Radioactivity and Nuclear Medicine

Radioactivity

• Radioactivity is the property of some naturally occurring unstable elements whose nuclei disintegrate, emitting alpha (α), beta (β), or gamma (γ) rays.
• The curie (Ci) is a unit of radioactivity, equivalent to 3.7 x 10¹⁰ disintegrations per second.
• The SI unit of radioactivity is the Becquerel (Bq), defined as one disintegration per second.
• Radioactive decay follows a fixed rate, often described by the half-life (t_1/2). This is the time taken for half of the radioactive nuclei to decay.
• Alpha particles are helium nuclei, positively charged, and have low penetration.
• Beta particles are negatively charged and possess higher penetration than alpha particles.
• Gamma rays are high-energy photons, similar to X-rays but with higher energy, and highly penetrating.
• Isotopes are nuclei of an element with different numbers of neutrons.
• Radioisotopes are radioactive isotopes of an element.

Nuclear Medicine

• Nuclear medicine involves administering small amounts of radioactive materials (radionuclides) for diagnostic purposes, providing information on various diseases.
• Radiopharmaceuticals are radionuclides used in nuclear medicine, emitting either gamma rays or positrons during decay.
• Positrons have short half-lives, requiring on-site accelerators.
• Tc-99m is a commonly used photon emitter, optimal for metabolic studies, with a 6-hour half-life.
• F-18 is a positron emitter, with a 110-minute half-life, used for specific studies.

Nuclear Medicine Imaging

• Nuclear medicine imaging techniques are categorized into:
  • Single Photon Emission Computed Tomography (SPECT)
  • Positron Emission Tomography (PET)
• Single photon emission imaging uses radionuclides that decay by gamma ray emission, creating a two-dimensional image using a gamma camera.
• Positron imaging uses radionuclides that decay by positron emission, producing two high-energy photons detected by a camera, forming three-dimensional Positron Emission Tomography (PET) images.

Radiotherapy

• Roentgen: A unit of exposure used for X-rays and gamma rays based on ionization in air.
• Rad: A unit of absorbed dose in radiation, quantifying energy absorption in matter.
• Gray (Gy): The SI unit of absorbed dose, equivalent to 100 rad.
• Rem: The unit of dose equivalent, considering radiation's biological effect. It's dose multiplied by quality factor.
• Principles of radiation therapy: Aim to damage tumors while minimizing harm to healthy tissues, often by directing multiple beams at the target.
• Ionizing radiation: Causes ionization of atoms by removing electrons, affecting molecules.
• Biological effect: The relative biological effect (RBE) is the ratio of doses of different types of radiation to achieve the same biological effect in tissues. Quality factor (QF) relates to RBE.
• LD50: The dose of radiation that is lethal to 50% of a population.

Radiotherapy Procedures

• External beam radiotherapy: Uses a radiation source outside the body, aiming radiation at the target from different angles.
• Brachytherapy (short distance treatment): Uses small, encapsulated radioactive sources placed close to or inside the tumour.

Radiation Protection

• Radiation therapy areas normally have thick concrete walls to shield against intense radiation.
• Treatment areas have door switches to automatically shut down the machine if a door is opened.
• Radiation sources need shielding.
• Measures to minimize staff radiation exposure: minimize time, maximize distance, use shielding.