Evaluation of voxel-based registration of 3-D power Doppler ultrasound and3-D magnetic resonance angiographic images of carotid arteries

Spatial registration and fusion of ultrasound (US) images with other modali ties may aid clinical interpretation. We implemented and evaluated on patie nt data an automated retrospective registration of magnetic resonance angio graphy (MRA) carotid bifurcation images with 3-D power Doppler ultrasound ( PD US) and indirectly with 3-D B-mode US. Volumes were initially thresholde d to reduce the uncorrelated noise signals. The registration algorithm subs equently maximized the mutual information measure between the PD US and 3-D MRA via iterative simplex search to find best "rigid body" transformation. We rated the performance of the algorithm visually on (n=5) clinical MRA a nd 3-D PD US datasets. We also evaluated quantitatively the effect of thres holding, initial misalignment of the paired volumes and the reproducibility registration. We investigated the effect of image artefacts by simulation experiments. Preregistration misalignments of up to 5 mm in the transaxial plane, up to 10 rum along the axis of the carotids and up to 40 degrees res ulted in 107 of 110 successful registrations, with translational and rotati onal errors of 0.32 mm +/- 0.3 mm and 1.6 +/-2.1 degrees. The algorithm was not affected by missing arterial segments of up to 8 mm in length. The ave rage registration time was 4 min. We conclude that the algorithm could be a pplied to 3-D US PD and MRA data for automated multimodality registration o f carotid vessels without the use of fiducials.