Diffusion MRI of the Body: Beyond PGSE-EPI.

Diffusion magnetic resonance imaging (dMRI) is a widely used clinical imaging technique which is sensitive to changes in tissue microstructure including cellularity, perfusion, and tissue damage. DMRI has been used in whole-body imaging as well as in targeted imaging of every major anatomical region. Most clinical dMRI applications use a standard acquisition technique: the pulsed gradient spin echo (PGSE) preparation with echo planar imaging (EPI) readout. PGSE creates strong diffusion contrast while removing the effect of T 2*, and EPI enables imaging of an entire 2D slice in a single shot. The PGSE-EPI method is rapid and robust; however, it is limited by low resolution and distortions typically caused by motion and susceptibility artifacts. Since the inception of dMRI, other methods have been used to sensitize diffusion and to read out the signal, which overcome different weaknesses of PGSE-EPI. These methods have largely been developed and used in the brain, but many of them have been applied to body dMRI as well. This review describes the major families of non-standard techniques used in body dMRI, covering both diffusion encoding strategies as well as image readout strategies. It then explores how these techniques have been applied in non-brain clinical applications and assesses the strengths and benefits of each method in various clinical contexts. Non-standard acquisition techniques have the potential to improve the value and efficiency of dMRI in the body, and further work to standardize and validate these sequences will enable their use in clinical contexts. Evidence Level: 3. Technical Efficacy: 2.