Spatial resolution positron range

Figure 6.1. Positrons travel a distance before annihilation in the absorber and the distance increases with positron energy. Since 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 Center for Medical Art Photography 2009. All Rights Reserved)...

Noncolinearity. Another factor of concern is the noncolinearity that arises from the deviation of the two annihilation photons from the exact 180° position. That is, two 511 keV photons are not emitted at exactly 180° after the annihilation process (Fig. 6.2), because of some small residual momentum of the positron at the end of the positron range. The maximum deviation from the 180° direction is 0.25° (i.e., 0.5° FWHM). Thus, the observed LOR between the two detectors does not intersect the point of annihilation, but is somewhat displaced from it, as illustrated in Fig. 6.2. This error (Ra) degrades the spatial resolution of the scanner and deteriorates with the distance between the two detectors. If D is the distance in cm between the two detectors (i.e., detector ring diameter), then Ra can be calculated from the point-spread-function as follows ... 

On the basis of the fundamental physics of positron decay, the limit for spatial resolution of a PET image using fluorine-18 (again, a nearly ideal isotope for PET imaging purposes) should be in the range of 0.5-0.75 mm. However, it is possible that in the future, software solutions that make corrections in image reconstruction for the loss of resolution due to the physical process of... 

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