A theoretical framework for the assessment of water fraction-dependent longitudinal decay rates and magnetisation transfer in membrane lipid phantoms

TL;DR

This paper introduces a theoretical MD-based model to predict water-dependent relaxation rates and magnetization transfer in liposome phantoms, enhancing understanding of MRI parameters in membrane systems.

Contribution

It presents a new MD-based approach and a hydration water fraction parameter to quantitatively analyze water interactions in liposomes using MRI data.

Findings

Liposome systems show similar behavior except PLPC with lower hydration and exchange rates.

The model accurately predicts water-dependent R1 relaxation rates.

Quantitative parameters help understand MRI signals in membrane-mimicking systems.

Abstract

Phantom systems consisting of liposome suspensions are widely employed to investigate quantitative MRI parameters mimicking cellular membranes. The proper physical understanding of the measurement results, however, requires proper models for liposomes and their interaction with the surrounding water molecules. Here, we present an MD-based approach for the theoretical prediction of R1=1/T1, the dependence of R1 on water concentration and the magnetization exchange between lipids and interacting water layer in lipids and lipid mixtures. Moreover, a new parameter is introduced which quantitatively measures the amount of hydration water (hydration water fraction, f_HW) based on conventional spoiled gradient echo MR acquisitions. Both f_HW and the magnetisation exchange rate between lipids and hydration water were determined quantitatively from spoiled gradient echo data. We observed that liposome systems behaved similarly, apart from PLPC which showed both lower hydration water fraction and lower exchange rate. The extracted parameters accurately predicted the measured water fraction-dependent R1 rates and allowed for a theoretical understanding of MR parameters in liposomes of different composition.