Biomedical Engineering Reference
In-Depth Information
2. Methods
2.1. Surgical
Preparation
Seven male Sprague-Dawley rats (250-300 g; Harlan, Indi-
anapolis, IN) were anesthetized with 1.2% isoflurane in oxy-
gen. Body temperature was monitored with a rectal probe
and maintained at 37. 0
0. 5 using a homeothermic feed-
back heating system (Baxter K-MOD100, Gaymar Industries).
Femoral arteries were cannulated with PE50 tubing for mean
arterial blood pressure (MAP) measurements and blood with-
drawal. Blood oxygen saturation was continuously monitored
by a pulse oximeter positioned on the hind limb. The phys-
iological parameters in control state were S
a
O
2
=
±
98
±
1%,
pH
±
10 mmHg. All procedures were approved by the research animal
committee of Massachusetts General Hospital, Harvard Medical
School.
=
7. 4
±
0. 08, P
a
CO
2
=
34
±
4 mmHg and MAP
=
110
2.2. FMRI Studies
fMRI experiments were performed using a 9.4 T/21 cm hori-
zontal bore (Magnex Scientific) using a Bruker Advance console
and custom-made surface-RF coil. In order to minimize motion
artifacts, the rat was secured to the RF coil by a bite bar resting
below the upper hard palate and over the snout along with a mask
for the delivery of anesthetic gas. Coronal localization of slices was
accomplished using an initially obtained mid-line sagittal slice and
comparing it with the sagittal section from a stereotaxic rat brain
atlas
(32)
. Five contiguous coronal slices were selected over the
region -5 mm to 0 from the Bregma covering the somatosen-
sory cortex, thalamus, caudate putamen and hippocampus. All
catheters for mean arterial pressure (MAP) measurement and
anesthesia delivery tubes were brought outside the magnet room
of the MR scanner. Anatomical images were obtained before
fMRI scanning using a RARE sequence with repetition time (TR)
=
1s,echotime(TE)
=
19 ms, 256
×
256 matrix and field-of-
view (FOV)
3.0 cm. For fMRI-BOLD measurements, a single
shot gradient EPI sequence was used to acquire multiple slices of
images using a 128
=
3.0
cm, slice thickness=1 mm. The resulting BOLD image had a spa-
tial resolution of 0. 24
×
128 matrix, TR/TE
=
2 s/15 ms, FOV
=
1mm
3
. Four hundred and fifty
images were obtained in about 15 minutes in each scanning ses-
sion. Studies were performed in sequential order starting with the
rest scan, followed by exsanguination, where 8 ml/kg of blood
was withdrawn from the femoral artery gradually in about 2-3
min. 8 ml/kg of blood was withdrawn since it led to a decrease
in MAP almost near the autoregulatory limits in the present anes-
thetized rat preparation. MR images were acquired 5 min after
×
0. 24
×
