Which imaging modality uses a radioactive-labeled substance to monitor physiological processes in the body?

The imaging modality that utilizes a radioactive-labeled substance to monitor physiological processes in the body is a PET scan, or Positron Emission Tomography scan. This technique involves the administration of a radiotracer, which is a form of glucose or other compounds tagged with a radioactive isotope. As these substances are metabolized by the body, they emit positrons, which collide with electrons and produce gamma rays. The PET scanner detects these gamma rays and uses them to create detailed images that reflect the functional activity of tissues and organs.

This capability is particularly beneficial in diagnosing conditions such as cancer, as it allows for the visualization of metabolic activity, which can indicate the presence of tumors even before structural changes occur. The sensitivity of PET scans in detecting changes at the cellular level sets it apart from other imaging techniques that primarily visualize anatomical structures rather than physiological processes.

In comparison, ultrasound imaging does not involve radioactive substances but instead uses sound waves to create images based on the reflection of sound. MRI (Magnetic Resonance Imaging) relies on magnetic fields and radio waves to produce high-resolution images of organs and tissues without ionizing radiation. CT scans (Computed Tomography) use X-rays to produce cross-sectional images of the body, also not involving radioactive tracers.