Neuroimaging Assessment of Traumatic Brain Injury - Animal Models of Acute Neurological Injuries: Injury and Mechanistic Assessments - page 263

Micro-CT uses an X-ray source and detector that rotate around
the stationary subject, generating hundreds of projection views
that are computer integrated to construct 3-dimensional anatomical
images. In the clinic, CT is the neuroimaging method of choice for
acute trauma patients because it can detect skull fractures and
intracranial bleeding, results can be obtained very quickly (from a
few seconds to a few minutes), and CT is generally compatible
with life-support in the acute state. Similarly, in the research setting
rapidity and cost-effectiveness are practical advantages of micro-
CT. A high-resolution micro-CT scanner can produce anatomical
images with spatial resolution reaching tens of micrometers.
Despite this excellent spatial resolution, CT images provide
relatively poor contrast to differentiate the structures in soft brain
tissue. Especially after mild-to-moderate brain trauma, micro-CT
may not detect certain tissue abnormalities that are visible with
other imaging techniques such as MRI. The potential risks of
irradiation exposure to both animal and researcher comprise a
further drawback of micro-CT imaging.
In future studies, injectable contrast media might help address
the problem of low tissue contrast with CT imaging. For example,
iodine-labeled contrast agents allow visualization of the cerebral
vasculature in exquisite detail, and could be employed to evaluate
vascular dysfunction and/or BBB disruption in experimental TBI.
Micro-CT scanners are now available from many commercial
sources (e.g., Scanco Medical, GE Healthcare, Siemens, Skyscan).
SPECT is a functional imaging approach which shows the dis-
tribution of radiolabeled tracer compounds in vivo. Unlike the
positron-emitting tracers used in PET imaging, SPECT tracers
(e.g., technetium-99 m, indium-111, iodine-131) emit gamma
rays. SPECT tracer isotopes are not naturally found in the body
and must be attached to the biological molecule of interest. SPECT
imaging is widely applied in clinical practice, although it is most
commonly used for cardiac imaging and relatively underemployed
for neurological evaluations. The spatial resolution of SPECT in
small animals is similar to PET at 1-2 mm.
A signifi cant advantage of SPECT over PET imaging is that,
on the one hand, SPECT tracers have much longer half-lives.
Accordingly, SPECT tracers are more readily available from com-
mercial sources and do not require an on-site cyclotron for pro-
duction, reducing operation costs. On the other hand, SPECT
frequently exhibits lower sensitivity than PET imaging. An addi-
tional drawback lies in the potential risk to the investigator and the
animal subjects from exposure to radioactivity, particularly in lon-
gitudinal studies requiring multiple injections of radiotracers.
SPECT imaging of intravenously administered technetium is
used clinically to assess regional cerebral blood fl ow (rCBF) and
could be applied in future studies to track rCBF defi ciencies after
experimental TBI. We caution that the utility of this approach,