The integrity of underground infrastructure of urban areas is crucial for the safety and the efficient operation of municipal systems. Sewer pipelines are subjected to harsh conditions, including mechanical stresses from traffic, soil, and groundwater pressure, leading to damage and the need for repair. Trenchless technologies using cured-in-place pipe (CIPP) liners have become popular for this purpose. These liners create a composite structure with the old pipeline with increased strength and ensure isolation from natural soil. Quality control of CIPP installations is vital, and this paper proposes a new method for assessing the integrity of repair sleeves using micro-computed tomography (mCT). The technique focuses on evaluating three-dimensional internal structure and the quality of resin impregnation within the liner. The study identifies significant variability in porosity among the tested specimens of different thicknesses. Apparent heterogeneity of the sleeves is revealed. In particular, layers of significantly increased porosity are indicated. The greater the thickness of the liner, the more pronounced heterogeneity of its internal structure. These findings underscore the necessity for improving impregnation quality of the textile liners used in CIPP. In this view, the paper primarily contributes to advancing the evaluation techniques of underground pipeline repairing sleeves, highlighting areas for further development of liner technology to enhance its strength and durability for extended, reliable service.