  Recently, X-ray microscopy (XRM) and light-sheet fluorescence microscopy
(LSFM) have emerged as two pivotal imaging tools in preclinical research on
bone remodeling diseases, offering micrometer-level resolution. Integrating
these complementary modalities provides a holistic view of bone
microstructures, facilitating function-oriented volume analysis across
different disease cycles. However, registering such independently acquired
large-scale volumes is extremely challenging under real and reference-free
scenarios. This paper presents a fast two-stage pipeline for volume
registration of XRM and LSFM. The first stage extracts the surface features and
employs two successive point cloud-based methods for coarse alignment. The
second stage fine-tunes the initial alignment using a modified
cross-correlation method, ensuring precise volumetric registration. Moreover,
we propose residual similarity as a novel metric to assess the alignment of two
complementary modalities. The results imply robust gradual improvement across
the stages. In the end, all correlating microstructures, particularly lacunae
in XRM and bone cells in LSFM, are precisely matched, enabling new insights
into bone diseases like osteoporosis which are a substantial burden in aging
societies.
