 The
vast majority of PET scans performed worldwide utilize the glucose
analog.
(fluoro-deoxyglucose, FDG) This glucose tracer is administered through
an IV injection where it mixes with native glucose.
The acquisition of PET images commences approximately 60 minutes after
injection. As cells take in native glucose to break down to ATP for
their energy requirements, a proportional amount of FDG is transported
and phosphorylated to fluoro-deoxyglucose-6-phosphate, where it is
effectively trapped.
The PET scanner then images the FDG concentration, essentially
providing tomographic images of glucose metabolism throughout the
body.
PET images are often presented as a series of orthogonal views.
- Axial
- Sagittal
- Coronal
Maximum Image Projection "MIP"
Thyroid and gastric glucose metabolism varies.
The myocardium can burn glucose, lactate or fatty acids for energy.
It can also appear "hot" on an FDG PET scan if the heart
is burning glucose or cooler if the heart is burning lactate or
fatty acids. The brain utilizes glucose exclusively for its energy
requirements.
Aggressive, fast growing tumors have an increased rate of glycolysis
and will appear as a “hot spot” on the PET scan. A semi-quantitative
number, the SUV (standardized uptake value), can be calculated for
each abnormality.
On subsequent PET scans, comparison of SUV’s helps determine
to what degree each tumor is responding to therapy.
Although FDG is currently the cornerstone of clinical PET, the future
progress in PET will come with the advent of new radiopharmaceuticals
that will reveal more specific biochemical activity in the body.
For example, note the PET methionine image which clearly delineates
a brain tumor (anaplastic astrocytome grade III) before any contrast
enhancement can be seen on the T1-weighted MR scans.
The last set of PET images are of F-18 labelled Fallypride, a D2
dopamine receptor antagonist. Note the differential uptake of this
PET tracer with maximum activity exhibited in the basal ganglia.
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