Christina E. Saikus

Interventional cardiovascular magnetic resonance imaging: a new opportunity for image-guided interventions.

Cardiovascular magnetic resonance (CMR) combines excellent soft-tissue contrast, multiplanar views, and dynamic imaging of cardiac function without ionizing radiation exposure. Interventional cardiovascular magnetic resonance (iCMR) leverages these features to enhance conventional interventional procedures or to enable novel ones. Although still awaiting clinical deployment, this young field has tremendous potential. We survey promising clinical applications for iCMR. Next, we discuss the technologies that allow CMR-guided interventions and, finally, what still needs to be done to bring them to the clinic.

Adaptive noise cancellation to suppress electrocardiography artifacts during real-time interventional MRI.

To develop a system for artifact suppression in electrocardiogram (ECG) recordings obtained during interventional real‐time magnetic resonance imaging (MRI).

MRI active guidewire with an embedded temperature probe and providing a distinct tip signal to enhance clinical safety

BackgroundThe field of interventional cardiovascular MRI is hampered by the unavailability of active guidewires that are both safe and conspicuous. Heating of conductive guidewires is difficult to predict in vivo and disruptive to measure using external probes. We describe a clinical-grade 0.035” (0.89 mm) guidewire for MRI right and left heart catheterization at 1.5 T that has an internal probe to monitor temperature in real-time, and that has both tip and shaft visibility as well as suitable flexibility.MethodsThe design has an internal fiberoptic temperature probe, as well as a distal solenoid to enhance tip visibility on a loopless antenna. We tested different tip-solenoid configurations to balance heating and signal profiles. We tested mechanical performance in vitro and in vivo in comparison with a popular clinical nitinol guidewire.ResultsThe solenoid displaced the point of maximal heating (“hot spot”) from the tip to a more proximal location where it can be measured without impairing guidewire flexion. Probe pullback allowed creation of lengthwise guidewire temperature maps that allowed rapid evaluation of design prototypes. Distal-only solenoid attachment offered the best compromise between tip visibility and heating among design candidates. When fixed at the hot spot, the internal probe consistently reflected the maximum temperature compared external probes.Real-time temperature monitoring was performed during porcine left heart catheterization. Heating was negligible using normal operating parameters (flip angle, 45°; SAR, 1.01 W/kg); the temperature increased by 4.2°C only during high RF power mode (flip angle, 90°; SAR, 3.96 W/kg) and only when the guidewire was isolated from blood cooling effects by an introducer sheath. The tip flexibility and in vivo performance of the final guidewire design were similar to a popular commercial guidewire.ConclusionsWe integrated a fiberoptic temperature probe inside a 0.035” MRI guidewire. Real-time monitoring helps detect deleterious heating during use, without impairing mechanical guidewire operation, and without impairing MRI visibility. We therefore need not rely on prediction to ensure safe clinical operation. Future implementations may modulate specific absorption rate (SAR) based on temperature feedback.

MRI-Guided Vascular Access with an Active Visualization Needle

To develop an approach to vascular access under magnetic resonance imaging (MRI), as a component of comprehensive MRI‐guided cardiovascular catheterization and intervention.

Interventional MRI using multiple 3D angiography roadmaps with real-time imaging.

To enhance real‐time magnetic resonance (MR)‐guided catheter navigation by overlaying colorized multiphase MR angiography (MRA) and cholangiopancreatography (MRCP) roadmaps in an anatomic context.

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

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

Visualization of active devices and automatic slice repositioning ("SnapTo") for MRI-guided interventions.

The accurate visualization of interventional devices is crucial for the safety and effectiveness of MRI‐guided interventional procedures. In this paper, we introduce an improvement to the visualization of active devices. The key component is a fast, robust method (“CurveFind”) that reconstructs the three‐dimensional trajectory of the device from projection images in a fraction of a second. CurveFind is an iterative prediction‐correction algorithm that acts on a product of orthogonal projection images. By varying step size and search direction, it is robust to signal inhomogeneities. At the touch of a key, the imaged slice is repositioned to contain the relevant section of the device (“SnapTo”), the curve of the device is plotted in a three‐dimensional display, and the point on a target slice, which the device will intersect, is displayed. These features have been incorporated into a real‐time MRI system. Experiments in vitro and in vivo (in a pig) have produced successful results using a variety of single‐ and multichannel devices designed to produce both spatially continuous and discrete signals. CurveFind is typically able to reconstruct the device curve, with an average error of approximately 2 mm, even in the case of complex geometries. Magn Reson Med 63:1070–1079, 2010.

Limitations of Closing Percutaneous Transthoracic Ventricular Access Ports Using a Commercial Collagen Vascular Closure Device

Introduction: Closed‐chest access and closure of direct cardiac punctures may enable a range of therapeutic procedures. We evaluate the safety and feasibility of closing percutaneous direct ventricular access sites using a commercial collagen‐based femoral artery closure device. Methods: Yorkshire swine underwent percutaneous transthoracic left ventricular access (n = 13). The access port was closed using a commercial collagen‐based vascular closure device (Angio‐Seal, St. Jude Medical) with or without prior separation of the pericardial layers by instillation of fluid into the pericardial space (“permissive pericardial tamponade”). After initial nonsurvival feasibility experiments (n = 6); animals underwent 1‐week (n = 3) or 6‐week follow‐up (n = 4). Results: In naïve animals, the collagen plug tended to deploy outside the parietal pericardium, where it failed to accomplish hemostasis. “Permissive pericardial tamponade” was created under MRI, and accomplished early hemostasis by allowing the collagen sponge to seat on the epicardial surface inside the pericardium. After successful closure, six of seven animals accumulated a large pericardial effusion 5 ± 1 days after closure. Despite percutaneous drainage during 6‐week follow‐up, the large pericardial effusion recurred in half, and was lethal in one. Conclusions: A commercial collagen‐based vascular closure device may achieve temporary but not durable hemostasis when closing a direct left ventricular puncture port, but only after intentional pericardial separation. These insights may contribute to development of a superior device solution. Elective clinical application of this device to close apical access ports should be avoided.

Real-time MRI guided percutaneous transthoracic left ventricular access and closure

Introduction and objective Percutaneous transthoracic left ventricular (LV) access and closure would be an enabling technology for a wide range of structural heart, electrophysiologic and proximal aorta procedures. We propose a real-time MRI guided approach to access and close the LV using novel active devices. Methods Sixteen Yorkshire swine underwent percutaneous transthoracic LV access and followed up to three months (n=8). All procedures were guided by real-time bSSFP MRI (Espree, Siemens) using two slices along the needle trajectory and a short-axis slice for cardiac monitoring. Transthoracic ventricular puncture was performed using a customized 18G active needle with integrated loop coil. Puncture trajectory was planned using realtime MRI to identify optimal LV puncture target and trajectory with respect to intraventricular and valvular structures (Figure 1A). After needle entry, an 18-French sheath was inserted and anticoagulation begun.

Visualization of dynamic active devices via adaptive undersampled projection imaging

Methods Knowledge of the device trajectory at a particular timeinstant is used to adapt the orientation of the two projection images from which the device trajectory at the next time-instant will be estimated. They are chosen to have independent and unobstructed views of the device, a small FOV in the phase-encoding direction, and allow for some device motion between the current time-instant and the next.