An apparatus adapted to scan a subsea pipeline and adapted to detect variations in density of the subsea pipeline, the apparatus comprising: a source of gamma radiation; a plurality of detectors, arranged to receive gamma radiation emitted by said source along a plurality of respective paths; wherein said apparatus is configured such that, in use, a structure to be scanned is positioned between said detectors and said source; collimation means comprising a plurality of collimator channels, each located between a respective detector and said source; and characterised in that: the apparatus comprises collimation adjustment means wherein said collimation adjustment means comprises at least one pair of plates or blocks, arranged to define a slit between their edges and extending across more than one said collimator channel; wherein, at least one said plate or block is adapted to move between a first position corresponding to a first configuration of said apparatus, in which said apparatus provides a first resolution in an axial direction substantially orthogonal to a plane passing through said source and said plurality of paths, and a second position corresponding to a second configuration of said apparatus, in which said collimation adjustment means is positioned between said collimation means and said subsea pipeline to be scanned such that said apparatus provides a second resolution in said axial direction; wherein said second resolution is smaller than said first resolution. a source of gamma radiation; a plurality of detectors, arranged to receive gamma radiation emitted by said source along a plurality of respective paths; wherein said apparatus is configured such that, in use, a structure to be scanned is positioned between said detectors and said source; collimation means comprising a plurality of collimator channels, each located between a respective detector and said source; and characterised in that: the apparatus comprises collimation adjustment means wherein said collimation adjustment means comprises at least one pair of plates or blocks, arranged to define a slit between their edges and extending across more than one said collimator channel; wherein, at least one said plate or block is adapted to move between a first position corresponding to a first configuration of said apparatus, in which said apparatus provides a first resolution in an axial direction substantially orthogonal to a plane passing through said source and said plurality of paths, and a second position corresponding to a second configuration of said apparatus, in which said collimation adjustment means is positioned between said collimation means and said subsea pipeline to be scanned such that said apparatus provides a second resolution in said axial direction; wherein said second resolution is smaller than said first resolution. the apparatus comprises collimation adjustment means wherein said collimation adjustment means comprises at least one pair of plates or blocks, arranged to define a slit between their edges and extending across more than one said collimator channel; wherein, at least one said plate or block is adapted to move between a first position corresponding to a first configuration of said apparatus, in which said apparatus provides a first resolution in an axial direction substantially orthogonal to a plane passing through said source and said plurality of paths, and a second position corresponding to a second configuration of said apparatus, in which said collimation adjustment means is positioned between said collimation means and said subsea pipeline to be scanned such that said apparatus provides a second resolution in said axial direction; wherein said second resolution is smaller than said first resolution.