Final answer:
Positron Emission Tomography (PET) is the imaging technique that uses radioactive materials for diagnostics, offering high-contrast images of organ function, differing from MRI, which uses magnetic fields and radio waves without radiation exposure.
Step-by-step explanation:
The medical imaging technique that involves injecting a small amount of radioactive material into the body and detecting gamma rays emitted from the positron-emitting radionuclides is known as Positron Emission Tomography (PET scan), not Computed Tomography (CT) as mistakenly identified in the question. This technique utilizes radiopharmaceuticals to create images of active blood flow and physiological activity of organs. Unlike MRI, which generates images using a magnetic field and radio waves and does not involve exposure to ionizing radiation, PET scans do involve exposure to a low dose of radiation for diagnostic purposes.
Diagnostic medical applications of PET scans are critical, as they provide details on both the structure and function of various body organs which can be crucial in the detection and treatment of diseases. PET is a type of nuclear medicine imaging that provides high contrast images, while CT and SPECT (Single-photon-emission computed tomography) are also nuclear imaging techniques that form three-dimensional images with good contrast though with relatively lower spatial resolution.