Diffusion MRI Acquisition, Part I: From Propagator to Image
Jana Hutter1 and Filip Szczepankiewicz2

1Biomedical Engineering, King's College London, London, United Kingdom, 2Radiology, Brigham and Women's Hospital, Boston, MA, United States

Synopsis

Single-shot Pulsed Gradient Spin Echo echo planar imaging remains the most commonly used sequence for diffusion MRI. However, recent years have seen numerous extensions. This lecture will introduce both the basic modular elements and more experimental novel approaches including modified diffusion preparations, read-out accelerations and combinations with additional contrasts such as relaxometry.

Target Audience:

Basic researchers and clinicians interested in practical diffusion MRI

Objectives/Outcomes:

Understanding how to get from diffusion propagator to diffusion-weighted image.

Familiarization with important imaging and diffusion sampling parameters.

Realizing important interactions between diffusion preparation and read-out.

Propagator to NMR signal

Based on the theory of the previous lecture, this lecture will describe the relationship between the diffusion propagator and the diffusion-weighted signal. We do so by considering the accrued phase of particles moving in a magnetic field gradient (Price 1997), and introduce how MRI experiments can be configured to probe diffusion by sensitizing the signal to the phase dispersion (Stejskal & Tanner 1965). We will arrive at the well known equation linking the signal to the strength of the diffusion encoding (b) and the apparent diffusion coefficient (D) according to S(b)/S(0) = exp(-bD).

From Voxel-wise signal to image

The next part will introduce Echo Planar Imaging (EPI) read-outs as the workhorse for diffusion MRI (Weber & Schrack 1995). Single-shot Echo Planar Imaging allows to acquire an entire slice of the diffusion volume in one shot. The length of the EPI read-out train is thereby defined by the desired resolution, the Field-of-View and the echo spacing - depending on hardware and acoustic noise constraints. The time required to sample an entire volume (repetition time TR) is composed of the time required for all slices. Reducing the length of the EPI train is possible by in-plane acceleration techniques focusing on acquiring a subset of k-space such as parallel Imaging (SENSE, GRAPPA) or partial Fourier. More recent developments include multiband imaging, effectively reducing TR by sampling multiple slices at the same time - thus reducing TR and thus acquisition time.

Interactions between Diffusion preparation and read-out

Finally, important interactions between the diffusion preparation and the read-out will be discussed and introduced. We will highlight the challenges and tradeoff considerations that commonly appear when setting up such experiments. Examples are hardware limits defining the accessible b-value - echo time combinations or the influence of the imaging gradients on the diffusion preparation.Practical exercises:Practical exercises will give participants the opportunity to reflect on the lecture content: The timings for a diffusion experiment together with accessible b-values and TEs will be derived first for a specific set parameters and hardware specification. Furthermore, the impact of hardware changes and novel acceleration techniques will be studied.

Acknowledgements

No acknowledgement found.

References

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Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)