Ruediger Marmulla

Image data acquisition and segmentation for accurate modeling of the calvarium

ABSTRACT 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. Keywords: Segmentation, Modeling, Accuracy, Image Guided Surgery

Neue Konzepte in der bildgestützten Chirurgie: automatische Patientenregistrierung anhand von Kiefer und Ohrmuschel

Mit Hilfe einer automatischen und markerlosen Patientenregistrierung auf der Basis natürlicher anatomischer Grenzflächen kann im Vorfeld eines computergestützten chirurgischen Eingriffs eine deutliche Reduktion von Strahlenbelastung und logistischem Aufwand erreicht werden, weil auf das Platzieren und Einmessen röntgensichtbarer Referenzmarker verzichtet werden kann. In einer klinischen Studie sollte überprüft werden, ob neben dem Gesicht auch die Ohrmuschel sowie der Ober- und Unterkiefer als anatomische Grenzfläche zur intraoperativen Registrierung der Patientenlage verwendet werden können. Vor einem chirurgischen Eingriff wurde die räumliche Lage von 20 Patienten mit Hilfe eines hochauflösenden 3D-Laserscans registriert und markerlos mit dem präoperativen CT-Datensatz korreliert. Indikation für den chirurgischen Eingriff waren Tumoren, skelettale Fehlbildungen und Fremdkörper. Die Ohrmuschel sowie der Ober- und Unterkiefer wurden dabei zur Registrierung der Patientenlage genutzt. Durch eine zusätzliche konventionelle markerbasierte Patientenregistrierung wurde die Genauigkeit dieser neuen—an sich markerlosen—Methode klinisch evaluiert. Die markerlose Patientenregistrierung auf der Basis natürlicher anatomischer Grenzflächen war im Bereich des Oberkiefers verlässlich möglich (Abweichung: 0,8±0,3 mm), im Unterkiefer haben Zunge und beweglicher Mundboden zu geometrischer Inkongruenz und mangelhafter Laserregistrierung geführt. Mit Hilfe der Ohrmuschelregistrierung war eine hohe Präzision zu erzielen, solange die Ohrmuschel während der CT-Bildgebung oder während des Laserscannens nicht deformiert wurde (Abweichung: 1,9±0,9 mm). Die übliche CT-Bildakquisition mit Kopfschale führte jedoch bei mehr als der Hälfte der Patienten zu temporären Ohrmuscheldeformierungen, die eine exakte Laserscanregistrierung unmöglich machte. Automatic and markerless patient registration based on natural anatomical interfaces may considerably reduce the radiation load and logistical input prior to computer-assisted surgical interventions, as it is not necessary to place and measure reference markers. The present study was to find out if, apart from the facial skin, also auricles as well as the upper and lower jaw can be used as anatomical interfaces for the intraoperative registration of the patient’s position. Prior to surgical intervention the positions of 20 patients were registered by a high-resolution 3D laser scan and correlated with the preoperative CT data set. Tumors, skeletal malformations, and foreign bodies were indications for surgical intervention. Auricles as well as the upper and lower jaw were used to register the patient’s positions. The accuracy of this—basically markerless—method was clinically evaluated through the additionally placed conventional registration markers. The markerless patient registration based on natural anatomical interfaces was successful in the upper jaw (deviation: 0.8±0.3 mm). The tongue and mobile floor of the mouth led to geometric incongruence and inadequate laser registration in the lower jaw. As far as the auricles were concerned, high accuracy could only be achieved as long as the auricles had not been deformed during CT imaging (deviation: 1.9±0.9 mm). The usual CT acquisition with a conventional head support, however, led to temporary auricular deformations in more than half of the patients, which made an exact laser scan registration impossible.

Technical experience from clinical studies with INPRES and a concept for a miniature augmented reality system

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 systems 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.