Radiation Effects and Radiation Safety

Radiation Effects and Radiation Safety

Radiation Units and Measurement

  • Two Systems of Measurement:
    • Traditional (British)
    • Systeme Internationale (SI)

SI Units vs Traditional Units

  • Exposure Units:

    • SI: Coulombs per kilogram (C/kg)
    • Traditional: Roentgen (R)
    • 1 R = 2.58 × 10⁻⁴ C/kg
    • 1 R produces 2.08 × 10⁹ ion pairs in 1 cm³ of air

  • Dose Units:

    • SI: Gray (Gy)
    • 1 Gy = 100 rad
    • Traditional: Radiation absorbed dose (rad)

  • Dose Equivalent Units:

    • SI: Sievert (Sv)
    • Sv = Gy × WR
    • 1 Sv = 100 rem
    • Traditional: Roentgen equivalent in man (rem)

Radiation Weighting Factors

  • Radiation Types and Corresponding Weighting Factors:
    • Photons: 1
    • Electrons and muons: 1
    • Protons and charged pions: 2
    • Alpha particles, fission fragments, heavy ions: 20
    • Neutrons: Energy-dependent function

Biological Effects of Radiation Exposure

Laws of Bergonié and Tribondeau

  • Cell sensitivity to radiation depends on:
    • Age: Younger cells are more sensitive.
    • Differentiation: Nonspecialized cells are more sensitive than specialized ones.
    • Metabolic rate: Rapidly metabolizing cells are more sensitive.
    • Mitotic rate: Cells that divide quickly are more sensitive.

Short-term Somatic Effects

  • Occur within 3 months of exposure
  • Associated with doses > 50 cGy
  • Categorized by affected systems: CNS, GI, hematological
  • Deterministic effects: Occur only after a specific threshold of exposure; severity is dose-dependent.

Long-term Effects

  • Latent effects; may take up to 30 years to appear.
  • Somatic Effects: Directly affect the irradiated individual.
  • Genetic Effects: Damage to reproductive cells, possibly affecting descendants.

Stochastic Effects

  • No threshold for occurrence; likelihood increases with dosage.
  • No correlation between dosage and severity.
  • Can occur from repeated small doses (e.g., in radiography).

Radiation Safety

Personnel Safety

  • Radiographers as “occupationally exposed” must avoid direct exposure to primary x-ray beam.

Principal Methods of Protection

  • Time: Minimize exposure time
  • Distance: Maximize distance from radiation source
  • Shielding: Always use shielding materials.

Personnel Monitoring

  • Dosimeters: Devices that monitor radiation exposure, worn with label facing outward at the collar.

Effective Dose Limits

  • Philosophy ALARA: As Low As Reasonably Achievable
    • Effective dose limit for occupational exposure: 50 mSv/year
    • Cumulative lifetime dose limit = 1 rem (10 mSv) x age of worker.
    • Specific EfD limits may apply to certain organs.

Patient Protection Principles

  • Minimize patient dose by:
    • Avoiding errors: double-check requisitions and IDs.
    • Avoiding repeats of unsatisfactory images.
    • Collimating to the smallest radiation field possible.
    • Using highest acceptable kVp to minimize mAs.
    • Maintaining at least 40 inches SID to limit exposure.
    • Providing appropriate shielding for sensitive organs (gonads, eyes, etc).

Gonad Shielding

  • Lead shields (0.5-mm lead) prevent unnecessary radiation to reproductive organs.
  • Required when:
    • Patient is of reproductive age or younger
    • Gonads are in primary radiation field
    • Shield does not interfere with the examination

New Scientific Research on Gonadal Shielding

  • Shielding not recommended for abdomen and pelvis radiography due to lack of protection from internal scatter.
  • Optimal safety achieved through collimation and correct technical factors.

Radiation and Pregnancy

  • Highest risk to fetus from radiation exposure during the first trimester.
  • Occupational limit for pregnant workers: 50 mSv whole-body radiation over pregnancy duration.
  • Female patients of childbearing age should be informed of potential radiation risks before exams.