Differential Image Based Robot to MRI Scanner Registration with Active Fiducial Markers for an MRI-Guided Robotic Catheter System

Abstract

In magnetic resonance imaging (MRI) guided robotic catheter ablation procedures, reliable tracking of the catheter within the MRI scanner is needed to safely navigate the catheter. This requires accurate registration of the catheter to the scanner. This paper presents a differential, multi-slice image-based registration approach utilizing active fiducial coils. The proposed method would be used to preoperatively register the MRI image space with the physical catheter space. In the proposed scheme, the registration is performed with the help of a registration frame, which has a set of embedded electromagnetic coils designed to actively create MRI image artifacts. These coils are detected in the MRI scanner's coordinate system by background subtraction. The detected coil locations in each slice are weighted by the artifact size and then registered to known ground truth coil locations in the catheter's coordinate system via least-squares fitting. The proposed approach is validated by using a set of target coils placed withing the workspace, employing multi-planar capabilities of the MRI scanner. The average registration and validation errors are respectively computed as 1.97 mm and 2.49 mm. The multi-slice approach is also compared to the single-slice method and shown to improve registration and validation by respectively 0.45 mm and 0.66 mm.

Fig. 1.

Schematic of the registration prototype used for the proposed approach. Yellow and purple circles respectively indicate the fiducial coils used for calibration and validation.

Fig. 2.

Visualizes the coordinate systems defined for the registration process. DCS and PCS are drawn outside the bore for a cleaner visualization.

Fig. 3.

Flowchart of the proposed registration method.

Fig. 4.

Foreground (upper left), background (upper right), background subtracted (lower left), and coil detection (lower right) images (coronal orientation) for the same slice.

Fig. 5.

Active fiducial coil artifacts in a multi-slice image (coronal orientation). Artifact size changes throughout the slices.

Fig. 6.

(a) Registration coil. (b) Validation coil. (c) Registration frame prototype used in the experiments.

Fig. 7.

Experiment setup inside a clinical MRI scanner. The Registration frame prototype is immersed in a phantom filled with distilled water doped with a gadolinium-based contrast agent. Phase array RF coils are placed on top of the prototype.

Fig. 8.

Spatial visualization of coils and their error ellipsoids. Error ellipsoid principle radii in x-y-z directions is proportional to the component errors for each individual coil. Each radii is scaled by a factor of 4 for better visualization. Pillars of the structure also displayed in solid blue lines. C1 : C5 labels the tree coils. Only the largest error among different imaging plane orientations are shown for tree coils.