- Open Access
Remote control catheter navigation: options for guidance under MRI
© Muller et al; licensee BioMed Central Ltd. 2012
- Received: 30 January 2012
- Accepted: 1 June 2012
- Published: 1 June 2012
Image-guided endovascular interventions have gained increasing popularity in clinical practice, and magnetic resonance imaging (MRI) is emerging as an attractive alternative to X-ray fluoroscopy for guiding such interventions. Steering catheters by remote control under MRI guidance offers unique challenges and opportunities.
In this review, the benefits and limitations of MRI-guided remote control intervention are addressed, and the tools for guiding such interventions in the magnetic environment are summarized. Designs for remote control catheter guidance include a catheter tip electromagnetic microcoil design, a ferromagnetic sphere-tipped catheter design, smart material-actuated catheters, and hydraulically actuated catheters. Remote control catheter guidance systems were compared and contrasted with respect to visualization, safety, and performance. Performance is characterized by bending angles achievable by the catheter, time to achieve bending, degree of rotation achievable, and miniaturization capacity of the design. Necessary improvements for furthering catheter design, especially for use in the MRI environment, are addressed, as are hurdles that must be overcome in order to make MRI guided endovascular procedures more accessible for regular use in clinical practice.
MR-guided endovascular interventions under remote control steering are in their infancy due to issues regarding safety and reliability. Additional experimental studies are needed prior to their use in humans.
- Interventional MRI
- Remote control catheter guidance
Image-guided endovascular interventions have gained increasing popularity in clinical practice as studies have shown consistently that these minimally invasive interventions are of equivalent or greater efficacy and confer lower morbidity when compared to traditional open surgical techniques [1–8]. A majority of interventions involve the use of a flexible catheter that is guided by the interventionalist into the appropriate vessels  under real-time X-ray fluoroscopy imaging. However, while X-ray fluoroscopy provides clear navigation in patent vessels and allows high spatial and temporal resolution, there are drawbacks to its use (such as lack of soft tissue visualization), making magnetic resonance imaging (MRI) an attractive alternative for remotely steering catheters during endovascular interventions. Although prior limitations to MRI guided interventions have been overcome in the last decade, including the development of near real-time dynamic MR “fluoroscopic” imaging sequences, steering catheters within blood vessels remains challenging as compared to steering of catheters under X-ray guidance. The use of MRI for endovascular catheter navigation is a growing field of study with significant clinical promise, and many innovative techniques for guiding a catheter in the magnetic field of the MRI scanner are being proposed and tested [10, 11]. The development of these augmented catheter guidance techniques stands to break down one of the most significant barriers to adoption of MRI as a real-time interventional guidance modality.
Potential advantages of MRI for guiding interventions
A clear advantage of using real-time MRI for intervention lies in the wealth of physiologic and structural information provided by the MR image itself. By visualizing the soft tissue surrounding a blood vessel as opposed to only the vessel lumen, the interventionalist may assess the function of an organ as a procedure is performed . For example, in the case of treating an acute ischemic stroke caused by thromboembolic occlusion of a cerebral artery, it is possible to visualize the ischemic penumbra (via MR perfusion imaging) surrounding the core infarct (via MR diffusion weighted imaging) such that clinical determination of whether to reopen an occluded artery can occur while the intervention is being executed . This real-time evaluation of tissue damage could prevent hemorrhage upon reopening an artery into brain tissue that is already infarcted . Additionally, MR perfusion and thermometry allow for monitoring the effects of procedures such as thermal and cryo-ablations . Real-time tissue visualization is useful in guiding a variety of interventions, including, for example: tumor embolization, aneurysm occlusion, angioplasty, and myocardial stem cell delivery .
MRI has at least two potential advantages over X-ray fluoroscopy that may increase safety for the patient and the physician. First, MRI does not necessarily involve the injection of iodinated contrast, which has been associated with complications, including nephrotoxicity and anaphylaxis . Although contrast-enhanced MRA involves the use of gadolinium or blood pool agents, noncontrast MRA techniques allow vascular visualization without the attendant risks for contrast administration. Second, whereas X-ray fluoroscopy uses significant doses of potentially damaging ionizing radiation, MRI uses only lower energy, non-ionizing radiation that has no known long term deleterious health consequences .
Challenges for MRI guidance of interventions
MRI has its own challenges that must be overcome before it is used as a first line real-time intervention guidance system. Traditional devices and robotic systems designed for use in operating rooms and X-ray fluoroscopy suites that use electromagnetic components, such as actuators and sensors, are rendered useless or dangerous in the clinical high magnetic field scanners. A device incompatible with the magnetic environment produces susceptibility artifact that can negatively affect image quality . Additionally, the long, narrow bore of many MR scanners dictates that any tool used in combination with MRI must be compact or attached to electronic controllers outside the 5 Gauss line of the MR scanner’s fringe field .
Challenges for endovascular catheter navigation
Maneuverability of a catheter for intravascular navigation in any imaging environment is key to reaching the target area; ability to steer a catheter thus affects to a great extent the length and success of the procedure. Difficulty in steering and control of the catheter increase the risk for complications, including vascular dissection, perforation, and thrombosis . Some of these risks can be offset by systemic heparinization, which is in routine use in clinical endovascular procedures performed today, though may itself increase procedural hemorrhage risk. For many procedures the catheter is guided from a safe entrance vessel (e.g., common femoral artery) to a target that is relatively far away in the body via branching or tortuous vessels. With a traditional catheter, the catheter tip is directed by manually rotating the catheter about its axis and pushing it forward into the desired vessel, often over a variably stiff coaxially placed guidewire. After the catheter has been navigated through several vascular turns, the torque at the proximal end is hindered  and control of the guidewire tip is limited . Additionally, it is difficult to manipulate a catheter tip through sharp turns, for instance entering a recurrent branch vessel whose origin is directed at a greater than 90 degree orientation to the parent vessel .
Because perforation and dissection when navigating vessels or treating tissue are safety concerns when performing an intervention, a pressure sensor at the tip of the catheter is desirable. This desire must be weighed against the additional bulk that such sensors would add to catheter tips, as well as the potential for RF heating of additional conducting wires. Thus, larger catheters for use in larger blood vessels are easier to conceptualize than microcatheters for use in small cerebral or coronary vessels. To be compatible with MRI, the options for such a pressure sensor are somewhat limited, but include aluminum force sensors  and fiber optics .
Temperature sensing is an essential component of many procedures, such as thermal and cryo-ablation. While MR thermometry is within the capability of the scanner and could be used to obtain a general idea of temperatures within tissues, it is limited by interference from blood flow or other movement and by local distortion of the magnetic field by ferromagnetic components of the catheter. A more specific temperature sensor at the catheter tip would enable safer use of the catheter for interventions in which the tip is heated or cooled, either to affect a target tissue as in ablation, or to induce a change in catheter orientation for navigation. This temperature sensor must be able to be miniaturized so not to add bulkiness to the catheter; thermocouples are an obvious first step to achieving temperature sensing on a small scale.
For performance purposes, steerable MR-compatible catheters that include a specialized catheter tip should be designed to achieve the shortest length tip possible so that the modification does not in itself limit too harshly the turning radius of the catheter tip. Limitation of turning radius occurs if the length of coil needed to generate sufficient magnetic moment for turning is too long to make the turn into a vessel. To fully utilize the capability of MRI for real-time navigation and soft tissue imaging, any device to navigate a catheter in an MR scanner should be operable without having to pause imaging for an extended period of time. For many designs, this combination of control and visualization may be achieved through a compromise in which signals to elicit a change in position of the catheter are interleaved with signals that give feedback and images describing the anatomy. Finally, an intuitive control system by which the interventionalist may operate the device and thereby efficiently guide the catheter to the target is crucial to the realization of faster and safer catheterization for intervention under MR guidance.
Comparison of Remote Control Mechanisms for Catheter Tip Steering
Remote Control Mechanism
Catheter Tip Microcoils
· Minimal image distortion when current is off
· Current carrying wires in catheter may undergo RF heating
· Microcoils can act as receiver coils for active imaging
· Microcoils may undergo resistive heating
· Multilayer coils add bulk and stiffness to catheter tip
Catheter Tip Ferromagnetic Beads
· Low potential for catheter tip heating
· Image distortion by beads may be difficult to eliminate
· Simple design improves manufacturability
· Separation between beads may make tip long and rigid
· Possible peripheral nerve stimulation by gradients used for steering
· Discontinuous jumps in tip deflection
Smart Material Actuators
· Easily miniaturizable
· May be difficult to deliver enough heat to change tip shape without heating adjacent tissue
· Prolonged cooling times to return to original shape
· High hysteresis and non-linearity limit deflection precision
· Could add contrast agent to hydraulic fluid for better visualization
· Susceptible to temperature changes and fatigue during long procedures, limiting reliability
· Easily miniaturizable
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