Biomedical Engineering Reference
In-Depth Information
constructed from single-mode fibers for flexibility. Light from a
broadband source (AFC Technologies, Canada) operating at an
output power of 30 mW and having a central mean wavelength
(
) of 50 nm is split into
sample and reference beams in the respective ratio of 19:1 by cou-
pler 1. Optical frequencies of the sample and reference beams are
shifted by acousto-optic modulators (AOM, Asahi glass, Japan).
We use AOMs to introduce a constant and stable phase delay
between the interfering beams. Both the reference and sample
beams, after passing through the circulators, illuminate the ref-
erence mirror and cortex, respectively. The reflected lights were
recombined at coupler 2. An interference beat signal that has a
beat frequency of 250 kHz is detected only when the path lengths
of the interferometer arms are matched to within the coherence
length of the source that is calculated to be 34
λ
0
)of1.31
μ
m and a spectral width (
λ
m in free space.
Heterodyne detection was done with a lock-in amplifier (EG&G,
USA) and the amplitude of the demodulated components was fed
into a computer via a 16-bit A/D converter. The reference mirror
M was mounted on a motorized stage and scanned at a speed of
2 mm/sec.
The sample arm viewing the animal side consisted of an objec-
tive lens of numerical aperture 0.08 and was also fitted with a
CCD camera. This allowed simultaneous viewing of the cortical
surface with the introduction of visible light from an auxiliary
laser source (wavelength 680 nm). The whole unit was mounted
on a manipulator unit (
Fig. 6.7C
) that has five degrees of free-
dom of translation along three axes and rotation and tilt (the
flexibility is needed for making the probing light beam normal
to the cortical surface). Galvano scanners were installed so as to
perform surface scans. The animal-related fluctuation in the sig-
nal was reduced by conducting measurements in synchronization
with heartbeat and respiration and by keeping the brain surface
immobile using agarose.
μ
Animals and Surgery
Each cat was anesthetized with a mixture of 70% N
2
O and 30%
O
2
supplemented with 1-2% isoflurane, paralyzed with Pancuro-
nium bromide (0.1 mg/Kg/hr), and artificially ventilated by a
respirator unit. Contact lenses were fitted to the eyes to pro-
tect the cornea from drying. The pupils of the eyes were dilated
with 0.5% tropicamide and 0.5% phenylephrine hydrochloride.
The head of the animal was held tightly by attaching it to a
metal rod. A stainless steel chamber (18 mm inner diameter) was
fixed onto the skull with dental acrylic cement by aseptic surgery
and was placed above area 18 (coordinates A10P5 of Horsheley).
After removal of the dura mater, the inside of the chamber was
