Comparison of 99mTc and 123I for thyroid imaging.

Thyroid images were obtained with 99mTc and 123I, using a scintillation camera and pinhole collimator, from 85 patients selected from over 1,000 patients predisposed to the development of thyroid neoplasms by prior radiotherapy. In 66 of 85 patients 99mTc and 123I gave essentially similar information, whereas in 19 patients some thyroid-image disparity was seen. In 11 of these 19, focal areas of increased 99mTc concentration were not seen with 123I. Radioiodide images showed slightly better contrast than the 99mTc images but required a longer imaging time and usually did not provide more information; in a few cases 99mTc showed abnormalities more readily. Regardless of which radionuclide was used, oblique views were needed to define some abnormalities. Currently 123I does not provide significant advantages over 99mTc for routine thyroid imaging. However, 123I is preferred in patients with very poor thyroid function and in those with suspected retrosternal thyroid tissue. In addition, areas of increased 99mTc concentration should also be studied with 123I.