An oral imaging template was developed to address the shortcomings of MR image data for image guided dental implant planning and placement. The template was conctructed as a gadolinium filled plastic shell to give contrast to the dentition and also to be accurately re-attachable for use in image guided dental implant placement. The result of segmentation and modelling of the dentition from MR Image data with the template was compared to plaster casts of the dentition. In a phantom study dental implant placement was performed based on MR image data. MR imaging with the contrast template allowed complete representation of the existing dentition. In the phantom study, a commercially available system for image guided dental implant placement was used. Transformation of the imaging contrast template into a surgical drill guide based on the MR image data resulted in pilot burr hole placement with an accuracy of 2 mm. MRI based imaging of the existing dentition for proper image guided planning is possible with the proposed template. Using the image data and the template resulted in less accurate pilot burr hole placement in comparison to CT-based image guided implant placement.
Image guided surgery typically relies on preoperatively acquired image data. The major disadvantage is that changes
that occur between image data acquisition and surgery, are not reflected by the image data. Furthermore, with the
beginning of surgery, the image data is not valid anymore. The use of an intraoperative Computer Tomography (CT)
suite is reported. The system consists of a single slice spiral CT scanner (Somatom Emotion, Siemens, Forchheim,
Germany) and a operating room table with a radiolucent board (AWIGS, Maquet, Rastatt, Germany) to put the patient
on. During CT scanning, the patient on the board is immobile, while the gantry of the CT scanner is moved on rails that
are embedded in the floor of the operating room. Image data can be transferred immediately via local area network to a
frameless stereotaxy system (VectorVision, Brainlab, Heimstetten, Germany). Furthermore, intraoperative image data
acquisition in connection with the navigation system can be used for automated patient to image registration. Using the
infrared camera of the navigation system, the position of the gantry can be measured during CT image data acquisition.
With the patient being tracked simultaneously, registration of the image data can be performed fully automatically. The
clinical use of intraoperative CT image data acquisition, the intraoperative workflow of the system, and the clinical
applications are demonstrated.
Image guided implantology using navigation systems is more accurate than manual dental implant insertion. The underlying image data are usually derived from computer tomography. The suitability of MR imaging for dental implant planning is a marginal issue so far. MRI data from cadaver heads were acquired using various MRI sequences. The data were assessed for the quality of anatomical imaging, geometric accuracy and susceptibility to dental metal artefacts. For dental implant planning, 3D models of the jaws were created. A software system for segmentation of the mandible and maxilla MRI data was implemented using c++, mitk, and qt. With the VIBE_15 sequence, image data with high geometric accuracy were acquired. Dental metal artefacts were lower than in CT data of the same heads. The segmentation of the jaws was feasible, in contrast to the segmentation of the dentition, since there is a lack of contrast to the intraoral soft tissue structures. MRI is a suitable method for imaging of the region of mouth and jaws. The geometric accuracy is excellent and the susceptibility to artefacts is low. However, there are yet two limitations: Firstly, the imaging of the dentition needs further improvement to allow accurate segmentation of these regions. Secondly, the sequence used in this study takes several minutes and hence is susceptible to motion artefacts.
INPRES, a system for Augmented Reality has been developed in the collaborative research center "Information Technology in Medicine - Computer- and Sensor-Aided Surgery". The system is based on see-through glasses. In extensive preclinical testing the system has proven its functionality and tests with volunteers had been performed successfully, based on MRI imaging. We report the surgeons view of the first use of the system for AR guided biopsy of a tumour near the skull base. Preoperative planning was performed based on CT image data. The information to be projected was the tumour volume and was segmented from image data. With the use of infrared cameras, the positions of patient and surgeon were tracked intraoperatively and the information on the glasses displays was updated accordingly. The systems proved its functionality under OR conditions in patient care: Augmented reality information could be visualized with sufficient accuracy for the surgical task. After intraoperative calibration by the surgeon, the biopsy was acquired successfully. The advantage of see through glasses is their flexibility. A virtual stereoscopic image can be set up wherever and whenever desired. A biopsy at a delicate location could be performed without the need for wide exposure. This means additional safety and lower operation related morbidity to the patient. The integration of the calibration-procedure of the glasses into the intraoperative workflow is of importance to the surgeon.
This paper is going to present a summary of our technical experience with the INPRES System -- an augmented reality system based upon a tracked see-through head-mounted display. With INPRES a complete augmented reality solution has been developed that has crucial advantages when compared with previous navigation systems. Using these techniques the surgeon does not need to turn his head from the patient to the computer monitor and vice versa. The system's purpose is to display virtual objects, e.g. cutting trajectories, tumours and risk-areas from computer-based surgical planning systems directly in the surgical site. The INPRES system was evaluated in several patient experiments in craniofacial surgery at the Department of Oral and Maxillofacial Surgery/University of Heidelberg. We will discuss the technical advantages as well as the limitations of INPRES and present two strategies as a result. On the one hand we will improve the existing and successful INPRES system with new hardware and a new calibration method to compensate for the stated disadvantage. On the other hand we will focus on miniaturized augmented reality systems and present a new concept based on fibre optics. This new system should be easily adaptable at surgical instruments and capable of projecting small structures. It consists of a source of light, a miniature TFT display, a fibre optic cable and a tool grip. Compared to established projection systems it has the capability of projecting into areas that are only accessible by a narrow path. No wide surgical exposure of the region is necessary for the use of augmented reality.
Accuracy of the patient-model is a critical point in robot assisted surgery. When performing craniotomies, the dura mater must not be perforated. Hence bone width is of particular interest. The influence of imaging and segmentation on accuracy of the width of the bone-model was investigated. A human cadaver head was scanned with a CT-scanner under a variety of image acquisition parameters. Bone was segmented from these image data sets using threshold based segmentation with different settings for the lower threshold. From these volume data sets surface models of the bone were generated. The real width of the bone of the skull was measured at several positions. Using fiducial marker registration, these measured values were compared to the corresponding positions in the bone-models. CT-scan imaging with a slice thickness and slice distance of 1.5 to 2mm and a segmentation of bone with a lower threshold of 300 or 400 Hounsfield Units resulted in models with an average accuracy of 0.4mm for bone-width. However, at some points these models were too thin by up to 0.9mm. More accurate models are needed. It has to be evaluated, whether CT imaging with higher resolution or more sophisticated segmentation algorithms can reduce the scatter.