Larmor frequency dependence on structural anisotropy of magnetically heterogeneous media

Abstract

Purpose: To investigate the effect of anisotropic magnetic microstructure on the measurable Larmor frequency offset in media with heterogeneous magnetic susceptibility. Specific objectives were (i) validation of recently developed theory for the case of fast diffusion and (ii) investigation of the transition between the regimes of fast and slow diffusion. Methods: Monte Carlo simulations in synthetic media. Results: Simulations demonstrate a perfect agreement with the previously developed theory for fast diffusion. Beyond this regime, the frequency offset shows a pronounced dependence on the medium microarchitecture and the diffusivity of NMR-reporting spins in relation to the magnitude of the susceptibility-induced magnetic field. Conclusion: While the effect of myelin in brain white matter is commonly treated assuming efficient diffusion narrowing, this regime does not hold for larger cells or higher magnetic susceptibility. In such a case, the effect essentially deviates from the prediction based on the assumption of diffusion narrowing.

What does FEXI measure?

“…For simulation, we used a C++ program, which had been previously developed for simulating transverse relaxation, diffusion, and the Larmor frequency shift 18,22–25 . The algorithm is based on nearest‐neighbor hopping on a cubic lattice.…”

“…For simulation, we used a C++ program, which had been previously developed for simulating transverse relaxation, diffusion, and the Larmor frequency shift. 18,[22][23][24][25] The algorithm is based on nearest-neighbor hopping on a cubic lattice. At each moment in time, a random walker (the simulated spin) moves, with equal probability, in one of six directions.…”

Larmor frequency shift from magnetized cylinders with arbitrary orientation distribution

“…Equation ( 44) is also in agreement with previous results derived for parallel cylinders. 44,58 For a microstructure with axial symmetry around b z,…”

Dynamic contrast-enhanced QSM for perfusion imaging: a systematic comparison of ΔR2*- and QSM-based contrast agent concentration time curves in blood and tissue

“…All CBF, CBV, and MTT estimates were evaluated in GM and WM ROIs based on segmentation using new segment in SPM8 ( https://www.fil.ion.ucl.ac.uk/spm/software/spm8/ ). For some voxels, the concentration time curves based on the QSM images showed a concentration decrease during the CA passage due to artifacts, most frequently seen in WM (possibly caused by the anisotropic susceptibility in WM [ 31 ]). This corresponds to negative peak concentrations, prohibiting calculation of physiologically reasonable perfusion parameters.…”