Magnetic Resonance-guided Active Catheter Tracking

Interventional device tracking under MRI via alternating current controlled inhomogeneities

PURPOSE

To introduce alternating current-controlled, conductive ink-printed marker that could be implemented with both custom and commercial interventional devices for device tracking under MRI using gradient echo, balanced SSFP, and turbo spin-echo sequences.

METHODS

Tracking markers were designed as solenoid coils and printed on heat shrink tubes using conductive ink. These markers were then placed on three MR-compatible test samples that are typically challenging to visualize during MRI scans. MRI visibility of markers was tested by applying alternating and direct current to the markers, and the effects of applied current parameters (amplitude, frequency) on marker artifacts were tested for three sequences (gradient echo, turbo spin echo, and balanced SSFP) in a gel phantom, using 0.55T and 1.5T MRI scanners. Furthermore, an MR-compatible current supply circuit was designed, and the performance of the current-controlled markers was tested in one postmortem animal experiment using the current supply circuit.

RESULTS

Direction and parameters of the applied current were determined to provide the highest conspicuity for all three sequences. Marker artifact size was controlled by adjusting the current amplitude, successfully. Visibility of a custom-designed, 20-gauge nitinol needle was increased in both in vitro and postmortem animal experiments using the current supply circuit.

CONCLUSION

Current-controlled conductive ink-printed markers can be placed on custom or commercial MR-compatible interventional tools and can provide an easy and effective solution to device tracking under MRI for three sequences by adjusting the applied current parameters with respect to pulse sequence parameters using the current supply circuit.

An interventional MRI guidewire combining profile and tip conspicuity for catheterization at 0.55T

PURPOSE

We describe a clinical grade, "active", monopole antenna-based metallic guidewire that has a continuous shaft-to-tip image profile, a pre-shaped tip-curve, standard 0.89 mm (0.035″) outer diameter, and a detachable connector for catheter exchange during cardiovascular catheterization at 0.55T.

METHODS

Electromagnetic simulations were performed to characterize the magnetic field around the antenna whip for continuous tip visibility. The active guidewire was manufactured using medical grade materials in an ISO Class 7 cleanroom. RF-induced heating of the active guidewire prototype was tested in one gel phantom per ASTM 2182-19a, alone and in tandem with clinical metal-braided catheters. Real-time MRI visibility was tested in one gel phantom and in-vivo in two swine. Mechanical performance was compared with commercial equivalents.

RESULTS

The active guidewire provided continuous "profile" shaft and tip visibility in-vitro and in-vivo, analogous to guidewire shaft-and-tip profiles under X-ray. The MRI signal signature matched simulation results. Maximum unscaled RF-induced temperature rise was 5.2°C and 6.5°C (3.47 W/kg local background specific absorption rate), alone and in tandem with a steel-braided catheter, respectively. Mechanical characteristics matched commercial comparator guidewires.

CONCLUSION

The active guidewire was clearly visible via real-time MRI at 0.55T and exhibits a favorable geometric sensitivity profile depicting the guidewire continuously from shaft-to-tip including a unique curved-tip signature. RF-induced heating is clinically acceptable. This design allows safe device navigation through luminal structures and heart chambers. The detachable connector allows delivery and exchange of cardiovascular catheters while maintaining guidewire position. This enhanced guidewire design affords the expected performance of X-ray guidewires during human MRI catheterization.

Experimental study of an intensity-modulated curvature sensor with high sensitivity based on microstructured optical fiber

Surface Plasmon Resonance (SPR) based fiber optic curvature sensors have the advantage of being insensitive to temperature and axial strain. However, they have the disadvantage of low sensitivity and small curvature detection range. To improve the performance of SPR curvature sensors, we propose an intensity-modulated microstructured optical fiber (MOF) curvature sensor. In this sensor, two no-core fibers (NCFs) are used as input-output couplers, and MOF with silver film deposited is used as sensing arms. The light in the cladding is used to excite the SPR, and the exciting resonant valley is extremely sensitive to slight bending changes. The performance of this sensor is investigated theoretically and experimentally. Numerical results show that its cladding pattern is more favorable in the excitation of SPR effects. Experimental results show that the cladding mode of MOF is very sensitive to curvature changes, thus giving it a great advantage in bending measurements. Its sensitivity reaches 0.18 dB/m^-1, and linearity reaches 0.995 in the curvature range of 0-30 m^-1. The sensor has the advantages of high sensitivity, low temperature and axial strain crosstalk, compact structure, and easy fabrication, which make it attractive in the field of bending sensing.

State of the Art and Future Opportunities in MRI-Guided Robot-Assisted Surgery and Interventions

Magnetic resonance imaging (MRI) can provide high-quality 3-D visualization of target anatomy, surrounding tissue, and instrumentation, but there are significant challenges in harnessing it for effectively guiding interventional procedures. Challenges include the strong static magnetic field, rapidly switching magnetic field gradients, high-power radio frequency pulses, sensitivity to electrical noise, and constrained space to operate within the bore of the scanner. MRI has a number of advantages over other medical imaging modalities, including no ionizing radiation, excellent soft-tissue contrast that allows for visualization of tumors and other features that are not readily visible by other modalities, true 3-D imaging capabilities, including the ability to image arbitrary scan plane geometry or perform volumetric imaging, and capability for multimodality sensing, including diffusion, dynamic contrast, blood flow, blood oxygenation, temperature, and tracking of biomarkers. The use of robotic assistants within the MRI bore, alongside the patient during imaging, enables intraoperative MR imaging (iMRI) to guide a surgical intervention in a closed-loop fashion that can include tracking of tissue deformation and target motion, localization of instrumentation, and monitoring of therapy delivery. With the ever-expanding clinical use of MRI, MRI-compatible robotic systems have been heralded as a new approach to assist interventional procedures to allow physicians to treat patients more accurately and effectively. Deploying robotic systems inside the bore synergizes the visual capability of MRI and the manipulation capability of robotic assistance, resulting in a closed-loop surgery architecture. This article details the challenges and history of robotic systems intended to operate in an MRI environment and outlines promising clinical applications and associated state-of-the-art MRI-compatible robotic systems and technology for making this possible.

A 20-gauge active needle design with thin-film printed circuitry for interventional MRI at 0.55T

PURPOSE

This work aims to fabricate RF antenna components on metallic needle surfaces using biocompatible polyester tubing and conductive ink to develop an active interventional MRI needle for clinical use at 0.55 Tesla.

METHODS

A custom computer numeric control-based conductive ink printing method was developed. Based on electromagnetic simulation results, thin-film RF antennas were printed with conductive ink and used to fabricate a medical grade, 20-gauge (0.87 mm outer diameter), 90-mm long active interventional MRI needle. The MRI visibility performance of the active needle prototype was tested in vitro in 1 gel phantom and in vivo in 1 swine. A nearly identical active needle constructed using a 44 American Wire Gauge insulated copper wire-wound RF receiver antenna was a comparator. The RF-induced heating risk was evaluated in a gel phantom per American Society for Testing and Materials (ASTM) 2182-19.

RESULTS

The active needle prototype with printed RF antenna was clearly visible both in vitro and in vivo under MRI. The maximum RF-induced temperature rise of prototypes with printed RF antenna and insulated copper wire antenna after a 3.96 W/kg, 15 min. long scan were 1.64°C and 8.21°C, respectively. The increase in needle diameter was 98 µm and 264 µm for prototypes with printed RF antenna and copper wire-wound antenna, respectively.

CONCLUSION

The active needle prototype with conductive ink printed antenna provides distinct device visibility under MRI. Variations on the needle surface are mitigated compared to use of a 44 American Wire Gauge copper wire. RF-induced heating tests support device RF safety under MRI. The proposed method enables fabrication of small diameter active interventional MRI devices having complex geometries, something previously difficult using conventional methods.

Separation method of bending and torsion in shape sensing based on FBG sensors array

This paper presents a theoretical method for separating bending and torsion of shape sensing sensor to improve sensing accuracy during its deformation. We design a kind of shape sensing sensor by encapsulating three fibers on the surface of a flexible rod and forming a triangular FBG sensors array. According to the configuration of FBG sensors array, we derive the relationship between bending curvature and bending strain, and set up a function about the packaging angle of FBG sensor and strain induced by torsion under different twist angles. Combined with the influence of bending and torsion on strain, we establish a nonlinear matrix equation resolving three unknown parameters including maximum strain, bending direction and wavelength shift induced by torsion and temperature. The three parameters are sufficient to separate bending and torsion, and acquire two scalar functions including curvature and torsion, which could describe 3D shape of rod according to Frenet-Serret formulas. Experimental results show that the relative average error of measurement about maximum strain, bending direction is respectively 2.65% and 0.86% when shape-sensing sensor is bent into an arc with a radius of 260 mm. The separating method also applied to 2D shape and 3D shape of reconstruction, and the absolute spatial position maximum error is respectively 3.79mm and 11.10mm when shape-sensing sensor with length 500mm is bent into arc shape with a radius 260mm and helical curve. The experiment results verify the feasibility of separating method, which would provide effective parameters for precise 3D reconstruction model of shape sensing sensor.

Advances in Real-Time MRI-Guided Electrophysiology

Purpose of Review

Theoretical benefits of real-time MRI guidance over conventional electrophysiology include contemporaneous 3D substrate assessment and accurate intra-procedural guidance and evaluation of ablation lesions. We review the unique challenges inherent to MRI-guided electrophysiology and how to translate the potential benefits in the treatment of cardiac arrhythmias.

Recent Findings

Over the last 5 years, there has been substantial progress, initially in animal models and more recently in clinical studies, to establish methods and develop workflows within the MR environment that resemble those of conventional electrophysiology laboratories. Real-time MRI-guided systems have been used to perform electroanatomic mapping and ablation in patients with atrial flutter, and there is interest in developing the technology to tackle more complex arrhythmias including atrial fibrillation and ventricular tachycardia.

Summary

Mainstream adoption of real-time MRI-guided electrophysiology will require demonstration of clinical benefit and will be aided by increased availability of devices suitable for use in the MRI environment.

Interventional magnetic resonance imaging guided carotid embolectomy using a novel resonant marker catheter: demonstration of preclinical feasibility

To assess the visualization and efficacy of a wireless resonant circuit (wRC) catheter system for carotid artery occlusion and embolectomy under real-time MRI guidance in vivo, and to compare MR imaging modality with x-ray for analysis of qualitative physiological measures of blood flow at baseline and after embolectomy. The wRC catheter system was constructed using a MR compatible PEEK fiber braided catheter (Penumbra, Inc, Alameda, CA) with a single insulated longitudinal copper loop soldered to a printed circuit board embedded within the catheter wall. In concordance with IACUC protocol (AN103047), in vivo carotid artery navigation and embolectomy were performed in four farm pigs (40-45 kg) under real-time MRI at 1.5T. Industry standard clots were introduced in incremental amounts until adequate arterial occlusion was noted in a total of n=13 arteries. Baseline vasculature and restoration of blood flow were confirmed via MR and x-ray imaging, and graded by the Thrombolysis in Cerebral Infarction (TICI) scale. Wilcoxon signed-rank tests were used to analyze differences in recanalization status between DSA and MRA imaging. Successful recanalizations (TICI 2b/3) were compared to clinical rates reported in literature via binomial tests. The wRC catheter system was visible both on 5° sagittal bSSFP and coronal GRE sequence. Successful recanalization was demonstrated in 11 of 13 occluded arteries by DSA analysis and 8 of 13 by MRA. Recanalization rates based on DSA (0.85) and MRA (0.62) were not significantly different from the clinical rate of mechanical aspiration thrombectomy reported in literature. Lastly, a Wilcoxon signed rank test indicated no significant difference between TICI scores analyzed by DSA and MRA. With demonstrated compatibility and visualization under MRI, the wRC catheter system is effective for in vivo endovascular embolectomy, suggesting progress towards clinical endovascular interventional MRI.