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

1-3Robotics and Media Institute, Korea Institute of Science and Technology1,3Department of Biomedical Engineering, University of Science and Technology

2 School of Mechanical Engineering, Georgia Institute of Technology1kstar1@kist.re.kr, 2hkim614@gatech.edu, 3jazzpian@kist.re.kr

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

1. IntroductionIn 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. DesignAs 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 PrismA 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.

(1)

(2)

(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

Fig. 2. References of refracted angles2.2 CatheterThe 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 ActuationThe 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

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

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. ExperimentTo 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 MethodA 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 ResultThe 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. DiscussionWe 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[1] V. Vitiello, S.-L. Lee, T. P. Cundy, and G.-Z. Yang,

“Emerging Robotic Platforms for Minimally Invasive Surgery,” IEEE Rev. Biomed. Eng., vol. 6, pp. 111-126, 2013.

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[3] Epstein J, Adler R. “Laser-assisted percutaneous endoscopic neurolysis,” Pain Physician, vol. 3, no.1, pp. 43-45, 2000.

[4] S. Patel, M. Rajadhyaksha, S. Kirov, Y. Li, and R. Toledo- Crow. “Endoscopic Laser Scalpel for Head and Neck Cancer Surgery,” Proc. Of SPIE, vol. 8207, pp. 82071S-1-82071S-11, 2012.