Tobias Ortmaier

Leibniz Universität Hannover, Hanover, Lower Saxony, Germany

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Publications (151)62.69 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We present a novel one-step calibration methodology for geometrical distortion correction for optical coherence tomography (OCT). A calibration standard especially designed for OCT is introduced, which consists of an array of inverse pyramidal structures. The use of multiple landmarks situated on four different height levels on the pyramids allow performing a 3D geometrical calibration. The calibration procedure itself is based on a parametric model of the OCT beam propagation. It is validated by experimental results and enables the reduction of systematic errors by more than one order of magnitude. In future, our results can improve OCT image reconstruction and interpretation for medical applications such as real time monitoring of surgery.
    No preview · Article · Jan 2016 · Journal of optics
  • S. Reiser · H. Feußner · D. Wilhelm · T. Ortmaier · R. Tamm · P. Meier · A. Meining

    No preview · Article · Dec 2015 · Endoskopie heute
  • Daniel Ramirez · Jens Kotlarski · Tobias Ortmaier
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    ABSTRACT: In this paper, an approach for the automatic structural synthesis of serial robot manipulators is proposed. Unlike existing methods, the parameters that can be used subsequently in a geometrical synthesis are obtained for every architecture as result of the presented approach. A modified motion vector is introduced in order to take into account different orientations of the robot base. This vector is used to generate a set of feasible architectures. Additionally, conditions for the evaluation of the presence of isomorphisms in this set of architectures are introduced. Finally, an example of a manipulator with four degrees of freedom is given to clarify the advantage of the proposed method.
    No preview · Conference Paper · Oct 2015
  • Christian Hansen · Kai Eggers · Jens Kotlarski · Tobias Ortmaier
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    ABSTRACT: The energy efficiency of high-dynamic servo drive applications often provides opportunities for improve- ment, since regenerative energy is mostly dissipated via brake choppers and is lost for later demands. Therefore, this paper discusses different methods for the energy demand and peak load reduction of DC-link coupled multi-axis servo drive systems by application of mechanical and electrical energy storage. Experimental results show that idle system axes can effectively be utilized for regenerative energy buffering to improve the system efficiency instead of applying expensive energy storage systems. Therefore, an extended model-based approach for the control of a rotational mechanical storage axis during high dynamics motion tasks of general multi-axis servo drive systems is presented. For comparison, the utilization of commercially available electrical capacitive storage extension is investigated. All methods are applicable to arbitrary multi-axis servo drive systems, including robotic manipulators, without adaption of the motion sequences.
    No preview · Conference Paper · Oct 2015
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    ABSTRACT: Purpose: Minimally invasive cochlear implantation is a novel surgical technique which requires highly accurate guidance of a drilling tool along a trajectory from the mastoid surface toward the basal turn of the cochlea. The authors propose a passive, reconfigurable, parallel robot which can be directly attached to bone anchors implanted in a patient's skull, avoiding the need for surgical tracking systems. Prior to clinical trials, methods are necessary to patient specifically optimize the configuration of the mechanism with respect to accuracy and stability. Furthermore, the achievable accuracy has to be determined experimentally. Methods: A comprehensive error model of the proposed mechanism is established, taking into account all relevant error sources identified in previous studies. Two optimization criteria to exploit the given task redundancy and reconfigurability of the passive robot are derived from the model. The achievable accuracy of the optimized robot configurations is first estimated with the help of a Monte Carlo simulation approach and finally evaluated in drilling experiments using synthetic temporal bone specimen. Results: Experimental results demonstrate that the bone-attached mechanism exhibits a mean targeting accuracy of [Formula: see text] mm under realistic conditions. A systematic targeting error is observed, which indicates that accurate identification of the passive robot's kinematic parameters could further reduce deviations from planned drill trajectories. Conclusion: The accuracy of the proposed mechanism demonstrates its suitability for minimally invasive cochlear implantation. Future work will focus on further evaluation experiments on temporal bone specimen.
    No preview · Article · Sep 2015 · International Journal of Computer Assisted Radiology and Surgery
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    ABSTRACT: Introduction:Visualization of inner ear structures is an useful subject for researchers and clinicians working on hearing pathologies. Optical Coherence Tomography as a high resolution imaging technology is applicable for visualization of surface and subsurface structures. In this experimental study, we aimed to evaluate inner ear anatomy in well prepared human labyrinthine. Methods:Three fresh human explanted temporal bones were trimmed, chemically decalcified with Ethylenediaminetetraacetic acid (EDTA) and mechanically drilled under visual control using Optical Coherence Tomography (OCT) in order to show the remained bone shell. After confirming the decalcification with CT scan, the samples scanned with OCT in different views. Oval window, round Window and remnant part of internal auditory canal and cochlear turn were investigated. Results:Preparation of the labyrinthine under OCT guidance was successfully performed down to a remaining bony layer of 300μm thickness. OCT images of the specimen enabled a detailed view on intra-cochlear anatomy. Conclusion:OCT is applicable in well prepared human inner ear and able to visualize soft tissue parts.
    Full-text · Article · Sep 2015 · Biomedizinische Technik/Biomedical Engineering
  • 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.
    No preview · Article · Jun 2015 · International Journal of Computer Assisted Radiology and Surgery
  • D. Beckmann · M. Schappler · M. Dagen · T. Ortmaier
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    ABSTRACT: This paper discusses flatness-based control approaches to reduce oscillations on the load side of mechanical motion systems. Choosing a suitable trajectory is one essential step in implementing a flatness-based control. Existing approaches use slow polynomials or mathematically complex Gevrey-functions to achieve the necessary differentiability. Our approach combines the oscillation reduction of flatness-based control and the dynamic and simplicity of higher order Scurve trajectories generated by a convolution based algorithm. The performance and robustness of the presented methods is experimentally validated with a linear flexible motion system.
    No preview · Article · Jun 2015
  • 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.
    No preview · Conference Paper · Jun 2015
  • Julian Öltjen · Jens Kotlarski · Tobias Ortmaier

    No preview · Conference Paper · Jun 2015
  • Daniel Ramirez · Jens Kotlarski · Tobias Ortmaier
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    ABSTRACT: In the present paper, an approach to generate a minimal set of serial manipulators that provide the required motion directions without redundant solutions is introduced. Besides the architectures, the Denavit-Hartenberg parameters that can be used as optimization parameters in a posterior geometrical synthesis are extracted. The motion directions are defined through a required motion vector which is modified in order to consider the orientation of the robot base. All suitable architectures are generated from a set of discrete Denavit-Hartenberg parameters. The method removes all isomorphisms from the set of suitable architectures significantly reducing the number of solutions, i.e. the computational effort. In order to illustrate the advantage of the proposed method, the number of solutions generated with and without detection of isomorphisms are given. Finally, an example of architectures with four degrees of freedom is given.
    No preview · Conference Paper · May 2015
  • S. Tappe · J. Kotlarski · T. Ortmaier · M. Dorbaum · A. Mertens · B. Ponick
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    ABSTRACT: Flexible endoscopes are a common instrument for different tasks in minimally invasive surgery. The motion and capability of resistance against manipulation forces of common endoscopes is restricted due to their flexibility. They tend to form loops and stress their surroundings. This paper proposes an active shaft concept based on a new actuation concept: the shaft is a hyper-redundant snake-like robot with binary electromagnetic actuators. This system combines good path following capabilities through actively controlling the whole endoscope body with good resistance with respect to manipulation forces through electromagnetism. The functional concept is presented and the influence of design parameters on kinematic characteristics such as workspace and radius of curvature is evaluated. Additionally, a set of kinematic design parameters is synthesized by minimizing the contouring error. Therefore, a path fitting algorithm is proposed.
    No preview · Article · Apr 2015
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    ABSTRACT: This work investigates combination of Optical Coherence Tomography and two cameras, observing a microscopic scene. Stereo vision provides realistic images, but is limited in terms of penetration depth. Optical Coherence Tomography (OCT) enables access to subcutaneous structures, but 3D-OCT volume data do not give the surgeon a familiar view. The extension of the stereo camera setup with OCT imaging combines the benefits of both modalities. In order to provide the surgeon with a convenient integration of OCT into the vision interface, we present an automated image processing analysis of OCT and stereo camera data as well as combined imaging as augmented reality visualization. Therefore, we care about OCT image noise, perform segmentation as well as develop proper registration objects and methods. The registration between stereo camera and OCT results in a Root Mean Square error of 284 μm as average of five measurements. The presented methods are fundamental for fusion of both imaging modalities. Augmented reality is shown as application of the results. Further developments lead to fused visualization of subcutaneous structures, as information of OCT images, into stereo vision.
    No preview · Article · Mar 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.
    No preview · Article · Mar 2015 · Medical Engineering & Physics
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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.
    No preview · Article · Mar 2015 · BioMed Research International
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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.
    No preview · Article · Mar 2015 · Surgical Innovation
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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.
    No preview · Article · Feb 2015 · Journal of Neurological Surgery, Part B: Skull Base
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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.
    No preview · Article · Feb 2015 · International Journal of Computer Assisted Radiology and Surgery
  • M. Wielitzka · M. Dagen · T. Ortmaier
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    ABSTRACT: In order to improve vehicle's active safety systems accurate knowledge about the vehicle's driving stability is necessary. Especially the exact determination of the side-slip angle can be of great importance, since it has major potential for improving current control algorithms. Therefore, a model-based methodology for online estimation of vehicle's lateral dynamics is presented, while generalizations of the Kalman Filter algorithm, the Extended and Unscented Kalman Filters are used due to the highly non-linear model behavior. The results of the introduced methodologies are presented for two different driving maneuvers and validated comparing to measurements taken with a VW Golf GTI. Furthermore, a qualitative comparison between Extended and Unscented Kalman Filter is realized.
    No preview · Article · Feb 2015 · Proceedings of the IEEE Conference on Decision and Control
  • A. Fuchs · S. Pengel · J. Bergmeier · L.A. Kahrs · T. Ortmaier
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    ABSTRACT: Laser surgery is an established clinical procedure in dental applications, soft tissue ablation, and ophthalmology. The presented experimental set-up for closed-loop control of laser bone ablation addresses a feedback system and enables safe ablation towards anatomical structures that usually would have high risk of damage. This study is based on combined working volumes of optical coherence tomography (OCT) and Er:YAG cutting laser. High level of automation in fast image data processing and tissue treatment enables reproducible results and shortens the time in the operating room. For registration of the two coordinate systems a cross-like incision is ablated with the Er:YAG laser and segmented with OCT in three distances. The resulting Er:YAG coordinate system is reconstructed. A parameter list defines multiple sets of laser parameters including discrete and specific ablation rates as ablation model. The control algorithm uses this model to plan corrective laser paths for each set of laser parameters and dynamically adapts the distance of the laser focus. With this iterative control cycle consisting of image processing, path planning, ablation, and moistening of tissue the target geometry and desired depth are approximated until no further corrective laser paths can be set. The achieved depth stays within the tolerances of the parameter set with the smallest ablation rate. Specimen trials with fresh porcine bone have been conducted to prove the functionality of the developed concept. Flat bottom surfaces and sharp edges of the outline without visual signs of thermal damage verify the feasibility of automated, OCT controlled laser bone ablation with minimal process time.
    No preview · Article · Jan 2015

Publication Stats

1k Citations
62.69 Total Impact Points


  • 2009-2016
    • Leibniz Universität Hannover
      • Institute of Mechatronic Systems
      Hanover, Lower Saxony, Germany
  • 2015
    • Friedrich-Alexander-University of Erlangen-Nürnberg
      Erlangen, Bavaria, Germany
  • 2010
    • Vanderbilt University
      • Department of Mechanical Engineering
      Nashville, Michigan, United States
  • 2000-2009
    • German Aerospace Center (DLR)
      • Institute of Robotics and Mechatronics
      Köln, North Rhine-Westphalia, Germany
  • 2005
    • Pierre and Marie Curie University - Paris 6
      • Institut des Systèmes Intelligents et de Robotique (ISIR)
      Lutetia Parisorum, Île-de-France, France
  • 2004
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France