Diagnostic methods in medicine: (e) diagnosis using PET scanning.

The idea

PET scanning is used to diagnose and monitor conditions by imaging metabolic activity. A positron-emitting tracer such as fluorine-18 labelled fluorodeoxyglucose is injected; cells with high metabolic demand, including many cancer cells, take up more of it. The scanner records coincidence gamma pairs and reconstructs a three-dimensional map of tracer concentration. Areas of high uptake appear as hot spots and may indicate tumours, active inflammation or specific brain activity. Because PET shows function rather than fine anatomy, it is often combined with CT or MRI so that metabolic hot spots can be located precisely. Diagnosis relies on comparing the pattern and intensity of uptake with normal expectations, and on clinical context. The technique exposes the patient to ionising radiation, so benefit must be weighed against risk.

What earns the marks

• A positron-emitting radiopharmaceutical is administered and its uptake reflects metabolic activity.
• The PET scanner detects coincidence gamma pairs and reconstructs a three-dimensional distribution of tracer.
• Regions of abnormally high uptake, called hot spots, may indicate tumours, inflammation or active brain regions.
• PET provides functional information and is often combined with CT or MRI for anatomical localisation.
• Diagnosis involves comparing the uptake pattern with normal expectations and considering the clinical context and radiation risk.

Common mistakes and how to put them right

• Thinking PET gives sharp anatomical detail on its own.

  Put it right: It primarily shows metabolic function and is often fused with CT or MRI.

• Assuming all high uptake is cancer.

  Put it right: Normal organs such as the brain, heart and kidneys can also show high uptake.

• Ignoring the radiation dose to the patient.

  Put it right: PET uses ionising radiation and the risk must be justified.

• Confusing the role of the tracer with the scanner.

  Put it right: The tracer accumulates in tissue, while the scanner detects the emitted gamma photons.

Exam tips

• State the clinical question PET can answer: where is metabolic activity abnormally high or low?
• Use the term hot spot for high uptake and link it to possible pathology.
• Mention that PET is often combined with CT or MRI to provide anatomical reference.