Advanced Diffusion Physics Applied to Body Imaging

Diffusion-weighted imaging (DWI) offers insight into cellular status, density, and structural organization by way of sequences sensitive to water mobility which is affected by these properties [1, 2]. Macroscopic tissue motion unrelated to diffusion can confound in vivo diffusion measurements therefore single-shot techniques are nearly exclusively used to mitigate extraneous motion, particularly in the abdomen. Microcirculation flow through randomlyoriented capillaries in the presence of diffusion-sensitization gradients will also appear as a hyper diffusion-like attenuation in the low b-value regime [3, 4]. In the high b-value extreme where diffusion-based contrast is high, quantitative measurement of signal attenuation with increasing b-value is susceptible to noise limitations [5]. While not as pronounced as in neuro tissue, water mobility in body tissues may be directional due to true underlying cyto-architecture or appear anisotropic due to residual bulk motion artefact. Unlike DWI of the brain, an effective fat suppression method is crucial for successful body DWI sequences. In summary, diffusion imaging of the body requires organ site-specific customization of protocols to deal with: perfusion contamination at low b-values, SNR limitations in the high b-value regime, shim quality and fat suppression over a large FOV, multi-axis measurements to properly quantify isotropic diffusion (eg. ADC) or anisotropic features, as well as possible synchronization with cardiac/respiration to reduce residual bulk motion errors. Recent technical acquisition enhancements have successfully addressed many of these issues such that DWI of the body has gained rapid growth in a variety of applications.