A screening system for screening items, such as aircraft-hold luggage, uses a conveyer system for carrying the items along a path and an array of sensors spaced along and around the path for determining the opacity to radiation in planes intersecting the path and the items. A signal processor uses signals from the sensors to detect contours of maximum local rate of change in opacity and from these reconstruct boundaries between the regions having different opacity to radiation and then determines whether or not one or more predetermined screening criteria are met.