Chapter 4: Radiation Quantities and Units

Chapter 4: Radiation Quantities and Units

A Brief History Review

Discovery

• November 8, 1895- German Physicist Wilhelm Conrad Roentgen discovered and used the first x-rays
• Clarence Madison Dally- the first American killed as a result of radiation-induced cancer
• Thomas Edision discontinued x-ray research
• From 1900 to 1930- the unit for measuring radiation exposure was called the skin erythema dose (the quantity of radiation causing diffuse redness after irradiation)

Definitions of Vocab

• Somatic damage- biologic damage to the body caused by ionizing radiation exposure
• Occupational exposure- exposure received by radiation workers while working (radiodermatitis)
• Tolerance dose- a radiation dose to which occupationally exposed persons could be continuously subjected without any apparent harmful effects.
• Threshold dose- dose of radiation lower than which an individual has a negligible chance of sustaining specific biologic damage.
• Roentgen- the original term for the quantitiy of "exposure"
• Late deterministic somatic effects/ late stochastic effects- side effects appear months to years after exposure
• can cause Genetic or Heritable effects
• effective dose (EfD) - based on energy depositied in biologic tissue by radiation
• accounts for type of radiation and sensitivity of tissues
• Sieverts (Sv)- SI unit of EfD
• Gray (Gy)- unit of absorbed dose

Early Definitions of quantities and units

• The International Commision on Radiation Units and Measurements (ICRU) was formed in 1925
• In 1928, a Second International Congress of Radiology was held in Sweden
• The early determinsistic somatic effects that appeared within minutes, hours, days, or weeks of the time of radiation exposure were believed to be preventable if doses to radiation workers were limited.
• by the early 1950s, maximum permissible dose (MPD) replaced the tolerance dose
• tolerance dose no longer accepted
• no amount of radiation is completely safe

Radiation Quantities and their Units of Measure

Absorbed dose

Air Kerma

• amount of energy per unit mass absorbed by an irradiated object
• the amount of energy depends on the anatomic structure of tissues
• The effective atomic number (Zeff) of a given biologic tissue is a "composite" or weighted average

• SI quantity used to express radiation concentration transferred to a point
• tissue kerma- the kinetic energy released in a unit mass of tissue
• "kinetic energy released in matter", "kinetic energy released in material", and "kinetic energy released per unit mass" all use the word "kerma"
• dose area product (DAP)- entire amount of energy delivered to the patient by the x-ray beam

Effective Dose

Exposure (X)

• provides a measure of the overall risk of exposure to humans from ionizing radiation
• incorporates both the effect of the type of radiation used and the variability in radio sensitivity of the specific organ or body part irradiated through the use of appropriate weighting factors
• The tissue weighting factor is a conceptual measure for the relative risk associated with irradiation of different body tissues

• - total electrical charge of one sign per nit mass that x-ray and gamma ray photons with energies
• exposure is based on a response produced when radiation interacts with a medium
• Coulomb (C)- basic unit of electrical charge
• exposure measured in Coulombs per kilogram (C/kg)

• Diagnostic imaging professionals must understand these quantities:
• Exposure (X)
• Air kerma
• Absorbed dose (D)
• Equivalent dose (EqD)
• Effective dose (EfD)

Equivalent Dose

• product of the average absorbed dose in a tissue or organ in the human body
• Stochastic effects are non threshold, randomly occurring biologic effects of ionizing radiation such as cancer and genetic abnormalities
• used for radiation protection purposes when a person receives exposure from various types of ionizing radiation
• determined and expressed in sieverts
• Equivalent dose is obtained by multiplying the absorbed dose (D) by the radiation weighting factor (Wr)
• EqD = D x Wr corresponds to Sv= Gy x Wr