37. The fundamentals of nuclear medicine imaging. Types of radiation, applications.

From greek.doctor
  • Fundamentals of nuclear medicine
    • Nuclear medicine is functional not structural imaging
      • Measures the function of organ or tissue, not the structure
      • Can be combined with conventional imaging to produce functional + structural images
        • For example PET/CT
    • A patient is given a short-lived radioactive material
      • This is called a radioactive isotope, radiotracer or radionuclide
      • Half-life of minutes or hours
    • The radionuclide is attached to a carrier compound (a pharmaceutical) which concentrates in certain tissues
      • This combination is called a radiopharmaceutical
      • Examples of radiopharmaceuticals
        • F18-FDG = fluorine-18 + fluorodeoxyglucose (FDG)
          • Fluorine-18 is radioactive (emits positrons)
          • FDG concentrates in tumors
        • 99mTc-pertechnetate = technetium-99m + pertechnetate
          • Technetium-99m emits gamma rays
            • 99mTc is the most widely used radioisotope
          • Pertechnetate concentrates in thyroid, brain
        • Iodine-131
          • No carrier compound needed; the radioactive isotope I-131 concentrates in the thyroid
          • I-131 emits both gamma and beta rays
          • Can be used both for thyroid imaging and destruction of thyroid cancer
    • The radiation emitted by the radiopharmaceutical is detected by a so-called gamma camera
    • Most nuclear medicine scans have a resolution of about 1 cm
      • Meaning that they can’t detect lesions smaller than that
    • Most common types
      • Single photon emission computed tomography (SPECT)
        • Less expensive than PET but worse contrast and resolution
        • Uses gamma-emitting radioisotopes
          • Technetium-99m
          • Iodine-123
        • Especially used in
          • Myocardial perfusion imaging
          • Bone imaging
          • Functional brain imaging
      • Positron emission tomography (PET)
        • More expensive than SPECT but better contrast and resolution
        • Metabolic changes in the heart, brain and tumours can be examined
        • Uses positron-emitting radioisotopes
          • 18F-FDG (most frequent)
          • Radioisotopes used in PET have ultrashort half-lives
            • They must therefore be made at the site of the examination
        • Indications
          • Diagnosis and follow-up of cancer
          • Locate hidden metastases
          • Detect recurrence of cancer
          • Measure brain activity
        • Physics
          • The patient is given an isotope which releases positrons
          • Inside the body these positrons will meet with electrons. They will annihilate each other
          • The annihilation produces gamma photons which are detected
      • Bone scan
      • Ventilation/perfusion scan
      • Cardiac scans
      • Thyroid scans
  • (Important) types of radiation
    • Beta particles
      • High-energy, high-speed electrons or positrons
      • Deliver high radiation dose to the patient
        • A disadvantage for imaging
        • An advantage for tissue destruction
    • Gamma waves
      • Lower energy
      • Good for imaging
      • Similar to x-rays