Neuroimaging Assessment of Traumatic Brain Injury - Animal Models of Acute Neurological Injuries: Injury and Mechanistic Assessments - page 262

immunostaining. Therefore, the development of a sensitive and
specifi c APP tracer for noninvasive neuroimaging would be of
considerable clinical interest.
The half-life of the tracer radioisotope is an important con-
sideration in the design of micro-PET studies. Although 18 F-FDG
is likely the most commonly used PET tracer, other 18 F-labeled
tracers are available, and biologically signifi cant isotopes such as
11 C, 13 N, 15 O, and 124 I can also be used. For tracers incorporating
longer half-life isotopes such as 18 F and 124 I (about 2 h to 4 days),
it is feasible to transport these tracers a short distance to the
imaging facility, even from nearby cities. However, the relatively
short half-life of 11 C, 13 N, or 15 O tracers (ranging from 2 to
20 min) requires an on-site cyclotron since the decay rate is too
rapid for adequate sensitivity.
In summary, particular strengths of micro-PET imaging include
the ability to assess metabolic function in the brain of a living
animal, the potential to assess neuronal response to a stimulus or
learning task, and the potential of novel tracers to provide insight
on a host of molecular functions in vivo. Disadvantages of micro-
PET include the necessity of working with radioactive materials,
the possible effects of administering radioactive tracers to animals,
particularly in studies that require multiple scans over time, and the
possible requirement for an on-site cyclotron. In addition, the spa-
tial resolution of micro-PET is relatively poor compared to MRI
and CT. To address this issue, an increasing number of studies are
using micro-PET in combination with MRI or CT, where the
functional PET imaging data can be co-registered with higher-
resolution images to provide the anatomical context. For most
laboratories, combined PET/MRI or PET/CT will involve separate
image acquisition systems; however, a single system that simultane-
ously acquires PET and MRI has recently been developed ( 25 ).
Stand-alone micro-PET scanners for animal research are now com-
mercially available from a number of manufacturers (e.g., Concord
Microsystems; Philips Medical Systems; GE Healthcare; Oxford
Positron Systems).
5. Micro-CT
and SPECT
Although recent advances in X-ray micro-computed tomography
(micro-CT) and high-resolution SPECT have increased the use of
these imaging techniques in many animal models of disease, to our
knowledge they have not been used in preclinical studies of TBI.
Nevertheless, these imaging modalities are among the most widely
available in the clinical setting and thus present an opportunity for
translational neurotrauma research. Here, we will briefl y consider
the general advantages and drawbacks of micro-CT and SPECT
and suggest potential applications for future studies.