Abstract: System for measuring layer thicknesses of a multi-layer pipe by measuring with a detector array (2) the attenuation of an X-ray transmitted though the pipe. According to the invention the detector array (2) comprises an array of detector elements D1, D2, D3, D4 with a collimator for defining the field of radiation in front of each detector element. The collimator has a narrow diaphragm aperture setting the resolution when the position of the pipe walls is to be determined. The defined field of radiation has an extent sufficient to radiate the four detector elements D1, D2, D3, D4 in parallel. In a suitable signal processing of the output signals from the detector elements D1, D2, D3, D4, eg by using the method of least squares, the thicknesses of the different layers may be fairly accurately determined.