The diagnosis of thyroid disease is usually determined from planar gamma-camera images of the distribution of 99Tcm pertechnetate using a pinhole or parallel-hole collimator. The high magnification possible for a small organ imaged with a pinhole collimator means that high spatial resolution is obtainable. However, the three-dimensional spatial distribution of cold or hot regions within the thyroid is difficult to determine. Pinhole camera images give only a qualitative indication of the functioning volume of thyroid tissue, a parameter vital for the accurate dosimetry of radioiodine therapy. Clinical examination can give only a poor estimate of anatomical volume which may relate only indirectly to the volume containing radioactivity. Hamburger and Hamburger (1985) have noted that dosimetry is often largely empirical due to the difficulty of estimating the mass of thyroid tissue irradiated. The limitations of planar scintigraphy can be Overcome by performing tomographic imaging. It has already been shown (Ott et al, 1986b) that positron emission tomography of the distribution of 124I within the thyroid can achieve a spatial resolution of the order of 1 cm throughout the section and yield an estimation of thyroid volume with ∼ 5% accuracy for uniform distribution of tracer and ∼ 12% accuracy for multinodular disease. The limiting factor affecting dosimetry was shown by this study to be estimation of the biological half-life rather than the error in estimating the volume of functioning tissue.
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Webb et al. (1986) studied this question.
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