Parameter Estimation

When enough points have been selected, we can estimate the transfer function between the two images. This is done by pressing the 'Est. Error' button of the point list interface (Figure 17).

Figure 17: Transformation parameters estimated

Since a transfer function has now been estimated, we can estimate the error on each landmark, by measuring the distance between the corresponding landmark of the standard image and the transformed coordinates of the reslice image (values in the right most column on Figure 17). If an 'edit' button of the point list interface is pressed, the seleced landmark is shown on the 3-pane windows, along with a projection of the same landmark transformed in from the other image (Figure 18).

Figure 18: Projection of 'minimal-error' point onto 3-pane views. Also available in zoom mode.

If one or more of the errors reported on the point list are large, this feature can be utilsed to readjust the point definitions. Note: You should never adjust a point which on the images seem well defined. Adjusting all points until zero error is achieved will only minimise the error of the points with respect to the estimated transfer function, the errors are not directly to the registration error between the two images, and are only useful for outlier detection.