Abstract: A mobile apparatus for and a method of non-contacting, nondestructive, on-line inspection of installed components of plants, facilities or other systems. The apparatus and method use penetrating radiation sources and detectors which can be placed about vertical or horizontal components while they remain in place and in use. The apparatus can operate in multiple scanning modes and the resulting data collected can be processed using both gauging and computerized axial tomographic (CAT) algorithms and techniques. In a gauging analysis, the apparatus and method make use of both a computer-implementable model of the general shape of the component scanned and a computer-generated model of the specific component scanned.