Transverse Relaxation (T2 & T2*): Mechanisms, Sensing & Quantifying
Cornelia Laule1
1University of British Columbia, Vancouver, BC, Canada

Synopsis

Relaxation is a fundamental concept in MRI as it plays a key role in determining image contrast for most MR sequences. T2 and T2* weighted imaging is common in clinical studies; however, some of the many factors which contribute to transverse relaxation-based contrast changes are still poorly understood. By measuring relaxation times accurately it is possible to extract quantitative information about microstructure from MR data. This talk will provide an overview of the processes of transverse relaxation, highlight some common pulse sequences used for quantitative assessment of relaxation and describe what factors influence and T2 and T2* in vivo.

Key points

  • Water environment in brain is heterogeneous
  • Transverse relaxation measures are influenced by many things including water content, iron, macromolecules, pH, B1 inhomogeneity, and exchange processes
  • T2 relaxation in central nervous system tissue cannot be described by a single number
  • Myelin water can tell us about the role of myelination in normal and abnormal brain
  • The field of relaxation is rapidly changing due to pulse sequence advances and new knowledge

Target Audience

Imaging scientists interested in the basics of transverse relaxation, with emphasis on central nervous system tissue examples.

Outcomes

Following the talk, attendees will have an improved understanding of:
  1. the processes responsible for transverse relaxation
  2. some common pulse sequences used for assessing T2 and T2* relaxation
  3. factors which influence transverse relaxation

Summary

Relaxation is a fundamental concept in magnetic resonance imaging (MRI) as it plays a key role in determining image contrast for most MR sequences. T2 and T2* weighted imaging is common in clinical studies; however, some of the many factors which contribute to transverse relaxation-based contrast changes are still poorly understood. By measuring relaxation times accurately it is possible to extract quantitative information about microstructure from MR data. This talk will provide an overview of the processes of transverse relaxation, highlight some common pulse sequences used for quantitative assessment of relaxation and describe what factors influence and T2 and T2* in vivo. T2 relaxation is also known as transverse or spin-spin relaxation. T2 describes the time constant for decay/dephasing of transverse magnetization MXY. T2 arises from heterogeneity of the local magnetic field experienced by each proton spin. Numerous approaches exist for assessing T2 quantitatively including 32 echo Carr Purcell Meiboom Gill (CPMG), 2D spiral EPI, linear combination, 3D multi-echo spin echo, gradient and spin echo, 3D T2prep spiral GRE, modified multi-slice CPMG, multiple gradient echo and mcDESPOT (see Alonso-Orbitz, Levesque and Pike, MRM 2015 and Lee et al JMRI 2020 for an excellent review). Factors that influence transverse relaxation include exchange, myelin debris, water content, iron, pH and B1 inhomogeneity.

Online Resources and Recommended Reading

ISMRM EDUCATIONAL SESSIONS
2009 – Quantitative Neuro-Anatomic and Functional Image Assessment
Quantitative Assessment of White Matter, Andrew Alexander
2009 – MR Physics for Physicists
Tissue Microstructure and Molecular Factors that Govern MRI Contrast, Christopher Quarles
2009 – Clinical MRI: From Physical Principles to Practical Protocols
Overview of MR Physics: T1, T2, Don Plewes
2010 – Quantitative MRI Approaches in Clinical Imaging
Quantitative Relaxometry, Sean Deoni
2011 – MR Physics for Physicists
Modern Methods for Accurate T1, T2 & Proton Density Mapping, Sean Deoni
2012 – Microstructural Imaging Techniques in the Brain
What Contrasts Can We Use to Probe Microstructure? Mark Does
Relaxivity Measurements, Alex MacKay
2012 – MR Properties of Tissue
Contributions to Relaxation Times & Diffusion, Gareth Barker
T1 properties of healthy and diseased tissue, Penny Gowland
T2 properties of healthy and diseased tissue, Bob Mulkern
2013 – Practical Quantitative Imaging
Relaxation Time, MT, Richard Dortch
2014 – MR Physics & Techniques for Clinicians
Spin Gymnastics 1 & 2, Walter Kucharczyk & Don Plewes
2019 – Quantitative MRI: Relaxometry
Beyond the Phenemenological Single Compartment Model, Sean Deoni
Implementation Considerations in Relaxometry, Martina Callaghan

ONLINE RESOURCES

  • http://www.cis.rit.edu/htbooks/mri (An online course on NMR and MRI with lots of animations)
  • http://www.magnetic-resonance.org/index.html (Textbook of the European MR Forum- excellent resource!)
  • http://mriquestions.com/ https://www.mr-tip.com/
  • https://users.fmrib.ox.ac.uk/~stuart/lectures/
  • https://www.youtube.com/watch?v=7aRKAXD4dAg (Paul Callaghan’s series of lectures on NMR and MRI- Highly recommended!

Acknowledgements

Thank you to the MRI study volunteers, caregivers and excellent MRI technologists at the UBC MRI Research Centre. Funding support is provided by the International Collaboration on Repair Discoveries (ICORD), the MS Society of Canada and NSERC.

References

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