Influence of patient height and weight and type of stress on myocardial count density during SPECT imaging with thallium-201 and technetium 99m-sestamibi

Background. The aim of this study was to determine the variability in myocardial activity with 99mTc-labeled sestamibi and 201Tl tomographic imaging, and to correlate this variability with patient anthropometric data and type of stress. Methods and Results. Conventional tomographic acquisition (all-purpose collimator, 30 views, 40 sec/view) was performed in 249 patients (155 with 99mTc-sestamibi, 84 with 201Tl). Normal myocardium was identified in three short-axis tomographic slices between the base and mid-ventricle. To exclude abnormal myocardium, all myocardial activity below an 85% threshold of peak counts was excluded. The average counts per pixel in the remaining myocardium was determined. All data were normalized to a collimator sensitivity of 8.1 counts/min/kBq and corrected for decay. Myocardial count densities in both 201Tl and 99mTc-sestamibi studies exhibited a high degree of variability and varied by factors of 3 to 4 and 2.5 to 3 respectively, between the 10th and 90th percentile of count densities. All 99mTc-sestamibi studies showed an inverse correlation (R ~ 0.7) between myocardial count density and patient weight. For stress 201Tl studies in male patients, a similar inverse correlation (R = 0.63) was found between patient weight and myocardial count density. No correlation was found for female patients. For rest 99mTc-sestamibi studies, the average myocardial count density was 1245 ± 297 counts/pixel for a 1.11 GBq (30 mCi) injection. This was similar with pharmacologic stress (1224 ± 297 counts/pixel per 1.11 GBq (30 mCi), but significantly higher with exercise (1569 ± 363 counts/pixel per 1.11 GBq (30 mCi) (p < 0.005). 201Tl studies averaged 327 ± 97 counts/pixel (111 MBq [3 mCi] injection) with exercise and 374 ± 135 counts/pixel/111 MBq (3 mCi) with pharmacologic stress (p = NS). Redistribution studies after exercise/pharmacologic stress (with a 37 MBq [1 mCi] reinjection) averaged 267 ± 83 counts/pixel (148 MBq [3 + 1 mCi]). Conclusions. Measured count densities in the myocardium demonstrate a high degree of variability that is only weakly correlated with patient anthropometric data. Nevertheless, knowledge of the mean values of myocardial activity as a function of administered dose may be useful in the optimization of rapid imaging procedures for 99mTc-based perfusion agents.