Basics of PET and SPECT Imaging - Correction Techniques in Emission Tomography - page 57

4.1.3 Scatter
Scatter does not only lead to a loss of photons for the projection data that
would have been counted originally. Also photons that would not have hit the
detector on their original path can be detected after scatter as mispositioned
events. Photons undergoing Rayleigh scattering do not lose energy; thus, they
cannot be distinguished by energy determination. In the energy ranges of
SPECT and PET, Compton scattering is the predominant scattering process.
In this case, photons loose energy, such that the number of registered scattered
events can be reduced by using narrow energy windows in the photon detection
process. Scattered events in PET are distributed across the whole field-of-view,
accumulating in dense areas, as can be seen in Figure 4.2, where reconstruction
was performed with attenuation correction but without scatter correction.
In the center of the phantom, the measured activity concentration is up to
15 kBq/ml while the real one is only 11 kBq/ml. The scattered events are
attributed more to the center of the phantom because in water, with which the
phantom was filled, Compton scatter occurs with a much higher probability
than in the air surrounding the phantom. In PET, scattered photons can lead
to coincidence detection outside the patient which is illustrated in Figure 4.5
on the left side. In SPECT, scattered events will lead to wrong events only in
the patient boundaries (Figure 4.5, right side). Scattered photons that would
lead to wrong events outside the patient have no chance to be detected due
to the parallel collimation as long as negligible scattering in air is assumed.
This can also be seen in Figures 4.3 and 4.4 where for PET the activity
concentration in the not-scatter-corrected image (bottom) is not equal to zero
FIGURE 4.5: In PET (left side) coincidence lines can be assigned wrongly
to lines outside the patient due to scatter in the body. In SPECT, due to
parallel collimation, scattered events can only appear within the boundaries
of the body.