Radiation Dose Flashcards
Absorbed radiation dose, strictly from x-rays is expressed as
Rad
Gray
Units used to measure the “effective dose” or “dose equivalent” of ANY type of ionizing radiation.
Rem
Sievert
1 Sv = ___ rem
100
1 rad = ____ rem
1
1 Gy = ____ rads
100
The amount of radiation in 1rad is equal to?
0.01 joules of energy per kg of matter (patient’s weight).
The single greatest natural exposure to radiation in the US comes from?
Radon
The average annual dose equivalent from natural sources is?
- 31 rem
3. 1 mSv
Man-made sources of radiation contribute how much annual radiation
- 31 rem
3. 1 mSv
CT scans contribute how much radiation annually
- 15 rem
1. 5 mSv
Other medical imaging contribute how much radiation annually
- 15 rem
1. 5 mSv
Other man-made radiation contribute how much radiation annually
- 10 rem
1. 0 mSv
What is the range of the effective dose from CT
About 1 to 10 mSv
“Low-dose CT scans” reduce levels by factors of?
1/2 to 1/5
For a body scan, the dose in the center of the patient may be _____ of the skin dose.
1/3
Typical effective dose for a Head CT
2 mSv
Typical effective dose for an Abdomen CT
8 mSv
Typical effective dose for 1 Radiography exposure
0.1 mSv
Typical effective dose for 5 Radiography exposures
0.5 mSv
Describe the radiation profile of a single slice
It is rounded.
This means there is more radiation applied in the center of the slice and less on the sides.
What does the penumbra, on a radiation profile mean?
The radiation is extended outside of the slice into neighboring tissue.
The radiation that leaks outside of the intended slice thickness is called?
Radiation penumbra
CT Dose Index (CTDI) is what?
An indicator of radiation dose which includes the radiation located within the intended slice thickness as well as that from the penumbra.
The CTDI does not factor in variation in dose due to?
Gaps or overlaps in adjacent slices
CTDI did not account for what?
Helical scanning,
Simultaneous acquisition of multiple slices in a single rotation, or cone beam irradiation.
Multiple Slice Average Dose (MSAD) is an indicator of radiation dose that?
Corrects the CTDI by factoring in gaps or overlaps between the slices or helices due to the selected table increment or pitch.
Since a slice obtained with MDCT is not individually collimated, where does the radiation penumbra occur?
Only the end slices of the slice volume
The CTDI 100 accounts for what?
- The possibility of having multiple slices in one rotation (MSDS)
- Slice thickness
CTDI(100) What does the 100 stand for?
100 Refers to the 100 mm pencil ionization chamber the physicists use to measure the radiation.
CTDI 100 Is typically measured where
At both the periphery and in the center of the CTDI Phantom
What is the disadvantage of CTDI 100
It only considers serial scans and does not consider helical scAns with varying pitches
CTDI W or weighted-CTDI is?
A weighted average of the center and peripheral CTDI 100 to arrive at a single measurement based on a single rotation of a multi row detector scanner
CTDI is an indicator of Average radiation dose along?
The x-direction and y-direction
CTDI w =
(1/3) CTDI100 center + (2/3) CTDI100 periphery
CTDI vol represents
The average dose in the central region of a multiple rotation exam
CTDI vol Accounts for
Helical studies performed on multi row detector scanners and it indicates the average radiation dose over all three directions.
Accounts for overlaps and gaps in the rotations
CTDI vol Does not account for what?
The number of rotations, the total length of the volume.
Ex: A limited volume pelvic exam would have the same CTDI as a abdomen pelvic exam
DLP dose length product Accounts for
(total number of slices) x (slice thickness)
DLP =
CTDI vol (mGy) x Total scan length (cm)
DLP Is expressed in what units
mGy-cm
DLP Does not consider what
The size of the patient or the body part exposed to radiation
What is the measurement of radiation dose that would allow risk assessment of inducing cancer?
Effective dose
Effective dose is calculated by
Multiplying the absorbed dose by a weighting factor assigned to each organ or body region.
If more than one area is exposed, then the total body effective dose is the sum of the effective dose for each individual exposed area.
Effective dose is expressed in what units
Sv
