1. A method of detecting a presence or an absence of at least one sheet explosive in an article of baggage containing a plurality of non-threat items from at least one x-ray image of the article of baggage, the method comprising acts of: determining a gradient field for at least a portion of the at least one image, the gradient field including a vector quantity indicating a gradient direction and gradient magnitude at each location for at least the portion of the at least one image; determining at least one characteristic of the gradient field including determining principal directions of variation of the gradient field; performing at least one comparison of the principal directions of variation to determine whether the at least one characteristic is consistent with characteristics of sheet explosives; and determining whether the at least one sheet explosive is present amongst the plurality of non-threat objects based, at least in part, on the at least one comparison.