1. The 12th Asian Conference on Computer Aided Surgery (ACCAS 2016)
Laser beam steering system for epiduroscopic laser treatment:
A feasibility study
Seong-il Kwon1
, Heechul Kim2
and Keri Kim3
1-3
Robotics and Media Institute, Korea Institute of Science and Technology
1,3
Department of Biomedical Engineering, University of Science and Technology
2
School of Mechanical Engineering, Georgia Institute of Technology
1
kstar1@kist.re.kr, 2
hkim614@gatech.edu, 3
jazzpian@kist.re.kr
1. Introduction
In order to minimize the pain on patients during and
after surgery, Minimally Invasive Surgery (MIS) [1] has
been widely applied to operations such as intervertebral
disc surgery that requires insertion of a thin and long
catheter into a body to inject drugs or irradiate laser [2,
3]. One of the methods is to insert a catheter from coccyx
through an epidural space. Such operation requires a
flexible catheter that enables bending mechanism of the
end tip and inserted optical fiber to perform
multidirectional irradiation. But the complexity of end tip
control and the damage on optical fiber due to bending to
a certain degree have remained as serious problems. Most
importantly, bending a catheter inside of spinal canal
often causes tissue damage and pain on patients by
touching nerves. Therefore, a function that controls
directions of irradiation without bending the catheter’s
end tip is desired. Past studies have been done to resolve
such issues, but enlarged system due to inserting a motor
into a surgical tool limits miniaturization [4].
In this paper, we present a new way of epiduroscopic
laser treatment implementing the beam steering
mechanism. The system includes applicability in the
curved canal, minimized multidirectional irradiation
control device, and motorized remote control.
2. Design
As shown in Fig. 1, our system is composed of three
separate components: a prism set, a catheter, and
actuators. The prism set is rotated along with the torque
coil from the motor. As the prism rotates, laser beam
through optical fiber from the laser source is able to
refract in accordance with any of 0˚, 30˚, and 60˚ prisms.
Both the torque coil and the optical fiber are guided
through the catheter to ensure stable alignment under any
deformation.
2.1 Prism
A key part of the system is the prism as it eliminates
the need to bend the end tip. BK-7, having refractive
index of approximately 1.5, was chosen as a material for
the prisms. As well, anti-reflection coating was applied to
ensure 99% penetration ratio of the laser. The angles
referring to prisms in this paper represent top vertex
angle as in Fig. 2.
Assuming that the laser passes through the prism
parallel to the base, ∠A can be easily obtained. Using
Snell’s law, the final refraction angle, ∠D - ∠A, can be
found by calculating ∠B, ∠C, and ∠D in chronological
order with Eq. (1), Eq. (2), and Eq. (3) respectively. The
refractive index of surroundings is set to 1.





 ∗
= −
n2
n1sinA
sinB 1
(1)
Β2ΑC −=
(2)





 ∗
= −
n3
n2sinC
sinD 1
(3)
The assembly shown in Fig. 1 includes three different
angles of prism: 0˚, 30˚, and 60˚. The purpose of putting
0˚ prism is to pass through the laser straight to a lesion,
and both 30˚ and 60˚ prisms are attached to each side of
the 0˚ prism to obtain interchangeable refraction angles as
necessary. For the prototype, we have built the prism set
with dimensions of 10mm width, 5mm length, and
4.34mm height.
Fig. 1. Full concept of the system with the assembly of
0˚, 30˚, and 60˚ prisms

2. The 12th Asian Conference on Computer Aided Surgery (ACCAS 2016)
Fig. 2. References of refracted angles
2.2 Catheter
The catheter used in the experiment was manufactured
temporarily by molding process. Inside of the catheter, a
torque coil and an optical fiber are placed throughout the
corresponding channels. The channels ensure a better
alignment of the prisms with respect to an optical fiber to
reduce unnecessary energy loss of the laser.
2.3 Actuation
The actuation of the system consists of the prism set, a
torque coil, a servo motor, and the ER chuck in the
gearbox. The torque coil (ACT ONE, ASAHI INTECC
Co., Nagoya, Japan) was chosen as a tool to actuate the
prisms because of its characteristics of being compliant to
any shape change and efficiency in transferring the
torque. The servo motor (Dynamixel MX-28, Robotis
Inc., Seoul, Korea) is geared with the chuck (ER-11
collet), therefore rotating the motor generates torque
through the ER chuck and the torque coil, and thus
rotating the prism set. A key feature of the actuation is
the communication between the servo motor and the
joystick connected with Arduino. A user controls the
joystick to rotate the motor as ordered, enabling a more
precise and automated prism control.
3. Experiment
To verify the advantages of BK-7 and anti-reflection
coating, we performed an experiment on how well the
beam refracts compared to theoretical values obtained by
Snell’s law. The overall representation of the experiment
is shown in Fig. 3.
Fig. 3. Whole representation of experiment set up
3.1 Method
A part of the catheter is held by a vise to pretend the
catheter is withheld inside of a body. The laser coming
out from the laser source passes through one of the
prisms and appears as a red dot on the NIR Detector card.
By controlling the joysticks to left and right, the
coordinates of the red dots produced by 0˚, 30˚, and 60˚
prisms could be tracked. 1310nm of near-infrared ray was
used.
3.2 Result
The beam refraction of each prism is depicted in Fig.
4. Without bending the end tip, the vertically shifted
coordinates of laser beam could be observed just by
changing the prisms.
Fig. 4. Refraction images (0˚, 30˚, 60˚ from left to right)
Table 1 numerically represents both theoretical and
measured values of refraction angles. The results showed
0%, 15.26%, and 12.55% of error for 0˚, 30˚, and 60˚
prisms respectively.
Table 1 Comparison between theoretical and
experimental data of refracted laser beam
Angle of prism Theoretical Experimental
30° 15.99° 18.43°
60° 47.10° 41.19°
4. Discussion
We have developed a system that can control the
directions of laser irradiation in addition to bending the
catheter’s end tip. The tool can be applied to a flexible
catheter used in curved canal and controlled remotely as
precise as possible because of motorized operation. Also,
the tool can minimize movements of the end tip. Thus, it
is expected to reduce pain on patients as interference
between the catheter and its near tissues decreases.
However, it is assumed that imperfect alignment of the
prism assembly itself and uncertain exit location of beam
on prisms caused such errors in experimental data.
The future work needs to focus on developing a
smarter system that increases the stability. For example,
if the ER chuck can rotate the torque coil by discrete
system, the alignment of individual prism and an optical
fiber can be more accurate. In addition, the automation of
the end tip on the catheter will ensure 2-DOFs beam
steering instead of current 1-DOF control. As well, we
plan to insert a micro camera through the torque coil to
control the beam in real-time by relying on the camera
vision only. Most importantly, reducing the dimension of
prisms will be attempted to be feasible in operations.
References

3. The 12th Asian Conference on Computer Aided Surgery (ACCAS 2016)
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