Nuclear Imaging by SPECT with 201Tl-Chloride
The initial uptake of 201Tl is mainly determined by
regional perfusion, whereas sustained uptake, over a longer
period of time, depends on cell membrane integrity and
thus myocyte viability. Although many protocols are available,
the 2 protocols most frequently used are stress–
redistribution–reinjection imaging and rest–redistribution
imaging. The first protocol provides information on both
stress-inducible ischemia and viability, whereas the latter
provides information only on viability.
Viability Criteria. Four markers of viability are normal
201Tl uptake (normal perfusion) at stress, stress defects with
redistribution (reversible defects) on 3- to 4-h delayed
images, redistribution in fixed defects on images at redistribution
after reinjection or on delayed rest images (Fig. 3)
(frequently a threshold of a 10% increase in tracer uptake is
used), and tracer uptake of greater than 50% on redistribution–
reinjection images or on delayed rest images. The first 3
markers reflect jeopardized but viable myocardium, but the
fourth marker is more complex. Frequently, segments with
tracer uptake of greater than 50% do not improve in function;
the reason for this observation is the presence of nontransmural
infarction, rather than jeopardized, hibernating
myocardium (assuming adequate revascularization). Segments
with nontransmural infarction contain viable tissue
(and thus frequently exhibit .50% tracer uptake) but are
not always capable of showing improvement in function
after revascularization because of the presence of fibrosis.
Prediction of Outcome. Thirty-three studies (22 with
rest–redistribution and 11 with a reinjection protocol) (total
of 858 patients) focused on the prediction of improvement
in regional function after revascularization (18). The mean
sensitivity and specificity in these studies were 86% and
59%, respectively (Table 4) (18). The lower specificity may
be related to the definition of viable myocardium; as stated
earlier, segments with tracer uptake of greater than 50% are
classified as viable but frequently contain subendocardial
scar tissue and may not improve in function after revascularization.
A higher accuracy for the prediction of improvement
in function was obtained when inducible ischemia
was present in segments with tracer uptake of greater than
50% (34).
Improvement in global LV function was evaluated in 5
studies with 96 patients; on average, the LVEF improved
from 30% to 38% in patients with viable myocardium. In
patients without viable myocardium, the LVEF remained
unchanged (29% vs. 31%) (Table 5).
Only one study focused on the prediction of improvement
in heart failure symptoms; Mule et al. (23) demonstrated that