The BRMD thyroid-neck phantom: design and construction

Counting efficiency of whole-body monitoring system using BOMAB and ANSI/IAEA thyroid phantom due to internal contamination of 131I

In this study, the effect of Indian reference BOttle MAnnikin aBsorber (BOMAB) neck with axial cavity and American National Standards Institute (ANSI)/International Atomic Energy Agency (IAEA) thyroid phantom using pencil sources of (133)Ba ((131)I simulant) on counting efficiency (CE) is seen experimentally in static geometry for whole-body monitoring system comprising 10.16-cm diameter and 7.62-cm-thick NaI(Tl) detector. The CE estimated using the neck part of BOMAB phantom is 50.2% lower in comparison with ANSI phantom. In rest of the studies FLUKA code is used for Monte Carlo simulations using ANSI/IAEA thyroid phantom. The simulation results are validated in static geometries with experimental CE and the differences are within 1.3%. It is observed that CE for pencil source distribution is 3.97% higher for (133)Ba in comparison with CE of (131)I source. Simulated CE for pencil source distribution is 4.7% lower in comparison with uniform source distribution in the volume of thyroid for (131)I. Since the radiation workers are of different physique; overlying tissue thickness (OTT) and neck-to-detector distance play an important role in the calculation of activity in thyroid. The CE decreases with increase in OTT and is found to be 5.5% lower if OTT is changed from 1.1 to 2 cm. Finally, the simulations are carried out to estimate the variation in CE due to variation in the neck-to-detector distance. The CE is 6.2% higher if the neck surface-to-detector distance is decreased from 21.4 to 20.4 cm and it goes on increasing up to 61.9% if the distance is decreased to 15.4 cm. In conclusion, the calibration of whole-body monitoring system for (131)I should be carried out with ANSI/IAEA thyroid phantom, the neck-to-detector distance controlled or the CE corrected for this, and the CE should be corrected for OTT.

In-vivo thyroid 125I monitoring method using imaging plate

A new in vivo thyroid monitoring system was devised with radioluminography using an imaging plate. The counting features of the system were investigated by using a neck-thyroid phantom. A clear thyroid image was obtained at thin prethyroid tissue thicknesses and short distances between IP and neck surface. The characteristic 125I thyroid image made it possible to visually confirm that the thyroid definitely accumulated 125I. The counting efficiency in 100xcps Bq(-1) of the imaging plate system was determined by setting a region of interest on the thyroid image resulting in a maximum efficiency of 0.5%. The higher efficiency was obtained for thinner tissue thickness overlaying the thyroid, smaller thyroids, and shorter distances between the imaging plate and neck surface. The imaging plate thyroid monitoring system was sensitive to thyroid 125I. Its minimum detection limit of 23 Bq for a 10-min measurement was considerably lower than the annual thyroid burden limit of 300 kBq.