Magnetic eye tracking: a new approach employing a planar transmitter

An open electromagnetic tracking framework applied to targeted liver tumour ablation

PURPOSE

Electromagnetic tracking is a core platform technology in the navigation and visualisation of image-guided procedures. The technology provides high tracking accuracy in non-line-of-sight environments, allowing instrument navigation in locations where optical tracking is not feasible. EMT can be beneficial in applications such as percutaneous radiofrequency ablation for the treatment of hepatic lesions where the needle tip may be obscured due to difficult liver environments (e.g subcutaneous fat or ablation artefacts). Advances in the field of EMT include novel methods of improving tracking system accuracy, precision and error compensation capabilities, though such system-level improvements cannot be readily incorporated in current therapy applications due to the 'blackbox' nature of commercial tracking solving algorithms.

METHODS

This paper defines a software framework to allow novel EMT designs, and improvements become part of the global design process for image-guided interventions. An exemplary framework is implemented in the Python programming language and demonstrated with the open-source Anser EMT system. The framework is applied in the preclinical setting though targeted liver ablation therapy on an animal model.

RESULTS

The developed framework was tested with the Anser EMT electromagnetic tracking platform. Liver tumour targeting was performed using the tracking framework with the CustusX navigation platform using commercially available electromagnetically tracked needles. Ablation of two tumours was performed with a commercially available ablation system. Necropsy of the tumours indicated ablations within 5 mm of the tumours.

CONCLUSIONS

An open-source framework for electromagnetic tracking was presented and effectively demonstrated in the preclinical setting. We believe that this framework provides a structure for future advancement in EMT system in and customised instrument design.

Distorter Characterisation Using Mutual Inductance in Electromagnetic Tracking

Electromagnetic tracking (EMT) is playing an increasingly important role in surgical navigation, medical robotics and virtual reality development as a positional and orientation reference. Though EMT is not restricted by line-of-sight requirements, measurement errors caused by magnetic distortions in the environment remain the technology's principal shortcoming. The characterisation, reduction and compensation of these errors is a broadly researched topic, with many developed techniques relying on auxiliary tracking hardware including redundant sensor arrays, optical and inertial tracking systems. This paper describes a novel method of detecting static magnetic distortions using only the magnetic field transmitting array. An existing transmitter design is modified to enable simultaneous transmission and reception of the generated magnetic field. A mutual inductance model is developed for this transmitter design in which deviations from control measurements indicate the location, magnitude and material of the field distorter to an approximate degree. While not directly compensating for errors, this work enables users of EMT systems to optimise placement of the magnetic transmitter by characterising a distorter's effect within the tracking volume without the use of additional hardware. The discrimination capabilities of this method may also allow researchers to apply material-specific compensation techniques to minimise position error in the clinical setting.

A simulator for advanced analysis of a 5-DOF EM tracking systems in use for image-guided surgery

PURPOSE

To assess the accuracy of medical electromagnetic tracking systems, reference positioning systems are generally required. Errors are unavoidable in such systems, and despite how tiny they may be, prevent the ground truth from being known. In this work, a simulator was developed and used to analyze the theoretical system performances in electromagnetic tracking.

METHODS

To simulate the entire tracking process, the magnetic dipole model, Faraday's law, and a mathematical optimization algorithm are applied. With the simulator, we optimized the spatial placement of the transmitter coils, analyzed the tracking accuracy by applying stochastic and optimized coil placement. Additionally, the performance of the calibration of transmitter coils' measurement error and Kalman filtering was tested.

RESULTS

The results show that, after optimizing the spatial arrangement of the transmitter coils, the tracking accuracy is significantly improved to a much higher level compared with applying statistical arrangement. The measurement errors of the transmitter coils' positions and orientations can be totally rectified by the developed calibration algorithm when no noises are introduced. The Kalman filter reduces the sensor jitter errors caused by noise, which potentially allows the EM tracking system to reach a larger volume of interest.

CONCLUSIONS

We proposed a simulator for advanced analysis in electromagnetic tracking without hardware requirements. Grounded on this, we performed an optimization of the spatial arrangement of the transmitter coils to improve the tracking accuracy further. The performances of the calibration algorithm and Kalman filtering were also evaluated. The developed simulator can also be applied for other analysis in electromagnetic tracking.

A low-computational approach on gaze estimation with eye touch system

Among various approaches to eye tracking systems, light-reflection based systems with non-imaging sensors, e.g., photodiodes or phototransistors, are known to have relatively low complexity; yet, they provide moderately accurate estimation of the point of gaze. In this paper, a low-computational approach on gaze estimation is proposed using the Eye Touch system, which is a light-reflection based eye tracking system, previously introduced by the authors. Based on the physical implementation of Eye Touch, the sensor measurements are now utilized in low-computational least-squares algorithms to estimate arbitrary gaze directions, unlike the existing light reflection-based systems, including the initial Eye Touch implementation, where only limited predefined regions were distinguished. The system also utilizes an effective pattern classification algorithm to be able to perform left, right, and double clicks based on respective eye winks with significantly high accuracy. In order to avoid accuracy problems for sensitive sensor biasing hardware, a robust custom microcontroller-based data acquisition system is developed. Consequently, the physical size and cost of the overall Eye Touch system are considerably reduced while the power efficiency is improved. The results of the experimental analysis over numerous subjects clearly indicate that the proposed eye tracking system can classify eye winks with 98% accuracy, and attain an accurate gaze direction with an average angular error of about 0.93 °. Due to its lightweight structure, competitive accuracy and low-computational requirements relative to video-based eye tracking systems, the proposed system is a promising human-computer interface for both stationary and mobile eye tracking applications.

An automatic saccadic eye movement detection in an optokinetic nystagmus signal

A saccade is one of the characteristic types of eye movements. The accurate detection and location of saccades in the signal representing the movement activity of the eyes are essential in medical applications. The main purpose of this paper is to present a new, robust approach to the detection of saccadic eye movements. The procedure is based on a so-called detection function, which is the result of the electronystagmographic (ENG) myriad signal filtering, nonlinear operation, and fuzzy median clustering. Smooth peaks of the detection function waveform correspond to the location of saccades in the ENG signal. The median fuzzy clustering-based method allows for calculating the amplitude threshold of the detection function, which improves the accurate saccade recognition. Both of these robust methods provide a two-step protection against outliers. The proposed algorithm was tested using artificial as well as real optokinetic nystagmus signals under different noise conditions. The results show the usefulness of the procedure when the precise detection and location of saccades are necessary.

Evaluation of a novel tracking system in a breathing lung model

We present the evaluation of an electromagnetic position tracking system for use with virtual bronchoscopy systems. Our system utilises a planar magnetic coil array and commercially available search coil sensors. Experimental results show the EM tracking accuracy to be in the range of 11.5mm, which is comparable to both commercial and research systems. The use of a bench-top breathing lung model is used to verify system operation in the in vitro setting. A novel fiducial-free registration method is implemented to reduce errors resulting from inaccurate landmark identification commonly associated with point-based registration. After registration, there is good agreement between the measured position of the sensor probe during endoscopic navigation and the lung airways as visualised in a 3D model of the phantom.