Ein Verfahren zur sicheren Visualisierung und Lokalisierung von Kathetern für MR-geführte intravaskuläre Prozeduren

Glass-Fiber-based MR-safe Guidewire for MR Imaging-guided Endovascular Interventions: In Vitro and Preclinical in Vivo Feasibility Study

Purpose To evaluate glass-fiber-based guidewires that are safe for magnetic resonance (MR) imaging-guided endovascular interventions by using a phantom and an in vivo swine model. Materials and Methods MR imaging-safe guidewires were made from micropultruded glass and/or aramid fibers and epoxy resin with diameters of 0.89 mm (0.035 inch) for standard and stiff guidewires and 0.36 mm (0.014 inch) for micro guidewires. MR imaging visibility and mechanical properties were assessed in a pulsatile flow model. After approval was obtained from the institutional animal care and use committee, MR imaging guidewires were evaluated for standard endovascular procedures in nine pigs. Real-time steady-state free-precession sequences were used for MR imaging-guided catheterization, balloon dilation, and stent implantation into aorto-iliac/visceral arteries and the vena cava (temporal resolution, five images per second; and spatial resolution, 150-mm field of view, and 128 × 128 matrix) with a 1.5-T clinical imager. Visualization with the guidewires was rated on a four-point scale, handling was rated on a three-point scale, and catheterization times for different vessel regions were determined by two interventional radiologists. Afterward, handling ratings and catheterization times were obtained for standard nitinol guidewires during x-ray-based fluoroscopy. Cannulation times, signal intensity in each vessel region, and visualization and handling ratings were measured for the MR imaging guidewires. Bland-Altman analysis was performed for inter- and intraobserver variability of cannulation time. Spearman rank correlation was used to compare handling of MR imaging guidewires and standard nitinol guidewires. Results MR imaging guidewires were characterized by good to excellent visibility, with a continuous artifact of 2 mm in diameter and 4 × 8-mm ball-shaped tip marker. Stiffness, flexibility, and guidance reflected comparable times for all in vitro and in vivo procedures with both the MR imaging and standard nitinol guidewires. Standard and micro MR imaging guidewires were most suitable for the iliac crossover maneuver. Phantom visceral artery cannulation was easier with standard and micro MR imaging guidewires. The stiff MR imaging guidewire provided the best support for cannulation of the swine aorta and vena cava. All interventional procedures were performed successfully without complications. Conclusion Preliminary results showed that the use of glass-fiber-based guidewires for evaluation of MR imaging-guided endovascular interventions is technically feasible and safe in a swine model, and potentially, in humans. ^© RSNA, 2017 Online supplemental material is available for this article.

Improvement of the MR imaging behavior of vascular implants

Vascular implants can cause significant MR image artifacts due to the material (susceptibility artifact) or the electromagnetic characteristics (RF artifact). These artifacts are caused by the distortion of the magnetic field and interferences with the radio frequency (RF) waves of the MR imaging process. Void or complete vanishing of signals occurs in close proximity or inside implants. The artifacts can be minimized by using a material with low magnetic susceptibility and a design of the implant which avoids electrical conductive loops. But not all designs can be made loop-free and non conductive. A resonant circuit tuned to the Larmor frequency of the MR tomography overcomes the RF artifact and thus improves the visualization of the implant lumen. The paper reviews the state-of-the-art technology of the MR-signal improvement in implants lumen, with particular regard to the use of resonant circuits such as stents or Vena Cava Filter (VCF), with resonators in 1.0 Tesla and 1.5Tesla MRT.

Magnetic resonance imaging-guided vascular interventions

Magnetic resonance imaging (MRI), which provides superior soft-tissue imaging and no known harmful effects, has the potential as an alternative modality to guide various medical interventions. This review will focus on MR-guided endovascular interventions and present its current state and future outlook. In the first technical part, enabling technologies such as developments in fast imaging, catheter devices, and visualization techniques are examined. This is followed by a clinical survey that includes proof-of-concept procedures in animals and initial experience in human subjects. In preclinical experiments, MRI has already proven to be valuable. For example, MRI has been used to guide and track targeted cell delivery into or around myocardial infarctions, to guide atrial septal puncture, and to guide the connection of portal and systemic venous circulations. Several investigational MR-guided procedures have already been reported in patients, such as MR-guided cardiac catheterization, invasive imaging of peripheral artery atheromata, selective intraarterial MR angiography, and preliminary angioplasty and stent placement. In addition, MR-assisted transjugular intrahepatic portosystemic shunt procedures in patients have been shown in a novel hybrid double-doughnut x-ray/MRI system. Numerous additional investigational human MR-guided endovascular procedures are now underway in several medical centers around the world. There are also significant hurdles: availability of clinical-grade devices, device-related safety issues, challenges to patient monitoring, and acoustic noise during imaging. The potential of endovascular interventional MRI is great because as a single modality, it combines 3-dimensional anatomic imaging, device localization, hemodynamics, tissue composition, and function.

Transmission line for improved RF safety of interventional devices

A new concept is proposed to improve the safety of transmission lines with respect to heating during RF transmission. It is based on the integration of transformers into the transmission line. The concept was applied to an active tracking device. Miniature transformers were designed, and two types of tracking devices were built based on a standard line and a transformer line. Temperature measurements were performed for both devices during high specific absorption rate (SAR) scanning, and the suppression of RF heating to a physiologically non-relevant level was demonstrated for the transformer device. The transmission properties of the transformer line were examined in simulations and RF measurements. Active tracking with the transformer device performed robustly in the phantom. Because of the favorable signal transmission properties of the tested device, it is expected that the concept can be applied to the construction of clinical devices for tracking and intravascular imaging, which are RF-safe under clinical SAR conditions. Since the transformer line has a large bandwidth, the concept may also be applied for RF-safe transmission of non-MR signals.

[Measurements of respiratory motion using fast magnetic resonance imaging and inductively-coupled marker coils]

The respiratory motion of the thoracic wall provides indirect information about the breathing displacement of the inner organs. To analyze the correlation between thoracic wall and lung motion for applications in radiation therapy, the breathing displacement of the lung is visualized with a fast gradient echo pulse sequence (trueFISP) at a rate of 2-3 images/sec. For quantification of the motion, a small inductively-coupled marker coil is attached to the chest wall and detected with a fast projection technique. Since the marker coil generates a flip angle amplification (factor 15) in its interior, very small nominal flip angles of 2 degrees can be used during the projection measurements which do not affect the image quality of the trueFISP images. Volunteer studies with the marker coil showed a good agreement with simultaneously acquired breathing belt data and position information extracted from the MR images. Whereas the breathing belt provided reliable data only within a certain dynamic range, the marker coil could detect also extreme breathing excursions with a precision better than 2 millimeters.

Interventional magnetic resonance angiography with no strings attached: wireless active catheter visualization

Active instrument visualization strategies for interventional MR angiography (MRA) require vascular instruments to be equipped with some type of radiofrequency (RF) coil or dipole RF antenna for MR signal detection. Such visualization strategies traditionally necessitate a connection to the scanner with either coaxial cable or laser fibers. In order to eliminate any wire connection, RF resonators that inductively couple their signal to MR surface coils were implemented into catheters to enable wireless active instrument visualization. Instrument background to contrast-to-noise ratio was systematically investigated as a function of the excitation flip angle. Signal coupling between the catheter RF coil and surface RF coils was evaluated qualitatively and quantitatively as a function of the catheter position and orientation with regard to the static magnetic field B0 and to the surface coils. In vivo evaluation of the instruments was performed in interventional MRA procedures on five pigs under MR guidance. Cartesian and projection reconstruction TrueFISP imaging enabled simultaneous visualization of the instruments and vascular morphology in real time. The implementation of RF resonators enabled robust visualization of the catheter curvature to the very tip. Additionally, the active visualization strategy does not require any wire connection to the scanner and thus does not hamper the interventionalist during the course of an intervention.

Catheter Visualization with Resonant Markers at MR Imaging–guided Deployment of Endovascular Stents in Swine

PURPOSE

To evaluate resonant circuits as markers for magnetic resonance (MR) imaging-guided placement of nitinol stents.

MATERIALS AND METHODS

The study was approved by the institutional animal research committee and complied with National Institutes of Health guidelines for care and use of laboratory animals. Resonant circuits similar to catheter markers used at conventional angiography were placed proximally and distally to a nitinol stent in a stent delivery system. Resonant circuits were tested in vitro and in vivo for signal intensity levels that would enable visualization during MR imaging-guided stent deployment. Experiments were conducted by using real-time imaging with a 1.5-T unit. Stents (n = 9) were deployed in the vena cava (n = 2), abdominal aorta (n = 2), isthmus of the aorta (n = 2), and carotid (n = 2) and iliac (n = 1) arteries in five pigs. After intervention, the site of the stent was investigated with balanced fast field-echo MR imaging and contrast material-enhanced MR angiography. Blood flow velocities were measured in the stent lumen and next to the stent with velocity-encoded cine MR imaging. Level of agreement was determined with Bland-Altman analysis.

RESULTS

During all interventions, resonant circuits provided highly visible MR signal that allowed fast and reliable visualization of the stent delivery system. Borders of loaded stents were clearly marked, which allowed precise stent placement in all experiments. Balanced fast field-echo MR imaging and contrast-enhanced MR angiography provided information about immediate postintervention position. Positions depicted on MR images were found accurate at postmortem examination. Results of Bland-Altman analysis showed good agreement between blood flow velocities measured in and next to the stent lumen, with a mean difference of -9 cm/sec +/- 5 (standard deviation).

CONCLUSION

Resonant circuits are well suited for use at deployment of endovascular stents.

Use of a Nonmetallic Guide Wire for Magnetic Resonance-Guided Coronary Artery Catheterization

RATIONALE AND OBJECTIVES

Metallic guide wires can be subject to substantial heating when used in the magnetic resonance (MR) environment. Therefore, animal experiments were performed to test the feasibility of a non-metallic and MR-safe guide wire with passive markers for catheterization of coronary arteries under MR guidance.

MATERIALS AND METHODS

Self-made guide wires consisting of a resin-microparticle compound covered by polytetrafluoroethylene were used to catheterize both coronary arteries of swine together with a non-braided catheter. Time needed for catheterization was recorded.

RESULTS

MR-guided coronary artery catheterization with passive visualization of a self-made non-metallic guide wire is possible. In average 141 seconds (SD 68) were needed to manipulate the guide wire together with a catheter from the carotid artery into the left or right coronary artery ostium.

CONCLUSION

Standard nitinol guide wires have to be considered unsafe for MR-guided interventions due to possible heating of electrical conducting structures in the MR environment. Passive visualization techniques allow MR-guided catheterization of small arteries like coronaries. However, there is the substantial disadvantage of obscuring the underlying anatomy of small vessels by the passive markers needed for real-time MR guidance.

In vivo safe catheter visualization and slice tracking using an optically detunable resonant marker

The purpose of this study was to test the in vivo feasibility of safe automatic catheter tracking based on an optically detunable resonant marker installed on the catheter tip, and also to test the compatibility of this approach with guidewire materials. The design of the resonant marker and the integration into the real-time MR environment is described. The catheter was used for real-time MR-guided catheterization of the aorta, left ventricle, and carotid in two swine. For in-plane visualization, the marker was repeatedly detuned. For automatic slice tracking, a projection difference measurement including detuning was interleaved with the imaging sequence. In vitro experiments were conducted to investigate the RF-safety of the marker and the effect of the guidewires on the signal intensity. For all orientations the marker provided excellent in vivo contrast using a radial steady-state free-precession sequence. Flashing of the marker by repetitive tuning/detuning further improved the in-plane visualization. Automatic slice tracking during real-time imaging was successfully performed. The plastic guidewires did not interfere with the marker, and detuning by guidewires containing nitinol could be compensated. In conclusion, automatic slice tracking as well as excellent in-plane visualization can be achieved with this approach and it is safe with respect to RF transmission.