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

Rigid registration is a key requirement to overlay anatomical structures from pre-operative magnetic resonance (MR) images onto live X-ray fluoroscopic images. Clinical applications are for instance radio frequency catheter ablation and transcatheter endomyocardial injection. We present a method for fully automatic marker-based detection and registration. The algorithm locates the markers in the MR volumetric data set automatically by combining homomorphic unsharp masking, adaptive thresholding, region growing, and shape analysis. In the X-ray images, the markers are localized in several 2-D images and then their 3-D positions are reconstructed. The 2-D X-ray localization is performed using adaptive thresholding and template matching. The 3-D X-ray marker positions are established using back projection. Rigid registration between the two 3-D point sets is performed by an iterative closest point algorithm. The 3-D registration error using a flat panel detector X-ray system is on average 1.5 ± 1.8 mm. The novelty of our approach is the incorporation of bundle adjustment to reduce the 2-D overlay errors. Bundle adjustment reduces the 2-D X-ray marker localization error from 0.6 ± 0.8 mm to 0.4 ± 0.5 mm. The target registration error of our proposed method evaluated on eight datasets is on average 1.4±3.5 mm. These targets were not used during registration. Accordingly, anatomical structures from MR are positioned more accurately.