INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 11, pp. 2015-2020 NOVEMBER 2013 / 2015

© KSPE and Springer 2013

Electromagnetic Tracking of Needle Intervention forSacral Nerve Stimulation Using the Image-GuidedSurgery Toolkit (IGSTK)

Youngjin Na1, Jong-Mo Seo2,3, and Jung Kim1,#

1 Department of Mechanical Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon, Republic of Korea, 305-7012 Department of Ophthalmology, School of Medicine, Seoul National University, Seoul, Republic of Korea

3 Electrical Engineering and Computer Science, Seoul National University College of Engineering, Seoul, Republic of Korea # Corresponding Author / E-mail: jungkim@kaist.ac.kr, TEL: +82-42-350-3231, FAX: +82-42-350-5230

KEYWORDS: Sacral nerve stimulation, Electromagnetic navigation, Needle insertion

Sacral nerve stimulation (SNS) is a therapeutic medical procedure that stimulates the sacral nerve using an electrode inserted near

the sacral nerve to treat the conditions of overactive bladder and fecal incontinence. The primary challenge in the SNS procedure

is that the physician inserts a needle to place an electrode through the narrow structure of the sacrum while relying on fluoroscopic

images. We developed a navigation system which relies on an off-the-shelf electromagnetic tracking device, preoperative CT images,

and a graphical user interface for needle insertion. Subjects inserted the needle into targets in a free metallic environment and in a

surgical environment with a sacral phantom model. The mean registration error in four trials was 0.90±0.081 mm in the free metallic

environment and 1.16±0.49 mm in the surgical environment. The mean user error (UE) between the needle tip position and a target

was 5.08±1.27 mm in the free metallic environment and 4.26±2.87 mm in the surgical environment. We believe that our navigation

system has potential to improve clinical sacral nerve stimulation treatment considerably. To support clinical application of this

method, several remaining issues pertaining to the developed navigation system in an in vivo environment must be resolved.

Manuscript received: January 6, 2013 / Accepted: September 2, 2013

1. Introduction

Sacral nerve stimulation (SNS) is a therapeutic medical procedure

to stimulate the sacral nerve using an electrode inserted near the sacral

nerve for overactive bladder and fecal incontinence1.1 The physician

inserts a needle via the sacral foramen to place an electrode. The

physician plans an insertion path to approach the sacral nerve and

tracks the needle tip position relying on fluoroscopic images from a C-

arm device. Electrode insertion near the sacral nerve is a challenging

technique because needle insertion highly relies on the physician's

experience and the ability to reconstruct 3D virtual images from 2D

fluoroscopic images of the patient. The physician imagines a 3D structure

based on 2D fluoroscopic images, which cannot provide the clear

images of the sacrum.2 Therefore, the physician requires the fluoroscopic

images repeatedly, and fluoroscopically guided procedures induce

radiation exposure to the physician and the patient. The short-term risk

is related to skin injury and the long-term effects can lead to infection.3

The physician plans the insertion path through the sacral foramen

instead of the sacral nerve because the sacral nerve is located beneath

the sacral foramen and not displayed in the fluoroscopic image. As a

result, the needle tip position is confirmed based on the relative distance

between the sacral foramen and the needle tip in fluoroscopic images

or observations of muscle activation stimulated by an electrode.4

Computer-assisted navigation can be a solution to improve the

precision of needle insertion. These systems provide the real-time needle

tip position with preoperative images and have several advantages, as

they shorten preoperative learning period, enable minimally invasive

procedures, and provide a quantitative comparison.5 Specifically, many

studies have been conducted regarding computer-assisted needle insertion

because needle insertion is one of the most common procedures for

biopsies, thermal ablation, and radiofrequency ablation. Previously

developed systems for needle insertion have focused on accuracy in

clinical trials. Banovac et al. tested a navigation system with an

electromagnetic tracking device for targeting in a physiologic phantom

and in a swine model.6,7 There was no difference in the planning time,

needle insertion accuracy or the procedure time between experienced

DOI: 10.1007/s12541-013-0274-9

2016 / NOVEMBER 2013 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 11

subjects and inexperienced subjects. Maier-Hein et al. assessed the

accuracy of needle insertion for CT-guided radiofrequency ablation in

vivo.8 In their results, non-experts achieved better performance than

experts. Cleary et al. developed an image-guided surgery toolkit (IGSTK),

which is a high-level open-source software toolkit, for researchers to

develop image-guided surgery applications.9 Yaniv et al. presented three

navigation systems for needle interventional procedures (vertebroplasty,

RF ablation of lung tumor, and lung biopsy) using IGSTK.10 In the three

navigation systems, they used an electromagnetic tracking device and

3-D images with preoperative CT images for visual feedback in clinical

cases. Electromagnetic tracking devices are used in many applications

to track the instrument position because they have advantages over an

optical tracking device, including no line-of-sight and no requirement

to consider needle deflection.11 The tracked sensor is easy to use with

conventional surgical instruments because the sensor can be easily

embedded into an instrument tip. However, the accuracy of an

electromagnetic tracking device undergoes distortion if metallic objects

or electrical devices are near the field generator and the sensor. Many

researchers have investigated methods to estimate the tracking error

and have worked to develop a compensation method to offset the error

of electromagnetic tracking devices.12-15

We proposed a navigation system to replace with the fluoroscopic

images in SNS. Based on the IGSTK, we added functions for SNS

procedures and tested the feasibility of the system in a surgical

environment. The proposed navigation system used an off-the-shelf

electromagnetic tracking device to track the needle tip position and

preoperative CT images for visual feedback. The accuracy of the

electromagnetic tracking device was measured in a free metallic

environment and in a surgical environment. We also tested it with a

phantom model and measured the user error, which is the distance

between a target point and the needle tip in an image coordinate.

2. Navigation System

2.1 Sacral Nerve Stimulation (SNS) procedures

It is important to determine an insertion point and confirm the

needle tip position. In the conventional method, the physician takes a

fluoroscopic image of the needle and the patient’s body from a C-arm

device and plans the needle insertion path. The physician predicts the

sacral foramen point as shown in Fig. 1 and then selects an insertion

point approximately 1.5 cm away from the sacral foramen center toward

the cranial direction and the insertion angle is about 45 degrees.16 The

third sacral (S3) nerve was a target nerve in Fig. 1. Therefore the

physician inserted the needle through the S3 foramen position as shown

in Fig. 2. Preoperative testing, known as a percutaneous nerve evaluation,

is a process of inserting a temporary stimulation electrode which is

connected to an external pulse generator. It generates a signal to stimulate

a nearby sacral nerve. If this neuromodulation has positive results for

the patient, a permanent implantation is performed. The temporary

stimulation electrode is replaced by the permanent stimulation electrode

and a generator is implanted in a subcutaneous pocket in the outer

quadrant of the buttock or the lower abdomen.

2.2 Electromagnetic tracking device

The developed navigation system uses an aurora electromagnetic

tracking device (Northern Digital Inc., Waterloo, ON, Canada) to track

the needle tip position. IGSTK provides in the form of classes for the

aurora device. The aurora device consists of a field generator, a control

unit, a sensor interface unit, and a sensor coil. The position and

orientation of the sensor coil are estimated by the induced voltage of

the sensor coil based on a field generator. The sensor coils, which

consist of three perpendicular coils embedded into an 18-gauge needle

(Aurora needle, 2-Part, 18G/150 mm, Chiba), coincide with the needle

axis. The tracking data provides three instances of position data and

two instances of orientation data (5DOF). For an accurate measurement,

the offset between the needle tip and the sensor location is considered

by means of pivot calibration.17

2.3 Registration

The rigid paired-point registration included in the IGSTK coincides

with the patient coordinate and the image coordinate based on the

position of each fiducial marker.18 The positions of the fiducial markers

in image coordinate (di) were recorded by manual selection in the

interface, while the positions of fiducial markers in patient coordinate

(mi) were measured by tapping with the needle tip. The rotational and

Fig. 1 Fluoroscopic images from the C-arm are used in SNS - Location

of sacral foramen was predicted lateral to the junction of the midline

and the lateral line in the top view (a) An inserted electrode was

displayed in the sagittal view (b) Supplementary images are available in

the literature16

Fig. 2 Location of third sacral (S3) foramen and needle insertion in

SNS (sagittal view)

INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 11 NOVEMBER 2013 / 2017

translational components between the two coordinate systems are

calculated using a least squares method to determine the rigid-body

registration. Two points sets {di} and {mi}, are related according to the

following equation:

(1)

Where and are point vectors in image coordinate and point vector

in patient coordinate, respectively. R is 3×3 rotation matrix. t is a

translation vector and vi is a noise vector. i is number of fiducial

markers.

R and t were optimized values by the least squares error criterion.

The fiducial registration error (FRE) is a useful equation for confirming

the rigidity of the registration.19 The FRE describes the distance error

at the fiducial markers between the two coordinates used in the

registration, as shown in Eq. (2)

(2)

2.4 Graphic User Interface

Four views are reformatted from the CT images. Three of these are

the axial, sagittal, and coronal view. The fourth view is a combination

of the axial and sagittal views to provide three-dimensional information.

The field of view is selected based on the position of the needle tip.

The user can select an entry point (blue dot) and a target point (red dot)

to plan an insertion path, which can be modified during the operation.

The estimated path is displayed to predict the next needle position if the

needle is inserted along the current angle and the ensuing needle

position. The planned path (green line) is displayed on four views of an

interface along with the estimated path (white line), as shown in Fig.

3. The alignment of the planned path and the estimated path are the

recommend case to secure accuracy during the needle insertion process.

3. Experimental Evaluation

We performed the needle insertion in a free metallic environment

and in a surgical environment. Free metallic environment was constructed

in laboratory. There is no metallic device and the navigation system was

located on the wooden desk to reduce the distortion by ferromagnetic

materials. In the surgical environment, the navigation system was located

near the surgical bed and C-arm was positioned next to the surgical bed

as shown in Fig. 4.

The test used the following procedure:

1) Preparation: The toroidal-shaped fiducial markers (IZI Medical

Products Corp., USA) are attached to a rigid region to decrease the

mobility of the fiducial markers and CT slice images are acquired.

2) Planning: To plan the needle path, the user selects an entry point

and a target point.

3) Patient-to-CT image registration: A rigid paired-point registration

process is used to align the image coordinate and the patient coordinate.

4) Needle guidance: The real-time needle position and planned path

are displayed on the graphic user interface. After needle insertion, the

user error (UE) is calculated by Eq. (3).

The UE, which is the distance between the planned target point and

the needle tip point,8 was used. It was calculated at target point as same

FRE in Eq. (2). The target point vector in image coordinate refers to pi

and mi is the target point vector in patient coordinate. The UE describes

di

Rmi

t vi

+ +=

FRE1

n--- Rm

it d

i–+

2

i=1

n

∑=

Fig. 3 User interface with four views of the phantom model: axial

Fig. 4 The phantom model experiment in a surgical environment

2018 / NOVEMBER 2013 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 11

how well the user can insert the needle using the developed navigation

system in the image coordinate.

(3)

First, the board was used to compare the difference of tracking

accuracy between the free metallic environment and the surgical

environment. The board is a rectangular acrylic board (200 mm×150

mm) with 63 measurement points as shown in Fig. 5. For registration,

four points (FM) at four corner points were used. After registration,

FRE was measured. 63 points of groove intersections were tapped with

the needle tip to measure the UE at each of the 63 points in the free

metallic environment and in the surgical environment. We could tap the

needle tip at the intersection point accurately, because we saw the

structure of the registration board and the needle tip.

Second, the phantom model was constructed as the similar structure

of the sacrum. The phantom model is a cubic-shaped model (200 mm×

200 mm×60 mm) and is filled with urethane foam. The toroidal-shaped

fiducial markers were attached on the bottom plate and the upper plate.

Four fiducial markers on the upper plate were used for the registration.

The other fiducial markers, which were attached on the bottom plate

60 mm away from the upper plate was used for the target points. The

fiducial markers on the bottom plate refer the sacral foramen. Subject

instructed to insert the needle into the center of the fiducial markers.

The UEs at the target point were measured from five subjects. Each

subject inserted the needle 10 trials in the free metallic environment. In

the surgical environment, one subject inserted 30 trials. During trial,

subject didn’t see the inside of the phantom model and only see the

outside and the fluoroscopic images from C-arm device.

4. Results

63 points were performed in the board with 4 trials. The results on

the board are represented in Fig. 6 and Table 1. The area and whiteness

of each marker refer the FRE. More white and larger circles indicate

higher UE. The mean FRE was 0.90±0.08 mm and the mean UE was

0.89±0.35 mm in the free metallic environment. In the surgical

environment, the mean FRE was 1.16±0.49 mm and the mean UE was

1.18±0.65 mm. The deviations of UE were different depending on the

positions of the points and whether or not the C-arm is used.

The results from the phantom model are represented in Table 2. The

mean FRE was 1.41±0.19 mm and the mean UE for five subjects was

5.08±1.27 mm in the free metallic environment. In the surgical

environment, the mean FRE was 0.76±0.11 mm and the mean UE was

4.26±2.87 mm from one subject (S6). The UE in the free metallic

UE1

n--- Rm

it p

i–+

2

i=1

n

∑=

Fig. 5 Top view of the board. Four points (FM) are used for registration

and 63 points of groove intersections in the board are touched by the

needle tip to assess the user error (UE)

Fig. 6 Deviations of the user error (UE) on the board’s surface (a) in the

free metallic environment and (b) in the surgical Top view of the board

Four points (FM) are used for registration and 63 points of groove

intersections in the board are touched by the needle tip to assess

Table 1 The UE and the FRE with the board in the free metallic

environment and in the surgical environment

Free metallic environment Surgical environment

Exp. UE (mm) FRE (mm) UE (mm) FRE (mm)

1 1.0±10.49 1.34 1.37±0.43 1.34

2 0.9±10.31 1.41 0.88±0.41 1.41

3 0.8±40.22 1.13 1.12±0.45 1.13

4 0.80±0.27 1.56 1.25±0.54 1.56

Mean 0.89±0.35 0.90±0.08 1.18±0.65 1.16±0.49

INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 11 NOVEMBER 2013 / 2019

environment did not differ from that in the surgical environment (Table

2).

5. Discussion and Conclusion

Sacral nerve stimulation (SNS) is a therapeutic medical procedure

to treat overactive bladder and fecal incontinence. In the conventional

SNS procedure, the physician uses 2D fluoroscopic images from a C-

arm device repeatedly to insert the needle near the sacral nerve. This

method solely depends on the physician’s experience and ability and

increases radiation exposure to the physician and the patient. To guide

the needle to the target point, we developed a navigation system during

SNS.

For the registration process, rigid paired-point registration was used

with skin fiducial markers. The centers of the fiducial markers were

tapped with the needle tip in the patient coordinate and localized on CT

images in the image coordinate. We measured the FRE in the free

metallic environment and in the surgical environment. The FRE is the

root mean square error, as expressed by Eq. (2). The measured FREs

were consistent in both environments (Table 1 and Table 2). The

registration error came from the misalignment of the centers of the

fiducial markers between the patient coordinate and the images

coordinate and the variation during the tapping at the centers of the

fiducial markers. And the electromagnetic tracking device contains the

inherent tracking error of the needle tip position. The mean position

error is 0.71±0.43 mm, and it can be changed due to the needle

orientation.21 The FRE can be also increased by the shifting and

movement of skin fiducial markers when used on humans or animals.

The developed navigation system was evaluated in the phantom

model. The UE, which is the distance between the planned target point

and the needle tip in the image coordinate, was measured in the free

metallic environment and in the surgical environment to validate the

feasibility of the system for needle guidance. The phantom model was

contracted as similar situation like the sacrum. The measured UE was

lower than the threshold values, which were the height and width of the

sacral foramen20; they were mainly related to human manipulation error

more than the errors of the navigation system and showed no significant

differences in the free metallic environment and the surgical environment.

From the experimental results, we compared the findings here with

related studies involving an electromagnetic tracking device for needle

guidance. Most studies reported the FRE for the misalignment between

the image coordinate and the patient coordinate. The experimental

environments have differences, such as the use of a phantom and a

swine model6 and a cadaver model21. Banovac et al.6 and Khan et al.21

measured the overall error, which is the distance between the needle tip

point and the target point in the control CT scan. Krücker et al.22

measured the system error, which is the distance in the image coordinate

between the needle tip position and the actual needle. There are

limitations in this study. We did not operate the whole SNS procedures

during experiments and only inserted the needle using the developed

navigation. After the needle insertion, it is necessary to identify the

electrode location from medical images or muscle reaction from an

electrical stimulation. Additionally, our developed navigation was tested

in the phantom model, not in vivo although it was tested in the surgical

environment.

The navigation system developed here allow for accurate needle

insertion in conditioned environments. Needle insertion was performed

in a free metallic environment and in a surgical environment. After the

insertion of the needle, the UE was measured for an evaluation of the

quality of the needle insertion. However, rectifying the overall error

remains a challenge, and in vivo accuracy assessments offer a topic for

future work. We believe that our navigation approach has the potential

to improve clinical treatments for SNS considerably. To support the

clinical application of this system, there are remaining issues to be

assessed regarding the developed navigation system before it can be

used in an in vivo environment.

ACKNOWLEDGEMENT

This research was supported by Basic Science Research Program

through the National Research Foundation of Korea (NRF) funded by

the Ministry of Science, ICT & Future Planning (2010-0022871).

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