Roentgen (R) [non-Standard International (SI) units]; as measured with ionization chamber in milliroentgen per hour (mR/hr)
Radiation intensity required to produce ionization charge of 0.000258 coulombs per kg of air (SI units)
Absorbed dose
Total amount of energy deposited in material
Any type of radiation, any radiation intensity, any material
Expressed as radiation absorbed dose (rad) (non-SI units), or gray (Gy) (SI units)
100 rads = 1 Gy; 1 rad = 10 mGy
1 rad = absorption of 100 ergs per g of tissue
In nuclear medicine, this depends on many factors
t1/2 of radiopharmaceutical
Biological t1/2 of radiopharmaceutical
Percentage taken up by body and differential uptake in organs
Type of radiation emitted by radiopharmaceutical
Absorbed dose for different nuclear medicine studies is estimated based on biodistribution studies in animals
Equivalent dose
Amount of radiation dose weighted based on damaging effects of type of radiation on tissue
Accounts for differences in types of radiation type and intensity
Based on relative biological effectiveness (RBE): Compares dose of standard radiation that yields biological response to dose of different radiation to produce similar biological response
e.g., alpha particle with mass and charge (weighting factor Q of 20) will do more damage than gamma photon without mass or charge (weighting factor Q of 1)
Measured in roentgen equivalent man (rem) (non-SI units), or sievert (Sv) (SI units)
100 rem = 1 Sv; 1 rem = 10 mSv
rem = rad x weighting factor (Q)
Effective dose
Equivalent dose while accounting for tissue/organ sensitivity and specific damage from radiation
Tissue-weighted sum of equivalent doses in all tissues/organs of body
e.g., tissue weighting factor (Wt) is high for breast tissue at 0.12 (12%) but low for brain tissue at 0.01 (1%); tissue weighting factors summed for all organs in body should add up to 1 (100%)
Measured in rem (non-SI units) or Sv (SI units)
Useful in estimating long-term biological risks to population from radiation exposure
Direct effects
Primarily from particulate and charged radiation (alpha, beta) where radiation directly damages cell
Indirect effects
Primarily from gamma and x-ray photons (no mass or charge) that primarily cause chemical changes from interactions with water and other molecules that lead to secondary chemical changes and indirect cell damage
CLINICAL IMPLICATIONS
Molecular Effects of Radiation
Deterministic Effects of Radiation
Stochastic Effects of Radiation
Dose-Response Models
Radiation Epidemiology
Internal Radiation Dosimetry
Selected References
NCRP Report No. 184: Medical Radiation Exposure of Patients in the United States, 2019
Hall E et al: Radiobiology for the Radiologist. 8th ed. Wolters Kluwer, 2018
NCRP Report No. 160: Ionizing Radiation Exposure of the Population of the United States, 2009
Cherry et al: Physics in Nuclear Medicine. 3rd ed. Saunders, 2003
Bolus NE: Basic review of radiation biology and terminology. J Nucl Med Technol. 29(2):67-73; test 76-7, 2001
Related Anatomy
Loading...
Related Differential Diagnoses
Loading...
References
Tables
Tables
KEY FACTS
Clinical Implications
TERMINOLOGY
Definitions
Radiation intensity
Roentgen (R) [non-Standard International (SI) units]; as measured with ionization chamber in milliroentgen per hour (mR/hr)
Radiation intensity required to produce ionization charge of 0.000258 coulombs per kg of air (SI units)
Absorbed dose
Total amount of energy deposited in material
Any type of radiation, any radiation intensity, any material
Expressed as radiation absorbed dose (rad) (non-SI units), or gray (Gy) (SI units)
100 rads = 1 Gy; 1 rad = 10 mGy
1 rad = absorption of 100 ergs per g of tissue
In nuclear medicine, this depends on many factors
t1/2 of radiopharmaceutical
Biological t1/2 of radiopharmaceutical
Percentage taken up by body and differential uptake in organs
Type of radiation emitted by radiopharmaceutical
Absorbed dose for different nuclear medicine studies is estimated based on biodistribution studies in animals
Equivalent dose
Amount of radiation dose weighted based on damaging effects of type of radiation on tissue
Accounts for differences in types of radiation type and intensity
Based on relative biological effectiveness (RBE): Compares dose of standard radiation that yields biological response to dose of different radiation to produce similar biological response
e.g., alpha particle with mass and charge (weighting factor Q of 20) will do more damage than gamma photon without mass or charge (weighting factor Q of 1)
Measured in roentgen equivalent man (rem) (non-SI units), or sievert (Sv) (SI units)
100 rem = 1 Sv; 1 rem = 10 mSv
rem = rad x weighting factor (Q)
Effective dose
Equivalent dose while accounting for tissue/organ sensitivity and specific damage from radiation
Tissue-weighted sum of equivalent doses in all tissues/organs of body
e.g., tissue weighting factor (Wt) is high for breast tissue at 0.12 (12%) but low for brain tissue at 0.01 (1%); tissue weighting factors summed for all organs in body should add up to 1 (100%)
Measured in rem (non-SI units) or Sv (SI units)
Useful in estimating long-term biological risks to population from radiation exposure
Direct effects
Primarily from particulate and charged radiation (alpha, beta) where radiation directly damages cell
Indirect effects
Primarily from gamma and x-ray photons (no mass or charge) that primarily cause chemical changes from interactions with water and other molecules that lead to secondary chemical changes and indirect cell damage
CLINICAL IMPLICATIONS
Molecular Effects of Radiation
Deterministic Effects of Radiation
Stochastic Effects of Radiation
Dose-Response Models
Radiation Epidemiology
Internal Radiation Dosimetry
Selected References
NCRP Report No. 184: Medical Radiation Exposure of Patients in the United States, 2019
Hall E et al: Radiobiology for the Radiologist. 8th ed. Wolters Kluwer, 2018
NCRP Report No. 160: Ionizing Radiation Exposure of the Population of the United States, 2009
Cherry et al: Physics in Nuclear Medicine. 3rd ed. Saunders, 2003
Bolus NE: Basic review of radiation biology and terminology. J Nucl Med Technol. 29(2):67-73; test 76-7, 2001
STATdx includes over 200,000 searchable images, including x-ray, CT, MR, and ultrasound images. To access all images, please log in or subscribe.