A Haptic Interface for Interventional Radiology∗

Dejan Ilic, Thomas Moix, Blaise Fracheboudand Prof. Hannes Bleuler

LSRO - Laboratoire de Systemes RobotiquesEPFL - Ecole Polytechnique de Lausanne

Ch. des Machines ME.B3, 1015 Lausanne, Switzerlanddejan.ilic@epfl.ch

MD. Ivan VecerinaXitact S.A.

Rue de Lausanne 45, 1110 Morges, Switzerlandivan.vecerina@xitact.com

Abstract— Low trauma, reduced costs, fast recovery are fewfactors why minimally invasive surgery (MIS) is taking overclassical surgical methods. Interventional Radiology (IR) isa MIS technique where thin tubular instruments are steeredthrough the patient’s vascular system under the X-ray imag-ing. These procedures demand highly trained and experiencedspecialists. A computer-based haptic interface is proposed asa quality training environment. The system is composed of avirtual reality (VR) environment and force feedback units forthe surgical instruments. This paper describes the 4 degreesof freedom (DOFs) haptic interface that has been developedfor this purpose.

Index Terms— Computer Assisted Surgery Simulator, In-terventional Radiology, Haptic Interface, Force-Feedback De-vices.

I. INTRODUCTION

A. Interventional Radiology

The minimally invasive technique that involves navi-gation of catheters and guidewires through the patient’svascular system is called Interventional Radiology (IR).Guidewires are very flexible cylindrical instruments that aresteered to the desired point in the patient’s vascular system.Catheters, tubular instruments, can afterward be insertedand driven over the guidewire directly to the interventionlocation. The control of the ongoing intervention and theinstrument positions are monitored with real-time X-rayimaging.IR is an extension into therapy of the diagnostic proce-dures (angiographies) of vascular diseases in the peripheral,cardiac and neuro areas. Immediate treatment is possiblethrough techniques including local drug injection, catheterballoon inflation (angioplasty) and stent (implant) place-ment among others [1].

B. Training methods in IR

Due to difficult hand-eye coordination, complicatedlooping and bending of the instruments and the risk ofvessel injury, these techniques need to be performed byhighly trained and experienced specialists. Some especiallyrisky procedures, such as carotid stent placement are evenexpected to require specific certification and training.

∗The research is supported by the Swiss National Science Foundationwithin the framework NCCR CO-ME (COmputer aided and image guidedMEdical interventions) and by the Swiss Innovation Promotion AgencyKTI/CTI

Only few mock devices are available to the radiologists.The training efficiency of such systems remains limited.Therefore, practice is often limited to the traditional ”seeone, do one, teach one” model with animal training as analternative, hindered by high costs and ethical issues.

C. Computer-Assisted Training Systems

Computer-assisted training systems are an interestingalternative for surgeons. Just as flight simulators recreatethe behavior of a plane, different weather conditions andcritical situations, surgery simulators offer an environmentfor realistic training of medical interventions [2]. Themedical procedure takes place in a virtual reality (VR)environment with the visual rendering of the imagingdevice used by doctors. The physical user-interface to thesurgeon, the ”joystick” of the simulator, consists of hapticforce-feedback devices with the appearance and feel ofsurgical tools [3].Several clinical studies have already highlighted the bene-fits of such training systems for the surgeons ([4], [5]).

II. THE PROPOSED HAPTIC INTERFACE

A. Requirements

Based on human factor studies [6] and simpleexperiments, the following hardware requirements havebeen identified.The rotation and the translation of each instrument haveto be measured, in total 4 degrees of freedom (DOFs).Active force feedback must be provided for both DOF ofeach instrument. The sensors requirements and the forcefeedback resolution are the following:

• Force and torque resolution shall be of 0.02 N and0.04 mNm respectively.

• In IR procedures forces and torques, estimated for a4 French (1 French = 1/3 mm) catheter, are in therange of ±2 N and ±4.5 mNm.

Furthermore, a haptic device has to meet severe controlaccuracy requirements as human hand sensitivity has amaximum of less than 1 µm amplitude at 300 Hz [7].

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B. State of the Art

Over the last decade, several computer-assisted trainingsystems for IR have been developed and some of themcommercialized. These systems propose haptic interfacesthat rely on two technical approaches.The first is based on friction drives directly acting on thetools ([8], [9], [10]). The actuators can be rolls or balls.The main advantage of this solution is the simplicity tocombine tracking (often code wheels with optical encoders)and actuation. The major drawbacks of friction drives areslippage, the required adjustment which can limit tool in-terchangeability, the inertial dynamic behavior and frictionthat needs to be compensated for.The second approach uses linear stages to follow the instru-ment’s translational movement ([11], [12], [13], [14]). Insuch arrangement, the surgical tool is fixed to the carriageof the linear stage. Therefore, tool interchangeability isimpossible. Rotation of the surgical tool must be measuredand actuated on the carriage leading to heavy structuresthat decrease the system bandwidth.Some of the proposed systems only include braking feed-back on the surgical instruments [9] which implies thatcertain movements of the tools experienced by surgeonscannot be rendered. A practical example would be whena guiding catheter (i.e. large and preshaped to fit intoan anatomical site) jumps out of place. The deformationenergy that was stocked in the catheter is then releasedand can create back and forth movements.As already mentioned, the catheter is steered overthe guidewire, meaning that during an intervention theguidewire can in certain conditions be entirely inside thecatheter. The tracking of the guidewire position, a socalled ”tube in tube” problem, becomes difficult. The work-around that has been used in other reported systems is theuse of separate haptic devices for each surgical instrument.The devices are spaced by the desired stroke of the trainingsystem. That strategy implies non-realistic behavior duringinsertion or withdrawal of an instrument and is particularyundesirable in peripheral areas (legs and arms) where theinsertion point on the body is very near to the interventionlocation.

C. Working Principle

The proposed haptic interface is separated in two hapticdevices. A catheter unit is fixed at the entrance of thesystem. It measures translational and rotational move-ments and applies force feedback on a catheter. A mobileguidewire unit tracks the motion of a guidewire whileapplying force feedback. The guidewire presence is de-tected at the entrance of the simulator and then grasped bya micro-gripper. Forces and movements of the guidewiremoving inside the catheter are then transmitted through thegripper and along a rod on to the external guidewire unit.The proposed solution is illustrated in figure 1.

D. Catheter Haptic Device

The central part of the device consists of two rubbercoated cylinders. The catheter is guided between them in

Catheter UnitCatheter

Guidewire Unit

Guidewire

Linear Stage

Gripper

Fig. 1. The proposed architecture

a friction drive arrangement as depicted in figure 2. Aframeless DC torque motor is mounted in direct driveon one of the cylinders, thus applying an active forcefeedback for the translation of the catheter. The cylindersare preloaded by springs (7 N force) thus avoiding slip-page. The friction between the catheter and the cylinders issufficient for different instrument diameters without defor-mation induced by preloading forces that would impeachthe movement of the guidewire inside the catheter. Hence,the proposed friction drive is more compact and lighterthan equivalent solutions that have been proposed for hapticdevices [15]. A second DC motor is linked with a beltto the cylinder system providing a force feedback for therotational DOF. Two force sensors, one in each cylinder,have been integrated in the friction drive to enable inertiaand friction active compensation. Both sensors consist ofan arrangement of optical reflective distance sensors mea-suring the deformation of a mechanical structure. Sensorsare placed as close as possible to the point of contactwith the catheter to provide the most direct measurementpossible. Hence precision losses and non-linearities due tobacklash, friction, vibrations between mechanical elementsare minimized.The first force sensor is directly integrated into the passive

cylinder measuring the torque transmitted by the rotationof the catheter. It consists of an inner and an outer ring thatare hold together with two disc springs. The stiffness of thestructure is very high in all directions except the axial one.A force applied to the outer ring induces a deflection on thedisc springs measured by two reflective infrared sensors.Their output signals are added to eliminate perturbationsdue to structure misalignment on the cylinder axis.The translational force transmitted by the catheter is mea-

sured with the second sensor that is part of the motorizedcylinder. The mechanical structure of the sensor is manu-factured in an unique piece by electro-erosion (figure 3).

Catheter

Active Cylinder

Passive Cylinder

Catheter Guides

Fig. 2. The friction drive arrangement

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Fig. 3. The mechanical structure of the translational force sensor

The structure is composed of two rings connected with fourbeams [16]. The inner ring is screwed on the main frameof the friction drive arrangement while the outer ring isattached to the stator of the torque motor. When a torqueis applied on one of the rings, the beams deflect. The beamsdeflection is measured on two reflective target located onthe outer ring of the torque sensor by two infrared sensorsas illustrated in figure 4. The two infrared optical sensorsoutputs are subtracted to eliminate unwanted perturbation.The calibration results are presented in figure 5. Theresolution of measured forces and torques of the catheterare respectively of 0.02 N over ±2 N and 0.04 mNmover ±20 mNm. The system was designed for a maximumspeed of 1 m/s for the translation and 2rps for the rotationand a maximum acceleration of 10 m/s2 and 5 rps2. Thesevalues were estimated on observations of actual procedures.The whole friction drive system is set into rotation when thecatheter is turning. Therefore, an electrical slip ring is usedto power the torque motor and the sensors. The amplifiedoutput signals from the sensors are routed through the slipring. The system is designed to be light, without backlash,and to display high resonance frequencies.The whole haptic interface is fixed on a base plate. The

distance between the insertion point of the catheter andwhere the force feedback occurs is of about 30 mm.The weight of the system that will be required to beput into rotation is less than 300 g. The inertia of the

Outer Ring

Reflective SensorInner Ring

Stopper

Fig. 4. The translational force sensor

Force (mN)

Out

put V

oltag

e (V)

-2000 -1500 -1000 -500 0 500 1000 1500 2000 2500-2.50

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

2.50

Fig. 5. Calibration curve of the translational force sensor

complete system in rotation is 1.4 10−4 kgm2, includingthe inertia of the rotors. The realized prototype of theproposed hardware haptic interface for catheter is presentedin figure 6.

Fig. 6. The catheter haptic interface

E. Guidewire Haptic Device

The guidewire haptic interface is based on a linear stage.The carriage of the device is mounted on a miniature guide-way and linked to a belt system. The belt is connected to aDC motor that provides force feedback for the translationalDOF of the guidewire.The carriage holds the actuation system to operate themicro-gripper used to grasp the guidewire inside thecatheter. The gripper is composed of a long 1.1 mmdiameter outer tube with an internal 0.7 mm diameter wire.A gripper head, a small tube with cuts, is glued at the distalend of the internal wire. Due to its dimensions the grippercan be inserted and moved into a 4 French catheter. Byrelative movement of the outer tube to the wire, the outertube can constrain or relieve the gripper head, thus closingor opening the gripper. The relative movement of the tubeis created by a small DC motor and transmitted to the tubeby a wire arrangement. The rotation of the outer tube andinner wire is decoupled in the transmission. It is thereforestill possible to measure the rotation of the guidewire withan optical encoder attached to the inner wire of the gripper.A magnetic particle brake provides passive force feedbackon rotation of the inner wire of the gripper and thus tothe guidewire. The prototype gripper presented in figure 8

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Fig. 7. Close-up view of the micro-gripper head

grasps a 0.035 inch diameter guidewire with applied axialpull forces up to 3 N .A force sensor has been integrated on the carriage for

friction compensation of the guideway system on thetranslational DOF. The force applied to the guidewire bythe user is transmitted through the rod of the gripper to thecarriage. When the gripper is actuated (i.e. the guidewireis grasped) a mechanical part of the transmission, whichis linked to the outer tube, is pushed against a compliantmaterial. This material is used as a preloaded spring forforce sensing. A reflective target is glued at the end of theouter tube on the carriage side. Two infrared sensors areused to measure the displacement of the target. The outputsignals of the sensors are added to cancel mechanicalunbalance. The sensor resolution is of 5 mN over ±2.5 N .Thanks to careful material choice and mechanical design,

the carriage’s weight is just under 200 g.

F. Control Strategy

Both haptic devices are linked to an electronic inter-face that includes AD converters, quadrature decoders andmotor drivers. The electronic interface is linked to thecomputer where the VR simulation is running. A low-level software interface formats the measured data and runsthe control algorithms. A generalized impedance controlarchitecture adapted to the VR interface has been developed[17]. Computed torque techniques have been implementedon the system. The non-linear input linearization strategy

Fig. 8. Detail of the guidewire haptic interface carriage

Fig. 9. The complete haptic interface with the control electronic

[18] includes dynamic and (Coulomb) friction with a PDcontroller. Parameters of the controllers have been extractedfrom theoretical models and identification techniques. Theforce feedback control loop has a sampling rate of 1 kHz.

G. Simulation software

The software for interfacing to the hardware implementsthe same software interface as the commercially availableXitact CHP device, an open platform for IR simulatorsavailable from Xitact S.A. Simulation software developedin our own institution, by the Simulation group at CIMIT[19] and commercial offerings that rely on Xitact’s platformare therefore compatible with our system.

III. CONCLUSION

Based on analysis of requirements and state of the art incomputer-assisted training systems for IR, a new hardwarehaptic interface is proposed. The prototype system offersa realistic behavior, adapts to real or slightly modifiedinstruments, and allows their complete withdrawal.Further work includes refinement to the current prototypeand its control architecture. Test and validation studies ofthe proposed hardware platform with a variety of simula-tion software will be carried out by medical partners.

ACKNOWLEDGMENT

The authors would like to thank the CIMIT Group atMGH, USA, and Xitact S.A, Switzerland, for their inputsand software support.

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