In situ X-ray microtomography was used to characterize the changes in internal structure of an expanded thermoplastic polyurethane (eTPU) bead foam used in footwear midsole during compression. Quantitative data on cell size and volume changes based on conventional segmentation were computed. Image correlation on the 4D datasets, i.e. a 3D tomogram collected at various points during deformation, was performed to map the strain fields to aid in analysis of observed deformation. Morphological and structural characterization revealed the gradient porosity and ligament thickness and its influence on the global mechanical behaviour of individual bead foam. The correlation of changes in void size and shape, identified areas of potentially weak regions. Strain maps derived by using voids as markers were adequate to capture the instabilities in local cell collapse. The smaller cells at the surface densified more readily providing more resistance to deformation. In addition, the thicker ligaments in the centre of the bead also aided resistance to deformation. Finally, our results showed the initiation of failure in cell structure was influenced by the heterogeneities around the immediate neighbours in a cluster of voids.