Neuroimaging Assessment of Traumatic Brain Injury - Animal Models of Acute Neurological Injuries: Injury and Mechanistic Assessments - page 257

and for in vivo tracking of transplanted stem cells after TBI ( 13 ).
Future studies could make use of a new class of “responsive” contrast
agents, which change relaxivity (and thus become detectable on
MRI) in response to specifi c cellular or molecular events such as
enzyme activity, changes in pH, or receptor binding (reviewed in
ref. ( 14 )). With the right contrast marker, MRI could in theory be
used as an in vivo functional assay for nearly any event of interest in
the pathological sequelae of TBI, making this a highly promising
area for future research.
Dedicated research MRI systems for small animals are now
available (Bruker Biospin; Varian Inc.) with magnet strengths typi-
cally ranging from 4.7 to 14 T. Some MRI systems allow simultane-
ous imaging of multiple animals for higher experimental throughput.
Although greater magnetic fi eld strength provides greater signal-to-
noise ratio and improves the spatial resolution of images, high fi eld
strength systems require a large amount of dedicated laboratory
space. Potential alternatives are the recently developed “benchtop”
MRI systems, which are small and portable but offer more limited
resolution at 1-1.5 T (MR Solutions; Stoelting).

3. Magnetic
Resonance
Spectroscopy

In addition to the structural and functional imaging approaches
described above, MR provides a quantitative, noninvasive approach
for measuring endogenous and possibly introduced neurochemicals.
Similar to the hydrogen nucleus (proton) that is the source of the
MRI signal, certain other naturally occurring nuclei also have a mag-
netic resonance signal. Of particular biological signifi cance are car-
bon ( 13 C), nitrogen ( 15 N), and phosphorus ( 31 P) nuclei. As described
in the MRI section, resonant nuclei induce a signal in the scanner
detector. In contrast to the spatial information encoded in MRI,
MRS focuses on the discrete resonant frequencies of specifi c nuclei,
which are dependent on the applied magnetic fi eld strength. Each
nuclear isotope has a characteristic frequency range. For example at
9.4 T, hydrogen (protons) resonate around 400 MHz. In compari-
son, carbon resonates around 100 MHz. In contrast to the anatomic
data generated from MRI, magnetic resonance spectroscopic (MRS)
data usually come in the form of a spectrum where the distinct
frequencies correspond to individual neurochemicals within a dis-
crete volume of tissue (voxel; see Fig. 2 ). Each peak in the spec-
trum represents an individual chemical environment. Proton MRS
voxels in animal models are usually chosen to be small enough to fi t
within a single brain hemisphere, typically about 10 mm 3 . Although
multivoxel acquisitions are possible and are commonly used in larger
human brains, they are technically more challenging in animal
models.