CT scan and MRI machines: what patients should know

A CT scanner uses X-rays and usually has a short, open ring. An MRI scanner uses a strong magnet and radio waves and commonly has a longer tunnel. The requested examination depends on the body area and the clinical question.

During CT, the table moves through the scanner's ring while an X-ray source rotates around you. CT can show bones, lungs, bleeding and many organs clearly. MRI collects signals from tissues inside a magnetic field; it can give particularly detailed views of the brain, spinal cord, joints and many soft tissues.

Both examinations require you to lie still. MRI is noisy, so hearing protection is provided, and the enclosed space may feel uncomfortable. Some examinations use a contrast agent to make particular tissues or blood vessels easier to distinguish. CT and MRI contrast agents are not interchangeable.

Follow the imaging centre's instructions about food, drink and regular medicines. Tell the team about pregnancy or possible pregnancy, kidney problems, previous contrast reactions, and any difficulty lying flat. For MRI, report a pacemaker, implanted device, metal fragment, surgical clip or medicine patch before entering the scan room.

Remove metal items when instructed. If enclosed spaces make you anxious, contact the centre before the appointment; options may include additional explanation, positioning support or prescribed medicine. Arrange transport if you are told that a sedating medicine will be used.

Tell staff immediately if you develop trouble breathing, swelling, widespread rash, severe dizziness or sudden pain during contrast administration. After leaving, seek urgent medical care for breathing difficulty or swelling of the face or throat. For a mild rash, vomiting or worsening discomfort after contrast, contact the imaging service promptly for advice.

Ask which scanner is being used, what question the images should answer, whether contrast is planned, and whether an implant needs clearance. You can also ask how results will be shared and who will explain any next step.