The effect of linear and micro-circular shear flow on diffusion MR measurements

Objectives Diffusion weighted MRI measures molecular displacements by tagging spins phase according to their posthon, consequently, incoherent displacements cause dephasing, which results in a measurable signal attenuation. Usually, in clinical and biological applications, this incoherent motion is attributed to molecular diffusion (i.e., Brownian motion) within hindered and restricted domains. However, any incoherent displacements (however, non-random), such as shear flow or shearing motion, may affect dephasing and attenuation of the DW-MRI signal. These could originate from pressure, thermal or electrical gradients within the tissue, cell streaming, molecular transport, ionic currents, etc. In this work we study, analytically and experimentally, the effect of shear flow on conventional diffusion MRI measurements. We calculate the contribution of the shear to signal attenuation and demonstrate how non-Brownian displacements can be erroneously interpreted as arising from molecular diffusion.