Characterizing Healthy and Diseased White Matter Using Quantitative Magnetization Transfer and Multicomponent T2 Relaxometry: A Unified View via a Four-Pool Model

Ives R. Levesque and G. Bruce Pike

Magn Reson Med, 2009; 62(6):1487-1496

Nuclear magnetic resonance relaxation and magnetization transfer in cerebral white matter can be described using a four-pool model: two for water protons (in separate myelin and intra/extracellular compartments) and two for protons associated with the lipids and proteins of biologic membranes (of myelin and nonmyelin semisolids). This model was used to gain insight into the observations from multicomponent quantitative T₂ relaxometry and quantitative magnetization transfer imaging, both based on simplified white matter models and experimentally feasible in vivo. Using a set of coupled Bloch equations describing the behavior of the magnetization in a four-pool model of white matter, simulations of the quantitative T₂ relaxometry and quantitative magnetization transfer imaging techniques were performed. Pathology-inspired modifications were made to the four-pool model to gauge their impact on quantitative T₂ relaxometry and quantitative magnetization transfer imaging observations. Our results show that changes in the rate of water movement between microanatomic compartments may impact otherwise stable quantitative T₂ relaxometry observations; that the measure of the quantitative magnetization transfer imaging-based semisolid pool population is robust, despite the presence of two distinct semisolid components; and that quantitative magnetization transfer imaging compartment size estimates are not influenced by changes in the T₂ of the intra/extracellular water pool. The four-pool model, while impractical for in vivo characterization, yields important insight into the interpretation of changes observed with these quantitative MRI methods based on simplified models of white matter.