Introduction To Radiologic Technology Practice Test

Emission-based imaging relies on radiopharmaceuticals that decay inside the body and emit detectable radiation. Detectors capture these emitted photons (such as gamma rays or positron-annihilation photons) and use them to form images that reflect how a tracer distributes and behaves in tissues. This approach provides functional information about physiology, metabolism, and receptor activity, not just structure. For example, gamma cameras and SPECT detect gamma rays from Tc-99m–labeled compounds to image perfusion and function, while PET detects pairs of photons from positron emitters like F-18 to show metabolic processes. In contrast, ultrasound uses reflected sound waves, MRI uses signals from hydrogen nuclei in a magnetic field, and CT relies on X-ray transmission through tissues.