Georg Eggers

Artefacts in magnetic resonance imaging caused by dental material

Abstract.A common problem in computer tomography (CT) based imaging of the oral cavity is artefacts caused by dental restorations. The aim of this study was to investigate whether magnetic resonance imaging (MRI) of the oral cavity would be less affected than CT by artefacts caused by typical dental restorative alloys. In order to assess the extent of artefact generation, corresponding MRI scans of the same anatomic region with and without dental metal restorations were matched using a stereotactic frame. MRI imaging of the oral and maxillofacial region could be performed without reduction of the image quality by metallic dental restorations made from titanium, gold or amalgam. Dental restorations made from titanium, gold or amalgam did not reduce the image quality of the MRI sequence used in imaging of the oral and maxillofacial region for dental implant planning. In this respect MRI is superior to CT in implant planning.

Geometric accuracy of digital volume tomography and conventional computed tomography

Digital volume tomography is a recently established imaging method that is based on the principle of cone beam computed tomography (CBCT). One of its main applications is imaging in dental and maxillofacial surgery. The objective of this study was to compare the geometric accuracy of digital volume tomographic imaging with that of conventional CT and to assess the suitability for image-guided operating. A calibration cube with a defined pattern of tubes inside was scanned using CT and digital volume tomography. Spatial accuracy was analysed with a software evaluation tool. The positions of the intersections of the tubes were detected in imaging data and registered to the real positions in the calibration body. The deviation was calculated and compared for CT and digital volume tomography. Resolution of spatial images was similar for both methods. However, the spatial accuracy in digital volume tomography was slightly lower than that of CT but still in the submillimetric range. The accuracy was better in the middle, but lower in the margins of the volume. This is a disadvantage in technical image quality, but does not affect the diagnostic image quality. The geometric accuracy is sufficient for digital volume tomography-based image-guided surgery.

Accuracy assessment for navigated maxillo-facial surgery using an electromagnetic tracking device

PURPOSE To evaluate the accuracy and the usability of an electromagnetic tracking device in maxillo-facial surgery through testing on a phantom skull under operating room (OR) conditions. MATERIAL AND METHODS A standard plastic skull phantom was equipped with a custom made model of the maxilla and with target markers and dental brackets. Imaging was performed with a computed tomography (CT) scanner. The extent and robustness of the electromagnetic tracking systems target registration error (TRE) was evaluated under various conditions. RESULTS For each measurement a total of 243 registrations were performed with 5 point registration and 4374 registrations with 6 point registration. The average target registration error for the 5 point registration under OR conditions was 2.1 mm (SD 0.86) and 1.03 (SD 0.53) for the 6 point registration. Metallic instruments applied to the skull increased the TRE significantly in both registration methods. CONCLUSION The electromagnetic tracking device showed a high accuracy and performed stable in both registration methods. Electromagnetic interference due to metallic instruments was significant but the extent of TRE was still acceptable in comparison to optical navigation devices. A benefit of EM tracking is the absence of line-of-sight hindrance. The test setting simulating OR conditions has proven suitable for further studies.

The accuracy of image guided surgery based on cone beam computer tomography image data

OBJECTIVE The objective of this study was to verify if accurate patient-to-image registration for precision navigation in maxillofacial surgery is possible based on cone beam computed tomography (CBCT) image data. STUDY DESIGN A maxillary registration template was placed on a standard plastic skull phantom that was equipped with a custom made model of the maxilla and with target markers. Imaging was performed with a CBCT device (Newtom 9000 Digital Volume Tomograph (DVT), QR s.r.l., Verona, Italy) and a computed tomography (CT) scanner (Somatom 4, Siemens, Forchheim, Germany). Using an infrared navigation system (Polaris, NDI, Waterloo, Ontario), multiple pair-point registration of both image data sets and the phantom were performed. The target registration error (TRE) was evaluated. RESULTS A total of 243 registrations were performed for either image data set. The spatial distribution of TRE on the skull showed increasing inaccuracy with growing distance from the registration markers. The average target registration error was 1.50 +/- 0.82 mm with CBCT and 1.57 +/- 0.84 mm with CT image data and did not differ significantly. Error distribution correlated strongly between CT- and CBCT-based registration. CONCLUSIONS The overall registration accuracy based on CBCT image data was similar to CT. The strong correlation of the geometric distribution of TRE between CT- and CBCT-based measurements proves that CBCT can be equivalent to CT in image-guided maxillofacial surgery.

Histological features and malignant transformation of inverted papilloma

Inverted papilloma (IP) is a primarily benign epithelial neoplasm with a propensity to transform to squamous cell carcinoma (SCC). Neither the etiology of IP nor the factors responsible for malignant transformation are fully known to date. A considerable number of histopathological grading systems have been suggested. It was the aim of this study to find histological parameters in IP that allow a prognosis concerning the occurrence of malignant transformation. In a group of 93 cases of IP, the patients’ records, histological specimens and a questionnaire were evaluated. Thirteen patients had suffered a recurrence of a previously treated IP. SCC in the IP had been diagnosed in 12 of the cases. The histological specimen showed increased counts of mitosis and dyscariosis in the IP. All other histological parameters did not show a significant difference between the groups. History and symptoms proved to be nonspecific as well. Patients in the group with SCC were significantly older at the first diagnosis of IP than patients without carcinoma. This group also included a higher portion of male patients. Patients fulfilling the named histological criteria combined with male gender and higher age are a high-risk group in the necessary long-term follow-up.

Template-based registration for image-guided skull base surgery

Objectives To evaluate whether patient-to-image registration with the use of a maxillary template is sufficiently accurate for image guided skull base surgery. Study Design and Setting In an experimental phantom study, pair-point registration of a skull phantom to its CT image data was performed with 243 different configurations of a maxillary template with markers. Then artificial skull mounted target markers were located with an infrared tracking device as used in navigation systems. Results The average target registration error was 1.57 mm in the anterior skull base (95% confidence interval, 1.53 to 1.61 mm), but 3.31 mm in the lateral skull base (95% confidence interval, 3.26 to 3.37 mm). Conclusions Fiducial marker registration based on a maxillary template is sufficiently accurate for image-guided surgery in the anterior skull base, but not for the lateral skull base. Significance Template-based registration is an accurate yet noninvasive registration method for frontal skull base surgery.

Clinical use of navigation based on cone-beam computer tomography in maxillofacial surgery

Image-guidance in maxillofacial surgery is based predominantly on computed tomographic (CT) images. Its main disadvantage is the considerable amount of radiation to which the patient is exposed, and dental metal artefacts. Recently, a new class of devices based on the concept of cone-beam computed tomography (CBCT) has been introduced for maxillofacial imaging, which we have investigated. In a clinical study, the first seven patients to be operated using a navigation system based on CBCT images, were evaluated. In all cases patient to image recording was uneventful and the surgical objective was reached. The guidance given by the navigation system was helpful. CBCT is an alternative to conventional CT, gives a lower dose of radiation, and costs less. Limitations in the quality of the images and the size of the field of view may restrict its use. It is suitable for image-guided surgery using a navigation system as long as the images show enough of the relevant anatomy and pathology.

PROJECTOR-BASED AUGMENTED REALITY FOR INTUITIVE INTRAOPERATIVE GUIDANCE IN IMAGE-GUIDED 3D INTERSTITIAL BRACHYTHERAPY

PURPOSE The aim of this study is to implement augmented reality in real-time image-guided interstitial brachytherapy to allow an intuitive real-time intraoperative orientation. METHODS AND MATERIALS The developed system consists of a common video projector, two high-resolution charge coupled device cameras, and an off-the-shelf notebook. The projector was used as a scanning device by projecting coded-light patterns to register the patient and superimpose the operating field with planning data and additional information in arbitrary colors. Subsequent movements of the nonfixed patient were detected by means of stereoscopically tracking passive markers attached to the patient. RESULTS In a first clinical study, we evaluated the whole process chain from image acquisition to data projection and determined overall accuracy with 10 patients undergoing implantation. The described method enabled the surgeon to visualize planning data on top of any preoperatively segmented and triangulated surface (skin) with direct line of sight during the operation. Furthermore, the tracking system allowed dynamic adjustment of the data to the patients current position and therefore eliminated the need for rigid fixation. Because of soft-part displacement, we obtained an average deviation of 1.1 mm by moving the patient, whereas changing the projectors position resulted in an average deviation of 0.9 mm. Mean deviation of all needles of an implant was 1.4 mm (range, 0.3-2.7 mm). CONCLUSIONS The developed low-cost augmented-reality system proved to be accurate and feasible in interstitial brachytherapy. The system meets clinical demands and enables intuitive real-time intraoperative orientation and monitoring of needle implantation.

Knowledge-based situation interpretation for context-aware augmented reality in dental implant surgery

The objective of this research is to develop and evaluate a context-aware Augmented Reality system which filters content based on the local context of the surgical instrument. We optically track positions of the patient and the instrument and interpret this data to recognize the phase of the operation. Depending on the result, an appropriate visualization is generated and displayed. For the interpretation, we combine a rule-based, deductive approach and a case-based, inductive one. Both rely on a description-logic based ontology. In phantom experiments the system was used to support implant positioning in models of the mandible. It recognized the phase correctly and provided an appropriate visualization about 85% of the time. The knowledge-based concept for intraoperative assistance proved capable of generating useful visualizations in a timely manner. However, further work is necessary to improve accuracy and reduce the deviation from the actual and planned implant positions.

Development and First Patient Trial of a Surgical Robot for Complex Trajectory Milling

Objective: Todays surgical robots normally perform “simple” trajectories, e.g., assisting as tool-holding devices in neurosurgery, or milling linear paths for cavities in total hip replacement. From a clinical point of view, it is still a complex undertaking to implement robots in the operating room. Until now, robot systems have not been used in patient trials to mill “complex” trajectories, which involve many positional and orientation changes and are often necessary in cranio-maxillofacial (CMF) surgery. This paper presents the RobaCKa surgical robot system, which allows more precise execution of surgical interventions and milling of “complex” trajectories. Materials and Methods: The main components of the RobaCKa system are a (former) CASPAR robot system, a POLARIS system, and a force-torque sensor. Results: In the first patient trial (April 2003) the planned trajectory was executed with an error of 0.66 ± 0.2 mm. Conclusions: The use of former industrial robots for surgical applications is possible but complex. The advantages are improved precision and quality and the possibility of documentation. The use of such systems is normally limited to research institutions or large clinics, because it is hardly possible to implement the necessary technical and logistic efforts in routine surgical work.