Branching ratio

What is “branching ratio”?

PET scanner detects only 511 keV photons. Different isotopes used in PET have variable fraction of other than β+ decay events (non-pure positron emitters), which are not detected by PET as coincidence events, but which may add to the scatter. This has to be taken into account when calibrating the scanner and the blood samples by dividing the measured values by the branching ratio. Image reconstruction and scatter correction methods may need to be adjusted to provide quantitative images despite of the additional gamma rays, or even to use the additional gamma rays to improve the sensitivity of PET imaging (Lubberink and Herzog, 2011; Braad et al., 2015; Conti and Eriksson, 2016; Lin et al., 2016).

Table 1. Branching ratios for PET isotopes
Isotope Branching ratio
C-11 0.998
Cu-64 0.1752 or 0.174(1
N-13 0.998
O-15 0.999
F-18 0.967
Ga-68/Ge-68 0.891
Rb-82 0.950
Zr-89 0.22
I-124 0.26

1) Reference for the branching factor of 0.1752 for 64Cu is Monographie BIPM-5, Table of Radionuclides, Vol. 6, 2011. However, in Inveon data branching factor 0.174 is used.

What data is corrected for the branching ratio?

Dose calibrators

All dose calibrators (in both radiochemistry and imaging laboratories) take into consideration the branching ratio. Therefore, the reported values of injected doses and specific radioactivities are always corrected.

Blood samples

Manual blood samples (and calibration samples) are corrected for branching ratio in the Imaging unit clinical laboratory. In practice, each isotope has its own calibration coefficient: higher for germanium-68 and fluorine-18 than for carbon-11 and oxygen-15.

Continuous blood data from on-line sampler are corrected for branching ratio during the calibration by program blo2kbq.

PET images

GE Advance and HR+ produced images that were corrected for branching ratio during image reconstruction.

ECAT 931 images are corrected for branching ratio when they are calibrated (normally during reconstruction), if the scanning date is Jan-01 1997 or later. Images scanned before that are not corrected for branching ratio by the calibration program. Because ECAT 931 calibration images are not themselves calibrated, they are also not corrected for branching ratio.

Inveon images contain branching factor in the header, but the factor is not included in the frame scaling factor in the original interfile format.

If PET image is produced using wrong isotope setting, isotope information must be corrected into sinograms or raw data and images must be re-reconstructed, otherwise wrong branching ratio is applied even though decay is corrected.

See also:


Monographie BIPM-5, Table of Radionuclides, Vol. 6, 2011.

Table of Isotopes, Sixth edition, edited by C.M. Lederer, J.M. Hollander, I. Perlman. Wiley, 1967.

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Updated at: 2018-09-28
Written by: Vesa Oikonen, Tuula Tolvanen, Mika Teräs