1. A method of detecting threat objects represented in three-dimensional reconstructed computed tomography data, wherein said three-dimensional tomography data comprises a set of voxels, and wherein a set of pre-computed shape histograms of known threats and a pre-defined threshold are provided, the said method comprises the following steps: a. identifying voxels corresponding to a potential threat object; b. generating a projection from the said voxels corresponding to a potential threat object; c. obtaining the contour of the boundary of the said potential threat object from the said projection; d. computing a shape histogram of the said contour; e. computing the difference measure between the said shape histogram and the said set of pre-computed shape histograms; and f. declaring that said potential threat object is a threat if said difference measure is less than the said pre-defined threshold.