1. A method for performing security screening at a checkpoint, including: a) providing an X-ray imaging system having a scanning area; b) providing a supporting device for supporting articles to be scanned in the scanning area, wherein the supporting device has at least two reference areas manifesting respective X-ray signatures when exposed to X-rays, the X-ray signatures being distinguishable from one another; c) placing an article to be scanned on the supporting device; d) introducing the article to be scanned in the scanning area while the article is supported by the supporting device; e) using the X-ray imaging system for deriving the X-ray signatures of the reference areas and for obtaining an X-ray image of the article while the supporting device is in the scanning area; f) using the X-ray signatures to derive X-ray attenuation information from the X-ray image; g) using the X-ray attenuation information in determining if the article is a security threat.