Contrast medium

In diagnostic imaging, contrast media are "substances used to allow enhanced visualization of tissues."

Contrast media include radiocontrast agents used for x-rays as well as other agents not based on the transmission of X-rays through the body, such as gadolinium for magnetic resonance imaging, and preparations that circulate microbubbles through the blood for contrast with ultrasonography.

Not contrast agents per se, but yet other forms of medical imaging, such as single photon emission computed tomography (SPECT) and positron emission tomography (PET), are also introduced into the patient's body, but are sources of radiation rather than radiopaque materials to external radiation]].