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.

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Electromagnetic tracking in medicine--a review of technology, validation, and applications.

A navigation or motion tracking system includes components associated with particular sensors, which are decoupled from a tracking component that takes advantage of information in the sensor measurements. The architecture of this system enables development of sensor-specific components independently of the tracking component, and enables sensors and their associated components to be added or removed without having to re-implement the tracking component. In a software implementation of the system, sensor-specific software components may be dynamically incorporated into the system and the tracking component is then automatically configured to take advantage of measurements from the corresponding sensors without having to modify the tracking component.

Tracking, auto-calibration, and map-building system

Highly accurate spatial measurement systems are among the enabling technologies that
have made image-guided surgery possible in modern operating theaters. Assessing the
accuracies of such systems is subject to much ambiguity, though. The underlying
mathematical models that convert raw sensor data into position and orientation
measurements of sufficient accuracy complicate matters by providing measurements
having non-uniform error distributions throughout their measurement volumes.
Users are typically unaware of these issues, as they are usually presented with only
a few specifications based on some "representative" statistics that were themselves
derived using various data reduction methods. As a result, much of the important
underlying information is lost. Further, manufacturers of spatial measurement
systems often choose protocols and statistical measures that emphasize the strengths
of their systems and diminish their limitations. Such protocols often do not reflect
the end users' intended applications very well. Users and integrators thus need to
understand many aspects of spatial metrology in choosing spatial measurement systems
that are appropriate for their intended applications. We examine the issues by
discussing some of the protocols and their statistical measures typically used by
manufacturers. The statistical measures for a given protocol can be affected by many
factors, including the volume size, region of interest, and the amount and type of
data collected. We also discuss how different system configurations can affect the
accuracy. Single-marker and rigid body calibration results are presented, along with
a discussion of some of the various factors that affect their accuracy. Although the
findings presented here were obtained using the NDI Polaris optical tracking systems,
many are applicable to spatial measurement systems in general.

Accuracy assessment and interpretation for optical tracking systems

Contemporary imaging modalities can now provide the surgeon with high quality three- and four-dimensional images depicting not only normal anatomy and pathology, but also vascularity and function. A key component of image-guided surgery (IGS) is the ability to register multi-modal pre-operative images to each other and to the patient. The other important component of IGS is the ability to track instruments in real time during the procedure and to display them as part of a realistic model of the operative volume. Stereoscopic, virtual- and augmented-reality techniques have been implemented to enhance the visualization and guidance process. For the most part, IGS relies on the assumption that the pre-operatively acquired images used to guide the surgery accurately represent the morphology of the tissue during the procedure. This assumption may not necessarily be valid, and so intra-operative real-time imaging using interventional MRI, ultrasound, video and electrophysiological recordings are often employed to ameliorate this situation. Although IGS is now in extensive routine clinical use in neurosurgery and is gaining ground in other surgical disciplines, there remain many drawbacks that must be overcome before it can be employed in more general minimally-invasive procedures. This review overviews the roots of IGS in neurosurgery, provides examples of its use outside the brain, discusses the infrastructure required for successful implementation of IGS approaches and outlines the challenges that must be overcome for IGS to advance further.

https://iopscience.iop.org/article/10.1088/0031-9155/51/14/R01/pdf

Image-guidance for surgical procedures

A LUS robotic surgical system is trainable by a surgeon to automatically move a LUS probe in a desired fashion upon command so that the surgeon does not have to do so manually during a minimally invasive surgical procedure. A sequence of 2D ultrasound image slices captured by the LUS probe according to stored instructions are processable into a 3D ultrasound computer model of an anatomic structure, which may be displayed as a 3D or 2D overlay to a camera view or in a PIP as selected by the surgeon or programmed to assist the surgeon in inspecting an anatomic structure for abnormalities. Virtual fixtures are definable so as to assist the surgeon in accurately guiding a tool to a target on the displayed ultrasound image.

Laparoscopic ultrasound robotic surgical system

Electromagnetic tracking systems have found increasing use in medical applications during the last few years. As with most non-trivial spatial measurement systems, the complex determination of positions and orientations from their underlying raw sensor measurements results in complicated, non-uniform error distributions over the specified measurement volume. This makes it difficult to unambiguously determine accuracy and performance assessments that allow users to judge the suitability of these systems for their particular needs. Various assessment protocols generally emphasize different measurement aspects that typically arise in clinical use. This can easily lead to inconclusive or even contradictory conclusions. We examine some of the major issues involved and discuss three useful calibration protocols. The measurement accuracy of a system can be described in terms of its 'trueness' and its 'precision'. Often, the two are strongly coupled and cannot be easily determined independently. We present a method that allows the two to be disentangled, so that the resultant trueness properly represents the systematic, non-reducible part of the measurement error, and the resultant precision (or repeatability) represents only the statistical, reducible part. Although the discussion is given largely within the context of electromagnetic tracking systems, many of the results are applicable to measurement systems in general.

https://iopscience.iop.org/article/10.1088/0031-9155/48/14/314/pdf

Accuracy assessment protocols for electromagnetic tracking systems

Accuracy assessment protocols for elektromagnetic tracking systems.

A system for determining spatial position and/or orientation of one or more objects. The system includes an optical subsystem and a non-optical subsystem. The optical subsystem includes optical subsystem light sources mounted to one or more of the objects and an optical subsystem sensor adapted to detect energy from the optical subsystem light sources. The optical subsystem has an optical subsystem coordinate system in a fixed relationship with the optical subsystem sensor. The optical subsystem sensor produces position and/or orientation signals relative to the optical subsystem coordinate system in response to optical subsystem light source detected energy. The non-optical subsystem has a non-optical coordinate system and is adapted to produce position and/or orientation signals of one or more of the objects relative to the non-optical subsystem coordinate system. A coupling arrangement is provided for producing position and/or orientation signals indicative of the position and/or orientation of the selected one of the optical or non-optical subsystems relative to the coordinate system of the other one of the optical or non-optical subsystems. A processor is responsive to signals produced by the coupling arrangement and the individual subsystem sensors for determining the position and/or orientation of one or more of the objects relative to some conveniently defined coordinate system.

System for determining spatial position and/or orientation of one or more objects

A system for determining the position, orientation and system gain factor of a probe includes a plurality of magnetic field sources and at least one magnetic field sensor, such that a combination of a magnetic field sensor and a magnetic field source generates a unique measured magnetic field value. The system includes a probe whose gain, position, and orientation affect these unique measured magnetic field values. A processor is configured to receive and iteratively process these unique measured magnetic field values to determine a system gain factor indicative of the gain of the probe and a plurality of location factors indicative of the position and orientation of the probe. The number of unique measured magnetic field values generated must be at least equal to the sum of the number of gain and location factors calculated.

A gain factor and position determination system

A process detects distortions to magnetic location or orientation determinations. The process includes measuring a plurality of magnetic field values, determining one of a probe's location and a probe's orientation from an extremum of an optimization function. The measured values depend on the probe's location and orientation. The optimization function depends on the measured field values and field values calculated from a model. The process also includes indicating that a distortion to the determining exists in response to the extremum belonging to a preselected range.

Stefan R. Kirsch

Papers

Accuracy assessment protocols for electromagnetic tracking systems

Accuracy assessment protocols for elektromagnetic tracking systems.

System for determining spatial position and/or orientation of one or more objects

A gain factor and position determination system

Errors in systems using magnetic fields to locate objects