High-resolution 3-dimensional micro-CT imaging of the nucleus pulposus using a novel contrast agent

Objective The nucleus pulpous (NP) in the intervertebral disc (IVD) is the first structure to exhibit degenerative changes during IVD degeneration (IDD). Currently, micro-computed tomography (micro-CT) imaging of the NP is a limiting factor in detecting IDD at an early stage. In this study, we used potassium iodide (KI) as a contrast agent to stain the NP in the IVDs of the tails and spines of mice and rats and imaged them with a micro-CT scanner. Methods We collected tails and spines from 12-week-old mice and rats and stained them with KI followed by micro-CT imaging. To induce IDD, we performed caudal disc needle puncture surgery (NPS) in age and sex-matched mice (n=5) and stained the tail with KI for imaging with micro-CT to study disc degeneration. Results KI successfully stained and helped visualize the 3D structure of the NP, with X-ray attenuation properties comparable to bone. Contrast staining with KI enabled accurate and reproducible quantification of disc height and NP volume. The cross-sectional micro-CT images of NPS discs were indistinguishable from the histological findings of the same sample and showed similar degenerative changes in the NP. Additionally, we estimated that up to 15 sagittal sections, each 5μm thick with 75μm spacing, would be needed to fully assess disc degeneration. Conclusion This study demonstrated that KI can be used to positively stain NP in the intact tail or spine and provide qualitative and quantitative data without any adverse effect on the histological processing of the samples.