Insignificance of active flow for neural diffusion weighted imaging: A negative result

Purpose: To provide a biophysical basis to estimate the effect of cytoplasmic flow in neurons, and assess their contribution to the drop in the Apparent Diffusion Coefficient (ADC) in a nerve tissue following extreme conditions, such as brain injury and epileptic seizures. Methods: Three mechanisms are treated using the relevant physics of hydrodynamics and electrostatics: cargo induced streaming, electroosmosis, and membrane swelling. Results: We begin by discussing the lack of experimental evidence on the necessary velocities required to influence the Magnetic Resonance (MR) experiments. This is followed by demonstrating that cargo induced streaming, a widely known phenomenon in plant cells, has a minor effect on the ADC in neurons. Subsequently, we suggest and analyze two additional mechanisms that may induce fluid displacement in neurons, and are related to the electrical activity: electroosmosis and membrane swelling. Conclusion: Although these mechanisms may induce interesting fluid displacements, these cannot explain the significant drop in the ADC. We conclude by outlining the criteria that any future mechanism should meet to have an influence on standard diffusion-MR measurements. Magn Reson Med 78:746–753, 2017.