Introduction
This topic covers contrast agents used in diagnostic imaging: what they are, how they are grouped, when they are given, when they must be avoided, and which side reactions the radiologist and the clinical team must recognise and treat. Knowledge of the agent groups links directly to CT, MRI and ultrasound, which are covered in the preceding topics.
Definition and classification
A contrast agent is a substance given to increase the difference in signal between a target tissue and its surroundings. Agents are classified by the imaging method they serve and by their chemical and physical properties, mainly osmolality and whether they are ionic. Osmolality matters because high-osmolality agents cause more side effects.
Iodinated agents are the main group for X-ray and CT. Gadolinium agents are paramagnetic and used intravenously in MRI. Ultrasound agents are microbubbles that enhance the blood-pool echo. Negative oral agents such as barium sulfate are not absorbed, while water-soluble iodinated agents can be used orally when there is a risk of aspiration or leakage.