Nuclear medicine (nuclear radiology) is a medical specialty involving the application of radioactive substances in the diagnosis and treatment of disease. Nuclear imaging is, in a sense, radiology done inside out, because it records radiation emitted from within the body rather than radiation that is transmitted through the body from external sources like X-ray generators. In addition, nuclear medicine scans differ from radiology, as the emphasis is not on imaging anatomy, but on the function. For this reason, it is called a physiological imaging modality. Single photon emission computed tomography (SPECT) and positron emission tomography (PET) scans are the two most common imaging modalities in nuclear medicine. Nuclear medicine procedures are the major clinical applications of molecular imaging and molecular therapy.
Diagnostic medical imaging
Diagnostic In nuclear medicine imaging, radiopharmaceuticals are taken internally, for example, through inhalation, intravenously, or orally. Then, external detectors (gamma cameras) capture and form images from the radiation emitted by the radiopharmaceuticals. This process is unlike a diagnostic X-ray, where external radiation is passed through the body to form an image. There are several techniques of diagnostic nuclear medicine.
2D: Scintigraphy ("scint") is the use of internal radionuclides to create two-dimensional images.
3D: SPECT is a 3D tomographic technique that uses gamma camera data from many projections and can be reconstructed in different planes. Positron emission tomography (PET) uses coincidence detection to image functional processes.
Nuclear medicine tests differ from most other imaging modalities in that nuclear medicine scans primarily show the physiological function of the system being investigated as opposed to traditional anatomical imaging such as CT or MRI. Nuclear medicine imaging studies are generally more organ-, tissue- or disease-specific (e.g.: lungs scan, heart scan, bone scan, brain scan, tumor, infection, Parkinson etc.) than those in conventional radiology imaging, which focus on a particular section of the body (e.g.: chest X-ray, abdomen/pelvis CT scan, head CT scan, etc.). In addition, there are nuclear medicine studies that allow imaging of the whole body based on certain cellular receptors or functions. Examples are whole body PET scans or PET/CT scans, gallium scans, indium white blood cell scans, MIBG and octreotide scans.
While the ability of nuclear metabolism to image disease processes from differences in metabolism is unsurpassed, it is not unique. Certain techniques such as fMRI image tissues (particularly cerebral tissues) by blood flow and thus show metabolism. Also, contrast-enhancement techniques in both CT and MRI show regions of tissue that are handling pharmaceuticals differently, due to an inflammatory process. Diagnostic tests in nuclear medicine exploit the way that the body handles substances differently when there is disease or pathology present. The radionuclide introduced into the body is often chemically bound to a complex that acts characteristically within the body; this is commonly known as a tracer. In the presence of disease, a tracer will often be distributed around the body and/or processed differently. For example, the ligand methylene-diphosphonate (MDP) can be preferentially taken up by bone. By chemically attaching technetium-99m to MDP, radioactivity can be transported and attached to bone via the hydroxyapatite for imaging. Any increased physiological function, such as due to a fracture in the bone, will usually mean increased concentration of the tracer. This often results in the appearance of a "hot spot", which is a focal increase in radio accumulation or a general increase in radio accumulation throughout the physiological system. Some disease processes result in the exclusion of a tracer, resulting in the appearance of a "cold spot". Many tracer complexes have been developed to image or treat many different organs, glands, and physiological processes.
Procedures Some of the most common clinical nuclear medicine procedures are: glucose metabolic imaging with F-fluorodeoxyglucose (FDG) for cancer, myocardial perfusion imaging for coronary artery disease, and skeletal imaging for both benign and malignant bone disease. Other non clinical, common procedures they perform include: brain perfusion and glucose metabolic imaging for seizure and dementia, blood pool imaging for myocardial function and gastrointestinal bleeding, gastric emptying studies for gastroparesis, hepatobiliary imaging for acute cholecystitis and gallbladder dysfunction, lymphoscintigraphy for sentinel lymph node biopsy, parathyroid imaging for hyperparathyroidism, pulmonary perfusion and ventilation imaging for pulmonary embolism, renal function imaging for various renal disorders, thyroid imaging for hyperthyroidism, thyroid whole body imaging for thyroid cancer, urinary tract imaging for vesicoureteral reflux, and white blood cell studies for infection. Less commonly performed procedures include: octreotide (pentetreotide) or NETSPOT (gallium 68) imaging for somatostatin receptors found on the surface of many tumors, meta-iodobenzylguanidine (MIBG) imaging for neuroendocrine tumors, heat-damaged red blood cell imaging for identifying ectopic splenic tissue, and gastric mucosa imaging for Meckel's diverticulum (especially in pediatric medicine).
Hybrid scanning techniques In some centers, the nuclear medicine scans can be superimposed, using software or hybrid cameras, on images from modalities such as CT or MRI to highlight the part of the body in which the radiopharmaceutical is concentrated. This practice is often referred to as image fusion or co-registration, for example SPECT/CT and PET/CT. The fusion imaging technique in nuclear medicine provides information about the anatomy and function, which would otherwise be unavailable or would require a more invasive procedure or surgery.
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