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