Enhancing electromagnetic tracking accuracy in medical applications using pre-trained witness sensor distortion models

PURPOSE

Electromagnetic tracking (EMT) accuracy is affected by the presence of surrounding metallic materials. In this work, we propose measuring the magnetic field's variation due to distortion at a witness position to localise the instrument causing distortion based on a pre-trained model and without additional sensors attached to it.

METHODS

Two experiments were performed to demonstrate possible applications of the technique proposed. In the first case, the distortion introduced by an ultrasound (US) probe was characterised and subsequently used to track the probe position on a line. In the second application, the measurement was used to estimate the distance of an interventional fluoroscopy C-arm machine and apply the correct compensation model.

RESULTS

Tracking of the US probe using the proposed method was demonstrated with millimetric accuracy. The distortion created by the C-arm caused errors in the order of centimetres, which were reduced to 1.52 mm RMS after compensation.

CONCLUSIONS

The distortion profile associated with medical equipment was pre-characterised and used in applications such as object tracking and error compensation map selection. In the current study, the movement was limited to one degree of freedom (1 DOF) and simple analytical functions were used to model the magnetic distortion. Future work will explore advanced AI models to extend the method to 6 DOF tracking using multiple witness sensors.

A novel approach to wireless electromagnetic tracking using frequency modulation radio communication

PURPOSE

Electromagnetic tracking (EMT) is beneficial in image-guided interventions to reduce the use of ionising radiation-based imaging techniques. Enabling wirelessly tracked sensors will increase the usability of these systems for catheter tracking and patient registration systems. This work introduces a novel method of wirelessly transmitting sensor data using a frequency modulation (FM) radio.

METHODS

The proposed technique was tested using the open-source Anser EMT system. An electromagnetic sensor was connected in parallel to an FM transmitter prototype and wired directly to the Anser system for comparison. The performance of the FM transmitter was evaluated on a grid of 125 test points using an optical tracking system as a gold standard.

RESULTS

An average position accuracy of 1.61 ± 0.68 mm and angular rotation accuracy of 0.04° for the FM transmitted sensor signal was obtained over a 30 cm × 30 cm × 30 cm volume, in comparison with the 1.14 ± 0.80 mm, 0.04° accuracy previously reported for the Anser system. The FM transmitted sensor signal had an average resolved position precision of 0.95 mm while the directly wired signal was found to have an average precision of 1.09 mm. A very low frequency (∼ 5 mHz) oscillation in the wirelessly transmitted signal was observed and compensated for by performing a dynamic scaling of the magnetic field model used for solving the sensor pose.

CONCLUSIONS

We demonstrate that FM transmission of an electromagnetic sensor signal can be used to achieve similar tracking performance to a wired sensor. FM transmission for wireless EMT is a viable alternative to digital sampling and transmission over Bluetooth. Future work will create an integrated wireless sensor node using FM communication that is compatible with existing EMT systems.

Intraoperative compensation of magnetic field distortions for fluoroscopic and electromagnetic hybrid navigation

Purpose

Hybrid navigation is a promising technique which combines the benefits of optical or electromagnetic tracking (EMT) and fluoroscopy imaging. Unfortunately, the fluoroscopy system is a source of metallic distortion for the EMT system. In this work, we present a new method for intraoperative calibration and real-time compensation of dynamic field distortions. The method was tested in the presence of a fluoroscopy C-arm, and sub-millimetre errors were obtained after distortion correction.

Methods

A hybrid navigation scenario was created by combining the open-source electromagnetic tracking system Anser EMT and a commercial fluoroscopy C-arm. The electromagnetic field generator was placed directly on top of the X-ray collimator, which introduced significant field distortion. Magnetic sensors were placed at known positions to capture the magnetic distortion, and virtual magnetic dipole sources were used to model the distortion magnetic field. The accuracy of the compensated EMT model was tested on a grid of test points.

Results

Error reduction was demonstrated from 12.01 to 0.35 mm and from 25.03 to 0.49 mm, for horizontal and vertical sensor orientations, respectively, over a volume of 16 × 16 × 6 cm. It is proposed that such sub-millimetre tracking errors meet the needs of most endoscopic navigation tasks.

Conclusions

We describe a method to model a magnetic field in real time, based on redundant electromagnetic field measurements, and we apply it to compensate for the distortion introduced by a fluoroscopy C-arm. The main limitation of the approach is the requirement for a high number of sensors, with possible occlusion of the operative space. Solutions might come from miniaturisation and wireless sensing.

Peripheral tumour targeting using open-source virtual bronchoscopy with electromagnetic tracking: a multi-user pre-clinical study

Objectives: The goal was to demonstrate the utility of open-source tracking and visualisation tools in the targeting of lung cancer.Material and methods: The study demonstrates the first deployment of the Anser electromagnetic (EM) tracking system with the CustusX image-guided interventional research platform to navigate using an endobronchial catheter to injected tumour targets. Live animal investigations validated the deployment and targeting of peripheral tumour models using an innovative tumour marking routine.Results: Novel tumour model deployment was successfully achieved at all eight target sites across two live animal investigations without pneumothorax. Virtual bronchoscopy with tracking successfully guided the tracked catheter to 2-12 mm from the target tumour site. Deployment of a novel marker was achieved at all eight sites providing a reliable measure of targeting accuracy. Targeting accuracy within 10 mm was achieved in 7/8 sites and in all cases, the virtual target distance at marker deployment was within the range subsequently measured with x-ray.Conclusions: Endobronchial targeting of peripheral airway targets is feasible using existing open-source technology. Notwithstanding the shortcomings of current commercial platforms, technological improvements in EM tracking and registration accuracy fostered by open-source technology may provide the impetus for widespread clinical uptake of electromagnetic navigation in bronchoscopy.

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.

Loss of flexion during bronchoscopy: a physical experiment and case study of commercially available systems

During routine endobronchial ultrasound-guided transbronchial needle aspiration (EBUS-TBNA) procedures, especially with biopsy of lymph nodes in or around the left upper lobe, frequent reports have noted the loss of ultrasound image and needle angulation leading to an inability to biopsy nodes visualised by EBUS. The aim of this research was to investigate and compare this loss of angulation with commercially available scopes. Bench-top experiments and a clinical case study demonstrated the varying loss of scope angulation, flexibility and manoeuvrability with different scopes and biopsy instruments leading to procedural implications. Improvements in both the EBUS scope and needle characteristics are required to overcome this limitation which has implications in bronchoscope navigation and the diagnostic yield of EBUS-TBNA.

Evaluation of Endoscopically Deployed Radiopaque Tumor Models in Bronchoscopy

BACKGROUND

Radiopaque markers and soft tissue models have been used extensively in clinical applications to target cancerous lesions and to calibrate and characterize imaging systems. However, the development of radiopaque, soft tissue models for pulmonary lesions is yet to be optimized. Such a material may improve endoscopic training techniques and also be useful to evaluate bronchoscopy navigation systems by the targeting and sampling of tumor models with computed tomography.

METHODS

This study investigates a modified agarose-based model and a novel contrast-infused tripe model to create clinically relevant pulmonary tumor models. An iodine-enhanced agarose model presents an injectable solution with high image contrast under computed tomography capable of reaching distal bronchial airways. The tripe solution presents a cheap and easily deployed method to quickly establish a fiducial marker that may be used during bronchial imaging system training and evaluation.

RESULTS

The iodine-enriched agarose model demonstrates desirable mechanical characteristics ex vivo, but has a number of limitations when administered in a live setting. The tripe solution presents a far more effective in vivo pulmonary tumor model and offers an effective radiopaque marker. However, the size of the tripe tumor samples required for effective insertion limits its ability to reach more distal airways. An iterative testing process was used to optimize the model composition, culminating in live animal investigations (n=3).

CONCLUSION

Both contrast-infused agarose and tripe models present a promising analog to a pulmonary lesion and may act as a radiopaque marker for bronchoscopic training and biopsy evaluation.

Optimizing parameters of an open-source airway segmentation algorithm using different CT images

BACKGROUND

Computed tomography (CT) helps physicians locate and diagnose pathological conditions. In some conditions, having an airway segmentation method which facilitates reconstruction of the airway from chest CT images can help hugely in the assessment of lung diseases. Many efforts have been made to develop airway segmentation algorithms, but methods are usually not optimized to be reliable across different CT scan parameters.

METHODS

In this paper, we present a simple and reliable semi-automatic algorithm which can segment tracheal and bronchial anatomy using the open-source 3D Slicer platform. The method is based on a region growing approach where trachea, right and left bronchi are cropped and segmented independently using three different thresholds. The algorithm and its parameters have been optimized to be efficient across different CT scan acquisition parameters. The performance of the proposed method has been evaluated on EXACT'09 cases and local clinical cases as well as on a breathing pig lung phantom using multiple scans and changing parameters. In particular, to investigate multiple scan parameters reconstruction kernel, radiation dose and slice thickness have been considered. Volume, branch count, branch length and leakage presence have been evaluated. A new method for leakage evaluation has been developed and correlation between segmentation metrics and CT acquisition parameters has been considered.

RESULTS

All the considered cases have been segmented successfully with good results in terms of leakage presence. Results on clinical data are comparable to other teams' methods, as obtained by evaluation against the EXACT09 challenge, whereas results obtained from the phantom prove the reliability of the method across multiple CT platforms and acquisition parameters. As expected, slice thickness is the parameter affecting the results the most, whereas reconstruction kernel and radiation dose seem not to particularly affect airway segmentation.

CONCLUSION

The system represents the first open-source airway segmentation platform. The quantitative evaluation approach presented represents the first repeatable system evaluation tool for like-for-like comparison between different airway segmentation platforms. Results suggest that the algorithm can be considered stable across multiple CT platforms and acquisition parameters and can be considered as a starting point for the development of a complete airway segmentation algorithm.

Magnetic compression in gastrointestinal and bilioenteric anastomosis: how much force?

AIM

The concept of compression alimentary anastomosis is well established. Recently, magnetic axial alignment pressures have been encompassed within such device constructs. We quantify the magnetic compression force and pressure required to successfully achieve gastrointestinal and bilioenteric anastomosis by in-depth interrogation of the reported literature.

METHODS

Reports of successful deployment and proof of anastomotic patency on survival were scrutinized to quantify the necessary dimensions and strengths of magnetic devices in (a) gastroenteral anastomosis in live porcine models and (b) bilioenteric anastomosis in the clinical setting. Using a calculatory tool developed for this work (magnetic force determination algorithm, MAGDA), ideal magnetic force and compression pressure were quantified from successful reports with regard to their variance by intermagnet separation.

RESULTS

Optimized ranges for both compression force and pressure were determined for successful porcine gastroenteral and clinical bilioenteric anastomoses. For gastroenteral anastomoses (porcine investigations), an optimized compression force between 2.55 and 3.57 kg at 2-mm intermagnet separation is recommended. The associated compression pressure should not exceed 60 N/cm(2). Successful bilioenteric anastomoses is best clinically achieved with intermagnet compression of 18 to 31 g and associated pressures between 1 and 3.5 N/mm(2) (at 2-mm intermagnet separation).

CONCLUSION

The creation of magnetic compression anastomoses using permanent magnets demonstrates a remarkable resilience to variations in magnetic force and pressure exertion. However, inappropriate selection of compression characteristics and magnet dimensions may incur difficulties. Recommendations of this work and the availability of the free online tool (http://magda.ucc.ie/) may facilitate a factor of robustness in the design and refinement of future devices.

Simulating Magnetic Nanoparticle Behavior in Low-field MRI under Transverse Rotating Fields and Imposed Fluid Flow

In the presence of alternating-sinusoidal or rotating magnetic fields, magnetic nanoparticles will act to realign their magnetic moment with the applied magnetic field. The realignment is characterized by the nanoparticle's time constant, τ. As the magnetic field frequency is increased, the nanoparticle's magnetic moment lags the applied magnetic field at a constant angle for a given frequency, Ω, in rad/s. Associated with this misalignment is a power dissipation that increases the bulk magnetic fluid's temperature which has been utilized as a method of magnetic nanoparticle hyperthermia, particularly suited for cancer in low-perfusion tissue (e.g., breast) where temperature increases of between 4°C and 7°C above the ambient in vivo temperature cause tumor hyperthermia. This work examines the rise in the magnetic fluid's temperature in the MRI environment which is characterized by a large DC field, B(0). Theoretical analysis and simulation is used to predict the effect of both alternating-sinusoidal and rotating magnetic fields transverse to B(0). Results are presented for the expected temperature increase in small tumors (~1 cm radius) over an appropriate range of magnetic fluid concentrations (0.002 to 0.01 solid volume fraction) and nanoparticle radii (1 to 10 nm). The results indicate that significant heating can take place, even in low-field MRI systems where magnetic fluid saturation is not significant, with careful The goal of this work is to examine, by means of analysis and simulation, the concept of interactive fluid magnetization using the dynamic behavior of superparamagnetic iron oxide nanoparticle suspensions in the MRI environment. In addition to the usual magnetic fields associated with MRI, a rotating magnetic field is applied transverse to the main B(0) field of the MRI. Additional or modified magnetic fields have been previously proposed for hyperthermia and targeted drug delivery within MRI. Analytical predictions and numerical simulations of the transverse rotating magnetic field in the presence of B(0) are investigated to demonstrate the effect of Ω, the rotating field frequency, and the magnetic field amplitude on the fluid suspension magnetization. The transverse magnetization due to the rotating transverse field shows strong dependence on the characteristic time constant of the fluid suspension, τ. The analysis shows that as the rotating field frequency increases so that Ωτ approaches unity, the transverse fluid magnetization vector is significantly non-aligned with the applied rotating field and the magnetization's magnitude is a strong function of the field frequency. In this frequency range, the fluid's transverse magnetization is controlled by the applied field which is determined by the operator. The phenomenon, which is due to the physical rotation of the magnetic nanoparticles in the suspension, is demonstrated analytically when the nanoparticles are present in high concentrations (1 to 3% solid volume fractions) more typical of hyperthermia rather than in clinical imaging applications, and in low MRI field strengths (such as open MRI systems), where the magnetic nanoparticles are not magnetically saturated. The effect of imposed Poiseuille flow in a planar channel geometry and changing nanoparticle concentration is examined. The work represents the first known attempt to analyze the dynamic behavior of magnetic nanoparticles in the MRI environment including the effects of the magnetic nanoparticle spin-velocity. It is shown that the magnitude of the transverse magnetization is a strong function of the rotating transverse field frequency. Interactive fluid magnetization effects are predicted due to non-uniform fluid magnetization in planar Poiseuille flow with high nanoparticle concentrations.

A magnetic retrieval system for stents in the pancreaticobiliary tree

Clinical endoscopic intervention of the pancreaticobiliary tree [endoscopic retrograde cholangiopancreatography (ERCP)] often concludes with the insertion of a temporary plastic stent to reduce the risk of post-ERCP complications by promoting continued flow of bile and pancreatic fluids. This stent is later removed once the patient has fully recovered, but today this necessitates a second endoscopic intervention. The final goal of this work is to obviate the second intervention. This is to be achieved by adding a magnetic ring to the stent such that the stent is removed using a hand-held magnet, held in a suitable position ex vivo . This paper details the design, optimization, and both ex vivo and in vivo testing of the magnetized stent and hand-held magnet, which has been accomplished to date. The optimized design for the hand-held magnet and the modified stent with a magnetic attachment performs in line with simulated expectations, and successful retrieval is achieved in the porcine ex vivo setting at 9-10 cm separation. This is comparable to the mean target capture distance of 10 cm between the entry point to the biliary system and the closest cutaneous surface, determined from random review of clinical fluoroscopies in ten human patients. Subsequently, the system was successfully tested in vivo in the acute porcine model, where retrieval at an estimated separation of 5-6 cm was captured on endoscopic video. These initial results indicate that the system may represent a promising approach for the elimination of a second endoscopic procedures following placement of pancreatic and biliary stents.

Heating in the MRI environment due to superparamagnetic fluid suspensions in a rotating magnetic field

In the presence of alternating-sinusoidal or rotating magnetic fields, magnetic nanoparticles will act to realign their magnetic moment with the applied magnetic field. The realignment is characterized by the nanoparticle's time constant, τ. As the magnetic field frequency is increased, the nanoparticle's magnetic moment lags the applied magnetic field at a constant angle for a given frequency, Ω, in rad/s. Associated with this misalignment is a power dissipation that increases the bulk magnetic fluid's temperature which has been utilized as a method of magnetic nanoparticle hyperthermia, particularly suited for cancer in low-perfusion tissue (e.g., breast) where temperature increases of between 4°C and 7°C above the ambient in vivo temperature cause tumor hyperthermia. This work examines the rise in the magnetic fluid's temperature in the MRI environment which is characterized by a large DC field, B(0). Theoretical analysis and simulation is used to predict the effect of both alternating-sinusoidal and rotating magnetic fields transverse to B(0). Results are presented for the expected temperature increase in small tumors (~1 cm radius) over an appropriate range of magnetic fluid concentrations (0.002 to 0.01 solid volume fraction) and nanoparticle radii (1 to 10 nm). The results indicate that significant heating can take place, even in low-field MRI systems where magnetic fluid saturation is not significant, with careful selection of the rotating or sinusoidal field parameters (field frequency and amplitude). The work indicates that it may be feasible to combine low-field MRI with a magnetic hyperthermia system using superparamagnetic iron oxide nanoparticles.

Measuring SPIO and Gd contrast agent magnetization using 3 T MRI

Traditional methods of measuring magnetization in magnetic fluid samples, such as vibrating sample magnetometry (VSM), are typically limited to maximum field strengths of about 1 T. This work demonstrates the ability of MRI to measure the magnetization associated with two commercial MRI contrast agents at 3 T by comparing analytical solutions to experimental imaging results for the field pattern associated with agents in cylindrical vials. The results of the VSM and fitted MRI data match closely. The method represents an improvement over VSM measurements since results are attainable at imaging field strengths. The agents investigated are Feridex, a superparamagnetic iron oxide suspension used primarily for liver imaging, and Magnevist, a paramagnetic, gadolinium-based compound used for tumors, inflammation and vascular lesions. MR imaging of the agents took place in sealed cylindrical vials in the presence of a surrounding volume of deionized water where the effects of the contrast agents had a measurable effect on the water's magnetization in the vicinity of the compartment of contrast agent. A pair of phase images were used to reconstruct a B(0) fieldmap. The resultant B(0) maps in the water region, corrected for shimming and container edge effects, were used to predict the agent's magnetization at 3 T. The results were compared with the results from VSM measurements up to 1.2 T and close correlation was observed. The technique should be of interest to those seeking quantification of the magnetization associated with magnetic suspensions beyond the traditional scope of VSM. The magnetization needs to be sufficiently strong (M(s) >or= 50 Am(2)/kg Fe for Feridex and X(m) >or=5 x 10(-5) m(3)/kg Gd for Magnevist) for a measurable dipole field in the surrounding water. For this reason, the technique is mostly suitable for undiluted agents.