An iodine-125 thyroid measurement method

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Abstract
A two-probe technique has been developed to measure 125I in the thyroid. In this technique simultaneous emissions from a source are detected as coincidence events between two probes rather than as a sum peak in a single probe (the sum peak method). The background count rate when counting coincidence events between probes is substantially lower than the background in the sum peak region of a single probe. Theoretical equations which enable the activity of a source to be calculated are given. The applicability of these equations to thyroid measurements has been verified by measuring sources of various sizes, so that the geometric distribution and self-absorption of the sources were varied beyond the limits likely to be encountered in thyroid monitoring. Measurements on simulated thyroid showed a variation in measured activity from +2% at a 5 cm source-to-detector distance to -4% at 10 cm. Measurements on a simulated thyroid containing 310 Bq (8.4nCi) for a 500 s counting time gave a standard deviation of 21%. When the activity of the source was increased to 5500 Bq (149 nCi) the standard deviation was 6.2%.

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