Decay correction
Half-lives of PET isotopes
The literature gives discrepant half-lives (T1/2) for the radioactive isotopes. Currently the reference used in Turku PET Centre is:
Table of Isotopes, Sixth edition, edited by C.M. Lederer, J.M. Hollander, I. Perlman. WILEY, 1967.
| Isotope | Half-life (sec) | Half-life (min) | Half-life (h) | Half-life (days) |
| C-11 | 1224 | 20.4 | 0.34 | |
| N-13 | 599 | 9.98 | ||
| O-15 | 123 | 2.05 | ||
| F-18 | 6588 | 109.8 | 1.83 | |
| Cu-62 | 582 | 9.7 | ||
| Ga-68 | 40080 | 68.0 | 1.13333 | |
| Ge-68 | 2.376E7 | 396000 | 6600 | 275 |
| Br-76 | 58700 | 978 | 16.3 | |
| Rb-82 | 75 | 1.25 |
Formulas for decay correction
Before correction for physical decay, data must be corrected for detector dead-time and background radioactivity.
The sample radioactivity (At) at the measurement time t can be corrected to the radioactivity at the injection time (A0) using equation:
For PET measurements the decay during the frame time must be considered (t1 and t2 are the start and end times of PET frame; A0 is the decay corrected radioactivity, and At is the non-corrected radioactivity):
Software for decay correction in TPC
The isotopes, and the corresponding values for half-life and λ, that
are identified by most of our in-house software, can be listed with
halflife.
Decay correction of PET images
Images can be decay corrected, isotope setting changed, or prior decay correction
removed, using program edecay.
By default, all PET images are decay corrected to the scan start time. If scan was not started at the injection time, the frame times and decay must be corrected to the true injection time prior to any other calculation with the dynamic image or sinogram; see information on correcting delayed scans.
However, if PET scanning was performed in separate sessions, follow this link: Catenation of PET images or sinograms.
Decay correction of TACs
For plasma/blood and PET TACs the decay correction, or removal of decay
correction, can be done using program
dftdecay.
If PET scan was not started at the injection time but later, and if the
dynamic image was not corrected for this (see above), then the times and
physical decay of radioactivity concentrations DFT file can be corrected using
program
dfttime with option -d.
If PET scanning was performed in separate sessions, and the dynamic images
were not joined together (see above), the separate DFT files can still be joined
together: first you must correct the times and radioactivity concentrations in
the late imaging DFT file(s) using program
dfttime
with option -d, and then catenate the DFT files
using dftcat.
If dynamic PET images are available, the time difference required by program
dfttime can be achieved using program
egetstrt.
