Cao ZongJian, Corley James H, Allison Jerry
Department of Radiology, Medical College of Georgia, 1120 15th Street, Augusta, GA 30912, USA.
J Nucl Med Technol. 2003 Dec;31(4):210-5.
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.
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.
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).
(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.
研究了在保护辐射工作人员和公众免受¹⁸F辐射方面的两个主要问题——¹⁸F源的距离和铅屏蔽，还研究了¹⁸F源对相邻γ相机计数率的影响。
使用电离室在有无铅屏蔽的情况下，于不同距离测量¹⁸F小瓶和充水圆柱体的剂量率。此外，在¹⁸F圆柱体存在的情况下，测量γ相机在不同探测器方向、距离和能量窗下的计数率。
空气中点源或扩展源的剂量率与距源距离的平方成反比。在2米处，一个370MBq的¹⁸F源在任何单小时内的剂量率小于20微戈瑞，这是无限制区域的限值。使用0.318厘米厚铅屏蔽时剂量率降至约60%，使用5.08厘米厚铅屏蔽时剂量率降至约4%；这些速率高于使用窄束衰减公式估算的速率。来自铅砖和周围结构的散射光子和特征X射线可能导致了这一结果。距离增加33%导致的剂量率降低与用0.318厘米厚铅屏蔽源的效果相似。在距一个185MBq的¹⁸F源3米处，当探测器面对源时，Orbiter相机在⁹⁹ᵐTc能窗内的计数率约为120000/分钟。该速率与典型的⁹⁹ᵐTc临床研究速率（约200000/分钟）相当。仅当距离增加到11米且探测器不面对源时，计数率才降至本底水平（3234/分钟）。计数率还取决于γ相机的能量窗。在Vertex相机上，¹⁸F在⁹⁹ᵐTc能窗内的计数率与在²⁰¹Tl（或⁶⁷Ga）能窗内的计数率之比为1:1.7（或1:2.7）。
¹⁸F剂量率可随距离显著降低。铅屏蔽效果不如预期。¹⁸F源应与γ相机保持足够远的距离，以避免图像质量受污染。
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