18F protection issues: human and gamma-camera considerations.
OBJECTIVE
Two main issues in protecting radiation workers and the general public from (18)F radiation-distance from and lead shielding for an (18)F source-were investigated. We also examined the effect of an (18)F source on the counting rate of a neighboring gamma-camera.


METHODS
The dose rates of an (18)F 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 gamma-cameras in the presence of the (18)F 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 (18)F source was less than 20 micro Gy 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 (18)F source, the counting rate in the (99m)Tc 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 (99m)Tc clinical study ( approximately 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 gamma-camera. On a Vertex camera, the counting rate of (18)F in the (99m)Tc window versus that in the (201)Tl (or (67)Ga) window was 1:1.7 (or 1:2.7).


CONCLUSIONS
(18)F dose rate can be significantly reduced with distance. Lead shielding is not as effective as was predicted. (18)F sources should be kept substantial distances away from gamma-cameras to avoid contamination of image quality.
Cao
ZongJian
Z
Department of Radiology, Medical College of Georgia, 1120 15th Street, Augusta, GA 30912, USA. zcao@mail.mcg.edu
Corley
James H
JH
Allison
Jerry
J
D004781:Environmental Exposure / Q000032:analysis*; D019544:Equipment Failure Analysis; D019788:Fluorodeoxyglucose F18 / Q000032:analysis*; D015902:Gamma Cameras; D006801:Humans; D016273:Occupational Exposure / Q000032:analysis; D011829:Radiation Dosage; D011832:Radiation Injuries / Q000517:prevention & control; D011835:Radiation Protection / Q000295:instrumentation* / Q000379:methods*; D011874:Radiometry / Q000295:instrumentation / Q000379:methods*; D019275:Radiopharmaceuticals / Q000032:analysis; D018570:Risk Assessment / Q000379:methods*; D017751:Safety Management / Q000379:methods*
D019275:Radiopharmaceuticals; D019788:Fluorodeoxyglucose F18