Active MR visualization of a vascular guidewire in vivo

Laser-Generated Leaky Acoustic Wave Imaging for Interventional Guidewire Guidance

Ultrasound (US) is widely used to visualize both tissue and the positions of surgical instruments in real time during surgery. Previously we proposed a new method to exploit US imaging and laser-generated leaky acoustic waves (LAWs) for needle visualization. Although successful, that method only detects the position of a needle tip, with the location of the entire needle deduced from knowing that the needle is straight. The purpose of the current study was to develop a beamforming-based method for the direct visualization of objects. The approach can be applied to objects with arbitrary shapes, such as the guidewires that are commonly used in interventional guidance. With this method, illumination by a short laser pulse generates photoacoustic waves at the top of the guidewire that propagate down its metal surface. These waves then leak into the surrounding tissue, which can be detected by a US array transducer. The time of flight consists of two parts: 1) the propagation time of the guided waves on the guidewire and 2) the propagation time of the US that leaks into the tissue. In principle, an image of the guidewire can be formed based on array beamforming by taking the propagation time on the metal into consideration. Furthermore, we introduced directional filtering and a matched filter to compress the dispersion signal associated with long propagation times. The results showed that guidewires could be detected at depths of at least 70 mm. The maximum detectable angle was 56.3°. LAW imaging with a 1268-mm-long guidewire was also demonstrated. The proposed method has considerable potential in new clinical applications.

Safe guidewire visualization using the modes of a PTx transmit array MR system

Purpose

MRI‐guided cardiovascular intervention using standard metal guidewires can produce focal tissue heating caused by induced radiofrequency guidewire currents. It has been shown that safe operation is made possible by using parallel transmit radiofrequency coils driven in the null current mode, which does not induce radiofrequency currents and hence allows safe tissue visualization. We propose that the maximum current modes, usually considered unsafe, be used at very low power levels to visualize conductive wires, and we investigate pulse sequences best suited for this application.

Methods

Spoiled gradient echo, balanced steady‐state free precession, and turbo spin echo sequences were evaluated for their ability to visualize a conductive guidewire embedded in a gel phantom when run in maximum current modes at very low power level. Temperature at the guidewire tip was monitored for safety assessment.

Results

Excellent guidewire visualization could be achieved using maximum current modes excitation, with the turbo spin echo sequence giving the best image quality. Although turbo spin echo is usually considered to be a high‐power sequence, our method reduced all pulses to 1% amplitude (0.01% power), and heating was not detected. In addition, visualization of background tissue can be achieved using null current mode, also with no recorded heating at the guidewire tip even when running at 100% (reported) specific absorption rate.

Conclusion

Parallel transmit is a promising approach for both guidewire and tissue visualization using maximum and null current modes, respectively, for interventional cardiac MRI. Such systems can switch excitation mode instantaneously, allowing for flexible integration into interactive sequences.

MR-guided Cardiac Interventions

Diagnostic and interventional cardiac catheterization is routinely used in the diagnosis and treatment of congenital heart disease. There are well-established concerns regarding the risk of radiation exposure to patients and staff, particularly in children given the cumulative effects of repeat exposure. Magnetic resonance imaging (MRI) offers the advantage of being able to provide better soft tissue visualization, tissue characterization, and quantification of ventricular volumes and vascular flow. Initial work using MRI catheterization employed fusion of x-ray and MRI techniques, with x-ray fluoroscopy to guide catheter placement and subsequent MRI assessment for anatomical and hemodynamic assessment. Image overlay of 3D previously acquired MRI datasets with live fluoroscopic imaging has also been used to guide catheter procedures.Hybrid x-ray and MRI-guided catheterization paved the way for clinical application and validation of this technique in the assessment of pulmonary vascular resistance and pharmacological stress studies. Purely MRI-guided catheterization also proved possible with passive catheter tracking. First-in-man MRI-guided cardiac catheter interventions were possible due to the development of MRI-compatible guidewires, but halted due to guidewire limitations.More recent developments in passive and active catheter tracking have led to improved visualization of catheters for MRI-guided catheterization. Improvements in hardware and software have also increased image quality and scanning times with better interactive tools for the operator in the MRI catheter suite to navigate through the anatomy as required in real time. This has expanded to MRI-guided electrophysiology studies and radiofrequency ablation in humans. Animal studies show promise for the utility of MRI-guided interventional catheterization. Ongoing investment and development of MRI-compatible guidewires will pave the way for MRI-guided diagnostic and interventional catheterization coming into the mainstream.

Interventional CMR: Clinical applications and future directions

Interventional cardiovascular magnetic resonance (iCMR) promises to enable radiation-free catheterization procedures and to enhance contemporary image guidance for structural heart and electrophysiological interventions. However, clinical translation of exciting pre-clinical interventions has been limited by availability of devices that are safe to use in the magnetic resonance (MR) environment. We discuss challenges and solutions for clinical translation, including MR-conditional and MR-safe device design, and how to configure an interventional suite. We review the recent advances that have already enabled diagnostic MR right heart catheterization and simple electrophysiologic ablation to be performed in humans and explore future clinical applications.

Optimizing accuracy and precision of micro-coil localization in active-MR tracking

OBJECTIVE

To examine whether a centroid peak detection algorithm and micro-transmit tracking improve the accuracy and precision of active-MR tracking when combined with previously published strategies of Hadamard Multiplexing and Phase-field Dithering.

MATERIALS AND METHODS

The dipole magnetic field of a solenoid tracking coil was modeled and MR spin excitation using both a uniform body coil and the tracking coil was simulated for 5329 orientations of the solenoid coil with respect to B0. A lumenless micro-coil was built onto a rotation platform and MR-tracking accuracy and precision were experimentally assessed for 576 orientations within a 1.5-T MRI scanner. Peak identification strategies (i.e. maximum pixel detection and the centroid pixel method) and transmit modes (body transmit and micro-transmit tracking) were employed and localization accuracy was assessed for each orientation in both simulation and experimentation.

RESULTS

The simultaneous use of the centroid pixel method, micro-transmit tracking, Phase-field Dithering, and Hadamard Multiplexing resulted in high MR tracking accuracy and precision: 0.52±0.41 mm and 0.34 mm respectively. Furthermore, all four methods combined offered a tracking error less than the size of the micro-coil, despite the lack of a signal source within the micro-coil.

CONCLUSIONS

Micro-transmit tracking and the centroid pixel method improve MR tracking accuracy and precision when combined with Phase-field Dithering and Hadamard Multiplexing compared to using Phase-field Dithering and Hadamard Multiplexing alone.

Magnetic Resonance-guided Active Catheter Tracking

Several advantages of MR imaging compared with other imaging modalities have provided the rationale for increased attention to MR-guided interventions, including its excellent soft tissue contrast, its capability to show both anatomic and functional information, and no use of ionizing radiation. An important aspect of MR-guided intervention is to provide visualization and navigation of interventional devices relative to the surrounding tissues. This article focuses on the methods for MR-guided active tracking in catheter-based interventions. Practical issues about implementation of active catheter tracking in a clinical setting are discussed and several current application examples are highlighted.

MR imaging-guided cardiovascular interventions in young children

Diagnostic cardiac catheterization procedures in children have been largely replaced by magnetic resonance (MR) imaging studies. However, when invasive catheterization is required, MR imaging has a significant role to play, when combined with invasive pressure measurements. Beyond the established reduction to the radiation dose involved, solely MR image-guided or MR image-assisted catheterization procedures can accurately address clinical questions, such as estimation of pulmonary vascular resistance and cardiac output response to stress, without needing to perform laborious measurements that are prone to errors. This article describes MR image-guided or MR image-assisted cardiac catheterization procedures for diagnosis and intervention in children.

Control of intravascular catheters using an array of active steering coils

PURPOSE

To extend the concept of deflecting the tip of a catheter with the magnetic force created in an MRI system through the use of an array of independently controllable steering coils located in the catheter tip, and to present methods for visualization of the catheter and/or surrounding areas while the catheter is deflected.

METHODS

An array of steering coils made of 42-gauge wire was built over a 2.5 Fr (0.83 mm) fiber braided microcatheter. Two of the coils were 70 turn axial coils separated by 1 cm, and the third was a 15-turn square side coil that was 2 x 4 mm2. Each coil was driven independently by a pulse width modulation (PWM) current source controlled by a microprocessor that received commands from a MATLAB routine that dynamically set current amplitude and direction for each coil. The catheter was immersed in a water phantom containing 1% Gd-DTPA that was placed at the isocenter of a 1.5 T MRI scanner. Deflections of the catheter tip were measured from image-based data obtained with a real-time radio frequency (RF) spoiled gradient echo sequence (GRE). The small local magnetic fields generated by the steering coils were exploited to generate a hyperintense signal at the catheter tip by using a modified GRE sequence that did not include slice-select rewinding gradients. Imaging and excitation modes were implemented by synchronizing the excitation of the steering coil array with the scanner by ensuring that no current was driven through the coils during the data acquisition window; this allowed visualization of the surrounding tissue while not affecting the desired catheter position.

RESULTS

Deflections as large as 2.5 cm were measured when exciting the steering coils sequentially with a 100 mA maximum current per coil. When exciting a single axial coil, the deflection was half this value with 30% higher current. A hyperintense catheter tip useful for catheter tracking was obtained by imaging with the modified GRE sequence. Clear visualization of the areas surrounding the catheter was obtained by using the excitation and imaging mode even with a repetition time (TR) as small as 10 ms.

CONCLUSIONS

A new system for catheter steering is presented that allows large deflections through the use of an integrated array of steering coils. Additionally, two imaging techniques for tracking the catheter tip and visualization of surrounding areas, without interference from the active catheter, were shown. Together the demonstrated steerable catheter, control system and the imaging techniques will ultimately contribute to the development of a steerable system for interventional MRI procedures.

CMR for characterization of the myocardium in acute coronary syndromes

The utility of cardiac magnetic resonance imaging (CMR) as a diagnostic technique is well established. CMR enables tissue characterization, distinction between myocardial scar tissue and viable tissue, and evaluation of myocardial perfusion and contractile function. To date, CMR has been mostly applied in the assessment of stable disease; however, a role for CMR in the acute setting is also emerging. An accurate appraisal of the myocardium with CMR in the first hours after the onset of chest pain could provide supporting information to standard diagnostic tools, such as electrocardiography and measurement of blood biomarkers, which could help guide the selection of appropriate treatment. The aims of this integrated approach include positive identification of an ischemic syndrome, estimation of downstream areas at risk of damage, evaluation of epicardial artery patency and small vessel integrity, quantification of infarct size, and determination of myocardial function. This Review critically evaluates both established and emerging CMR techniques, and relates the imaging findings to the underlying pathophysiological processes in acute coronary syndromes. A more thorough understanding of CMR techniques will clarify their potential clinical applications and limitations, and assess the practicality of CMR in the setting of acute coronary syndromes, where early intervention is crucial to save myocardium at risk of irreversible injury.

Toward true 3D visualization of active catheters using compressed sensing

A crucial requirement in MR-guided interventions is the visualization of catheter devices in real time. However, true 3D visualization of the full length of catheters has hitherto been impossible given scan time constraints. Compressed sensing (CS) has recently been proposed as a method to accelerate MR imaging of sparse objects. Images acquired with active interventional devices exhibit a high CNR and are inherently sparse, therefore rendering CS ideally suited for accelerating data acquisition. A framework for true visualization of active catheters in 3D is proposed employing CS to gain high undersampling factors making real-time applications feasible. Constraints are introduced taking into account prior knowledge of catheter geometry and catheter motion over time to improve and accelerate image reconstruction. The potential of the method is demonstrated using computer simulations and phantom experiments and in vivo feasibility is demonstrated in a pig experiment.

MR-guided intravascular interventions: techniques and applications

Magnetic resonance imaging (MRI) offers several advantages over other imaging modalities that make it an attractive imaging tool for diagnostic and therapeutic procedures. This tremendous potential of MRI has provided the rationale for increased attention toward MR-guided endovascular interventions. MR guidance has been used recently to navigate endovascular catheters and deliver stents, vena cava filters, embolization materials, and septum closure devices. However, its potential goes beyond just copying existing procedures toward the development of new minimally invasive techniques that cannot be performed with conventional guiding techniques. Because of technical limitations and safety issues associated with some of the currently available devices, a limited number of clinical studies have been performed so far. The overall success for this developing field requires considerable interdisciplinary research within both the interventional and the MR community. Only through a combined effort can this complex technology find its way into clinical practice. This review discusses the hardware and software improvements that have helped to advance endovascular interventions under MR imaging guidance from a pure research tool to become a clinical reality. In addition, technical and safety issues specific to endovascular MR image guidance will be described and practical applications will be shown that take advantage of the benefits of MR for endovascular interventions.

Magnetic resonance-compatible robotic and mechatronics systems for image-guided interventions and rehabilitation: a review study

The continuous technological progress of magnetic resonance imaging (MRI), as well as its widespread clinical use as a highly sensitive tool in diagnostics and advanced brain research, has brought a high demand for the development of magnetic resonance (MR)-compatible robotic/mechatronic systems. Revolutionary robots guided by real-time three-dimensional (3-D)-MRI allow reliable and precise minimally invasive interventions with relatively short recovery times. Dedicated robotic interfaces used in conjunction with fMRI allow neuroscientists to investigate the brain mechanisms of manipulation and motor learning, as well as to improve rehabilitation therapies. This paper gives an overview of the motivation, advantages, technical challenges, and existing prototypes for MR-compatible robotic/mechatronic devices.

An overview on the advances in cardiovascular interventional MR imaging

Interventional cardiovascular magnetic resonance imaging (iCMR) represents a new discipline whose systematic development will foster minimally invasive interventional procedures without radiation exposure. New generations of open, wide and short bore MR scanners and real time sequences made cardiovascular intervention possible. MR compatible endovascular catheters and guide-wires are needed for delivery of devices such as stents or atrial septal defect (ASD) closures. Catheter tracking is based on active and passive approaches. Currently performed MR-guided procedures are used to monitor, navigate and track endovascular catheters and to deliver local therapeutic agents to targets, such as infarcted myocardium and vascular walls. Heating of endovascular MR catheters, guide-wires and devices during imaging still presents high safety risks. MR contrast media improve the capabilities of MR imaging by enhancing blood signal, pathologic targets (such as myocardial infarctions and atherosclerotic plaques), endovascular catheters and by tracking injected therapeutic agents. Labeling injected soluble therapeutic agents, genes or cells with MR contrast media enables interventionalists to ensure the administration of the drugs in the target and to trace their distribution in the targets. The future clinical use of this iCMR technique requires (1) high spatial and temporal resolution imaging, (2) special catheters and devices and (3) effective therapeutic agents, genes or cells. These conditions are available at a low scale at the present time and need to be developed in the near future. Such progress will lead to improved patient care and minimize invasiveness.

Dynamic imaging of contrast-enhanced coronary vessels with a magnetization prepared rotated stripe keyhole acquisition

PURPOSE

To evaluate dynamic coronary imaging based on a magnetization prepared contrast-enhanced (CE) rotated stripe keyhole acquisition scheme.

MATERIALS AND METHODS

Background suppression of long T(1) tissue was used so that the k-space would be selectively dominated by the contribution of the CE vessel. The phase-encoding axis was then adjusted parallel to the long axis of the vessel to sample the significant power spectrum of the vessel. The performance of this approach was evaluated by means of computer simulations and experimental studies on phantoms and a pig model instrumented with an intracoronary catheter for infusion of contrast media.

RESULTS

Computer simulations and phantom studies demonstrated that by rotating the gradient axes to match the k-space pattern of the frequency spectrum, one can reduce the keyhole band to 20% of the full k-space while preserving the structure's lumen width and sharpness. In vivo studies validated those findings, and dynamic angiograms of the CE coronary arteries were obtained as rapidly as 140 msec per image, with an in-plane spatial resolution of 1.5 mm.

CONCLUSION

With efficient background suppression, a rotated stripes keyhole acquisition can efficiently acquire the significant k-space of a CE vessel, and provide improved vessel definition with a reduced acquisition matrices scheme.

Image-guided and -monitored renal artery stenting using only MRI

PURPOSE

To demonstrate the ability of a unique interventional MR system to be used safely and effectively as the only imaging modality for all phases of MR-guided stent-supported angioplasty.

MATERIALS AND METHODS

An experimental disease model of renal stenosis was created in six pigs. An interventional MR system, which employed previously reported tools for real-time catheter tracking with automated scan-plane positioning, adaptive image parameters, and radial true-FISP imaging with steady-state precession (True-FISP) imaging coupled with a high-speed reconstruction technique, was then used to guide all phases of the intervention, including: guidewire and catheter insertion, stent deployment, and confirmation of therapeutic success. Pre- and postprocedural X-ray imaging was used as a gold standard to validate the experimental results.

RESULTS

All of the stent-supported angioplasty interventions were a technical success and were performed without complications. The average postoperative residual stenosis was 14.9%. The image guidance enabled the stents to be deployed with an accuracy of 0.98 +/- 0.69 mm. Additionally, using this interventional MRI system to guide renal artery stenting significantly reduces the procedure time, as compared to using X-ray fluoroscopy.

CONCLUSION

This study has clearly demonstrated the first successful treatment of renal artery stenting in an experimental animal model solely under MRI guidance and monitoring.

Advances in interventional cardiovascular MRI

Because of its superior soft tissue imaging, MRI has become a valuable diagnostic tool in cardiovascular disease. These strengths make MRI attractive to guide therapeutic catheter-based procedures, both conventional and novel. We review how to configure an interventional MRI suite, how MRI catheter devices differ from conventional radiographic catheters, and finally developments in preclinical and investigational clinical applications.

A polymer-based MR-compatible guidewire: a study to explore new prospects for interventional peripheral magnetic resonance angiography (ipMRA)

PURPOSE

To introduce a newly developed polymer-based and magnetic resonance (MR)-compatible guidewire and to explore its capabilities with respect to interventional peripheral magnetic resonance angiography (ipMRA) in a flow phantom.

MATERIALS AND METHODS

The guidewire is based on a polyetheretherketone (PEEK) polymer core, and small iron particles are embedded in its coating. A passive device tracking technique was designed utilizing a susceptibility artifact induced by the wire in images acquired with a balanced steady-state free precession (b-SSFP) sequence using small flip angles. The position of the guidewire tip was determined from image intensity maxima and overlayed onto a roadmap in near real-time. Guidewire tracking and balloon angioplasty of an artificial stenosis were attempted in two configurations of a flow phantom.

RESULTS

Successful passive guidewire tracking was performed for all phantom configurations. Robustness and accuracy of the tracking technique were sufficient for phantom studies. A balloon catheter was placed into the stenosis using the guidewire under complete MR guidance, and subsequent balloon angioplasty yielded improved flow conditions.

CONCLUSION

The new guidewire is well-suited for clinical application due to an absence of the risk of core fracture and its atraumatic flexible tip. It opens novel prospects for the realization of ipMRA in humans that need to be explored in further studies.

Interventional and intraoperative MRI at low field scanner--a review

Magnetic resonance imaging (MRI) is a cutting edge imaging modality in detecting diseases and pathologic tissue. The superior soft tissue contrast in MRI allows better definition of the pathology. MRI is increasingly used for guiding, monitoring and controlling percutaneous procedures and surgery. The rapid development of interventional techniques in radiology has led to integration of imaging with computers, new therapy devices and operating room like conditions. This has projected as faster and more accurate imaging and hence more demanding procedures have been applied to the repertoire of the interventional radiologist. In combining features of various other imaging modalities and adding some more into them, interventional MRI (IMRI) has potential to take further the interventional radiology techniques, minimally invasive therapies and surgery. The term "Interventional MRI" consists in short all those procedures, which are performed under MRI guidance. These procedures can be either percutaneous or open surgical of nature. One of the limiting factors in implementing MRI as guidance modality for interventional procedures has been the fact, that most widely used magnet design, a cylindrical magnet, is not ideal for guiding procedures as it does not allow direct access to the patient. Open, low field scanners usually operating around 0.2 T, offer this feature. Clumsy hardware, bad patient access, slow image update frequency and strong magnetic fields have been other limiting factors for interventional MRI. However, the advantages of MRI as an imaging modality have been so obvious that considerable development has taken place in the 20-year history of MRI. The image quality has become better, ever faster software, new innovative sequences, better MRI hardware and increased computing power have accelerated imaging speed and image quality to a totally new level. Perhaps the most important feature in the recent development has been the introduction of open configuration low field MRI devices in the early 1990s; this enabled direct patient access and utilization of the MRI as an interventional device. This article reviews the current status of interventional and intraoperative MRI with special emphasis in low field surrounding.

MR-guided interventional procedures: a review

Magnetic resonance imaging (MRI) has emerged as a potential guidance tool for a variety of procedures. Diagnostic and therapeutic procedures using either open surgical or percutaneous access are performed. They span from simple lesion targeting and biopsy to complex applications requiring multiple tasks performed simultaneously or in rapid succession. These tasks include instrument guidance and therapy monitoring as well as procedural follow-up. The interventional use of MRI (IMRI) is increasing steadily. This article reviews the prerequisites, systems, and clinical interventional procedures of IMRI.

Endovascular interventional MRI

MR guidance has been used recently to navigate endovascular catheters and deliver stents in large (aorta and pulmonary) and small (coronary, renal, and femoral) arteries, place ASD closure devices, deliver pulmonary valve stents, guide cardiac RF ablations, and perform intramyocardial injections. However, MR visualization of a stent lumen is still a problem and requires more attention. Because of technical limitations and safety concerns associated with the prototype devices used, limited numbers of clinical studies have been performed. Considerable development is necessary to overcome the challenges and take advantage of the benefits that MR has to offer for endovascular interventions. In this article we review the current state of the art and address the topic partly by referring to our own experiments and presenting our recent illustrations.

Real-time magnetic resonance-guided endovascular repair of experimental abdominal aortic aneurysm in swine

OBJECTIVES

This study tested the hypotheses that endografts can be visualized and navigated in vivo solely under real-time magnetic resonance imaging (rtMRI) guidance to repair experimental abdominal aortic aneurysms (AAA) in swine, and that MRI can provide immediate assessment of endograft apposition and aneurysm exclusion.

BACKGROUND

Endovascular repair for AAA is limited by endoleak caused by inflow or outflow malapposition. The ability of rtMRI to image soft tissue and flow may improve on X-ray guidance of this procedure.

METHODS

Infrarenal AAA was created in swine by balloon overstretch. We used one passive commercial endograft, imaged based on metal-induced MRI artifacts, and several types of homemade active endografts, incorporating MRI receiver coils (antennae). Custom interactive rtMRI features included color coding the catheter-antenna signals individually, simultaneous multislice imaging, and real-time three-dimensional rendering.

RESULTS

Eleven repairs were performed solely using rtMRI, simultaneously depicting the device and soft-tissue pathology during endograft deployment. Active devices proved most useful. Intraprocedural MRI provided anatomic confirmation of stent strut apposition and functional corroboration of aneurysm exclusion and restoration of laminar flow in successful cases. In two cases, there was clear evidence of contrast accumulation in the aneurysm sac, denoting endoleak.

CONCLUSIONS

Endovascular AAA repair is feasible under rtMRI guidance. Active endografts facilitate device visualization and complement the soft tissue contrast afforded by MRI for precise positioning and deployment. Magnetic resonance imaging also permits immediate post-procedural anatomic and functional evaluation of successful aneurysm exclusion.

Invasive human magnetic resonance imaging: feasibility during revascularization in a combined XMR suite

We tested the feasibility and safety of invasive magnetic resonance imaging (MRI) during peripheral angioplasty. Real-time MRI can image soft tissue and may potentially guide therapeutic procedures without ionizing radiation or nephrotoxic contrast. MRI-guided diagnostic catheterization has been described recently, but safe and conspicuous catheter devices are not widely available. An active guidewire, which serves as an MRI receiver antenna, might be useful to guide catheterization or even to image atheroma. We describe a combined interventional suite offering both X-ray fluoroscopy and real-time MRI. We used a 0.030'' active guidewire receiver coil for invasive MRI after X-ray lesion traversal in patients undergoing percutaneous iliofemoral artery revascularization. Intravascular MRI was compared with noninvasive MRI, X-ray angiography, and intravascular ultrasound (IVUS). Seven eligible patients consented to participate, but three were excluded because of lengthy revascularization procedures. Four remaining patients safely underwent combined X-ray fluoroscopy and real-time magnetic resonance imaging (XMR) transport, continuous monitoring, and all imaging modalities. There was no device dislodgment, contamination or evidence of heating. The intravascular MRI coil was well visualized except at the tip, but did not provide superior mural imaging compared with IVUS. Therefore, because an adequate safety and workflow experience was obtained, enrollment was terminated after only four subjects. Invasive MRI is feasible and apparently safe during peripheral angioplasty. Patients can safely be transported and monitored in an XMR interventional suite. An active quarter-wavelength guidewire coil does not provide superior imaging compared with IVUS, but provides satisfactory guidewire visualization. These tools may prove useful for advanced therapeutic procedures in the future.

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.

A Prototype Manipulator for Magnetic Resonance-Guided Interventions Inside Standard Cylindrical Magnetic Resonance Imaging Scanners

The aim of this work is to develop a remotely controlled manipulator to perform minimally invasive diagnostic and therapeutic interventions in the abdominal and thoracic cavities, with real-time magnetic resonance imaging (MRI) guidance inside clinical cylindrical MR scanners. The manipulator is composed of a three degree of freedom Cartesian motion system, which resides outside the gantry of the scanner, and serves as the holder and global positioner of a three degree of freedom arm which extends inside the gantry of the scanner At its distal end, the arm's end-effector can carry an interventional tool such as a biopsy needle, which can be advanced to a desired depth by means of a seventh degree of freedom. These seven degrees of freedom, provided by the entire assembly, offer extended manipulability to the device and a wide envelope of operation to the user, who can select a trajectory suitable for the procedure. The device is constructed of nonmagnetic and nonconductive fiberglass, and carbon fiber composite materials, to minimize artifacts and distortion on the MR images as well as eliminate effects on its operation from the high magnetic field and the fast switching magnetic field gradients used in MR imaging. A user interface was developed for man-in-the-loop control of the device using real-time MR images. The user interface fuses all sensor signals (MR and manipulator information) in a visualization, planning, and control command environment. Path planning is performed with graphical tools for setting the trajectory of insertion of the interventional tool using multislice and/or three dimensional MR images which are refreshed in real time. The device control is performed with an embedded computer which runs real-time control software. The manipulator compatibility with the MR environment and image-guided operation was tested on a 1.5 T MR scanner.

Visualization and tracking of an inflatable balloon catheter using SSFP in a flow phantom and in the heart and great vessels of patients

Passive catheter tracking involves direct interaction between the device and its surroundings, creating a local signal loss or enhancement of the image. Using only standard balloon catheters filled with CO(2) and imaged with a steady-state free precession sequence, it was possible to visualize and passively track catheters in a flow phantom and in the heart and great vessels of 20 patients without any additional image processing. The phantom work demonstrated that it was advantageous to sacrifice spatial resolution in order to increase temporal resolution. Frame rates greater than 10/sec were necessary for ease of catheter manipulation. Although only the tip of the catheter was visualized, this technique proved to be effective in patients undergoing cardiac catheterization.

Active device tracking and high-resolution intravascular MRI using a novel catheter-based, opposed-solenoid phased array coil

A novel two-element, catheter-based phased array coil was designed and built for both active MR device tracking and high-resolution vessel wall imaging. The device consists of two independent solenoid coils that are wound in opposite directions, connected to separate receive channels, and mounted collinearly on an angiographic catheter. The elements were used independently or together for tracking or imaging applications, respectively. The array's dual functionality was tested on a clinical 1.5 T MRI scanner in vitro, in vivo, and in situ. During real-time catheter tracking, each element gave rise to a high-amplitude peak in the respective projection data, which enabled reliable and robust device tracking as well as automated slice positioning. In vivo microimaging with 240 microm in-plane resolution was achieved in 9 s using the device and TrueFISP imaging. Therefore, a single device was successfully implemented that met the combined requirements of intravascular device tracking and imaging.

Real-time catheter tracking and adaptive imaging

PURPOSE

To evaluate the performance of a real-time MR system for interventional procedures that adjusts specific image parameters in real time based on a catheter's speed of insertion.

MATERIALS AND METHODS

The system was implemented using only the hardware provided with a standard short-bore 1.5 T scanner (Siemens Magnetom Sonata) (with the exception of small tracking markers affixed to the catheter). The system tracks the position of an MR microcoil-instrumented catheter and automatically updates the scan plane's position and orientation, as well as other features, including, but not limited to, field of view, resolution, tip angle, and TE. A real-time feedback loop continuously localizes the tracking markers, updates the scan plane position and orientation, calculates the catheter's speed, adjusts the value of specific image parameters, then collects new image data, reconstructs an image, and provides it for immediate display. The system was evaluated in phantom and in vivo porcine experiments.

RESULTS

The system is able to accurately localize a moving catheter in the abdominal aorta, calculate the device speed, and respond by adjusting specified image parameters 98% of the time, with precision of approximately 2 mm and 1.5 degrees.

CONCLUSION

Simply slowing the speed of the catheter allows the clinician to adjust predetermined image parameters. This work also has the potential to build a degree of intelligence into the scanner, enabling it to react to changes in the clinical environment and automatically optimize specific image parameters.

MR-trackable intramyocardial injection catheter

There is growing interest in delivering cellular agents to infarcted myocardium to prevent postinfarction left ventricular remodeling. MRI can be effectively used to differentiate infarcted from healthy myocardium. MR-guided delivery of cellular agents/therapeutics is appealing because the therapeutics can be precisely targeted to the desired location within the infarct. In this study, a steerable intramyocardial injection catheter that can be actively tracked under MRI was developed and tested. The components of the catheter were arranged to form a loopless RF antenna receiver coil that enabled active tracking. Feasibility studies were performed in canine and porcine myocardial infarction models. Myocardial delayed-enhancement (MDE) imaging identified the infarcted myocardium, and real-time MRI was used to guide left ventricular catheterization from a carotid artery approach. The distal 35 cm of the catheter was seen under MRI with a bright signal at the distal tip of the catheter. The catheter was steered into position, the distal tip was apposed against the infarct, the needle was advanced, and a bolus of MR contrast agent and tissue marker dye was injected intramyocardially, as confirmed by imaging and postmortem histology. A pilot study involving intramyocardial delivery of magnetically labeled stem cells demonstrated the utility of the active injection catheter system.

Interventional MR Angiography with a Floating Table

A floating table was integrated into a setup for performance of interventional magnetic resonance (MR) angiography procedures with actively visualized catheters and biplanar real-time image fusion. The setup was evaluated by performing catheterizations in eight pigs. The floating table enabled the authors to follow actively visualized instruments in the pigs' vasculature during MR imaging-guided interventional angiography procedures while performing real-time biplanar MR imaging. Interventional MR angiography with a floating table enables the field of view to be moved along with the instrument tip to the region of interest and thus enhances the usability and flexibility of the interventional MR imaging setup.

Endovascular interventional magnetic resonance imaging

Minimally invasive interventional radiological procedures, such as balloon angioplasty, stent placement or coiling of aneurysms, play an increasingly important role in the treatment of patients suffering from vascular disease. The non-destructive nature of magnetic resonance imaging (MRI), its ability to combine the acquisition of high quality anatomical images and functional information, such as blood flow velocities, perfusion and diffusion, together with its inherent three dimensionality and tomographic imaging capacities, have been advocated as advantages of using the MRI technique for guidance of endovascular radiological interventions. Within this light, endovascular interventional MRI has emerged as an interesting and promising new branch of interventional radiology. In this review article, the authors will give an overview of the most important issues related to this field. In this context, we will focus on the prerequisites for endovascular interventional MRI to come to maturity. In particular, the various approaches for device tracking that were proposed will be discussed and categorized. Furthermore, dedicated MRI systems, safety and compatibility issues and promising applications that could become clinical practice in the future will be discussed.

Intravascular MR imaging and intravascular MR-guided interventions

Intravascular MR technology, using an intravascularly placed MR receiver probe to acquire high-resolution angiographic MR images (i.e. intravascular MR imaging) and to guide cardiovascular interventional therapies (i.e. intravascular MR-guided interventions), is a new, very attractive development in the field of MR imaging. The new technology offers unique advantages for cardiovascular imaging and interventions, including superior contrast capability and multiplanar imaging capabilities without the use of contrast agents and with no risk of ionizing radiation. Thecombination of intravascular MR techniques with other advanced MR imaging techniques, such as functional MR imaging, will open new avenues for the future comprehensive management of cardiovascular atherosclerotic disease. Further improvements in intravascular MR fluoroscopy with true real-time display, analogous to X-ray fluoroscopy, will dramatically establish the role of intravascular MR technology in modern medicine.

Use of internal coils for independent and direct MR imaging-guided endovascular device tracking

PURPOSE

To test the hypotheses that a single internal guide wire coil (i) permits independent and direct depiction of guide wires and catheters and (ii) improves catheter-tracking accuracy and depiction compared to external receiver coils.

MATERIALS AND METHODS

Standard 5-6-F angiographic catheters were filled with dilute 4% gadolinium chelate. A single 0.030-inch-diameter internal guide wire coil was placed inside the catheter. True fast imaging with steady-state precession was used to directly visualize the guide wire. Inversion recovery-prepared fast low-angle shot technique was used to track catheters over a thick slice. In phantom experiments, we compared catheter signal-to-noise ratios (SNRs) with the internal coil and a phased-array surface coil with use of the Wilcoxon signed-rank test. Tip-tracking accuracy was assessed with use of linear regression. In pigs (n = 7), catheters and guide wires were independently tracked in real time.

RESULTS

In phantoms, catheter SNR with the internal coil (12.0) was significantly greater than that with the surface coil (4.0; P =.001). Tip-tracking accuracy was also improved with use of the internal coil (R(2) = 0.94 vs 0.50). In swine vasculature, catheters and guide wires could be directly and independently tracked at 1.7-2.0 frames per second. Catheters were clearly visualized with use of the internal coil, with a typical catheter background contrast-to-noise ratio of 6.6. Catheters were not visible with use of the external coil because of the small catheter size compared to the slice thickness.

CONCLUSION

Internal guide wire coils permit independent and direct depiction of guide wires and catheters in vivo for MR imaging-guided endovascular interventions. They also improve catheter tracking accuracy and depiction compared to external coils.

Interventional MRA using actively visualized catheters, TrueFISP, and real-time image fusion

An integrated system for performing interventional magnetic resonance angiography (MRA) with actively visualized instruments and real-time image fusion was implemented on a 1.5 T scanner. True fast imaging with steady precession (TrueFISP) imaging provided high acquisition speed paired with high signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) for the simultaneous visualization of active instruments and arterial morphology. The system enabled simultaneous image reconstruction and image postprocessing of multiple receiver channels, with subsequent image fusion display in real time. Optional interleaved image acquisition in two planes provided additional important information for biplanar instrument guidance. Various vascular interventions, including selective catheterization and subsequent selective MRA of the abdominal aorta, renal arteries, superior mesenteric artery (SMA), hepatic artery, and aortic arch, were performed on 10 pigs under MR guidance. In terms of instrument contrast, image acquisition, reconstruction, and fusion speed, the setup represents a powerful platform for performing interventional MRA procedures.

Interventionelle MRA: Konzepte zur aktiven Visualisierung von Kathetern und Stents

A precondition to safe guidance of vascular guidewires and catheters during the course of magnetic resonance (MR)-guided vascular intervention is a high-contrast visualization of the instruments. The integration of miniature radiofrequency (RF) coils and coaxial cables into guidewires and catheters enables the reception of RF signal from the lumen of blood vessels, and thus the active visualization of the instruments. Moreover, metallic vascular implants (stents) can be modified to act as intravascular RF antennas that inductively couple their RF signal to a conventional surface RF coil. Such stent resonators show signal amplification inside the lumen of the stent and thus can be visualized with high contrast in MR images. Furthermore, once such a device has been implanted, the method offers the potential for non-invasive long-term follow-up of the stent patency.

RF heating due to conductive wires during MRI depends on the phase distribution of the transmit field

In many studies concerning wire heating during MR imaging, a "resonant wire length" that maximizes RF heating is determined. This may lead to the nonintuitive conclusion that adding more wire, so as to avoid this resonant length, will actually improve heating safety. Through a theoretical analysis using the method of moments, we show that this behavior depends on the phase distribution of the RF transmit field. If the RF transmit field has linear phase, with slope equal to the real part of the wavenumber in the tissue, long wires always heat more than short wires. In order to characterize the intrinsic safety of a device without reference to a specific body coil design, this maximum-tip heating phase distribution must be considered. Finally, adjusting the phase distribution of the electric field generated by an RF transmit coil may lead to an "implant-friendly" coil design.

Remote control of catheter tip deflection: an opportunity for interventional MRI

This study seeks to exploit the high magnetic field environment of a clinical MRI scanner and demonstrate the technical feasibility of developing a catheter whose tip can be remotely oriented within the magnetic field by applying a DC current to a coil wound around the catheter tip to generate a magnetic moment and consequent deflection. To achieve arbitrary three-dimensional deflections, a three-axis coil was wound on a 1.5 Fr cylindrical catheter. By applying DC currents in the 100 mA range, this catheter was successfully guided through a 3D phantom maze, mimicking the vasculature, under MR imaging guidance. Feasibility was demonstrated that the strong ambient magnetic field of the MR scanner offers a special opportunity to develop simple devices that can be remotely steered to sites of clinical interest.

Catheter-based endomyocardial injection with real-time magnetic resonance imaging

BACKGROUND

We tested the feasibility of targeted left ventricular (LV) mural injection using real-time MRI (rtMRI).

METHODS AND RESULTS

A 1.5T MRI scanner was customized with a fast reconstruction engine, transfemoral guiding catheter-receiver coil (GCC), MRI-compatible needle, and tableside consoles. Commercial real-time imaging software was customized to facilitate catheter navigation and visualization of injections at 4 completely refreshed frames per second. The aorta was traversed and the left ventricular cavity was entered under direct rtMRI guidance. Pigs underwent multiple injections with dilute gadolinium-DTPA. All myocardial segments were readily accessed. The active GCC and the passive Stiletto needle injector were readily visualized. More than 50 endomyocardial injections were performed with the aid of rtMRI; 81% were successful with this first-generation prototype.

CONCLUSION

Percutaneous endomyocardial drug delivery is feasible with the aid of rtMRI, which permits precise 3-dimensional localization of injection within the LV wall.

Multifunctional interventional devices for MRI: a combined electrophysiology/MRI catheter

The design and application of a two-wire electrophysiology (EP) catheter that simultaneously records the intracardiac electrogram and receives the MR signal for active catheter tracking is described. The catheter acts as a long loop receiver, allowing for visualization of the entire catheter length while simultaneously behaving as a traditional two-wire EP catheter, allowing for intracardiac electrogram recording and ablation. The application of the device is demonstrated by simultaneously tracking the catheter and recording the intracardiac electrogram in canine models using 7 and 10 frame/sec real-time imaging sequences. Using solely MR imaging, the entire catheter was visualized and guided from the jugular vein into the cardiac chambers, where the intracardiac electrogram was recorded. By combining several functions in a single, simple structure, the excellent tissue contrast and functional imaging capabilities of MR can be used to improve the efficacy of EP interventions. This catheter will facilitate MR-guided interventions and demonstrates the design of multifunctional interventional devices for use in MRI.

A brief review of hardware for catheter tracking in magnetic resonance imaging

Magnetic resonance imaging (MRI) has traditionally been used exclusively in a role for patient diagnosis. However, it is unlikely that this role is sufficient for its continued prominence in medical imaging. Instead, the more ambitious role in diagnosis and also therapy/intervention will occur as demand for minimally invasive procedures increases. Fortunately, with recent improvement in technical specifications and creative pulse sequence design, MRI systems can now provide high quality near-real-time images that facilitate a variety of image-guided procedures, many based around delivery via catheters. While X-ray opacity is not available as a means for detecting the progression of the catheter in MRI systems today, a variety of novel hardware devices have been designed and used for MRI catheter tracking. This report provides a brief review of some fundamental methods for catheter tracking in MRI.

A method for fast 3D tracking using tuned fiducial markers and a limited projection reconstruction FISP (LPR-FISP) sequence

This work demonstrates the feasibility of using wireless, tuned fiducial markers with a limited projection reconstruction-fast imaging with steady-state free precession sequence (LPR-FISP) to accurately obtain tracking information necessary for interactive scan plane selection in magnetic resonance imaging (MRI). The position and orientation of a rigid interventional device can be uniquely determined from the 3D coordinates of three fiducial markers mounted in a known configuration on the device. Three fiducial markers were tuned to the proton resonant frequency in a 0.2T open MR scanner and mounted to the surface of a cylindrical water phantom. An LPR-FISP sequence was developed to suppress the water phantom signal while preserving that of the fiducial markers through a nonselective low-tip-angle excitation and a dephaser gradient applied prior to data acquisition. A localization algorithm was developed to accurately calculate the 3D coordinates of the fiducial markers using four LPR-FISP projections in two orthogonal scan planes. The sequence repetition time (TR = 21 msec) and the limited projection set resulted in fast LPR-FISP coordinate acquisition times of approximately 170 msec with an accuracy (max error) of 3 mm on a 0.2T MR system. This fast, accurate tracking method provides the fundamental technology for interactive MRI scan plane definition for rigid interventional devices without the need for stereotactic cameras or reference frames.

Thermal injuries associated with MRI

Most physicians are aware of the absolute contraindications to magnetic resonance imaging (MRI). However, less familiar is the potential for an MRI-induced thermal or electrical burn associated with electrical monitoring devices. Although detailed studies concerning the burn hazard in MRI have not been reported, it is widely believed that direct electromagnetic induction in looped cables associated with the patient is responsible for the excessive heating and it is on this theory that present guidelines are based. Recent reports have however indicated that other mechanisms may cause the heating of metal, either in or on the patient. This document reviews numerous reported burn injuries sustained during MRI and addresses the underlying heating mechanisms possibly causing these events.

Investigation of the factors responsible for burns during MRI

Numerous reported burn injuries have been sustained during clinical MRI procedures. The aim of this study was to investigate the possible factors that may be responsible for such burns. Experiments were performed to investigate three possible mechanisms for causing heating in copper wire during MRI: direct electromagnetic induction in a conductive loop, induction in a resonant conducting loop, and electric field resonant coupling with a wire (the antenna effect). Maximum recorded temperature rises were 0.6 degrees C for the loop, 61.1 degrees C for the resonant loop, and 63.5 degrees C for the resonant antenna. These experimental findings suggest that, contrary to common belief, it is unlikely that direct induction in a conductive loop will result in thermal injury. Burn incidents are more likely to occur due to the formation of resonant conducting loops and from extended wires forming resonant antenna. The characteristics of resonance should be considered when formulating safety guidelines.

Catheter tracking for MR fluoroscopy: design of a transmit/receive coil for use with a nasogastric tube

A coil tuned to 21.3 MHz was incorporated into a nasogastric tube and used as a marker of tube position during magnetic resonance (MR) imaging in a 0.5-T scanner. Catheter tracking was investigated with the coil used in both transmit/receive and in receive-only modes. Data acquired from this coil were overlaid on images obtained using the body coil of the scanner. Visualization of the full length of the catheter with local high signal at the tip was achieved with a temporal resolution of approximately 1 second. J. Magn. Reson. Imaging 2001;13:127-130.

Development of an MR-safe tracking catheter with a laser-driven tip coil

We developed a magnetic resonance (MR)-safe tracking catheter using an optical fiber with a light-diffusing tip segment to transport laser energy through the catheter. This energy is converted to a DC current running through a small coil at the catheter tip. Our method is inherently MR-safe since the use of long conducting wires is avoided. The intravoxel dephasing induced by the tip coil was clearly visible for laser powers between 250 mW and 750 mW for all angular positions of the catheter. J. Magn. Reson. Imaging 2001;13:131-135.