Neuroimaging Assessment of Traumatic Brain Injury - Animal Models of Acute Neurological Injuries: Injury and Mechanistic Assessments - page 260

MRS acquisitions can be completed in the same scanning session
as an MRI study and the data sets are inherently registered in three-
dimensional space allowing direct comparison between structure
and metabolism. Moreover, compared to the radionuclide studies
described below, labeled MRS studies do not require radioactive
isotopes meaning that longitudinal studies on individual animals
are feasible. Another advantage of isotopically-enriched MRS is
that the metabolic fate of specifi c labeled nuclei can be followed as
the labeled compound is metabolized over time. In contrast, the
sensitivity of MRS is lower than the various forms of radionuclide
imaging resulting in poorer spatial and/or temporal resolution.
Finally, although radiolabeled analogs of many drugs are available
for PET and SPECT tracer studies, the relatively low concentra-
tions of drugs mean that analogous labeled studies using 13 C, for
example, are likely not feasible.
4. Micro-PET
Positron emission tomography (PET) is a functional imaging
modality which tracks the localization of positron-emitting radio-
active tracer molecules administered to the test subject. As the
radioactive tracer decays, each emitted positron collides with an
electron within the tissue nearby, producing 2 gamma rays travel-
ing in opposite directions. These gamma rays are detected by an
array of scintillation detectors surrounding the subject, and signals
are fed to a computer, which localizes the sources of the gamma
rays and combines these points into a 3-dimensional image. The
term “micro-PET” refers to a high-resolution PET system designed
specifi cally for imaging in small animal research models. The spatial
resolution of micro-PET scanners currently approaches 2 mm.
One of the most widely used tracer molecules for micro-PET
imaging is 18 F-fl uorodeoxyglucose ( 18 F-FDG). This radioactively
labeled glucose analog makes it possible to measure regional
changes in brain metabolism after TBI (Fig. 3 ). Intravenously
administered 18 F-FDG crosses the BBB, is taken up by active cells,
and is phosphorylated by hexokinase, creating a polar molecule
that cannot diffuse out of the cell. The regional accumulation of
18 F-FDG in the brain thus refl ects the local metabolic rate of glucose,
which in turn is linked to neuronal activity. Resting-state meta-
bolic depression identifi ed with the 18 F-FDG method correlates
with behavioral impairment in animal models of TBI ( 2 ) and
with Glasgow coma score after brain trauma in humans ( 21 ).
Future studies could make use of 18 F-FDG PET to investigate
trauma-induced changes in neuronal activation in response to a
specifi c sensory stimulus or learning task. If a stimulus is applied or
a behavioral task is completed during the tracer uptake period