tissue), and CT with intravenous contrast agents (to assess
scar tissue). These techniques are discussed in the following
text.
Nuclear Imaging by PET with 18F-FDG
Although various tracers have been used in combination
with PET (11C-acetate and 82Rb), 18F-FDG is the tracer
most frequently used to assess myocardial viability. 18FFDG
is used to evaluate cardiac glucose use, and the tracer
is a glucose analog (one OH group is replaced by an 18F
atom). The initial tracer uptake in myocytes is comparable
to glucose uptake. After phosphorylation, 18F-FDG-6-PO4
remains trapped in myocytes, and further metabolism is not
possible, thus providing a strong signal for imaging. Like
glucose uptake, cardiac 18F-FDG uptake is strongly influenced
by metabolic circumstances, in particular, by plasma
levels of insulin and free fatty acids. Although insulin
stimulates cardiac glucose (and 18F-FDG) uptake, free fatty
acids inhibit glucose (and 18F-FDG) accumulation. This
situation may be mimicked by either oral glucose loading
or hyperinsulinemic euglycemic clamping. The majority of
cardiac 18F-FDG studies have been performed after oral
glucose loading, which is a simple and effective approach.
The main shortcoming is that image quality for patients
with impaired glucose tolerance or overt diabetes is poor.
Hyperinsulinemic euglycemic clamping can overcome this
problem, but this approach is time-consuming and laborious.
An alternative approach may be the use of nicotinic
acid derivatives; initial studies have demonstrated adequate
image quality, even for patients with diabetes.
Viability Criteria. For the optimal assessment of viability,
integration of function, perfusion, and 18F-FDG uptake
is needed. Regions with contractile dysfunction can exhibit
4 patterns of perfusion and 18F-FDG uptake. Viable tissue
can display normal perfusion and 18F-FDG uptake (chronic
stunning) or reduced perfusion and preserved 18F-FDG
uptake (mismatch pattern, hibernation) (Fig. 2) (26); scar
tissue (perfusion–18F-FDG match) can be further divided
into subendocardial and transmural scars, depending on the
percentage of tracer uptake.
Prediction of Outcome. Twenty studies with 18F-FDG
PET (total of 598 patients) aimed to predict improvement in
regional function after revascularization (18). The mean
sensitivity and specificity in these studies were 93% and
58%, respectively (Table 4) (18). The majority of these
studies used combined information on perfusion and 18FFDG
uptake. Moreover, when the studies that used 18FFDG
alone (without a flow tracer) were excluded from the
analysis of pooled data, a sensitivity of 88% and a specificity
of 74% were obtained. Improvement in global LV
function was evaluated in 12 18F-FDG PET studies with
333 patients. On average, the LVEF improved from 37% to
47% in patients with viable myocardium. In patients without
viable myocardium, the LVEF remained unchanged
(39% vs. 40%) (Table 5) (26).
Two studies (19,22) evaluated the relationship between
the presence of viability on 18F-FDG PET before revascularization
and improvement in symptoms after revascularization.
Both studies indicated that improvement in heart
failure symptoms after revascularization occurred predominantly
in patients with viable myocardium. Seven 18F-FDG
PET studies with 619 patients evaluated long-term prognosis
in relation to treatment (medical and revascularization)
and viability (absent or present) (27–33). Analysis of
the pooled data demonstrated that the highest event rate
was observed in patients who had viable myocardium and