Further Insights into (ih)MT: Four-Pool Model Analysis Accounting for Dipolar Order in ex vivo White Matter

Motivation: Recent findings indicate that simplified two-pool models are inadequate for modelling MR behaviour of white matter, suggesting a sophisticated four-pool model (4PM) consisting of two aqueous and two non-aqueous reservoirs. Goal(s): To extend the four-pool model to include effects of dipolar order created by off-resonant RF pulses and to address the question of appropriate RF absorption lineshapes. Approach: Comprehensive set of MR relaxometry acquisitions was performed in fixated nervus opticus with a custom-made setup at physiological temperature. Results: The fit of the 'extended 4PM' demonstrates the existence of (at least) two dipolar order relaxation times (T1D) and remarkable deviations from super-Lorentzian absorption behaviour. Impact: Experimental evidence was provided for the need for an 'extended 4PM' with dipolar reservoirs. Systematic deviations in the z-spectra were resolved by a 'free lineshape' model, providing enhanced sensitivity to tissue microstructure (e.g., cylindrical arrangement of myelin).