Dmitry S.
Novikov1, Valerij G. Kiselev2
1Radiology, NYU School of Medicine, New
York, NY, United States; 2Medical Physics, Diagnostic Radiology,
Uniklinikum Freiburg, Freiburg, Germany
MR
signal is massively volume-averaged. Which parameters of tissue
microstructure can survive this averaging, and be quantified by MRI? An
answer is given by the effective medium description of tissues yielding the
voxel-averaged equation for the magnetization. Heterogeneous diffusivity,
relaxation rate and Larmor frequency offset give rise to corrections to the
magnetization dynamics. The quantifiable tissue parameters are the distinct
length scales on which the local diffusivity, relaxation rate and Larmor frequency
vary. The effective medium approach unifies diffusion and relaxation,
focussing on the single quantity whose frequency and wavevector dependence
contains all measurable information about tissue heterogeneity.