Types of Radiation

Ionizing and Non-Ionizing Radiation

Radiation is classified based on its ability to ionize atoms and molecules. This distinction is crucial in medical physics and radiation therapy.

• Ionizing radiation: Has enough energy to remove tightly bound electrons from atoms, creating ions. Examples include x-rays, gamma rays, alpha particles, beta particles, and neutrons. Ionizing radiation is used in diagnostic imaging and cancer therapy but can cause serious biological damage at high doses.

• Non-ionizing radiation: Does not have sufficient energy to ionize atoms. Examples include radio waves, microwaves, infrared, and visible light. Non-ionizing radiation typically causes thermal damage (e.g., burns).

Example: Ultraviolet light is on the border between non-ionizing and ionizing radiation; higher-energy UV can cause ionization and DNA damage.

Passage of Radiation Through Matter; Radiation Damage

Ionization and Biological Effects

When ionizing radiation passes through matter, it can ionize atoms and molecules, leading to chemical changes and biological damage. This is the basis for both the risks and therapeutic uses of radiation.

• Types of ionizing radiation: Alpha, beta, gamma rays; x-rays; protons, neutrons, pions.

• Damage mechanisms: Ionization can break chemical bonds, damage DNA, and denature proteins, potentially leading to cancer or cell death.

Example: Absorbed dose to the lens of the eye from a brain CT scan is measured in grays (Gy) or rads.

Measurement of Radiation—Dosimetry

Source Activity

Source activity quantifies the rate of radioactive decay, which is essential for calculating exposure and dose.

• Definition: Number of disintegrations per second.

• Units: Curie (Ci) and Becquerel (Bq)

Absorbed Dose

Absorbed dose measures the energy deposited by radiation in a material, typically biological tissue.

• Units: Rad and Gray (Gy)

Relative Biological Effectiveness (RBE)

RBE compares the biological damage caused by different types of radiation, relative to a standard (usually x-rays).

• Alpha particles have the highest RBE, causing the most biological damage per unit dose.

• Effective dose is calculated by multiplying the absorbed dose by the RBE.

Effective Dose

Effective dose adjusts the absorbed dose for both radiation type and organ sensitivity, providing a measure of overall risk.

• Units: Sievert (Sv)

• Used to set regulatory limits and assess long-term health risks.

Acute Radiation Syndromes (ARS)

Conditions and Stages

ARS occurs after exposure to high doses of ionizing radiation over a short period. The severity depends on dose, exposure time, and whether the radiation is external and penetrating.

• Threshold: Usually >0.7 Gy (70 rad)

• Stages: Prodromal, Latent, Manifest Illness, Recovery/Death

Classic ARS Syndromes

• Bone marrow syndrome: Occurs at 0.7–10 Gy; affects blood cell production.

• Gastrointestinal syndrome: Occurs at >10 Gy; damages GI tract, often fatal.

• Cardiovascular/CNS syndrome: Occurs at >50 Gy; rapid death due to neurological and cardiovascular failure.

Radiation Therapy

Principles and Application

Radiation therapy uses ionizing radiation to destroy cancer cells while minimizing damage to healthy tissue. Techniques include rotating the radiation source to distribute exposure.

• Goal: Maximize tumor dose, minimize healthy tissue dose.

• Methods: External beam, brachytherapy, rotational techniques.

Example: Rotating the source during treatment to target the tumor from multiple angles.

Tracers in Research and Medicine

Radioactive Isotopes for Diagnosis

Radioactive tracers are used for non-invasive diagnostic scans, detecting abnormal concentrations in tissues.

• Detection: Gamma-ray detectors measure emitted radiation.

• Applications: Tumor detection, organ function studies.

Emission Tomography: PET and SPECT

Principles of PET and SPECT

Both techniques use radioactive tracers to create images of physiological processes.

• PET (Positron Emission Tomography): Uses positron-emitting tracers; detects gamma rays from positron annihilation. Commonly used for cancer detection and monitoring.

• SPECT (Single Photon Emission Computed Tomography): Uses gamma-emitting tracers; detects gamma rays directly. Primarily used for heart disease diagnosis.

Comparison: PET provides higher resolution and sensitivity for metabolic activity; SPECT is more widely available and less expensive.

Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI)

Physical Principles

NMR and MRI are based on the behavior of nuclear spins in a magnetic field. Protons (hydrogen nuclei) align with or against the field, creating energy differences that can be probed with radiofrequency radiation.

• Spin states: Parallel or antiparallel to the magnetic field.

• Energy splitting: Proportional to field strength.

• Resonance: Absorption of RF radiation causes spin-flip transitions.

Example: MRI excites hydrogen nuclei in water and fat, producing detailed images of soft tissues.

Imaging Techniques

• MRI: Uses gradients in the magnetic field to localize signals and create images.

• NMR Spectroscopy: Provides molecular structure information based on chemical environment.

Summary Table: Key Units in Radiation Dosimetry

Additional info:

• Background radiation is typically 2–3 mSv per year.

• Occupational exposure limits for radiation workers are set at 50 mSv/year; for the general public, 0.5 mSv/year.

• Acute radiation syndrome requires a large, external, penetrating dose delivered in a short time.