Basics of PET and SPECT Imaging - Correction Techniques in Emission Tomography - page 64

TABLE 4.2: Maximum positron energy and mean and maximum positron
range in water for some typical PET isotopes (adapted from [5]).

Isotope E max [MeV] R max [mm] R mean [mm]

11 C

0.96 4.1 1.1

13 N

1.20 5.1 1.5

15 O

1.70 7.3 2.5

18 F

0.63 2.4 0.6

68 Ga

1.90 8.2 2.9

82 Rb

3.40 14.1 5.9

tion with mean and maximum range in body tissue (water equivalent). These
values can be found in Table 4.2 for the most important isotopes in PET.
Another limitation of the spatial resolution of a PET system originating
from the annihilation process itself is that the 180-degree angle between the
two annihilation photons is valid only when positron and electron have zero
momentum at the time of annihilation. Actually, in more than 65% of the
annihilation processes this is not true. The angular distribution of the annihi-
lation photons was found by Levin and Hoffman to be Gaussian shaped with
0.25 degree FWHM [5]. The effect on the spatial resolution of the PET system
due to this non-collinearity depends on the diameter of the detector ring and
is about 1.8 mm for a 80 cm ring system.

References
[1] ICRU Report 44: Tissue substitutes in radiation dosimetry and measure-
ment. Technical report, 1989.

[2] E. J. Hoffman, S. C. Huang, and M. E. Phelps. Quantitation in positron
emission computed tomography: 1. Effect of object size. Journal of Com-
puter Assisted Tomography, 3(3):299, 1979.

[3] J. H. Hubbell. Photon cross sections, attenuation coecients, and energy
absorption coecients from 10 keV to 100 GeV. US National Bureau of
Standards; for sale by the Supt. of Docs., US Govt. Print. Off., [Washing-
ton], 1969.

[4] J. H. Hubbell. Review of photon interaction cross section data in the
medical and biological context. Physics in Medicine and Biology, 44:R1,
1999.