For high accuracy applications the principle of planar tomography is also applied. The X-ray tube is shifted corresponding to fig. 5 parallel to the pipe axis only and the camera is located opposite to the X-ray tube below the weld to inspect. The camera is not shifted for each set of projections. The X-ray tube is shifted alog the surface parallel to the pipe axis and several hundred projections are taken for reconstruction.
Artifacts arise for volumetric indications, only due to the missing projection angle problem.
The used reconstruction is based on a modified filtered back projection. This algorithm is a typical analysis method and is applied for cross-sectional analysis of preselected regions of interest. 6 shows a typical indication of a crack in an austenitic pipe weld. The evaluation of indications for open cracks and undercuts may be distorted by artifacts. The exact knowledge of the surface allows the exact determination of the distance between outer wall and crack tip.