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

Abstract.

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.