• Determining the quality of radiation exposure or dose is termed dosimetry.
  • Dose is used to describe amount of energy absorbed per unit mass at site of interest.
  • Radiation absorbed dose (D).
  • Equivalent dose (HT).
  • Effective dose (E).
  • Collective effective dose or collective dose.
  • Dose rate.
  • Exposure or radioactivity.

This is the measure of amount of energy absorbed from the radiation beam per unit mass of tissue.

  • S I unit- Gray, (Gy) measured in joules/kg.
  • Sub unit- milligray, (mGy)(x10-3).
  • Original unit: rad, measured in ergs/g.
  • Conversion I Gy =100 rads.

It is used to compare the biologic effects of different types of radiation to a tissue or organ.

Equivalent dose= Radiation absorbed dose (D) X Radiation weighting factor (WR)

  • S I unit- Sievert (Sv).
  • Sub unit:
  1. millisievert (mSv) X10­-3
  2. microsievert (µSv)X 10-6
  • Original unit- rem.
  • Conversion 1 Sievert = 100rems.
  • The effective dose is used to estimate the risk in humans.
  • The international commission on radiological protection (ICRP) has allocated each tissue a numerical value known as tissue weighting factor based on its radio sensitivity.
  • Greater the risk, the higher tissue weighting factor.
  • Effective dose (E) = Equivalent dose (HT) X Tissue weighting factor(WR).
  • S I unit- Sievert (Sv).
  • Sub unit-millisievert (mSv).
  • This measure is used when considering the total effective dose to a population, from a particular investigation or a source of radiation.
  • Collective dose= effective dose(E) X Population
  • S I unit = man- sievert (man-sv)

Exposure is the measure of radiation quality, the capacity of radiation to ionized air.

  • S I unit= Air kerma.
  • Traditional unit: Roentgen(R).
  • Kerma measures the kinetic energy transferred from photons to electrons and is expressed in units of dose (Gy), where 1 Gy equals 1 joule/kg.
  • Kerma is the sum of the initial kinetic energies of all ionizing radiation (neutrons and photons) in a sample of matter, divided by the mass of the sample. 
  • It is the measurement of radioactivity (A) describes the decay rate of a sample of radioactive material.
  • S I unit= Becquerel (Bq)
  • The traditional unit is the curie (Ci).
  • 1Bq=2.7×10-11 Ci.
  • 1Ci=3.7×1010 Bq.
  1. Goaz PW, White SC Radiation physics. Book of oral radiology, principles and interpretation 2nd edition 1990;41-43.
  2. White SC, Pharaoh MJ Physics of ionizing radiation. Book of oral radiology principles and interpretations 5th edition 2009;20-21.
  3. Whaites E Dose units and dosimetry. Book of Essentials of dental radiography and radiology 3rd edition 2002;25-27.
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