Effect of time on liver clearance of technetium-99m-tetrofosmin in patients with acute chest pain: when should imaging begin?

Due to stable myocardial retention and technetium imaging characteristics, 99mTc-tetrofosmin has been considered potentially useful for acute chest pain imaging. Tetrofosmin also has favorable biokinetics with reported rapid liver clearance, 5 min poststress and 30-45 min post-rest injection. Since comparable data are not available, the effect of time on liver clearance was evaluated in patients with acute chest pain. One hundred six patients received an intravenous injection of 25-30 mCi 99mTc-tetrofosmin to evaluate acute chest pain. SPECT imaging was performed 15-120 min after injection of the tracer. Patient images were grouped according to the time of acquisition after acute injection: 15-30 min, 31-45 min, 46-60 min, 61-90 min and > 90 min. Quantitative analysis was performed of a similar anterior projection for each patient consisting of 6 X 6-pixel region of interest over the myocardium and adjacent liver. Average counts per pixel were determined and a heart/liver (H/Li) ratio was calculated. The mean H/Li ratio was < 1.0 for patient images acquired 15-45 min after injection, and > 1.0 for patient images acquired after 45 min. The difference was statistically significant (p < 0.05). Quantitative analysis suggests that the optimal imaging time should be at least 45 min after the injection of 99mTc-tetrofosmin to allow adequate liver clearance before image acquisition of acute chest pain syndromes.