¹⁸F protection issues: Human and γ-camera considerations

Objective: Two main issues in protecting radiation workers and the general public from ¹⁸F radiation - distance from and lead shielding for an ¹⁸F source-were investigated. We also examined the effect of an ¹⁸F source on the counting rate of a neighboring γ-camera. Methods: The dose rates of an ¹⁸F vial and a water-filled cylinder were measured using an ionization chamber at different distances with or without lead shielding. In addition, the counting rates of γ-cameras in the presence of the ¹⁸F cylinder were measured with different detector orientations, distances, and energy windows. Results: The dose rate of a point or an extended source in air was proportional to the inverse square of the distance from the source. At 2 m, the dose rate for a 370-MBq ¹⁸F source was less than 20 μGy in any single hour, which is the limit for unrestricted areas. The dose rate with 0.318-cm-thick lead shielding decreased to about 60%, and that with 5.08-cm-thick lead shielding decreased to about 4%; these rates were higher than those estimated using the narrow-beam attenuation formula. The scattered photons and characteristic x-rays from the lead brick and surrounding structures may have contributed to this result. The decrease in dose rate resulting from a 33% increase in distance was similar to the effect from shielding the source with 0.318-cm-thick lead. At 3 m from a 185-MBq ¹⁸F source, the counting rate in the ^99mTc window of an Orbiter camera was about 120,000/min when the detector faced the source. This rate was comparable to that of a typical ^99mTc clinical study (∼200,000/min). Only when the distance was increased to 11 m and the detector did not face the source did the counting rate decrease to the background level (3,234/min). The counting rate also depended on the energy window of the γ-camera. On a Vertex camera, the counting rate of ¹⁸F in the ^99mTc window versus that in the ²⁰¹Tl (or ⁶⁷Ga) window was 1:1.7 (or 1:2.7). Conclusion: ¹⁸F dose rate can be significantly reduced with distance. Lead shielding is not as effective as was predicted. ¹⁸F sources should be kept substantial distances away from γ-cameras to avoid contamination of image quality.