Sebastian Röhl

Comparative Validation of Single-Shot Optical Techniques for Laparoscopic 3-D Surface Reconstruction

Intra-operative imaging techniques for obtaining the shape and morphology of soft-tissue surfaces in vivo are a key enabling technology for advanced surgical systems. Different optical techniques for 3-D surface reconstruction in laparoscopy have been proposed, however, so far no quantitative and comparative validation has been performed. Furthermore, robustness of the methods to clinically important factors like smoke or bleeding has not yet been assessed. To address these issues, we have formed a joint international initiative with the aim of validating different state-of-the-art passive and active reconstruction methods in a comparative manner. In this comprehensive in vitro study, we investigated reconstruction accuracy using different organs with various shape and texture and also tested reconstruction robustness with respect to a number of factors like the pose of the endoscope as well as the amount of blood or smoke present in the scene. The study suggests complementary advantages of the different techniques with respect to accuracy, robustness, point density, hardware complexity and computation time. While reconstruction accuracy under ideal conditions was generally high, robustness is a remaining issue to be addressed. Future work should include sensor fusion and in vivo validation studies in a specific clinical context. To trigger further research in surface reconstruction, stereoscopic data of the study will be made publically available at www.open-CAS.com upon publication of the paper.

Dense GPU‐enhanced surface reconstruction from stereo endoscopic images for intraoperative registration

PURPOSE In laparoscopic surgery, soft tissue deformations substantially change the surgical site, thus impeding the use of preoperative planning during intraoperative navigation. Extracting depth information from endoscopic images and building a surface model of the surgical field-of-view is one way to represent this constantly deforming environment. The information can then be used for intraoperative registration. Stereo reconstruction is a typical problem within computer vision. However, most of the available methods do not fulfill the specific requirements in a minimally invasive setting such as the need of real-time performance, the problem of view-dependent specular reflections and large curved areas with partly homogeneous or periodic textures and occlusions. METHODS In this paper, the authors present an approach toward intraoperative surface reconstruction based on stereo endoscopic images. The authors describe our answer to this problem through correspondence analysis, disparity correction and refinement, 3D reconstruction, point cloud smoothing and meshing. Real-time performance is achieved by implementing the algorithms on the gpu. The authors also present a new hybrid cpu-gpu algorithm that unifies the advantages of the cpu and the gpu version. RESULTS In a comprehensive evaluation using in vivo data, in silico data from the literature and virtual data from a newly developed simulation environment, the cpu, the gpu, and the hybrid cpu-gpu versions of the surface reconstruction are compared to a cpu and a gpu algorithm from the literature. The recommended approach toward intraoperative surface reconstruction can be conducted in real-time depending on the image resolution (20 fps for the gpu and 14fps for the hybrid cpu-gpu version on resolution of 640 × 480). It is robust to homogeneous regions without texture, large image changes, noise or errors from camera calibration, and it reconstructs the surface down to sub millimeter accuracy. In all the experiments within the simulation environment, the mean distance to ground truth data is between 0.05 and 0.6 mm for the hybrid cpu-gpu version. The hybrid cpu-gpu algorithm shows a much more superior performance than its cpu and gpu counterpart (mean distance reduction 26% and 45%, respectively, for the experiments in the simulation environment). CONCLUSIONS The recommended approach for surface reconstruction is fast, robust, and accurate. It can represent changes in the intraoperative environment and can be used to adapt a preoperative model within the surgical site by registration of these two models.

Physics‐based shape matching for intraoperative image guidance

PURPOSE Soft-tissue deformations can severely degrade the validity of preoperative planning data during computer assisted interventions. Intraoperative imaging such as stereo endoscopic, time-of-flight or, laser range scanner data can be used to compensate these movements. In this context, the intraoperative surface has to be matched to the preoperative model. The shape matching is especially challenging in the intraoperative setting due to noisy sensor data, only partially visible surfaces, ambiguous shape descriptors, and real-time requirements. METHODS A novel physics-based shape matching (PBSM) approach to register intraoperatively acquired surface meshes to preoperative planning data is proposed. The key idea of the method is to describe the nonrigid registration process as an electrostatic-elastic problem, where an elastic body (preoperative model) that is electrically charged slides into an oppositely charged rigid shape (intraoperative surface). It is shown that the corresponding energy functional can be efficiently solved using the finite element (FE) method. It is also demonstrated how PBSM can be combined with rigid registration schemes for robust nonrigid registration of arbitrarily aligned surfaces. Furthermore, it is shown how the approach can be combined with landmark based methods and outline its application to image guidance in laparoscopic interventions. RESULTS A profound analysis of the PBSM scheme based on in silico and phantom data is presented. Simulation studies on several liver models show that the approach is robust to the initial rigid registration and to parameter variations. The studies also reveal that the method achieves submillimeter registration accuracy (mean error between 0.32 and 0.46 mm). An unoptimized, single core implementation of the approach achieves near real-time performance (2 TPS, 7-19 s total registration time). It outperforms established methods in terms of speed and accuracy. Furthermore, it is shown that the method is able to accurately match partial surfaces. Finally, a phantom experiment demonstrates how the method can be combined with stereo endoscopic imaging to provide nonrigid registration during laparoscopic interventions. CONCLUSIONS The PBSM approach for surface matching is fast, robust, and accurate. As the technique is based on a preoperative volumetric FE model, it naturally recovers the position of volumetric structures (e.g., tumors and vessels). It cannot only be used to recover soft-tissue deformations from intraoperative surface models but can also be combined with landmark data from volumetric imaging. In addition to applications in laparoscopic surgery, the method might prove useful in other areas that require soft-tissue registration from sparse intraoperative sensor data (e.g., radiation therapy).

Context-aware Augmented Reality in laparoscopic surgery

Augmented Reality is a promising paradigm for intraoperative assistance. Yet, apart from technical issues, a major obstacle to its clinical application is the man-machine interaction. Visualization of unnecessary, obsolete or redundant information may cause confusion and distraction, reducing usefulness and acceptance of the assistance system. We propose a system capable of automatically filtering available information based on recognized phases in the operating room. Our system offers a specific selection of available visualizations which suit the surgeons needs best. The system was implemented for use in laparoscopic liver and gallbladder surgery and evaluated in phantom experiments in conjunction with expert interviews.

Towards shape-based visual object categorization for humanoid robots

Humanoid robots should be able to grasp and handle objects in the environment, even if the objects are seen for the first time. A plausible solution to this problem is to categorize these objects into existing classes with associated actions and functional knowledge. So far, efforts on visual object categorization using humanoid robots have either been focused on appearance-based methods or have been restricted to object recognition without generalization capabilities. In this work, a shape model-based approach using stereo vision and machine learning for object categorization is introduced. The state-of-the-art features for shape matching and shape retrieval were evaluated and selectively transfered into the visual categorization. Visual sensing from different vantage points allows the reconstruction of 3D mesh models of the objects found in the scene by exploiting knowledge about the environment for model-based segmentation and registration. These reconstructed 3D mesh models were used for shape feature extraction for categorization and provide sufficient information for grasping and manipulation. Finally, the visual categorization was successfully performed with a variety of features and classifiers allowing proper categorization of unknown objects even when object appearance and shape substantially differ from the training set. Experimental evaluation with the humanoid robot ARMAR-IIIa is presented.

Real-time surface reconstruction from stereo endoscopic images for intraoperative registration

Minimally invasive surgery is a medically complex discipline that can heavily benefit from computer assistance. One way to assist the surgeon is to blend in useful information about the intervention into the surgical view using Augmented Reality. This information can be obtained during preoperative planning and integrated into a patient-tailored model of the intervention. Due to soft tissue deformation, intraoperative sensor data such as endoscopic images has to be acquired and non-rigidly registered with the preoperative model to adapt it to local changes. Here, we focus on a procedure that reconstructs the organ surface from stereo endoscopic images with millimeter accuracy in real-time. It deals with stereo camera calibration, pixel-based correspondence analysis, 3D reconstruction and point cloud meshing. Accuracy, robustness and speed are evaluated with images from a test setting as well as intraoperative images. We also present a workflow where the reconstructed surface model is registered with a preoperative model using an optical tracking system. As preliminary result, we show an initial overlay between an intraoperative and a preoperative surface model that leads to a successful rigid registration between these two models.

3D surface reconstruction for laparoscopic computer-assisted interventions: comparison of state-of-the-art methods

One of the main challenges related to computer-assisted laparoscopic surgery is the accurate registration of pre-operative planning images with patients anatomy. One popular approach for achieving this involves intraoperative 3D reconstruction of the target organs surface with methods based on multiple view geometry. The latter, however, require robust and fast algorithms for establishing correspondences between multiple images of the same scene. Recently, the first endoscope based on Time-of-Flight (ToF) camera technique was introduced. It generates dense range images with high update rates by continuously measuring the run-time of intensity modulated light. While this approach yielded promising results in initial experiments, the endoscopic ToF camera has not yet been evaluated in the context of related work. The aim of this paper was therefore to compare its performance with different state-of-the-art surface reconstruction methods on identical objects. For this purpose, surface data from a set of porcine organs as well as organ phantoms was acquired with four different cameras: a novel Time-of-Flight (ToF) endoscope, a standard ToF camera, a stereoscope, and a High Definition Television (HDTV) endoscope. The resulting reconstructed partial organ surfaces were then compared to corresponding ground truth shapes extracted from computed tomography (CT) data using a set of local and global distance metrics. The evaluation suggests that the ToF technique has high potential as means for intraoperative endoscopic surface registration.

Quadratic Corotated Finite Elements for Real-Time Soft Tissue Registration

Organ motion due to respiration and contact with surgical instruments can significantly degrade the accuracy of image-guided surgery. In most applications, the ensuing soft tissue deformations have to be compensated in order to register preoperative planning data to the patient. Biomechanical models can be used to perform registration based on sparse intraoperative sensor data. Using elasticity theory, the approach can be formulated as a boundary value problem with displacement boundary conditions. In this paper, we propose to use corotated finite elements (FE) with quadratic shape functions as a robust and accurate model for real-time soft-tissue registration. A detailed numerical analysis reveals that quadratic FE perform significantly better than linear corotated FE for high resolution meshes. We also show that the method achieves nearly the same registration accuracy as a complex nonlinear viscoelastic material model. Furthermore, a phantom experiment demonstrates how the model can be used for intraoperative liver registration.

Visual tracking of da Vinci instruments for laparoscopic surgery

Intraoperative tracking of laparoscopic instruments is a prerequisite to realize further assistance functions. Since endoscopic images are always available, this sensor input can be used to localize the instruments without special devices or robot kinematics. In this paper, we present an image-based markerless 3D tracking of different da Vinci instruments in near real-time without an explicit model. The method is based on different visual cues to segment the instrument tip, calculates a tip point and uses a multiple object particle filter for tracking. The accuracy and robustness is evaluated with in vivo data.

A biomechanical liver model for intraoperative soft tissue registration

Organ motion due to respiration and contact with surgical instruments can significantly degrade the accuracy of image guided surgery. In most applications the ensuing soft tissue deformations have to be compensated in order to register preoperative planning data to the patient. Biomechanical models can be used to perform an accurate registration based on sparse intraoperative sensor data. Using elasticity theory, the approach can be formulated as a boundary value problem with displacement boundary conditions. In this paper, several models of the liver from the literature and a new simplified model are evaluated with regards to their application to intraoperative soft tissue registration. We construct finite element models of a liver phantom using the different material laws. Thereafter, typical deformation pattern that occur during surgery are imposed by applying displacement boundary conditions. A comparative numerical study shows that the maximal registration error of all non-linear models stays below 1.1mm, while the linear model produces errors up to 3.9mm. It can be concluded that linear elastic models are not suitable for the registration of the liver and that a geometrically non-linear formulation has to be used. Although the stiffness parameters of the non-linear materials differ considerably, the calculated displacement fields are very similar. This suggests that a difficult patient-specific parameterization of the model might not be necessary for intraoperative soft tissue registration. We also demonstrate that the new simplified model achieves nearly the same registration accuracy as complex quasi-linear viscoelastic models.