Thyroid Scanning with Technetium 99m Pertechnetate

Technetium 99m pertechnetate (99mTcO4−) is a useful radioactive label for thyroid scanning. It is of particular merit when emphasis is placed upon a low dose of thyroid radiation, optimal resolution, and scanning of thyroid glands in which function is decreased or partly blocked. Harper et at. (4) reported the first pertechnetate scan of the thyroid in 1964, and the next year the same group published a paper on pertechnetate thyroid scans in a series of 86 patients (1). Recently, Quinn and Brand (6) reported on 767 patients in whom pertechnetate scans were obtained of the thyroid gland, incidental to brain scans. Certain aspects of this method of thyroid labeling deserve further elucidation and are described in the present report. Material and Method During a six-month period, the thyroid glands of 41 selected patients were scanned with pertechnetate. All patients with hypothyroidism or iatrogenically suppressed thyroid function who had insufficient uptake for 131I thyroid scanning were included in the series. Pertechnetate scans were done in euthyroid patients for three reasons: (a) The dosimetry in children is favorable. (b) The high count rates and ease of collimation allow better evaluation of small thyroid nodules. (c) In patients not able to return the following day for a twenty-four-hour, 131I thyroid scan, scanning is possible within an hour after administration of the label. 99mTcO4 −was administered intravenously in doses of 1 to 5 mCi. The usual dose in adults was 2 mCi and in children 1 mCi. After a period of thirty to sixty minutes, both dot and photoscans were obtained with a 3-inch rectilinear scanner.2 The pulse-height analyzer was set for a 126-to-154-keV window, the time constant at 0.02 second, and scanning speed at 30 or 44 em per minute. A 127-hole, finefocus collimator was employed. The correct setting of background suppression, which is of critical importance to the success of this method, is discussed later. The suppression levels were set at 10 to 70 per cent, allowing only a minimal imaging of nontarget areas. 131I thyroid scans are generally performed after twenty-four hours. By that time most of the labeled iodide in the thyroid gland has been organified, as thyroid hormone and serum levels of iodide are negligible. In contrast, scanning of the thyroid gland with pertechneta.te is performed about one hour after its intravenous administration, when relatively high nontarget count rates are always present. Selection of background suppression depends on the ratio of count rates over the gland to nontarget rates adjacent to it. Figure 1 illustrates some typical examples of target and nontarget count ratios found during programming of a pertechnetate thyroid scan. The background suppression in each instance was set appropriately. In practice, the background suppression is set just high enough to eliminate most of the nontarget counting from the scan.