated as some tissues are more radiosensitive than others.
when treating radiosensitive areas - pt receives a higher effective dose than it would a less radiosensitive tissue.
You have to figure out the contribution from each individual organ , multiply by the weighting factor and then add them up to reach the effective dose.
Radiation can be used for
Radiation saves lives…
- Early diagnosis
- Image guided surgery
- Radiotherapy
- Sterilisation
- — Blood products
- — Foodstuffs
- — Dressings
Harmful radiation effects:
- Cancer
- Cataracts
- Skin darkening / burns
- Hair falling out
- Sterility
2 types of effects of radiation on people
- Deterministic - acute effects
- Stochastic - Chronic + potential long term effects
Deterministic Effects of radiation:
Known that these will happen when re proceed past a certain threshold
Acute
- High doses over a short period of time
- Only occur above a certain threshold dose
- Severity increases with dose &dose-rate
- Effects include “radiation sickness”,
- erythema, epilation, loss of fertility, death
Stochastic Effects of radiation:
Chronic / long term effects
- Occur randomly at both high and low doses
- Occur randomly, but probability of effect depends on dose received - higher dose = higher probability
- Severity independent of dose
- Cancer induction
Stochastic Effect models
Cells and radiation sensitivity
Active cells are more sensitive to radiation (why children are more at rick to radiation)
- White blood cells, bone marrow, stomach lining, hair follicles
Inactive cells are much less sensitive
- Bone, muscle, tendon, nerves, skin
Ionisation and cellular water
X-rays can cause ionisation
Ionisation of cellular water can lead to molecular changes and the formation of chemicals that can damage chromosomes
X-rays can ionise atoms in tissue making free radicals which can damage DNA
Leads to changes in the structure and function of the cell
