X-ray security inspection machine (10) comprising an X-ray tunnel (40); a conveyor means (50) for conveying an article through the tunnel; an X-ray source for irradiating the article; and an X-ray detection means for detecting X-rays transmitted through the article. In one aspect, the detection means comprises a photodetector array module (20) actuatable between a first stowed configuration and a second deployed configuration. In a second aspect, the detection means comprises a first unit having a first photodetector array (22); and a second unit, having a second photodetector array (24), offset with respect to the first unit. The units are moveable relative to one another between a first arrangement where the arrays overlap to a first degree and a second arrangement where they overlap to a second, lower degree; preferably zero. Also, a conveyor belt-tracking device (100) comprising a guide frame (104) to receive the conveyor belt (116) and substantially to restrict its motion to a predetermined direction.