T1 & T1w/T2w
Christine Tardif1

1Montreal Neurological Institute, McGill University, Canada

Synopsis

This talk describes two methods used for imaging myelin content in vivo: T1 mapping and T1-weighted/T2-weighted (T1w/T2w) signal ratio imaging. Their advantages and limitations will be discussed in comparison to other techniques presented in the Myelin Imaging session.

Target audience

This educational talk targets a diverse audience of clinicians, neuroscientists and physicists who want to learn when and how to use T1 relaxometry and the T1w/T2w signal ratio to image myelin content, and how to interpret their findings.

Learning objectives

As a result of attending this talk, participants should be able to:

  • Describe the main techniques for T1 mapping;
  • List the advantages and limitations of the different T1 mapping techniques and T1w/T2w imaging, such as SNR efficiency and impact of RF field inhomogeneity;
  • Explain how these contrast mechanisms relate to brain tissue microstructure and myelin;
  • Identify applications where T1 mapping or T1w/T2w imaging would be most appropriate for probing myelin content in vivo.

T1 relaxometry

There are a variety of methods to measure the T1 longitudinal or spin-lattice relaxation time. I will first present the gold standard inversion recovery technique1 followed by more efficient methods that allow high-resolution whole brain T1 mapping within clinical scan times, such as the variable flip angle (VFA) method2 and MP2RAGE3. The advantages and disadvantages of the different techniques will be discussed, in particular their sensitivity to B1 transmission field inhomogeneity at high field strengths4,5.

T1w/T2w signal ratio

In addition to T1 mapping, I will also present the rationale for using the T1w/T2w signal ratio as a marker for myelination6, as well as the image post-processing required to minimize methodological biases7,8. This technique is semi-quantitative since the signal ratio depends on the T1-weighted and T2-weighted acquisition parameters chosen, such as the inversion time and effective echo time.

Biological specificity

The T1 relaxation time characterizes the exponential recovery of the longitudinal magnetization to its equilibrium state, and is affected by the interaction of spins with their surrounding lattice. T1 is thus sensitive to the macromolecular content (proteins and lipids) and water content of brain tissue9. The high proportion of lipids in the myelin sheath is very effective at shortening T110. In the healthy human brain T1 maps mainly reflect variations in myelin content, although T1 is also modulated by tissue iron content11 and myelinated axon caliber12. The relationship between myelin and T1 is weakened in areas of pathology where T1 can be affected by many concomitant microstructural changes, such as oedema, inflammation and iron deposition. The T1w/T2w signal ratio approach was originally proposed to enhance intra-cortical myelin contrast for the purpose of parcellating the cortex into distinct myeloarchitectonic areas13. Given the inclusion of T1w and T2w acquisition protocols in the Human Connectom Project, as well as other open datasets, the T1w/T2w signal ratio is increasingly used as an index of brain tissue microstructure. However, the biological interpretation of this metric in the context of disease is limited without a priori information about the underlying pathological features. T1w/T2w has been compared to other MRI-based myelin mapping techniques14,15,16, but studies comparing the signal ratio to myelin content and other microstructural features measured from histology are lacking.

Example applications

These two imaging techniques are less specific to myelin than the techniques presented earlier in this session: magnetization transfer and myelin water imaging. Nevertheless, they do have advantages that make them the most appropriate choice for certain applications. For instance, the required sequences are available on most MRI systems, and several post-processing tools are available to the community for analysis. Due to their high SNR efficiency, these two imaging techniques have been used at high and ultra-high field strengths to map cortical myeloarchitecture in vivo at sub-millimetre resolution7,17,18 and relate it to function19,20. T1 mapping and T1w/T2w imaging have also been used to study myelination during neurodevelopment21,22 and aging23,24, and demyelination in disease25,26,27.

Acknowledgements

No acknowledgement found.

References

  1. Barral, J. K. et al. A robust methodology for in vivo T1 mapping. Magn Reson Med 64, 1057-1067 (2010).
  2. Sigalovsky, I. S., Fischl, B. & Melcher, J. R. Mapping an intrinsic MR property of gray matter in auditory cortex of living humans: A possible marker for primary cortex and hemispheric differences. NeuroImage 32, 1524-1537 (2006).
  3. Marques, J. P. et al. MP2RAGE, a self bias-field corrected sequence for improved segmentation and T1-mapping at high field. NeuroImage 49, 1271-1281 (2010).
  4. Stikov, N. et al. On the accuracy of T1 mapping: Searching for common ground. Magn Reson Med 73, 514-522 (2015).
  5. Jutras, J. D., Wachowicz, K., Gilbert, G. & De Zanche, N. SNR efficiency of combined bipolar gradient echoes: Comparison of three-dimensional FLASH, MPRAGE, and multiparameter mapping with VFA-FLASH and MP2RAGE. Magn Reson Med 77, 2186-2202 (2017).
  6. Glasser, M. F. & Van Essen, D. C. Mapping human cortical areas in vivo based on myelin content as revealed by T1- and T2-weighted MRI. The Journal of Neuroscience 31, 11597-11616 (2011).
  7. Glasser, M. F. et al. The minimal preprocessing pipelines for the Human Connectome Project. NeuroImage 80, 105-124 (2013).
  8. Ganzetti, M., Wenderoth, N. & Mantini, D. Whole brain myelin mapping using T1- and T2-weighted MR imaging data. Frontiers in human neuroscience 8, 671-671 (2014).
  9. Fatouros, P. P., Marmarou, A., Kraft, K. A., Inao, S. & Schwarz, F. P. In vivo brain water determination by T1 measurements: effect of total water content, hydration fraction, and field strength. Magn Reson Med 17, 402-413 (1991).
  10. Koenig, S. H., Brown, R. D., Spiller, M. & Lundbom, N. Relaxometry of brain: Why white matter appears bright in MRI. Magn Reson Med 14, 482-495 (1990).
  11. Stüber, C. et al. Myelin and iron concentration in the human brain: A quantitative study of MRI contrast. NeuroImage 93, 95-106 (2014).
  12. Harkins, K. D. et al. The microstructural correlates of T1 in white matter. Magn Reson Med 75, 1341-5 (2016).
  13. Glasser, M. F. et al. A multi-modal parcellation of human cerebral cortex. Nature 536, 171-178 (2016).
  14. Uddin, M. N., Figley, T. D., Solar, K. G., Shatil, A. S. & Figley, C. R. Comparisons between multi-component myelin water fraction, T1w/T2w ratio, and diffusion tensor imaging measures in healthy human brain structures. Sci Rep 9, 2500 (2019).
  15. Arshad, M., Stanley, J. A. & Raz, N. Test-retest reliability and concurrent validity of in vivo myelin content indices: Myelin water fraction and calibrated T1 w/T2 w image ratio. Hum Brain Mapp 38, 1780-1790 (2017).
  16. Hagiwara, A. et al. Myelin Measurement: Comparison Between Simultaneous Tissue Relaxometry, Magnetization Transfer Saturation Index, and T1w/T2w Ratio Methods. Sci Rep 8, 10554 (2018).
  17. Waehnert, M. D. et al. A subject-specific framework for in vivo myeloarchitectonic analysis using high resolution quantitative MRI. Neuroimage 125, 94-107 (2016).
  18. Lutti, A., Dick, F., Sereno, M. I. & Weiskopf, N. Using high-resolution quantitative mapping of R1 as an index of cortical myelination. NeuroImage 93, Pt 2 176-188 (2014).
  19. Sereno, M. I., Lutti, A., Weiskopf, N. & Dick, F. Mapping the Human Cortical Surface by Combining Quantitative T1 with Retinotopy. Cerebral cortex 23, 2261-2268 (2013).
  20. Glasser, M. F., Goyal, M. S., Preuss, T. M., Raichle, M. E. & Van Essen, D. C. Trends and Properties of Human Cerebral Cortex: Correlations with Cortical Myelin Content. NeuroImage 93, 165-175 (2014).
  21. Rowley, C. D. et al. Age-related mapping of intracortical myelin from late adolescence to middle adulthood using T1-weighted MRI. Hum Brain Mapp (2017).
  22. Shafee, R., Buckner, R. L. & Fischl, B. Gray matter myelination of 1555 human brains using partial volume corrected MRI images. NeuroImage 105, 473-485 (2015).
  23. Grydeland, H., Walhovd, K. B., Tamnes, C. K., Westlye, L. T. & Fjell, A. M. Intracortical myelin links with performance variability across the human lifespan: results from T1- and T2-weighted MRI myelin mapping and diffusion tensor imaging. J Neurosci 33, 18618-18630 (2013).
  24. Yeatman, J. D., Wandell, B. a. & Mezer, A. a. Lifespan maturation and degeneration of human brain white matter. Nature communications 5, 4932-4932 (2014).
  25. Iwatani, J. et al. Use of T1-weighted/T2-weighted magnetic resonance ratio images to elucidate changes in the schizophrenic brain. Brain Behav 5, e00399 (2015).
  26. Yasuno, F. et al. Use of T1-weighted/T2-weighted magnetic resonance ratio to elucidate changes due to amyloid beta accumulation in cognitively normal subjects. Neuroimage Clin 13, 209-214 (2017).
  27. Bernhardt, B. C. et al. Preferential susceptibility of limbic cortices to microstructural damage in temporal lobe epilepsy: A quantitative T1 mapping study. Neuroimage 182, 294-303 (2018).
Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)