1. Apparatus for imaging the internal structure of a volume exhibiting an internal variation, comprising: a source of penetrating radiation and a two dimensional detector for that radiation, the source and the detector being arranged to produce a series of projected images of the volume; a reconstruction means for deriving information as to the three dimensional structure in the volume from selected images of the series; a selection means for selecting images with similar phase from the series for use by the reconstruction means; wherein the selection means is arranged to: collapse the images derived from the series from two dimensions to one dimension by summing the intensities of pixels along a dimension transverse to the one dimension, produce a further image from a composite of the one-dimensional images obtained from images in the series, analyse the further image for patterns, and select from the series images having like phase in that pattern.