Fluoride F18 decays by positron (+) emission and has a half-life of 109.7 minutes. Ninety-seven percent of the decay results in emission of a positron with a maximum energy of 633 keV and 3% of the decay results in electron capture with subsequent emission of characteristic X-rays of oxygen. The principal photons useful for diagnostic imaging are the 511 keV gamma photons, resulting from the interaction of the emitted positron with an electron (Table 2). Fluorine F18 atom decays to stable 18O-oxygen.
Table 2: Principal Emission Data for Fluoride F18 | Radiation/Emission | % per Disintegration | Mean Energy |
|---|
| Positron (+) | 96.73 | 249.8 keV |
| Gamma (±) | 193.46 | 511.0 keV |
The specific gamma ray constant (point source air kerma coefficient) for fluoride F18 is 5.7 R/hr/mCi (1.35 x 10 -6 Gy/hr/kBq) at 1 cm. The half-value layer (HVL) for the 511 keV photons is 4 mm lead (Pb). A range of values for the attenuation of radiation results from the interposition of various thickness of Pb. The range of attenuation coefficients for this radionuclide is shown in Table 3. For example, the interposition of an 8 mm thickness of Pb with a coefficient of attenuation of 0.25 will decrease the external radiation by 75%.
Table 3:Radiation Attenuation of 511 keV Photons by Lead (Pb) Shielding| Shield Thickness (Pb) mm | Coefficient of Attenuation |
|---|
| 0 | 0.00 |
| 4 | 0.50 |
| 8 | 0.25 |
| 13 | 0.10 |
| 26 | 0.01 |
| 39 | 0.001 |
| 52 | 0.0001 |
Table 4 lists the fraction of radioactivity remaining at selected time intervals from the calibration time. This information may be used to correct for physical decay of the radionuclide.
Table 4: Physical Decay Chart for Fluoride F18| Time Since Calibration | Fraction Remaining |
|---|
| 0 | 1.00 |
| 15 minutes | 0.909 |
| 30 minutes | 0.826 |
| 60 minutes | 0.683 |
| 110 minutes | 0.500 |
| 220 minutes | 0.250 |
| 440 minutes | 0.060 |
| 12 hours | 0.011 |
| 24 hours | 0.0001 |