Final answer:
Positron Emission Tomography (PET) is the imaging technique that uses injected radiotracers emitting gamma rays to create images showing the physiologic activity of organs.
Step-by-step explanation:
The imaging technique that uses injected radiotracers which emit energy in the form of gamma rays is Positron Emission Tomography (PET). In a PET scan, a radiotracer is introduced into the body, typically by injection into a vein. This radiotracer is a substance that emits positrons, which encounter electrons and annihilate to produce two gamma-ray photons. The gamma rays are then detected by the PET scanner, which works with a computer to create images showing the active blood flow and physiologic activity of the targeted organs. PET scans are highly effective in diagnosing conditions like heart disease, cancer, brain abnormalities, and more because they illustrate metabolic processes, not just static structures.