[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Radiology](https://durgical.com/en/radiology/) › [Nuclear medicine and interventional radiology](https://durgical.com/en/radiology/areas/nuclear/) › Nuclear medicine and PET: how they work # 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 Nov 13, 2026 ## Key points - Nuclear medicine images function; the tracer goes where a physiological process is active. - Gamma camera and SPECT detect gamma photons; PET detects pairs of photons from positron annihilation (coincidence). - FDG is a glucose analogue: hot in cancer but also in infection and inflammation. - PET/CT and SPECT/CT add CT for anatomy and attenuation correction. - Deauville scale in lymphoma: residual uptake compared with mediastinum and liver. ## 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. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/radiology/nuclear-medicine-and-pet-how-they-work/) ## Connections ## Related 5 - [How each imaging study makes a picture: X-ray, CT, ultrasound, MRI and nuclear medicine](https://durgical.com/en/radiology/how-each-imaging-study-makes-a-picture-x-ray-ct-ultrasound-mri-and-nuclear-medic/) - [Radiation in medical imaging: risk, ALARA and pregnancy](https://durgical.com/en/radiology/radiation-in-medical-imaging-risk-alara-and-pregnancy/) - [V/Q, bone and thyroid scans: when they are used](https://durgical.com/en/radiology/v-q-bone-and-thyroid-scans-when-they-are-used/) - [CT: windows, Hounsfield units and contrast phases](https://durgical.com/en/radiology/ct-windows-hounsfield-units-and-contrast-phases/) - [Choosing the right study: appropriateness and incidental findings](https://durgical.com/en/radiology/choosing-the-right-study-appropriateness-and-incidental-findings/) ## Related conditions 3 - [Hodgkin lymphoma](https://durgical.com/en/conditions/hodgkin-lymphoma/) - [Non-Hodgkin lymphoma](https://durgical.com/en/conditions/non-hodgkin-lymphoma/) - [Lung cancer](https://durgical.com/en/conditions/lung-cancer/) [← All of radiology](https://durgical.com/en/radiology/) 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](https://www.ncbi.nlm.nih.gov/books/NBK568731/) (StatPearls (NCBI Bookshelf), 2023); [Nuclear Medicine Instrumentation](https://www.ncbi.nlm.nih.gov/books/NBK597384/) (StatPearls (NCBI Bookshelf), 2023); [Nuclear Medicine PET/CT Lymphomas Assessment, Protocols, and Interpretation](https://www.ncbi.nlm.nih.gov/books/NBK585116/) (StatPearls (NCBI Bookshelf), 2026). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/radiology/nuclear-medicine-and-pet-how-they-work/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/radiology/nuclear-medicine-and-pet-how-they-work/