⚛️Intro to Applied Nuclear Physics Unit 10 – Nuclear Medicine & Medical Physics

Key Concepts and Principles

• Nuclear medicine utilizes radioactive isotopes for diagnostic imaging and therapeutic purposes
• Involves the application of physics principles to medical diagnosis, treatment, and research
• Radioactivity is the spontaneous emission of radiation from unstable atomic nuclei
• Half-life ($t_{1/2}$) represents the time required for half of a given quantity of a radioactive substance to decay
• Activity ($A$) measures the rate of radioactive decay, expressed in becquerels (Bq) or curies (Ci)
  • $1 \text{ Bq} = 1 \text{ decay/second}$
  • $1 \text{ Ci} = 3.7 \times 10^{10} \text{ Bq}$
• Absorbed dose quantifies the energy deposited by ionizing radiation per unit mass of matter, measured in grays (Gy)
• Linear energy transfer (LET) describes the energy deposition rate along the path of ionizing radiation

Radioactive Decay and Isotopes

• Radioactive decay occurs when an unstable atomic nucleus releases energy in the form of radiation
• Alpha decay involves the emission of an alpha particle (two protons and two neutrons)
• Beta decay involves the emission of a beta particle (electron or positron) and an antineutrino or neutrino
• Gamma decay involves the emission of high-energy photons (gamma rays) from an excited nuclear state
• Isotopes are variants of a chemical element with differing numbers of neutrons in their nuclei
• Radioisotopes are unstable isotopes that undergo radioactive decay, emitting radiation
• Common radioisotopes used in nuclear medicine include technetium-99m, iodine-131, and fluorine-18
  • Technetium-99m is widely used in bone scans and cardiac imaging
  • Iodine-131 is used for thyroid imaging and therapy
  • Fluorine-18 is used in positron emission tomography (PET) imaging

Radiation Detection and Measurement

• Radiation detectors convert the energy of ionizing radiation into measurable electrical signals
• Gas-filled detectors (ionization chambers, proportional counters, Geiger-Müller tubes) rely on the ionization of gas molecules by radiation
• Scintillation detectors use materials that emit light when exposed to ionizing radiation, which is then converted to electrical signals by photomultiplier tubes
• Semiconductor detectors (silicon, germanium) generate electron-hole pairs when exposed to radiation, producing an electrical signal
• Thermoluminescent dosimeters (TLDs) measure accumulated radiation dose using materials that emit light when heated after exposure to radiation
• Radiation spectrometry techniques (gamma spectroscopy, alpha spectroscopy) identify and quantify specific radionuclides based on their characteristic radiation energies
• Counting statistics (background counts, efficiency, dead time) must be considered when interpreting radiation measurements

Medical Imaging Techniques

• Radiography uses X-rays to produce two-dimensional images of internal structures
• Computed tomography (CT) generates cross-sectional images by rotating an X-ray source and detector array around the patient
• Single-photon emission computed tomography (SPECT) uses gamma-emitting radioisotopes to create three-dimensional images of functional processes
• Positron emission tomography (PET) detects coincident gamma rays from the annihilation of positrons emitted by radioisotopes, providing functional and metabolic information
• Magnetic resonance imaging (MRI) uses strong magnetic fields and radio waves to generate detailed images of soft tissues and organs
• Ultrasound imaging employs high-frequency sound waves to visualize internal structures in real-time
• Hybrid imaging techniques (PET/CT, SPECT/CT, PET/MRI) combine functional and anatomical information for improved diagnostic accuracy

Radiation Therapy Applications

• Radiation therapy uses ionizing radiation to destroy cancer cells and shrink tumors
• External beam radiation therapy (EBRT) delivers high-energy X-rays or particles from an external source
  • Linear accelerators (LINACs) generate high-energy X-rays or electrons for EBRT
  • Proton therapy uses proton beams to deliver precise radiation doses with reduced damage to surrounding tissues
• Brachytherapy involves the placement of radioactive sources directly inside or near the tumor
  • Intracavitary brachytherapy (gynecological cancers) and interstitial brachytherapy (prostate cancer) are common applications
• Radioisotope therapy uses targeted radionuclides to deliver therapeutic radiation doses to specific tissues or organs
  • Radioiodine therapy treats thyroid cancer and hyperthyroidism using iodine-131
  • Radium-223 is used for the treatment of bone metastases in prostate cancer patients
• Treatment planning systems optimize radiation dose distribution to maximize tumor coverage while minimizing damage to healthy tissues

Dosimetry and Safety Protocols

• Dosimetry is the measurement and calculation of radiation doses received by patients, workers, and the public
• Equivalent dose ($H_T$) accounts for the biological effectiveness of different types of radiation, measured in sieverts (Sv)
• Effective dose ($E$) represents the whole-body dose that would result in the same stochastic risk as the sum of equivalent doses to individual organs
• ALARA (As Low As Reasonably Achievable) principle guides radiation protection practices to minimize exposure
• Occupational dose limits for radiation workers are set by regulatory agencies to prevent deterministic effects and limit stochastic risks
• Personal protective equipment (lead aprons, thyroid shields, gloves) reduces radiation exposure to staff during procedures
• Shielding materials (lead, concrete, tungsten) attenuate radiation and protect personnel and the public from unnecessary exposure
• Quality assurance programs ensure the proper functioning and calibration of imaging and therapy equipment

Emerging Technologies and Future Trends

• Theranostics combines diagnostic imaging and targeted radionuclide therapy using the same molecular target
  • Peptide receptor radionuclide therapy (PRRT) targets somatostatin receptors in neuroendocrine tumors
  • Prostate-specific membrane antigen (PSMA) theranostics for prostate cancer
• Radiomics extracts quantitative features from medical images to aid in diagnosis, prognosis, and treatment response assessment
• Artificial intelligence and machine learning algorithms enhance image interpretation, segmentation, and treatment planning
• Nanoparticle-based radiopharmaceuticals enable targeted delivery of therapeutic radionuclides to specific tumor sites
• Miniaturization of imaging devices (portable SPECT, handheld ultrasound) improves accessibility and point-of-care diagnostics
• Radiogenomics investigates the relationship between imaging features and genomic data to personalize cancer treatment
• Adaptive radiation therapy adjusts treatment plans in response to anatomical changes during the course of therapy

Real-World Case Studies

• Myocardial perfusion imaging using technetium-99m sestamibi SPECT for the diagnosis of coronary artery disease
  • Evaluates blood flow to the heart muscle at rest and under stress
  • Identifies areas of reduced perfusion indicative of ischemia or infarction
• Radioiodine therapy for differentiated thyroid cancer
  • Administered orally as sodium iodide I-131
  • Concentrates in thyroid tissue, destroying residual cancer cells after thyroidectomy
• Yttrium-90 microsphere radioembolization for liver cancer
  • Microspheres loaded with yttrium-90 are injected into the hepatic artery
  • Selectively targets liver tumors while sparing healthy liver tissue
• Stereotactic radiosurgery (SRS) for brain metastases
  • Delivers high radiation doses to small, well-defined brain lesions
  • Achieves local tumor control while minimizing cognitive side effects
• PET/CT imaging with fluorine-18 fluorodeoxyglucose (FDG) for cancer staging and treatment response assessment
  • FDG uptake reflects increased glucose metabolism in malignant tumors
  • Monitors changes in tumor metabolic activity during and after therapy