CT, MRI and PET scans: understanding the difference

No single scan answers every cancer question. Clinicians select or combine these examinations to locate an abnormality, define its relationship to nearby structures, assess activity, plan treatment or compare changes over time.

CT uses X-rays to create cross-sectional anatomical images and can cover a large body area quickly. MRI uses a strong magnet and radio waves, making it useful when detailed contrast between soft tissues is needed. PET uses a small amount of radiotracer; areas of uptake are matched to anatomy, commonly with a CT acquired on the same scanner.

Increased tracer uptake is not automatically cancer because infection, inflammation and normal organs can also take up tracer. Likewise, a small lesion or a tumour with low tracer uptake may be less conspicuous. Reports therefore consider the scan pattern, clinical history and results from other tests, including pathology when available.

Follow the imaging centre's instructions because food, drink and medicine guidance differs by examination. Before PET, staff may give directions about eating, activity and diabetes medicines. Before contrast-enhanced CT or MRI, report kidney problems and previous contrast reactions.

Tell MRI staff about implants, metal fragments, medical devices, pregnancy and claustrophobia before entering the scan room. Bring prior images when requested and arrive with enough time for safety checks. Afterward, confirm when the report will reach the clinician who ordered it and whether another test is already planned.

During any scan, alert staff immediately if you develop breathing difficulty, throat tightness, facial swelling, faintness or severe pain. After leaving, emergency symptoms following an injection need urgent help. For the illness being investigated, seek prompt assessment for rapidly worsening symptoms such as severe breathlessness, new weakness, confusion, uncontrolled pain or repeated vomiting rather than waiting for the scan appointment.

Ask what clinical question the scan is intended to answer, which body areas will be covered and whether CT contrast, MRI contrast or a PET radiotracer will be used. Clarify how the new images will be compared with earlier scans and whether an uncertain finding would lead to repeat imaging, another imaging method or biopsy.