Robust automatic rigid registration of MRI and X‐ray using external fiducial markers for XFM‐guided interventional procedures

Abstract

Purpose: In X-ray fused with MRI, previously gathered roadmap MRI volume images are overlaid on live X-ray fluoroscopy images to help guide the clinician during an interventional procedure. The incorporation of MRI data allows for the visualization of soft tissue that is poorly visualized under X-ray. The widespread clinical use of this technique will require fully automating as many components as possible. While previous use of this method has required time-consuming manual intervention to register the two modalities, in this article, the authors present a fully automatic rigid-body registration method. Methods: External fiducial markers that are visible under these two complimentary imaging modalities were used to register the X-ray images with the roadmap MR images. The method has three components: ͑a͒ The identification of the 3D locations of the markers from a full 3D MR volume, ͑b͒ the identification of the 3D locations of the markers from a small number of 2D X-ray fluoroscopy images, and ͑c͒ finding the rigid-body transformation that registers the two point sets in the two modalities. For part ͑a͒, the localization of the markers from MR data, the MR volume image was thresholded, connected voxels were segmented and labeled, and the centroids of the connected components were computed. For part ͑b͒, the X-ray projection images, produced by an image intensifier, were first corrected for distortions. Binary mask images of the markers were created from the distortion-corrected X-ray projection images by applying edge detection, pattern recognition, and image morphological operations. The markers were localized in the X-ray frame using an iterative backprojection-based method which segments voxels in the volume of interest, discards false positives based on the previously computed edge-detected projections, and calculates the locations of the true markers as the centroids of the clusters of voxels that remain. For part ͑c͒, a variant of the iterative closest point method was used to find correspondences between and register the two sets of points computed from MR and X-ray data. This knowledge of the correspondence between the two point sets was used to refine, first, the X-ray marker localization and then the total rigid-body registration between modalities. The rigid-body registration was used to overlay the roadmap MR image onto the X-ray fluoroscopy projections. Results: In 35 separate experiments, the markers were correctly registered to each other in 100% of the cases. When half the number of X-ray projections was used ͑10 X-ray projections instead of 20͒, the markers were correctly registered in all 35 experiments. The method was also successful in all 35 experiments when the number of markers was ͑retrospectively͒ halved ͑from 16 to 8͒. The target registration error was computed in a phantom experiment to be less than 2.4 mm. In two in vivo experiments, targets ͑interventional devices with pointlike metallic structures͒ inside the heart were successfully registered between the two modalities. Conclusio...

Bundle adjustment for marker-based rigid MR/X-ray registration

“…Here, the mean RMS error for the best eight markers reduced from 3.04 mm to 0.75 mm. Both registration and localization errors are higher than in the literature [12], where a localization error of 0.32 mm and a registration error of 1.16 mm were reported, using an image intensifier system but comparable or lower than the reference paper using a flat panel detector system. Therefore, the algorithm applied on a flat panel detector system is sufficiently accurate for ventricular septal defects and electrophysiology procedures as well.…”

“…Most of the voxels have zero intensity. For this sparse problem, the volume can be filled quickly with ray-driven backprojection [16], [12]. In ray-driven backprojection, the 2-D marker centers are iteratively projected back on the volume, implying that only non-zero voxels have to be processed.…”

“…The use of only eight markers reflects the Trimmed ICP cost function, see Eq. (3), and simplifies comparison with the reference publication [12].…”

“…Conceptually, this paper is based on the publication of George et al, who used external fiducial markers that are visible in both modalities to calculate the registration [12]. This approach has high accuracy and requires only a few projections.…”

3D registration of MR and X-ray spine images using an articulated model

“…Previous works on on MRI/X-ray registration were applied to interventional imaging [84,85,86], angiographic imaging [87,88,89,90,91,92,93], cerebral [93,94,86,95], cardiac and cardiovascular imaging [96,97,98,99,100,101], and several other fields [102,103,104,105,106,107]. A detailed review of MRI/X-ray and other 2D/3D registration techniques can be found in [108].…”