Endovascular Magnetic Resonance Imaging Applications
The size, motion, and contrast characteristics of the target are major determinants for an effective implementation of an MR-guided procedure. It is attractive to target large and thick-walled peripheral arteries, which are relatively immobile, because images can be acquired at acceptable frame rates with relatively high SNR and spatial resolution. Structures that can be imaged within a single plane (such as straight segments of iliac arteries) are more appropriate for MR-guided interventions compared with moving and tortuous structures such as coronary arteries, which are even difficult to image in a non–real-time mode of MRI.
Speuntrup et al91
demonstrated device navigation and delivery of passively visualized stainless steel stents in the coronary arteries of healthy swine. The devices and proximal coronary arteries were visible using SSFP sequences at 1.5 T. Cardiac and respiratory motion and the small size of coronary arteries especially in diseased states, make this procedure very difficult. Unless there is a major technologic advancement, it is fair to say that coronary artery interventions are not going to be in the forefront for interventional MRI in the near future. Although it gives much more functional information (eg, viability and perfusion) for cardiac myocardial tissue, MR is not near the temporal (15–30 fps) and spatial (200 μm) resolution currently enjoyed by x-ray fluoroscopy. This is unfortunate because coronary diseases are one of the most important health problems in the world.
The most common and straightforward MRI-guided vascular applications have been transluminal angioplasty42,43,92-96
and stent deployment45,48,49,52,97,98
in peripheral arteries. These have been conducted in various animal models. Similarly, investigators have reported placement of vena cava filters99-102
and transcatheter visceral embolization.103,104
Other groups have reported rtMRI catheter manipulation using selective arteriography, tracking microcoil–based catheters for selective carotid artery catheterization,105
passive catheters over active guide wires for selective coronary arteriography,106
or active catheters for selective visceral artery catheterization.107
These provided important proofs of concept toward clinical development, and a few human examples of peripheral artery angioplasty and stent deployment have already been reported.44,108
These and other human experience in MR-guided interventions will be reviewed at the end of this section.
Critics have asserted that angioplasty and stent deployment do not justify changing imaging modalities to MRI. Certain applications, however, may be particularly well-suited for MRI guidance.
Raval et al45
performed MR-guided stenting of aortic coarctation in a pig model. They deliberately used oversized balloons and showed that continuous imaging could reveal catastrophic aortic rupture as it happens. This might offer a potential safety advantage over conventional radiography. In another application, the vessel trajectories in total arterial occlusion cannot be determined using radiography because the occluded lumen will not fill with contrast to become visible; on the other hand, MRI can visualize vascular spaces even if they are completely occluded. In a pig model of chronic total occlusion of peripheral arteries, Raval et al109
navigated an active recanalizing guide wire under MR guidance. Magnetic resonance imaging allowed the operators to see and traverse occlusions while remaining within the walls of the vessels.
Aortic aneurysm might present a complex tortuous 3-dimensional structure that could be difficult to visualize using x-ray imaging. Magnetic resonance imaging as a single imaging modality can be used in planning, device deployment, and anatomic and hemodynamic assessment before, during, and after the procedure. In pig models, this was demonstrated first using passive110
and later using custom active devices with endograft stent ().111
Treatment under MRI has been shown to restore normal lumen contour and laminar flow in the vessel. More importantly, MRI could demonstrate device apposition and allow interrogation for endoleak.112
Recently, unmodified, passive stent graft devices have been used under MR guidance in an animal model of thoracic aortic dissection.113
Magnetic resonance imaging could reveal the true and false lumena of the dissected aorta, guide stent-graft deployment, and immediately demonstrate stent-graft obliteration of the false lumen.
The ventricular myocardium is an attractive target for interventional procedures because its large, thick-walled structure is depicted in high contrast in MRI despite cardiac and respiratory motion. Several cardiac procedures ranging from cardiac catheterization to myocardial injections have been performed under MR guidance and will be examined next.
Schalla et al114
conducted a comprehensive diagnostic cardiac catheterization procedure in a porcine model of atrial septal defect under MR guidance. Using catheters containing tracking microcoils embedded near the tip, they were easily able to accomplish catheterization of the left and right parts of heart, including continuous intracavitary pressure monitoring and blood sampling (). Shunt fractions were then measured using phase-contrast MRI.
FIGURE 5 Diagnostic cardiovascular catheterization in swine using a tracking-microcoil active catheter. The ‘+’ marker overlaid on the image indicates the tip of the catheter as it advances from the inferior vena cava (A) into the right atrium (more ...)
Several research groups have used MRI to deliver cells and other materials into specified targets in normal and infarcted animal hearts.30,115-119
Multiple slices could be rendered in 3 dimensions to represent their true relationship to each other. Comparable visualization in a beating heart is not available with any other modality, even during open-chest surgery (). When the injectate includes a contrast agent, dispersion of the injected material can be seen directly. This could be valuable for confirming successful delivery, for assuring confluence of treated volumes, and for avoiding overlapping injections. Targeting, in theory, can be based on wall motion, delayed hyperenhancement (infarction), perfusion defects, strain maps, or any other contrast mechanism.
FIGURE 6 Multislice view of targeted myocardial injection of iron-labeled mesenchymal stromal cells into a small myocardial infarction. This is a rotated view showing both the long axis and short axis of the myocardium. The injection guiding catheter is shown (more ...)
Several groups demonstrated the value of combined tissue and device imaging for the precise placement of prosthetic devices in the heart. A passively visualized nitinol occluder device was positioned to treat porcine models of atrial septal defect, which could be combined with hemodynamics assessment using phase-contrast MRI.120-122
A preliminary experience of deployment of a passively visualized, nitinol-based aortic valve prosthesis has been shown from a transfemoral approach in healthy swine.123
Under x-ray guidance, several catheter-based valve treatments are being field tested already.124
Magnetic resonance imaging might be particularly useful in guiding the deployment of such devices in relationship to coronary artery origins and other critical anatomic structures. Atrial septal puncture could provide a direct access to mitral valve for more complex treatments, even replacement, in the future.
Currently, various therapeutic cardiac electrophysiology procedures are conducted using catheter techniques without real image guidance; for example, simple endocardial surface maps are generated using electromagnetic catheter localization techniques during electrophysiological recordings. These rough surfaces might be useful only as roadmaps. They do not account for respiratory and other dynamic changes in cardiac tissues. Therefore, most catheter-based ablation procedures are ultimately guided by multichannel intracardiac electrograms. Surgical exposure often proves more simple and effective, although considerably more morbid for the patient.125
Magnetic resonance imaging might provide similar or even better visualization and might enable image-guided transcatheter ablation of cardiac arrhythmia. Magnetic resonance imaging may be particularly useful for visualizing lines of continuity (corresponding to functional electrophysiologic block) after the delivery of ablative energy.126
Unlike myocardial injections, the treatment is already known to be effective, and there is a larger group of patients and physicians who could potentially benefit from such procedures.
Initial work on MRI-guided cardiac electrophysiologic procedures is encouraging. Preliminary catheter-tracking experiments using active catheters with specific electronic filters, allowing them to acquire local intracardiac electrograms, have been reported.127
Magnetic resonance imaging allows for the characterization of ablated myocardium immediately and over time.128
Additional groups have presented animal experiments positioning electrophysiology catheters in an MRI system ().76,129,130
Several laboratories are currently working on image-guided therapeutic myocardial ablation in animal models.
FIGURE 7 MRI-guided electrophysiologic mapping using active-tracking microcoils. A, MR imaging of this porcine model of healed myocardial infarction reveals the hyperenhanced anterior wall scar (blue arrows), to which an MRI-compatible mapping catheter was manipulated (more ...)
Magnetic resonance imaging may enable interventions not restricted to normal lumen spaces. Arepally et al131
conducted image-guided puncture of the cardiac interatrial septum, a procedure currently conducted primarily using tactile feedback under x-ray guidance. This septal puncture could be followed by balloon septostomy.132
Although, similarly, guidance is afforded by intracardiac or transesophageal US, these are important preclinical steps toward more elaborate procedures guided by MRI. Magnetic resonance imaging also allows quantitative assessment of the resulting intracardiac shunts.
Following the theme of going beyond vessel lumen spaces, Kee et al have conducted preclinical50
transjugular intrahepatic portosystemic shunt procedures using a unique double-doughnut MRI configuration containing an integrated flat-panel x-ray fluoroscopy system. Magnetic resonance imaging reduced the number of transhepatic needle punctures compared with historical controls. Arepally et al134,135
conducted even more difficult preclinical experiments in creating a catheter-based mesocaval shunt outside the liver capsule. These investigators hope to access the splenic and pancreatic venous system using this approach for future biological treatments. This sort of extraanatomic bypass, once made available to nonsurgeons, has the potential to revolutionize mechanical revascularization.
Several groups have already reported investigational MR-guided procedures in patients. Razavi et al39
have conducted diagnostic cardiac catheterization in children using a combined x-ray/MRI (XMR) environment. During MR-guided cardiac catheterization in humans, they examined total pulmonary arterial compliance using MR flow quantification and invasive pressure monitoring.136,137
The same group is also conducting x-ray fused with MRI procedures, in which previous MRI datasets are combined with real-time x-ray fluoroscopy to conduct therapeutic procedures.138
Kuehne et al40
have conducted diagnostic cardiac catheterization procedures using passive catheter devices under rtMRI. Three groups have reported invasive imaging of peripheral artery atheromata using profile-design active guidewire receiver coils.68-70
The used guidewire probably adds little to surface coils for diagnostic MRI of atherosclerosis,139
but it might have value in delivering interventional devices. High-quality, selective intraarterial MR angiography140,141
and preliminary revascularization procedures have been reported using passive devices in the iliac and femoral arteries.44,51,142
In addition, MR-assisted transjugular intrahepatic portosystemic shunt procedures in patients have been shown in a novel hybrid double-doughnut XMR system.133
Human examples of peripheral artery angioplasty and stenting have been mentioned before,44,108
but these used only passive devices. Numerous additional investigational human MR-guided endovascular procedures are now under way in several medical centers around the world.