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Non-EPI Pulse Sequences for Diffusion MRI 

Non-EPI Pulse Sequences for Diffusion MRI
Non-EPI Pulse Sequences for Diffusion MRI

James G. Pipe

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date: 19 March 2018

Single-Shot echo planar imaging (SSEPI) is the most common data collection method for diffusion weighted MRI primarily because it is relatively immune to the undesired phase caused by motion during diffusion weighting, and secondarily because it is fast, allowing collection of data with many different diffusion weightings in a reasonable scan time. The most serious disadvantage of SSEPI methods is the extreme sensitivity to inhomogeneous fields, which leads to geometric warping in areas of susceptibility changes (e.g. air/tissue and bone/tissue interfaces) and in the presence of the system eddy currents commonly produced by the diffusion weighting gradients. Alternative single-shot methods include line-scanning, a one-dimensional (1D) method which avoids warping artifact, but which can be slow and noisy, and single-shot fast spin-echo (SSFSE), which adds refocusing radio-frequency (RF) pulses during data collection to eliminate warping. The challenge to SSFSE is that these added RF pulses greatly prolong the data acquisition, are prone to create patient heating, and can exhibit poor performance in signal refocusing due to the unique nature of diffusion weighting. Multi-shot methods require some method of estimating the undesired, motion-induced phase between each TR, and effectively accounting for it during image reconstruction. Several different data measurement methods have been proposed to solve this problem. Alternative methods, such as stimulated echo imaging (STEAM) and steady-state free precession (SSFP) MRI can also be used with diffusion weighting. Each method has benefits and challenges, as outlined in this chapter.

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