14. Biomedical Applications Flashcards
What does radiation do to tissue as it penetrates it?
It loses energy within it and ionise molecules and atoms
What are the two types of damage that radiation can cause when passing through the skin?
- Direct damage - by altering cellular strucutre
2. Indirect damage - creates free radicals which cause cell damage
What is a free radical?
An electrically neutral atom or molecule that has an unpaired electron
- Unpaired electron is highly reactive in tissue and can cause cell structure damage
What is the oxygen effect?
Irradiation of oxygen molecules can alter the original strucutre as well as create a new radical
- Increases radiation efficiency
What does the cell ability for self repair depend on?
How radiation is recieved over time
- Also big initial dose is worse than the same dose spread out over time
- Also depends on the organ and the area over which the radiation is recieved
Define the absorbed dose, D
Amount of energy absorbed by a unit mass measured in Grays (Gy)
1Gy = 1J/kg
What is the equivalent dose, H_T?
H_T = w_R * D_T,R
w_R - weighing factor
D_T,R - absorbed dose for given radiation
Unit in Sieverts = J/kg
What is the weighting factor, w_R?
Tells us how the different types of radiation can have different severities on the cell for the equivalent dose
e.g. photon, electron = 1, alpha particle= 20
What is the effective dose, E?
E = sum w_T H_T
w_T is the organ sensitivity weighting factor
e.g. gonads is 0.2, skin is 0.01
Units again in Sieverts
Describe the difference between the abosrbed dose, the equivalent dose and the effective dose
Absorbed - Just a number on how much is absorbed
Equivalent - Weights the radiation in terms of the irradiating particle
Effective - Weights the radiation on the irradiating particle and the organ
For an external source, what is the most important term in considering the dose recieved
Only the energy of the radiation
For an internal source, what else must be considered?
Half life of source and the fraction of energy deposited
What are the main sources of external radiation for humans?
Natural sources such as cosmic rays and common radioisotopes (85% of annual dosage)
- Can also get internal exposure from food
What are the main sources of internal radiation for humans?
Breathing in radon gas
Give some examples of ALARA in radiation protection
- Plan work in advance to reduce exposure
- Increase distance from source
- SHielding
When might a worker need to wear a passive or active dosiometer
Passive if radiation recieved will be above 30% of natural sources over a period of time
Active if possibility of a signficant dose in a short period of time
How are x rays used in medical applications?
Measure the attenuation as they penetrate the body by measuring the ratio of intensity in front and behind the patient e.g. calcium in bones absorbs better than tissue
How does a CT scan work and what is a downside?
3D images containing depth information can be constructed from several 2D images taken at different angles
- Patient exposed to more radiation
What is a PET scan?
Positron emission topography
- Inject a beta plus emitter
- positron will annihilate into two photons and define a line of sight as they travel in opposite directions
- By observing many annihilations, can observe where things happen
What is the main concern when using radiation to irradiate tumours, and what is a solution to this?
Tumours are less oxygenated than the surrounding tissue
- Tissue is more sensitive than the tumour to radiation
- Want to irradiate the tumour with a highly powerful, focussed dose to kill it and cause minimal damage to tissue around it
Describe using a gamma ray of photons to destory a tumour and the advantages and disadvantages
Generate a collimated beam of photons from a linear accelerator - Readily available to do
- However most of them deposit their energy very early in the material, meaning any tissue in front of the tumour would recieve a higher dose than the actual tumour
- Must irradiate the tumour from different sides to optimise tumour dose/healthy cells
Why would electrons be used for a tumour on the surface rather than photons?
Photons can penetrate the body easier than electrons
- For a surface tumour, electrons can spare the tissue behind it from irratiation when compared to photons
When would hadron therapy be beneficial?
They lose most of their energy in their narrow bragg peak at the end of their depth penetration
- Damage infront of and behind the tumour is less than in the tumour
