Neuroimaging Assessment of Traumatic Brain Injury - Animal Models of Acute Neurological Injuries: Injury and Mechanistic Assessments - page 256

more importantly, researchers can also follow the tissue effects of
novel drug treatments or rehabilitative interventions in longitu-
dinal studies of brain injury and recovery.
A key advantage of MRI in basic research is the potential for
direct clinical comparison, since MRI scanning is commonly used
in the clinical workup of TBI patients. Although other neuroim-
aging techniques such as CT are also used routinely in the clinic,
the wide variety of image weighting options make MRI the most
successful approach for detecting injury-related abnormalities in
brain tissue. A further advantage is that MRI does not require the
use of ionizing radiation or radioactive contrast agents.
The conventional MRI techniques introduced above yield
primarily anatomical data that, when considered alone, provide
limited functional insight into degenerative and recovery processes
after TBI. However, this past limitation of MRI has been rapidly
changing with the implementation of novel techniques and
applications. Many of these newer MRI approaches have been
relatively underutilized in experimental TBI research, but they
offer considerable promise for future investigations.
Diffusion tensor imaging (DTI) is one specialized MRI tech-
nique with great potential for assessing brain injury mechanisms.
DTI measures water diffusion in the brain with spatial mapping of
the fractional anisotropy (a measure of how directional, or aniso-
tropic, the diffusion is) and apparent diffusion coeffi cient (an estimate
of the average magnitude of water movement). Interest in this
imaging approach has grown in recent years, as DTI is more
sensitive than conventional MRI to white matter injury. White
matter changes observed with DTI after brain injury may refl ect
both axonal degeneration and reactive plasticity ( 9, 10 ).
Manganese-enhanced MRI (MEMRI) is another emerging
MRI approach that allows visualization of white matter tracts and
functionally connected circuits within the brain. After systemic or
localized administration of manganese chloride, Mn 2+ is taken up
by active neurons through voltage-gated calcium channels and
distributed via anterograde and retrograde axonal transport.
Transynaptic exchange of Mn 2+ also takes place between function-
ally connected neurons. Since Mn 2+ is a paramagnetic agent that
alters the appearance of T1-weighted MRI, the distribution of
Mn 2+ shows the white matter architecture. Decreases in the rate of
transfer between connected regions can indicate neuronal dysfunc-
tion accompanying neurodegeneration ( 11 ). Thus, future studies
could utilize MEMRI to assess white matter integrity and plastic-
ity, neuronal activity, and axonal transport dynamics after TBI.
A third novel MRI approach is the emerging fi eld of Molecular
MRI, in which a specifi c molecule or drug, receptor, or cell type is
tagged with a contrast agent so that its distribution may be visualized
with in vivo imaging. Molecular MRI has been used for assessing
the endogenous macrophage response after experimental TBI ( 12 )