Radiation Quantities and Units

Reasons for Familiarity with Radiation Quantities and Units

• Need to develop standards for measuring and limiting radiation exposure.
• Awareness of potential harmful effects of ionizing radiation.
• Desire of the medical community to reduce radiation exposure throughout the world by developing standards for measuring and limiting this exposure.
• Diagnostic imaging personnel should be familiar with radiation quantities and units.

Historical Evolution of Radiation Quantities and Units

• Discovery of x-rays
  • November 8, 1895
  • Wilhelm Conrad Roentgen
  • University of Wurzburg, in Bavaria
  • Made using a Crookes tube
  • Paper coated with barium platinocyanide

First X-ray Image

• Mrs. Roentgen’s hand.

First American Radiation Fatality

• Clarence Madison Dally (1865–1904), the first American radiation fatality.
• Assistant to Thomas A. Edison, seen holding his hand over a box containing an x-ray tube while Edison examines the hand through a fluoroscope that he invented.

Somatic Damage

• Result of excessive occupational radiation exposure for early pioneers and excessive exposure of patients
  • Radiodermatitis
  • Cancer
  • Blood disorders

Investigation of Methods for Reducing Radiation Exposure

• Medical community was alarmed by the increasing number of radiation injuries reported
• Creation of the British X-ray and Radium Protection Committee in 1921
  • Purpose of the committee
  • Handicap faced by the committee
  • Outcome of fulfilling committee responsibilities

Skin Erythema Dose

• Unit used from 1900 to 1930 to measure radiation exposure
• Problems encountered in using the skin erythema dose as a way to measure radiation exposure
• Need to find a more reliable unit
• New unit selected to be based on some exactly measurable effect produced by radiation, such as ionization of atoms or energy absorbed in the irradiated object

Early Definition of Quantities and Units

• First International Congress of Radiology, London, England, 1925
• International Commission of Radiation Units and Measurements (ICRU) formed in 1925
• Second International Congress of Radiology, Stockholm, Sweden, 1928
  • Acceptance of the roentgen (R) as a unit of exposure
  • ICRU charged to define the R
  • Establishment of the International X-ray and Radium Protection Committee (predecessor of the ICRP)

Effects of Ionizing Radiation

• Early Tissue Reactions
  • Nausea
  • Fatigue
  • Diffuse redness of the skin
  • Loss of hair
  • Intestinal disorders
  • Fever
  • Blood disorders
  • Shedding of the outer layer of skin
• Late Tissue Reactions
  • Cataract formation
  • Fibrosis
  • Organ atrophy
  • Loss of parenchymal cells
  • Reduced fertility
  • Sterility
• Stochastic Effects
  • Cancer
  • Genetic (hereditary) effects

Keeping Occupational Radiation Exposure Below a Tolerance Dose

• Concept of tolerance dose (threshold dose)
  • Tolerance dose used for radiation protection purposes in 1930s
  • U.S. Advisory Committee on X-ray and Radium Protection is formed to formulate recommendations for radiation control
  • 0.2 R is recommended as a tolerance daily dose limit in 1934
  • 0.1 R is recommended as a tolerance daily dose limit in 1936
• A search for a more reliable unit to replace tolerance dose began as scientists started to recognize the late tissue reactions and stochastic effects of ionizing radiation and the possibility of genetic, or heritable, effects.
  • Focus on finding ways to minimize the risk of sustaining damage caused by radiation exposure
  • R becomes internationally accepted as the unit of measurement for exposure to x-radiation and gamma radiation in 1937.
• U.S. Advisory Committee of X-ray and Radium Protection becomes known as the National Council on Radiation Protection and Measurements.
• International system of units (SI) is developed.
• Early 1950s: maximum permissible dose (MPD) replaces tolerance dose for radiation protection purposes
• R is redefined in 1962 to increase accuracy and acceptability of this unit for radiation exposure.

The Modern Era of Radiation Protection

• Tolerance dose replaced by MPD in early 1950s.
• Dose limits were calculated and established in the 1970s to ensure that the risk from radiation exposure acquired on the job did not exceed risks encountered in “safe” occupations.
• ICRU adopted SI units for use with ionizing radiation in 1980.
• NCRP adopted SI units for use in 1985.
• ICRP adopted the term, effective dose, in 1991.

Overview of Radiation Quantities and Their Units of Measure Presently in Use

Traditional Nonmetric Units Gradually Becoming Obsolete

• Radiation quantities
  • Exposure
  • Absorbed dose (D)
  • Equivalent dose (EqD)
  • Effective dose (EfD)
• Units of measure
  • R
  • Rad
  • Rem
  • Rem

SI Units Presently in Use

• Radiation quantities
  • Exposure
  • Air kerma
  • Absorbed dose (D)
  • Equivalent dose (EqD)
  • Effective Dose (EfD)
• Radiation units
  • Coulombs per kilogram (C/kg)
  • Gray (Gy)
  • Centigray (cGy)
  • Milligray (mGy)
  • Sievert (Sv)

Exposure (X)

• Exposure is the total electric charge of one sign, either all plus or all minus, per unit mass that x- ray and gamma ray photons with energies up to 3 million electron volts (MeV) generated in dry (i.e., nonhumid) air at standard temperature ($22° C$) and pressure ($760 mm Hg$ or 1 atmosphere at sea level).
• It is a radiation quantity “that expresses the concentration of radiation delivered to a specific area, such as the surface of the human body.”

Precise Measurement of Radiation Exposure in Radiography

• Total amount of ionization (charge) an x-ray beam produces in a known mass of air must be obtained.
• This type of direct measurement is accomplished in an accredited dosimetry calibration laboratory by using a standard, or free-air, ionization chamber.
• Away from the laboratory much smaller and less complicated instruments are used.
• Units used away from the laboratory must be periodically recalibrated in a standardization laboratory against a free-air chamber.

Standard, or Free-Air, Ionization Chamber

• This device determines radiation exposure by measuring the amount of ionization (charge) an x-ray beam produces within its air collection volume.
• The instrument consists of a box containing a known quantity of air, two oppositely charged metal plates, and an electrometer, an instrument that measures the total amount of charge collected on the positively charged metal plate.
• The chamber measures the total amount of electrical charge of all the electrons produced during the ionization of a specific volume of air at standard atmospheric pressure and temperature.
• The electrical charge is measured in units called coulombs (C) (charge of an electron = –$1.6 × 10^{-19}$ C).
• A collected electrical charge of $2.58 × 10^{-4}$ C/kg of irradiated air constitutes an exposure of 1 roentgen (R).

C/kg

• Coulomb (C)
  • The basic unit of electric charge
  • It is equal to the “amount” of electrical charge moving past a point in a conductor in 1 second when an electric current amounting to 1 ampere is used.
• Ampere
  • The SI unit of electric current
• SI unit of measure for the radiation quantity, exposure, is equal to an electric charge of 1 C produced in a kilogram of dry air by ionizing radiation.

Air Kerma

• SI quantity used to express how energy is transferred from a beam of radiation to a material such as the patient’s skin.
• Gradually replacing the traditional quantity, exposure
• Denotes a calculation of radiation intensity in air
• Quantity that can be used to express x-ray tube output and inputs to image receptors
• Acronym for
  • Kinetic energy released in air
  • Kinetic energy released in material
  • Kinetic energy released per unit mass

Air Kerma (Cont.)

• Expressed in metric units of joule per kilogram (J/kg)
• May be stated in Gy
• When the Gy is used to indicate kinetic radiation energy deposited or absorbed in a mass of air, it is written as Gya.
• When the Gy is used to indicate kinetic radiation energy deposited or absorbed in a mass of tissue, it is written as Gyt.

Dose Area Product (DAP)

• Is the sum total of air kerma over the exposed area of the patient’s surface, or a measure of the amount of radiant energy that has been thrust into a portion of the patients body surface
• Is usually specified in units of mGy-cm2

Absorbed Dose (D)

• This quantity is the amount of energy per unit mass absorbed by an irradiated object.
• It is responsible for any biologic damage resulting from exposure of the tissues to radiation.
• Some structures in the body can absorb more radiant energy than others.

Absorbed Dose (D) (Cont.)

• The amount of energy absorbed by a structure depends on the
  • Atomic number (Z) of the tissue comprising the structure
  • Mass density of the tissue
  • Energy of the incident photon
• The SI unit of absorbed dose is the Gy.

Equivalence of Radiation-Produced Damage From Different Sources of Ionizing Radiation

• Equal absorbed doses of different types of radiation produce different amounts of biologic damage in body tissue.
• The concept of dose equivalence takes this biologic impact into consideration by using a specific modifying, or quality, factor to adjust the absorbed dose value.

Quality Factors for Different Types of lonizing Radiation

• X-ray photons: 1
• Beta particles: 1
• Gamma photons: 1
• Thermal neutrons: 5
• Fast neutrons: 20
• High-energy external protons: 1
• Low-energy internal protons
• Alpha particles: 20
• Multiple charged particles of unknown energy: 20
  • Protons produced as a result of neutrons interacting with the nuclei of tissue molecules.

EqD

• Is the product of the average absorbed dose in a tissue or organ in the human body and its associated WR chosen for the type and energy of the radiation in question.
• Used for radiation protection purposes when a person received exposure from various types of ionizing radiation
• For measuring biologic effects may be determined and expressed in Sv or in a subunit of the Sv
• $EqD = D × W<em>R$ and $Sv = Gy × W</em>R$

Radiation Weighting Factor ($W_R$)

• Must be used when determining EqD
• Is a dimensionless factor (a multiplier) used for radiation protection purposes to account for differences in biologic impact among various types of ionizing radiation
• Places risks associated with biologic effects on a common scale

Radiation Weighting Factors for Different Types and Energies of Ionizing Radiation

• X-ray and gamma ray photons and electrons (every energy): 1
• Neutrons
  • energy <10 keV: 5
  • 10 keV-100 keV: 10
  • >100 keV-2 MeV: 20
  • >2 MeV-20 MeV: 10
  • >20 MeV: 5
• Protons: 2
• Alpha particles: 20

EfD

• Provides a measure of the overall risk of exposure to humans from ionizing radiation
• “The sum of the weighted equivalent doses for all irradiated tissues or organs” (NCRP Report No. 116)
• Incorporates both the effect of the type of radiation used and the variability in radiosensitivity of the organ or body part irradiated through the use of appropriate weighting factors
  • These factors quantify the overall potential harm to those biologic components and the risk of developing a radiation- induced cancer or, for the reproductive organs, the risk of genetic damage.
• $EfD = D × W<em>R × W</em>T$

Tissue Weighting Factor ($W_T$)

• Takes into account the relative detriment to each specific organ and tissue
• Used in the calculation of EfD
• A value that denotes the percentage of the summed stochastic (cancer plus genetic) risk stemming from irradiation of tissue (T) to the all- inclusive risk, when the entire body is irradiated in a uniform fashion
• Accounts for the risk to the entire organism brought on by irradiation of individual tissues and organs

Collective EfD

• Used in radiation protection to describe internal and external dose measurements
• Quantity used to describe radiation exposure of a population or group from low doses of different sources of ionizing radiation
• Determined as the product of the average EfD for an individual belonging to the exposed population or group and the number of persons exposed
• Person-sievert is the radiation unit for this quantity.

Determining Collective Effective Dose Using the Radiation Unit Person-Sievert

• Example: If 200 people receive an average effective dose of 0.25 Sv, the collective effective dose (ColEfD) is $200 × 0.25 = 50$ person-sieverts.

Total Effective Dose Equivalent (TEDE)

• A radiation dosimetry quantity that was defined by the NRC to monitor and control human exposure to ionizing radiation
• Described by NRC regulations as “the sum of effective dose equivalent from external radiation exposure and a quantity called committed effective dose equivalent (CEDE) from internal radiation exposures.”