Physics 2 Rida Flashcards
What are the 3 things that are interlinked when it comes to image quality?
(If u make one better, u often make another worse)
Contrast resolution
Unsharpness
Noise
Define contrast resolution
The ability of the system to distinguish objects w a small signal difference to background
What is our main source of contrast and why?
Photoelectric effect (PE), cos when this occurs, we dont get a scattered photon
Equation for probability of PE
PE directly prop to Zcubed over Ecubed
At higher energies, is the PE effect more or less likely?
Less
What is beam quality?
Contrast is influenced by beam energy & filtration & target material
(i.e. the shape of the spectrum)
At what beam energy is the Compton effect more likely?
Higher
Why is the Compton effect more likely at higher beam energies?
Cos the photoelectric effect is less likely
In order to increase the contrast, what can we do?
• Decrease the kV, (although we know that this increases the patient dose)
• Reduce the scattering volume lea. compression or smaller tield size)
• Change patient orientation (put object of interest closest to the film)
Define unsharpness
The ability of the system to resolve small objects
The larger the focal spot the … (penumbra)
Larger the penumbra on the edge of an object
Why cant we have a tiny focal spot?
Anode gets v hot, lots of energy on it.
So instead, we angle the anode to artificially create a smaller focal spot (improved resolution)
Angling also = creating a large area for the e- to collide with (reducing heat)
Shallow angle =? Focal spot
Steep angle=? Focal spot
Shallow- larger focal spot
Steep- smaller focal spot
Noise
Fluctuations in image signal
Noise directly prop to?
1 / square root of n
n= number of x-rays
When can you no longer detect the contrast differences?
If the fluctuations are as great as the variations in contrast
How to reduce noise?
Must increase the number of photons forming the image cos noise is linked to statistical fluctuations in the number of photons reaching each part of the image
-inc the dose (inc the mAs)
-stop more of the photons (detector with greater stopping efficiency)
-use more photons per pixel (increase the pixel size)
How does larger pixels reduce noise
Each individual pixel has more X-rays incident on it
How does larger pixels mean low resolution?
Smaller objects may not be resolved
Main unit for measuring radiation dose is called?
Effective dose
What is effective dose?
Absorbed dose (gray)
Energy absorbed per unit mass
J/kg= Gy
The amount of energy transferred by the radiation into the absorbing matter, per unit mass (doesn’t account for the type of radiation or the type of absorbing matter
Which effective dose accounts for the type of radiation and the body part irradiated?
Equivalent dose (sieverts)
List alpha beta and gamma in order of ionising strength
Most - alpha
Beta
Least- gamma
List alpha beta and gamma in order of penetrative strength
Most- gamma, thru hand and into concrete
Beta- thru a hand
Alpha- not thru hand
Weighting factors of alpha beta and gamma
Alpha- x20
Beta- x1
Gamma- x1
What type of radiation is more likely to damage DNA?
A more ionising type
Would a given absorbed dose of alpha radiation give a higher or lower equivalent dose than the same absorbed dose of x-rays
Higher
What is the weighting factor for breast, bone marrow, colon, lung and stomach tissue
0.12
What is the weighting factor for Gonads tissue
0.08
What is the weighting factor for bladder, oesophagus, liver, thyroid tissue
0.04
What is the weighting factor for bone surface, brain, salivary glands, skin tissue
0.01
What is the weighting factor for remaining tissues
0.12
Effective dose=
Sum of contributions from each individual organ
If breast tissue is exposed to radiation, the patient will receive a higher effective dose than if a less radiosensitive organ was exposed (e.g. the brain).
What factors make xray and gamma more ionising
Freq- high
Wavelength- short
Sources of ionising radiation in the environment
Hospitals
Food- bananas
Ground (terrestrial radiation)
Space (cosmic radiation)
The average total radiation dose (including man-made sources) per person in the UK throughout 2010?
2.7mSv
Radon & Theron
48%
Intakes of radionuclides excluding radon
11%
Weapons fallout
0.2%
Occupational exposure
0.2%
Discharge from power stations
0.1%
Medical
16%
Terrestrial gamma radiation
13%
Cosmic radiation
12%
Natural background?
83.5%
Man made
16.5%
Diagnostic nuclear medicine
6%
Interventional (non-CT)
7%
Angiography (non-CT)
4%
Therapeutic procedures
3%
Conventional xray (inc dental)
18%
Lots of conventional X-rays, but low dose
22.9 million Apr17- Mar 18 in England
Computerised tomography (CT)
62%
Fewer CT scans but higher dose
5.15million Apr 17- Mar18 in Eng
Average annual dose to UK individual from diagnostic radiology is approx
0.44mSv
Positives of radiation
• Early diagnosis
• Image guided surgery
• Radiotherapy
•Sterilisation
-Blood products
- Foodstuffs
- Dressinas
Negatives of radiation
• Cancer
• Cataracts
• Skin darkening / burns
• Hair falling out
• Sterility
Deterministic (acute) effects of radiation?
- 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 (chronic, potential long term effects) of radiation
- Occur at high and low doses
- Occur randomly, but probability of effect depends on dose received
- Severity independent of dose
- Cancer induction
What dose will cause cataract induction?
More than 500 mSv
What dose will cause skin reddening?
More than 2000 mSv
What dose will cause temporary hair loss?
More than 3000 mSv
What dose will cause ulceration?
More than 24000 mSv
What dose is sufficient to kill 50% of the population ?
More than 5000 (to entire body)
What are the different models to estimate the risk for stochastic effect?
Linear threshold
Quadratic
Wasted dose
Hormesis
Linear, no threshold
What does the linear no threshold model say?
There is no threshold for induction of cancer
Even at low doses, there is a possibility of inducing cancer
When are cells most at risk of damage?
During the replication stage of cell cycle
G2 & mitosis
Which cells are more sensitive to radiation?
Active cells
- White blood cells, bone marrow, stomach lining, hair follicles
Which cells are less sensitive to radiation?
Inactive cells
- Bone, muscles, tendon, nerves, skin
Why are children at more risk from radiation?
They are growing and so there is more cell replication
Children have more years of life left to experience secondary cancers (often a number of decades after radiation exposure)
Why is X-ray and cells a bad combo
Cells largely composed of water
X-rays can cause ionisation
Ionisation of cellular water can lead to molecular changes and the formation of chemicals that can damage chromosomes
Leading to the changes in the structure and function of the cell
What is made when X-rays ionise atoms in tissues?
Free radicals- which can damage DNA
What can we use effective dose to calculate?
A risk of lifetime cancer induction
On average, what the chance of inducing fatal cancer per 1,000mSv effective dose
5.5%
Equating to a 1 in a million chance of inducing cancer as a result of a chest xray
Why are women more at risk of radiation?
Women have a higher amount of radiosensitive tissue compared to men
Individuals will not accept risks of death if they are as high as ?
But will accept?
1 in 100 per year
Will accept: 1 in 1000 per year (for enjoyment or reward, eg occupational limit)
How much radiation is in a dental xray?
0.005 mSv
V v little (0.01 in Brazil nuts!)
What 2 factors is it important to consider when assessing medical radiation doses?
Doses from medical exposures to radiation in context alongside other sources of radiation exposure
