Nuclear Medicine Scanners

• Nuclear medicine scanners are only detectors that pick up energy emitted from radioisotopes given to patients either orally or intravenously.
• No radiation is emitted from the scanner.
• Scanners count the number of photons and convert the emissions into images.
• In x-ray or CT imaging, radiation is externally generated, passes through the patient, and is captured by the detector on the other side.
• In nuclear imaging, the radioactive isotopes are ingested or injected, and subsequent radiation is internally generated and externally detected as radioactive decay occurs.

 

Example of gamma camera

• Positron Emission Topography (PET) scanners are different from gamma cameras.
• They detect positron emitting isotopes.
• Positron = beta particle with positive charge.
• Positron is emitted from the nucleus and after a short distance will collide with an electron, causing an "annihilation" event.
• Annihilation creates two photons traveling in opposite directions which the PET scanner detects and converts into an image.
• PET detector has to detect both photons traveling in opposite directions at  nearly the same time for it to be counted.
• PET scanners also include a CT scanner, which can be used to create "fused" images.

Annihilation Event

1. Positron is emitted from the nucleus.
2. Electron and Positron collide, and an annihilation event occurs.
3. Two identical photons are generated, traveling in opposite directions.
4. Two signals are captured by the detector ring simultaneously.