the ac and / or pc landmarks are identified in a midsagittal mri image by firstly identifying structures in the brain as groups of pixels in a radiological image which have an intensity in ranges defined by one or more thresholds and which obey predefined geometrical criteria . the thresholds are varied until the predefined geometrical criteria are met . initial estimates of the position of the ac and / or pc are derived from the identified structures . these estimates can be improved in various ways , especially by making use of axial and / or corona / radiological images in planes including , and / or proximate to , the initial estimated position of the ac and / or pc .