Nuclear Medicine Basics
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Direct answer
Nuclear medicine imaging records the physiological distribution of injected, inhaled or ingested radiopharmaceuticals — a radionuclide tagged to a targeting molecule — using the gamma emissions detected by a gamma camera (planar or SPECT), or positron annihilation photons in PET. Technetium-99m is the workhorse radionuclide: a six-hour half-life, a 140 keV gamma photon ideal for cameras, and versatile labelling as pertechnetate, MDP, sestamibi, macroaggregated albumin and more. Unlike anatomical imaging, nuclear studies show function — for example bone turnover on a bone scan, superior to radiographs for detecting metastases, stress fractures and osteomyelitis earlier — which is the conceptual axis NEET-PG tests.
What you must remember
- Technetium-99m: half-life about 6 hours, gamma emission of 140 keV, eluted from a molybdenum-99 generator; ideal physics and cheap availability make it the most used radionuclide.
- Radiopharmaceutical = radionuclide + pharmaceutical carrier; the carrier determines organ uptake, the isotope provides the signal.
- Gamma camera (Anger camera): sodium iodide crystal coupled to photomultiplier tubes converts gamma rays to images; SPECT adds rotational acquisition for tomographic slices; PET detects 511 keV annihilation photons from positron emitters such as fluorine-18.
- Bone scan: Tc-99m MDP; whole-body survey for skeletal metastases (breast, prostate, lung), shows stress fractures and osteomyelitis earlier than radiographs; uptake reflects osteoblastic activity.
- Thyroid: Tc-99m pertechnetate or iodine-123 imaging classifies nodules — hot (autonomous, rarely malignant) versus cold (needs further evaluation); radioiodine uptake studies separate Graves disease (high diffuse uptake) from thyroiditis (low uptake); I-131 is therapy for hyperthyroidism and differentiated thyroid cancer, contraindicated in pregnancy.
- Meckel scan: Tc-99m pertechnetate taken up by ectopic gastric mucosa — the non-invasive test of choice for a Meckel diverticulum bleeding in children.
- Renal: DTPA and MAG3 studies assess split renal function and obstruction with diuretic challenge; MUGA measures ejection fraction; V/Q scans detect mismatched perfusion defects in pulmonary embolism.
- Sentinel lymph node mapping with Tc-99m colloid guides breast cancer and melanoma surgery; gallium-67 and FDG-PET image inflammation and tumour metabolism.
- Dosimetry note: children and pregnancy require careful justification; breastfeeding interruption is advised after certain radiopharmaceuticals, including radioiodine.
Common confusion
Students blur nuclear imaging with X-ray imaging: radiographs and CT send radiation through the patient from an external source, whereas nuclear medicine installs the source inside the patient and records where it localises — physiological rather than purely anatomical information. A second confusion is uptake pattern versus diagnosis: a bone scan detects osteoblastic response, so purely lytic lesions (myeloma) may be missed, and a superscan of metastases can be mistaken for a normal high-quality study if bladder and renal activity are not checked.
Exam-focused takeaway
Expect facts about Tc-99m (half-life six hours, 140 keV), the composition of a radiopharmaceutical, gamma camera versus PET detection, and named scans: MDP bone scan indications, Meckel scan tracer and rationale, hot versus cold thyroid nodules, and the lambda-panda gallium signs of sarcoidosis. Radioiodine contraindications in pregnancy and breastfeeding are favourite one-liners.
Frequently asked questions
Why is technetium-99m the most used radionuclide?
Its six-hour half-life limits dose, the 140 keV gamma energy suits gamma cameras, it has no beta emission, and it is conveniently eluted daily from a molybdenum generator while labelling to many carriers.
What is the difference between SPECT and PET?
SPECT uses gamma-emitting tracers with cameras rotating around the patient, while PET detects the two 511 keV photons produced when positron emitters such as fluorine-18 annihilate, giving higher sensitivity and quantification at higher cost.
What is a Meckel scan?
A Tc-99m pertechnetate study visualising ectopic gastric mucosa within a Meckel diverticulum, the non-invasive investigation of choice for unexplained lower gastrointestinal bleeding in children.
How does a bone scan detect disease earlier than radiographs?
Tc-99m MDP localises to osteoblastic turnover, which rises before 30-50% of bone mineral is lost — the threshold for radiographic visibility — enabling earlier detection of metastases, stress fractures and osteomyelitis.
What do hot and cold nodules mean on a thyroid scan?
Hot nodules concentrate tracer, suppress surrounding thyroid and are almost never malignant, whereas cold nodules show a photon-deficient area and require ultrasound with fine-needle aspiration to exclude carcinoma.