What Is Nuclear Medicine?

Nuclear medicine is a medical specialty that uses radioactive materials, called radiotracers or radiopharmaceuticals, to diagnose and treat disease. Unlike conventional imaging modalities that record anatomical structure, nuclear medicine imaging captures physiological function: how organs metabolize glucose, how blood flows through the heart, how receptors are distributed in the brain. A small quantity of a radiotracer is administered to a patient, typically by intravenous injection, and the gamma-ray emissions from its decay are detected outside the body to produce functional images of internal organs and tissues.

The field draws on nuclear physics for its understanding of radioactive decay, on chemistry for synthesizing labeled molecules that target specific biological pathways, on detector engineering for building the gamma cameras and tomographic scanners that capture the emissions, and on medical physics for ensuring accurate dosimetry and image quality. The two dominant imaging modalities are single-photon emission computed tomography (SPECT) and positron emission tomography (PET).

Radiotracers and Radiopharmaceuticals

A radiotracer consists of a biologically active molecule labeled with a radioactive isotope. The molecule determines where in the body the tracer accumulates; the isotope determines what kind of radiation is emitted and how quickly the tracer decays. SPECT relies on gamma-emitting isotopes, most commonly technetium-99m (99mTc), which has a six-hour half-life and emits 140 keV gamma rays, as well as iodine-123, indium-111, thallium-201, and gallium-67. PET uses positron-emitting isotopes including fluorine-18 (the basis of FDG, fluorodeoxyglucose), carbon-11, nitrogen-13, and oxygen-15. When a positron annihilates with a nearby electron, it produces two 511 keV gamma rays traveling in opposite directions, which PET scanners detect in coincidence to localize the annihilation event with higher spatial resolution than SPECT.

Research published in PMC on radiopharmaceuticals for PET and SPECT documents how the development of new labeled compounds over the past decade has expanded nuclear medicine's reach into targeted cancer therapy, neurodegeneration, and infection imaging, with theranostic agents that use one isotope for diagnosis and a paired isotope for treatment becoming a major area of clinical interest.

Gamma-Ray Imaging Instrumentation

The gamma camera, introduced by Hal Anger in 1958, remains the hardware foundation of SPECT. It consists of a lead collimator that restricts the direction of accepted photons, a sodium iodide (NaI) scintillator crystal, an array of photomultiplier tubes that localize where light flashes occur, and pulse-processing electronics that convert detected events into position and energy estimates. Energy resolution, the ability to discriminate the primary gamma rays of interest from scattered photons of lower energy, is a key performance parameter: a 99mTc study uses a 15–20 percent energy window centered on 140 keV, and poor energy resolution increases scatter contamination and degrades image contrast.

PET scanners replace the collimator with electronic coincidence timing, allowing much higher detection efficiency. Modern scanners use lutetium oxyorthosilicate (LSO) or similar fast scintillator crystals and silicon photomultipliers, achieving coincidence timing resolutions below 200 picoseconds. The NIBIB resource on nuclear medicine describes these imaging technologies and their diagnostic roles across clinical specialties.

Hybrid scanners that combine PET or SPECT with CT or MRI in a single examination are now standard in major medical centers, allowing anatomical structure and functional signal to be registered precisely.

Applications

Nuclear medicine has applications in a wide range of clinical and research fields, including:

• Oncology staging, treatment planning, and therapy response monitoring using FDG-PET
• Cardiology assessment of myocardial perfusion and viability
• Neurology imaging of dopamine transporter density in Parkinson's disease and amyloid plaques in Alzheimer's disease
• Thyroid disease diagnosis and radioiodine therapy for hyperthyroidism and thyroid cancer
• Bone scanning for metastatic disease and skeletal infection
• Theranostic cancer treatment using isotope pairs such as lutetium-177 and actinium-225

For engineering literature on scanner design and reconstruction algorithms, IEEE Xplore's Nuclear Science and Medical Imaging conference proceedings represent the primary research archive.