1. A method of classifying an object represented in computed tomography (CT) data for a region comprising: identifying a plurality of volume elements in the CT data each volume element being associated with a density value; for each of a plurality of volume elements, comparing the density value of the volume element to a volume element density range; if the density value of the volume element is within said volume element density range, identifying the volume element as an object volume element; combining a plurality of identified object volume elements into an object; using the density values associated with the combined object volume elements, calculating a density of the object; identifying an object density range which contains the calculated density of the object; selecting an object mass range based on the calculated density of the object wherein the selected object mass range is associated with the object density range; calculating a mass of the object; comparing the mass of the object with the selected object mass range; if the mass of the object is within the selected object mass range, classifying the object as belonging to a predetermined class of objects.