Identify a non-invasive technique used in radiopharmacy.

Positron Emission Tomography (PET) is a non-invasive imaging technique that allows for the visualization of metabolic processes in the body. It utilizes radiotracers that emit positrons, which collide with electrons in the body, producing gamma rays that are detected by the PET scanner. This imaging method is particularly valuable in diagnosing and monitoring various medical conditions, including cancer, as it provides insight into the biological function of tissues and organs, rather than just anatomical structure.

One of the key advantages of PET is its ability to detect changes at the cellular level, making it an essential tool in both research and clinical settings. The non-invasive nature of PET means that it typically does not require any surgical procedures or in-depth physical interventions, which can increase patient comfort and safety during diagnostic assessments.

Other options listed, while all important imaging modalities, may not fit the context of the question regarding non-invasive techniques as clearly as PET does. Magnetic resonance imaging (MRI) and computed tomography (CT) are also non-invasive, but PET is distinct in its focus on metabolic activity. X-ray imaging is primarily used for structural visualization and involves exposure to ionizing radiation, yet it may not fully capture the functional dynamics of tissues like PET does.