PET-CT vs MRI: what each scan is used to find

The first question is not which scan is better but what needs answering. If the question is "what exactly does the tumour or injury look like and what surrounds it?", MRI is usually stronger. If it is "is there disease elsewhere in the body, or is this lump active?", PET-CT is more often the tool.

This is why a person can reasonably have both, and why asking for one in place of the other can miss the point of the pathway.

MRI places you in a strong magnetic field and uses radio pulses to make hydrogen atoms in water and fat emit signals. A computer converts these signals into detailed images of organs, brain, spinal cord, joints and soft tissue. Contrast agent can be given through a vein to highlight vessels and abnormal tissue.

PET-CT starts with an injection of a tracer, most commonly a form of glucose with a radioactive label (FDG), although other tracers are used for specific cancers such as prostate. Active cells take up more tracer. After an uptake period you lie in a scanner that detects the tracer and also performs a CT, so hot spots can be placed on a precise anatomical map.

Guidelines for individual cancers set out which imaging is used when. For example, prostate cancer pathways commonly use MRI of the prostate to locate and grade the tumour locally, and specialised PET tracers for suspected spread in selected cases. Lymphoma and many lung cancers use PET-CT for staging. Brain tumours rely mainly on MRI.

Outside cancer, PET-CT is used for some infection and inflammation questions and for some heart and brain assessments, while MRI is far more widely used for neurological, musculoskeletal and cardiac structure questions. Availability also matters, since PET-CT scanners are fewer in number and often need booking in advance.

• You need a detailed look at one region (brain, pelvis, spine, joint): MRI is normally the better match.

• You or your team need to know whether disease has spread, or whether a treated tumour is still active: PET-CT is normally more useful.

• You are anxious about radiation or cannot be exposed (pregnancy): MRI is preferred where it can answer the question.

• You cannot lie still or fit in the scanner: discuss sedation, open MRI or alternative imaging with the referring team.

• Your specialist has recommended one: ask why, and whether the other will also be needed.

PET-CT is not a general health screen. It is not a routine way to find hidden disease in people without symptoms, and a positive result in someone with no reason for suspicion often leads to further tests of uncertain value.

MRI is not weaker simply because it shows no function; in many organs it is more precise than PET-CT. And a "clear" PET-CT does not rule out all small deposits, because very small tumours can fall below its resolution.

• What specific decision will this scan inform?

• Do I need to stop eating, change medication or manage my diabetes before the scan?

• Will the scan be reported by a specialist radiologist or nuclear medicine doctor?

• If the result is uncertain, what is the next step?