A Model for Magnetization Transfer in Tissues

Abstract

Magnetization transfer in several tissues is measured and successfully modeled using a two-pool model of exchange. The line shape for the semi-solid pool is characterized by a superLorentzian and the liquid pool by a Lorentzian. The tissues investigated were white and gray matter, optic nerve, muscle, and liver. All tissues the authors studied are characterized by the same model but differ in the parameter values of the model. Blood and cerebral spinal fluid (CSF) were also investigated. The two-pool model with a Lorentzian line shape for both the semi-solid and liquid pools modeled the magnetization transfer in blood. In CSF, as expected, there is no measurable exchange of magnetization. The T2B associated with the semi-solid pool was short (approximately 10 microseconds) for all tissues indicating a fairly rigid semi-solid pool. In addition characterization of the line shape as superLorentzian indicates molecules such as integral membrane proteins or lipids in membranes are likely molecules participating in the exchange. Conversely, in blood large globular proteins are indicated due to the Lorentzian nature of the semi-solid pool and a T2B approximately 300 microseconds.

Magnetization transfer in MRI: a review

“…[16][17][18] Agreement between calculations and measured data to within 1-3% has allowed quantitative parameterization of biophysical models of MT in tissues. Agar was the first model system to be studied in detail.…”

Magnetization transfer MRS

“…Swanson 26 as well as Henkelman et al 27 emphasized that for certain models or tissues the immobile pool B cannot be adequately described by a Lorentzian; in such cases the assumption of Gaussian 27 or super-Lorentzian line shape 28 can give a better explanation of MT experiments. Besides the problem of choosing an appropriate line shape model, the question of whether a two-pool model is sufficient to fit the experimental data of MT experiments has arisen.…”

Characterizing healthy and diseased white matter using quantitative magnetization transfer and multicomponent T2 relaxometry: A unified view via a four‐pool model

“…The MT ratio is a semiquantitative index that presents a limited view of the MT effect. More detailed analysis of pulsed, off-resonance MT data using the two-pool model of tissue (6,7) has been extended to in vivo imaging, in particular in human WM (8)(9)(10). The resulting method, termed quantitative MT imaging (QMTI), allows mapping of the parameters of the two-pool (liquid and semisolid) model of tissue: the ratio of semisolid to liquid protons F, the first-order forward and reverse exchange rate constants k f and k r (where k r ϭ k f /F), the spin-lattice relaxation rate R 1f of the free pool (R 1f ϭ 1/T 1f ), the spin-spin relaxation time constant of the free pool T 2f , and the "T 2 " of the restricted pool, T 2r (inversely related to the width of the restricted pool resonance).…”