=Paper=
{{Paper
|id=Vol-1476/paper1
|storemode=property
|title=Supporting Navigated Surgery with Pan-Tilt Controlled Laser Pointer
|pdfUrl=https://ceur-ws.org/Vol-1476/Proceedings_CURAC_2011_Paper_1.pdf
|volume=Vol-1476
|dblpUrl=https://dblp.org/rec/conf/curac/ChenOMP11
}}
==Supporting Navigated Surgery with Pan-Tilt Controlled Laser Pointer==
https://ceur-ws.org/Vol-1476/Proceedings_CURAC_2011_Paper_1.pdf
10. CURAC-Jahrestagung, 15. - 16. September 2011, Magdeburg
Supporting Navigated Surgery with Pan-Tilt Controlled Laser Pointer
L. Chen¹, D. Ojdanić², K. Michels¹, H.О. Peitgen²
¹ University Bremen, Institute of Automation, Bremen, Germany
² Fraunhofer MEVIS, Bremen, Germany
Contact: longquan.chen@uni-bremen.de
Abstract:
This work introduces a laser guidance system for improving the performance of surgical navigation. It consists of a
laser pointer mounted on a pan-tilt platform calibrated with the tracking system from the navigation platform. With two
pan-tilt rotation axes, the laser beam of a green laser pointer can reach any location in 3D space. The calibration
between the pan-tilt platform and the tracking system is done by using Levenberg-Marquardt and Least-Squares
Estimation of Transformation algorithms. The registration between organ and 3D virtual model is obtained by the
tracking system and therefore, the laser is also implicitly registered. Therefore, important points or the resection path of
the 3D model can be projected precisely on the surface of the organ with the laser pointer.
Keywords: Pan-Tilt Platform, 3D Tracking, Least-Squares Estimation of Transformation algorithm, Levenberg-
Marquardt algorithm
1 Problem
In a navigated surgery, the planning model is presented on a screen in front of the surgeon. Without the visual
augmentation, it is error-prone for the surgeon to fuse the planning models with the organ mentally. One of the most
common methods of augmenting reality with 3D models is the use of projectors [1]. After registering the 3D planning
model to the liver, the projector can display vessels or tumours onto the liver in detail, but the brightness of the
projector is a significant disadvantage in an operating room. The brightness of a normal projector is only 2×103 LUX,
low compared to the brightness of a surgical light, which is approximately 4×104 LUX. The surgeon has to turn the light
off and on during the operation in order to see the images projected on the liver. However, a laser pointer can easily
achieve 2×105 LUX with only 1 mW output power, suitable for a complementary device for surgery navigation.
Moreover, a pan-tilt platform is cost effective, light, and easy to transport and attach to the navigation system.
Furthermore, being able to display a point on an organ quickly and precisely offers a good solution for many tasks in
navigated surgery, e.g., marking critical points, entry points, resection lines (in combination with drive motion), and
assessing registration precision.
2 Methods
Laser guidance systems are often proposed in medical contexts, although only few have been used in combination with
a tracking system. The setup proposed in [2] uses two laser beam shooters to project two parallel lines onto the
cylindrical surface of the surgical tools, and in [3] a six axes robot is used to manipulate the laser pointer. While these
systems aim to target the position and orientation of the surgical tool with a rather complex setup, the goal of this work
is to provide a precise positioning tool by using only two rotation axes and one laser pointer, in combination with
appropriate modeling and calibration methods.
Figure 1 shows the system setup. The pan-tilt platform with the laser pointer hangs on the navigation system from
CAScination [7]. Nevertheless, for the purpose of this work, the navigation software developed at Fraunhofer MEVIS
was used. The navigation system consists of two parts – a display with the 3D planning model and an NDI Polaris
tracking system. After landmark registration procedure, the tracking camera tracks the surgical instrument. The
positions of the instrument and the planned data are displayed in real-time on the interactive screen. To augment liver
surface with some important points from 3D model, two steps are necessary – modeling and calibration.
After the above two steps, the coordinates of points in the 3D planning model are converted first to the tracking system
and then to the pan-tilt platform. The pan-tilt platform calculates the required rotation angles so that the laser beam
points at the corresponding points on the liver.
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10. CURAC-Jahrestagung, 15. - 16. September 2011, Magdeburg
Figure 1: Pan-tilt platform fixed on the Figure 2: Kinematic modeling of pan-tilt
navigation system. platform with laser pointer.
Modeling
The transformation matrix between the laser pointer and the mounting plate is calculated using the following method
(explained in figure2): Rotate the pan-tilt axes so that the laser beam points at the grid board and record the coordinates
of this point with respect to the base of the pan-tilt platform, after which the coordinates of this point with respect to the
mounting plate can be calculated using the rotation angles. The tracking system is used to measure the distance between
the pointer outlet and the point on the board, and the distance can represent the coordinates of this point in the laser
coordinate system. In the same way, acquire coordinates of four points in both coordinate systems. Thereafter, using the
least-squares estimation of transformation algorithm [4], the transformation matrix between the laser pointer and the
mounting plate on the pan-tilt platform can be calculated. After modeling, the whole pan-tilt platform will be mounted
on the navigation system.
Calibration
At the beginning of the process, the registration between the tracking system and the 3D model is achieved by matching
two point patterns acquired in both coordinate systems.
To acquire the transformation matrix between the tracking system and the pan-tilt platform, the coordinates of four
points in both coordinate systems should be obtained. The key problem of this project is to calculate the coordinates of
four points in the pan-tilt coordinate system.
The solution is this: The laser pointer points at four arbitrary points one by one (showed in figure 4); the angles of two
rotation axes with respect to each point are received from the pan-tilt platform. Simultaneously, use the tracking
instrument to acquire the coordinates of each point with respect to the tracking coordinate system. Then the distances of
any two points can be calculated using the acquired coordinates.
Knowing the distances and angles, six equations with four variables {KA, KB, KC, KD} can be formulated. To solve this
nonlinear over-determined equation set, the Levenberg-Marquardt algorithm is used [5]. After calculating {KA, KB, KC,
KD}, the coordinates of these four points with respect to the pan-tilt base can be calculated using angle information
again.
Finally, as the coordinates of these four points in both coordinate systems are available, the calibration of pan-tilt
platform and tracking system is achieved by using the least-squares estimation of transformation algorithm.
Registration of Calculate Apply Calibration of
Laser beam points The coordinates
tracking distances Levenberg- pan-tilt platform
at 4 points one by of 4 points in
system and 3D between any Marquardt and tracking
one pan-tilt platform
model two points method system
Get the
Tracking
rotation angles
system reads
of pan-tilt
the coordinate
platform
Figure 3: Calibration procedure between laser system and tracking system.
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10. CURAC-Jahrestagung, 15. - 16. September 2011, Magdeburg
Figure 4: Calibration between the laser system and the tracking system.
Figure 5: Augmenting one point defined in 3D planning model onto the liver.
The pan-tilt platform is now implicitly registered with 3D model; the predefined points in the model can be augmented
onto the liver (shown in figure 5).
3 Results
To evaluate the calibration, a millimetre grid board and a tracking instrument are used.
The evaluation method is explained in figure 5: Put the tracking instrument on the cross of a millimetre board and make
the laser beam follow the instrument tip. Due to inaccuracy in modeling, calibration error, and resolution limits of the
pan-tilt platform, the laser beam will be shifted a slight amount from the instrument tip. The distance between the
instrument tip and the laser beam is defined as the error. Altogether, 144 measurements were recorded, for which six
calibrations were done. In each of the four predefined regions on the millimetre board, six errors were measured.
Table 1 Mean - variance (mm-mm2) of error in testing.
Region 1 Region 2 Region 3 Region 4 Average
Calibration 1 2.2-0.47 1.6-0.09 1.3-0.01 1.9-0.06 1.75-0.16
Calibration 2 0.8-0.03 0.7-0.02 0.8-0.02 0.9-0.01 0.8-0.02
Calibration 3 1.3-0.02 0.7-0.01 0.7-0.01 1.4-0.04 1.03-0.02
Calibration 4 1.3-0.04 0.8-0.01 0.9-0.02 1.4-0.04 1.1-0.03
Calibration 5 1.5-0.01 0.5-0.01 0.6-0.01 2.1-0.02 1.18-0.01
Calibration 6 1.6-0.1 1.1-0.01 0.9-0.02 2.1-0.03 1.43-0.04
Average 1.45-0.11 0.9-0.025 0.87-0.015 1.63-0.03 1.21-0.045
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10. CURAC-Jahrestagung, 15. - 16. September 2011, Magdeburg
Figure 6: Evaluation method – Left: Instrument tip with laser mark; Middle: The laser mark with cross at the centre;
Right: Error between two crosses.
The total average mean and variance of the 144 measurements are 1.21 mm and 0.045 mm2; however, the laser mark
has an elliptic size of 5 x 3 mm, which is much larger than the error. So, a better laser pointer will be used in the future
development to match the accuracy of the calibration.
4 Discussion
The proposed laser guidance system offers a meaningful improvement of navigation systems, in that the user can easily
see the critical points on the organ. The surgeon can choose a point on the 3D virtual model, for which the laser pointer
will efficiently display that point. Furthermore, the pointer can slowly follow a line marked on the virtual model, e.g.,
resection line, which gives seamless guidance for the surgeon. Also, visually assessing the registration error could fit
well in overall workflow: A visible organ landmark can be targeted with the laser pointer.
Compared to a projector, the laser guidance system is not capable of showing the anatomy of the organ in detail.
However, it can compete with the brightness of the light in the operating room. From another point of view, the laser
system can complement a projector.
Another open issue is the organ motion, which is often present in navigated surgery. Until now, only static models were
used in the lab environment. Further development of this system would take organ motion into account. If the motion is
known or measurable, registration could be updated in real time. In this way, the laser beam will synchronise with the
organ’s motion and continuously point at the defined target.
5 Reference
[1] Hansen, “Illustrative Visualization of 3D Planning Models for Augmented Reality in Liver Surgery”,
International Journal of Computer Assisted Radiology and Surgery, volume 5, 133-141, 2010.
[2] S Lavall′ee, J Troccaz, P Sautot, et al., “Computer-Assisted Spinal Surgery Using Anatomy-
Based Registration”, The MIT Press, Cambridge, Massachusetts, pp.425-449, 1996.
[3] Toshihiko Sasama, Nobuhiko Sugano, “A Novel Laser Guidance System for Alignment of Linear Surgical
Tools”. MICCAI 2002, LNCS 2489, pp. 125–132, 2002.
[4] Shinji Umeyama, “Least-Squares Estimation of Transformation Parameters between Two Point Patterns”,
IEEE Transaction on Pattern Analysis and Machine Intelligence, 1991.
[5] Ajoy K. Palit, Dobrivoje Popovic, “Computational Intelligence in Time Series Forecasting”, Springer, 2005
[6] Ruby Shamir, Ruby Shamir, “An augmented reality guidance probe and method for image-guided surgical
navigation”, IEEE International symposium on robotics and automation, ISRA’2006, August, 2006.
[7] M. Peterhans, A. Vom Berg, B. Dagon, D. Inderbitzin, “A navigation system for open liver surgery: design,
workflow and first clinical applications”, The International Journal of Medical Robotics and Computer
Assisted Surgery, Volume 7, Issue1, page 7-16, March, 2011.
[8] CAScination, http://www.cascination.ch/Home.html
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