Tobias Ortmaier

Friedrich-Alexander-University of Erlangen-Nürnberg, Erlangen, Bavaria, Germany

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Publications (79)43.45 Total impact

  • Andreas Schoob · Dennis Kundrat · Lüder A Kahrs · Tobias Ortmaier
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    ABSTRACT: Processing stereoscopic image data is an emerging field. Especially in microsurgery that requires sub-millimeter accuracy, application of stereo-based methods on endoscopic and microscopic scenarios is of major interest. In this context, direct comparison of stereo-based surface reconstruction applied to several camera settings is presented. A method for stereo matching is proposed and validated on in-vitro data. Demonstrating suitability for surgical scenarios, this method is applied to two custom-made stereo cameras, a miniaturized, bendable surgical endoscope and an operating microscope. Reconstruction accuracy is assessed on a custom-made reference sample. Subsequent to its fabrication, a coordinate measuring arm is used to acquire ground truth. Next, the sample is positioned by a robot at varying distances to each camera. Surface estimation is performed, while the specimen is localized based on. markers. Finally, the error between estimated surface and ground truth is computed. Sample measurement with the coordinate measuring arm yields reliable ground truth data with a root-mean-square error of [Formula: see text]. Overall surface reconstruction with analyzed cameras is quantified by a root-mean-square error of less than 0.18 mm. Microscope setting with the highest magnification yields the most accurate measurement, while the maximum deviation does not exceed 0.5 mm. Custom-made stereo cameras perform similar but with outliers of increased magnitude. Miniaturized, bendable surgical endoscope produces the maximum error of approximately [Formula: see text]. Reconstruction results reveal that microscopic imaging outperforms investigated chip-on-the-tip solutions, i.e., at higher magnification. Nonetheless, custom-made cameras are suitable for application in microsurgery. Although reconstruction with the miniaturized endoscope is more inaccurate, it provides a good trade-off between accuracy, outer dimensions and accessibility to hard-to-reach surgical sites.
    International Journal of Computer Assisted Radiology and Surgery 06/2015; DOI:10.1007/s11548-015-1240-z · 1.66 Impact Factor
  • Christian Hansen · Kai Eggers · Jens Kotlarski · Tobias Ortmaier
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    ABSTRACT: Positioning tasks of multi-axis servo drive mechanisms typically lead to high energy demands, especially if lossy operating points are applied and/or recuperated break energy, e.g. during deceleration phases, is not effectively reused. A trajectory optimization approach based on the particle swarm algorithm is presented for the adaption of multi-axis positioning tasks during system run-time. Established path planning methods (including the possibility of minimum time motion) are applied, that are adapted by only two parameters per axis and positioning task. In this manner, idle-times that often exist between the motion cycles and/or energy exchange via coupled inverter DC-links are utilized to reduce energy demands and improve system efficiency. In contrast to existing offline trajectory optimization procedures, the method is able to adapt changing motion tasks during system run-time within only few movement cycles. Experimental results prove that, depending on the use case and the chosen optimization constraints, energy losses are effectively reduced, brake chopper dissipation often is even completely avoidable and, hence, total energy demands are distinctly reduced. The approach is applicable to different multi-axis configurations and enables to considerable energy savings without additional hardware invest.
    IEEE Conference on Industrial Electronics and Applications, Auckland, New Zealand; 06/2015
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    ABSTRACT: Bone-attached robots and microstereotactic frames, intended for deep brain stimulation and minimally invasive cochlear implantation, typically attach to a patient's skull via bone anchors. A rigid and reliable link between such devices and the skull is mandatory in order to fulfill the high accuracy demands of minimally invasive procedures while maintaining patient safety. In this paper, a method is presented to experimentally characterize the mechanical properties of the anchor-bone linkage. A custom-built universal testing machine is used to measure the pullout strength as well as the spring constants of bone anchors seated in four different bone substitutes as well as in human cranial bone. Furthermore, the angles at which forces act on the bone anchors are varied to simulate realistic conditions. Based on the experimental results, a substitute material that has mechanical properties similar to those of cranial bone is identified. The results further reveal that the pullout strength of the investigated anchor design is sufficient with respect to the proposed application. However, both the measured load capacity as well as the spring constants vary depending on the load angles. Based on these findings, an alternative bone anchor design is presented and experimentally validated. Furthermore, the results serve as a basis for stiffness simulation and optimization of bone-attached microstereotactic frames. Copyright © 2015 IPEM. Published by Elsevier Ltd. All rights reserved.
    Medical Engineering & Physics 03/2015; 37(5). DOI:10.1016/j.medengphy.2015.02.012 · 1.84 Impact Factor
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    ABSTRACT: To develop skills sufficient for hearing preservation cochlear implant surgery, surgeons need to perform several electrode insertion trials in ex vivo temporal bones, thereby consuming relatively expensive electrode carriers. The objectives of this study were to evaluate the insertion characteristics of cochlear electrodes in a plastic scala tympani model and to fabricate radio opaque polymer filament dummy electrodes of equivalent mechanical properties. In addition, this study should aid the design and development of new cochlear electrodes. Automated insertion force measurement is a new technique to reproducibly analyze and evaluate the insertion dynamics and mechanical characteristics of an electrode. Mechanical properties of MED-EL’s FLEX28, FLEX24, and FLEX20 electrodes were assessed with the help of an automated insertion tool. Statistical analysis of the overall mechanical behavior of the electrodes and factors influencing the insertion force are discussed. Radio opaque dummy electrodes of comparable characteristics were fabricated based on insertion force measurements. The platinum-iridium wires were replaced by polymer filament to provide sufficient stiffness to the electrodes and to eradicate the metallic artifacts in X-ray and computed tomography (CT) images. These low-cost dummy electrodes are cheap alternatives for surgical training and for in vitro, ex vivo, and in vivo research purposes.
    BioMed Research International 03/2015; 2015. DOI:10.1155/2015/574209 · 2.71 Impact Factor
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    ABSTRACT: Purpose. Though already proclaimed about 7 years ago, natural orifice transluminal endoscopic surgery (NOTES) is still in its early stages. A multidisciplinary working team tried to analyze the technical obstacles and identify potential solutions. Methods. After a comprehensive review of the literature, a group of 3 surgeons, 1 gastroenterologist, 10 engineers, and 1 representative of biomedical industry defined the most important deficiencies within the system and then compiled as well as evaluated innovative technologies that could be used to help overcome these problems. These technologies were classified with regard to the time needed for their implementation and associated hindrances, where priority is based on the level of impact and significance that it would make. Results. Both visualization and actuation require significant improvement. Advanced illumination, mist elimination, image stabilization, view extension, 3-dimensional stereoscopy, and augmented reality are feasible options and could optimize visual information. Advanced mechatronic platforms with miniaturized, powerful actuators, and intuitive human-machine interfaces could optimize dexterity, as long as enabling technologies are used. The latter include depth maps in real time, precise navigation, fast pattern recognition, partial autonomy, and cognition systems. Conclusion. The majority of functional deficiencies that still exist in NOTES platforms could be overcome by a broad range of already existing or emerging enabling technologies. To combine them in an optimal manner, a permanent dialogue between researchers and clinicians is mandatory. © The Author(s) 2015.
    Surgical Innovation 03/2015; DOI:10.1177/1553350615573578 · 1.34 Impact Factor
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    ABSTRACT: Introduction: Internal auditory canal (IAC) tumors are one of the most common lesions in the skull base area. Image guided surgery significantly improved the safety and accuracy of surgery and decreased the complications. Available image guided systems do not provide submillimeteric accuracy in standard setting, as required for skull base surgery. Optical coherence tomography (OCT) is a high resolution imaging system. It captures micrometer resolution, three-dimensional images. OCT is an interferometric technique, typically employing near-infrared light. The use of relatively long wavelength light allows it to penetrate into the scattering medium. It can reject the background signal and so build up clear 3D images. We intend to use the OCT as a more accurate, real-time guiding system for different IAC approaches. Material and Methods: At first, a novel and specific combined OCT and drill holder were designed. Accurate angel between OCT view and drill tip was a critical point. Then, the standard IAC approaches (translabyrinthine [TL], retrosigmoid [RS], and middle cranial fossa [MCF]) were done on the different human cadavers. Guiding by OCT image, the important structures were navigated. At the end of the procedure, the entire of IAC was exposed safely and vital structure saved. Result: OCT images provide a very high accurate image of subsurface area. Image resolution was 10µm. The acceptable image depth was 300 to 500 µm that is enough for blue lining the semicircular canals or exposing the cochlea, vestibule, and IAC contents. Handling device was very helpful to fix the OCT view on surgical site and provide continuous image. Conclusion: OCT can use as a real-time navigation system to find the important landmark during difficult surgeries. More accurate and fine surgery is possible using this system.
    02/2015; 76(S 01). DOI:10.1055/s-0035-1546512
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    ABSTRACT: During guided drilling for minimally invasive cochlear implantation and related applications, typically forces and torques act on the employed tool guides, which result from both the surgeon's interaction and the bone drilling process. Such loads propagate through the rigid mechanisms and result in deformations of compliant parts, which in turn affect the achievable accuracy. In this paper, the order of magnitude as well as the factors influencing such loads are studied experimentally to facilitate design and optimization of future drill guide prototypes. The experimental setup to evaluate the occurring loads comprises two six degree of freedom force/torque sensors: one mounted between a manually operated, linearly guided drill handpiece and one below the specimens into which the drilling is carried out. This setup is used to analyze the influences of drilling tool geometry, spindle speed as well as experience of the operator on the resulting loads. The results reveal that using a spiral drill results in lower process loads compared with a surgical Lindemann mill. Moreover, in this study, an experienced surgeon applied lower interaction forces compared with untrained volunteers. The measured values further indicate that both the intraoperative handling of the bone-attached drill guide as well as the tool removal after completing the hole can be expected to cause temporary load peaks which exceed the values acquired during the drilling procedure itself. The results obtained using the proposed experimental setup serve as realistic design criteria with respect to the development of future drill guide prototypes. Furthermore, the given values can be used to parameterize simulations for profound stiffness analyses of existing mechanisms.
    International Journal of Computer Assisted Radiology and Surgery 02/2015; DOI:10.1007/s11548-015-1153-x · 1.66 Impact Factor
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    Joel Greenyer · Christian Hansen · Jens Kotlarski · Tobias Ortmaier
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    ABSTRACT: Due to continuously rising electricity prices, the energy efficiency of production systems is an increasingly important factor in industrial manufacturing. One goal is therefore to reuse the braking energy of manipulator axes. The acceleration and deceleration phases of axes must be synchronized such that, ideally, the braking energy can be reused immediately within the system. In our current technique for the energy-efficient motion planning of concurrent movements of axes, we do not yet consider changing the discrete control logic to reorder movement sequences. This is difficult, since this reordering must not violate critical system requirements. In this paper, we outline a new technique for automatically synthesizing energy-efficient discrete controllers from a scenario-based specification of a production system, which we enrich with information about consumed and generated energy. Last, we provide an outlook on challenging open research problems.
    12/2014; 15:388–397. DOI:10.1016/j.protcy.2014.09.093
  • Kai Eggers · Christian Hansen · Jens Kotlarski · Tobias Ortmaier
    Automated Systems and Technologies, Gabsen, Germany; 10/2014
  • Christian Hansen · Kai Eggers · Jens Kotlarski · Tobias Ortmaier
    31st International Symposium on Automation and Robotics in Construction and Mining, Sydney, Australia; 07/2014
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    ABSTRACT: Purpose The integration of a robot into an image-guided therapy system is still a time consuming process, due to the lack of a well-accepted standard for interdevice communication. The aim of this project is to simplify this procedure by developing an open interface based on three interface classes: state control, visualisation, and sensor. A state machine on the robot control is added to the concept because the robot has its own workflow during surgical procedures, which differs from the workflow of the surgeon. Methods A KUKA Light Weight Robot is integrated into the medical technology environment of the Institute of Mechatronic Systems as a proof of concept. Therefore, 3D Slicer was used as visualisation and state control software. For the network communication the OpenIGTLink protocol was implemented. In order to achieve high rate control of the robot the “KUKA Sunrise. Connectivity SmartServo” package was used. An exemplary state machine providing states typically used by image-guided therapy interventions, was implemented. Two interface classes, which allow for a direct use of OpenIGTLink for robot control on the one hand and visualisation on the other hand were developed. Additionally, a 3D Slicer module was written to operate the state control. Results Utilising the described software concept the state machine could be operated by the 3D Slicer module with 20 Hz cycle rate and no data loss was detected during a test phase of approximately \(270\,s\) (13,640 packages). Furthermore, the current robot pose could be sent with more than 60 Hz. No influence on the performance of the state machine by the communication thread could be measured. Conclusion Simplified integration was achieved by using only one programming context for the implementation of the state machine, the interfaces, and the robot control. Eventually, the exemplary state machine can be easily expanded by adding new states.
    International Journal of Computer Assisted Radiology and Surgery 06/2014; 10(3). DOI:10.1007/s11548-014-1081-1 · 1.66 Impact Factor
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    ABSTRACT: Purpose Introducing computational methods to laser surgery are an emerging field. Focusing on endoscopic laser interventions, a novel approach is presented to enhance intraoperative incision planning and laser focusing by means of tissue surface information obtained by stereoscopic vision. Methods Tissue surface is estimated with stereo-based methods using nonparametric image transforms. Subsequently, laser-to-camera registration is obtained by ablating a pattern on tissue substitutes and performing a principle component analysis for precise laser axis estimation. Furthermore, a virtual laser view is computed utilizing trifocal transfer. Depth-based laser focus adaptation is integrated into a custom experimental laser setup in order to achieve optimal ablation morphology. Experimental validation is conducted on tissue substitutes and ex vivo animal tissue. Results Laser-to-camera registration gives an error between planning and ablation of less than 0.2 mm. As a result, the laser workspace can accurately be highlighted within the live views and incision planning can directly be performed. Experiments related to laser focus adaptation demonstrate that ablation geometry can be kept almost uniform within a depth range of 7.9 mm, whereas cutting quality significantly decreases when the laser is defocused. Conclusions An automatic laser focus adjustment on tissue surfaces based on stereoscopic scene information is feasible and has the potential to become an effective methodology for optimal ablation. Laser-to-camera registration facilitates advanced surgical planning for prospective user interfaces and augmented reality extensions.
    International Journal of Computer Assisted Radiology and Surgery 05/2014; 10(2). DOI:10.1007/s11548-014-1077-x · 1.66 Impact Factor
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    ABSTRACT: Die Entwicklung und Erprobung von Elektrodenträgern, die für eine modiolusnahe Endlage mit einer auf dem Formgedächtniseffekt basierenden Aktorik funktionalisiert sind, stellt besondere Anforderungen an die experimentelle Erprobung unter reproduzierbaren Laborbedingungen. Elementar ist dabei die einstellbar temperierte Lagerung von künstlichen Modellen der Cochlea oder Felsenbeinpräparaten, da der Formgedächtniseffekt temperaturgesteuert ausgelöst wird. Vorgestellt wird ein eigens dafür entwickelter Versuchsstand bestehend aus einem thermostatgesteuerten Wasserbecken, eine eigens entwickelte Halterung für Felsenbeinpräparate und Cochleamodelle sowie ein Insertionstool zur automatisierten und damit reproduzierbaren Insertion der Elektrodenträger. Der Implantatvorschub wird dabei reproduzierbar mittels Linearaktoren realisiert. Eine stereooptische Navigationskamera erlaubt die Positionierung des Insertionstools relativ zum Cochleamodell bzw. Humanpräparat basierend auf bildgestützten Planungsdaten für einen optimierten Insertionswinkel. Das Insertionstool ist dazu über einen Ausleger an einem Hexapod befestigt, dessen einzustellende Beinlängen für jede Insertion individuell über ein Matlabprogramm berechnet werden können. Eine modifizierte Felsenbeinhalterung erlaubt zusätzlich die Ausrichtung des Felsenbeinpräparats bei gleichzeitig verlässlicher Umströmung des Präparats mit dem Temperiermedium. Eine wasserfest gekapselte USB-Mikroskopkamera dient der Dokumentation der Insertionsversuche. Der beschriebene Versuchsaufbau konnte unter Laborbedingungen erfolgreich bei Insertionsversuchen erprobt werden und ist aktuell bei der Entwicklung neuartiger, temperaturgeregelt formverändernder Elektroden im Einsatz. Weitere Insertionsversuche in Felsenbeinpräparate sind geplant.
    85. Jahresversammlung der Deutschen Gesellschaft für Hals-Nasen-Ohren-Heilkunde, Kopf- und Hals-Chirurgie e. V., Dortmund; 05/2014
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    ABSTRACT: Minimally invasive cochlear implantation is a surgical technique which requires drilling a canal from the mastoid surface toward the basal turn of the cochlea. The choice of an appropriate drilling strategy is hypothesized to have significant influence on the achievable targeting accuracy. Therefore, a method is presented to analyze the contribution of the drilling process and drilling tool to the targeting error isolated from other error sources. The experimental setup to evaluate the borehole accuracy comprises a drill handpiece attached to a linear slide as well as a highly accurate coordinate measuring machine (CMM). Based on the specific requirements of the minimally invasive cochlear access, three drilling strategies, mainly characterized by different drill tools, are derived. The strategies are evaluated by drilling into synthetic temporal bone substitutes containing air-filled cavities to simulate mastoid cells. Deviations from the desired drill trajectories are determined based on measurements using the CMM. Using the experimental setup, a total of 144 holes were drilled for accuracy evaluation. Errors resulting from the drilling process depend on the specific geometry of the tool as well as the angle at which the drill contacts the bone surface. Furthermore, there is a risk of the drill bit deflecting due to synthetic mastoid cells. A single-flute gun drill combined with a pilot drill of the same diameter provided the best results for simulated minimally invasive cochlear implantation, based on an experimental method that may be used for testing further drilling process improvements.
    International Journal of Computer Assisted Radiology and Surgery 04/2014; 9(6). DOI:10.1007/s11548-014-0997-9 · 1.66 Impact Factor
  • Jesus Diaz Diaz · Mauro H. Riva · Omid Majdani · Tobias Ortmaier
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    ABSTRACT: In recent years, optical coherence tomography (OCT) has gained increasing attention not only as an imaging device, but also as a navigation system for surgical interventions. This approach demands to register intraoperative OCT to pre-operative computed tomography (CT) data. In this study, we evaluate algorithms for multi-modal image registration of OCT and CT data of a human temporal bone specimen. We focus on similarity measures that are common in this field, e.g., normalized mutual information, normalized cross correlation, and iterative closest point. We evaluate and compare their accuracies to the relatively new normal distribution transform (NDT), that is very common in simultaneous localization and mapping applications, but is not widely used in image registration. Matching is realized considering appropriate image pre-processing, the aforementioned similarity measures, and local optimization algorithms, as well as line search optimization. For evaluation purpose, the results of a point-based registration with fiducial landmarks are regarded as ground truth. First results indicate that state of the art similarity functions do not perform with the desired accuracy, when applied to unprocessed image data. In contrast, NDT seems to achieve higher registration accuracy.
    SPIE Medical Imaging; 03/2014
  • SPIE Medical Imaging; 03/2014
  • Jesús Díaz Díaz · Mauro H. Riva · Omid Majdani · Tobias Ortmaier
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    ABSTRACT: The choice of a navigation system highly depends on the medical intervention and its accuracy demands. The most commonly used systems for image guided surgery (IGS) are based on optical and magnetic tracking systems. This paper compares two optical systems in terms of accuracy: state of the art triangulation-based optical tracking (OT) and optical coherence tomography (OCT). We use an experimental setup with a combined OCT and cutting laser, and an external OT. We simulate a robotic assisted surgical intervention, including planning, navigation, and processing, and compare the accuracies reached at a specific target with each navigation system.
    Proceedings of SPIE - The International Society for Optical Engineering 02/2014; DOI:10.1117/12.2040263 · 0.20 Impact Factor
  • Christian Hansen · Jens Kotlarski · Tobias Ortmaier
    International Journal of Mechatronics and Automation 01/2014; 4(3):147. DOI:10.1504/IJMA.2014.064096
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    ABSTRACT: Designing a Kalman filter requires knowledge about the stochastic part of the system. Thus, disturbances affecting states and measurements should be known. However, in practical application these disturbances are usually unknown. In this contribution a modification of the autocovariance least-square method is presented. This method converts the measurement and process noise covariance estimation problem into a least squares functional, which can be solved with a Landweber iteration to regularize the ill-posed problem. Then, a tuned Kalman filter gain can be calculated. A simulative evaluation is introduced to prove the method regarding robustness against modeling error and variance of the estimates.
  • Christian Hansen · Jens Kotlarski · Tobias Ortmaier
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    ABSTRACT: The improvement of energy efficiency in automation and robotic systems still has high significance in engineering research. This paper presents experimental results of minimum energy trajectory optimization for multi-axis manipulators using electrical energy exchange via the shared inverter DC link. Thus, existing simulation findings demonstrating the potential of the proposed minimum energy optimization approach are validated. In this context, a global optimization method is applied utilizing an advanced cost function formulation that is based on an energy flow model including all servo drive components. After the validation of the utilized system model, increased efficiency improvements are proven and compared to established methods. The presented approach as well as the experimental results are fully transferable to any kind of multi-axis system comprising DC link energy supply.
    16th International Conference on Advanced Robotics, Montevideo, Uruguay; 11/2013