Editing Single-photon emission computed tomography (section)

==Principles==
{{Unreferenced section|date=January 2014}}
[[File:TEMP-Siemens.jpg|thumb|A Siemens brand SPECT scanner, consisting of two gamma cameras]]
Instead of just "taking a picture of anatomical structures", a SPECT scan monitors level of biological activity at each place in the 3-D region analyzed. Emissions from the radionuclide indicate amounts of blood flow in the capillaries of the imaged regions.  In the same way that a plain [[Radiography|X-ray]] is a 2-dimensional (2-D) view of a 3-dimensional structure, the image obtained by a [[gamma camera]] is a 2-D view of 3-D distribution of a [[radionuclide]].

SPECT imaging is performed by using a gamma camera to acquire multiple 2-D images (also called [[graphical projection|projections]]), from multiple angles. A computer is then used to apply a [[tomographic reconstruction]] algorithm to the multiple projections, yielding a 3-D data set. This data set may then be manipulated to show thin slices along any chosen axis of the body, similar to those obtained from other tomographic techniques, such as [[magnetic resonance imaging]] (MRI), [[X-ray computed tomography]] (X-ray CT), and [[positron emission tomography]] (PET).

SPECT is similar to PET in its use of radioactive tracer material and detection of gamma rays. In contrast with PET, the tracers used in SPECT emit gamma radiation that is measured directly, whereas PET tracers emit positrons that annihilate with electrons up to a few millimeters away, causing two gamma photons to be emitted in opposite directions. A PET scanner detects these emissions "coincident" in time, which provides more radiation event localization information and, thus, higher spatial resolution images than SPECT (which has about 1&nbsp;cm resolution). SPECT scans are significantly less expensive than PET scans, in part because they are able to use longer-lived and more easily obtained radioisotopes than PET.

Because SPECT acquisition is very similar to planar gamma camera imaging, the same [[Radiopharmacology|radiopharmaceuticals]] may be used. If a patient is examined in another type of nuclear medicine scan, but the images are non-diagnostic, it may be possible to proceed straight to SPECT by moving the patient to a SPECT instrument, or even by simply reconfiguring the camera for SPECT image acquisition while the patient remains on the table.

[[File:SPECT CT.JPG|thumb|left|SPECT machine performing a total body bone scan. The patient lies on a table that slides through the machine, while a pair of gamma cameras rotate around her.]]
To acquire SPECT images, the gamma camera is rotated around the patient. Projections are acquired at defined points during the rotation, typically every 3–6 degrees. In most cases, a full 360-degree rotation is used to obtain an optimal reconstruction. The time taken to obtain each projection is also variable, but 15–20 seconds is typical. This gives a total scan time of 15–20 minutes.

Multi-headed gamma cameras can accelerate acquisition. For example, a dual-headed camera can be used with heads spaced 180 degrees apart, allowing two projections to be acquired simultaneously, with each head requiring 180 degrees of rotation. Triple-head cameras with 120-degree spacing are also used.

Cardiac [[Gated SPECT|gated acquisitions]] are possible with SPECT, just as with planar imaging techniques such as [[Radionuclide angiography|multi gated acquisition scan]] (MUGA). Triggered by [[Electrocardiography|electrocardiogram]] (EKG) to obtain differential information about the heart in various parts of its cycle, gated myocardial SPECT can be used to obtain quantitative information about myocardial perfusion, thickness, and contractility of the myocardium during various parts of the cardiac cycle, and also to allow calculation of [[Ejection fraction|left ventricular ejection fraction]], stroke volume, and cardiac output.