Interventional cardiovascular magnetic resonance: still tantalizing

Abstract

The often touted advantages of MR guidance remain largely unrealized for cardiovascular interventional procedures in patients. Many procedures have been simulated in animal models. We argue these opportunities for clinical interventional MR will be met in the near future. This paper reviews technical and clinical considerations and offers advice on how to implement a clinical-grade interventional cardiovascular MR (iCMR) laboratory. We caution that this reflects our personal view of the "state of the art."

Figure 1

This is an example of "adaptive" projection navigation. Two dimensional slices are volume-rendered to show their three-dimensional position. In addition, the active catheter device is shown in superimposed projection-mode. The viewing angle of the two-dimensional slices can interactively be changed [A-C], but in this mode the projection angle is always perpendicular to the selected viewing angle. In this example, an active myocardial injection catheter is navigated toward an infarct border. The catheter contains two receiver coils: a small coil at the tip (red) and a loopless antenna along the shaft (green). A yellow dot is applied by the operator to mark the tissue target. The volume rendering is rotated manually during the procedure to view the device trajectory from different angles, conveying three-dimensional information [14]. In A and B, the catheter tip is outside the imaging plane; in C the catheter tip is in-plane and coincides with the selected target.

Figure 2

A combined X-ray and MRI interventional suite. The two systems can be used independently or together, when the radiofrequency-shielded and leaded doors are retracted. Patients can be transferred between labs on a cradle that slides along a motorized table. Each room can display real-time images, instantaneous hemodynamics, and imaging control to the operator. Both share a common control room for support staff [45].

Figure 3

MRI catheter designs. (A) Conventional X-ray catheters are maneuverable in part because they contain steel braiding that distorts MR images. (B) With steel braiding removed, the same "passive" catheter is inconspicuous. (C) An "active" version of the same catheter in (B) has a loopless-design MRI receiver coil along its length, and another loopless-design guidewire receiver coil inside. Both are conspicuous along their shaft or profile. (D) A clinical grade balloon-tipped catheter can be filled with CO2 gas and appears as a black spot (dashed white arrow). This catheter has been manipulated from the femoral vein to the right ventricle in a patient (courtesy Reza Razavi, King's College London). (E) "Active tracking" in a porcine right ventricle (courtesy Steffen Weiss, Phillips Research Europe). A computer-synthesized cross-hair indicates the catheter position on an interleaved image. (F) Multiple channel "active imaging" catheter prototype where the shaft is seen in blue, proximal and distal tip microcoil markers are seen in green, and the catheter bend is seen as an orange signal.

Figure 4

A comparison of passive (A) and active (B) catheter visualization techniques during rtMRI stent angioplasty in a porcine model of aortic coarctation. Panel A uses a passive PTFE-coated 0.035" nitinol guidewire, while panel B uses a loopless active 0.030" guidewire. The platinum-iridium (Cheatham-Platinum, Numed, Inc) stent is crimped on a dilute gadolinium-filled balloon (white arrows). The devices are more conspicuous and the procedures more straightforward using the active guidewire technique [51].

Figure 5

A promising technique (reverse polarization) to impart "signatures" to inductively-coupled intravascular devices. This is tested in a phantom using phased-array surface coils and real-time MRI. (A) Forward polarized mode shows the catheter and background. (B) Reverse polarization method distinguishes the inductively coupled receiver from background. The signal can be colorized and overlaid (C) as if it were an active catheter using a separate receiver chain. Courtesy Ergin Atalar, Bilkent University.

Figure 6

Real time MRI guided endograft treatment of aortic dissection in swine. The dissection flap (arrowheads) separates true lumen (TL) and false lumen (FL). (A & B) The passive endograft (arrows) has entered the false lumen and thereafter is redirected. Following endograft deployment, the false lumen is obliterated (Panel C). Courtesy Holger Eggebrecht, University Essen [96].

Figure 7

Clinical demonstration of iCMR guided therapy of aortic coarctation. (A) The large arrow points to the coarctation lesion. (B) A balloon dilatation catheter (double arrows) is positioned across the narrowing. (C and D) The balloon is inflated with a T2*-shortening agent. Panels E (aortic diameter) and F (pressure gradient) show favorable results of iCMR guided balloon angioplasty in 4 out of 5 patients (pre/post indicates measured before/after intervention). Courtesy Titus Kuehne, Charité Hospital [22].

Figure 8

Intracardiac electrograms are obtained during rtMRI in dogs without (left) and with (right) filtering using an MRI-compatible cardiac electrophysiology system. The same system was used in two human patients. Courtesy of Henry R. Halperin, MD, Johns Hopkins University [144].

Figure 9

Active-tracking microcoils are positioned in an MRI system using a prior time-resolved three-dimensional cardiac MR image. Left atrium (around pulmonary veins) in a pig is evaluated using MRI compatible electrophysiology catheter and sheath system (Panels A & B). Ablation sites are mapped in an interactive three-dimensional display (Panel C) and seen on necropsy (Panel D). Courtesy of Ehud Schmidt, PhD, General Electric [36].

Figure 10

X-ray Fused with MRI (XFM) guided repair of a membranous ventricular septal defect in swine [167]. Panel A shows traditional X-ray contrast left ventriculogram highlighting ventricular septal defect (VSD; thick, white arrow). Panel B shows turbo spin-echo 3 chamber view, which highlights the VSD (thick, white arrow) in the same orientation as X-ray image in panel A. MRI regions of interest were segmented manually to be presented to the operator as a static image overlay on real-time X-ray fluoroscopic images. Panel C is a schematic depicting a new XFM guided antegrade approach to crossing the VSD in order to close the defect with a nitinol implant. XFM roadmap (Panel D) elements include the right ventricular endocardial contour (yellow), the left ventricular epicardial contour (green), the left ventricular endocardial contour (blue), the VSD tract (red outline and black arrow), aortic root (red), and aortic valve (blue annulus with white leaflets). The guidewire (white arrowhead) is delivered across the VSD and into the aorta.