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Nuclear medicine and PET: how they work

  • Modality

A tiny amount of a radioactive tracer goes into the body, collects where a process is active, and a camera outside the body pictures that activity.

Updated

What it is

Nuclear medicine images function rather than shape. A radiopharmaceutical (a radioactive atom attached to a carrier molecule) is injected, swallowed or inhaled; the carrier takes it to a tissue that uses that molecule, and the radiation it gives off is detected outside the body. In conventional scintigraphy the tracer emits gamma photons, most often technetium-99m, which a gamma camera records as flat (planar) images or, rotating around the patient, as cross-sectional SPECT. In PET the tracer emits positrons; each positron meets an electron and the two vanish into a pair of photons that fly off in opposite directions, and a ring of detectors records the pairs that arrive at the same time (coincidence). The commonest PET tracer is FDG, a glucose analogue taken up by cells that use much glucose, such as many cancers, active inflammation, brain and heart. Hybrid machines (SPECT/CT, PET/CT) add a CT in the same session to place each hot spot on the anatomy and to correct for tissue attenuation.

Key facts

  • Nuclear medicine shows physiology; CT and MRI show anatomy. Hybrid imaging combines both.
  • FDG uptake is not specific for cancer: infection, inflammation, healing tissue and brown fat also take it up.
  • Normal FDG uptake is seen in brain, heart (variable), kidneys, ureters and bladder (excretion), and often bowel and muscle.
  • The SUV (standardised uptake value) is a semi-quantitative measure of uptake; it varies with the scanner, timing and blood glucose.
  • In lymphoma, the Deauville five-point scale compares residual uptake with the mediastinal blood pool and the liver; scores 1 to 3 usually mean complete metabolic response.

How the image is made

  • Tracer: a radioactive isotope bound to a carrier that follows a physiological pathway (bone turnover, glucose use, thyroid iodine trapping, lung perfusion).
  • Gamma camera: a collimator lets through only photons travelling in one direction; a scintillation crystal turns them into flashes of light; photomultiplier tubes turn the light into an electrical signal and a position.
  • SPECT: the gamma camera rotates around the patient and a computer rebuilds slices, as CT does, from the gamma photons.
  • PET: positron and electron annihilate into two photons emitted about 180 degrees apart; detectors on opposite sides of the ring that fire together define a line along which the event happened.
  • Hybrid PET/CT and SPECT/CT: the CT gives anatomy and an attenuation map, so uptake is located and measured more accurately.
  • Technetium-99m has a half-life of about six hours and comes from an on-site generator; fluorine-18 (used in FDG) has a half-life of under two hours and is made in a cyclotron.

What it is good for

  • Bone scintigraphy: metastases, occult and stress fractures, osteomyelitis.
  • Ventilation-perfusion (V/Q) lung scan: pulmonary embolism when CT angiography is not suitable.
  • Thyroid scintigraphy: the cause of hyperthyroidism and whether a nodule is functioning («hot»).
  • FDG PET/CT: staging and treatment response of lymphoma and many solid cancers, fever of unknown origin, some infections and vasculitis.
  • Others: myocardial perfusion, renal function and drainage, hepatobiliary scan for acute cholecystitis, gastrointestinal bleeding, sentinel lymph node mapping.

Limitations and risks

  • Radiation: a bone scan or FDG PET/CT carries an effective dose in the range of one or several CT scans; PET/CT adds the radiation of its CT.
  • False positives on FDG PET: infection, granulomatous disease, recent surgery, radiotherapy or biopsy, and bone marrow stimulated by growth factors.
  • False negatives on FDG PET: small lesions, low-grade or mucinous tumours, some well-differentiated cancers, and high blood glucose at the time of injection.
  • Spatial resolution is lower than CT or MRI; small lesions near a site of intense normal uptake can be missed.
  • Pregnancy and breastfeeding need a check before any tracer: some studies are postponed, and breastfeeding may need a pause depending on the tracer.
  • Patients must arrive prepared: fasting and glucose control for FDG, stopping some medicines or iodine sources for thyroid studies, good hydration and frequent voiding for bone scans.

Next step

  • Write the clinical question and the diagnosis being staged or followed; nuclear medicine chooses the tracer and protocol from it.
  • For FDG PET/CT: fasting instructions, blood glucose control in diabetes, and the dates of the last chemotherapy, radiotherapy, surgery or growth-factor treatment.
  • Ask about pregnancy and breastfeeding in every woman of reproductive age before booking.
  • An unexpected hot spot: correlate with CT, MRI or biopsy before acting; discuss with the nuclear medicine physician.

Test yourself

Think of the answer, then tap the question to see it.

What does a PET scanner actually detect?

Pairs of photons emitted in opposite directions when a positron from the tracer annihilates with an electron; detectors that fire at the same time (coincidence) locate the event.

Why can an FDG PET scan be positive in a patient without cancer?

FDG is a glucose analogue taken up by any cell with high glucose use, including infection, inflammation, healing tissue and brown fat.

What does the CT part add to a PET/CT study?

It places each area of uptake on the anatomy and provides the attenuation correction, so uptake is located and measured more accurately.

Which preparation matters most before an FDG PET/CT and why?

Fasting and blood glucose control: high glucose competes with FDG and lowers uptake in tumours, causing false negatives.

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Connections

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Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.

Sources: Nuclear Medicine Physics (StatPearls (NCBI Bookshelf), 2023); Nuclear Medicine Instrumentation (StatPearls (NCBI Bookshelf), 2023); Nuclear Medicine PET/CT Lymphomas Assessment, Protocols, and Interpretation (StatPearls (NCBI Bookshelf), 2026).

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Representative model, for educational purposes only.