Patient Dosimetry (Erin) Flashcards
In Xray:
Does a higher tube current (mA) increase or decrease patient dose?
Increases patient dose
In Xray:
Does a longer exposure time (s) increase or decrease patient dose?
Increases patient dose
In Xray:
Does a wider collimation increase or decrease patient dose?
Increase patient dose (reduces scatter and irradiated area)
In Xray:
Dose a smaller FOV increase or decrease patient dose?
Decreases
In Xray:
does a higher kVP increase or decrease patient dose?
Increases patient doseA
List the 4 beam properties that increase patient dose in xray?
Higher mA
higher kVp
Larger FOV
Wider collimation
In Xray:
Dose a larger FOV increase or decrease patient dose?
Increases
In Xray:
Does a lower tube current (mA) increase or decrease patient dose?
Decreases
In Xray:
Does a shorter exposure time (s) increase or decrease patient dose?
Decreases
In Xray:
Does filtration increase or decrease patient dose?
Decreases
In Xray:
Does the use of a grid increase or decrease patient dose?
Increases
In Xray:
Does reduced receptor sensitivity increase or decrease patient dose?
IncreasesN
List the 3 scanner properties that increase patient dose in Xray
No filtration
Use of a grid
Reduced receptor sensitivity
In Xray:
Does NO filtration increase or decrease patient dose?
Increases
In Xray:
Does a lack of grid increase or decrease patient dose?
Decreases
In Xray:
Does increased receptor sensitivity increase or decrease patient dose?
Decreases
List the 2 patient properties which increase patient dose in xray imaging
Closer to focal spot (xray source)
Larger patient habitus (larger skin surface to absorb maximum dose)
Does moving the patient further from the focal spot increase or decrease patient dose in xray?
Decreases
In Xray:
Does moving the patient closer to focal spot increase or decrease patient dose?
Increases
In fluoroscopy:
Does a lower kVp increase or decrease patient dose?
Increases (a less penetrating beam means more radiation absorbed particularly on the skin)
In fluoroscopy:
Does continuous or pulsed technique give a higher patient does?
Continuous
In fluoroscopy:
Does a larger area of collimation increase or decrease patient dose?
Increases
In fluoroscopy:
Does a smaller area of collimation increase or decrease patient dose?
Decreases
In fluoroscopy:
Does the use of a grid increase or decrease patient dose?
Increase
In fluoroscopy:
Does NO grid increase or decrease patient dose?
Decrease
In fluoroscopy:
Does increase electrical magnification (zoom modes) increase or decrease patient dose?
Increase
In fluoroscopy:
Does increased geometric magnification (moving patient closer to the source) increase or decrease patient dose?
Increase
In fluoroscopy:
Does an obese patient receive increased or decreased dose?
Increased
List the 3 scanner properties that increase dose in fluoroscopy
- Use of a grid
- Increased electrical magnification (zoom mode)
- Increased geometric magnification (i.e moving patient closer to the source, or detector closer to source)
In CT:
Does higher mA increase or decrease patient dose?
Increases
In CT:
Does lower mA increase or decrease patient dose?
Decreases
In CT:
Does higher kV increase or decrease patient dose?
Increases
In CT:
Does lower kV increase or decrease patient dose?
Decreases
In CT:
Does longer exposure time increase or decrease patient dose?
Increase
In CT:
Does shorter exposure time increase or decrease patient dose?
Decrease
In CT:
Does NOT using mA modulation increase or decrease patient dose?
Increases
In CT:
Does using mA modulation increase or decrease patient dose?
Decrease
In CT:
Does wider collimation increase or decrease patient dose?
Increases
However if collimation is too small the system will compensate by increasing mA/kVp
In CT:
Does decreasing pitch increase or decrease patient dose?
Increases
In CT:
Does increasing pitch increase or decrease patient dose?
Decreases
In CT:
Does a smaller patient receive increases or decreased dose?
Increased
More xrays will penetrate to the centre and deposit a higher dose
NB a larger patient will receive more total xrays but dose is measured per unit mass
In Nuclear Imaging:
Does an increased amount of injected activity increase or decrease patient dose?
Increases
In Nuclear Imaging:
Does an decreased amount of injected activity increase or decrease patient dose?
Decreases
In Nuclear Imaging:
What is the result of reduced patient drinking/urination on patient dose?
Increased patient dose (slower loss of activity from bladder)
In Nuclear Imaging:
What is the result of increased patient drinking/urination on patient dose?
Decreased patient dose
What is the effective dose of a cardiac CT?
12mSv
What is the effective dose of a CT abdomen pelvis
10mSv
What is the effective dose of a CT head?
2mSv
LAC equation?
μLAC = 0.693* / HVL
MAC equation?
μMAC = μLAC/ρ
To calculate the intensity of a beam (I) of x-ray photons after they have passed through a filter…(equation)
I = Io e-μd
e = energy
u = LAC
d = thickness of filter
Describe how radiation can directly affect biological molecules
Atoms within the DNA molecule are directly ionised by radiation leading to molecular damage
Breaking atomic bonds or causing base alteration
Describe how radiation can indirectly affect biological molecules
Water molecules interact with radiation to create free radicals which then migrate to the DNA molecule and cause molecular damage (breaking atomic bonds or causing base alteration)
How is absorbed dose (D) measured (formula)
D = E/m
E = energy deposited in a medium
M = mass of medium
What unit is absorbed dose measured in
Gray (Gy)
What is 1Gy equal to in J/kg
1Gy = 1J/kg
What is 1Gy in mGy
1Gy = 1000mGy (milligray)
What is 1mGy in uGy
1mGy = 1000 uGy (microgray)
What is the radiation weighting factor of X-rays and gamma photons
1
What is the radiation weighting factors of electrons and/positrons
1
What is the weighting factor of alpha particles
20
How is equivalent dose calculated
D (absorbed dose) x Radiation weighting factor
Do alpha particles have a high or low LET
High LET
Do beta particles have a high or low LET
Low LET
What units is equivalent dose measured in
Sv
(1Sv = 1000mSv)
How is effective dose calculated
Sum of (Equivalent dose x tissue weighting factor)
What is the tissue weighting factor for bone marrow, breast, colon, lungs and stomach
0.12
What is the tissue weighting factor for the gonads
0.08
What is the tissue weighting factor for the bladder, liver, oesophagus and thyroid
0.04
What is the tissue weighting factor for brain, bone surfaces skin and salivary glands
0.01
Rank in order from most tissue sensitive to least tissue sensitivity
Gonads
Salivary glands
Liver
Bone marrow
Most Sensitive
- Bone marrow
- Gonads
- Liver
- Salivary Glands
Least sensitive
What are the 3 damage effects can radiation have on DNA
No biological effect (tissue repaired)
Cell death
Cell mutation
What are some examples of deterministic effects
Cataract formation
Skin erythema/necrosis/ulceration
Bone marrow suppression
Sterility (can be temporary or permanent depending on dose)
Death
When do deterministic effects typically occur
Soon after exposure
What are deterministic effects due to
Cell death
How does the severity of deterministic effects change with increasing dose
Severity of deterministic effects increase with increasing dose (above a certain threshold)
How does the probability of stochastic effects change with increasing dose
Probability of stochastic effects increases with increasing dose
What does ALARP stand for
As low as reasonably practicable
When do stochastic effects typically occur
Many years after exposure
What is the main stochastic effect we are concerned about
Cancer
What is the cancer risk for exposure to 1mSv
1 in 20,000 for 1mSv exposure
What is the Gy threshold for skin necrosis
10Gy
What is the Gy threshold for cataract formation
0.5Gy
What is the Gy threshold for bone marrow suppression
0.5Gy
What is the Gy threshold for skin erythema
2Gy
What is the Gy threshold for Testes - temporary sterility
0.15 Gy
What is the threshold for stochastic effects
NO THRESHOLD
What is the excess childhood cancer risk per 1mSv of exposure
1 in 13,000 per mSv
What is the natural risk of paediatric cancer
1 in 500
What foetal dose (in mSv) will double the natural risk of paediatric cancer
25 mSv
What is the typical effective dose of an extremity xray
<0.01 mSv
What is the typical effective dose of chest xray
0.1 mSv
What is the typical effective dose of a lumbar spine xray
1.3 mSv
What is the typical effective dose of barium enema
7 mSv
What is the typical effective dose of CT head
2 mSv
What is the typical effective dose of a CTAP
10 mSv
What is the typical effective dose of a CT Chest
8 mSv
What is the typical effective dose of a bone scan (NM)
4 mSv
What is the typical effective dose of PDG PET Scan
8 mSv
What is the typical effective dose of a lung perfusion scan (NM)
1 mSv
For a point source what law does the dose rate follow
Inverse square law
What is the inverse square law equation
1/d(squared)
Doubling your distance from an xray source decreases your dose by a factor of ………
4
How much protection do lead aprons provide from the primary xray beam
NONE
They protect against scattered radiation but not the primary beam
What materials are commonly used for radiation shielding
Lead and tungsten
What crystals are TLD made from
Lithium fluoride
Or
Calcium fluoride
Why is lithium fluoride used for TLD
Because has a similar atomic number to soft tissue
How do TLD work?
Small crystals which absorb ionising radiation and emit light when exposed to heat
Can TLD be reused
Yes
Usually worn for 1-2 months then sent away for processing
Then reused
How many TLDs does a body dosimeter comprise of?
2
What are the functions of the 2 TLDs in a body dosimeter
One TLD measures skin dose
The other TLD measures dose at a depth of 1cm
What does KERMA stand for
Kinetic energy released to matter
What is KERMA very similar to
Absorbed dose (especially at lower energies)
What piece of equipment allows air kerma to be measures
Ionisation chamber
Is the dose area product linked to stochastic or deterministic risk
Stochastic
What is the typical exposure from a long haul flight
0.08mSv
What is the largest source of background radiation
Exposure to radon
With a lead thickness of 0.25mm what % of photons are transmitted
5%
With a lead thickness of 0.35mm what % of photons are transmitted
3%
With a lead thickness of 0.5mm What % of photons are transmitted
1.5%
What particles does radon gas produce
Alpha particles
What unit is Kerma measured in
J/kg-1
if someone is a classified worker how long must their dose records be kept for
Minimum 30 years
For how long do classified workers need dose records kept?
How often do they need a medical exam?
How old do they have to be?
Must be >18yrs
Have their dose records kept for minimum of 30 years
Undergo a medical examination before they are classified then annually thereafter
All procedures must have a DAP or dose administered recorded T or F
True
How is the DRL decided
Every procedure must have a DAP or dose administered recorded
From all these recordings a local DRL is set as the third quartile of the median dose
National DRL set as the third quartile of all the relevant local DRL
Can TLD differentiate between the type of radiation you are exposed to?
No
Can TLD measure dose rate?
No
Can TLD be used to measure eye doses?
Yes as they can be made into various shapes, they can be used for assessment of example eye or finger doses
Can TLD be reused?
Yes
Can TLD be used to measure superficial and deep doses?
Yes
Are film dosimetry badges able to differentiate between the different types of radiation exposure?
Yes
Can film badges differentiated between the different energies of photons?
Yes
What are film badges made from
Silver halide
Advantages of film badges
- Cheap
- Can distinguish between different energies of photons
- Can measure doses from different types of radiation
- Provide a permanent record
C- Accurate for exposures > 100 millirem
Can film badges be reused?
No
Which are more expensive TLD or film badges?
TLD
Can TLD store dose records?
No once information is read out it is lost
Are TLD or film badges more sensitivty
Relatively similar
Can TLD provide a direct reading of dose
No
Can TLD provide a permanent record of dose?
No
Can TLD measure radioactive contamination?
No
How many times can the dose of a TLD be read?
Once
Are TLD affected by environmental factors?
Yes
What is the sensitivity of film badges in mSv
0.1-0.2mSv
Can film badges be used to measure finger dose
No
TLD
Do film badges provide permanent record of exposure
Yes
How often are film badges usually replaced?
1 month
Due to fogging
Do film badges measure the effective dose T/F?
False
Measure absorbed dose
What are the most commonly used detectors in electronic dosimeters?
Silicone diode detector
What is the most sensitive type of dosimeter
Electronic dosimeters
Sensitive to the nearest 1uSv
What is a drawback to electronic dosimeters
Expensive
How many more times sensitive are electronic dosimeters compared to TLD
100 times
Benefits of electronic dosimeters
- Can provide direct electronic readout
-Can provide live/real time readouts - Don’t need processing like TLD
Can electronic dosimeters measure dose and dose rates
Yes
Do electronic dosimeters provide a direct reading
Yes
Are electronic dosimeters good for measuring pregnancy dose
Yes
What is the effective dose for an AXR
0.7mSv
What is the effective dose for a dental xray
0.004/0.005mSv
What is the effective dose of a barium swallow
0.2mSv
Air kerma rate (mGy/s) can be measured using…?
An ionisation chamber, at a known distance from the X-ray tube
In CT: what is beam modulation?
A method of reducing the dose by altering the mA dynamically, depending on what part of the body you’re at (done automatically by a computer)