Physics Flashcards
“Joint Commission (JC) Sentinel Event is declared for unintended cumulative dose of
15 Gy to a single field (>1500 rads)
“Leakage radiation from the x-ray tube housing is limited to”
1 mGy/h at a distance of 1 meter
Max entrance air kerma (AK) rate in fluoro (non diagnostic) allowed by FDA is
100 mGy/min
No limit for DSA
Fetal dose of > \_\_\_\_ are assoc. with 2w: fetal death 3-8w: congenital malformation 9-15w: growth or mental retardation
100 mGy
A ____ fetal absorbed dose during the first trimester can result in x3.5 inc risk of childhood CA
10 mGy
Dose rate limit of an unrestricted area
0.02 mSv in an hour
Annual radiation dose limit (total body of adult occupational worker in the US):
50 mSv
Avg risk of mortality from radiaton induced cancer in the gen pop
5% per Sv
Avg Abd/Pelvic CT exam dose
9 mSv
Natural background radiation in the United States
3 mSv/year
natural exposure: radon, solar etc.
Standard voltage for a CXR
120 kVp
Approximate power used to obtain an image with a plain film radiography
100 kW
Approximate power used to obtain a mammogram
3 kW or 30 kW? fix this
Appropriate entrance air kerma for an abdominal radiographs?
entrance skin dose?
3 mGy
~ 5mGy (usually about 50% more)
Pediatric protocol change for CT
Decrease kVP (eg from 120 - 80), decrease mA, decrease SFOV. Sometimes slight increase in mA to decrease shot noise.
Protocol change for CT in obese patients
Increase kVp to 140, and 120 for CTA
Protocol change for CT in angiography or CECT
Set kVp = 100 (Increases vessel and tissue contrast by choosing kVp so that kVavg is just above the Kedge of iodine (33keV) but even higher than DSAs 70 kVp to avoid grainy images.
Standard phantom sizes for CT
head ___
abdomen____
16 cm
32 cm
Effective dose equation
K x DLP
Where K accounts for tissue being scanned (brain vs breast)
Absorbed dose (mGy): Definition?
Energy absorbed per mass in J/kg.
CTDIvol
Equivalent dose (mSv):
Multiply the absorbed dose by the RADIATION weighing factor (Wr).
Allows to compare radiation types by cancer risk
Radiation weighing factor for XRays
1
so, equivalent = absorbed
Radiation weighing for alpha rays
20
Radiation weighing for gamma rays
1 (Same as XR)
Effective dose
Multiply absorbed dose by TISSUE weighing factor. Accounts for tissue type irradiated (more radiosensitive tissues have larger Wt)
1 Rem = ____ mSV
10
Gamma should be shielded using
High Z material such as lead. Concrete can be used as building material.
Termination should only be considered for fetal absorbed dose of ____ between the ____ weeks of gestation
> 50 mGy, 2-15
Avg person in the US is exposed to approximately ____ mSv annually
6.2
~half from natural, half from medical
Rad units operated above > 70kVp should contain a filtration of at least how much of aluminum equivalent
2.5mm
Fetal absorbed doses of more than ____ occurring during ____ of gestation may result in fetal death.
25mGy, 3-8w
The lethal dose in 50% of humans in 60 days is referred to as LD50/60 and occurs at what level of radiation?
3.5-7 Gy. Depends on level of medical care following exposure.
What radiation induced cancer has the shortest latency period?
Leukemia (5-15yrs)
vs solid tumors 20-60yrs
No deterministic effects from radiation have been proven below which does level?
0.1 Gy
Improve streak artifact by
increasing kVp
DLP equation
CTDIvol x scan length
CTDIvol is estimated using
an acrylic phantom.
Overestimated in obese pts and underestimates in peds.
Of the four basic dose units ________ is the closest measure of cancer risk; it accounts for both radiation type and tissue type.
effective dose
Kerma-Area Product (KAP) equation
“KAP = AK x (Area Exposed)” measured in Gy-cm2
KAP relates to _____ risk while AK relates to ______ risk.
stochastic (cancer); deterministic
