in the anterior and posterior positions. This causes loss of data and hence
loss of spatial resolution in these projections. To circumvent this problem,
many modern cameras are designed to include a feature called noncircular
orbit(NCO) (i.e., to follow the body contour) that moves the camera heads
such that the detector remains at the same distance close to the body
contour at all angles.
Data collection can be made in either continuous motion or “step-and-
shoot” mode. In continuous acquisition, the detector rotates continuously
at a constant speed around the patient, and the acquired data are later
binned into the number of segments equal to the number of projections
desired. In the step-and-shoot mode, the detector moves around the patient
at selected incremental angles (e.g., 3°) and collects the data for the pro-
jection at each angle.
Image Reconstruction
Data collected in two-dimensional projections give planar images of the
object at each projection angle. To obtain information along the depth of
the object, tomographic images are reconstructed using these projections.
Two common methods of image reconstruction using the acquired data are
the backprojection method and the iterative method, of which the former
is the more popular, although lately the latter is gaining more attention.
Both methods are described below.
Simple Backprojection
The principle of simple backprojection in image reconstruction is illustrated
in Figure 12.3, in which three projection views are obtained by a gamma
camera at three equidistant angles around an object containing two sources
of activity designated by black dots. In the two-dimensional data acquisi-
tion, each pixel count in a projection represents the sum of all counts along
the straight-line path through the depth of the object (Fig. 12.3A). Recon-
struction is performed by assigning each pixel count of a given projection
in the acquisition matrix to all pixels along the line of collection (perpen-
dicular to the detector face) in the reconstruction matrix (Fig. 12.3B). This
156 12. Single Photon Emission Computed Tomography
Table12.1. Relationship of sensitivity and time of imaging for 180° and 360° acqui-
sitions for different camera head configurations.
180° Acquisition 360° Acquisition
Acquisition Relative Acquisition Relative
Camera Type time (min) sensitivity time (min) sensitivity
Single-head 15 1 15 1
Dual-head (heads at 180°) 15 1 7.5 2
Dual-head (heads at 90°) 7.5 2 7.5 2
Triple-head (heads at 120°) 10 1.5 5 3