Biomedical Engineering Reference
In-Depth Information
depth
(31,32)
. In this technique, light from a low-coherence light
source is focused onto the tissue and reflectivity of the internal
microstructures at different depths is measured by an interferom-
eter, thus providing a map of the structural profile of the tissue.
As described in the previous section on conventional OISI, in
addition to the oxy- and deoxy-hemoglobin related absorption
changes, scattering changes also contribute to the intrinsic sig-
nal measured with OCT. Scattering changes could result from
changes in the size of the scatterer or the density of the scatterer,
or both. During neural activation, secondary physiological struc-
tural changes such as those in photoreceptors described above
(13)
, capillary dilation
(12,33)
, change in the density of red blood
cells
(34)
and swelling of glial cells
(35)
can occur
(36)
. We expect
that the changes in scattering characteristics would result in an
activity-dependent reflectivity change, and that the sensitivity of
OCT to refractive index changes would make it theoretically sen-
sitive to the scattering changes such as those happening during
neural activation. We refer to this technique as functional OCT
(fOCT) and its signal as fOCT signal.
To demonstrate the potential of OCT in functional studies,
we used primary visual cortex (V1) of cats to confirm that detec-
tion of a stimulus-specific reflectivity change is feasible
(36, 37)
.
The reliability of the technique was demonstrated by compar-
ison with results of conventional OSIS and multi-unit activity
recorded electrophysiologically. Recently, supporting evidence for
the potential of OCT in functional studies has been reported in
squid
(38)
and in retina
(28-30)
.
4. Brief
Introduction for
Optical Coherence
Tomography (OCT)
Figure 6.7A
shows a simple schematic of the principles of an
interferometer: a broad-spectrum light source is divided by a half
mirror into two beams, one illuminating the reference mirror and
the other illuminating a turbid medium such as the cortex. The
light reflected back from the reference mirror and the cortex are
recombined at the half mirror to reach the detector. The reflected
light beams would interfere only if their total light path length
difference (Lr-Ls) is within the coherence length of the source,
or, in other words, if the light reaching the detector has temporal
correlation. The extent of temporal correlation is determined by
autocorrelation of the source, and can be described in terms of
the spectral width (Proportional to
2
λ
0
/λ
) of the source, where
λ
0
is the central mean wavelength and
the spectral width. For
the case of the light source used in OCT, the coherence length is
on the order of a few
λ
m. So, by having a
mechanism to move the reference mirror, it is possible to dissect
μ
m to a few tens of
μ
