Multi-Resolution

Currently, four different scales are used in our method: 8mm, 4mm, 2mm, 1mm. At each scale, the two images are resampled, after initial pre-blurring (using a Gaussian with FWHM equal to the ratio of the final and initial voxel sizes), so that they have isotropic voxels of size equal to the scale size. Note that an exception to this occurs if the scale is smaller than the data resolution, in which case the data is resampled to isotropic voxels of scale closest to the data resolution. Furthermore, skew and anisotropic scaling changes are much less prominent than rotational, translational and global scaling changes and so their effects are difficult to estimate reliably at low resolutions. Consequently, only similarity transformations (rigid-body + global scaling) are estimated at the 8mm and 4mm scales.