Calibration and count rate performance of NaI(Tl) gamma counters for quantifying the activity ⁸⁹Zr...

Quantification of low activity samples using gamma counters is a common technique when investigating the biodistribution of experimental radiopharmaceuticals. Several human and pre-clinical trials have investigated ⁸⁹Zr labeled compounds, utilizing gamma counter to quantify activity in blood or animal samples. While the performance of gamma counters has been investigated for other positron emitting radionuclides, the more complex spectrum, including a 909 keV gamma emission, and low positron fraction of ⁸⁹Zr may reduce both counting efficiency while increasing the severity of count rate losses.

The purpose of this study is to evaluate both the counting efficiency and the effect of count rate losses on the quantitation of activity for ⁸⁹Zr using NaI(Tl) gamma counters.

Sources of known activity (initially 78.3 kBq) were measured in a gamma counter as they decayed over approximately 15 days. Observed count rates were fit with a paralyzable detector model for two separate energy windows, 511 keV ± 10% and 909 keV ± 10%, as well as their sum. Counting efficiency in the absence of losses as well as the effective deadtime for each dataset were calculated.

Count rates for each energy window were well fit with a paralyzable model with R² > 0.999. Counting efficiency (mean ± 95% confidence interval) was 10.07% ± 0.07%, 17.38% ± 0.13%, and 27.46% ± 0.16% for the 511 keV, 909 keV, and summed energy windows respectively. Effective dead time (mean, 95% confidence interval) was 14.31 µs, (13.30 µs-15.30 µs), 7.11 µs (6.68 µs-7.55 µs), and 4.77 µs (4.48 µs-5.06 µs) for the 511 keV, 909 keV, and summed energy windows respectively.

Counting efficiency and count rate performance for ⁸⁹Zr are poorer than for other positron emitting radionuclides. Count rate losses follow the paralyzable model and are correctible.