Patient-specific, high-fidelity Lagrangian motion of cerebrospinal fluid in the spinal canal.

The transport of intrathecally administered drugs in the spinal canal is mainly driven by the mean Lagrangian motion of cerebrospinal fluid (CSF). In this study, we numerically compute, for the first time, the mean Lagrangian drift along an anatomically complete spinal canal, incorporating key microanatomical features obtained from magnetic resonance imaging and ex vivo measurements—without simplifications. These elements are accounted for using the immersed boundary method (IBM) with volumetric penalization, which significantly reduces preprocessing time while maintaining accurate velocity fields compared with traditional body-fitted approaches. Comparisons with direct numerical simulations confirm that the mean Lagrangian motion is the dominant advective transport mechanism along the canal. The approach represents a step forward toward a robust, efficient, reduced-order model applicable within clinically relevant timescales.