Hybrid-State Free Precession in Nuclear Magnetic Resonance

TL;DR

This paper introduces the hybrid state in nuclear magnetic resonance, combining adiabatic and transient dynamics to enable robust, efficient imaging and quantification of spin relaxation times, with potential applications in brain MRI.

Contribution

It defines the hybrid state concept, deriving adiabaticity conditions that simplify spin dynamics and improve robustness in magnetic resonance imaging.

Findings

Hybrid state captures spin dynamics with a single parameter.

Optimized drive enhances robustness and efficiency in relaxation time measurement.

Application demonstrated in human brain MRI.

Abstract

The dynamics of large spin-1/2 ensembles in the presence of a varying magnetic field are commonly described by the Bloch equation. Most magnetic field variations result in unintuitive spin dynamics, which are sensitive to small deviations in the driving field. Although simplistic field variations can produce robust dynamics, the captured information content is impoverished. Here, we identify adiabaticity conditions that span a rich experiment design space with tractable dynamics. These adiabaticity conditions trap the spin dynamics in a one-dimensional subspace. Namely, the dynamics is captured by the absolute value of the magnetization, which is in a transient state, while its direction adiabatically follows the steady state. We define the hybrid state as the co-existence of these two states and identify the polar angle as the effective driving force of the spin dynamics. As an example, we optimize this drive for robust and efficient quantification of spin relaxation times and utilize it for magnetic resonance imaging of the human brain.