Activity determination of ¹⁸F using liquid scintillation beta-efficiency extrapolation and non-extrapolation methods

Highlights

• Absolute standardization of F-18 using beta-efficiency extrapolation and-non extrapolation methods.

• Liquid scintillation coincidence counting.

• Double-tube efficiency adapted for threshold above second monopeak for non-extrapolation method.

• Excellent agreement between the methods.

Abstract

The absolute activity of ¹⁸F was determined using 4$\mathrm{\pi \beta }-\mathrm{\gamma }$ liquid scintillation coincidence counting using beta-efficiency extrapolation. An ionization chamber factor was determined for use during a SIRTI comparison, for which results are presented. A non-extrapolation method based on a detection efficiency analysis was also employed using an adaptation of the double-phototube coincidence efficiency for a threshold above the second monopeak. Results and uncertainty budgets for the two methods are presented and discussed.

Introduction

The National Metrology Institute of South Africa (NMISA) performed its first absolute standardization of a¹⁸F solution. The method employed 4πβ-γ coincidence counting using liquid scintillation (LS) in the beta-plus channel. The result of the standardization was used to determine a calibration factor for ¹⁸F for the NMISA secondary standard ionization chamber. One month later NMISA participated in the BIPM.RI(II)–K4.F-18 International Reference System Transfer Instrument (SIRTI) comparison (Michotte et al., 2017), for which NMISA was only the fourth participant at that time.

Fluorine-18 is the most common positron emission tomography (PET) radiopharmaceutical used for diagnostics. Literature indicates that ¹⁸F is a simple radionuclide to standardize (Roteta et al., 2006; Rodrigues et al., 2011; Kim et al., 2012) due to its straightforward decay scheme and high β⁺ maximum energy of 633.9 keV resulting in a high efficiency in the liquid scintillator. Experimentally, the biggest challenge is measurement logistics due to its short half-life of 1.8 h, which was overcome at NMISA by using a fifteen-channel double- and triple-coincidence unit for simultaneous measurements at various electronic thresholds (Simpson and Meyer, 1988), to allow for activity determination through the liquid scintillation efficiency extrapolation technique.

We present a detection efficiency analysis of the 4πβ-γ coincidence counting method, accounting for both 511 keV annihilation photons, which is more complete than that currently found in the literature (Roteta et al., 2006; Rodrigues et al., 2011) where only one 511 keV photon is considered. The high β⁺-efficiency leads to a small extrapolation range and corresponding uncertainty.

Additionally, a non-extrapolation method (Simpson and Morris, 2004; van Rooy et al., 2016) was employed to analyse data corresponding to only the first threshold above the second monopeak, advantageous if multi-discrimination data acquisition is not possible. The detection efficiency analysis indicated that the measured liquid scintillation efficiency is a function of the positron- and 511 keV Compton electron efficiency. These efficiencies were determined from the theoretical formulae for the double-phototube coincidence efficiencies, modified for a threshold set above the second monopeak, and using the theoretical positron- and Compton spectra. Subsequently, the activity was determined.