Chapter 4 Patient Dosimetry Flashcards
is kV and mAs enough to characterize radiation on the patient?
No, also need tube design and filtration
what is Kair
intensity of x-ray beam
number of x-ray photons per square millimeter
how does Kair fall off with increasing distance
IS law
how does Kair changed with mAs and tube voltage?
proportional to mAs
supralinearly with voltage
how does Kair change with filtration
more filtration decreases Kair
how does adding 3 mm of Al to a 80 kV beam reduce Kair?
by 50%
2 things needed to characterize radiation incident on patients
quality and quantity of x-ray beam
what is radiation intensity incident on a patient requird to generate a good image?
entrance Kair
what does entrance Kair depend on
patient thickness, composition
patient size
beam quality
entrance Kair for skull radiograph, PA chest x-ray, and lateral lumbar spine
skull: 1 mGy
chest: 0.1 mGy
spine: 10 mGy
what is entrance Kair rate for for fluoro on 23 cm wide patient?
10 mGy/min
depends on FOV, frame rate, selected dose level
kerma area product
aka dose area product
- total amount of radiation incident on patient
- product of entrance Kair and area of cross-sectional beam
- independent of measurement location
median KAP in radiographic imaging
1 Gy cm^2
KAP for fluoro guided GI studies and urologic procedures
20 Gy cm^2
KAP in interventional radiology
200 Gy cm^2
what is meant by superficial doses
doses absorbed by skin, scalp, and eye lens
why does an entrance Kair of 1 mGy result in superficial skin dose of up to 1.5 mGy?
- tissues are higher Z than air (10% higher)
- backscatter can increase superficial tissue doses by up to 40 %
usual radiography skin dose
< 10 mGy
usual fluoro skin dose
< 500 mGy
usual interventional radiology skin dose
> 500 mGy
-radiation burns, epilation, cataracts are possible
how do organ doses change with Kair?
increase in proportion to Kair
how are organ doses affected by beam quality for a given Kair?
-increase with beam quality due to more penetration
what is embryo dose for abdominal radiograph
1/3 Kair for AP projection
about 1 mGy
for PA and lateral projections, 1/6 and 1/20 Kair
are embryo doses cumulative
yes
dose rate at embryo for fluoro
1.5 mGy/min
embryo doses in abdominal/pelvic CT
25 mGy
embryo doses in chest CT
- 1 mGy
- mostly from internal scatter
- lead aprons don’t provide benefit
what are genetically significant doses?
dose metrics that quantify potential genetic damage
GSD in US
- 3 mGy
- takes into account dose received by gonads and how many offspring an individual is likely to produce
downside of use of gonad shields
have to repeat exam if poorly placed
what is integral dose
total energy imparted to a patient
what is integral dose for 1 Gy imparted to 70 kg patient?
70 J
i.e. 1 J/kg * 70 kg
chest x-ray integral dose
0.002 J
abdominal radiograph integral dose
0.02 J
head CT integral dose
0.15 J
body CT integral dose
0.5 J
what is plotted on cell survival curve
plots surviving fraction as function of radiation dose
is energy absorbed by cells sufficient to predict biologic damage?
No
for example, alpha particles cause more biologic damage than x-rays
why does some radiation cause more biologic damaga than others?
for example, alpha particles result in a more concentrated pattern of energy deposition than x-rays which produce a more diffuse pattern
i.e. alpha particles have higher linear energy transfer (denser pattern of energy deposition)
unit of LET
keV/um
LET of alpha particles
100 keV/um
LET of x-rays, gamma rays, beta particles
1 keV/um
radiation weighting factor
higher LET radiation have higher weighing factors
higher Wr= more biologic damage at the same dose
Wr for x-rays, gamma rays, beta particles
1
Wr of protons
2
Wr of alpha particles
20
Wr of neutrons
1-20 depending on energy
equivalent dose
absorved dose X radiation weighting factor
lets you compare different types of radiation
-expressed in Sv
radiation detriment
defined by International Commission on Radiologic Protection (ICRP)
-judges relative importance of fatal cancers, non fatal cancers, and genetic effects in future generations
tissue weighting factor Wt
fractional contribution of each organ to the total detriment (uniform whole-body radiation)
Wt values are age and sex averages
indicators of radiosensitivity of an organ
what are remainder organs
adrenals gall bladder heart kidneys pancreas prostate small intestine thymus uterus/cervix
Wt for red bone marrow, lung, stomach, breast, remainder organs
0.12
detriment is cancer
Wt for gonads
0.08
detriment is hereditary
Wt for bladder, liver, esophagus, thyroid
0.04
detriment is cancer
Wt for skin, bone surfaces, salivary glands, brian
Wt is 0.01
detriment is cancer
what is effective dose
accounts for equivalent dose to every organ as well as each organ’s relative radiosensitivity
- presented in mSv
- multiply equivalent dose (H) to an organ by the organ weighting factor (Wt) summed for all irradiated organs
- effective dose is uniform whole body dose that results in same patient detriment
effective dose for lateral skull radiograph
0.03 mSv
effective dose for PA chest radiograph
0.015 mSv
effective dose for AP Abdomen radiograph
0.5 mSv
effective dose for head CT
1.5 mSv
effective dose for chest CT
5 mSv
effective dose for abdomen/pelvic CT
6 mSv
very low vs low vs moderate vs high effective dose ranges
very low: < 0.1 mSv
low: 0.1 - 1 mSv (most radiographic exams)
moderate: 1 -10 mSv (GI studies, some CT exams)
highL > 10 mSv : interventional studies, some CT exams
uniquitous natural background radiation
includes cosmic, internal, and terrestrial activity
-~ 1 mSv/yr in US
effective dose from cosmic radiation
0.4 mSv/y
effective dose from internal primordial radionuclides (40K and 14C)
0.4 mSv/y
effective dose from terrestrial activity
0.3 mSv/yr
dose from transcontinental US flight
0.03 mSv
additional dose to air crews
5 mSv/yr
space travel dose
0.01 mSv/yr
what does radon emit
alpha particles
progeny of radon are radioactive and attach to aerosols that are inhaled and deposited in the lungs
-15% of lung cancers due to exposure to radon
average annual effective dose from radon in US
2 mSv/yr
depends on location
total yearly natural background effective dose US
3 mSv
population-averaged incidence of fatal cancer from radiation
4 %/Sv cancer incidence (fatal + non fatal) is 10%/Sv
nominal cancer detriment
5.5 %/Sv
total radiation detriment (cancer + hereditary effects)
6 %/Sv
what does effective dose not account for?
age sex
risk is higher for newborn than in retiree
mid organ radiation vs exit for abdomen
middle radiation about 10% of entrance Kair, , exit is 1 % of entrance Kair
are absorbed organ doses (mSv) always numerically equal to organ equivalent dose (mGy)?
yes
