Zoltan Nagy

Magnetic resonance imaging (MRI) has become the prime imaging technique for in vivo examination of the brain. In addition to anatomical and functional MRI, diffusion MRI (dMRI) is widely used in both clinics and research to assess tissue structure and fiber directions, particularly in the nervous system. While diffusion tensor imaging is the most widespread approach for assessing orientation measures, other, more sophisticated models have also been proposed. Validation of dMRI is, however, a challenging endeavor that requires specialized test samples. Here it is shown that two‐photon polymerization (2PP) 3D printing allows for manufacturing such test objects, a.k.a. phantoms. After upscaling the 2PP fabrication process, 3D structures at high spatial resolution and sufficient size to image in a human 7T MRI scanner are created. These phantoms reliably mimic human white matter and thus enable the systematic validation and verification of dMRI data and their analyses. The 3D‐printed structures include up to 51 000 microchannels that mimic the diffusion behavior of larger axons, with a cross‐section of 12 × 12 µm 2 each, in parallel and crossing arrangements. The acquired dMRI data demonstrates and verifies the utility of these novel brain phantoms.