Nuclear medicine is a medical speciality that uses radioactive substances, called radiopharmaceuticals, for diagnosis, functional imaging and treatment of disease. Unlike conventional anatomical imaging, which primarily shows structures such as bones, organs and blood vessels, nuclear medicine can reveal physiological processes by following where a radioactive tracer travels and how it behaves inside the body. A radiopharmaceutical normally consists of a radionuclide attached to a biologically active molecule chosen because it participates in or accumulates in a particular physiological process.
The radionuclide provides a detectable radioactive signal or therapeutic radiation, while the molecular component determines biological distribution. Diagnostic nuclear medicine commonly uses gamma emitting radionuclides for single photon emission computed tomography and positron emitting radionuclides for positron emission tomography. Therapeutic nuclear medicine uses radionuclides that deliver radiation to diseased tissue, including beta or alpha particle emitters in appropriately selected treatments. The central concept is therefore molecular or physiological targeting rather than simply anatomical photography.
In diagnostic nuclear medicine, a small amount of a radiopharmaceutical is administered by injection, ingestion, inhalation or another appropriate route. The substance then distributes according to its chemical and biological properties. External detectors measure radiation escaping from the body, and computer systems reconstruct the measurements into images or quantitative maps. In planar scintigraphy, a gamma camera detects gamma photons and forms two dimensional images.
In SPECT, multiple gamma camera views acquired around the patient are reconstructed into three dimensional information about tracer distribution. In PET, a positron emitting radionuclide produces a positron that eventually annihilates with an electron, generating two approximately oppositely directed photons. PET scanners detect coincident photons and reconstruct their likely line of origin. Nuclear medicine images can therefore show processes such as glucose metabolism, blood flow, bone turnover, receptor expression and organ function. Hybrid systems such as PET CT and SPECT CT combine functional information with anatomical localisation, while PET MRI combines molecular imaging with magnetic resonance information.
Nuclear medicine depends on carefully selected radionuclides with suitable physical and biological properties. Technetium 99m is one of the most widely used diagnostic radionuclides because it emits useful gamma radiation, has a relatively short physical half life of about six hours and can be incorporated into numerous radiopharmaceuticals. Fluorine 18 is particularly important in PET and is used in fluorodeoxyglucose, commonly abbreviated FDG, which resembles glucose and can reveal differences in glucose metabolism.
Other radionuclides include iodine 123 for certain thyroid imaging applications and gallium 68 for several PET radiopharmaceuticals. Therapeutic radionuclides include iodine 131, lutetium 177 and yttrium 90, among others. The choice of radionuclide involves balancing radiation type, energy, physical half life, chemical behaviour, biological half life and the intended clinical purpose. A radionuclide that is excellent for imaging may be unsuitable for therapy because diagnostic imaging seeks enough detectable radiation with minimal unnecessary dose, whereas therapy seeks sufficient radiation deposition in diseased tissue to damage target cells.
Therapeutic nuclear medicine uses radioactive decay deliberately to damage pathological tissue. The basic principle is targeted radionuclide therapy, in which a radiopharmaceutical preferentially accumulates in a tumour, organ or other biological target and delivers ionising radiation locally. Radioiodine therapy is a classic example because iodine is naturally concentrated by thyroid tissue. Iodine 131 emits beta particles capable of delivering therapeutic dose to thyroid cells and also emits gamma radiation that can permit imaging and monitoring.
More recently, radioligand therapies have used molecules that bind specific receptors or antigens expressed by tumours. Lutetium 177 labelled compounds can deliver beta radiation to targeted cells, with the emitted radiation potentially affecting nearby tumour cells as well. Alpha particle therapy uses high linear energy transfer radiation with very short ranges, potentially producing highly concentrated damage near the radionuclide. Therapeutic nuclear medicine therefore depends heavily on molecular biology and target expression. Before some treatments, diagnostic imaging with a related radiopharmaceutical is used to establish whether sufficient target is present for therapy, creating a diagnostic and therapeutic pairing sometimes described as theranostics.
Nuclear medicine has major applications in oncology, cardiology, endocrinology, neurology, nephrology, infection imaging and skeletal medicine. In oncology, PET can help identify metabolically active disease, stage malignancy, assess treatment response and investigate suspected recurrence, although the interpretation depends on the tracer and clinical context. In cardiology, myocardial perfusion imaging can evaluate blood flow to heart muscle under stress and rest conditions.
Bone scintigraphy can detect areas of altered bone turnover and has historically been important for identifying skeletal metastases and other bone abnormalities. Thyroid nuclear medicine can assess gland function and distinguish different patterns of hormone production. Renal nuclear medicine can provide information about renal perfusion, filtration and drainage. Neurological PET tracers can investigate glucose metabolism, amyloid deposition or other molecular characteristics. These examples demonstrate the defining strength of nuclear medicine: it can provide information about function and molecular activity that may change before obvious anatomical abnormalities become apparent.
Nuclear medicine involves radiation exposure, but the administered activity is carefully selected to achieve the required diagnostic or therapeutic objective while managing radiation risk. Diagnostic radiopharmaceuticals are generally administered at activities designed to produce adequate image quality with an appropriately low radiation dose. Many diagnostic radionuclides have relatively short physical half lives, and biological clearance can further reduce exposure after the tracer leaves the body.
Patients receiving radiopharmaceuticals may temporarily emit radiation and may excrete radioactive material, so particular instructions can sometimes be required depending on the radionuclide and treatment. Therapeutic administrations can involve much higher activities and consequently require more substantial radiation protection arrangements. Radiation safety also covers staff, carers and members of the public. The risk assessment depends on radionuclide, activity, patient physiology, treatment type and duration of exposure. Nuclear medicine therefore uses radioactive materials under controlled conditions rather than treating radioactivity as inherently incompatible with medical care.
The development of nuclear medicine arose from the convergence of nuclear physics, chemistry, physiology, detector technology and medicine during the twentieth century. Early medical uses of radioisotopes demonstrated that radioactive tracers could follow biological pathways, while advances in scintillation detectors and computer reconstruction enabled increasingly sophisticated imaging. The development of the gamma camera greatly expanded diagnostic imaging, and SPECT provided three dimensional functional information. PET later transformed molecular imaging by enabling quantitative investigation of biochemical processes using positron emitting tracers.
Cyclotrons and radionuclide generators have supported the production and availability of short lived radionuclides. Modern nuclear medicine increasingly integrates imaging with computational reconstruction, attenuation correction, quantitative analysis and personalised treatment planning. Radiopharmaceutical development has also moved beyond broad physiological tracers towards molecules targeting receptors, transporters, enzymes and disease associated proteins. Consequently, nuclear medicine has evolved from relatively simple tracer studies into a field capable of interrogating complex molecular biology in living patients.
Nuclear medicine is distinct from radiology even though the two fields overlap and hybrid imaging combines them. Radiology generally uses externally generated X rays, magnetic fields, radiofrequency waves or ultrasound to interrogate the body, whereas nuclear medicine introduces a radioactive substance into the patient and detects radiation emitted from within the body. This difference produces different strengths and limitations. Nuclear medicine can provide highly specific functional information but often has lower spatial resolution than some purely anatomical imaging methods.
CT provides excellent anatomical detail and can detect structural abnormalities rapidly, while PET or SPECT can reveal metabolic or physiological abnormalities that may not be obvious anatomically. Hybrid systems address this complementarity by registering functional and anatomical information. Nuclear medicine therefore should not be understood simply as taking pictures with radiation. It is a discipline based on tracer kinetics, radiochemistry, nuclear decay, detector physics, image reconstruction, biological targeting, radiation dosimetry and clinical interpretation, with both diagnostic and therapeutic roles.
