Abstract

T1, T2 and magnetization transfer values are dependent on brain microstructure characteristics. Therefore, it should be possible to learn about brain microstructure from measuring T1, T2 and MT. This talk will highlight four tissue properties accessible from these MR parameters: water content, intra/extracellular water content, myelin content and axonal diameter.
There is a relationship between T1 times and water content in brain; however this is not a robust method for measuring water content since T1 is also influenced by other tissue microstructure constituents such as iron and myelin. A preferred approach to measure water content is to use T1 corrections in order to create pure proton density images. Following normalization to a water standard such as cerebral spinal fluid, the proton density images provide accurate water content values.
It is challenging, but possible, to separate intra and extracellular water contents using MR; this has been done 1) by using contrast agents in animal models, 2) by separating T2* components and 3) by joint T2/diffusion experiments. Pure diffusion measurements may be a more promising approach for separating intra and extracellular water MR signals.
There is a large literature on the use of MR to measure myelin content. T1w/T2w, short TE imaging, myelin water imaging, magnetization transfer ratio, macromolecular proton fraction and inhomogeneous magnetization transfer will be discussed here. Each of these approaches provides a slightly different perspective on the MR signal from myelin. Most of these approaches have been validated by histology.
Finally, MR can enable estimation of axonal diameters; perhaps the most useful diameter measurement is the g-ratio which is the ratio of the inner to the outer diameters of the myelin sheath. A recently developed approach for estimating the g-ratio combined data the myelin volume fraction (from magnetization transfer or T2) and the fibre volume fraction (from diffusion).
Several of the above measurement techniques (water content and myelin) can be acquired in a clinically realistic imaging time.

Acknowledgements

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References

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