EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Diffusion-weighted magnetic resonance imaging (DWI) is a non-invasive in vivo imaging technique that is based on mapping and characterizing multiscale diffusion processes in biological tissues. It is a powerful scientific and diagnostic tool, which enables the exploration of white matter in the human brain. Increasingly complex models to analyze DWI data and to create brain tractographies are proposed. To test and verify those models, objects with a known ground truth that reliably mimic different axon configurations, so called brain phantoms, are needed.
Two-Photon Polymerization (2PP) is a versatile tool within additive manufacturing technologies that enables the fabrication of objects with resolutions in the sub-micrometer range by utilizing nonlinear absorption of femtosecond laser pulses to induce cross-linking with a high spatial and temporal control. The high-resolution nature of 2PP allows it to create phantoms with complex networks of microchannels that mimic axons. However, to perform DWI measurements the imaged phantoms need to show an overall volume of several mm3. Creating large-volume 2PP structures while preserving sub-micrometre features is challenging due to long structuring times and limitations in set-up and material properties. The 2PP fabrication process was up-scaled and optimized in regard of throughput, feature size and material properties enabling the production of phantoms of a volume in the range of millimeters traversed by liquid diffused microchannels reaching physiological relevant sizes and a high packing density. DWI measurements with a 7T microgradient system showed the expected anisotropy and the tractographies were reconstructed correctly, proving the utility of large-volume 2PP structures as ground truth phantoms.