Myocardial thallium-201 kinetics in normal and ischemic myocardium.

Abstract

SUMMARY The initial myocardial uptake of thallium-201 depends on myocardial blood flow distribution. The phenomenon of delayed thallium redistribution after transiently or chronically altered myocardial perfusion has been described. The net myocardial accumulation of thallium-201 after injection depends upon the net balance between continuing myocardial extraction from low levels of recirculating thallium in the blood compartment and the net rate of efflux of thallium from the myocardium into the extracardiac blood pool. These experiments were designed to measure separately the myocardial extraction and intrinsic myocardial efflux of thallium-201 at normal and at reduced rates of myocardial blood flow. The average myocardial extraction fraction at normal blood flow in 10 anesthetized dogs was 82 ± 6% (±SD) at normal coronary arterial perfusion pressures and increased insignificantly, to 85 ± 7%, at coronary perfusion pressures of 10-35 mm Hg. At normal coronary arterial perfusion pressures in 12 additional dogs, the intrinsic thallium washout in the absence of systemic recirculation had a half-time (T½h) of 54 ± 7 minutes. The intrinsic cellular washout rate began to increase as distal perfusion pressures fell below 60 mm Hg and increased markedly to a T½ of 300 minutes at perfusion pressures of 25-30 mm Hg. A second, more rapid component of intrinsic thallium washout (T' 2.5 minutes) representing approximately 7% of the total initially extracted myocardial thallium was observed. The faster washout component is presumed to be due to washout of interstitial thallium unextracted by myocardial cells, whereas the slower component is presumed due to intracellular washout. The net clearance time of thallium measured after 'Lv. injection is much longer than the intrinsic myocardial cellular washout rate because of continuous replacement of myocardial thallium from systemic recirculation. Myocardial redistribution of thallium-201 in states of chronically reduced perfusion cannot be the result of increased myocardial extraction efficiency, but rather, is the result of the slower intrinsic cellular washout rate at reduced perfusion levels.DESPITE the widespread use of thallium-201 perfusion scintigraphy in clinical situations, there remain significant gaps in our knowledge concerning the kinetics of the radionuclide in normal and ischemic myocardium. The initial distribution of thallium in the myocardium immediately after i.v. injection is the result of both blood flow delivery of the tracer to the heart and the extraction of the tracer by the myocardium. ' 2 The delayed or equilibrium distribution, particularly under conditions of altered perfusion, is less well understood. The determination of relative myocardial thallium concentration as a function of time in sequential imaging studies is being used increasingly to detect and evaluate coronary artery disease.1' 3-9 The clinical use of delayed thallium redistribution imaging has brought about an acute need for an improved understanding of the mechanism of thallium kinet...

Effects of dipyridamole-induced vasodilation on myocardial uptake and clearance kinetics of thallium-201.

“…The influence of hypoperfusion on initial extraction and subsequent intrinsic washout of 201TI was previously examined in a canine preparation by means of a graded reduction in transmural myocardial flow produced by a hydraulic occluder around the LAD. 26 The intrinsic washout rate of 20 figure 7.…”

Current status of myocardial perfusion imaging radiopharmaceuticals for SPECT and PET imaging modalities

“…1,9 It decays by electron capture with a physical half-life of 73 hours and has x-rays of energy 67-82 kEv and gamma rays of 135-167 kEv. 5,10 Tl-201 has lower image quality than technetium labelled agents that limited its use. 8 About 85% of the administered amount of Tl-201 is extracted by the myocardial cell following the first coronary capillary circulation, which is higher than the Tc-99m labeled MPI radiotracers.…”

“…5,12,24 F-18 labeled radiotracers may be produced at regional cyclotrons in similar way as F-18-FDG. 10 F-18 FBnTP, is a member of a new class of positronemitting lipophilic phosphonium cation, which can pass biological membranes by passive diffusion. 12 It is localized intact mitochondria due to the negative mitochondrial membrane potential, like the Tc-99m sestamibi and Tc-99m tetrafosmin.…”

Assessment of myocardial perfusion with Tc-99m: image is everything

“…3 Experimental models clarified the effect of variable coronary blood flow and myocardial injury on thallium uptake and clearance. 4,5 Moderate tracer washout resulting in ''redistribution'' permitted the differentiation of ischemic from infarcted myocardium with a practical single day imaging protocol, 6 and Tl-201 was approved by the U.S. Food and Drug Administration (FDA) for clinical use in 1977. However, despite widespread clinical use, the relatively long half-life and suboptimal photon energy of Tl-201 for Anger camera imaging led to the search for perfusion agents labeled with Tc-99m.…”