Ionizing Radiation Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
J Nucl Med. 2010 Mar;51(3):448-53. doi: 10.2967/jnumed.109.070300.
We have developed a methodology for calibrating (68)Ge radioactivity content in a commercially available calibration source for activity calibrators in a way that is traceable to the national standard. Additionally, the source was cross-calibrated for equivalent (18)F content by direct comparison with the national standard for (18)F in the same geometry.
Sources containing standardized (68)GeCl(4) or (18)F-FDG solutions were prepared at the National Institute of Standards and Technology (NIST) with mock syringe blanks used in the construction of a commercially available epoxy-based (68)Ge calibration source. These sources and several NIST-constructed epoxy-based (68)Ge mock syringes were then used as artifact standards to determine calibration factors for NIST-maintained activity calibrators and secondary standard ionization chambers to enable calibration of the actual commercial sources. A direct comparison between the solution-based (68)Ge sources and the (18)F-FDG sources allowed for an empiric determination of the relative response for these radionuclides in several commercial activity calibrators. Potential measurement effects due to differences between the solution composition and the epoxy and theoretic (68)Ge-to-(18)F response ratios were studied by Monte Carlo simulation.
The calibration factors developed in this study enabled NIST to calibrate epoxy-based mock syringe sources with a relative combined standard uncertainty of 0.52%. The direct comparisons of the (68)Ge and (18)F standards in the various ionization chambers allowed the activity to be expressed in terms of equivalent (18)F activity with a relative combined standard uncertainty of about 0.9%.
The ability for NIST to calibrate these epoxy-based mock syringes enabled, for the first time to our knowledge, the direct traceability to the national (68)Ge standard to be established for this type of source. Through a direct comparison with the NIST (18)F standard, the determination of the relative response ratios in activity calibrators enabled the equivalent (18)F activity to be determined in a way that was also traceable to the national (18)F activity standard.
我们开发了一种方法来校准商业可用的放射性活度校准器中的（68）Ge 放射性含量，使其与国家标准可追溯。此外，通过与同几何形状的国家标准（18）F 进行直接比较，对该源进行了（18）F 含量的交叉校准。
在国家标准与技术研究所（NIST）中，使用在商业上可获得的基于环氧树脂的（68）Ge 校准源的构造中使用的模拟注射器空白，制备含有标准化（68）GeCl（4）或（18）F-FDG 溶液的源。然后，将这些源和几个由 NIST 构造的基于环氧树脂的（68）Ge 模拟注射器用作人工标准，以确定 NIST 维护的放射性活度校准器和二级标准电离室的校准因子，从而可以校准实际的商业源。基于溶液的（68）Ge 源与（18）F-FDG 源之间的直接比较允许在几个商业放射性活度校准器中确定这些放射性核素的相对响应。通过蒙特卡罗模拟研究了由于溶液组成与环氧树脂之间的差异以及理论上的（68）Ge 与（18）F 响应比引起的潜在测量效应。
本研究中开发的校准因子使 NIST 能够以相对综合标准不确定度为 0.52%校准基于环氧树脂的模拟注射器源。在各种电离室中对（68）Ge 和（18）F 标准的直接比较允许以等效（18）F 活度表示活度，其相对综合标准不确定度约为 0.9%。
NIST 校准这些基于环氧树脂的模拟注射器的能力首次使我们能够为这种类型的源建立与国家标准（68）Ge 标准的直接可追溯性。通过与 NIST（18）F 标准的直接比较，确定放射性活度校准器中的相对响应比使得可以以与国家标准（18）F 活度标准同样可追溯的方式确定等效（18）F 活度。
