Challenges in MRI Catheterization
The developments in MRI catheterization have been subject to several reviews and editorials, with an enthusiasm for the technique stressing the excellent potential of this technique within routine clinical practice. However, MRI catheterization has been slow to develop over the past decade and its current use remains limited to a few centers.
The challenges to this development have remained fairly similar in that time, with recent advances outlined below. These relate to the MRI environment, scanning capabilities and the availability of MRI-compatible guide wires, catheters and devices.
MRI Environment
Most of the centers practicing XMR/MRI catheterization have purpose-built XMR/MRI laboratories, which are expensive and require a skilled multidisciplinary team, which includes clinicians with expertise in MRI and cardiac catheterization supported by MRI physicists and other technical specialists to establish a successful clinical program. Where this is not available, centers can employ a combination of x-ray-guided catheterization in a separate laboratory, with subsequent transfer of the patient to the MRI suite with MRI-compatible catheters in situ for diagnostic studies provided patient safety is not compromised. Access to the patient during catheterization has also been limited by the bore size of the scanner, but this is less of an issue with the newer generation of MRI scanners with a wider bore. Additionally, improved MRI-compatible hardware is being developed for hemodynamic monitoring of patients.
Catheter Tracking
Visualization of the catheters under MRI guidance has also been challenging. We employed passive catheter tracking in our unit, using the susceptibility artefact and resultant signal void from the carbon dioxide-filled balloon catheter balloon tips to track the catheter tip for manipulation. Other groups have advocated using gadolinium-filled balloon tips in place of carbon dioxide for improved catheter tip visualization and steering. Passive catheter tracking, however, lacks the ability to visualize the entire shaft of the catheter, which makes it susceptible to unrecognized coiling and twisting of the catheter. The alternative is active catheter tracking where the device is electrically connected to the MRI scanner and has a coil or antenna that is able to transmit or transmit and receive. Ratnayaka et al. have summarized the key aspects of comparison between active and passive catheter tracking. Active catheter tracking offers the best prospect for improved dynamic visualization and with new technology, that addresses safety issues, should become much more widely used in the future.
MRI-Guided Interventions
In the past MR-guided interventions have only been performed in animals due to lack of MR compatible and safe interventional equipment. These have included atrial septal defect device closure, transeptal puncture, stenting of aortic coarctation and percutaneous implantation of aortic valve. However, the lack of appropriate hardware has limited its application in humans where MRI remained an adjunct to x-ray guidance. The availability of a new MR-compatible and safe fiberglass passive guide wire led to a successful preclinical trial of solely MR-guided interventions leading to a first-in-man intervention performed by our group and applied to several other patients as described above. Although we were able to demonstrate the feasibility of performing solely MR-guided diagnostic cardiac catheterizations, the prototype guide wire did not have adequate mechanical properties for safe application despite satisfactory preclinical testing.
Catheter & Guidewire Development
There needs to be further developments in MR-guided catheter steering, which can be achieved either by using guide wires as mentioned previously or catheter development. Recent catheter developments to aid steering and visualization in the MRI environment include catheter tip ferromagnetic beads, catheter tip microcoils and microfluidic hydraulic catheters. The imaging of the latter is dependent on the nature of the hydraulic fluid. Smart material actuators in catheter development provide better steering without the benefit of improved visualization.
Further developments in this field are ongoing and new devices are being developed with improved mechanical properties. Once these devices have obtained regulatory approval then solely MRI-guided cardiac catheterization both for diagnostic and interventional purposes will become much more feasible.
Image Registration & Overlay
The increasingly complex nature of procedures will ultimately benefit from integrating a wide range of imaging modalities to impart a comprehensive view of the cardiac structures for the interventionists. It is crucial to consistently maintain correct alignment of the images from the different modalities being used. Image registration techniques are constantly improving to allow overlay of echocardiographic, MRI or CT images and fluoroscopy into the same image with improved alignment and minimal motion artefact from respiration.
Other Applications
Progress has been made since the first clinical application of MRI electrophysiology, with advances in its use limited to animals. Newer catheters with full diagnostic electrophysiology functionality with active tracking have been developed. Sommer et al. recently published the first series of patients with real-time MRI-guided placement of multiple catheters with subsequent performance of stimulation maneuvers in five adult patients. These developments are highly relevant to patients with congenital heart following complex surgery where there is a recognized burden of arrhythmia. Additionally, intramyocardial stem cell or gene treatments with MRI-guided and XMR catheterization has been shown to be technically possible in previous animal models and offers promising potential for due to the advantages in soft tissue visualization.