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A telerobotic haptic system for minimally invasive stereotactic neurosurgery

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A telerobotic haptic system for minimally invasive stereotactic neurosurgery
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  ORIGINAL ARTICLE A telerobotic haptic system for minimally invasivestereotactic neurosurgery A Rossi, A Trevisani, V Zanotto A Rossi, A Trevisani and V Zanotto Department of Innovation in Mechanics and Management, Universita` di Padova, ItalyCorrespondence to: A Rossi, E-mail: aldo.rossi@unipd.it  Abstract  Medical robotics and computer assisted surgery are feasible and promising applications of robotic technology,whose main goals are surgical augmentation, information enhancement and improved surgical action.Neurosurgery probably presents the most major challenges, and can considerably benefit from theintroduction of computers and robots to guide surgical procedures. This paper presents an innovative master-slave haptic robotic system for minimally invasive neurosurgery, which can help surgeons overcome humanshortcomings and perform more accurate, repeatable, and reliable stereotactic neurosurgery. The system,named LANS, consists of a slave mechatronic actuator and a haptic master. The slave is designed to movelinearly a laser pointer, a biopsy needle or a low-energy X-ray emitter along a pre-planned axis. The toolinsertion into the brain is guided by the surgeon through the haptic master which also provides forcefeedback to the operator. Not only can the haptic master reproduce the contact force between the surgicaltool and the treated tissue, but it can also produce virtual forces aimed at assisting surgeons during theoperations. Experiments have been conducted to prove the soundness and accuracy of the overall systemmechanical design and to assess the effectiveness of the control schemes synthesized for the master and theslave. Keywords:  Medical robotics, computer assisted surgery, stereotactic neurosurgery, haptic display, robot design, X-rayPaper accepted: 19 November 2004Published online: 15 January 2005. Available from:  www.roboticpublications.comDOI:  10.1581/mrcas.2005.010209 INTRODUCTION Medical robotics and computer assisted surgery(CAS) span the broad areas of medical science andengineering, and aim at developing innovativedevices and tools for performing surgical proceduresin a more precise and effective way.Indeed, robotic systems can perform somesurgical tasks with more precision, repeatabilityand delicacy than a human could. However decisionmaking during any intervention is still requiredby surgeons, and consequently, at present, robotscannot replace physicians entirely. Nonetheless, thealready proved and recognized effective interactionbetween robotic systems and physicians hasproduced an increasingly worldwide interest inthe area of medical robotics, and in particular inneurosurgery. An up-to-date overview of the mostsignificant researches conducted in this field canbe found in Zamorano  (1) and in the referencestherein.Generally speaking, there are four areas in surgeryin which the use of robots is particularly useful:diagnostic procedures, teaching-training, telesurgeryand therapy  (2) . In diagnostic procedures for example, robotic tools can be currently employedto perform ultrasound and computer axial tomo-graphy (CAT)-guided biopsies. Additionally, newpossibilities are arising in surgical instruction andtraining, since movements and procedures canbe faithfully reproduced making use of tactilefeedback and virtual reality  (3, 4) . The use of hapticinterfaces, providing tactile and force feedback, 64 Int J Medical Robotics and Computer Assisted Surgery 2005;1(2):64–75  E 2005 Robotic Publications Ltd. www.roboticpublications.com   joined with telemetry and robotic technologies arealso likely to expand the development of telesurgeryprocedures.Telesurgery provides surgeons with new capabil-ities, with the aim of performing less invasive andmore accurate surgical interventions, reducing post-operative hospital stay and improving clinical out-comes. In particular, minimally invasive surgery(MIS) is acknowledged to be a revolutionarysurgical technique  (5) . Minimally invasive proce-dures use remotely controlled robots to enablesurgeons to operate inside patients without makinglarge incisions. The main advantage of this techni-que is a substantial reduction of tissue trauma, whichis a main cause of post-operative pain and longer hospital stays.Among all the cited areas, therapy is certainly themost promising field of application of robotictechnologies to surgery and will be the subject of extensive future research  (2) . This paper presents anew telerobotic system for minimally invasiveneurosurgical therapy, designed and manufacturedat the Department of Innovation in Mechanicsand Management of the University of Padova.The system, named LANS (Linear Actuator for NeuroSurgery) has been conceived specifically toperform biopsies and neurosurgical interventionsby means of a miniature X-ray source (the PRS,Photon Radiosurgery System, by Carl Zeiss),whose emitting tip must be placed accurately insidethe patient’s brain.The LANS robotic system comprises a hapticmaster module, operated by the surgeon, and a slavemechatronic module that makes a PRS probe, or abiopsy needle, move along a predefined emissionaxis in accordance with the master position decidedby the surgeon. So as to orient the LANS alongthe established emission axis, a NeuroMate robot isemployed in a frame-based configuration whichensures the highest possible accuracy.The system has been designed by assuming thatduring the surgical operation only the LANS (whichis very accurate, and provides the surgeon with forcefeedback) is in active mode while the NeuroMateis powered off. This allows many of the problemsassociated with the complex nature of this surgicaltherapy to be overcome. Moreover, very preciseand repeatable movements of the biopsy needle andof the X-ray source can be obtained, thus improvingthe overall intervention outcomes.The proposed system therefore greatly enlargesthe NeuroMate performances when dealing withthose neurosurgical therapies making use of toolsthat must be accurately and delicately inserted intothe brain. In particular, the tool positioningaccuracy and repeatability is increased by more thanone order of magnitude and a very precise anddelicate insertion of the tool is assured.Furthermore, the fact that surgeons do not movethe tool directly, but through a master input handle,gives the possibility of both scaling the motion(therefore improving human dexterity limits), andproviding them with force feedback. Force feedbackand control, is not only important to recreaterealistic feelings during the intervention but alsobecause the surgeon’s instrument guidance can beeffectively and naturally improved by exertingsuitable virtual forces on the master handle. Suchforces prevent incorrect movements, associatedwith anxiety, fatigue or age, which might seriouslydamage healthy brain tissues. Thus, by means of thehaptic master, the surgeon can be placed in contactwith a virtual environment where the surgical task isconsiderably simplified. NEUROSURGERY BY MEANS OF PRS:ADVANTAGES AND OPEN ENGINEERINGISSUES Conventional radiosurgery is based on the use of highly collimated external beams or on the useof interstitial radiation (IR) with radioisotopes.The use of external beams requires a preventivehistological diagnosis, in addition to expensivestructures and dedicated facilities. IR with radio-isotopes, on the other hand, presents the usualinconveniences associated with the manipulation of radioactive sources.The PRS is a new miniaturized radio-surgicaldevice for the IR of brain lesions  (6, 7) which canconsiderably reduce the aforementioned drawbacks,if appropriately employed. The device emits lowenergy X-rays from the tip of a cylindrical probe.The probe length is 10 cm, the diameter 3,2 mmand the dose distribution almost spherical  (8, 9) , asshown in Figure 1.The use of PRS in neurosurgery providesconsiderable advantages in comparison with tradi-tional techniques  (10, 11) . In particular, comparedwith conventional radiosurgery, IR with PRSprovides dosimetric advantages since adjustable doserates and steep dose gradients can be obtained by A telerobotic haptic system for minimally invasive stereotactic neurosurgery  65 E 2005 Robotic Publications Ltd. Int J Medical Robotics and Computer Assisted Surgery 2005;1(2):64–75 www.roboticpublications.com  adapting the electron acceleration potential.Additionally, compared with radiosurgical techni-ques with external beams, PRS represents a morecost-effective radiosurgical tumour treatment whichcan be executed immediately after the stereotacticbiopsy without requiring specific radiation-protection measures or dedicated facilities. Inparticular, measurements aimed at determining theradiation exposure level in the environment duringPRS IR have demonstrated that no particular radiation-protection measures are needed  (7) .From the radiobiological point of view, IR withPRS has the aim of delivering a necrotizing dose of radiation in the tumour volume, minimizing thedose in the surrounding tissues. Imaging andpathological studies on post-mortem specimensshowed that in the region corresponding to thetreated volume, a spherical area of coagulativenecrosis appears, involving both a vascular net andparenchyma sharply demarcated at the periphery of the area of apparently undamaged tissue. Suchan effect is obtained rapidly (i.e. 24 hours after irradiation)  (12–16) . These observations support theeffectiveness of IR with PRS in the treatment of selected spherical shaped tumours.On the other hand, the possibility of inducingonly spherical necrotic areas is one of the major limits of the PRS. As a matter of fact, tumours areoften irregularly shaped and consequently, in order to employ the PRS, it is necessary to perform a setof treatments of suitable intensity through apredefined path of points of emission. The accuracyin the execution of such a treatment procedure(both in terms of steepness of the dose gradient, andof positioning precision in the points of emission)becomes of paramount importance so as to keep itreally minimally invasive, and justifies the develop-ment and the use of robotic devices, like the onepresented in this paper.Currently, the PRS is prevalently positioned andoriented using stereotactic headframes, by means of a long-lasting procedure preceding the intervention.The position and orientation of the PRS emissionaxis are determined on the basis of the knownpositions of the entry-point and of the tumour lesion (target). Then, the insertion of the probe iscontrolled manually through a graduated rack andpinion mechanism. It follows that the position andorientation of the X-ray emitter during the opera-tion are significantly influenced by the surgeon’sskill and expertise. Clearly, the higher the position-ing accuracy and the ability to move the PRSduring the insertion, the lower the invasiveness of the intervention. In this regard, it is well known thatnot only is the use of stereotactic headframes painfulfor patients but also the manoeuvres for introducinginstruments into the brain (e.g. biopsy needles andradiation source probes) may often be traumatic  (12) .The use of robotic systems to position, orientateand guide the PRS can guarantee a much higher level of accuracy than stereotactic headframe-basedtechniques. Hence, it is essential to design medicalrobots that support and improve surgeons’ capabil-ities.A first possible robotic approach to the executionof the PRS IR might involve the direct use of theNeuroMate which has been specifically developedfor neurosurgery, and has been proved to be highlyuseful to hold, orient and stabilize a variety of conventional surgical devices such as drills, laser probes and electrodes  (1) . However, the NeuroMatehas some significant technical limitations whichmake its use unreliable for the insertion of PRSprobes and biopsy needles into the brain. In fact,precision, accuracy, repeatability and force controlare all critical in these surgical procedures, and inparticular in PRS IR, not only because it can bevery traumatic, but also because any deflection of the probe is to be avoided to prevent the dissipationof the beam energy and consequently reduction of the beneficial therapeutic effects. Probe deflectionsmay be chiefly caused by the contacts of the probewith the skull at the entry-point. However, thelimited accuracy (0.86 mm ¡ 0.32 mm)  (17) of theNeuroMate and the lack of force control makeit impossible to prevent or monitor the probe Figure 1  Schematic representation of the PRS sphericaldose distribution within the treated tissue. 66  Rossi, Trevisani, Zanotto Int J Medical Robotics and Computer Assisted Surgery 2005;1(2):64–75  E 2005 Robotic Publications Ltd. www.roboticpublications.com  deflections and hence to ensure an actually mini-mally invasive procedure.Nonetheless the NeuroMate can be very usefulalso in PRS IR since it can be effectively employedto hold and orientate statically robotic systemsspecifically designed for the accurate insertion of biopsy needles and PRS probes into the brain. Inaccordance with the aforementioned requirements,these robotic systems, besides being very accurate,must allow the simultaneous measurement of thecontact force between the surgical tool and thetreated tissue. Such a force, properly scaled, has thento be fed back to the surgeon by means of a suitablehaptic interface used to guide the tool insertion.This is the robotic configuration proposed andadopted in this work.In the following sections the two chief compo-nents of the LANS are presented: the mechatronictool actuator (the slave module) and the hapticinterface (the master module). THE TOOL ACTUATOR The most important LANS component is amechatronic device which can be mounted directlyon the NeuroMate (see Figure 2, red box) andallows the movement of surgical instruments withhigh accuracy along the axis defined by the entry-point and the target established for the therapy. Thechief components of the designed tool actuator areshown in Figure 3.Basically, the tool actuator comprises two mainparts: an aluminium alloy base frame, which can befixed to the NeuroMate arm, and a moving carriagewith a neurosurgical tool holder linked to a loadcell.A current-controlled DC mini-motor, placed atan extremity of the frame, drives a miniature ballscrew through a synchronous belt (velocity ratio12:28). The backlash-free preloaded single nut of the ball screw shifts the carriage. The actuator isdevoid of a braking system because no backdrivingoccurs.Motion linearity is ensured by a minirailrecirculating ball guideway. The angular positionof the DC motor is measured by a shaft-positionoptical encoder whose resolution is 400 pulses per revolution. The overall accuracy and repeatability of the tool actuator are better than 0.05 mm.In order to guarantee patients’ safety, the carriagemotion range has been restricted to 90 mm bymeans of two mechanical stops incorporated in thesystem frame.Appropriate mechanical adapters have beendesigned to couple neurosurgical tools, such as laser pointers (Figure 4) biopsy needles (Figure 5) andPRS sources (Figure 6) to the tool holder mountedon the carriage. A micro-sensor has been inserted inthe tool holder so as to assure continuous monitor-ing of the correct placing of the adapters, and henceof the surgical tools.The tool holder is directly linked to a homemadealuminium load cell. The load cell can measure thecontact forces between the tools and the cerebral Figure 2  The LANS tool actuator mounted on theNeuroMate. Figure 3  The chief components of the LANS toolactuator. A telerobotic haptic system for minimally invasive stereotactic neurosurgery  67 E 2005 Robotic Publications Ltd. Int J Medical Robotics and Computer Assisted Surgery 2005;1(2):64–75 www.roboticpublications.com  tissues with a resolution of 1/50 N. These forces,suitably amplified by the LANS controller, are fedback to the surgeon through the handle of the hapticconsole.For safety and hygienic reasons the actuator hasbeen provided with a stainless steel external cover.The total weight of the LANS slave module is1.8 kg.The technical specifications of the overall slavesystem are summarized in Table 1 THE HAPTIC INTERFACE The haptic master developed for the LANS isshown in Figure 7 and basically consists of a 40 mmdiameter haptic handle positioned on an ergonomicconsole. Using the handle the surgeon can guide thesurgical tool along a pre-planned axis and can feelthe interaction forces between the tool and thecerebral tissue.The interface meets the important requirement of being almost transparent to the operator   (18) , in thesense that the dynamic behaviour of the mechanismcan be hardly perceived by the surgeon. The forcesare reproduced by means of a current-controlledDC motor directly coupled to the handle and whoseinertia and friction are very small. The angular  Figure 4  The laser pointer mounted on the tool actuator. Figure 5  The mechanical adapter specifically designedto couple biopsy needles to the tool holder. Figure 6  The PRS mounted on the tool actuator. Table 1  Technical specifications of the tool actuator Tool movement range [mm] 90Movement linearity [mm] 0.01 Accuracy [mm]  ¡ 0.05Max. carriage speed [mm/s] 13Max. pay load [N] 15Tool holder axis max. pitch displacement [deg.]  , 0.02 Figure 7  The LANS haptic interface and visual devices. 68  Rossi, Trevisani, Zanotto Int J Medical Robotics and Computer Assisted Surgery 2005;1(2):64–75  E 2005 Robotic Publications Ltd. www.roboticpublications.com
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