Branching ratio
What is "branching ratio"?
PET scanner detects only 511 keV photons. Different isotopes used in PET have variable fraction of other decay events, which are not detected by PET. This has to be taken into account when calibrating the scanner and the blood samples by dividing the measured values by the branching ratio.
| Isotope | Branching ratio |
|---|---|
| C-11 | 0.998 |
| N-13 | 0.998 |
| O-15 | 0.999 |
| F-18 | 0.967 |
| Ga-68 Ge-68 |
0.891 |
| Rb-82 | 0.950 |
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 blood 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+ produce images that are 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.
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.
