Technetium-99m Q4: a prototype cationic perfusion radiotracer with myocardial washout

Technetium-99m Q4 is derived from an existing mixed ligand myocardial tracer (99mTc-Q3) by the addition of an ester group to promote myocardial washout. Six subjects with single-vessel coronary disease documented by angiography and/or Q wave myocardial infarction documented by electrocardiography were studied with 99mTc-Q4 injection during exercise and with comparative thallium-201 tomography. Six healthy volunteers were also studied with 99mTc-Q4 imaging following injection at peak exercise. Tomographic images with 99mTc-Q4 and 201Tl each provided correct assessment of the presence or absence of coronary disease in 22 of 30 myocardial segments (73.3%). Six myocardial segments showed defect reversibility with 99mTc-Q4, whereas 14 segments showed reversibility with 201Tl, but the latter included three segments with no angiographic or electrocardiographic evidence of disease. In both normals and subjects with coronary artery disease, significant global washout of 99mTc-Q4 was observed over 4 h. For five patients with angiographic evidence of unrevascularized coronary artery stenosis, the ischemic to normal zone count ratio increased from 0. 782+/-0.107 at 45 min postexercise to 0.891+/-0.115 at 4 h postexercise (P = 0.016), suggesting occurrence of differential washout. It is concluded that addition of an ester group functionality to a previously studied mixed ligand cardiac tracer promotes global and regional myocardial tracer washout. Nevertheless, demonstration of perfusion defect reversibility with comparable frequency to that observed with 201Tl stress and reinjection images, required separate injections of 99mTc-Q4 at peak stress and at rest.