Technology preview: X-ray fused with magnetic resonance during invasive cardiovascular procedures

Abstract

Background: We have developed and validated a system for real-time X-ray fused with magnetic resonance imaging, MRI (XFM), to guide catheter procedures with high spatial precision. Our implementation overlays roadmaps-MRI-derived soft-tissue features of interest-onto conventional X-ray fluoroscopy. We report our initial clinical experience applying XFM, using external fiducial markers, electrocardiogram (ECG)- gating, and automated real-time correction for gantry and table movement.

Methods: This prospective case series for technical development was approved by the NHLBI Institutional Review Board and included 19 subjects. Multimodality external fiducial markers were affixed to patients' skin before MRI, which included contrast-enhanced, 3D T1-weighted, or breath-held and ECG-gated 2D steady state free precession imaging at 1.5T. MRI-derived roadmaps were manually segmented while patients were transferred to a calibrated X-ray fluoroscopy system. Image spaces were registered using the fiducial markers and thereafter permitted unrestricted gantry rotation, table panning, and magnification changes. Static and ECG-gated MRI data were transformed from 3D to 2D to correspond with gantry and table position and combined with live X-ray images.

Results: Clinical procedures included graft coronary arteriography, right ventricular free-wall biopsy, and iliac and femoral artery recanalization and stenting. MRI roadmaps improved operator confidence, and in the biopsy cases, outperformed the best available alternative imaging modality. Registration errors were increased when external fiducial markers were affixed to more mobile skin positions, such as over the abdomen.

Conclusion: XFM using external fiducial markers is feasible during X-ray guided catheter treatments. Multimodality image fusion may prove a useful adjunct to invasive cardiovascular procedures.

Fig. 1

Staff and patient workflow during XFM procedures. Baseline MRI is performed after fiducial markers are affixed to patients’ skin. Although the patient is transferred to the X-ray system, the fiducial markers and MRI regions of interest are manually extracted. The fiducial markers are identified on X-ray and image fields are registered. Afterward, the 3D and 2D images are transformed and combined during live X-ray imaging.

Fig. 2

XFM-guided biopsy of a mass in the right ventricular free wall. Serial thin section SSFP MR images (panels A–C) with manually segmented regions of interest depicting the right ventricular endocardial border (green) and the mass in free wall (red). Panel D shows an intracardiac echocardiogram of the bioptome (arrow) engaging the mass (arrowhead). Panel E shows the XFM image. External fiducial markers are affixed to the skin (example marked with arrowhead). The intracardiac ultrasound catheter is positioned in the right atrium (arrow). The MRI-derived regions of interest (red and green) are overlaid on live X-ray and corroborate positioning of the bioptome.

Fig. 3

Triangulating the position of the bioptome. Panels A–B show two different fluoroscopic projections and corresponding 3D MRI-derived regions of interest (C, D). The right ventricular endocardial border is depicted in green, the mass in red, and the tip of the biopsy forceps is depicted as a yellow dot. The bioptome is discontiguous with the mass (see text). Panels E–J provide an explanation for the spurious normal biopsy finding. Review of MRI (panels E and F) reveals a papillary muscle (blue) posterior to the mass (red). Revised XFM (G–H) incorporates the papillary muscle. Panels I–J shows the 3D image of the mass (red), papillary muscle (blue), and triangulated bioptome position (yellow dot) indicating that the specimens were obtained from the papillary muscle and not from the mass.

Fig. 4

XFM using a 3D contrast-enhanced MR angiogram. (A) shows a retrograde L iliac radiocontrast angiogram in a contralateral oblique projection. (B) shows the regions of interest (adventitial borders, green) derived from a T1-weighted noncontrast MRI. (C) shows the XFM of both X-ray and MR angiograms. The correspondence is high.

Fig. 5

Unsatisfactory registration during an iliac artery revascularization procedure. Panel A shows the misregistration of a contrast-enhanced MR angiogram with X-ray during XFM. Multiple fiducial markers are positioned on the abdominal skin. Panel B shows another patient with MR-derived arterial adventitial contours overlaid during recanalization of occlusive iliac in-stent restenosis. Both presumably reflect error introduced by nonrigid deformation and respiratory motion of the fiducial markers.

Fig. 6

XFM-guided recanalization and stenting of R superficial femoral artery occlusion. A: Baseline composite contrast-enhanced MR angiogram shows R SFA occlusion and contra-lateral popliteal and tibial occlusion. B: A descending series of axial T1-weighted MR images permit contrast-independent arterial contours to be segmented (green). C and D: The MRI-derived contours are combined in XFM with the live X-ray images during guidewire recanalization. Note that the automatic gantry position correction permits the same XFM contours to be applied throughout the occlusion, which spans two X-ray fields of view. E: X-ray image after stenting. F: XFM during final poststent angiography.

Fig. 7

XFM during graft coronary arteriography. An injection sequence of a right internal mammary artery during ECG-gated XFM with continuous table panning. The corresponding numbered phase of the cardiac cycle is correctly represented by the MRI-derived regions of interest, irrespective of table position, throughout the injection. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]