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

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 less than 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.