Simultaneous Multi-Angular Relaxometry of Tissue With MRI (SMART MRI): Theoretical Background and Proof of Concept

Purpose: Accurate measurement of tissue-specific relaxation parameters is an ultimate goal of quantitative MRI. The objective of this study is to introduce a new technique, simultaneous multiangular relaxometry of tissue with MRI (SMART MRI), which provides naturally coregistered quantitative spin density, longitudinal and transverse relaxation rate constant maps along with parameters characterizing magnetization transfer (MT) effects. Theory and Methods: SMART MRI is based on a gradientrecalled echo MRI sequence with multiple flip angles and multiple gradient echoes and a derived theoretical expression for the MR signal generated in this experimental conditions. The theory, based on Bloch-McConnell equations, takes into consideration cross-relaxation between two water pools: free and bound to macromolecules. It describes the role of cross-relaxation effects in formation of longitudinal and transverse relaxation of free water signal, thus providing background for measurements of these effects without using MT pulses. Bayesian analysis is used to optimize SMART MRI sequence parameters. Results: Data obtained on three participants demonstrate feasibility of the proposed approach. Conclusion: SMART MRI provides quantitative measurements of longitudinal and transverse relaxation rate constants of free water signal affected by cross-relaxation effects. It also provides information on some essential MT parameters without requiring off-resonance MT pulses. (C) 2016 International Society for Magnetic Resonance in Medicine