12
Single Photon Emission Computed Tomography
153Tomographic Imaging
Conventional gamma cameras provide two-dimensional planar images of
three-dimensional objects. Structural information in the third dimension,
depth, is obscured by superimposition of all data along this direction.
Although imaging of the object in different projections (posterior, anterior,
lateral, and oblique) gives some information about the depth of a structure,
precise assessment of the depth of a structure in an object is made by tomo-
graphic scanners. The prime objective of these scanners is to display the
images of the activity distribution in different sections of the object at dif-
ferent depths.
The principle of tomographic imaging in nuclear medicine is based on
the detection of radiations from the patient at different angles around the
patient. It is called emission computed tomography(ECT), which is based
on mathematical algorithms, and provides images at distinct depths (slices)
of the object (Fig. 12.1). In contrast, in transmission tomography, a radi-
ation source (x-rays or a radioactive source) projects an intense beam of
radiation photons through the patient’s body, and the transmitted beam is
detected by the detector and further processed for image formation.
In nuclear medicine, two types of ECT have been in practice based on
the type of radionuclides used:single photon emission computed tomogra-
phy(SPECT), which uses g-emitting radionuclides such as 99mTc,^123 I,^67 Ga,
and^111 In, and positron emission tomography(PET), which uses b+-emitting
radionuclides such as^11 C,^13 N,^15 O,^18 F,^68 Ga, and^82 Rb. SPECT is described
in detail in this chapter and PET in Chapter 13.
Single Photon Emission Computed Tomography
The most common SPECT system consists of a typical gamma camera with
one to three NaI(Tl) detector heads mounted on a gantry, an online com-
puter for acquisition and processing of data, and a display system (Fig. 12.2).