The co-registration error can be estimated by performing a number of additional iterative co-registrations, each iteration having a starting point dictated by the found co-registration transformation, and using a set of initial parameters different to that of the co-registration being tested. The spread in the resulting co-registrations can then be used as the estimate of the co-registration error. The variations in the set of initial parameters can include (i) the candidate transformation which the iteration uses as its starting point, adopting starting points that are offset from the co-registration being tested, (ii) the sample points used for the mutual information metric (or whichever metric is used to optimise the transformation), and (iii) the region-of-interest that is selected. Ideally, all three are varied to some extent within the plurality of additional co-registrations that are performed, and an average value is taken.
Estimation of co-registration errors