1. Apparatus for the diagnosis of body structures into which an essentially monochromatic gamma-emitting radioactive isotope has been introduce, wherein said gamma-emitting radioactive isotope emits gamma photons at a specific energy and wherein some of said gamma photons lose energy due to Compton scattering in said body structures so that on emerging from said body structures they have second energies below said specific energy, the apparatus comprising, in sequence, a collimator through which photons emanating from the body structure being studied are directed; a first position sensitive photon detector, said first photon detector being adapted to absorb a fraction lying between 10 and 50% of both the scattered and non-scattered photons incident thereon; a filter comprising an element with an atomic number not less than 74 and having an absorption edge at an energy just below said specific energy but above said second energies, whereby said filter preferentially absorbs photons emanating from but not scattered within said body structures and preferentially transmits scattered photons; a second position sensitive photon detector positioned after said filter in line with said body structures, said first detector and said filter for detecting scattered photons passing through said filter and analysing means for producing an image due to non-scattered photons from the image of scattered photons formed by the second detector and the image of scattered and non-scattered photons formed by the first detector.