Single-photon emission computed tomography (SPECT) is a nuclear medicine imaging technique using gamma rays. It may be used with any gamma-emitting isotope, including Tc. In the use of technetium-99m, the radioisotope is administered to the patient and the escaping gamma rays are incident upon a moving gamma camera which computes and processes the image. To acquire SPECT images, the gamma camera is rotated around the patient. Projections are acquired at defined points during the rotation, typically every three to six degrees. In most cases, a full 360° rotation is used to obtain an optimal reconstruction. The time taken to obtain each projection is also variable, but 15–20 seconds are typical. This gives a total scan time of 15–20 minutes.
The technetium-99m radioisotope is used predominantly in bone and brain scans. For bone scans, the pertechnetate ion is used directly, as it is taken up by osteoblasts attempting to heal a skeletal injury, or (in some cases) as a reaction of these cells to a tumor (either primary or metastatic) in the bone. In brain scanning, Tc is attached to the chelating agent HMPAO to create technetium (Tc) exametazime, an agent which localizes in the brain according to region blood flow, making it useful for the detection of stroke and dementing illnesses that decrease regional brain flow and metabolism.
Most recently, technetium-99m scintigraphy has been combined with CT coregistration technology to produce SPECT/CT scans. These employ the same radioligands and have the same uses as SPECT scanning, but are able to provide even finer 3-D localization of high-uptake tissues, in cases where finer resolution is needed. An example is the sestamibi parathyroid scan which is performed using the Tc radioligand sestamibi, and can be done in either SPECT or SPECT/CT machines.