is called simple backprojection. When many projections are backprojected,
a final image is produced as shown in Figure 12.3C.
Backprojection can be better explained in terms of data acquisition in
the computer matrix. Suppose the data are collected in a 4 ×4 acquisition
matrix, as shown in Figure 12.4A. In this matrix, each row represents a slice,
projection, or profile of a certain thickness and is backprojected individu-
ally. Each row consists of four pixels. For example, the first row has pixels
A 1 ,B 1 ,C 1 , and D 1. Counts in each pixel are considered to be the sum of all
counts along the depth of the view. In the backprojection technique, a new
reconstruction matrix of the same size (i.e., 4 ×4) is designed so that counts
in pixel A 1 of the acquisition matrix are added to each pixel of the first
column of the reconstruction matrix (Fig. 12.4B). Similarly, counts from
pixels B 1 ,C 1 , and D 1 are added to each pixel of the second, third, and fourth
columns of the reconstruction matrix, respectively.
Next, suppose a lateral view (90°) of the same object is taken, and the
data are again stored in a 4 ×4 acquisition matrix. The first row of pixels
(A 2 ,B 2 ,C 2 , and D 2 ) in the 90° acquisition matrix is shown in Figure 12.4C.
Counts from pixel A 2 are added to each pixel of the first row of the same
reconstruction matrix, counts from pixel B 2 to the second row, counts from
pixel C 2 to the third row, and so on. If more views are taken at angles
between 0° and 90°, or any other angle greater than 90° and stored in 4 ×
4 acquisition matrices, then the first row data of all these views can be
Single Photon Emission Computed Tomography 157Fig. 12.3. Basic principle of reconstruction of an image by the backprojection
technique. (A) An object with two “hot” spots (solid spheres) is viewed at three pro-
jections (at 120° angles). Each pixel count in a projection represents the sum of all
counts along the straight-line path through the depth of the object. (B) Collected
data are used to reconstruct the image by backprojection. (C) When many views
are obtained, the reconstructed image represents the activity distribution with “hot”
spots. (D) Blurring effect described by 1/rfunction where ris the distance away
from the central point.