What to know about CT scans and MRI scans

CT is often useful when speed matters or when the chest, lungs, bones, or abdomen must be assessed. MRI can provide clearer separation between many soft tissues, including the brain, spinal cord, muscles, and pelvic organs. The appropriate scan depends on the body area, the clinical question, previous images, and whether contrast is suitable.

A CT scanner takes X-ray measurements from different angles and a computer combines them into slice-like views. An MRI scanner changes magnetic fields and detects signals from tissues; it does not use ionising radiation. CT examinations are generally shorter and the machine is more open around the body. MRI examinations commonly take longer, make loud knocking sounds, and require you to remain inside the scanner bore. Either test may be performed without contrast or with a contrast agent chosen for that imaging method.

Follow the imaging centre's instructions about food, drink, medicines, and arrival time. Before either scan, mention previous reactions to contrast, kidney problems, pregnancy or possible pregnancy, and difficulty lying flat. Before MRI, disclose a pacemaker, implanted device, metal fragment, surgical clip, hearing implant, or medicine patch because staff must confirm magnetic safety. Remove metal items when instructed. Tell the team beforehand if enclosed spaces cause distress; they can discuss positioning, reassurance, or prescribed medicine and whether another scanner design is appropriate.

After contrast, seek urgent medical help for trouble breathing, swelling of the face or throat, fainting, or a rapidly worsening widespread rash. Tell imaging staff immediately about burning, marked pain, or increasing swelling around an injection site. Symptoms that prompted imaging can also require emergency assessment: sudden weakness on one side, new difficulty speaking, a seizure, severe chest pain, or sudden breathlessness should not wait for a scheduled scan or routine result discussion.

Ask what specific question the scan is intended to answer, whether earlier images are available for comparison, and whether contrast is planned. You can also ask how the result will influence the next decision, who will explain the report, and what to do if you cannot remain still or complete the examination. If both CT and MRI are requested, ask what distinct information each is expected to provide.