A method of determining a distance of a source point to a surface of an object in three-dimensional space, wherein the object is represented by a decision rule and a plurality of volume elements arranged in slices comprising rows and columns, wherein each volume element carries at least one value, and wherein the decision rule determines membership of each volume element to the object in accordance with the at least one value of the volume element, the method comprising: determining a contour of a representation of the object within each slice; locating on the contour of each slice a homing point closest to the source point; fitting a curve to the located homing points; determining a distance from the source point to the fitted curve; and equating the determined distance as the distance of the source point to the surface of the object.