Myocardial 99m Tc-Tetrofosmin Uptake During Adenosine-Induced Vasodilatation With Either a Critical or Mild Coronary Stenosis

BACKGROUND: Clinical studies have shown a comparable coronary stenosis detection rate between 99mTc-tetrofosmin and 201Tl but with smaller defect magnitudes for 99mTc-tetrofosmin. The goals of this study were to measure the first-pass extraction fraction (EF) of 99mTc-tetrofosmin in canine myocardium and to compare 99mTc-tetrofosmin with 201Tl uptake during adenosine-induced vasodilatation in dogs with various degrees of coronary stenosis. METHODS AND RESULTS: EF was calculated in 4 anesthetized, open-chest dogs after intracoronary administration of 125I-labeled albumin and 99mTc-tetrofosmin. In another 16 dogs with either critical (n=6) or mild (n= 10) left anterior descending coronary artery (LAD) stenoses, 201Tl and 99mTc-tetrofosmin were administered during adenosine infusion (250 microg x kg(-1) x min(-1)). Dogs were killed 5 minutes later, and tracer activities were determined by ex vivo imaging of heart slices and by well counting. Mean 99mTc-tetrofosmin EF was 54.0+/-3.7%. In the 6 critical-stenosis dogs, the LAD-to-left circumflex artery (LCx) microsphere flow ratio was 0.22+/-0.02 with adenosine. The LAD-to-LCx activity ratios were 0.37+/-0.04 for 201Tl and 0.67+/-0.05 for 99mTc-tetrofosmin (P<.01). For the 10 mild-stenosis dogs, the LAD-to-LCx flow ratio was 0.44+/-0.05. The 201Tl activity ratio was 0.58+/-0.04, compared with 0.81+/-0.04 for 99mTc-tetrofosmin (P<.01). Thus, in both groups, 99mTc-tetrofosmin uptake underestimated the flow disparity more than 201Tl. Similarly, magnitudes of ex vivo image defects were significantly greater for 201Tl than for 99mTc-tetrofosmin in both groups. CONCLUSIONS: In this canine model, relative underperfusion with adenosine stress is better resolved with 201Tl than with 99mTc-tetrofosmin and may be explained by the lower EF for 99mTc tetrofosmin. With clinical imaging, greater 201Tl attenuation and redistribution may lessen this advantage.