Wolfgang Birkfellner

Calibration of tracking systems in a surgical environment

The purpose of this paper was to assess to what extent an optical tracking system (OTS) used for position determination in computer-aided surgery (CAS) can be enhanced by combining it with a direct current (DC) driven electromagnetic tracking system (EMTS). The main advantage of the EMTS is the fact that it is not dependent on a free line-of-sight. Unfortunately, the accuracy of the EMTS is highly affected by nearby ferromagnetic materials. The authors have explored to what extent the influence of the metallic equipment in the operating room (OR) can be compensated by collecting precise information on the nonlinear local error in the EMTS by using the OTS for setting up a calibration look-up table. After calibration of the EMTS and registration of the sensor systems in the OR the authors have found the average euclidean deviation in position readings between the DC tracker and the OTS reduced from 2.9/spl plusmn/1.0 mm to 2.1/spl plusmn/0.8 mm within a half-sphere of 530-mm radius around the magnetic field emitter. Furthermore the authors have found the calibration to be stable after re-registration of the sensors under varying conditions such as different heights of the OR table and varying positions of the OR equipment over a longer time interval. These results encourage the further development of a hybrid magnetooptical tracker for computer-aided surgery where the electromagnetic tracker acts as an auxiliary source of position information for the optical system. Strategies for enhancing the reliability of the proposed hybrid magnetooptic tracker by detecting artifacts induced by mobile ferromagnetic objects such as surgical tools are discussed.

A head-mounted operating binocular for augmented reality visualization in medicine - design and initial evaluation

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 realworld 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 binocular, for AR. In this paper, 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 16 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 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.

Systematic distortions in magnetic position digitizers

Medical devices equipped with position sensors enable applications like image guided surgical interventions, reconstruction of three-dimensional 3D ultrasound (US) images, and virtual or augmented reality systems. The acquisition of three-dimensional position data in real time is one of the key technologies in this field. The systematic distortions induced by various metals, surgical tools, and US scan probes in different commercial electromagnetic tracking systems were assessed in the presented work. A precise nonmetallic six degree-of-freedom measurement rack was built that allowed a quantitative comparison of different electromagnetic trackers. Also, their performance in the presence of large metallic structures was quantified in a phantom study on an acrylic skull model in an operating room (OR). The trackers used were alternating current (ac) and direct current (dc) based systems. The ac trackers were, on average, distorted by 0.7 mm and 0.5 degree by metallic objects positioned at a distance greater than 120 mm between the geometrical center of the sample and the sensor. In the OR environment, the ac system exhibits mean errors of 3.2 +/- 2.4 mm and 2.9 degrees +/- 1.9 degrees. The dc trackers are more sensitive to distortions caused by ferromagnetic materials (averaged value: 1.6 mm and 0.5 degree beyond a distance of 120 mm). The dc tracker shows no distortions from other conductive materials but was less accurate in the OR environment (typical error: 6.4 +/- 2.5 mm and 4.9 degrees +/- 2.0 degrees). At distances smaller than approximately 100 mm between sample and sensor error increases quickly. It is also apparent from our measurements that the influence of US scan probes is governed by their shielding material. The results show that surgical instruments not containing conductive material are to be preferred when using an ac tracker. Nonferromagnetic instruments should be used with dc trackers. Static distortions caused by the OR environment have to be compensated by precise calibration methods.

Design and application of an assessment protocol for electromagnetic tracking systems.

This paper defines a simple protocol for competitive and quantified evaluation of electromagnetic tracking systems such as the NDI Aurora (A) and Ascension microBIRD with dipole transmitter (B). It establishes new methods and a new phantom design which assesses the reproducibility and allows comparability with different tracking systems in a consistent environment. A machined base plate was designed and manufactured in which a 50 mm grid of holes was precisely drilled for position measurements. In the center a circle of 32 equispaced holes enables the accurate measurement of rotation. The sensors can be clamped in a small mount which fits into pairs of grid holes on the base plate. Relative positional/orientational errors are found by subtracting the known distances/rotations between the machined locations from the differences of the mean observed positions/rotation. To measure the influence of metallic objects we inserted rods made of steel (SST 303, SST 416), aluminum, and bronze into the sensitive volume between sensor and emitter. We calculated the fiducial registration error and fiducial location error with a standard stylus calibration for both tracking systems and assessed two different methods of stylus calibration. The positional jitter amounted to 0.14 mm(A) and 0.08 mm(B). A relative positional error of 0.96mm±0.68mm, range -0.06 mm; 2.23 mm(A) and 1.14mm±0.78mm, range -3.72 mm; 1.57 mm(B) for a given distance of 50 mm was found. The relative rotation error was found to be 0.51° (A)/0.04° (B). The most relevant distortion caused by metallic objects results from SST 416. The maximum error 4.2mm(A)∕⩾100mm(B) occurs when the rod is close to the sensor(20 mm). While (B) is more sensitive with respect to metallic objects, (A) is less accurate concerning orientation measurements. (B) showed a systematic error when distances are calculated.

Evaluation of a miniature electromagnetic position tracker

The advent of miniaturized electromagnetic digitizers opens a variety of potential clinical applications for computer aided interventions using flexible instruments; endoscopes or catheters can easily be tracked within the body. With respect to the new applications, the systematic distortions induced by various materials such as closed metallic loops, wire guides, catheters, and ultrasound scan heads were systematically evaluated in this paper for a new commercial tracking system. We employed the electromagnetic tracking system Aurora (Mednetix/CH, NDI/Can); data were acquired using the serial port of a PC running SuSE Linux 7.1 (SuSE, Gmbh, Nürnberg). Objects introduced into the digitizer volume included wire loops of different diameters, wire guides, optical tracking tools, an ultrasonic (US) scan head, an endoscope with radial ultrasound scan head and various other objects used in operating rooms and interventional suites. Beyond this, we determined the influence of a C-arm fluoroscopy unit. To quantify the reliability of the system, the miniaturized sensor was mounted on a nonmetallic measurement rack while the transmitter was fixed at three different distances within the digitizer range. The tracker was shown to be more sensitive to distortions caused by materials close to the emitter (average distortion error 13.6 mm +/- 16.6 mm for wire loops positioned at a distance between 100 mm and 200 mm from the emitter). Distortions caused by materials near the sensor (distances smaller than 100 mm) are small (typical error 2.2 mm +/- 1.9 mm). The C-arm fluoroscopy unit caused considerable distortions and limits the reliability of the tracker (distortion error 18.6 mm +/- 24.9 mm). Distortions resulting from the US scan head are high at distances smaller than about 100 mm from the emitter. The distortions also increase when the scan head is positioned horizontally and close to the sensor (average error 4.1 mm +/- 1.5 mm when the scan head is positioned within a distance of 100 mm from the sensor). The distortions are slightly higher when the ultrasound machine is switched on. We also evaluated the influence of common medical instruments on distance measurements. For these measurements the average deviation from the known distance of 200 mm amounted to 3.0 mm +/- 1.5 mm (undistorted distance measurement 1.5 mm +/- 0.3 mm). The deviations also depend on the relative orientation between emitter and sensor. The results demonstrate that the miniature tracking system opens up new perspectives with regard to surgery applications where a flexible instrument is to be tracked within the body. Significant distortions caused by metallic objects only occur in the worst cases, for example, in the presence of a closed, unisiolated wire loop or a C-arm fluorescence unit close to the emitter and which can be avoided by suitable usage.

Electromagnetic Tracking in Medicine—A Review of Technology, Validation, and Applications

Object tracking is a key enabling technology in the context of computer-assisted medical interventions. Allowing the continuous localization of medical instruments and patient anatomy, it is a prerequisite for providing instrument guidance to subsurface anatomical structures. The only widely used technique that enables real-time tracking of small objects without line-of-sight restrictions is electromagnetic (EM) tracking. While EM tracking has been the subject of many research efforts, clinical applications have been slow to emerge. The aim of this review paper is therefore to provide insight into the future potential and limitations of EM tracking for medical use. We describe the basic working principles of EM tracking systems, list the main sources of error, and summarize the published studies on tracking accuracy, precision and robustness along with the corresponding validation protocols proposed. State-of-the-art approaches to error compensation are also reviewed in depth. Finally, an overview of the clinical applications addressed with EM tracking is given. Throughout the paper, we report not only on scientific progress, but also provide a review on commercial systems. Given the continuous debate on the applicability of EM tracking in medicine, this paper provides a timely overview of the state-of-the-art in the field.

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.

Stability of miniature electromagnetic tracking systems

This study aims at a comparative evaluation of two recently introduced electromagnetic tracking systems under reproducible simulated operating-room (OR) conditions: the recently launched Medtronic StealthStation, Treon-EM and the NDI Aurora. We investigate if and to what extent these systems provide improved performance and stability in the presence of surgical instruments as possible sources of distortions compared with earlier reports on electromagnetic tracking technology. To investigate possible distortions under pseudo-realistic OR conditions, a large Langenbeck hook, a dental drill with its handle and an ultrasonic (US) scanhead are fixed on a special measurement rack at variable distances from the navigation sensor. The position measurements made by the Treon-EM were least affected by the presence of the instruments. The lengths of the mean deviation vectors were 0.21 mm for the Langenbeck hook, 0.23 mm for the drill with handle and 0.56 mm for the US scanhead. The Aurora was influenced by the three sources of distortion to a higher degree. A mean deviation vector of 1.44 mm length was observed in the vicinity of the Langenbeck hook, 0.53 mm length with the drill and 2.37 mm due to the US scanhead. The maximum of the root mean squared error (RMSE) for all coordinates in the presence of the Langenbeck hook was 0.3 mm for the Treon and 2.1 mm for the Aurora; the drill caused a maximum RMSE of 0.2 mm with the Treon and 1.2 mm with the Aurora. In the presence of the US scanhead, the maximum RMSE was 1.4 mm for the Treon and 5.1 mm for the Aurora. The new generation of electromagnetic tracking systems has significantly improved compared to common systems that were available in the middle of the 1990s and has reached a high level of technical development. We conclude that, in general, both systems are suitable for routine clinical application.

Wobbled splatting—a fast perspective volume rendering method for simulation of x-ray images from CT

3D/2D registration, the automatic assignment of a global rigid-body transformation matching the coordinate systems of patient and preoperative volume scan using projection images, is an important topic in image-guided therapy and radiation oncology. A crucial part of most 3D/2D registration algorithms is the fast computation of digitally rendered radiographs (DRRs) to be compared iteratively to radiographs or portal images. Since registration is an iterative process, fast generation of DRRs-which are perspective summed voxel renderings-is desired. In this note, we present a simple and rapid method for generation of DRRs based on splat rendering. As opposed to conventional splatting, antialiasing of the resulting images is not achieved by means of computing a discrete point spread function (a so-called footprint), but by stochastic distortion of either the voxel positions in the volume scan or by the simulation of a focal spot of the x-ray tube with non-zero diameter. Our method generates slightly blurred DRRs suitable for registration purposes at framerates of approximately 10 Hz when rendering volume images with a size of 30 MB.

Evaluation of a new electromagnetic tracking system using a standardized assessment protocol

This note uses a published protocol to evaluate a newly released 6 degrees of freedom electromagnetic tracking system (Aurora, Northern Digital Inc.). A practice for performance monitoring over time is also proposed. The protocol uses a machined base plate to measure relative error in position and orientation as well as the influence of metallic objects in the operating volume. Positional jitter (E(RMS)) was found to be 0.17 mm +/- 0.19 mm. A relative positional error of 0.25 mm +/- 0.22 mm at 50 mm offsets and 0.97 mm +/- 1.01 mm at 300 mm offsets was found. The mean of the relative rotation error was found to be 0.20 degrees +/- 0.14 degrees with respect to the axial and 0.91 degrees +/- 0.68 degrees for the longitudinal rotation. The most significant distortion caused by metallic objects is caused by 400-series stainless steel. A 9.4 mm maximum error occurred when the rod was closest to the emitter, 10 mm away. The improvement compared to older generations of the Aurora with respect to accuracy is substantial.