IEEE Robotics & Automation Magazine - June 2017 - 69

Validation of OCT Distance
Servoing
To obtain high-quality pCLE images,
as shown in Figure 6, the probe should
be placed in gentle contact with the tissue at all times. In the probe holder, the
OCT probe is positioned with an axial
offset, slightly higher than the pCLE
probe. Optimal pCLE images should
therefore be obtained when the distance to the tissue, given by the position of the top surface in the OCT
cross-sectional image, is at a fixed
value around this distance (the exact
distance required is determined experimentally). To evaluate how effective
the OCT distance servoing is at maintaining continuous tissue contact, and
hence good image quality, we used a
translation stage to move a phantom in
a cyclic motion with a constant linear
velocity along the axial direction of the
OCT probe (both toward and away
from the probe). Trials were performed

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0.93 mm

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mosaicing process. The first row of Figure 4(e) and (f) shows that the servoing
is able to recover following the unexpected phantom motion. To confirm
the capability of the system under unexpected motion of the tissue, the tissue
was placed on a motorized translation
stage that was moved laterally with different velocities during the robotic
scanning. The mosaic results are shown
in Figure 5. The maximal velocity of the
lateral motion that would not cause failure (discontinuities in the mosaic
image) was 0.5 mm/s. When the velocity was increased further to 0.63 mm/s,
there was an obvious discontinuity
when the system attempted to correct
the motion and continue the mosaicing.
Improving the system to deal with faster
motion would require an increase in the
mosaic-processing frame rate, which is
currently set to 10 fps. However, this is
not limited in practice by the mosaic
algorithm but by the use of a tendondriven robot that is not capable of fast
and accurate motion. If the mosaic
frame rate were increased without an
increase in velocity, the mosaic algorithm would fail, as the positional shift
between frames would be too small to
detect accurately.

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Kinematic Trajectory

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Mosaic Image

Figure 4. An illustration of how pCLE servoing improves mosaicing results: (a), (c), (e), and
(g) show the scanning trajectory based on kinematic reading of the robot's end effector.
(b), (d), (f), and (h) show the corresponding mosaic results on the same phantom. (a) and
(b) show the result of using only the kinematic model, while (c) and (d) use closed-loop
scanning. (e) and (f) show the effect of unexpected motion on the mosaic image. (f) shows
the unexpected motion in the red-dashed circle before the closed-loop servoing recovers
and returns the probe to the desired trajectory relative to the tissue surface. The diameter
of the spiral trajectory from the kinematic model is about 0.93 mm and from the pCLE
servoing on the printed pattern and porcine tissue is 1.41 mm and 1.85 mm, respectively.

JUNE 2017

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IEEE ROBOTICS & AUTOMATION MAGAZINE

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69
Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - June 2017
