18F Protection Issues: Human and -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 -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 -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 Gy in any single hour, which is the limit for unrestricted areas. The dose rate with 0.318cm-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 -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 -cameras to avoid contamination of image quality.