Comparison of Thallium-201 Resting Redistribution With Technetium-99m–Sestamibi Uptake and Functional Response to Dobutamine for Assessment of Myocardial Viability
Authors: Sansoy Vedat , Glover David K. , Watson Denny D. , Ruiz Mirta , Smith William H. , Simanis Juris P. , Beller George A. 
Published: June 2012

Background
201
Tl scintigraphy is useful for determination of viability in patients with coronary artery disease and depressed left ventricular function. Whether
99m
Tc sestamibi is adequate for viability detection remains controversial. The primary goal of this study was to compare
99m
Tc-sestamibi uptake with
201
Tl uptake in canine models of sustained low flow and regional asynergy for determination of viability. A secondary objective was to compare myocardial uptake of these tracers with the functional response to low-dose dobutamine.

Methods and Results
In protocol 1, 14 open-chested, anesthetized dogs with a 50% reduction in resting left anterior descending coronary artery (LAD) flow underwent 1 hour of transient LAD occlusion followed by reperfusion through the severe stenosis. Then 1.0 mCi of
201
Tl was injected, and serial imaging was performed 5 minutes and 2 hours later. After acquisition of the delayed
201
Tl image, 10 mCi of
99m
Tc sestamibi was injected, and imaging was repeated 45 minutes later. No significant difference was seen between the
201
Tl defect ratio (LAD/left circumflex coronary artery [LCx]) on redistribution images (0.62±0.02) and
99m
Tc-sestamibi defect ratio (0.60±0.02). Similarly, LAD/LCx activity ratios by gamma-well counting were comparable (0.62±0.02 versus 0.59±0.04) and reflected the flow decrement. Systolic thickening was −11±3% at the time of tracer injection. In protocol 2, 16 dogs underwent serial
201
Tl and
99m
Tc-sestamibi imaging during a 50% reduction in LAD flow with no superimposed transient LAD occlusion. In this model, the
99m
Tc-sestamibi LAD/LCx image defect ratio (0.61±0.03) was significantly less than the
201
Tl redistribution image defect ratio (0.66±0.03,
P
<.01). In 10 dogs, the stenosis was released, resulting in a significant increase in systolic thickening (
P
=.003), which increased further in response to 5 μg · kg
−1
· min
−1
of dobutamine (
P
=.02). In contrast, thickening increased only from −7±3% to 2±4% (
P
=.004) in response to dobutamine infusion in the remaining 6 dogs with persistent severe LAD stenoses. In protocol 3, 5 dogs received both
201
Tl and
99m
Tc sestamibi to compare the degree of delayed redistribution between tracers at 2 hours. The LAD/LCx microsphere flow ratios when
201
Tl and
99m
Tc sestamibi were injected were 0.44±0.06 and 0.43±0.05 (
P
=NS), respectively. The LAD/LCx activity ratio by gamma-well counting was greater for
201
Tl (0.56±0.08) than
99m
Tc sestamibi (0.50±0.07) at 2 hours of redistribution (
P
<.05), indicating greater redistribution for
201
Tl. The LAD/LCx
99m
Tc-sestamibi defect ratios on serial imaging improved from 0.49±0.07 to 0.52±0.07 (
P
=.0005), consistent with a slight amount of
99m
Tc-sestamibi redistribution. In protocol 4, no difference between
201
Tl and
99m
Tc-sestamibi defect magnitudes was seen in 4 dogs undergoing 3 hours of total LAD occlusion and ligation of visible coronary collaterals. Infarct size was 68±19% of the risk area.

Conclusions
Although
99m
Tc-sestamibi and
201
Tl defect magnitudes and regional activities were comparable in dogs with sustained low coronary flows and superimposed subendocardial infarctions and in dogs with large infarctions, approximately 5% more
201
Tl than
99m
Tc-sestamibi uptake was observed in dogs with chronic low flow and severe systolic dysfunction. Substantial
99m
Tc-sestamibi uptake in asynergic zones was observed in this low-flow model, with some slight resting
99m
Tc-sestamibi redistribution observed on serial images. Systolic thickening was negligibly enhanced during dobutamine infusion in dogs with sustained low flow, whereas
201
Tl uptake was only mildly reduced.