An apparatus for scanning a structure to detect density differences between different structure parts, the apparatus includes a source of gamma radiation, a plurality of first detectors arranged to detect radiation emitted by the source along a plurality of respective first paths, a plurality of second detectors arranged to detect radiation emitted by the source along a plurality of respective second paths. Each first and second path is substantially aligned with a respective radius of a circle centered on the source, and an angular separation between at least two neighboring first paths is S. The first and second detectors and the source are arranged for rotation in a fixed relationship with respect to each other, about an axis of rotation located between the source and detectors, wherein the axis of rotation is substantially orthogonal to the circle. An asymmetric collimator block directs gamma radiation from the source to the detectors.
