A theoretical interpretation of diffusion weighted and intravoxel incoherent motion imaging for cerebrospinal fluid flow.

Purpose: Diffusion-weighted imaging (DWI) and intravoxel incoherent motion (IVIM) imaging are well-established approaches for evaluating cerebrospinal fluid (CSF) flow in subarachnoid and perivascular spaces, and have recently been applied to study ventricular CSF flow. However, DWI does not directly measure flow velocity, and the physical implications of DWI measurements are unclear. This study aimed to provide a theoretical interpretation of the DWI and IVIM imaging of CSF flow velocity fields.

Theory: The general semi-analytical form of the signal attenuations caused by fluid flow and the resultant apparent diffusion coefficient were derived from the Bloch-Torrey equation for arbitrary $b$ values.

Methods: The fundamental properties of the signal attenuation in laminar flow velocity fields were investigated. A Monte Carlo simulation of the IVIM parameter estimation was performed based on these signal attenuations, taking background noise into consideration.

Results: The developed theoretical framework indicates that signal attenuations in DWI detect intravoxel flow velocity standard deviations ranging from approximately 0.1 to 10 mm/s within the range of practical scan parameter settings. The lower bounds of the DWI flow profiles appeared where the flow effect was an order of magnitude lower than the molecular diffusion effects, even when $b$ increased. The IVIM fitting parameters reflected the flow effects of the signal attenuations despite an inconsistency with the original IVIM model assumptions.

Conclusion: The physical implications of signal attenuation in DWI have been theoretically clarified. This framework provides a useful basis for understanding CSF flow dynamics and considering appropriate imaging settings.