Neuroimaging Assessment of Traumatic Brain Injury - Animal Models of Acute Neurological Injuries: Injury and Mechanistic Assessments - page 254

approaches provide high-resolution views of anatomic structure,
vasculature and perfusion, axonal tracts, and metabolism. PET and
SPECT through the use of introduced labeled tracers investigate
metabolism as well as distribution of pharmaceutical agents. Finally,
CT is sensitive to bone injury as well as blood products.
Noninvasive neuroimaging in animal models has evolved in
concert with clinical radiological development. MRI studies in a
controlled cortical impact model of TBI were reported as early as
1995 ( 1 ) and a PET study in a fl uid percussion injury model
appeared in 2000 ( 2 ). Although most neuroimaging studies in TBI
have been carried out in rats and mice, some investigators have
developed models based on larger animals, including pigs ( 3 ).
Imaging approaches have several characteristics that make
them appealing for TBI research. In general, they (1) are noninva-
sive, meaning that longitudinal studies are feasible, (2) can provide
structural, functional, and metabolic data, (3) acquire a three-
dimensional data set that can be reformatted in any arbitrary orien-
tation in post hoc analysis, and (4) generate digital image data that
can be re-analyzed to answer different questions. Finally, the fact
that imaging sequences are largely similar to clinical imaging
approaches often provides a translational capability for use in
humans.

2. Magnetic
Resonance
Imaging

Magnetic resonance imaging (MRI) provides a detailed character-
ization of the pathoanatomical features of brain trauma in the
living animal. An MRI scanner uses a strong magnetic fi eld to align
hydrogen nuclei within the target tissue, creating a net magnetiza-
tion. Radiofrequency (RF) pulses are then applied to alter the
alignment of the magnetization, resulting in an electromagnetic
signal that can be detected by the scanner and reconstructed into
detailed anatomical images. Spatial information is encoded by
manipulating magnetic fi eld gradients that are superimposed on
the main magnetic fi eld during acquisition. MR image resolution
can reach tens of micrometers for neuroimaging of laboratory
animals in vivo.
The appearance of an image can be varied by altering the
timing between the RF pulses within the acquisition sequence.
By varying this image “weighting,” the visible contrast between
tissue structures can be highlighted in a predictable and controlled
way. Using these different sequences, several basic types of images
can be produced with any small animal MRI system. For studies of
experimental TBI, T1-weighted imaging is particularly useful to
visualize intraparenchymal hemorrhage, which appears hypointense
(darker) compared to normal surrounding tissue ( 4 ). Since