Robot assisted needle interventions for brain surgery

Tiago Salgueiro

Under supervision of Prof. Jorge MartinsDep. of Mechanical Engineering, IST, Lisbon, Portugal

December 2014

Abstract

Brain diseases affect a sizable amount of people worldwide, being the brain surgery an attractive butchallenging therapeutic procedure for treatment. The fast evolution in the robotics field can providemore precise and less invasive tools to overcome certain limitations of the conventional brain surgeries.In this work, a new robotic solution involving the KUKA LWR 4+ was proposed and fully implementedin order to perform brain surgeries involving any kind of targeting within the brain. The robot’spositioning task is assisted by a co-manipulation setup between the surgeon and a virtual impedanceenvironment. A simulation of a brain biopsy to the red nucleus was performed using the developedrobotic system. A Needle Path Planning software was also designed to choose the desired target pointalong with the planning of the needle’s trajectory. Our findings revealed that this procedure has a totalpositioning error of 6.3 mm. In this work a user-friendly and intuitive robotic system was successfullycreated providing a new alternative tool for the brain robotic surgery.Keywords: Brain Surgery, Robotic Brain Biopsy, Co-manipulation, Virtual Impedance Control,KUKA LWR 4+ Kinematics

1. Introduction

Brain cancer statistics demonstrate that, through-out both sexes and a standardized age, in Europethis disease affects 6.6 people per 100,000 popula-tion (total of 57099 people) and have a mortalityrate of 4.9 people per 100,000 population (total of44979 people), occupying the sixteenth position inthe most common cancer ranking. The statisticsalso show that in Portugal 7.1 people per 100,000population (total of 932 people) suffer from braincancer and it has a mortality rate of 5.0 people per100,000 population (total of 718 people)[1, 2].

Diagnosis of this disease strongly relies on neu-roimaging techniques like magnetic resonance imag-ing (MRI) and computed tomography scans(CT-scan). Although these methods are good surveyingfor brain tumors, to obtain a definitive diagnosis, anhistopathological examination of the tumor tissue isessential. In order to obtain a sample of this tissuea brain biopsy must be performed [3]. Currently,there are two standard procedures for performingthis surgery, the stereotactic brain biopsy and theneuronavigation assisted brain biopsy.

Neuronavigation is a method that allows the sur-geon to observe the various brain structures thatare not visible otherwise, thus making it possible to”navigate” through the brain. This method offers,not only, a way of locating tumors within the brain,

but also the possibility to plan the best needle tra-jectory to reach them [4]. This surgery relies ontwo optical digitizer cameras and a light-emittingdiodes (LEDs) setup that allows the tracking of thepatient’s head as well as the biopsy needle duringthe entire procedure. For this, LEDs must be at-tached to the needle and the Mayfield frame, whichis a tool that holds the head in place in these surg-eries [5].

Another type of procedure that makes possiblethe biopsy of lesions/tumors deeply embedded inthe brain is the stereotactic brain biopsy. Thesurgery consists in defining a working space aroundthe patient’s head by setting up a cartesian coordi-nate system [6]. This can be achieved by attaching alight-weight frame to the skull [7]. The stereotacticbrain biopsy is a technique largely used worldwide,since the stereotactic apparatus provides a safe andstatic environment for this type of surgery and canachieve errors around 0.5 mm.

With the advancements in the robotics field, to-day, more precise and user friendly robots exist.The state of art technology in the field of neuro-surgery comprises 3 major commercialized robots,the ROSA

TM

, the Neuromate R© and the neuroArm.

ROSATM

is a versatile instrument that can as-sist neurosurgeons in a wide range of surgical pro-cedures to the brain, including the brain biopsy.This robot can behave in two different ways during

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the surgery. It can position the surgical instrumentautomatically according to the pre-planned trajec-tory, or it can be co-manipulated. This last featureallows the surgeon to move the robot manually, butconstraining, at the same time, the movement tothe preprogrammed trajectory. The safety mech-anisms are further enhanced by the security zonefeature, available on ROSA

TM

. This functionalityconfines all the robot’s motions to a preoperativelyplanned cone defined along the trajectory. All theseROSA

TM

’s characteristics ensure that the surgicalinstrument stays aligned with the planned trajec-tory and that the entire procedure achieves the min-imum invasion possible[8, 9, 10, 11].

The Neuromate R© robot is another state of theart equipment, specifically developed for stereotac-tic brain surgeries, whether frame based or frame-less. It offers different options in terms of thesurgery itself. If the surgical procedure is a biopsy,for example, the surgeon can command the robotto position the biopsy needle in two distinct ways.One approach is to use the endoscopic navigationoption to insert the needle. In this case the sur-geon can control the insertion with a remote con-trol (tele-manipulation) while observing the insideof the brain [12, 13]. Another approach is to com-mand the robot to position the needle automati-cally. In either method, a burr hole in the skull’sbone must be opened, before inserting the biopsyneedle. A laser pointer mounted on the robot’s armis used to correctly position the burr hole.

The neuroArm robot was developed by Gar-nette Sutherland and Associates and, currently, itis the most sophisticated master-slave system usedin brain surgery [13]. This robotic system com-prises two seven degrees of freedom (DOF) roboticarms , a main system controller and a workstation(human-machine interface), where the surgeon iscapable of controlling the robot in an extremely in-teractive way [14, 15]. The neuroArm allows bothstereotactic and microsurgical procedures and it ischaracterized by being fully MRI compatible andbe teleoperated from a workstation situated out-side the operating room. Fully MRI compatiblemeans that it can operate while an MRI scanneris acquiring images of the brain [16]. This fea-ture allows procedures to be performed in an en-vironment with real-time update of the brain’s po-sition, which enables the correction of the deforma-tion/displacement of the brain during surgical pro-cedures (Brain Shift). This workstation includesone high-definition medical monitor and two six-DOF haptic devices that provide, respectively, vi-sual and tactile feedback of the robot’s work envi-ronment [17]. The medical monitor receives imagesfrom two high-definition cameras present inside theoperating block, which enable the surgeon to have

full-depth perception and spatial orientation, thusgranting plain 3D display capabilities. The use ofsix-DOF haptic devices adds many functionalitiesto the robot including the filtration of the surgeon’shand tremor, the scaling of the surgeon’s motionsand the force-feedback. The later tries to mimic thetactile feel that the surgeon would get as if he wasperforming the surgery in the conventional way[15].

The objective of this work is to develop a com-plete robotic solution that can improve and replacethe conventional brain surgeries that are performednowadays. In this line of thinking, the robot shouldperform the surgical procedure with the help ofthe surgeon, in a cooperative way. The proposedrobotic solution to improve brain surgery is to usea robotic arm, the KUKA LWR 4+, to do the po-sitioning of the needle during the surgery. In anykind of surgery it is very important that the sur-gical environment be as safe as possible. For thispurpose a virtual impedance environment was im-plemented. This consists in an impedance cone thatconstrains the robot’s movement to a safety zone.Although the robot is manipulated by the surgeon,the robot also constrains the surgeons movementto the cone-shaped zone. This relation betweenrobot and surgeon is called co-manipulation. Theimpedance cone is aligned with the orientation of apreoperatively planned needle’s trajectory and thecone vertex is positioned at a safe distance fromthe patient’s head. As the needle’s tip passes thecone vertex the robot locks its orientation and onlyallows the needle to move along the predefined tra-jectory. When the needle’s tip reaches the targetpoint the robot stops and forbids any movementfurther down the planned trajectory. While thiswhole process is in course, the surgeon can observethe robot’s virtual environment in a 3D simulator.This includes the virtual cone defined as the safeworking zone. In order to offer the surgeon a train-ing instrument and a way to exploit the featuresand capabilities of the robot, the 3D simulator canalso be used in offline mode.

2. Materials

2.1. KUKA LWR 4+

The KUKA lightweight robot (LWR) 4+ is theproduct of a research collaboration between the In-stitute of Robotics and Mechatronics at the GermanAerospace Center (DLR) and the KUKA RoboterGmbH. The KUKA’s key features are its 1:1 power-to-weight ratio and its kinematic redundancy thatmakes it resemble the human arm. These propertiesalong with its flexibility to execute tasks, give theKUKA the position of one of the most advancedrobot in the human-robot interaction field. TheKUKA can be commanded in two different ways, by

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means of the KUKA Fast Research Interface (FRI)or by directly controller console via KUKA RobotLanguage (KRL). Both of these languages use stan-dard functions of KUKA industrial robots which of-fer three types of control methodologies, the JointImpedance control, Cartesian Impedance Controland the Joint Position Control.

2.2. Brain Phantom

In this work, a human brain inside an acrylic boxwas provided by Dr. Herculano Carvalho from Hos-pital Santa Maria. An MRI of the phantom wasperformed so as to grant knowledge of the deepanatomical structures of brain. The purpose of thephantom’s box is to play the role of the human skullallowing the creation of a reference frame for thebrain.

3. Developed Software

3.1. Needle Path Planning Software

The preoperative planning is a very important stepof the surgical procedure. At this stage, the sur-geon must plan and define the trajectory that theneedle will travel to reach the desired target point.This trajectory must be carefully delineated sinceit must avoid relevant areas of the brain. In or-der to make this process as simple and intuitiveas possible a user interface was developed underMatlab R© R2014a (The MathWorks, Inc.; Natick,Massachusetts, United States) (Figure 1). The de-velopment of some features available on this soft-ware were based on the work of Teodoro et al. [18].The working principle behind this interface is verystraightforward and follows a logical sequence ofsteps: Load an MRI file and its 3D reconstructionfile; Choose the desired target point andChoose aneedle’s entry point.

First, to setup the GUI the surgeon must load theMRI file, as well as its 3D reconstruction file. To dothis, the surgeon just needs to press the menu File->Open and File->Load 3D Model buttons presentin the menu bar and choose the correct files. Rightafter the program finishes loading both files, thevarious brain slices are displayed in three windowson the left side of the GUI, whereas the 3D model ofthe brain can be observed on the right side window.The brain slices are displayed in three views thatcorrespond to the axial, the sagittal and the coronalplanes. There are also slide bars on the windows’sides that can be used to scroll through the brainslices. When the slide bar is used the slice number isupdated and displayed in the Section Views panel.At this point the surgeon can use the Colormap,Contrast and Brightness buttons in the menu barto improve the image quality.

Usually, for the surgeon’s better understanding of

the brain’s anatomy by looking at the MRI images,he needs the planes where the images are displayedto be orthogonal to a plane that intersects the an-terior (AC) and posterior (PC) commissures of thebrain (AC-PC line). This is another functionalitythat the software provides. To rearrange the dis-played brain slices according to the AC-PC line,the surgeon can use the small slide bar present atthe right-top side of the coronal and sagittal planes.The images should be rotated till the AC-PC linebecomes horizontal. This process is usually per-formed in the sagittal view since it is the most in-tuitive view to perform this task. When an image isrotated, the images in the other planes are also up-dated to match that same rotation. The next stepconsists in selecting the desired target point. To dothis the surgeon should use the Choose Target Pointbutton present in the Selection Tools panel. Whenthe button is clicked the mouse gains a cross shapeform, then the surgeon only needs to click the de-sired target point in the view that is most intuitiveto him. Every time that a point is chosen in one ofthe views, the projection of that same point in theother views is also displayed. The desired targetpoint appears in the three views as a red dot.

After defining the target point the user canchoose the needle’s entry point. Once again theuser should go to the Selection Tools panel andclick the Define Needle Trajectory button. Whenthat button is clicked the program allows the sur-geon to choose an entry point at the brain’s surface.This can be done in the right window where the 3Dmodel of the brain is displayed. To select this pointthe surgeon just needs to click the brain’s surfaceand a green line representing the needle’s trajectorywill be displayed. The user can hold down the leftmouse button to re-position the entry point. Afterdefining the needle trajectory the user can click inthe Show Needle Trajectory button to display thepath in the three plane views. This tool exhibitsthe intersection of the chosen slice with the tra-jectory line, thus producing an intersection point(displayed as a green point in the section views).This tool enables the surgeon to check if the chosentrajectory passes through some important area ofthe brain, thus allowing him to correct the speci-fied needle path. Apart from the buttons alreadymentioned, the Selection Tools panel also displaysthe select target and entry points.

There is one more panel that also contains usefulfunctionalities, which is the 3D View panel. In itthe surgeon can change the transparency of the 3Dbrain model, display the target point in the 3D view(appears in red) and observe a given brain slice. Touse this last feature, the check boxes named aftereach section view can be clicked to show or hidethe respective slice. If the check box is active the

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Figure 1: Illustration of the developed Needle Path Planning Software.

surgeon can then use the slide bar to scroll throughthe brain slices. This feature is very useful sinceit provides the surgeon an intuitive way of avoidingimportant brain structures when defining the needletrajectory.

3.2. CHAI3D Virtual Environment

In order to have a tool that can represent accu-rately the robot working environment and a virtualimpedance control, a 3D simulator was developed.This software was developed in Microsoft VisualStudio R© 2010 (Microsoft Corporation; Redmond,Washington, United States) with the CHAI3DC++ libraries [19], and uses the KUKA FRI com-munication protocol as an interface between thePC and the KUKA robot (Figure 2). Two hap-tic devices were integrated with the CHAI3D, theOmega.6 haptic device (Force Dimension; Switzer-land) [20], which is six-DOF pointer (three for theposition and three for the orientation), and theKUKA LWR 4+. Both haptic devices were attachedto the 3D model of the biopsy needle used in thiswork, since it is the needle’s tip position and ori-entation that we want to track and control. Thevirtual environment also contains a 3D model ofthe robot and a model of the needle’s holder. Thissurgical virtual environment has two main goals:

• It is intended to be a tool for the surgeon tohave a feel of the brain biopsy procedure (of-fline mode).

• A tool for the surgeon to use in a co-manipulation procedure. The software creates

an impedance environment that, with the helpof the surgeon, will guide the robot to the pre-planned needle’s orientation and the target po-sition within the brain. The robot will be thehaptic tool which will interact with the virtualand real environments.

3.2.1. Surgical Co-Manipulation Mode

This mode allows the surgeon to perform the sur-gical procedure. It uses the FRI API to make theconnection between the computer and the KUKAcontroller. This mode is automatically set at thestart up of the program if the robot is connected tothe PC. Before the program launches the visual in-terface, it will send the robot to a pre-programmedposition, in cartesian impedance control, where therobot can work safely. After this, the visual in-terface window opens and the program is ready touse. In this mode the robot is the haptic deviceused. If the surgeon moves the robot the 3D modelof the robot with the needle holder and needle willmove accordingly in the virtual environment, tofully mimic the real robot’s movements. This wasachieved by implementing the direct kinematics ofthe KUKA [21].

After the program has successfully opened andthe robot is in its starting position, the surgicalprocedure can begin. This process can be summa-rized in two steps, the registration of the phantomand the positioning of the needle in the target pointvia an impedance environment. One thing to havein mind is that the preoperative planning of thesurgery, i.e., define the target point and the nee-dle’s trajectory in the needle path planning soft-

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ware, must be performed before the surgical proce-dure starts.

Figure 2: Image of the developed CHAI3D virtualenvironment.

Registration Procedure

The registration procedure is very simple andstraightforward. To start it the surgeon shouldpress the J button in the keyboard and a messagein command window will show that the program isready to start the registration procedure. After thebutton is pressed the surgeon must not move therobot since it takes approximately five seconds forthe robot to change from the cartesian impedancecontrol to the joint impedance control mode. In thismode all the joint’s stiffness are set to zero, and therobot is now in gravity compensation. This meansthat the robot is completely maneuverable and onlyapplies the necessary force to maintain its pose. Toperform a successful registration the surgeon mustacquire points that belong to five faces of the phan-tom. To do this he must use the needle’s holdertool spherical part. The methodology for acquir-ing the points is quite simple. The surgeon mustfirst lightly press the tool’s spherical part againstthe cube’s face, and then press the Space Bar sothat the program starts recording the tools position.During the acquisition the surgeon must performslow movements and go through all the face’s area,in order to minimize the error inherent to the reg-istration process. When these conditions are met,the surgeon must press the Space Bar to stop theprogram from acquiring more points. The sequenceof planes to be acquired is left, front, right, backand bottom.

After finishing the point acquisition, the programperforms all the necessary calculations to find thegeometric transformation between the MRI’s phan-tom and registered phantom [22]. Afterwards, thebrain model along with a representation of the boxappear in the virtual environment. The impedancecone also appears together with a sphere represent-

ing the desired target, chosen with the Needle’sPath Planning interface. The cone is aligned withthe needle’s trajectory and its vertex is set 20 cmfrom the chosen brain’s entry point along that sametrajectory. At this point, the surgical setup is readyto perform the surgery.

When the registration is over the robot changesthe controls again from joint impedance control tocartesian impedance control to get ready for the co-manipulation surgical procedure. To activate thevirtual impedance environment mechanism the sur-geon should click the 3 button on the keyboard.

Virtual Impedance Positioning

The impedance environment implementation wasbased on the Doctoral Thesis of Pedro Pires [23].The virtual impedance positioning control, which isillustrated in Figure 3, can be split in three parts:

• While the tool center point (TCP) is in thesafety zone and it has not passed the cone ver-tex (A).

• While the TCP is between the cone vertex andthe target position, along the needle trajectory(B).

• When the TCP reaches the target point (C).

For the purpose of implementation, a virtualimpedance environment between the robot tool andthe pre-planned needle trajectory axis, ZBrain, wascreated. This system attracts the tool to correctlyalign it with the needle trajectory. The distancebetween the target point and the needle’s tip is ex-pressed in the brain reference frame (which has itsorigin at the desired target point), along the nee-dle’s trajectory and can be defined as follows:

pBrain

Tool= R0T

Brain(p0Tool − p0

Brain) (1)

Since the XTool axis represents the orientation ofthe needle in the tool reference frame, the distancebetween the TCP and the target point, along theneedle trajectory corresponds to the x componentof pBrain

Tool ({pBrainTool }x). The other two elements of

pBrainTool represent the directions where the stiffness

values will vary.The impedance control law applied in this work

consists in changing the spring stiffness values as afunction of {pBrain

Tool }x. The equation that definesthis control is as follows:

KCStiffV al = KCStiffMax −KCStiffMax · · ·

· · ·4

√{pBrain

Tool }x4√limp

(2)

where {pBrainTool }x ≤ limp ∧ {pBrain

Tool }x > 0. Ascan be deducted from eq. 2, when {pBrain

Tool }x is

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greater than a predefined distance, limp the valueof KCStiffV al is set to zero, making the robot enterthe gravity compensation mode. To calculate theapplied force that the virtual spring exerts in theTCP, eq. 3 was used.

FKCStiffV al= R0

Brain

[0 {pBrain

Tool }y {pBrainTool }z

]· · ·

· · ·KCStiffV al

(√{pBrain

Tool }2

y + {pBrainTool }2z

)(3)

To this last equation, a dissipative force functionwith damping coefficient factor ξ was added, thusobtaining

FImp = FKCStiffV al+ D(ξ) (4)

Figure 3: Two dimensional illustration of the vir-tual impedance environment.

The fact that the XTool axis of the tool referenceframe is aligned with the needle orientation, allowsthe simplification of the kCStiff vector calculations,making the diagonal stiffness matrix become

KCStiffImp = diag([0 KCStiffV al KCStiffV al · · ·

· · ·KCStiffV al KCStiffV al 0])

(5)Note that in the KCStiffImp matrix diagonal, thefirst three entries correspond to the stiffness val-ues in the x, y and z directions, and the last threeentries express the stiffness values for the rotationalong the z, y and x axis, respectively. Since thestiffness is null in the needle trajectory direction,the robot will not pull towards the target point. Forthat purpose, it must be the surgeon that performsthat task.

The control law described above is only usedwhile the TCP has not yet passed through the conevertex. When the needle’ tip does pass that point

the stiffness is set to maximum in the axial direc-tions and it is set to zero in the predefined needle’strajectory direction. With this, the diagonal of thestiffness matrix becomes

KCStiffImp = diag([0 KCStiffMax KCStiffMax · · ·

· · ·KCStiffMax KCStiffMax KCStiffMax

])

(6)According to eq. 6 the robot is constrained fromperforming any rotation at it can only move alongthe needle’s trajectory. Finally, when the robotreaches the desired target all entries of KCStiffImp

are set to the maximum stiffness values thus pre-venting the robot from moving further. At thispoint the surgeon can perform the biopsy from thedesired anatomical structure.

One more functionality that this system providesis the ability to change the robot configuration whileit maintains the needle position and orientation(robot’s null space). This is a very useful featurethat allows the surgeon to modify and optimize thework space.

3.2.2. Training Mode

Another goal of this software is to offer the surgeona way to practice the surgery and test the capabil-ities not only of the co-manipulation procedure aswell as the virtual environment itself. When thisprogram is started it checks if any haptic device isplugged to the computer. If the robot is not de-tected then the program automatically starts theoffline mode. In this mode only the omega.6 hap-tic is available to the surgeon. With this hapticdevice the surgeon can control the needle’s orien-tation and its tip position. When the needle’s tipmoves the 3D robot model also moves mimickingthe robot’s real movements. This was achieved byimplementing the forward and inverse kinematics ofthe KUKA [21]. Each time the surgeon moves thejoystick of the omega.6 its position and orientationare retrieved and the inverse kinematics are calcu-lated to discover the set of angle joints that pro-duces that same needle’s pose. After finding the setof angles, the program uses the forward kinematicsto update the position and orientation of each ofthe robot’s joints, giving the sensation of the realrobot’s movements.

This mode also allows the surgeon to feel theforce exerted on the robot when it enters in contactwith some virtual structure or when the impedancemode is turned on. For this, the model of the brainand the cone used in the impedance mode are alsoloaded on the program start up. Since this is just atest mode, the brain is placed in a predefined posi-tion and a default target and needle trajectory arechosen so as to set the impedance cone accordingly.

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The virtual impedance system setup is exactly thesame as the one from the co-manipulation surgicalmode.

4. Results

The developed robotic solution was applied to thebrain biopsy surgery. For this, the phantom with ahuman brain inside was used. The procedure wasperformed by the surgeon in order to fully validatethe robotic solution.

To obtain the results that will be presented a se-quence of steps from the Surgical Co-ManipulationMode were followed. In a preliminary step the Nee-dle Path Planning software was used to set the de-sired target along with the trajectory that the nee-dle must follow. The chosen target was the rednucleus, which is a 6 mm size anatomical structuresituated in the midbrain next to the substantia ni-gra and it is involved in motor coordination.

To perform the surgical procedure, the robot wasconnected to the computer to serve as an hapticdevice. After the robot reached its starting posi-tion the registration procedure was started. Thisprocess had to be performed by two persons, onethat clicked the Space Bar button and another thatmaneuvered the robot while the program was ac-quiring the points from the phantom’s faces. Theorder for the planes’ point retrieval was the follow-ing, left plane → front plane → right plane → backplane → bottom plane. An illustration of this pro-cess is shown in Figure 4. Since the bottom face ofthe box can not be reached, the table’s surface wasused for the same purpose. This can be done sincethe acquired points are fitted to planes. The topplane is not acquired since the box can be openedduring the registration procedure. The top planewas discovered with the knowledge of the box’s di-mensions.

After all point were acquired, it took about tenseconds for the program to load the MRI data andperform all the calculations required to discover thetransformation between the acquired physical boxand the virtual MRI box. In order to calculate theerrors inherent to the registration procedure, thedifference between correspondent box vertexes werecalculated and the results are presented in Table 1.These errors were measured in the reference frameof the physical box so that a meaningful geometricevaluation could be performed.

(a) Left Side (b) Front Side (c) Right Side

(d) Back Side (e) Bottom Side

Figure 4: Illustration of the registration procedure.The sequence of the process is from the image a) toimage e).

Table 1: Errors from the registration procedure.Results in mm.Vertex |xi| |yi| |zi| RMSEi

1 1.4506 0.3929 0.1326 1.50872 1.0502 1.3273 0.6045 1.79733 0.3539 0.8381 0.1406 0.92054 1.2593 0.3628 0.6038 1.44295 0.3104 0.5817 0.1422 0.67456 0.7227 0.1246 0.5869 0.93937 0.5232 0.3864 0.1414 0.66568 0.3776 0.5662 0.5948 0.9039

Mean 0.7560 0.5725 0.3683 1.1066

In the table 1 it can be observed that the meanerrors in the x, y, and z directions were, respec-tively, 0.7530 mm, 0.5725 mm and 0.3683 mm. Themean value of the root mean square errors (RMSE)was 1.1066 mm.

Right after the program finishes all the necessarycalculations associated to the registration, the vir-tual impedance environment was created in the 3Dsimulator. Before clicking the 3 keyboard button,the robot was set in the safe working space and wasroughly aligned to the pre-planned needle trajec-tory. This intermediate step can be seen as a safetyprocedure and it is done to certify that the robotdoes not start in a no-go zone.

Following the previous safety check, the posi-tioning of the needle was performed via the co-manipulation virtual impedance environment. Inthis step the surgeon pulled the robot towards thethe desired target point and the virtual impedanceenvironment correctly guided the robot to the de-sired position. When the robot reached the targetposition it stopped in the target point with an offset

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of no more that 0.5 mm. This offset is the distancebetween the needle’s tip and the target point mea-sured in the virtual environment. During this wholeprocess the 3D virtual environment was frequentlychecked for a better understanding of the needle’slocation inside the brain.

Figure 5: Photograph of the robot correctly posi-tioning the needle in the desired target.

After the needle was correctly positioned the sur-geon removed two pieces of brain tissue from thetarget and injected a contrast mixture made of bar-ium sulfate, which provides contrast in the CT scan,and magnesium sulfate, which provides contrast inthe MRI. This contrast was also injected every 2 cmwhile the needle was being removed. Such processwas performed so that the needle trajectory can beeasily detected in the postoperative images. Oneimportant thing that must be noted is that duringthis process the robot managed to keep a constantposition and the effects of tremor inherent to thesurgeons hand were significantly attenuated withnothing more than the virtual impedance controlthat was implemented. The postoperative imagesfrom the brain can be observed in the figure thatfollows.

Figure 6: Postoperative images from the biopsiedbrain.

After carefully analyzing the postoperative MRIit was concluded that there was no significantchanges in the brain position nor any distortion ofits structures. The error in the biopsy needle’s po-sitioning was then calculated and the results are

shown in Table 2.

Table 2: Errors from the biopsy needle’s position-ing. Comparison between the pre and postoperativeMRIs. Results shown in mm.

|x| |y| |z| RMSE1.7± 0.5 4.9± 0.5 3.6± 0.5 6.3

The errors shown in the table 2 were also calcu-lated in the reference frame of the physical box andthe total positioning error obtained for the roboticsystem was 6.3 mm. The axial deviation from thechosen target (center of the right red nucleus) has amagnitude of 1.7 mm in the x direction and 4.9 mmin the y direction and the depth error has a valueof 3.6 mm. These results where somehow expected,and are justified by accumulation of errors startingwith the robot kinematic calibration. The enhance-ment of this robotic solution must now be done infuture work. For the robot to biopsy the red nu-cleus, an accuracy of approximately 2 mm shouldhave been obtained. In a real surgery, a poorer ac-curacy than 2 mm to biopsy a structure of 6 mmsize is far too risky to even consider performing it.

The three main sources of error in the performedsurgical procedure lie in the tool calibration (1.7mm), the registration method (1.1 mm) and theneedle’s positioning (0.5 mm). Another source oferror can be due to some deviation of the biopsyneedle, that might have occurred when it perforatedthe brain, since it was significantly stiffer than a liv-ing one. The knowledge of these errors can providea good guess where the robotic solution can be im-proved.

5. Conclusions

The presented work focused on developing a com-plete robotic solution that can effectively positiona needle in a desired target inside the brain. Us-ing a robotic system in this type of surgeries bringsmany advantages when compared to the conven-tional brain surgeries, namely, it is capable of re-moving the tremor of the human hand, it canachieve a higher level of accuracy and it allows thedecrease of the burr hole size. A secondary butalso important objective of this work was to testthe capabilities of the KUKA LWR 4+ in a co-manipulative surgery. Although the robotic systempresented was tested on a brain biopsy surgery itcan also be used in similar procedures that involvesome kind of positioning, like the placement of anelectrode in DBS.

As a first product of this work, a software forchoosing the desired target, along with the defini-tion of the trajectory that the needle should travel

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to reach that same target was developed. This userinterface has a wide range of functionalities, whichgoes from improving of image quality to the redef-inition of the planes in which the MRI images aredisplayed.

The ultimate product of this work is the roboticsystem developed for brain surgeries. This sys-tem comprises a 3D virtual environment that canbe used, not only as a surgical tool but also as atraining tool for surgeons. For this last part theomega.6 haptic was used to serve as a substitutefor the robot.

The developed virtual impedance control solutionis a smart, safe, and helpful way for the surgeon toguide the robot to the desired target position. Thissolution provides a safe environment to be used insurgery, since a security zone was implemented thatstops the robot to enter the no-go zone even if thesurgeon forces it. The robotic solution was testedin a procedure that tried to simulate a brain biopsyas realistic as possible. For this, the phantom witha human brain inside was used as a patient. Thisprocedure consisted in three main steps. First, thechoosing of the desired target position and needletrajectory via the Needle Path Planning Software;second, the registration of the phantom; and third,the guidance of the needle to the target point.

The total error of the entire procedure has a valueof 6.3 mm, while the errors obtained in the x, y andz directions are, respectively, 1.7 mm, 4.9 mm and3.6 mm. These are positive results, since the mainobjective of this work was to develop a good androbust foundation for a robotic solution and not aperfect one. The three main sources of error thatinfluenced the final needle position were the reg-istration method, the needle’s positioning and thetool calibration. The RMSE from the registrationprocess was of 1.1 mm, which can be explained bydisplacement of the table and/or the phantom dur-ing the procedure. The needle’s positioning errormeasured in the virtual environment was of approx-imately 0.5 mm and the tool calibration error wasof 1.7 mm. From these, one can extrapolate thatthe main source of error is in fact the last one. Thisis a good sign for this project, since it is the easiestto correct. To do this an optical tracking systemcould be employed to reduce this error as much aspossible. In order to reduce the error inherent to theregistration procedure a force control could be im-plemented. Since this means that the robot wouldkeep a constant force applied in the box’s faces, theoscillations that occur while the registration proce-dure is being performed would be greatly reduced.

In sum, the final outcome of this work is very pos-itive and it constitutes a good foundation for fur-ther development and enhancement of this roboticsurgical system. The advantages of using the devel-

oped system can be resumed to the fact that it isa very slender and compact solution that can pro-vide a fast and easy way to perform brain surgeries.One other advantage that this robotic solution hasin respect to the other surgical robots presented inthe state of art is that the registration procedure isdone with the robot itself, no other tools are needed,thus making it a more cost effective product.

6. Future work

My suggestions and recommendations for the conti-nuity of this project so as to improve the developedrobotic solution are:

• Integrate an optical tracking system to improvethe robot’s tool calibration error.

• Implement a force control to decrease the errorassociated to the registration procedure.

• Implement a tele-manipulation functionalitythat would allow the scaling of the surgeon’shand movements.

• Implement a brain shift correction module thatwould update the brain volume in real time anddetect if any of the brain’s structures movedfrom its original position.

• Implement a registration algorithm that canperform the registration of a real patient, viaa 3D reconstruction of the head and a contourof the patient’s face.

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