Efficacy of thyroid blockade on thyroid radioiodine uptake in 123I-mIBG imaging.

UNLABELLED: Although iodinated radiopharmaceuticals usually contain a small quantity of unbound iodine, it is difficult to establish the degree to which thyroid activity on scintigraphic images reflects uptake of free radioiodine. The objective of the present study was to examine the effectiveness of thyroid blockade in subjects undergoing (123)I-meta-iodobenzylguanidine (mIBG) imaging and to estimate the relative contribution of bound and unbound radioiodine to imaging findings. METHODS: All subjects were participants in prospective trials of (123)I-mIBG cardiac imaging in which pretreatment with thyroid blockade was optional unless locally required. In a pilot project, 15 subjects (6 blocked) had thyroid uptake measured at 4 h using a probe system. Fifteen-minute (early) and 4-h (late) anterior planar chest images that included the thyroid region were visually scored for thyroid uptake (scale of 0-4) in another group of 152 subjects (98 blocked). Quantitative analysis based on thyroid regions of interest was performed on anterior planar images from a further sample of 669 subjects (442 blocked). For all 3 investigations, quantitative comparisons of thyroid uptake were made between the blocked and nonblocked subjects. RESULTS: There was no statistical difference between probe uptake of the 6 blocked and 9 nonblocked subjects. However, in the second series, mean visual score on the late images was significantly lower for blocked than nonblocked subjects (P < 0.001). In the region-of-interest analyses, net thyroid counts were significantly higher on the late images of nonblocked subjects (P < 0.0001), and compared with early images, 87% of subjects who received blockade showed decreased or unchanged counts whereas 75% of nonblocked subjects had increased net thyroid activity. In nonblocked subjects, an estimated 79% of thyroid counts on late images could be attributed to unbound (123)I. CONCLUSION: On the basis of 3 different methods for assessing thyroid uptake of (123)I, use of thyroid blockade pretreatment in (123)I-mIBG imaging prevents increase of thyroid activity over time because of uptake of unbound (123)I. In most subjects, there is a low level of (123)I-mIBG thyroid activity that probably represents specific uptake in sympathetic nerve terminals.

Normalization of abnormal T-waves during stress testing does not identify patients with reversible perfusion defects.

OBJECTIVE: To determine if T-wave normalization during exercise or dobutamine stress testing identified patients with myocardial ischemia as indicated by reversible perfusion defects. METHODS: Exercise or dobutamine stress tests with perfusion scintigraphy were performed in 1,173 patients with abnormal T-waves on their baseline electrocardiograms. The results of perfusion scintigraphy were compared in patients with and without stress-induced T-wave normalization. RESULTS: Only 33 of 270 patients with reversible perfusion defects (12.2%) had T-wave normalization during stress while 76.4% of 140 patients who had T-wave normalization during stress did not have a reversible perfusion defect. Results were similar for patients who did or did not reach 85% of their maximal predicted heart rate, for patients with and without Q-wave infarction on the baseline EKG and for patients who did or did not have ischemic ST-segment depression during stress. CONCLUSIONS: T-wave normalization during stress testing has low sensitivity and poor positive predictive value for stress-induced reversible myocardial ischemia.