Intraoperative Adaptation and Visualization of Preoperative Risk Analyses for Oncologic Liver Surgery Christian Hansen*a , Stefan Schlichtingb , Stephan Zidowitza , Alexander K¨hna , o Milo Hindennach a , Markus Kleemannb , and Heinz-Otto Peitgena aMeVis Research - Center for Medical Image Computing, Bremen, Germany; b u University Hospital Schleswig-Holstein, Department of Surgery, L¨beck, Germany ABSTRACT Tumor resections from the liver are complex surgical interventions. With recent planning software, risk analyses based on individual liver anatomy can be carried out preoperatively. However, additional tumors within the liver are frequently detected during oncological interventions using intraoperative ultrasound. These tumors are not visible in preoperative data and their existence may require changes to the resection strategy. We propose a novel method that allows an intraoperative risk analysis adaptation by merging newly detected tumors with a preoperative risk analysis. To determine the exact positions and sizes of these tumors we make use of a navigated ultrasound-system. A fast communication protocol enables our application to exchange crucial data with this navigation system during an intervention. A further motivation for our work is to improve the visual presentation of a moving ultrasound plane within a complex 3D planning model including vascular systems, tumors, and organ surfaces. In case the ultrasound plane is located inside the liver, occlusion of the ultrasound plane by the planning model is an inevitable problem for the applied visualization technique. Our system allows the surgeon to focus on the ultrasound image while perceiving context-relevant planning information. To improve orientation ability and distance perception, we include additional depth cues by applying new illustrative visualization algorithms. Preliminary evaluations conﬁrm that in case of intraoperatively detected tumors a risk analysis adaptation is beneﬁcial for precise liver surgery. Our new GPU-based visualization approach provides the surgeon with a simultaneous visualization of planning models and navigated 2D ultrasound data while minimizing occlusion problems. Keywords: Visualization, Intraoperative Imaging, Treatment Planning, Ultrasound Guidance 1. INTRODUCTION With regard to complex oncological resections, a computer-assisted risk analysis is important for precise and safe liver surgery. Based upon security margins around each tumor, disturbances of blood supply and drainage within the remaining liver parenchyma can be computed, quantiﬁed and visualized preoperatively.1 However, using intraoperative ultrasound during oncological interventions, approximately 20% of patients with primary liver malignancies or metastases show additional tumors.2–4 Although the maximum diameter of these intraoperative ﬁndings is mostly smaller than 15 mm, changes to the resection strategy may be necessary, especially if the new tumors are adjacent to hepatic vessels. In such cases, an intraoperational tool for automatic risk analysis adaptation would assist the surgeons in optimizing the resection strategy. We propose a new application, the Intraoperative Planning Assistant, which allows a risk analysis adaptation by merging intraoperatively detected tumors with a preoperative risk analysis. To determine the exact positions and sizes of newly found tumors, we make use of an navigated ultrasound-system.5 Data exchange between Intraoperative Planning Assistant and navigation system during an intervention is realized using a fast XML-based communication protocol. Furthermore, an adequate visualization of the ultrasound plane relative to the preoperative model facilitates the navigation for the surgeon and is beneﬁcial for precise liver surgery. Major drawbacks of existing concepts are occlusions of the ultrasound plane by the 3D planning model and fade-out of crucial context information. * Corresponding author. E-mail: Christian.Hansen@mevis.de; Tel.: ++49 421-2187722 We introduce a new visualization approach, that allowing a focus-view on the ultrasound image while preserving context-relevant planning information and depth cues. The paper is organized as follows: First, we review related work. After describing the applied registration technique, we introduce a new approach for intraoperative adaptation of risk analysis in the case of intraoperatively found tumors. Furthermore, we introduce requirements for an adequate, intraoperative visualization and describe a new visualization pipeline. Finally, we present our results, draw conclusion and discuss future work. 2. RELATED WORK The work described in this paper extends and combines existing concepts in medical image processing and visualization. In this section we provide an overview of related work in the ﬁeld of intraoperative adaptation and registration, as well as illustrative and intraoperative visualization. 2.1 Intraoperative Adaptation and Registration An initial concept for intraoperative adaptation of risk analysis for liver surgery was presented by Ritter et al.6 They propose to determine tumor attributes like position and size manually by means of mouse interactions on a notebook inside the operation room (OR). Their approach is not designed for the use in sterile areas and does not make use of a navigation system to determine tumor attributes. Since our approach is based on an ultrasound- based navigation system, the success of an intraoperative risk analysis adaptation correlates with the accuracy of the applied intraoperative registration technique. Maier-Hein et al.7 developed a motion simulator for CT-guided liver interventions allowing a real-time adaptation of preoperative calculated tumor positions. Their system is u of particular importance for radio-frequency ablations where liver immobilization is hard to archive. Z¨hlke et al.8 presented a model-based technique based on Self Organized Maps, using extracted vessel branchings from preoperative (CT) and intraoperative (tracked 2D Doppler ultrasound) data as input. The method outputs a rigid registration matrix. Lange et al.9 combined the Iterative Closest Point algorithm and multilevel B-Splines u to achieve a non-rigid registration of preoperative CT/MRI and intraoperative 3D ultrasound. Similar to Z¨hlke et al. the algorithm is based on the vessel center lines of both image modalities. Papenberg et al.10 introduced a landmark-based registration approach for ultrasound and CT-data. It combines the distance measure of the normalized gradient ﬁeld with a penalizer that forces the deformation to fulﬁll the landmark condition using an aﬃne linear deformation model. 2.2 Illustrative Visualization The rendering of overlapping transparent iso-surfaces like the segmentation result of a vascular tree often gener- ates a complex output which is diﬃcult to interpret. Selective accentuation of shape features using illustrative techniques like hatching strokes or silhouettes can reduce visual complexity. Visual complexity can further be decreased by viewing important objects in focus while less important structures are visualized as context in- formation or even discarded. In recent years a variety of non-photorealistic methods have been proposed to increase expressiveness of illustrations, e.g. focus & context techniques, hatching strokes or silhouettes. Detailed overviews have been given by Strothotte et al.11 and Gooch et al.12 In the ﬁeld of medical visualization these illustrative methods were adapted, improved, and combined to allow an eﬀective exploration of medical data sets. Our visualization approach was inspired by a sequence of user studies accomplish by Interrante et al.13 They evaluated the eﬀect of displaying surface texture in combination with overlapping transparent surfaces and showed that non-photorealistic techniques are an adequate solution to support evident recognition of layered surfaces. Corresponding to these studies, Fischer et al.14 introduced a pipeline for illustrative rendering of overlapping iso-surfaces. Hatching, silhouette extraction, and depth-peeling are combined in such a way that a simultaneous inspection of inner and outer structures of an object is possible. The pipeline allows a manual deﬁnition of focus regions by specifying a secondary geometry, but is limited to two iso-surface layers. Tietjen et al.15 describes a focus & context rendering approach for medical visualization that combines silhouettes, direct volume rendering, and surface shading. Diﬀerent styles are used to control the appearance of anatomical structures according to their relevance for the visualization. A context-preserving volume rendering model that automatically enhances important information in a volume data set using illustrative volume rendering was introduced by Bruckner et al.16 In order to reduce the opacity in less important data regions, they use a function of shading intensity, gradient magnitude, distance to the eye point, and previously accumulated opacity. A user-controlled focus & context technique for volume rendering was presented by Kr¨ger et al.17 Changing attributes like size and u location of the focus or weight for the context allows the user to control the appearance of a medical visualization using a point-and-click interface. 2.3 Intraoperative Visualization Previous publications in the ﬁeld of medical visualization primary focus on preoperative exploration of medical data. A detailed overview of these techniques can be found in the book of Preim et al.18 Since the availability of navigation support in the OR has risen in the last years, intraoperative visualization becomes more interesting. One of the ﬁrst approaches for intraoperative visualization in liver surgery with clinical relevance was pre- sented by Lange et al.9 Registrated preoperative models of vessels, tumors, the liver surface, and the resection plane are rendered as colored intersection lines onto the top and perpendicular slice of a 3D ultrasound image. The current position of these two slices in relation to the preoperative data is presented in an extra viewer. However,the approach does not consider an adequate visualization in a single view in order to avoid frequent comparisons. Burns et al.19 presented an approach for simultaneous visualization of ultrasound and medical volume data. To provide a clear view on the ultrasound plane, they removed occluding volume data based on an importance function. Furthermore, they apply illustrative rendering techniques like silhouette enhancement and Gooch cool- to-warm shading to provide visual distinction between anatomic structures. Although their aim is similar to ours, we use segmented iso-surface objects with additional, color-coded planning information (e.g. distance to relevant structures like tumors or organ surface) for intraoperative visualization. Compared to Burns et al. we do not clip any planning data, but carefully fade out planning data that occludes the view between camera and ultrasound plane by applying illustrative rendering techniques. 3. METHODS In this section we brieﬂy review prior work concerning the underlying risk analyses algorithm for planning models. Afterwards, we present a new approach for intraoperative risk analyses adaption and describe the applied registration technique. Furthermore, we deﬁne requirements for an intraoperative visualization and introduce a novel rendering pipeline that allows an adequate visualization of adapted risk analyses together with intraoperative 2D ultrasound. 3.1 From Voxels to Risk Analyses The generation of 3D planning models including risk analyses is part of the software assistant MeVisLiverAnalyzer which integrates all steps to analyze contrast-enhanced CT data for preoperative planning in liver surgery.20 We brieﬂy describe the important steps for vascular analysis and visualization: Skeletonization with a topology- preserving thinning algorithm yielding an exact centerline and the radius at each voxel of the skeleton. A graph analysis transforms the vessel skeleton in a directed, acyclic graph where nodes represent branchings.21 Based on the assumption of a circular cross-section of vasculature, smooth transitions at branchings and rounded ends are produced by means of convolution surfaces.22 Tumors and liver surface are semi-automatically segmented using Live Wire23 and transformed to a triangle mesh. For the visualization of aﬀected vessels the spatial relations between tumors and vessels have to be analyzed. Therefore, certain security margins around the tumor are assumed. Computing the diﬀerence between the tumor segmentation mask and the eroded tumor segmentation mask (3x3x3 erosion ﬁlter) delivers the border of the tumors. Distance transformation is applied to all border voxels, thus generating the security margins.1 Finally, a graph traversion identiﬁes aﬀected voxels of depending vascular branches according to the speciﬁed security margins and determines the vertex colors for the convolution surfaces. Based on the eight major branches of the portal vein (giving a classiﬁcation similar to Couinaud’s scheme) vascular territories and their volumes are approximated using a Voronoi Tessellation. An iso-surface renderer generates separate triangle meshes and sets vertex colors. While vascular territories show the relation between liver parenchyma and supplying/draining branches, territories at risk show the liver parenchyma of impaired blood in- or outﬂow. Thus, the calculation of territories at risk (as the aﬀected vessels) has to be adapted when new tumor ﬁndings are made during operation. 3.2 Intraoperative Adaption of Risk Analyses If additional tumors are found during surgery, tumor size, and position are directly deﬁned in the 2D ultrasound image on the navigation system’s display which is located in front of the situs. For simplicity, tumors are assumed to be spherical in shape and it is therefore suﬃcient to draw a circle around the tumor. After tumor attributes have been determined, the navigation system sends an XML-request to the Intraoperative Planning Assistant, containing the new tumor radius and position. In case the request is accepted, the tumor is added to the planning model and a risk analysis adaptation is performed by the Intraoperative Planning Assistant. To create a new risk analysis, an Euclidean Distance Transformation is applied to calculate the distances between vessels and tumors. Aﬀected vessels are identiﬁed and relabeled depending on the predeﬁned security margins (Fig. 1a, 1b). (a) (b) Figure 1. (a) Preoperative risk analysis for the portal vein vessels and dependent subtrees for diﬀerent security margins. Three standard security margins around the tumors are chosen and displayed in red (5mm), yellow (10mm) and green (15mm). The red sphere on the ultrasound plane shows an intraoperatively detected tumor before adapting the risk analysis. (b) Adapted risk analysis including relabeled vessels. (a) (b) Figure 2. (a) Preoperative territories at risk that are supplied by the aﬀected vessels. (b) Adapted risk analysis showing the updated territories at risk. Tumor and risk margins are the same as in Fig. 1. Furthermore, a Voronoi Tessellation with respect to the vascular system is performed to approximate the volume of the parenchyma supplied or drained by the aﬀected vessels (Fig. 2a, 2b). After the computation has ﬁnished, the Intraoperative Planning Assistant sends a response to the navigation system and the updated planning results are immediately available on the navigation display. Using a multi-scale approach for the underlying volume data, our algorithm for risk analysis adaptation can choose a trade oﬀ between accuracy and computation time. Figure 3 illustrates the XML-based exchange protocol, deﬁned between Intraoperative Planning Assistant and navigation system. Intraoperative Planning Assistant navigation system request planning data allocate planning data data location of planning data use tracked ultrasound to look for new lesions tumor position and radius define tumor attributes acknowledge adapt risk analysis use tracked ultrasound inform about new planning data acknowledge download new planning data Figure 3. Exchange protocol, deﬁned between Intraoperative Planning Assistant and navigation system. 3.3 Intraoperative Registration In order to calculate the exact position of a newly detected tumor, a registration between the intraoperative ultrasound images and the preoperative planning data is required. We implemented a registration method that is robust, executable in real-time and interactively adaptable. It requires the surgeons to deﬁne a small set of corresponding markers in both the preoperative radiological data and the intraoperative ultrasound images using anatomic landmarks like vessel branchings. By minimizing total squared distances of the corresponding markers, we compute an aﬃne transformation and apply it to the preoperative data. 3.4 Intraoperative Visualization In case the ultrasound plane is located inside the liver, occlusion of the ultrasound plane by the planning model is an inevitable problem for the applied visualization technique. Presenting the ultrasound image without planning data in a second window results in time-consuming comparisons for the observer because necessary information is not presented in a single image. Applying transparency or clipping operations on occluding objects removes important context information and results in a loss of spatial information and clearness. To address these problems, we deﬁne three guiding requirements to ensure clinical applicability of our method: 1. Diagnostic Usability: While the whole visualization is presented in a single view, ultrasound information should always be visible, even if it would be occluded by planning data or parts thereof. 2. Orientation Aid: Spatial relations between ultrasound plane and planning data should be clearly per- ceivable without rotating or translating the camera. 3. Error Identiﬁcation: Since recent intraoperative registration methods make a compromise between real- time and error-prone computation, a clear hint to perceive registration errors should be provided. The aim of visualization approach is to provide an adequate presentation of planning models and ultrasound data in a single view. Therefore, we combine three extended rendering techniques in a GPU-accelerated rendering pipeline (Fig. 4). This pipeline consists of the following four steps in which the ﬁrst three steps directly address the mentioned requirements: 3.4.1 Focus, context and mask image generation The main objective of our visualization approach is to provide the surgeon with a focus-view on the ultrasound plane and a context-view on the planning model in order to ensure diagnostic usability. Since the focus lies on the ultrasound plane, we create a mask by ﬁrst rendering this plane with disabled texturing onto an oﬀ- screen texture M. For the generation of the focus and context views, we render the colored and shaded planning models onto two oﬀ-screen textures F and C. For F, the models are transparent. Correct order-independent transparency is achieved via depth-peeling.24 Using the mask M to combine F and C would lead to an abrupt change of rendering styles at the border of the ultrasound plane. This causes a wrong spatial interpretation due to receiving the impression that the ultrasound plane occludes the vessels. We found that a gradual transition between focus and context images avoids this misinterpretation. To achieve this, the mask M is of low resolution. When sampling from M, we use fast bi-cubic interpolation as described by Hadwiger et al.25 to obtain a smooth gradient. Thus, we avoid several blurring passes which are required when a higher resolution is utilized. 3.4.2 Silhouette and hatch stroke generation We found that transparent layered surfaces in combination with shape-accentuating strokes give a context- preserving view of the planning data while providing orientation aid and minimizing occlusion problems. Fur- thermore, we suggest varying line width of silhouettes depending on the distance to the ultrasound plane to support depth perception. In a ﬁrst step, we render the depth values Dstored of the vessels into an oﬀ-screen texture. While D is the relative distance between the current vertex and the ultrasound plane, and R describes the diameter of the bounding sphere, we use a vertex shader to translate all vertices of the model by the factor DR in opposite direction to their normal vector. This causes a shrinking of the model depending on the distance to the ultrasound plane. The corresponding fragment shader computes the diﬀerence of the current depth value Orientation Aid Discretized Z-Buffer Discretized Z-Buffer NPR-Image Colored NPR-Image of shrinked model N If (N=black) Nα+C(1-α) αC Dstored (D Z stored-Z)*σ clipping Diagnostic Usabilty Error Identification C F Cα+F(1-α) + α ultrasound image & intersection contours Context image Focus image (opaque) (transparent) Rescale & cubic interpolation ultrasound plane as 16x16 texture Figure 4. Pipeline overview showing intermediate rendering results together with the associated requirements. Z and Dstored which delivers a distance encoding silhouette in image space precision, i.e., the thickness decreases with increasing distance to the ultrasound plane. We extend the method by applying hatching strokes within the same rendering step, using the hatching algorithm introduced by Ritter et al.26 3.4.3 Intersection contour generation This is done by ﬁrst cutting an image representation of the ultrasound plane out of a 3D binary vessel mask (applying a multiplanar reformation). Then a marching squares algorithm is used to ﬁnd all iso-contours in the image. Thus, the registration result can be compared with the current ultrasound image (error identiﬁcation). 3.4.4 Compositing in screen space A fragment program starts by computing the output color colM odel using a convex combination of the current F and C samples. The value sampled from the mask image M is used as the weight α. Recall that we use bi- cubic interpolation when sampling from M to achieve a smooth transition between F and C. Next, the program computes the contribution from N to the current fragment color. If the sample of N is black the program outputs colN instead of colM odel , where colN is determined by blending black with the color of C using α. Figure 6 shows a resulting image generated with the technique described above. 4. RESULTS So far, preoperative risk analyses for oncologic liver surgery were of limited value during interventions in case of intraoperatively found tumors. For the ﬁrst time, we provide surgeons with an intraoperational tool for risk analyses adaptation that can be integrated into a surgical workﬂow. Preliminary evaluations (Fig. 5) conﬁrm that in case of newly discovered tumors an adaptation of the preoperative risk analysis is beneﬁcial for precise liver surgery. Our application, in combination with a navigated ultrasound-system, oﬀers crucial decision support and is easy to use. Using an XML-based communication protocol, the provided service can be utilized by other systems as well. Furthermore, we present methods for simultaneous visualization of preoperative planning models and nav- igated 2D ultrasound. Our new GPU-accelerated visualization approach is guided by requirements of clinical applicability and provides the surgeon with a focus-view on a moving ultrasound plane within a complex, inter- weaving 3D planning model (Fig. 6). Unlike previous visualization approaches, we minimize occlusions of the ultrasound plane by the 3D planning model and avoid fade-out of crucial context information. With a prototype implementation of our pipeline on the developer platform MeVisLab∗ we achieve interactive framerates (16fps on a GeForce 7900 GT) with a 2 GHz CPU using a model with 203k vertices (four depth-peel passes and one anti-antialiasing pass). We expect real-time framerates after optimization of the code. 5. DISCUSSION AND FUTURE WORK Preliminary evaluations with ex-vivo pig livers have been performed. The purpose was to prove the feasibility of our concept in collaboration with liver surgeons as well as an analysis of the surgical workﬂow for the intra- operative risk analysis adaptation. For the near future we plan to evaluate our system under realistic conditions during oncological interventions with a focus on accuracy. Therefore, we will replace the spherical approximation of tumors by a tumor segmentation algorithm applied on intraoperative 3D ultrasound data using a predeﬁned sphere as the region of interest. Since our system uses a rigid registration approach, which does not provide an adequate accuracy for clinical use, we want to extend our system by an intensity-based elastic registration.10 Regarding clinical applicability, a trade-oﬀ between registration accuracy and computation time needs to be found. Concerning our new visualization approach, we expect additional clues from a quantitative user study which is currently performed in cooperation with clinical partners. Furthermore we will extend the approach with an automatic viewpoint selection algorithm27 that computes optimal camera positions for a 3D scene with respect to clinical parameters, like newly found tumors. ∗ For more information visit www.mevislab.de (a) (b) Figure 5. (a) Intraoperative Planning Assistant showing the risk analysis for a pig’s liver with artiﬁcial tumors before applying a risk analysis adaptation. (b) Preliminary evaluations in the OR using the Intraoperative Planning Assistant and the ultrasound-based navigation system for laparoscopic liver surgery. Figure 6. Simultaneous visualization of preoperative planning data and intraoperative 2D ultrasound. The close-up view on the right hand side shows the highlighted intersection areas and depth-accentuating hatching strokes. While silhouettes encode the distance to the ultrasound plane, a gradual transition from opaque to transparent rendering (around the focused ultrasound plane) avoids misinterpretation of spatial relations. ACKNOWLEDGMENTS This work was funded by the Federal Ministry of Education and Research (SOMIT-FUSION project FKZ 01—BE03C). The authors thank Mathias Markert, Stefan Weber (Technical University Munich, Germany), u Armin Besirevic, Volker Martens (University of L¨beck, Germany), Holger Bourquain, Horst Hahn, Olaf Kon- rad, Guido Prause, and Felix Ritter (MeVis Research, Bremen, Germany) for their valuable ideas and support e in performing this work. We also express gratitude to Wolfram Lamad´ (RBK Stuttgart, Germany) and all involved physicians of the FUSION-project for fruitful discussion and clinical advice. REFERENCES 1. B. Preim, H. Bourquain, D. Selle, K. Oldhafer, and H.-O. 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