Target audience

Researchers and clinicians who have basic knowledge of diffusion and relaxation MRI and are interested in diffusion time- and echo time-dependent diffusion MRI.

Diffusion time-dependent MRI

Diffusion time determines how water molecules travel physiologically relevant distances by self-diffusion alone. With different diffusion times, water molecules experience with different degrees of restriction/hindrance at different length scales. Therefore, diffusion time provides an opportunity to tune the sensitivity of diffusion MRI measurements, which, in turn, assists better characterizing biological tissue microstructure.

Theory
The room mean square displacement of diffusion is determined by $$$\sqrt(2Dt_{diff})$$$ , where $$$D$$$ is the diffusion coefficient and $$$t_{diff}$$$ is diffusion time. Therefore, the diffusion time in diffusion MRI experiments determines how long water molecules can diffuse and encounter different amounts of obstacles such as membranes and organelles. With appropriately chosen diffusion time, the sensitivity of diffusion MRI can be adjusted. However, the dependence on diffusion time is very complicated and the specific dependency may vary in different diffusion time ranges. In the short diffusion time range, water molecule diffusion is mainly influenced by barrier surfaces, so the surface-to-volume ratio can be probed. In the long diffusion time range, diffusion is significantly influenced by the microstructural organization. Note that the short or long diffusion time is relative to the sizes of diffusion compartments, which increases the complexity of the problem.

Practical approaches to extend achievable diffusion time ranges
The conventional achievable diffusion time range is usually within 20 – 100 ms on human scanners, depending on the available gradient coil performance and targeted b values. This range sometimes cannot meet many needs in practice. There are numerous practical approaches to extend the range of achievable diffusion times.

1. Hardware. The hardware limits, e.g., the maximum gradient strength and slew rates, and usually the dominant limitations to achievable diffusion times. High-performance gradient coils available on animal and human scanners can significantly enhance the ability to probe a broader range of diffusion times.
2. Pulse sequence. With appropriately designed diffusion MRI pulse sequences, particularly diffusion gradient waveforms, it is possible to achieve broader ranges of diffusion times. For example, the oscillating gradient spin-echo (OGSE) and the stimulated echo acquisition mode (STEAM) diffusion MRI methods have been used to achieve short and long diffusion times, respectively.

Applications of diffusion time-dependent MRI
As a general MRI method, the diffusion time-dependent MRI could have man potential applications. We will review two main applications, i.e., neuroimaging and cancer imaging, and focus on how diffusion time dependence provides additional information to enhance the ability to probe microstructure.

Echo time-dependent MRI

In diffusion MRI, the T2-weighted b=0 images are usually used to reduce the influences of relaxation times. However, biological tissues always contain multiple diffusion compartments (e.g., intra- and extra-cellular spaces) and it is still not fully clear the differences in diffusion and relaxation properties of different compartments. This may provide a confounding source to diffusion measurements, but, on the other end, also provides an opportunity to provide additional information that may assist better characterizing biological tissues. We will briefly introduce the development of echo time-dependent diffusion MRI and its possible applications.

Acknowledgements

No acknowledgement found.

References

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