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tissue and the high, achievable resolution, a transmis-
sion version of the OCT was designed.
The Michelson interferometer usually used in
classical OCT, with which backscattered light is an-
alyzed interferometrically, is replaced by a Mach–
Zender interferometer in which forward scattered
light is to be analyzed.
In order to generate interference patterns with a
sufficiently high contrast, some approximations must
be fulfilled. On the one hand, the once scattered rays
of the sample beam must be dominant over the multi-
ple scattered rays. The setup in use also assumes that
the examined tissue can be approximated as a plane–
parallel plate.
The approximation that only single scattering can
be measured limits the achievable result, since for
multiple scattered light the formula 1 is no longer
valid. Multiple scattering would result in the wrong
depth of the scattering point, so that a clear assign-
ment of the scattering point is no longer possible. This
would manifest itself in a decrease in the contrast in
the generated interference image. With a sufficiently
large scattering angle, however, as can be seen in fig-
ure 2, it can be assumed that the amplitude of the mul-
tiple scattered light drops sufficiently quickly so that
the reduction in the achievable contrast still allows the
result to be clearly interpreted.
In a numerical simulation it was shown that the
proposed setup is suitable for generating interference
from rays that are scattered at different depths of the
tissue. In addition, the proof of concept was provided
that the proposed design for carrying out an A–scan
works in principle. However, the tissue cannot be
scanned in full depth, since, above a certain depth,
scattered rays are totally reflected on the exit surface.
In addition, the NA of the A–Scan–lens limits the
measurable range of the representable scattered rays.
In a simplified experimental setup, the proof of
concept was achieved and demonstrated that interfer-
ence patterns can be generated with scattered rays
in transmission in a Mach–Zehnder interferometer.
Therefore this method is in principle able to function
as the core of a tOCT.
5 CONCLUSION & FURTHER
CHALLENGES
Possibilities for optimization were identified in both
the numerical and the experimental setup.
The scan depth is limited by the total reflection
occurring and by the NA of the A–scan lens. The for-
mer can be reduced by using a contact gel, similar to
ultrasound, and the latter appears to be reducible by
using a field lens in the position of the pinhole. In
addition, a numerical analysis should be carried out
with more realistic dissemination models for tissue
obtained from laboratory experiments. The Gaussian
model used in the current simulation may not reflect
reality adequately.
The influence of different split ratios of the beam
splitter should be examined and optimized numeri-
cally and there is still no tolerance analysis of the
components used.
In the next step, a tOCT including a complete
A–scan will be implemented experimentally. In this
setup it is to be examined how the A–scan can be
implemented in practical application. The possible
choice of a second pinhole in front of the beam splitter
(k) from figure 5 and the scanning speed will depend
on this.
Furthermore, it has to be analyzed in experimen-
tal investigations whether the back–calculation is also
possible from the generated interference patterns for
the illuminated structure. This analysis will be based
on measurements of scattered light on tissue samples.
The maximum depth of penetration will depend
on the wavelength used, the intensity of the light used
and the tissue being screened. These examinations are
to be carried out in the future.
In addition, the usability of different light sources,
especially with regard to the desired wavelength
used, should be investigated. Multi–wavelength scans
could significantly improve the resolution of the scans
generated.
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