and the angle of orientation (f) of the LOR (i.e., the angle between rand
the vertical axis of the field). A matrix of an appropriate size is chosen
defined by the r,fcoordinates, rather than by X-,Y-coordinates used in
SPECT data acquisition, and counts in each LOR are stored in the corre-
sponding pixel in the matrix. If we plot the distance ron the x-axis and the
angle fon the y-axis, then the coincidence event along the LOR (r,f) will
be assigned at the cross-point of rand fvalues (Fig. 13.6B). In a given pro-
jection, adjacent detector pairs constitute parallel LORs (at different r
values in Fig. 13.6A) at the same angle of orientation. The plot of these
LORs will be seen as a horizontal row at angle f. Similarly LORs from dif-
ferent projections (i.e., at different angles f) for the same rvalues can be
plotted, which will give a vertical line. When all projections around the field
of view are considered, the plot of the LORs at different projection angles
and rvalues will result in the shaded area in Figure 13.6B, which is called
the sinogram. A typical normal sinogram is shown in Figure 13.7.
The sinogram represents a single slice of data for a transverse FOV
obtained from a single ring of the PET scanner. PET data are acquired
directly into a sinogram in a matrix of appropriate size in the computer.
Each pixel corresponds to a particular LOR characterized by (r,f) con-
taining all coincidence counts detected by the detector pair along the LOR.
Data can be collected in both static and dynamic imaging using either the
frame mode or the list mode, described in Chapter 11.
Because PET scanners are axially fixed, whole-body imaging is accom-
plished by the use of a computer-controlled bed-table that moves along the
axis of the scanner. The whole-body scan of the patient is obtained at dif-
ferent axial positions of the bed.
Data Acquisition 193Fig. 13.7. A typical normal sinogram indicating all detectors are working properly.