T1: Principles & Methods
Carl Michal1
1University of British Columbia, Canada

Synopsis

T1 relaxation describes the approach of nuclear spin magnetization to thermal equilibrium and plays a fundamental role in every MRI scan. Relaxation in heterogeneous systems like tissue are complicated by exchange of magnetization amongst different proton pools. This presentation will begin with an introduction to the physical mechanisms of T1 relaxation relevant to MRI in vivo. We will describe several methods for characterizing T1 and consider the impact of magnetization transfer on these measurements. Examples from white matter brain tissue demonstrate that easily misinterpreted effects can be understood as consequences of exchange.

Target Audience

Clinicians and researchers interested in understanding the physical origin of longitudinal (T1) nuclear spin relaxation, methods to measure T1 relaxation, and the effects of magnetization exchange on relaxation measurements.

Outcomes

Following the talk, attendees will have improved understanding of:
  1. The microscopic processes involved in T1 relaxation
  2. T1 relaxation in an exchanging, heterogeneous, multi-compartment system
  3. Techniques such as inversion recovery, saturation recovery, Look-Locker, and variable flip-angle to measure T1 relaxation.
  4. How the narrow-band pulses typically used in MRI complicate the interpretation of T1 measurements.

Summary

Longitudinal, or T1, relaxation plays a fundamental role in every MRI scan as it describes the approach to equilibrium of the nuclear spin magnetization. T1 relaxation arises due to fluctuations in the local fields experienced by the spins. These fields may arise from dipolar couplings to neighbouring spins, couplings to paramagnetic centers (eg iron), or other nuclear-spin interactions such as chemical shifts or J couplings. The strength and timescale of the fluctuations, along with the field strength, determine T1 in simple systems, where fluctuations on timescales of approximately the inverse of the Larmor frequency produce the fastest relaxation. In heterogeneous materials like tissue, the exchange of magnetization between spin pools alters the relaxation behaviour in important, and potentially surprising ways that depend on the details of the measurement method used. Of particular importance is the fact that most pulses used on clinical MRI instruments are “soft” pulses, in the sense that they fully excite only the aqueous magnetization, and only partially excite the nuclear spins of non-aqueous, semi-solid pools, such as those in lipid membranes or large macromolecules.

Numerous methods for measuring T1 relaxation have been developed. Inversion recovery, where the initial spin magnetization is inverted and recovery to equilibrium is monitored, is a simple, standard method, but is relatively slow. Saturation recovery can be performed more rapidly but has a reduced dynamic range. Lock-Locker and variable flip-angle methods can be faster still. In practice, different methods often give different results for T1 measurements and multi-component T1 relaxation is frequently observed. Easily misinterpreted effects can be understood as consequences of exchange. T1 measurements in white matter tissue made with solid-state NMR instrumentation help to illustrate and unravel some of these issues.

Acknowledgements

Research support from the Discovery Grants program of the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.

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Proc. Intl. Soc. Mag. Reson. Med. 30 (2022)