IRMER Flashcards
What is the absorbed dose
Absorbed radiation per kg
What is the effective dose
The measure of risk. Whole body effective dose is 1 and then divided per risk per organ.
What is the dose equivalent
The measure of the harmfulness of the absorbed dose.
What are the units for the dose equivalent and effective dose
Sievert, Sv.
How does radiation affect DNA
Directly by knocking out an e- from the DNA or indirectly by creating free radicals from ionising water.
Stochastic effects
Increased dose doesn’t increase the severity of the effects but increases the probability of getting dose caused effects e.g. cancer. No threshold.
Deterministic effects
Increased dose = increased the severity of effects, when dose is above a threshold. The threshold is different for different tissues.
What safety measures can be used to reduce the dose or protect people from radiation
Reduce distance, reduce exposure time, use lead to shield, monitor radiation levels on staff and surrounding, have a controlled area (6mV).
What material/amount does it need to stop each radiation particle
a = 1cm of air or 1 mm of matter b = a few mm of aluminium y = a few cms of lead x-ray = a few cms of lead (less than for y)
Structure of an x-ray tube
The cathode (-) releases e- The anode (+) is hit by e- so needs to have a high mp and high atomic number/density to absorb the e-, e.g. tungsten, with a copper tube to absorb heat.
Rectification
AC becomes DC and x3. Negative waves flipped so twice as many and all positive and then everything x3.
Bremsstrahlung
When e- are rapidly decelerated by the atom’s +ve nucleus and lose lots of energy and release a photon. Different E of the photons depending on the number of protons in the atom. Continuous.
Measuring attenuation
I = Io x e^-ut
u is the liner attenuation coefficient
Io is the monoenergetic beam
What do all the radiation particles have an effect on
Fluorescent effect on phosphor screens and an effect on photographic film
Relationship between current and intensity and quality of the image
mA proportional to intensity
No change in quality of image
Relationship between voltage and intensity and quality of the image
Intensity is proportional to kVp^2
Increase voltage increases the quality of the image
Relationship between z/atomic number of the target tissue and intensity and quality of the image
Intensity is proportional to Z
No change in the quality of the image
Relationship between filtering and intensity and quality of the image
Intensity reduces, quality increased when filtering applied
Relationship between rectifying and intensity and quality of the image
Rectifying the current increases the intensity and the quality of the image
Details for filtration of the x-rays
Use 1.5mm of aluminium, absorbs the low energy photons that increase the dose but reduce the image quality. A legal requirement.
What happens if u reduce the Voltage
Reduced transmission, increased dose, more attenuation, less Compton scattering, better contrast of the image.
What happens if u increase the voltage
Higher energy photons so more transmission so a smaller dose (fewer photons absorbed by the body) and a better quality beam, but more Compton scattering and reduced contrast of the image.
Increased photoelectric effect.
How can you increase the quality of the beam
Increase the voltage or add filtration
How can you double the intensity/number of photons
Double the current
Explain the photoelectric effect
High energy photons are absorbed by an e- in an atom. If the energy of the photon is higher than the electron binding energy, the electron is released (photoelectron)
Structure of the film
Emulsion layer contains silver halide crystals.
Structure of the screen
Reflective layer reflects the light photons back to the screen.
Phosphor screen where photoelectric effect happens, x-rays absorbed and more light photons emitted.
Film in the middle.
How does the screen work
Phosphor screen converts x-rays into more light photons. These hit the screen and are absorbed and cause blackening. Interaction in the phosphor and photons spread out in all directions so when they hit the film they have spread out so the image is larger.
Importance of matching up fluorescent materials to films
Different fluorescent materials produce different colours of light and different films are more sensitive to different colours of light.
What is speed
Speed is the sensitivity of the film/amount of blackening of crystals.
What factors does the speed depend on
Thickness of film
Size of crystals (bigger = more sensitive but reduced resolution)
The efficiency of conversion of x-rays to photons
Light absorbing dyes on the film - reduce speed but increase the resolution.
Speed is inversely proportional to the resolution.
Latent image formation
- X-ray absorbed by silver halide and makes the halide release an e- which moves to a defect or impurity in the crystal = a sensitivity speck.
- e- attracts silver ions which become atoms and attract more e- and the process is repeated until small deposits of silver in crystals hit by x-rays.
- These deposits are more sensitive to the developer so get blackened more.
Developing process
- Alkaline added which converts the rest of the silver ions into atoms. Crystals sensitised by the x-rays will be affected more.
- Washed off so that it doesn’t neutralise the acid and to stop the alkaline from acting and blackening all the film.
- Acid added to harden the image and remove unreacted silver halide.
- Washed off to prevent staining
Latent image fading
When some silver atoms converted back to ions
Factors affecting the subject contrast
Voltage of tube
Contrasting agents e.g. iodine
Grids stop scattering
Factors affecting film contrast
Optical density e.g. 1 = 10% of light transmitted, 2 = 1% of light.
How does the latitude of dose/optical density graph affect the image
Wider latitude = a range of doses with the same optical density/blackening so reduced contrast
What is film fog and how do you reduce it
Blackening due to background/natural sources. Reduce by keeping the film in a cold dark room away from radiation sources.
