Interventional cardiovascular magnetic resonance: still tantalizing

Interventional cardiovascular magnetic resonance: state-of-the-art

Transcatheter cardiovascular interventions increasingly rely on advanced imaging. X-ray fluoroscopy provides excellent visualization of catheters and devices, but poor visualization of anatomy. In contrast, magnetic resonance imaging (MRI) provides excellent visualization of anatomy and can generate real-time imaging with frame rates similar to X-ray fluoroscopy. Realization of MRI as a primary imaging modality for cardiovascular interventions has been slow, largely because existing guidewires, catheters and other devices create imaging artifacts and can heat dangerously. Nonetheless, numerous clinical centers have started interventional cardiovascular magnetic resonance (iCMR) programs for invasive hemodynamic studies or electrophysiology procedures to leverage the clear advantages of MRI tissue characterization, to quantify cardiac chamber function and flow, and to avoid ionizing radiation exposure. Clinical implementation of more complex cardiovascular interventions has been challenging because catheters and other tools require re-engineering for safety and conspicuity in the iCMR environment. However, recent innovations in scanner and interventional device technology, in particular availability of high performance low-field MRI scanners could be the inflection point, enabling a new generation of iCMR procedures. In this review we review these technical considerations, summarize contemporary clinical iCMR experience, and consider potential future applications.

RF-induced heating of interventional devices at 23.66 MHz

OBJECTIVE

Low-field MRI systems are expected to cause less RF heating in conventional interventional devices due to lower Larmor frequency. We systematically evaluate RF-induced heating of commonly used intravascular devices at the Larmor frequency of a 0.55 T system (23.66 MHz) with a focus on the effect of patient size, target organ, and device position on maximum temperature rise.

MATERIALS AND METHODS

To assess RF-induced heating, high-resolution measurements of the electric field, temperature, and transfer function were combined. Realistic device trajectories were derived from vascular models to evaluate the variation of the temperature increase as a function of the device trajectory. At a low-field RF test bench, the effects of patient size and positioning, target organ (liver and heart) and body coil type were measured for six commonly used interventional devices (two guidewires, two catheters, an applicator and a biopsy needle).

RESULTS

Electric field mapping shows that the hotspots are not necessarily localized at the device tip. Of all procedures, the liver catheterizations showed the lowest heating, and a modification of the transmit body coil could further reduce the temperature increase. For common commercial needles no significant heating was measured at the needle tip. Comparable local SAR values were found in the temperature measurements and the TF-based calculations.

CONCLUSION

At low fields, interventions with shorter insertion lengths such as hepatic catheterizations result in less RF-induced heating than coronary interventions. The maximum temperature increase depends on body coil design.

MRI-guided endovascular intervention: current methods and future potential

INTRODUCTION

Image-guided endovascular interventions, performed using the insertion and navigation of catheters through the vasculature, have been increasing in number over the years, as minimally invasive procedures continue to replace invasive surgical procedures. Such endovascular interventions are almost exclusively performed under x-ray fluoroscopy, which has the best spatial and temporal resolution of all clinical imaging modalities. Magnetic resonance imaging (MRI) offers unique advantages and could be an attractive alternative to conventional x-ray guidance, but also brings with it distinctive challenges.

AREAS COVERED

In this review, the benefits and limitations of MRI-guided endovascular interventions are addressed, systems and devices for guiding such interventions are summarized, and clinical applications are discussed.

EXPERT OPINION

MRI-guided endovascular interventions are still relatively new to the interventional radiology field, since significant technical hurdles remain to justify significant costs and demonstrate safety, design, and robustness. Clinical applications of MRI-guided interventions are promising but their full potential may not be realized until proper tools designed to function in the MRI environment are available. Translational research and further preclinical studies are needed before MRI-guided interventions will be practical in a clinical interventional setting.

Sustainable low-field cardiovascular magnetic resonance in changing healthcare systems

Cardiovascular disease continues to be a major burden facing healthcare systems worldwide. In the developed world, cardiovascular magnetic resonance (CMR) is a well-established non-invasive imaging modality in the diagnosis of cardiovascular disease. However, there is significant global inequality in availability and access to CMR due to its high cost, technical demands as well as existing disparities in healthcare and technical infrastructures across high-income and low-income countries. Recent renewed interest in low-field CMR has been spurred by the clinical need to provide sustainable imaging technology capable of yielding diagnosticquality images whilst also being tailored to the local populations and healthcare ecosystems. This review aims to evaluate the technical, practical and cost considerations of low field CMR whilst also exploring the key barriers to implementing sustainable MRI in both the developing and developed world.

Role of Cross-Sectional Imaging in Pediatric Interventional Cardiac Catheterization

Management of congenital heart disease (CHD) has recently increased utilization of cross-sectional imaging to plan percutaneous interventions. Cardiac computed tomography (CT) and cardiac magnetic resonance (CMR) imaging have become indispensable tools for pre-procedural planning prior to intervention in the pediatric cardiac catheterization lab. In this article, we review several common indications for referral and the impact of cross-sectional imaging on procedural planning, success, and patient surveillance.

MRI-Guided Cardiac Catheterization in Congenital Heart Disease: How to Get Started

PURPOSE OF REVIEW

Cardiac magnetic resonance imaging provides radiation-free, 3-dimensional soft tissue visualization with adjunct hemodynamic data, making it a promising candidate for image-guided transcatheter interventions. This review focuses on the benefits and background of real-time magnetic resonance imaging (MRI)-guided cardiac catheterization, guidance on starting a clinical program, and recent research developments.

RECENT FINDINGS

Interventional cardiac magnetic resonance (iCMR) has an established track record with the first entirely MRI-guided cardiac catheterization for congenital heart disease reported nearly 20 years ago. Since then, many centers have embarked upon clinical iCMR programs primarily performing diagnostic MRI-guided cardiac catheterization. There have also been limited reports of successful real-time MRI-guided transcatheter interventions. Growing experience in performing cardiac catheterization in the magnetic resonance environment has facilitated practical workflows appropriate for efficiency-focused cardiac catheterization laboratories. Most exciting developments in imaging technology, MRI-compatible equipment and MRI-guided novel transcatheter interventions have been limited to preclinical research. Many of these research developments are ready for clinical translation. With increasing iCMR clinical experience and translation of preclinical research innovations, the time to make the leap to radiation-free procedures is now.

Interventional cardiac magnetic resonance imaging: current applications, technology readiness level, and future perspectives

BACKGROUND

Cardiac magnetic resonance (CMR) provides excellent temporal and spatial resolution, tissue characterization, and flow measurements. This enables major advantages when guiding cardiac invasive procedures compared with X-ray fluoroscopy or ultrasound guidance. However, clinical implementation is limited due to limited availability of technological advancements in magnetic resonance imaging (MRI) compatible equipment. A systematic review of the available literature on past and present applications of interventional MR and its technology readiness level (TRL) was performed, also suggesting future applications.

METHODS

A structured literature search was performed using PubMed. Search terms were focused on interventional CMR, cardiac catheterization, and other cardiac invasive procedures. All search results were screened for relevance by language, title, and abstract. TRL was adjusted for use in this article, level 1 being in a hypothetical stage and level 9 being widespread clinical translation. The papers were categorized by the type of procedure and the TRL was estimated.

RESULTS

Of 466 papers, 117 papers met the inclusion criteria. TRL was most frequently estimated at level 5 meaning only applicable to in vivo animal studies. Diagnostic right heart catheterization and cavotricuspid isthmus ablation had the highest TRL of 8, meaning proven feasibility and efficacy in a series of humans.

CONCLUSION

This article shows that interventional CMR has a potential widespread application although clinical translation is at a modest level with TRL usually at 5. Future development should be directed toward availability of MR-compatible equipment and further improvement of the CMR techniques. This could lead to increased TRL of interventional CMR providing better treatment.

Interventional cardiac MRI using an add-on parallel transmit MR system: In vivo experience in sheep

PURPOSE

We present in vivo testing of a parallel transmit system intended for interventional MR-guided cardiac procedures.

METHODS

The parallel transmit system was connected in-line with a conventional 1.5 Tesla MRI system to transmit and receive on an 8-coil array. The system used a current sensor for real-time feedback to achieve real-time current control by determining coupling and null modes. Experiments were conducted on 4 Charmoise sheep weighing 33.9-45.0 kg with nitinol guidewires placed under X-ray fluoroscopy in the atrium or ventricle of the heart via the femoral vein. Heating tests were done in vivo and post-mortem with a high RF power imaging sequence using the coupling mode. Anatomical imaging was done using a combination of null modes optimized to produce a useable B₁ field in the heart.

RESULTS

Anatomical imaging produced cine images of the heart comparable in quality to imaging with the quad mode (all channels with the same amplitude and phase). Maximum observed temperature increases occurred when insulation was stripped from the wire tip. These were 4.1℃ and 0.4℃ for the coupling mode and null modes, respectively for the in vivo case; increasing to 6.0℃ and 1.3℃, respectively for the ex vivo case, because cooling from blood flow is removed. Heating < 0.1℃ was observed when insulation was not stripped from guidewire tips. In all tests, the parallel transmit system managed to reduce the temperature at the guidewire tip.

CONCLUSION

We have demonstrated the first in vivo usage of an auxiliary parallel transmit system employing active feedback-based current control for interventional MRI with a conventional MRI scanner.

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.

Real-time device tracking under MRI using an acousto-optic active marker

PURPOSE

This work aims to demonstrate the use of an "active" acousto-optic marker with enhanced visibility and reduced radiofrequency (RF) -induced heating for interventional MRI.

METHODS

The acousto-optic marker was fabricated using bulk piezoelectric crystal and π-phase shifted fiber Bragg grating (FBGs) and coupled to a distal receiver coil on an 8F catheter. The received MR signal is transmitted over an optical fiber to mitigate RF-induced heating. A photodetector converts the optical signal into electrical signal, which is used as the input signal to the MRI receiver plug. Acousto-optic markers were characterized in phantom studies. RF-induced heating risk was evaluated according to ASTM 2182 standard. In vivo real-time tracking capability was tested in an animal model under a 0.55T scanner.

RESULTS

Signal-to-noise ratio (SNR) levels suitable for real-time tracking were obtained by using high sensitivity FBG and piezoelectric transducer with resonance matched to Larmor frequency. Single and multiple marker coils integrated to 8F catheters were readout for position and orientation tracking by a single acousto-optic sensor. RF-induced heating was significantly reduced compared to a coax cable connected reference marker. Real-time distal tip tracking of an active device was demonstrated in an animal model with a standard real-time cardiac MR sequence.

CONCLUSION

Acousto-optic markers provide sufficient SNR with a simple structure for real-time device tracking. RF-induced heating is significantly reduced compared to conventional active markers. Also, multiple RF receiver coils connected on an acousto-optic modulator can be used on a single catheter for determining catheter orientation and shape.

Susceptibility artifacts from metallic markers and cardiac catheterization devices on a high-performance 0.55 T MRI system

INTRODUCTION

Visualization of passive devices during MRI-guided catheterizations often relies on a susceptibility artifact from the device itself or added susceptibility markers that impart a unique imaging signature. High-performance low field MRI systems offer reduced RF-induced heating of metallic devices during MRI-guided invasive procedures, but susceptibility artifacts are expected to diminish with field strength, reducing device visualization. In this study, field strength and orientation dependence of artifacts from susceptibility markers and metallic guidewires were evaluated using a prototype high-performance 0.55 T MRI system.

MATERIALS AND METHODS

Artifact volume from nitinol and stainless steel passive susceptibility markers was quantified using histogram analysis of pixel intensities from three-dimensional gradient echo images at 0.55 T, 1.5 T and 3 T. In addition, visibility of commercially available clinical catheterization devices was compared between 0.55 T and 1.5 T using real-time bSSFP in phantoms and in vivo.

RESULTS

A low-tensile strength stainless-steel marker produced field strength- and orientation-dependent artifact size (1.7 cm³, 1.95 cm³, 2.21 cm³ at 0.55 T, 1.5 T, 3 T, respectively). Whereas, a high-tensile strength steel marker, of the same alloy, produced field strength- and orientation-independent artifact size (3.35 cm³, 3.41 cm³, 3.42 cm³ at 0.55 T, 1.5 T, 3 T, respectively). Visibility of commercially available nitinol guidewires was reduced at 0.55 T, but imaging signature could be maintained using high-susceptibility stainless steel markers.

DISCUSSION AND CONCLUSION

High-susceptibility stainless-steel markers generate field-independent artifacts between 0.55 T, 1.5 T and 3 T, indicating magnetic saturation at fields <0.55 T. Thus, artifact size can be tailored such that interventional devices produce identical imaging signatures across field strengths.

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.

Invasive cardiovascular magnetic resonance (iCMR) for diagnostic right and left heart catheterization using an MR-conditional guidewire and passive visualization in congenital heart disease

BACKGROUND

Today's standard of care, in the congenital heart disease (CHD) population, involves performing cardiac catheterization under x-ray fluoroscopy and cardiac magnetic resonance (CMR) imaging separately. The unique ability of CMR to provide real-time functional imaging in multiple views without ionizing radiation exposure has the potential to be a powerful tool for diagnostic and interventional procedures. Limiting fluoroscopic radiation exposure remains a challenge for pediatric interventional cardiologists. This pilot study's objective is to establish feasibility of right (RHC) and left heart catheterization (LHC) during invasive CMR (iCMR) procedures at our institution in the CHD population. Furthermore, we aim to improve simultaneous visualization of the catheter balloon tip, MR-conditional guidewire, and cardiac/vessel anatomy during iCMR procedures.

METHODS

Subjects with CHD were enrolled in a pilot study for iCMR procedures at 1.5 T with an MR-conditional guidewire. The CMR area is located adjacent to a standard catheterization laboratory. Using the interactive scanning mode for real-time control of the imaging location, a dilute gadolinium-filled balloon-tip catheter was used in combination with an MR-conditional guidewire to obtain cardiac saturations and hemodynamics. A recently developed catheter tracking technique using a real-time single-shot balanced steady-state free precession (bSSFP), flip angle (FA) 35-45°, echo time (TE) 1.3 ms, repetition time (TR) 2.7 ms, 40° partial saturation (pSAT) pre-pulse was used to visualize the gadolinium-filled balloon, MR-conditional guidewire, and cardiac structures simultaneously. MR-conditional guidewire visualization was enabled due to susceptibility artifact created by distal markers. Pre-clinical phantom testing was performed to determine the optimum imaging FA-pSAT combination.

RESULTS

The iCMR procedure was successfully performed to completion in 31/34 (91%) subjects between August 1st, 2017 to December 13th, 2018. Median age and weight were 7.7 years and 25.2 kg (range: 3 months - 33 years and 8 - 80 kg). Twenty-one subjects had single ventricle (SV) anatomy: one subject was referred for pre-Glenn evaluation, 11 were pre-Fontan evaluations and 9 post-Fontan evaluations for protein losing enteropathy (PLE) and/or cyanosis. Thirteen subjects had bi-ventricular (BiV) anatomy, 4 were referred for coarctation of the aorta (CoA) evaluations, 3 underwent vaso-reactivity testing with inhaled nitric oxide, 3 investigated RV volume dimensions, two underwent branch PA stenosis evaluation, and the remaining subject was status post heart transplant. No catheter related complications were encountered. Average time taken for first pass RHC, LHC/aortic pull back, and to cross the Fontan fenestration was 5.2, 3.0, and 6.5 min, respectively. Total success rate to obtain required data points to complete Fick principle calculations for all patients was 331/337 (98%). Subjects were transferred to the x-ray fluoroscopy lab if further intervention was required including Fontan fenestration device closure, balloon angioplasty of pulmonary arteries/conduits, CoA stenting, and/or coiling of aortopulmonary (AP) collaterals. Starting with subject #10, an MR-conditional guidewire was used in all subsequent subjects (15 SV and 10 BiV) with a success rate of 96% (24/25). Real-time CMR-guided RHC (25/25 subjects, 100%), retrograde and prograde LHC/aortic pull back (24/25 subjects, 96%), CoA crossing (3/4 subjects, 75%) and Fontan fenestration test occlusion (2/3 subjects, 67%) were successfully performed in the majority of subjects when an MR-conditional guidewire was utilized.

CONCLUSION

Feasibility for detailed diagnostic RHC, LHC, and Fontan fenestration test occlusion iCMR procedures in SV and BiV pediatric subjects with complex CHD is demonstrated with the aid of an MR-conditional guidewire. A novel real-time pSAT GRE sequence with optimized FA-pSAT angle has facilitated simultaneous visualization of the catheter balloon tip, MR-conditional guidewire, and cardiac/vessel anatomy during iCMR procedures.

MRI Catheterization: Ready for Broad Adoption

In recent years, interventional cardiac magnetic resonance imaging (iCMR) has evolved from attractive theory to clinical routine at several centers. Real-time cardiac magnetic resonance imaging (CMR fluoroscopy) adds value by combining soft-tissue visualization, concurrent hemodynamic measurement, and freedom from radiation. Clinical iCMR applications are expanding because of advances in catheter devices and imaging. In the near future, iCMR promises novel procedures otherwise unsafe under standalone X-Ray guidance.

MR safety watchdog for active catheters: Wireless impedance control with real-time feedback

PURPOSE

To dynamically minimize radiofrequency (RF)-induced heating of an active catheter through an automatic change of the termination impedance.

METHODS

A prototype wireless module was designed that modifies the input impedance of an active catheter to keep the temperature rise during MRI below a threshold, ΔT_max . The wireless module (MR safety watchdog; MRsWD) measures the local temperature at the catheter tip using either a built-in thermistor or external data from a fiber-optical thermometer. It automatically changes the catheter input impedance until the temperature rise during MRI is minimized. If ΔT_max is exceeded, RF transmission is blocked by a feedback system.

RESULTS

The thermistor and fiber-optical thermometer provided consistent temperature data in a phantom experiment. During MRI, the MRsWD was able to reduce the maximum temperature rise by 25% when operated in real-time feedback mode.

CONCLUSION

This study demonstrates the technical feasibility of an MRsWD as an alternative or complementary approach to reduce RF-induced heating of active interventional devices. The automatic MRsWD can reduce heating using direct temperature measurements at the tip of the catheter. Given that temperature measurements are intrinsically slow, for a clinical implementation, a faster feedback parameter would be required such as the RF currents along the catheter or scattered electric fields at the tip.

Opportunities in Interventional and Diagnostic Imaging by Using High-Performance Low-Field-Strength MRI

Background Commercial low-field-strength MRI systems are generally not equipped with state-of-the-art MRI hardware, and are not suitable for demanding imaging techniques. An MRI system was developed that combines low field strength (0.55 T) with high-performance imaging technology. Purpose To evaluate applications of a high-performance low-field-strength MRI system, specifically MRI-guided cardiovascular catheterizations with metallic devices, diagnostic imaging in high-susceptibility regions, and efficient image acquisition strategies. Materials and Methods A commercial 1.5-T MRI system was modified to operate at 0.55 T while maintaining high-performance hardware, shielded gradients (45 mT/m; 200 T/m/sec), and advanced imaging methods. MRI was performed between January 2018 and April 2019. T1, T2, and T2* were measured at 0.55 T; relaxivity of exogenous contrast agents was measured; and clinical applications advantageous at low field were evaluated. Results There were 83 0.55-T MRI examinations performed in study participants (45 women; mean age, 34 years ± 13). On average, T1 was 32% shorter, T2 was 26% longer, and T2* was 40% longer at 0.55 T compared with 1.5 T. Nine metallic interventional devices were found to be intrinsically safe at 0.55 T (<1°C heating) and MRI-guided right heart catheterization was performed in seven study participants with commercial metallic guidewires. Compared with 1.5 T, reduced image distortion was shown in lungs, upper airway, cranial sinuses, and intestines because of improved field homogeneity. Oxygen inhalation generated lung signal enhancement of 19% ± 11 (standard deviation) at 0.55 T compared with 7.6% ± 6.3 at 1.5 T (P = .02; five participants) because of the increased T1 relaxivity of oxygen (4.7e-4 mmHg-1sec-1). Efficient spiral image acquisitions were amenable to low field strength and generated increased signal-to-noise ratio compared with Cartesian acquisitions (P < .02). Representative imaging of the brain, spine, abdomen, and heart generated good image quality with this system. Conclusion This initial study suggests that high-performance low-field-strength MRI offers advantages for MRI-guided catheterizations with metal devices, MRI in high-susceptibility regions, and efficient imaging. © RSNA, 2019 Online supplemental material is available for this article. See also the editorial by Grist in this issue.

Anesthetic considerations for magnetic resonance imaging-guided right-heart catheterization in pediatric patients: A single institution experience

Cardiac catheterization is an integral part of medical management for pediatric patients with congenital heart disease. Owing to age and lack of cooperation in children who need this procedure, general anesthesia is typically required. These patients have increased anesthesia risk secondary to cardiac pathology. Furthermore, multiple catheterization procedures result in exposure to harmful ionizing radiation. Magnetic resonance imaging-guided right-heart catheterization offers decreased radiation exposure and diagnostic imaging benefits over traditional fluoroscopy but potentially increases anesthetic complexity and risk. We describe our early experience with anesthetic techniques and challenges for pediatric magnetic resonance imaging-guided right-heart catheterization.

Physiological Recording in the MRI Environment (PRiME): MRI-Compatible Hemodynamic Recording System

Hemodynamic recording during interventional cardiovascular procedures is essential for procedural guidance, monitoring patient status, and collection of diagnostic information. Recent advances have made interventions guided by magnetic resonance imaging (MRI) possible and attractive in certain clinical scenarios. However, in the MRI environment, electromagnetic interference (EMI) can cause severe distortions and artifacts in acquired hemodynamic waveforms. The primary aim of this paper was to develop and validate a system to minimize EMI on electrocardiogram (ECG) and invasive blood pressure (IBP) signals. A system was developed which incorporated commercial MRI compatible ECG leads and pressure transducers, custom electronics, user interface, and adaptive signal processing. Measurements were made on pediatric patients (N = 6) during MRI-guided catheterization. Real-time interactive scanning, which is known to produce significant EMI due to fast gradient switching and varying imaging plane orientations, was selected for testing. The effectiveness of the adaptive algorithms was determined by measuring the reduction of noise peaks, amplitude of noise peaks, and false QRS triggers. During real-time gradient-intensive imaging sequences, peak noise amplitude was reduced by 80% and false QRS triggers were reduced to a median of 0. There was no detectable interference on the IBP channels. A hemodynamic recording system front-end was successfully developed and deployed, which enabled high-fidelity recording of ECG and IBP during MRI scanning. The schematics and assembly instructions are publicly available to facilitate implementation at other institutions. Researchers and clinicians are provided a critical tool in investigating and implementing MRI guided interventional cardiovascular procedures.

Catadioptric Stereo Tracking for Three Dimensional Shape Measurement of MRI Guided Catheters

The recent introduction of Magnetic Resonance Imaging (MRI)-actuated steerable catheters lays the ground work for increasing the efficacy of cardiac catheter procedures. The MRI, while capable of imaging the catheter for tracking and control, does not fulfill all of the needs required to identify and develop a complete catheter model. Specifially, the frequency response of the catheter must be identified to ensure stable control of the catheter system. This requires a higher frequency imaging than the MRI can achieve. This work uses a catadioptric stereo camera system consisting of a mirror and a single camera in order to track a MRI actuated catheter inside a MRI machine. The catadioptric system works in parallel to the MRI and is capable of recording the catheter at 60 fps for post processing. The accuracy of the catadioptric system is verified in imaging conditions that would be found inside the MRI. The stereo camera is then used to track a catheter as it is actuated inside the MRI.

Endovascular MR-guided Renal Embolization by Using a Magnetically Assisted Remote-controlled Catheter System

Purpose To assess the feasibility of a magnetically assisted remote-controlled (MARC) catheter system under magnetic resonance (MR) imaging guidance for performing a simple endovascular procedure (ie, renal artery embolization) in vivo and to compare with x-ray guidance to determine the value of MR imaging guidance and the specific areas where the MARC system can be improved. Materials and Methods In concordance with the Institutional Animal Care and Use Committee protocol, in vivo renal artery navigation and embolization were tested in three farm pigs (mean weight 43 kg ± 2 [standard deviation]) under real-time MR imaging at 1.5 T. The MARC catheter device was constructed by using an intramural copper-braided catheter connected to a laser-lithographed saddle coil at the distal tip. Interventionalists controlled an in-room cart that delivered electrical current to deflect the catheter in the MR imager. Contralateral kidneys were similarly embolized under x-ray guidance by using standard clinical catheters and guidewires. Changes in renal artery flow and perfusion were measured before and after embolization by using velocity-encoded and perfusion MR imaging. Catheter navigation times, renal parenchymal perfusion, and renal artery flow rates were measured for MR-guided and x-ray-guided embolization procedures and are presented as means ± standard deviation in this pilot study. Results Embolization was successful in all six kidneys under both x-ray and MR imaging guidance. Mean catheterization time with MR guidance was 93 seconds ± 56, compared with 60 seconds ± 22 for x-ray guidance. Mean changes in perfusion rates were 4.9 au/sec ± 0.8 versus 4.6 au/sec ± 0.6, and mean changes in renal flow rate were 2.1 mL/min/g ± 0.2 versus 1.9 mL/min/g ± 0.2 with MR imaging and x-ray guidance, respectively. Conclusion The MARC catheter system is feasible for renal artery catheterization and embolization under real-time MR imaging in vivo, and quantitative physiologic measures under MR imaging guidance were similar to those measured under x-ray guidance, suggesting that the MARC catheter system could be used for endovascular procedures with interventional MR imaging. (©) RSNA, 2016.

Interventional-Cardiovascular MR: Role of the Interventional MR Technologist

BACKGROUND

Interventional-cardiovascular magnetic resonance (iCMR) is a promising clinical tool for adults and children who need a comprehensive hemodynamic catheterization of the heart. Magnetic resonance (MR) imaging-guided cardiac catheterization offers radiation-free examination with increased soft tissue contrast and unconstrained imaging planes for catheter guidance. The interventional MR technologist plays an important role in the care of patients undergoing such procedures. It is therefore helpful for technologists to understand the unique iCMR preprocedural preparation, procedural and imaging workflows, and management of emergencies. The authors report their team's experience from the National Institutes of Health Clinical Center and a collaborating pediatric site.

Magnetic resonance imaging compatible remote catheter navigation system with 3 degrees of freedom

PURPOSE

To facilitate MRI-guided catheterization procedures, we present an MRI-compatible remote catheter navigation system that allows remote navigation of steerable catheters with 3 degrees of freedom.

METHODS

The system consists of a user interface (master), a robot (slave), and an ultrasonic motor control servomechanism. The interventionalist applies conventional motions (axial, radial and plunger manipulations) on an input catheter in the master unit; this user input is measured and used by the servomechanism to control a compact catheter manipulating robot, such that it replicates the interventionalist's input motion on the patient catheter. The performance of the system was evaluated in terms of MRI compatibility (SNR and artifact), feasibility of remote navigation under real-time MRI guidance, and motion replication accuracy.

RESULTS

Real-time MRI experiments demonstrated that catheter was successfully navigated remotely to desired target references in all 3 degrees of freedom. The system had an absolute value error of [Formula: see text]1 mm in axial catheter motion replication over 30 mm of travel and [Formula: see text] for radial catheter motion replication over [Formula: see text]. The worst case SNR drop was observed to be [Formula: see text]3 %; the robot did not introduce any artifacts in the MR images.

CONCLUSION

An MRI-compatible compact remote catheter navigation system has been developed that allows remote navigation of steerable catheters with 3 degrees of freedom. The proposed system allows for safe and accurate remote catheter navigation, within conventional closed-bore scanners, without degrading MR image quality.

Positive contrast spiral imaging for visualization of commercial nitinol guidewires with reduced heating

BACKGROUND

CMR-guidance has the potential to improve tissue visualization during cardiovascular catheterization procedures and to reduce ionizing radiation exposure, but a lack of commercially available CMR guidewires limits widespread adoption. Standard metallic guidewires are considered to be unsafe in CMR due to risks of RF-induced heating. Here, we propose the use of RF-efficient gradient echo (GRE) spiral imaging for reduced guidewire heating (low flip angle, long readout), in combination with positive contrast for guidewire visualization.

METHODS

A GRE spiral sequence with 8 interleaves was used for imaging. Positive contrast was achieved using through-slice dephasing such that the guidewire appeared bright and the background signal suppressed. Positive contrast images were interleaved with anatomical images, and real-time image processing was used to produce a color overlay of the guidewire on the anatomy. Temperature was measured with a fiber-optic probe attached to the guidewire in an acrylic gel phantom and in vivo.

RESULTS

Left heart catheterization was performed on swine using the real-time color overlay for procedural guidance with a frame rate of 6.25 frames/second. Using our standard Cartesian real-time imaging (flip angle 60°), temperature increases up to 50 °C (phantom) and 4 °C (in vivo) were observed. In comparison, spiral GRE images (8 interleaves, flip angle 10°) generated negligible heating measuring 0.37 °C (phantom) and 0.06 °C (in vivo).

CONCLUSIONS

The ability to use commercial metallic guidewires safely during CMR-guided catheterization could potentially expedite clinical translation of these methods.

Controlling radiofrequency-induced currents in guidewires using parallel transmit

PURPOSE

Elongated conductors, such as pacemaker leads, neurostimulator leads, and conductive guidewires used for interventional procedures can couple to the MRI radiofrequency (RF) transmit field, potentially causing dangerous tissue heating. The purpose of this study was to demonstrate the feasibility of using parallel transmit to control induced RF currents in elongated conductors, thereby reducing the RF heating hazard.

METHODS

Phantom experiments were performed on a four-channel parallel transmit system at 1.5T. Parallel transmit "null mode" excitations that induce minimal wire current were designed using coupling measurements derived from axial B1 (+) maps. The resulting current reduction performance was evaluated with B1 (+) maps, current sensor measurements, and fluoroptic temperature probe measurements.

RESULTS

Null mode excitations reduced the maximum coupling mode current by factors ranging from 2 to 80. For the straight wire experiment, a current null imposed at a single wire location was sufficient to reduce tip heating below detectable levels. For longer insertion lengths and a curved geometry, imposing current nulls at two wire locations resulted in more distributed current reduction along the wire length.

CONCLUSION

Parallel transmit can be used to create excitations that induce minimal RF current in elongated conductors, thereby decreasing the RF heating risk, while still allowing visualization of the surrounding volume.

Magnetic Resonance Imaging-Guided Cardiac Interventions

Performing intraoperative cardiovascular procedures inside an MR imaging scanner can potentially provide substantial advantage in clinical outcomes by reducing the risk and increasing the success rate relative to the way such procedures are performed today, in which the primary surgical guidance is provided by X-ray fluoroscopy, by electromagnetically tracked intraoperative devices, and by ultrasound. Both noninvasive and invasive cardiologists are becoming increasingly familiar with the capabilities of MR imaging for providing anatomic and physiologic information that is unequaled by other modalities. As a result, researchers began performing animal (preclinical) interventions in the cardiovascular system in the early 1990s.

Magnetic Resonance Sequences and Rapid Acquisition for MR-Guided Interventions

Interventional MR uses rapid imaging to guide diagnostic and therapeutic procedures. One of the attractions of MR-guidance is the abundance of inherent contrast mechanisms available. Dynamic procedural guidance with real-time imaging has pushed the limits of MR technology, demanding rapid acquisition and reconstruction paired with interactive control and device visualization. This article reviews the technical aspects of real-time MR sequences that enable MR-guided interventions.

Dual echo positive contrast bSSFP for real-time visualization of passive devices during magnetic resonance guided cardiovascular catheterization

BACKGROUND

Cardiovascular magnetic resonance (CMR) guided cardiovascular catheterizations can potentially reduce ionizing radiation exposure and enable new interventions. Commercially available paramagnetic X-Ray devices create a small signal void in CMR images, which is ambiguous and insufficient to guide catheterization procedures. This work aims to improve real-time CMR of off-the-shelf X-Ray devices by developing a real-time positive contrast sequence with color overlay of the device onto anatomy.

METHODS

A dual-echo bSSFP sequence was used to generate both a dephased positive contrast image and bSSFP image simultaneously. A variable flip angle scheme was implemented to reduce the specific absorption rate (SAR) and hence device heating. Image processing was used to isolate the device from background signal, and the device was overlaid in color on the anatomy, mimicking active device visualization. Proof-of-concept experiments were performed using a commercially available nitinol guidewire for left heart catheterization in Yorkshire swine.

RESULTS

The dual echo pulse sequence generated a temporal resolution of 175 ms (5.7 frames/second) with GRAPPA acceleration factor 4. Image processing was performed in real-time and color overlay of the device on the anatomy was displayed to the operator with no latency. The color overlay accurately depicted the guidewire location, with minimal background contamination, during left heart catheterization.

CONCLUSIONS

The ability to effectively visualize commercially available X-Ray devices during CMR-guided cardiovascular catheterizations, combined with safe low-SAR pulse sequences, could potentially expedite the clinical translation of interventional CMR.

Wireless mobile technology to improve workflow and feasibility of MR-guided percutaneous interventions

PURPOSE

A wireless interactive display and control device combined with a platform-independent web-based user interface (UI) was developed to improve the workflow for interventional magnetic resonance imaging (iMRI).

METHODS

The iMRI-UI enables image acquisition of up to three independent slices using various pulse sequences with different contrast weighting. Pulse sequence, scan geometry and related parameters can be changed on the fly via the iMRI-UI using a tablet computer for improved lesion detection and interventional device targeting. The iMRI-UI was validated for core biopsies with a liver phantom ([Formula: see text] [Formula: see text] 40) and Thiel soft-embalmed human cadavers ([Formula: see text] [Formula: see text] 24) in a clinical 1.5T MRI scanner.

RESULTS

The iMRI-UI components and setup were tested and found conditionally MRI-safe to use according to current ASTM standards. Despite minor temporary touch screen interference at a close distance to the bore ([Formula: see text]20 cm), no other issues regarding quality or imaging artefacts were observed. The 3D root-mean-square distance error was [Formula: see text] (phantom)/[Formula: see text] mm (cadaver), and overall procedure times ranged between 12 and 22 (phantom)/20 and 55 min (cadaver).

CONCLUSION

The wireless iMRI-UI control setup enabled fast and accurate interventional biopsy needle placements along complex trajectories and improved the workflow for percutaneous interventions under MRI guidance in a preclinical trial.

Using a low-amplitude RF pulse at echo time (LARFET) for device localization in MRI

We describe a new method for frequency down-conversion of MR signals acquired with the radio-frequency projections method for device localization. A low-amplitude, off-center RF pulse applied simultaneously with the echo signal is utilized as the reference for frequency down-conversion. Because of the low-amplitude and large offset from the Larmor frequency, the RF pulse minimally interfered with magnetic resonance of protons. We conducted an experiment with the coil placed at different positions to verify this concept. The down-converted signal was transformed into optical signal and transmitted via fiber-optic cable to a receiver unit placed outside the scanner room. The position of the coil could then be determined by the frequency analysis of this down-converted signal and superimposed on previously acquired MR images for comparison. Because of minimal positional errors (≤ 0.8 mm), this new device localization method may be adequate for most interventional MRI applications.

Preclinical feasibility of a technology framework for MRI-guided iliac angioplasty

PURPOSE

Interventional MRI has significant potential for image guidance of iliac angioplasty and related vascular procedures. A technology framework with in-room image display, control, communication and MRI-guided intervention techniques was designed and tested for its potential to provide safe, fast and efficient MRI-guided angioplasty of the iliac arteries.

METHODS

A 1.5-T MRI scanner was adapted for interactive imaging during endovascular procedures using new or modified interventional devices such as guidewires and catheters. A perfused vascular phantom was used for testing. Pre-, intra- and post-procedural visualization and measurement of vascular morphology and flow was implemented. A detailed analysis of X-ray fluoroscopic angiography workflow was conducted and applied. Two interventional radiologists and one physician in training performed 39 procedures. All procedures were timed and analyzed.

RESULTS

MRI-guided iliac angioplasty procedures were successfully performed with progressive adaptation of techniques and workflow. The workflow, setup and protocol enabled a reduction in table time for a dedicated MRI-guided procedure to 6 min 33 s with a mean procedure time of 9 min 2 s, comparable to the mean procedure time of 8 min 42 s for the standard X-ray-guided procedure.

CONCLUSIONS

MRI-guided iliac vascular interventions were found to be feasible and practical using this framework and optimized workflow. In particular, the real-time flow analysis was found to be helpful for pre- and post-interventional assessments. Design optimization of the catheters and in vivo experiments are required before clinical evaluation.

Interventional device visualization with toroidal transceiver and optically coupled current sensor for radiofrequency safety monitoring

PURPOSE

The development of catheters and guidewires that are safe from radiofrequency (RF) -induced heating and clearly visible against background tissue is a major challenge in interventional MRI. An interventional imaging approach using a toroidal transmit-receive (transceive) coil is presented. This toroidal transceiver allows controlled, low levels of RF current to flow in the catheter/guidewire for visualization, and can be used with conductive interventional devices that have a localized low-impedance tip contact.

METHODS

Toroidal transceivers were built, and phantom experiments were performed to quantify transmit power levels required for device visibility and to detect heating hazards. Imaging experiments in a pig cadaver tested the extendibility to higher field strength and nonphantom settings. A photonically powered optically coupled toroidal current sensor for monitoring induced RF currents was built, calibrated, and tested using an independent image-based current estimation method.

RESULTS

Results indicate that high signal-to-noise ratio visualization is achievable using milliwatts of transmit power-power levels orders of magnitude lower than levels that induce measurable heating in phantom tests. Agreement between image-based current estimates and RF current sensor measurements validates sensor accuracy.

CONCLUSION

The toroidal transceiver, integrated with power and current sensing, could offer a promising platform for safe and effective interventional device visualization.

Magnetically assisted remote-controlled endovascular catheter for interventional MR imaging: in vitro navigation at 1.5 T versus X-ray fluoroscopy

PURPOSE

To compare in vitro navigation of a magnetically assisted remote-controlled (MARC) catheter under real-time magnetic resonance (MR) imaging with manual navigation under MR imaging and standard x-ray guidance in endovascular catheterization procedures in an abdominal aortic phantom.

MATERIALS AND METHODS

The 2-mm-diameter custom clinical-grade microcatheter prototype with a solenoid coil at the distal tip was deflected with a foot pedal actuator used to deliver 300 mA of positive or negative current. Investigators navigated the catheter into branch vessels in a custom cryogel abdominal aortic phantom. This was repeated under MR imaging guidance without magnetic assistance and under conventional x-ray fluoroscopy. MR experiments were performed at 1.5 T by using a balanced steady-state free precession sequence. The mean procedure times and percentage success data were determined and analyzed with a linear mixed-effects regression analysis.

RESULTS

The catheter was clearly visible under real-time MR imaging. One hundred ninety-two (80%) of 240 turns were successfully completed with magnetically assisted guidance versus 144 (60%) of 240 turns with nonassisted guidance (P < .001) and 119 (74%) of 160 turns with standard x-ray guidance (P = .028). Overall mean procedure time was shorter with magnetically assisted than with nonassisted guidance under MR imaging (37 seconds ± 6 [standard error of the mean] vs 55 seconds ± 3, P < .001), and time was comparable between magnetically assisted and standard x-ray guidance (37 seconds ± 6 vs 44 seconds ± 3, P = .045). When stratified by angle of branch vessel, magnetic assistance was faster than nonassisted MR guidance at turns of 45°, 60°, and 75°.

CONCLUSION

In this study, a MARC catheter for endovascular navigation under real-time MR imaging guidance was developed and tested. For catheterization of branch vessels arising at large angles, magnetically assisted catheterization was faster than manual catheterization under MR imaging guidance and was comparable to standard x-ray guidance.

Design and evaluation of a hybrid radiofrequency applicator for magnetic resonance imaging and RF induced hyperthermia: electromagnetic field simulations up to 14.0 Tesla and proof-of-concept at 7.0 Tesla

This work demonstrates the feasibility of a hybrid radiofrequency (RF) applicator that supports magnetic resonance (MR) imaging and MR controlled targeted RF heating at ultrahigh magnetic fields (B₀≥7.0T). For this purpose a virtual and an experimental configuration of an 8-channel transmit/receive (TX/RX) hybrid RF applicator was designed. For TX/RX bow tie antenna electric dipoles were employed. Electromagnetic field simulations (EMF) were performed to study RF heating versus RF wavelength (frequency range: 64 MHz (1.5T) to 600 MHz (14.0T)). The experimental version of the applicator was implemented at B₀ = 7.0T. The applicators feasibility for targeted RF heating was evaluated in EMF simulations and in phantom studies. Temperature co-simulations were conducted in phantoms and in a human voxel model. Our results demonstrate that higher frequencies afford a reduction in the size of specific absorption rate (SAR) hotspots. At 7T (298 MHz) the hybrid applicator yielded a 50% iso-contour SAR (iso-SAR-50%) hotspot with a diameter of 43 mm. At 600 MHz an iso-SAR-50% hotspot of 26 mm in diameter was observed. RF power deposition per RF input power was found to increase with B₀ which makes targeted RF heating more efficient at higher frequencies. The applicator was capable of generating deep-seated temperature hotspots in phantoms. The feasibility of 2D steering of a SAR/temperature hotspot to a target location was demonstrated by the induction of a focal temperature increase (ΔT = 8.1 K) in an off-center region of the phantom. Temperature simulations in the human brain performed at 298 MHz showed a maximum temperature increase to 48.6C for a deep-seated hotspot in the brain with a size of (19×23×32)mm³ iso-temperature-90%. The hybrid applicator provided imaging capabilities that facilitate high spatial resolution brain MRI. To conclude, this study outlines the technical underpinnings and demonstrates the basic feasibility of an 8-channel hybrid TX/RX applicator that supports MR imaging, MR thermometry and targeted RF heating in one device.

Tracking the position and rotational orientation of a catheter using a transmit array system

A new method for detecting the rotational orientation and tracking the position of an inductively coupled radio frequency (ICRF) coil using a transmit array system is proposed. The method employs a conventional body birdcage coil, but the quadrature hybrid is eliminated so that the two excitation channels can be used separately. The transmit array system provides RF excitations such that the body birdcage coil creates linearly polarized and changing RF pulses instead of a conventional rotational forward-polarized excitation. The receive coils and their operations are not modified. Inductively coupled RF coils are constructed on catheters for detecting rotational orientation and for tracking purposes. Signals from the anatomy and from tissue close to the ICRF coil are different due to the new RF excitation scheme: the ICRF coil can be separated from the anatomy in real time, and after doing so, a color-coded image is reconstructed. More importantly, this novel method enables a real-time calculation of the absolute rotational orientation of an ICRF coil constructed on a catheter. Modified FLASH and TrueFISP sequences are used for the experiments. The acquired images from this technique show the feasibility of different applications, such as catheter tracking. Furthermore, applications where knowledge of the rotational orientation of the catheter is important, such as magnetic resonance-guided endoluminal-focused ultrasound, RF ablation, side-looking optical imaging, and catheters with side ports for needles, become feasible with this method.

Real-time multipoint gastrointestinal 19-fluorine catheter tracking

PURPOSE

To develop MR based real-time gastrointestinal 19-Fluorine (19F) catheter tracking and visualization allowing for real-time detection and feedback of 3D catheter shape and movement as well as catheter-driven adjustments of 1H imaging geometry parameters.

METHODS

Data were acquired on a 3T clinical system using 3D Golden Angle radial sampling. Two gastrointestinal catheters incorporating four fiducial 19F markers (65 or 50 µL marker volume) were tracked while being pulled through a gel phantom by an operator inside the MR room with velocities of 2-18 mm/s. During continuous acquisition, k-space profiles were transferred in real-time to an external computer for concurrent reconstruction of 3D 19F images and detection and visualization of marker positions. Based on αthe marker positions, automatic adjustments of 1H imaging planes to facilitate targeted anatomical scanning was implemented.

RESULTS

Mean tracking reliabilities were 94.5 and 83.6% (catheters 1 and 2) for temporal resolutions 185-740 ms. Reconstruction times of 196 ms were achieved. Real-time visual feedback allowed the operator to accurately control the catheter movement. Catheter-guidance for 1H imaging was reliable.

CONCLUSION

The presented real-time 19F MR based framework for the tracking of 19F labeled devices is applicable to combined 19F and 1H MRI guidance of gastrointestinal devices in vivo.

Transthoracic delivery of large devices into the left ventricle through the right ventricle and interventricular septum: preclinical feasibility

BACKGROUND

We aim to deliver large appliances into the left ventricle through the right ventricle and across the interventricular septum. This transthoracic access route exploits immediate recoil of the septum, and lower transmyocardial pressure gradient across the right versus left ventricular free wall. The route may enhance safety and allow subxiphoid rather than intercostal traversal.

METHODS

The entire procedure was performed under real-time CMR guidance. An "active" CMR needle crossed the chest, right ventricular free wall, and then the interventricular septum to deliver a guidewire then used to deliver an 18Fr introducer. Afterwards, the right ventricular free wall was closed with a nitinol occluder. Immediate closure and late healing of the unrepaired septum and free wall were assessed by oximetry, angiography, CMR, and necropsy up to four weeks afterwards.

RESULTS

The procedure was successful in 9 of 11 pigs. One failed because of refractory ventricular fibrillation upon needle entry, and the other because of inadequate guidewire support. In all ten attempts, the right ventricular free wall was closed without hemopericardium. There was neither immediate nor late shunt on oximetry, X-ray angiography, or CMR. The interventricular septal tract fibrosed completely. Transventricular trajectories planned on human CT scans suggest comparable intracavitary working space and less acute entry angles than a conventional atrial transseptal approach.

CONCLUSION

Large closed-chest access ports can be introduced across the right ventricular free wall and interventricular septum into the left ventricle. The septum recoils immediately and heals completely without repair. A nitinol occluder immediately seals the right ventricular wall. The entry angle is more favorable to introduce, for example, prosthetic mitral valves than a conventional atrial transseptal approach.

Real-time MRI-guided right heart catheterization in adults using passive catheters

AIMS

Real-time MRI creates images with superb tissue contrast that may enable radiation-free catheterization. Simple procedures are the first step towards novel interventional procedures. We aim to perform comprehensive transfemoral diagnostic right heart catheterization in an unselected cohort of patients entirely using MRI guidance.

METHODS AND RESULTS

We performed X-ray and MRI-guided transfemoral right heart catheterization in consecutive patients undergoing clinical cardiac catheterization. We sampled both cavae and both pulmonary arteries. We compared success rate, time to perform key steps, and catheter visibility among X-ray and MRI procedures using air-filled or gadolinium-filled balloon-tipped catheters. Sixteen subjects (four with shunt, nine with coronary artery disease, three with other) underwent paired X-ray and MRI catheterization. Complete guidewire-free catheterization was possible in 15 of 16 under both. MRI using gadolinium-filled balloons was at least as successful as X-ray in all procedure steps, more successful than MRI using air-filled balloons, and better than both in entering the left pulmonary artery. Total catheterization time and individual procedure steps required approximately the same amount of time irrespective of image guidance modality. Catheter conspicuity was best under X-ray and next-best using gadolinium-filled MRI balloons.

CONCLUSION

In this early experience, comprehensive transfemoral right heart catheterization appears feasible using only MRI for imaging guidance. Gadolinium-filled balloon catheters were more conspicuous than air-filled ones. Further workflow and device enhancement are necessary for clinical adoption.

Virtual dye angiography: flow visualization for MRI-guided interventions

In magnetic resonance imaging-guided cardiovascular interventional procedures, it is valuable to be able to visualize blood flow immediately and interactively in selected regions. In particular, it is useful to assess normal or pathological communications between specific heart chambers and vessels. Phase-contrast velocity mapping is not suitable for this purpose as it requires too much data and is not capable of determining directly if blood originating in one location travels to a nearby location. This article presents a novel flow visualization method called virtual dye angiography that enables visualization of blood flow analogous to selective catheter angiography. The method uses two-dimensional radio frequency pulses to achieve interactive, intermittent, targeted saturation of a localized region of the blood pool. The flow of the saturated spins is observed directly on real-time images or, in an enhanced manner, using ECG synchronized background subtraction. The modular nature of the technique allows for easy and seamless integration into a real-time, interactive imaging system with minimal overhead. We present initial results in animals and in a healthy human volunteer.

A deflectable guiding catheter for real-time MRI-guided interventions

PURPOSE

To design a deflectable guiding catheter that omits long metallic components yet preserves mechanical properties to facilitate therapeutic interventional MRI procedures.

MATERIALS AND METHODS

The catheter shaft incorporated Kevlar braiding. A 180° deflection was attained with a 5-cm nitinol slotted tube, a nitinol spring, and a Kevlar pull string. We tested three designs: passive, passive incorporating an inductively coupled coil, and active receiver. We characterized mechanical properties, MRI properties, RF induced heating, and in vivo performance in swine.

RESULTS

Torque and tip deflection force were satisfactory. Representative procedures included hepatic and azygos vein access, laser cardiac septostomy, and atrial septal defect crossing. Visualization was best in the active configuration, delineating profile and tip orientation. The passive configuration could be used in tandem with an active guidewire to overcome its limited conspicuity. There was no RF-induced heating in all configurations under expected use conditions in vitro and in vivo.

CONCLUSION

Kevlar and short nitinol component substitutions preserved mechanical properties. The active design offered the best visibility and usability but reintroduced metal conductors. We describe versatile deflectable guiding catheters with a 0.057" lumen for interventional MRI catheterization. Implementations are feasible using active, inductive, and passive visualization strategies to suit application requirements.

Closed-chest transthoracic magnetic resonance imaging-guided ventricular septal defect closure in swine

OBJECTIVES

The aim of this study was to close ventricular septal defects (VSDs) directly through the chest wall using magnetic resonance imaging (MRI) guidance, without cardiopulmonary bypass, sternotomy, or radiation exposure.

BACKGROUND

Surgical, percutaneous, and hybrid management of VSD each have limitations and known morbidity.

METHODS

Percutaneous muscular VSDs were created in 10 naive Yorkshire swine using a transjugular laser catheter. Under real-time MRI guidance, a direct transthoracic vascular access sheath was introduced through the chest into the heart along a trajectory suitable for VSD access and closure. Through this transthoracic sheath, muscular VSDs were occluded using a commercial nitinol device. Finally, the right ventricular free wall was closed using a commercial collagen plug intended for arterial closure.

RESULTS

Anterior, posterior, and mid-muscular VSDs (6.8 ± 1.8 mm) were created. VSDs were closed successfully in all animals. The transthoracic access sheath was displaced in 2, both fatal. Thereafter, we tested an intracameral retention sheath to prevent this complication. Right ventricular access ports were closed successfully in all, and after as many as 30 days, healed successfully.

CONCLUSIONS

Real-time MRI guidance allowed closed-chest transthoracic perventricular muscular VSD closure in a clinically meaningful animal model. Once applied to patients, this approach may avoid traditional surgical, percutaneous, or open-chest transcatheter ("hybrid") risks.

Reverse polarized inductive coupling to transmit and receive radiofrequency coil arrays

In this study, the reverse polarization method is implemented using transmit and receive arrays to improve the visibility of the interventional devices. Linearly polarized signal sources--inductively and receptively coupled radiofrequency coils--are used in the experimental setups to demonstrate the ability of the method to separate these sources from a forward polarized anatomy signal. Two different applications of the reverse polarization method are presented here: (a) catheter tracking and (b) fiducial marker visualization, in both of which transmit and receive arrays are used. The performance of the reverse polarization method was further tested with phantom and volunteer studies, and the results proved the feasibility of this method with transmit and receive arrays.