In this study we report on the first continuous records of three dimensional firn structure reconstructions using archive pieces of firn cores that cover the whole depth range of firn starting from surface down to the transition to bubbly ice. The CT-measurements have been performed on 1m-core segments with the means of the AWI-X-ray-computer tomograph especially designed for ice applications. Flyby recording in helical mode under a time-optimized measurement protocol enabled us to reduce the scanning time to 25 minutes per meter firn. The reconstructed volumes have a spatial resolution of 120 µm, giving about 1 million cross-section images per firn core. The analytical work flow includes three steps of pre-processing with denoising, image segmentation and a manual check for outliers at break positions and a layer-wise (5.5 mm thick) calculation of several geometrical parameters like density, ice and pore cluster sizes, intercept lengths, autocorrelation functions, structural anisotropy, Euler number (connectivity), coordination number, bubble number density, closed and open porosity. The method was applied to archive pieces of EGRIP-S6 (firn air pumping site, 2018, North-East-Greenland, 75.6°N ,35.9°W), B40 (Kohnen station, 2013, Dronning Maud Land, East Antarctica, 75.0°S, 0.1°E) and B51 (CoFi-Traverse 2013, Dronning Maud Land, East Antarctica, 75.1°S ,15.4°E). Temperature and accumulation rates at the different core sites vary between -30°C and -50°C and 130 mm w.eq./a and 40 mm w.eq./a respectively. The selected sites cover a wide range of recent environmental conditions of polar regions. In this contribution we present several fundamental relationships between the derived geometrical parameters. The evolution of firn structure with depth will be discussed in respect to the dominant processes acting at different sintering/densification stages. The potential value for densification modelling, gas transport and enclosure modelling will be highlighted.