Simultaneous Mapping of Longitudinal and Transverse Relaxation Times
Rahel Heule1
1High Field Magnetic Resonance, Max Planck Institute for Biological Cybernetics, Tübingen, Germany

Synopsis

This talk gives a technical overview about acquisition strategies suited to simultaneously map longitudinal and transverse relaxation times. Special focus is on fast joint T1 and T2 quantification based on two main classes: magnetization-prepared (MP) schemes with steady-state free precession (SSFP) readout and multi-contrast imaging in the steady state. Possible acquisition approaches sampling multiple gradient echoes to simultaneously obtain the effective transverse relaxation time T2* alongside either T1 or T2 are introduced briefly as well.

Target audience

Researchers and clinicians interested in fast MR acquisition methods for simultaneous quantification of longitudinal and transverse relaxation times.

Purpose

The rapid and accurate simultaneous quantification of multiple relaxation parameters based on the same acquisition scheme is a long-standing goal in the field of MR relaxometry. Multi-property techniques are often more efficient compared to sequential approaches and offer the calculation of intrinsically co-registered maps with identical motion state as well as matched chemical shift or distortion artifacts. Jointly quantifying multiple fundamental MR parameters such as relaxation times has gained further interest since a variety of typical clinically relevant image contrasts can be synthesized from the derived maps, e.g. magnetization-prepared rapid gradient echo (MPRAGE)-like or fluid-attenuated inversion recovery (FLAIR)-like contrasts. Furthermore, the correlation between relaxation parameters in particular regions-of-interest can be investigated.

Outline of educational talk

In the past years, various acquisition techniques have been presented to simultaneously quantify longitudinal and transverse relaxation times.

T1 and T2
The proposed joint T1 and T2 mapping methods can be classified into three main groups: transient phase imaging after magnetization preparation with steady-state free precession (SSFP) readouts, multi-contrast steady-state experiments, and magnetic resonance fingerprinting (MRF). This talk focuses on techniques falling in the first two categories. The acquired signal evolution of many of these methods can well be approximated by analytical solutions facilitating the parameter estimation process.

Magnetization-prepared SSFP imaging
Magnetization-prepared (MP) techniques sample the signal recovery time course after an inversion pulse using rapid SSFP modules for readout. The recovery curves provide high T1 sensitivity while the SSFP readouts, e.g. balanced SSFP (as employed with IR-bSSFP (1-4)) or double-echo steady state (DESS) (as employed with MP-DESS (5)), introduce T2 sensitivity.

Multi-contrast SSFP sequences
SSFP imaging produces signal amplitudes with intrinsic mixed dependence on both T1 and T2, which can be utilized for combined mapping. The possible SSFP acquisition schemes enabling simultaneous T1 and T2 relaxometry can be subdivided into the three following groups:

  • Multi-pathway nonbalanced SSFP imaging offers the acquisition of several steady-state configurations including higher order modes in a single scan. For simultaneous T1 and T2 estimation, it is sufficient to acquire the two lowest-order SSFP-FID modes (F0, F1) and the lowest-order SSFP-Echo mode (F-1) – an approach known as triple-echo steady-state (TESS) relaxometry (6,7).
  • A series of phase-cycled balanced SSFP scans allows sampling the characteristic tissue-specific frequency profile. T1 and T2 relaxation times can be estimated from the bSSFP profile by using motion-insensitive rapid configuration relaxometry (MIRACLE) (8) or an ellipse fitting approach termed PLANET (9).
  • Nonbalanced SSFP imaging with variable RF phase shift increments φ can generate nearly ideally spoiled signal amplitudes with strong T1 weighting as well as partially spoiled signal amplitudes with additional T2 weighting depending on the choice of φ (10). Based on a series of nonbalanced SSFP experiments with varying flip angles and φ values, simultaneous estimation of T1 and T2 is feasible (11).

T1 and T2*
Generally, combined quantification of T1 and T2* can be achieved by modifying dedicated T1 mapping methods for the acquisition of multiple gradient echoes within each repetition time. This concept will be described by the example of a multi-echo MP2RAGE sequence (12).

T2 and T2*
Knowledge of both T2 and T2* allows decomposing the effective transverse relaxation time T2* into an irreversible component (T2) and a reversible component (T2′). To this end, a spin echo sequence can be combined with a multi-echo gradient-echo acquisition in order to sample the FID and/or ascending and descending spin echoes (13-15).

Parameter estimation based on MR fingerprinting will be discussed in another lecture of this session.

Acknowledgements

No acknowledgement found.

References

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Proc. Intl. Soc. Mag. Reson. Med. 28 (2020)