Reconstruction method. If the filter backprojection method is used for
reconstruction, the choice of filter with a selected cut-off frequency
degrades spatial resolution. This error (Kr) is reported to be a factor of
1.2 to 1.5 depending on the type of filter used.
Location of the detector. The use of block detectors rather than single
detectors introduces an error in the (X,Y) positioning of the detector
pair, which causes degradation of the spatial resolution. This value (Rl)
is approximately 2.2 mm for BGO detectors, and somewhat less for LSO
detectors.
Combining all these factors, the spatial resolution of a PET scanner is
given by
RK RRRR=× +++ripa^2222 l (13.9)Performance of PET Scanners 203Observed LOR
Fig. 13.13. Noncolinearity of 511-keV annihilation photons. Because there is some
residual momentum associated with the positron, the two annihilation photons are
not emitted exactly at 180°, but at a slight deviation from 180°. Two detectors detect
these photons in a straight line, which is slightly deviated from the original annihi-
lation line. The maximum deviation is ±0.25°. (Reprinted with the permission of the
Cleveland Clinic Foundation.)