186 13. Positron Emission Tomography
ularly if the annihilation occurs at the edge of the FOV. This also adds to
the timing resolution. For a whole-body scanner with a diameter of 1 m, the
maximum distance traveled by one photon can be as large as 1 m, and
the other photon travels almost no distance, if the annihilation occurs at the
edge of the FOV. Because the velocity of light is 3 × 108 m/s, the difference
between the arrival times of the two photons is about 3 to 4 ns (time to
travel 1 m). This is the limit of timing resolution of a PET scanner with
1-m diameter.
After annihilation, two timing signals A and B are formed with timing
width, say t, depending on the scanner system. Signal B may arrive at one
detector just tahead of (Fig. 13.3A) or tbehind (Fig. 13.3B) the arrival of
signal A, and both signals will be counted in coincidence. This is the extreme
A B
D
105 °^90 °
C
Fig. 13.2. Different configurations of PET scanners. (A) A circular full-ring scanner.
(B) A partial ring scanner with a 15° angular shift between two blocks of detectors.
(C) Continuous detectors using curve plates of Nal (TI). (D) Hexagonal array of
quadrant-sharing panel detectors. (Reprinted with the permission of the Cleveland
Clinic Foundation.)