A medical viewing system having means, for analysing and visualising medical image data corresponding to folded surfaces, comprises means of segmentation of the image data to identify the object surface, means for approximating the object surface by a reference surface, and means for detecting folded portions of the object surface including means for determining points of the reference surface at which a normal to a zone forming a patch on the reference surface intersects with the object surface. If there are more than one point of intersection, then that patch corresponds to a folded portion of the object surface. Fold-portion patches are assigned a code such that, on display or printing of an image (RP) corresponding to the reference surface, the fold-portions will be flagged (for example by coloured or patterned regions (HP). Other fold-attribute data can also be determined, coded and displayed: for example, the number and location of intersections between the first surface and the normal to the fold-portion patch, the distance to the first intersection, the distance between the second and third intersections, etc.