12.7: Imaging Studies II: Positron Emission Tomography and Scintigraphy

Positron Emission Tomography (PET) is a medical imaging technique that provides crucial insights into the body's physiological functions at a molecular level. It is an indispensable resource for diagnosing, staging, and monitoring various illnesses, notably cancer, neurological disorders, and cardiovascular conditions.

Fundamental Principles of PET

1. Radioactive Tracer: PET involves using biologically active molecules labeled with radioactive isotopes, known as tracers or radiotracers. The primary tracer is fluorodeoxyglucose (FDG), a glucose analog labeled with the radioactive isotope Fluorine-18.
2. Emission of Positrons: After administration into the body, these tracers undergo radioactive decay, emitting positrons. The emitted positrons travel a short distance within the body and interact with electrons, resulting in annihilation. This annihilation produces two gamma photons that travel in opposite directions.
3. Detection of Gamma Rays: The annihilation event generates a pair of gamma photons. The PET scanner, a ring-shaped machine that encircles the patient, detects these gamma photons.
4. Image Reconstruction: The scanner's detectors capture the emitted gamma photons, and a computer processes this data to construct detailed, three-dimensional images of the tracer distribution within the body.

Scintigraphy, also known as nuclear scintigraphy or gamma scintigraphy, is an imaging technique used primarily for gastrointestinal imaging. It employs the principles of nuclear medicine to assess gastrointestinal function and identify pathology. Scintigraphy provides functional information about various organs and systems in the body, making it particularly valuable in diagnosing and monitoring multiple diseases and conditions.

Fundamental Principles of Scintigraphy

1. Radioactive Tracers: Scintigraphy involves administering radiopharmaceuticals, which are substances tagged with a radioactive isotope. These tracers emit gamma radiation, which is detectable from outside the body.
2. Radioactive Isotope Labeling: Specific blood cells, like red blood cells or leukocytes, are labeled with a radioactive isotope. For instance, Technetium-99m (Tc-99m), a radioactive isotope, is favored for its short half-life (about 6 hours), reducing radiation exposure. Tc-99 m pairs well with different compounds to examine various gastrointestinal areas. For example, Tc-99m-labeled sulfur colloid is used for liver-spleen scans, and Tc-99m-labeled red blood cells are used for gastrointestinal bleeding studies.
3. Blood Sampling and Re-Injection: A sample of the patient's blood is mixed with the radioactive substance and then reinjected into the bloodstream.
4. Monitoring Distribution: The distribution of labeled blood cells is monitored over periods from 24 to 48 hours.
5. Gamma Camera Detection: Gamma cameras capture gamma radiation. These devices consist of a detector head with a scintillation crystal, which converts gamma rays into light, and photomultiplier tubes, which transform and amplify the light signals into electrical signals.
6. Image Formation: The gamma camera moves around the patient, capturing images from different angles. A computer then processes these signals to form two-dimensional images representing the radioactive tracer distribution within the body.