2025/09/30 by Anders Dyhr Sandgaard, Rafael Neto Henriques, Sandgaard, Anders Dyhr +5
Medicine · #Advanced MRI Techniques and Applications #Advanced Neuroimaging Techniques and Applications #Biological Physics (physics.bio-ph) #FOS: Physical sciences #Medical Physics (physics.med-ph)
paper · doi:10.48550/arxiv.2509.26267
openalex publication_date 2025/09/30 · openalex created_date 2025/10/19 · openalex updated_date 2026/08/01
In biological tissue, MR transverse relaxation stems from mechanisms spanning multiple scales, from molecular dipole-dipole interactions to mesoscopic field variations driven by tissue microstructure. While mesoscopic relaxation reflects cellular organization, its dynamics in white matter, specifically its dependence on axonal orientation and echo time, remain less investigated. This study employs theoretical frameworks and Monte-Carlo simulations using 3D EM-based white matter (WM) geometries to investigate how compartmentalization and realistic morphology drive these effects. Specifically, we simulate intra-axonal relaxation driven by magnetic fields induced by realistic axonal myelin sheaths and intra-axonal spheres as a model of iron-containing mitochondria. Our results confirm that orientation dependence of mesoscopic relaxation in WM is detectable and agrees with experimental observations. The time-dependence aligns with 1-dimensional short-range structural disorder, but at clinical echo times, this signature may be masked by dominant molecular relaxation. This work moves beyond idealized models to aid the development of more specific biophysical models of mesoscopic relaxation to achieve better biomarkers for neurodegenerative disease.