Clinical indications for digital volume tomography in oral and maxillofacial surgery.

Digital volume tomography (DVT, NewTom, NewTom AG, Marburg, Germany) is a novel technique for maxillofacial imaging at a lower radiation dose and lower cost than CT. We describe four clinical cases to illustrate its potential advantages.

Intraoperative guidance of pre-planned bone deformations with a surface scanning system.

Computer- and robot-based systems to support interventions become more and more important in modem surgery. In general these systems provide methods to plan an intervention pre-operatively and to execute it with support from a autonomous robot-system. Due to the principle restriction of a robot to comparatively simple work steps, there are some complex work steps which the surgeon may plan but which he/she has to execute manually. In craniofacial surgery osteotomised bone segments are deformed by hand to a shape given by the planning system. We support the execution of pre-planned deformation by comparison of the actual shape of an object with the target shape. The actual shape is obtained intra-operatively with a surface scanning device, the deviation from the target shape are visualised by projecting colour-coded error values directly on the object to be deformed. The surgeon uses these projections to adjust further deformation steps. The system is therefore able to validate the correct execution of planned deformations, especially of bony structures.

Daily patient set-up control in radiation therapy by coded light projection.

Advances in conformal radiation therapy to control disease via dose escalation are challenged by set-up uncertainties. Recently, techniques have been developed to use surface features to evaluate the patient's position and correct it where necessary. The aim of this study was to use the patient's surface as a tool for daily set-up control and monitoring. We use a surface scanner based on the projection of coded light to receive--in a daily routine--a large amount of surface points which enables us to register the CT-based planning data with the patients current position. By superimposing current and planned volumes, a volume of congruency was obtained. An error below 1 mm was considered acceptable. In cases where set-up was not satisfactory a map of the surface comparison was evaluated showing the areas of missing alignment. According to this information a manual repositioning was performed. This procedure was repeated until the error was acceptable. No more then 3 repetitions where necessary to obtain an acceptable result. The whole procedure including registration, calculation and visualization took about 20 sec for one repetition. The use of structured light projection in the daily set-up control and monitoring proved to be a noninvasive, easy, quick, inexpensive and reliable solution.

Integration of intraoperative radiotherapy (IORT) dose distribution into the postoperative CT-based external beam radiotherapy (EBRT) treatment planing.

In the treatment of malignant disease external beam radiation therapy (EBRT) is often combined with surgery. Intraoperative radiotherapy (IORT) improves the local control by dose escalation. For reasons of recording, improvement and security of the intervention, it would be necessary to merge the IORT-dose distribution with the postoperative CT-based EBRT-planing. The aim of this work was to develop a method to reconstruct the IORT field and register it with the postoperative planing CT. This enables the reconstruction of the IORT dose distribution and merge it with the CT-based EBRT planing data. We use a surface scanner to receive a large amount of surface points which enables us reconstruct the IORT-field and to register it with the CT-based EBRT planning data. Scanning and calculation time is not over 2 seconds, depending mainly on the CPU power. The error of a single point is below 1 mm. The density of the point cloud is approx. 4 per mm2. In this paper we give an overview of our experimental setup and the accuracy of the method.

Statistical analysis of the morphology of three-dimensional objects and pathologic structures using spherical harmonics.

To support diagnosis and therapy, it is a fundamental aim of medical image processing to describe morphological characteristics of pathological structures or image objects in general. Different authors propose quantitative methods of description like bounding boxes[1], fourier descriptors[2] or contour moments[3]. Unfortunately, these methods either don't supply a complete, respectively precise description of the object or only operate on two-dimensional images. Among the range of application are systems to classify lung nodules [4] or to help the diagnosis of brain tumors [5]. In this paper we present a method to analyze the morphology or shape of any three-dimensional object in order to describe it mathematically well-defined. We show how the description can be used to perform statistical operations on morphologies. The method presented in this paper was developed to assist the planning of craniofacial surgery. We analyze the shape of a given set of skull CT-data and use the mathematical description to statistically calculate the average shape of the skulls.

Intuitive operation planning based on force feedback.

In the craniofacial surgery image-slices obtained from tomographies are used for planning and simulation of surgical interventions. Using these image-slices three-dimensional geometric models can be reconstructed, representing bones and soft tissue. However, for planning complex surgical interventions simulation methods are needed additionally to the pure visualisation. Exemplary, planning a Frontal Orbital Advancement (FOA) operation the cutting trajectories, their depth and orientation in each point, the drill hole, the position, orientation and deformation of bones, etc. are the point of interest. Especially for intraoperative execution supported by navigation systems or robots. In order to be able to plan such complex interventions with the help of computers, geometrical and haptical models must be generated from the image-slices. On the one hand these models represent the anatomical structures exactly, on the other hand they are needed for the simulation of the different activities, which have to be performed during the surgical intervention like drilling, milling, deforming, positioning, etc. Beside the geometrical and haptical models methods for interactions must be supplied to the physician for an accurate and intuitive planning of the surgical intervention. We developed such an operation planning system which is already used in clinical practise in Heidelberg.

A pattern catalogue of surgical interventions for computer-supported operation planning.

In this paper we present a new operation planning system which was evaluated in the clinic for Cranio-Maxillo-Facial-Surgery at the University of Heidelberg. In opposite to commercial systems our goal was, that the system considers the complete surgical intervention and not only a single procedure of it. A second goal was, that the system enables managing of complex operations, independent of which way the intervention will be intraoperatively performed (without technical support, with passive navigation support or active support by robots). Our system supports the surgeon during the preoperative planning as well as during the intraoperative execution phase. Therefore we developed a course model by which the managing of surgical interventions is possible. The focus of this paper is on this course model. At first we introduce instruction graphs and describe the structure of each activity observing its attributes and their context. Additionally, various surgical scopes will be presented which enable the surgeon to select one view among different ones of the individual operation procedures in accordance to medical and technical knowledge as well as in accordance to different degrees of abstraction. At last we demonstrate operation patterns, used as expert knowledge.

Clinical evaluation of a highly accurate algorithm for CT bone contour segmentation.

Planning, visualisation and intraoperative navigation in a robot assisted environment for craniofacial surgery require highly accurate methods for the segmentation of bone structures in CT data. Clinical systems are still based on time consuming interactive methods like the seed-point segmentation. Faster methods with no need for interactivity lacks in precision. In the following we will present an automatic and highly accurate algorithm for the segmentation of bone contours in CT data. It is based on an algorithm for the automatic calculation of a grey-value tissue relation model for CT and MRI data.

A real-time CORBA based system architecture for robot assisted craniofacial surgery.

We present the concept of a system architecture for the computer aided craniofacial surgery. The architecture is based on CORBA, an industrial standard specification for the development of distributed applications. Our concept includes a fundamental behaviour oriented communication model and some fundamental software safety considerations. We've developed a standard library for the integration of new services and devices into our system architecture. It decreases development time noticeably. We tested the performance and usability of our concept on an evaluation set up consisting of a surgery robot system, an infrared navigation system, a force-torque sensor and a visualisation software, obtaining excellent results. Future work will consist in the integration of further devices and the extension of our safety concept. An accurate clinical evaluation will take place continuously.

Texture mapping based visualisation methods for the manipulation of CT data: interaction and ergonomics.

The manipulation of large CT datasets needs fast visualisation methods for a comfortable user interaction. Modern visualisation techniques make use texture hardware in graphics workstations extensively. In the following we will present an interactive tool for the positioning of anatomical landmarks in CT datasets of non-pathological children. The tool includes a fast visualisation of CT cross sections based on a texture mapping technique and an interactive three-dimensional view of the segmented CT dataset.

[Computer-assisted oral, maxillary and facial surgery]. / Computerunterstützte Mund-, Kiefer- und Gesichtschirurgie.

BACKGROUND: Methods from the area of virtual reality are used in oral and maxillofacial surgery for the planning and three-dimensional individual simulation of surgeries. SIMULATION: In order to simulate complex surgeries with the aid of a computer, the diagnostic image data and especially various imaging modalities (CT, MRT, US) must be arranged in relation to each other, thus enabling rapid switching between the various modalities as well as the viewing of mixed images. Segmenting techniques for the reconstruction of three-dimensional representations of soft-tissue and osseous areas are required. We must develop ergonomic and intuitively useable interaction methods for the surgeon, thus allowing for precise and fast entry of the planned surgical intervention in the planning and simulation phase. SURGERY: During the surgical phase, instrument navigation tools offer the surgeon interactive support through operation guidance and control of potential dangers. This feature is already available today. Future intraoperative assistance will take the form of such passive tools for the support of intraoperative orientation as well as so-called tracking systems (semi-active systems) which accompany and support the surgeons' work. The final form are robots which execute specific steps completely autonomously. DISCUSSION: The techniques of virtual reality keep gaining in importance for medical applications. Many applications are still being developed or are still in the form of a prototype. However, it is already clear that developments in this area will have a considerable effect on the surgeon's routine work.

3D norm data: the first step towards semiautomatic virtual craniofacial surgery.

When planning craniofacial surgical interventions, the ideal appearance of the patient is very important. The final appearance should be as close as possible to that which the patient would have if he/she were without defects. Our first step towards achieving this is to build a database containing sets of three-dimensional CT images that allows for comparison of the shape of a patient with defects to the typical shape of an age- and sex-matched "average" person without defects. We started to collect CT data from patients without pathologies and, in co-operation with two radiology institutes (in Mannheim and Heidelberg), over 100 CT data sets have now been collected and classified according to age and sex. It is necessary to choose an appropriate statistical method to calculate the norm data from the different data sets. Based on the statistical method, an age- and sex-matched "average" model of the anatomy will be created.

CT-based 3D-planning for dental implantology.

A system for 3D-planning for dental implantology is described. Since exact knowledge of the position of the nervus alv. inf. is critical, we present an algorithm for automated detection of this nerve, which requires only very little initial user interaction. To allow interactive implant placement on comparatively low-cost pc hardware we developed hybrid visualization techniques, which refrain from using large texture memory and raster engines.