In vivo x-ray fluorescence analysis (XRF) of the thyroid iodine content -: Influence of measurement geometries on the iodine Kα signal

The thyroid gland is unique in that it stores a large amount of iodine, an element suitable for x-ray fluorescence analysis (XRF). Knowledge of the amount of iodine stored in the thyroid in the iodine pool may contribute to the understanding of thyroid diseases and possibly predict treatment outcome. XRF is a non-invasive method which can be used for direct evaluations of the iodine pool in vitro as well as in vivo. However, in contrast to in vitro studies, which involve a measurement geometry that is well known and can be carefully controlled, complications may arise when using the technique in vivo. In this study, Monte Carlo (MC) simulations were used for finding an optimal measurement geometry regarding source and detector collimator apertures. The relative importance of primary and scattered radiation in the production of iodine K alpha x-rays for a simulated thyroid gland at different depths in the neck was investigated. The results showed that incoherently scattered radiation may be a considerable contributor of Ka photons in commonly encountered in vivo geometries. Calibrations should thus be performed with precise knowledge of the measurement geometry and it is important that corrections are made for primary photon fluence but also for photon scattering in the neck. Copyright (C) 2007 John Wiley & Sons, Ltd.