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Estimation of Thyroid Depth and Correction for I-123 Uptake Measurements

Veterans Administration Hospital, San Francisco, California

Correspondence: For reprints contact: Peter M. Martin, Veterans Administration Hospital, 4150 Clement St., San Francisco, CA 94121.

ABSTRACT

A new technique has been developed to correct I-123 uptake measurements for the effect of gland depth. The method uses the effect of differential tissue absorption and/or scatter of photons of different energies, and measures the ratio of counts of the primary I-123 emission to the counts of the tellurium K shell x-ray to determine a depth-correction factor. A comparison of this new method against three previously reported methods indicates that the present method provides the most sensitive index of gland depth. The method is sensitive to depth changes, is not significantly dependent on detector distance (p > 0.05) for distances greater than 18 cm, and is not dependent on gland size (p > 0.25), within the range 20–40 ml. In a group of 40 patients, the ORINS phantom method was found to underestimate the mean gland depth by about 1 cm, thereby causing an average uptake error of 23%. The application of the depth-correction factor was found to change the interpretation of uptake estimates in approximately 10% of the cases in this limited series.