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Basics of diagnostic nuclear medicine

In nuclear medicine a radioactive tracer is injected that accumulates according to function (glucose uptake, bone formation, bile excretion), and the radiation leaving the body is imaged — Tc-99m with a gamma camera and SPECT, and positron emitters with PET.

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Why it matters in the exam

The physical basis is asked often. Tc-99m emits 140 keV gamma rays, has a half-life of about 6 hours and is obtained from a Mo-99 generator — so it is the commonest tracer. In PET the isotope emits a positron that annihilates with an electron and creates two 511 keV photons in opposite directions; the scanner records them in coincidence. F-18 is the commonest isotope, with a half-life of about 110 minutes. FDG is a glucose analogue trapped in the cell after phosphorylation; the quantitative value is the SUV.

FDG pitfalls are a standard question. False positives: inflammation, infection, granulomatous disease (sarcoidosis, tuberculosis), brown fat, active muscle and after surgery or radiation. False negatives: a lesion smaller than 8 mm, low-grade tumour, mucinous tumour, lepidic lung adenocarcinoma, well-differentiated NET (where Ga-68 DOTATATE is used), well-differentiated HCC and most prostate cancers (where PSMA is used). High blood glucose lowers sensitivity, so fasting and glucose in range are required.

Common studies: bone scintigraphy with Tc-99m MDP shows osteoblastic activity — good for prostate and breast metastases, but myeloma is missed because the lesions are purely lytic, so in myeloma low-dose whole-body CT or MRI is used. HIDA scan for suspected acute cholecystitis: if the gallbladder does not fill within one hour (or after morphine) — the study is positive. A Meckel scan with Tc-99m pertechnetate detects ectopic gastric mucosa. MAG3 assesses renal obstruction with a diuretic, and DMSA shows cortical renal scars. A thyroid scan separates a "hot" nodule (almost always benign) from a "cold" one. Tc-99m MAA is used for lung perfusion scanning and also to calculate liver–lung shunting before Y-90 treatment. MIBG — neuroblastoma and phaeochromocytoma. In lactation: some diagnostic tracers need a short interruption, and I-131 needs complete cessation.

Key points

Tc-99m: 140 keV, about 6 hours.
PET: two 511 keV photons in coincidence.
FDG — inflammation positive, small lesions and well-differentiated NET negative.
Bone scan shows bone formation; myeloma is missed.
HIDA — gallbladder not filling = acute cholecystitis.

Summary table

Study	Tracer	What it shows	Pitfall
PET-CT	F-18 FDG	Glucose uptake	Inflammation positive; lesion below 8 mm negative
Bone scan	Tc-99m MDP	Osteoblastic activity	Myeloma is missed
HIDA	Tc-99m IDA analogue	Bile excretion and gallbladder filling	No filling within one hour = cholecystitis
Meckel scan	Tc-99m pertechnetate	Ectopic gastric mucosa	Intestinal bleeding in a child
Lung perfusion	Tc-99m MAA	Perfusion; shunt before Y-90	Also liver treatment planning

In the full version for subscribers

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Sources

EANM Procedure Guidelines for Tumour Imaging: FDG PET/CT 2.0, 2015 — Patient preparation and interpretation pitfalls
SNMMI Procedure Standard for Bone Scintigraphy, 2023 — Indications and interpretation
SNMMI Procedure Standard for Hepatobiliary Scintigraphy, 2021 — Acute cholecystitis protocol

Sources are given for further reading; the text is written afresh and not copied from them.

Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.