PET image formation depends on the physical process occurring when a positron emitted by the radionuclide comes to rest in tissue. What happens next, and what does the detector system register?

• A Two 511 keV photons emitted nearly 180 degrees apart, registered by electronic coincidence detection ✓
• B Single photon of variable energy released, registered by SPECT-type collimators
• C Auger electrons absorbed locally, producing heat measured by thermoluminescence
• D Internal conversion electron escaping the body and detected directly

Explanation

After travelling a short range, the positron annihilates with an electron, converting their combined mass into two photons of 511 keV each, emitted in almost exactly opposite directions. Ring detectors register pairs arriving within nanoseconds of each other (coincidence detection), and the line joining the two hits defines the annihilation site. This electronic collimation removes the need for lead collimators, giving PET its superior resolution compared with SPECT.

Reference: Christensen's Introduction to the Physics of Diagnostic Radiology, 4th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.