Cerebral hypoxia-ischemia remains a major contributor to
perinatal morbidity and mortality. It is estimated that between
0.2 to 0.4% of full-term infants and up to 60% of premature
infants experience asphyxiation at or before birth. Research
efforts are focused on the pathophysiologic mechanisms for the
development of the cerebral tissue damage resulting from
cerebral hypoxia-ischemia with the hope that standardized
detection and treatment protocols may be developed and
implemented in the human newborn. Towards this end it is
necessary to identify methods to prevent these neurological
deficits in the immature brain. Our laboratory utilizes 1H
and 31P NMR spectroscopy and imaging to evaluate
therapeutic regimens and the temporal evolution of neonatal
hypoxia-ischemia in immature rats. Proton imaging can be used to
detect brain regions sensitive to vasogenic edema (T2-weighted
imaging) and regions experiencing cytotoxic edema (DWI,
Diffusion-Weighted Imaging, sensitive to restrictions in the
translational displacement of intrinsic tissue water). The model
consists of both right common carotid artery ligation followed
by exposure to 3 h of hypoxia (8% O2, balance N2).
A combination of both hypoxia and ischemia is needed to cause
permanent damage in this neonatal model of hypoxia-ischemia.
31P spectroscopy is used to monitor high-energy
metabolites during hypoxia-ischemia. The time course of these
changes can be used to predict which animals will proceed to
infarction. We have shown that modest hypothermia and
pre-injection with dexamethasone preserves these high-energy
metabolites during hypoxia-ischemia, and provides neurological
protection. We have developed an imaging probe for our 9.4T
vertical bore magnet. Samples are changed from the bottom of the
magnet within 5 minutes. This probe is tuned through the change
in effective inductance of the RF coil with an RF shield. There
is no physical-electrical connection with the outside world,
providing extremely homogenous B1 fields. With the
collaboration of the Chemistry Deptartment and Drs. Springer and
Latour at Brookhaven National Laboratory, we have designed and
constructed a 10 cm diameter (100 G/cm) 3-axis gradient coil,
that can be used in our whole body 3T magnet, for microimaging.
Below our examples of in vivo 1H images of 12g (7 d
old) neonatal rat pups obtained at both 400 and 125 MHz.
Diffusion-Weighted Imaging and Neonatal Hypoxia-Ischemia
 Diagram
of intracellular space (ICS) and extracellular space (ECS)
volume changes that accompany a disruption in ion homeostasis.
As high energy metabolites are depleted (ATP & PCr), the
transmembrane ionic pumps shut down. Osmotically obligated water
causes the ICS to increase by 12% and the ECS to decrease by
50%. Reduction in the ECS causes an increase in tortuosity. This
increased tortuosity causes a reduction in the apparent
diffusion coefficient (ADC) of water which results in
hyperintensity in DWIs.
 MRI at 5
d post HI and comparison with morphological brain sections.
Notic DWI hypointensity corresponds to cystic infarct (cell
lysis) with concomitant increase in the ADC.
The percent hemisphere lesion area (%HLA) was determined in two
different slices in the neonatal rat pup brain (n=12) by two
different methods. The %HLA of DWI hyperintensity at 1 h post-HI
is compared to triphenyltetrazolium chloride (TTC) staining at
18 h post-HI. As the exposure to 8% O2 is decreased
to 2 h the predictive value of DWI becomes diminished. There is
an excellent correlation for animals exposed to 3 h HI:
%HLATTC = 0.79 * %HLADWI + 7.0, R2
= 0.71, p = 0.0006
 Logistic
regression is used to predict the probability of neuropathologic
damage from MRI. Brain regions were scored as: 0 = no damage, 1
= non-cystic, and 2 = cystic infarcts. MRI parameters (ADC
value, T2 hyperintensity) were normalized to the
contralateral hemisphere for 7 different brain regions. These
results are for ratpups (n = 12) undergoing 3 h of HI. The ADC
at 1 h post-HI is a good indicator of which brains will have
neuropathologic damage at 5 d post HI. T2
hyperintensity at 42 h post-HI is a good discriminator of
whether infarcts will be cystic or non-cystic 5 days post-HI.
These results cannot be extrapolated to shorter episodes of HI.
Reducing
motion artifact is crucial for high resolution DWI. A 12 g,
7-day-old rat pup is difficult to restrain. We use a mold made
from dental registration material to secure the shoulders and
fill up the volume of the RF coil. The mold and the rat pup are
then rolled up in an overhead transparency and placed into the
imaging probe. This method has provided successful restraint
without discomfort to the animal. anesthesia is still required
for high quality DWI.
Photograph depicting RF coil, RF shield used for tuning, and the
inductive couple used for matching.
 Low Pass
Birdcage RF coil for application at 9.4 T. No physical or
electrical connection is needed. This provides a resonant cavity
with extremely homogenous B1 field. The coil is
inexpensively built from disc capacitors and the coil elements
are constructed from the leads of the capacitors.
  The
imaging probe insert contains the RF coil, RF shield, inductive
coupler, heater, thermocouples, and anesthesia port.
Diffusion-weighted magnetic resonance images of a 7-day-old
neonatal rat pup subjected to 3 h of hypoxia-ischemia to
demonstrated image quality @ 9.4 T. Images were acquired 1 h
post hypoxia-ischemia (HI) with a pulsed field gradient
multi-slice interleaved spin-echo imaging sequence with TR/TE =
2000/70 ms, field of view = 24 mm, pixel resolution = 128 x 128,
slice thickness = 1.0 mm, slice separation = 1.0 mm, number of
averages = 2, bandwidth = 25 kHz, and b-value = 1000 s/mm2.
Hyperintense region is ipsilateral to the ligation and reflects
restricted diffusion of endogenous tissue water subsequent to
cytotoxic edema.
 100 G/cm
10 cm diameter 3-axis Gradient Set for Bruker 3T.
Multislice sagittal T2-weighted images with 30 x 40
mm field of view and 1.5 mm slice thickness.
Multislice T2-weighted coronal images with 24.0 mm
field of view and 1.5 mm slice thickness. |
