A multiobserver comparison of ^99mTcO₄ and ¹²³I thyroid imaging

Abstract

The thyroids of forty patients were imaged using 2 mCi (74 MBq) ^99mTc pertechnetate (^99mTcO₄) followed within one week by 2 mCi (74 MBq) ¹²³I Iodide. The images obtained were evaluated by eight observers for 6 morphological criteria and assigned to 6 diagnostic categories with a confidence grading on a seven level scale (grade 1 being that for maximum confidence). Images were obtained with ¹²³I for the same counts and the same times as those with ^99mTcO₄ and compared using an index in which the number of diagnostic categories and the mean confidence grading within each category were taken into account. The mean index obtained for ^99mTcO₄ images (9.2) was significantly greater (P>0.05, thus representing a lower observer confidence and less interobserver diagnostic agreement) than the mean indices for ¹²³I equal count images (5.6) and ¹²³I equal time images (6.8). The diagnoses made on the basis of ¹²³I images were more frequently concordant with the final diagnosis after six months follow up (75% for equal counts and 77% for equal time) than those with ^99mTcO₄ (63%). The radioisotope of iodine most suited to modern nuclear medicine instrumentation and with the most favourable radiation dosimetry is ¹²³I which is recommended for those areas of thyroid imaging where iodine is superior.