EPI as Workhorse for Diffusion, Perfusion, fMRI....
Penny Gowland1

1Sir Peter Mansfield Imaging Centre, United Kingdom

Synopsis

Traditionally MRI is used to produce high quality, high resolution images of the human anatomy. However it is also has the capacity to capture a range of dynamic processes in the body, and one of the best imaging sequences for doing this is EPI. This talk will consider the advantages and disadvantages of EPI as a readout scheme, its use in quantitative imaging and in imaging dynamic processes.

Target audience

Early career MR physicists and other scientists using MRI in their research.

Learning objectives

Participants will have a better understanding of the value of MRI and in particular EPI for quantitative imaging and dynamic imaging, as well as the challenges of using EPI readouts.

The advantages and disadvantages of EPI

EPI is the fastest imaging sequence (typically less than 50 ms to readout an image) and has a very high signal to noise per unit time1. It only requires a single RF pulse to create an image readout so it minimally perturbs the longitudinal magnetization making it an ideal sequence to use for quantitative imaging. It also has intrinsically low SAR. However EPI has a low bandwidth per pixel so that it is prone to fat shift artefacts and to distortions and signal pile up due to susceptibility artefacts or other field inhomogeities. Furthermore any errors in the gradient waveforms or sequence timing can lead to the ‘niquist artefact’2. It also generally creates high acoustic noise.

The use of EPI to study moving objects

One of the first applications of EPI was to study systems undergoing unpredictable motion such as the fetus3 and the gastrointestinal tract. To some extent HASTE or BTFE has replaced the use of EPI in these cases though EPI will still have relevance at low field, where susceptibility artefacts are not severe or where low SAR is required.

The use of EPI in dynamic imaging

The very high speed and low SAR of EPI makes it ideal for studying dynamic process, most obviously in fMRI which exploits the high sensitivity of the EPI signal to local changes in magnetic susceptibility caused by variations in blood oxygenation4, although also depends on other physiological parameters5. Sensitivity to susceptibility variations increases with field strength and this has to some extent driven the interest in 7T MRI. However EPI can also be used to study other dynamic processes such as first pass gadolinium studies6.

The use of EPI in quantitative imaging

As stated above since EPI encodes an image from a single FID or spin echo, it minimally perturbs the longitudinal recovery making it a very simple sequence for use in quantitative imaging. Its most widespread in this mode is in mapping water self-diffusion7, which has grown into the field of diffusion tensor imaging of anatomical connectivity in the brain and diffusion weighted imaging for clinical detection of peripheral nerves and also tumors, including perfusion related signals in tumours8,9. However it is also the standard sequence used in arterial spin labelling for the measurement of perfusion where the limited perturbation of the magnetization in the slice is key to providing maximum sensitivity to inflowing blood. However EPI is also useful for measuring T110 and it is particularly valuable for measuring T2 avoiding the systematic errors caused by imperfect RF pulses that affect most MRI measurements of T211.

Acknowledgements

Everyone at Nottingham now and in the past.

References

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Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)