Critical point drying of brain tissue for X-ray phase contrast imaging

X-ray phase contrast tomography is emerging as a powerful method for imaging large volumes of brain tissue at sub-cellular resolution. However, current sample preparation methods are largely inherited from visible light or electron microscopy workflows and hence are not optimised to exploit the full potential of X-ray contrast mechanisms. Here we propose to replace interstitial material by air to enhance X-ray phase contrast of the ultrastructural features. We used critical point drying (CPD) of heavy metal-stained mouse brain tissue to produce mechanically stable samples with preserved ultrastructure and enhanced refractive index boundaries, a nanofoam-like material that remains compatible with follow-up conventional resin embedding. Using two complementary synchrotron-based setups, a high-throughput microtomography beamline (P14, DESY) and a nanoscale holographic tomography beamline (ID16A, ESRF), we found that CPD samples consistently showed 2-4x stronger phase-shift signal than conventional resin-embedded tissue. The contrast gain remained consistent across samples, imaging conditions, and beamlines. Our results suggest that CPD offers a versatile route for preparing tissue for subcellular and ultrastructural-resolution X-ray imaging. It retains structural detail while improving signal, and is compatible with other processing procedures like femtosecond laser milling or electron microscopy, paving the path for biological tissue imaging beyond the mm3 scale.