Rolf Ewers

Computer-aided navigation in secondary reconstruction of post-traumatic deformities of the zygoma

Augmented reality technology was used in 5 patients for secondary reconstruction of post-traumatic unilateral deformities of the zygomaticomaxillary complex. Three electromagnetic sensors interfaced to a computer-aided navigation system (ARTMA Biomedical Inc.) were utilized. The computer navigation procedure was planned by drawing graphic lines on the CT scan at the level of the zygomatic arch, representing the outer surface of the zygoma. The desired position of the displaced zygoma was planned by mirroring from the healthy side, using a virtual mid-sagittal plane. These virtual graphics were presented intraoperatively on a TV monitor and also on the surgeons see-through head-mounted display. Correct reduction was assumed when the virtual line representing the position of the zygoma before the osteotomy reached the virtual line defined preoperatively as the desired position. The advantages of the technique presented are that a complete exposure of the zygomatic bone is no longer necessary, and coronal and subciliary incisions may be avoided unless enophthalmos correction has to be carried out, which was in fact necessary in 2 patients. The results of zygomatic reconstruction have been satisfactory in all 5 patients.

Positioning of dental implants using computer-aided navigation and an optical tracking system: case report and presentation of a new method

A navigation system for computer-aided surgery (Virtual Patient System, VPS) has been described in previous studies for different indications in oral and maxillofacial surgery. The aim of the system is the intraoperative transfer of preoperative planning on radiographs or CT scans on the patient, in real-time, and independent of the position of the patients head. Until now an electromagnetic tracking system has been used for intra-operative position measurement. For placement of dental implants, the electromagnetic tracking system is not suitable since the motor of the implant drill leads to a considerable distortion of the magnetic field, thus direct visualization of drilling the implant socket was not possible. To overcome this problem, an optical tracking system which is not disturbed by conductive materials was integrated in the VPS system. The first patient operated on with this system had a posttraumatic loss of the upper incisors; three implants have been placed according to the prosthetic axis previously planned on radiographs and CT scans. The experience gained in this intervention led to the conclusion that computer-aided surgery provides a valuable tool in implant dentistry.

Lactosorb panel and screws for repair of large orbital floor defects

In a series of five patients with extensive fractures of the orbital floor, we used a biodegradable sheet for bridging of the bony defects. To achieve optimal support of the orbital contents in their anatomically correct position, we fixed the sheet with at least two resorbable screws to the infraorbital rim. This new technique appears to be superior to conventional methods because it offers reproducible results without the need for secondary interventions.

Computed intraoperative navigation guidance-a preliminary report on a new technique

OBJECTIVEnTo assess the value of a computer-assisted three-dimensional guidance system (Virtual Patient System) in maxillofacial operations.nnnDESIGNnLaboratory and open clinical study.nnnSETTINGnTeaching Hospital, Austria.nnnSUBJECTSn6 patients undergoing various procedures including removal of foreign body (n=3) and biopsy, maxillary advancement, and insertion of implants (n=1 each).nnnINTERVENTIONSnStorage of computed tomographic (CT) pictures on an optical disc, and imposition of intraoperative video images on to these. The resulting display is shown to the surgeon on a micromonitor in his head-up display for guidance during the operations.nnnMAIN OUTCOME MEASURESnTo improve orientation during complex or minimally invasive maxillofacial procedures and to make such operations easier and less traumatic.nnnRESULTSnSuccessful transferral of computed navigation technology into an operation room environment and positive evaluation of the method by the surgeons involved.nnnCONCLUSIONSnComputer-assisted three-dimensional guidance systems have the potential for making complex or minimally invasive procedures easier to do, thereby reducing postoperative morbidity.

Development of the Varioscope AR. A see-through HMD for computer-aided surgery

In computer-aided surgery (CAS), an undesired side-effect of the necessity of handling sophisticated equipment in the operating room is the fact that the surgeons attention is drawn from the operating field, since surgical progress is partially monitored on the computers screen. Augmented reality (AR), the overlay of computer-generated graphics over a real-world scene, provides a possibility to solve this problem. The technical problems associated with this approach, such as viewing of the scenery within a common focal range on the head-mounted display (HMD) or latency in the display on the HMD, have, however, kept AR from widespread usage in CAS. The concept of the Varioscope AR, a lightweight head-mounted operating microscope used as a HMD, is introduced. The registration of the patient to the pre-operative image data, as well as pre-operative planning, take place on VISIT, a surgical navigation system developed at our hospital. Tracking of the HMD and stereoscopic visualisation take place on a separate POSIX.4-compliant real-time operating system running on PC hardware. We were able to overcome the technical problems described above; our work resulted in an AR visualisation system with an update rate of 6 Hz and a latency below 130 ms. It integrates seamlessly into a surgical navigation system and provides a common focus for both virtual and real-world objects. First evaluations of the photogrammetric 2D/3D registration have resulted in a match of 1.7 pixels on the HMD display. The Varioscope AR with its real-time visualisation unit is a major step towards the introduction of AR into clinical routine.

Interventional video tomography

Interventional Video Tomography (IVT) is a new imaging modality for Image Directed Surgery to visualize in real-time intraoperatively the spatial position of surgical instruments relative to the patients anatomy. The video imaging detector is based on a special camera equipped with an optical viewing and lighting system and electronic 3D sensors. When combined with an endoscope it is used for examining the inside of cavities or hollow organs of the body from many different angles. The surface topography of objects is reconstructed from a sequence of monocular video or endoscopic images. To increase accuracy and speed of the reconstruction the relative movement between objects and endoscope is continuously tracked by electronic sensors. The IVT image sequence represents a 4D data set in stereotactic space and contains image, surface topography and motion data. In ENT surgery an IVT image sequence of the planned and so far accessible surgical path is acquired prior to surgery. To simulate the surgical procedure the cross sectional imaging data is superimposed with the digitally stored IVT image sequence. During surgery the video sequence component of the IVT simulation is substituted by the live video source. The IVT technology makes obsolete the use of 3D digitizing probes for the patient image coordinate transformation. The image fusion of medical imaging data with live video sources is the first practical use of augmented reality in medicine. During surgery a head-up display is used to overlay real-time reformatted cross sectional imaging data with the live video image.

Current status of the Varioscope AR, a head-mounted operating microscope for computer-aided surgery

Computer-aided surgery (CAS), the intraoperative application of biomedical visualization techniques, appears to be one of the most promising fields of application for augmented reality (AR), the display of additional computer generated graphics over a real-world scene. Typically a device such as a head-mounted display (HMD) is used for AR. However considerable technical problems connected with AR have limited the intraoperative application of HMDs up to now. One of the difficulties in using HMDs is the requirement for a common optical focal plane for both the real-world scene and the computer generated image, and acceptance of the HMD by the user in a surgical environment. In order to increase the clinical acceptance of AR, we have adapted the Varioscope (Life Optics, Vienna), a miniature, cost-effective head-mounted operating microscope, for AR. In this work, we present the basic design of the modified HMD, and the method and results of an extensive laboratory study for photogrammetric calibration of the Varioscopes computer displays to a real-world scene. In a series of sixteen calibrations with varying zoom factors and object distances, mean calibration error was found to be 1.24/spl plusmn/0.38 pixels or 0.12/spl plusmn/0.05 mm for a 640/spl times/480 display. Maximum error accounted for 3.33/spl plusmn/1.04 pixels or 0.33/spl plusmn/0.12 mm. The location of a position measurement probe of an optical tracking system was transformed to the display with an error of less than I mm in the real world in 56% of all cases. For the remaining cases, error was below 2 mm. We conclude that the accuracy achieved in our experiments is sufficient for a wide range of CAS applications.

Calibration of projection parameters in the varioscope AR, a head-mounted display for augmented-reality visualization in image-guided therapy

Computer-aided surgery (CAS), the intraoperative application of biomedical visualization techniques, appears to be one of the most promising fields of application for augmented reality (AR), the display of additional computer generated graphics over a real-world scene. Typically a device such as a head-mounted display (HMD) is used for AR. However, considerable technical problems connected with AR have limited the intraoperative application of HMDs up to now. One of the difficulties in using HMDs is the requirement for a common optical focal plane for both the real-world scene and the computer generated image, and acceptance of the HMD by the user in a surgical environment. In order to increase the clinical acceptance of AR, we have adapted the Varioscope (Life Optics, Vienna), a miniature, cost- effective head-mounted operating microscope, for AR. In this work, we present the basic design of the modified HMD, and the method and results of an extensive laboratory study for photogrammetric calibration of the Varioscopes computer displays to a real-world scene. In a series of sixteen calibrations with varying zoom factors and object distances, mean calibration error was found to be 1.24+/- 0.38 pixels or 0.12+/- 0.05 mm for a 640 x 480 display. Maximum error accounted for 3.33+/- 1.04 pixels or 0.33+/- 0.12 mm. The location of a position measurement probe of an optical tracking system was transformed to the display with an error of less than 1 mm in the real world in 56% of all cases. For the remaining cases, error was below 2 mm. We conclude that the accuracy achieved in our experiments is sufficient for a wide range of CAS applications.

PC-based control unit for a head-mounted operating microscope for augmented-reality visualization in surgical navigation

Two main concepts of Head Mounted Displays (HMD) for augmented reality (AR) visualization exist, the optical and video-see through type. Several research groups have pursued both approaches for utilizing HMDs for computer aided surgery. While the hardware requirements for a video see through HMD to achieve acceptable time delay and frame rate seem to be enormous the clinical acceptance of such a device is doubtful from a practical point of view. Starting from previous work in displaying additional computer-generated graphics in operating microscopes, we have adapted a miniature head mounted operating microscope for AR by integrating two very small computer displays. To calibrate the projection parameters of this so called Varioscope AR we have used Tsais Algorithm for camera calibration. Connection to a surgical navigation system was performed by defining an open interface to the control unit of the Varioscope AR. The control unit consists of a standard PC with a dual head graphics adapter to render and display the desired augmentation of the scene. We connected this control unit to a computer aided surgery (CAS) system by the TCP/IP interface. In this paper we present the control unit for the HMD and its software design. We tested two different optical tracking systems, the Flashpoint (Image Guided Technologies, Boulder, CO), which provided about 10 frames per second, and the Polaris (Northern Digital, Ontario, Canada) which provided at least 30 frames per second, both with a time delay of one frame.