STRESS INJECTION PROTOCOLS FOR MYOCARDIAL SCINTIGRAPHY WITH TC-99(M)-SESTAMIBI COMPARED WITH TL-201 - IMPLICATIONS OF EARLY POSTSTRESS KINETICS/

Stress/injection protocols developed for myocardial perfusion imaging with Tl-201 may not be optimal for Tc-99(m)-sestamibi (MIBI), an agent with lower myocardial extraction and higher abdominal uptake; prolong ation of exercise after radiotracer injection might improve these rela tive drawbacks of MIBI. We compared the kinetics of MIBI and Tl-201 by acquiring dynamic planar images for 5-7 min after a bolus injection ( n = 180 studies) with stress performed by supine bicycle exercise alon e, intravenous dipyridamole or combined stress. Routine or prolonged p rotocols involved continuation of exercise for 1 or 2.5 min respective ly after tracer appearance in the heart. Subsequently, the perfusion i mages obtained were categorized as normal or showing significant defec ts. Myocardial uptake of MIBI, normalized for injected dose, body weig ht and camera sensitivity, was only 40% of that for Tl-201; there were no differences based on test mode or scan result for either perfusion tracer. During the second minute after injection, the cavity/myocardi al ratios, an index of blood pool activity, were elevated with MIBI by 25% when compared with Tl-201 (p < 0.001). During the third minute, c avity activity was again higher with MIBI, but only in those subjects with abnormal scans. The prolonged exercise phase did not prevent prog ressive accumulation in the abdomen, but did allow cavity levels to de cline before termination of exercise. The prolonged protocol may ensur e that myocardial uptake of MIBI is completed during peak blood flow, and therefore is recommended for stress with exercise or with dipyrida mole and exercise in combination.