'Hot spot' detection of ischemic myocardium in vivo by gamma camera imaging

Background - ⁹⁹Tc-HL91 is a new hypoxia imaging that demonstrates increase uptake and retention in globally hypoxic myocardium in vitro. The purpose of this study was to determine whether ⁹⁹Tc-HL91 could detect regional ischemia in vivo by gamma camera imaging. Methods and Results - Eight open-chest dogs with left circumflex (LCx) stenoses were studied. Injection of 5 mCi of ⁹⁹Tc-HL91 and microspheres was followed by imaging over 4 hours. Heart slices were imagined, then stained with triphenytetrazolium chloride (TTC), and tissues were well-counted. TTC staining demonstrated no injury. Mean LCx blood flow was 0.32±0.04 mL · min^-1 · g^-1, and mean left anterior descending coronary artery (LAD) flow was 0.96±0.02 mL · min^-1 · g^-1 (ratio, 0.33). 'Hot spots' were detected in 8 of 8 experiments in vivo within 60 minutes and improved over 4 hours. Region of interest analysis of LCx/LAD activity ratios demonstrated significant increases within 30 minutes (final ratios, 3.0;P<0.05). LCx and LAD washout curves demonstrated significant differences within 15 minutes. Washout curves were biexponential over 1 hour, followed by linear retention from 1 to 4 hours. Four-hour fractional retention was 0.12 for LAD and 0.44 for LCx (P<0.01). Myocardial flow versus tracer uptake demonstrated 2 phases: phase 1 (flow, 0.05 to 0.7 mL · min^-1 · g^-1) had an inverse linear correlation (r=0.80); phase 2, (flow,>0.7 mL · min^-1 · g^-1) had no correlation. ISchemic heart/liver ratios remained near 1.0 for 4 hours. Conclusions - ⁹⁹Tc-HL91 positively identifies regional myocardial ischemia in a canine model using ⁹⁹Tc imaging. Quantitative techniques allowed identification of ischemic myocardium within 15 minutes of tracer administration.