Technetium Pertechnetate as a Thyroid Scanning Agent

131I is the accepted thyroid scanning agent in spite of the substantial radiation dose administered to the gland. Other isotopes of iodine (1,2) have been suggested but none have proved ideal. When Harper (3) introduced technetium 99m it was shown that satisfactory thyroid scans were possible with a marked reduction in the radiation dose (4–6). Because of this advantage we have compared the two isotopes for routine use as thyroid scanning agents. Method We evaluated the usefulness of technetium by performing thyroid scans with both 99mTcO4 and 131I on ninety-eight patients. First we made a 99mTc scan one hour after the intravenous administration of one to two millicuries of 99mTc. Then we administered orally fifty microcuries of 131I and made a scan with this nuclide the next day. At least a twenty-four-hour I31I uptake was measured. Both scans were performed with a scanner3 that had a crystal 5.1 cm thick and 7.6 cm in diameter. A 721-hole focused collimator, especially designed for use with low energy-emitting isotopes (7), was used for the 99mTc scans. The FWHM (full width at half maximum)—the width of the counting rate profile for a point source in air—for the 99mTc collimator is 0.66 cm at a focal length of 8.1 cm. A 61-hole focused collimator with a 0.6-cm FWHM for 131I at a focal length of 5.8 cm was used for the 131I scans. A scanning speed of 1.5 cm per second could be used with 99mTc as compared to a slower scanning speed of 0.5 cm per second required with 131I for acceptable picture quality. Results Scans of ninety-eight patients were analyzed to see if those with technetium provided the same information as those with 131I. Observation and subjective comparison of the two scans by five different observers was the basis for this analysis. In eighty-three of the ninety-eight patients 99mTc scans were comparable or superior to those with 131I. Two normal thyroid scans made on successive days with these two nuclides are shown in Figure 1. The count rate over the thyroid with 99mTc averaged from two to ten times the count rate with 131I in this group of patients. Included in this group were seventeen patients who had a single “cold” area found on the scan. This “cold” area corresponded to a palpable thyroid nodule in all cases. This type of lesion is well shown with 99mTc (Fig. 2). The nodule could be seen equally well with either isotope in eight of these scans but it was easier to define the borders of the nodule with 99mTc in the other nine. Figure 3 shows a nodule better defined with 99mTc than with 131I.