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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:

Eq 1 for physical decay

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):

Eq 2 for physical decay

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.

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