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Cardio Diabetes Medicine 2017 373

but still viable. Such tissue is often hypokinetic or aki- mic insulin clamp technique.
netic but returns to normal or near-normal function if Under fasting conditions the normal myo-
blood flow is restored. Consequently, in patients with cardium primarily utilises fatty acids, while glucose
severely impaired ventricular function, combined with utilisation and thus FDG uptake becomes minimal.
measurements of myocardial perfusion & glucose Under this condition, ischaemic myocardium with en-
metabolism have been advocated. It may be note- hanced glucose metabolism markedly takes up FDG,
worthy to mention that eating fat & protein shifts the which is displayed on PET images as hot spots. The
heart away from using glucose as an energy source.
problem of imaging under fasting conditions is the
heterogeneous distribution of FDG in to normal myo-
Metabolic tracers. cardium. Usually the lateral wall of the left ventricle
Fluorine-18 FDG, C-11 palmitate, & C-11 acetate are tends to show higher FDG uptake than the septum.
typical examples of PET radiopharmaceuticals used High background activity with low tissue FDG uptake
for metabolic cardiac studies. ‘Deoxyglucose’ is an also leads to poor image quality. For these reasons
analog of glucose that can be labeled with F-18, a FDG PET under fasting condition, the interpretation
cyclotron produced radionuclide, to form F18-FDG. Its showed carefully be done for clinical viability assess-
myocardial uptake reflects overall myocardial utiliza- ment.
tion of glucose. Palmitate is a naturally occurring fat-
ty acid that can be chemically synthesized & labeled The most commonly used protocol is oral glucose
with C-11, a cyclotron-produced radionuclide with a loading. Ingestion of about 50-70g of glucose stim-
physical half-life of approximately 20.4 minutes. Its ulates insulin secretion & increases the FDG uptake
myocardial uptake & clearance reflect the myocardial into normal myocardium to near maximum. This en-
utilization of fatty acids. hances the image quality with homogenous myocar-
dial FDG uptake. Euglycaemic insulin clamping is an
alternative technique to oral glucose loading and is
Fluorine-18 FDG metabolic Imaging slightly more complex but guarantees more stable
18F- FDG is a glucose analogue which crosses the & controlled metabolic conditions. FDG uptake in to
capillary and sarcolemmal membrane at a rate pro- normal & ischaemic but viable myocardium is en-
portional to that of glucose. Following myocardial hanced and negative FDG uptake is considered to
uptake, FDG is phosphorylated to FDG phosphate indicate scar tissue.
and is then trapped in the myocardium unlike phos-
phorylated glucose. Regional myocardial uptake of Thus hibernating myocardium therefore would
FDG therefore reflects relative distribution of region- demonstrate increased FDG uptake in the fasting
al rates of exogenous glucose utilization, unlike the state unlike the surrounding normal myocardium. But
brain which mainly depends upon glucose metabo- in the post prandial state (with oral glucose loading)
lism. The myocardium is an omnivore. There are three hibernating myocardium would demonstrate FDG up-
major circulating substances which affect the myo- take. Therefore either preserved or even enhanced
cardial metabolism: blood glucose, FFAs and insulin. FDG uptake in dysfunctional myocardial regions rep-
Many other factors, such as the fasting period & age, resent presence of myocardial viability by the help of
also affect the FDG uptake. Even hospitalization sta- most popular criterion of flow-metabolism mismatch
tus (in patient or out patient) has an influence on the methods. However using this criterion requires a per-
myocardial FDG uptake . fusion image preferably acquired either with PET or
SPECT perfusion study.
This complexity interferes with the interpretation of
data obtained from FDG-PET imaging. To accommo- Regional dysfunction due to stunned myocardium
date this complexity of metabolism there are several may be manifested by normal blood flow & normal,
different protocols for myocardial FDG PET imaging. enhanced or reduced glucose utilization using FDG
These protocols can be divided into two major cat- & flow images. Only criteria to diagnose this myo-
egories. cardium is the presence of regional myocardial wall
motion abnormalities.
I) One is imaging under low myocardial glucose
metabolism which includes imaging under fasting
conditions.
II) The other protocol images the myocardium under
high to maximum myocardial glucose metabolism
which includes oral glucose loading & the euglyce-

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