An apparatus for inspecting integrity of a multi-barrier wellbore is described. The apparatus includes at least one source to generate radiation to impinge a target volume of the wellbore. The apparatus includes a source collimator having a plurality of alternating blocking channels and passing channels to direct radiation to impinge the target volume, such that the radiation directed from each passing channel forms a plurality of field of views extending radially into the target volume. The apparatus further includes at least one detector to receive backscatter rays arising from each respective field of view from the plurality of field of views and to generate an image representative of an inspected portion of the wellbore. The apparatus is useful for inspecting very small volumes in the multiple barriers of the wellbore and determine the integrity of the wellbore based on the different densities in the image of the inspected portion.