Detector size. In the absence of collimators, unlike in SPECT, the intrin-
sic resolution Riis the predominant factor in spatial resolution, which is
related to the detector size din multidetector PET scanners. It is nor-
mally given by dat the face of the detector and by d/2midway between
the two detectors.
Positron range. A positron with energy travels a distance in tissue losing
energy before it almost comes to rest and then combines with an elec-
tron to produce two 511-keV photons. The locations of positron emission
and annihilation are separated by the effective range of the positron (Fig.
13.12), which adds to the uncertainty of X,Ypositioning of the detector
pair. This error Rpincreases with the positron energy and decreases with
the tissue density. This value is reported to be 0.2 mm for^18 F and 2.6 mm
for^82 Rb in tissue (Tarantola et al., 2003).
Noncolinearity. Positrons at the end of their range still possess some resid-
ual momentum and, therefore, the two annihilation photons are not
emitted exactly 180°, but at slight deviation. This deviation from 180° is
±0.25° at maximum. Because of this deviation, the LOR is displaced from
the point of annihilation (Fig. 13.13) and, thus, an error Rais introduced
in the spatial resolution of the PET scanner. This value deteriorates with
the diameter of the ring and is estimated to be
Ra=0.0022D (13.8)
202 13. Positron Emission Tomography
AnnihilationNucleus
p
nβ+
Positron
effctive
range511 keV
photon511 keV
photone-
β+Fig. 13.12. Positrons travel a distance before annihilation in the absorber and the
distance increases with positron energy. Because positrons with different energies
travel in zigzag directions, the effective range is the shortest distance between the
nucleus and the direction of 511-keV photons. This effective range degrades
the spatial resolution of the PET scanner. (Reprinted with the permission of the
Cleveland Clinic Foundation.)