In nuclear medicine imaging, radiopharmaceuticals are taken internally, for example 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. ''Scintigraphy'' ("scint") is the use of internal radioisotopes to create two-dimensional images. ''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 diagnostic tests 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 or tissue specific (e.g.: lungs scan, heart scan, bone scan, brain scan, 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 scan 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 which 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.
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
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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