Ventriculo-peritoneal shunts are used for the treatment of hydrocephalus in pediatric patients. Monitoring the condition of CSF flow through the shunt is vital for identifying possible shunt failure. We propose an MRI-based monitoring technique, Shunt-FENSI, for accurate, quantitative and non-invasive shunt flow measurement. The Shunt-FENSI technique uses pseudocontinuous sub-voxel labeling to build up signal due to flow within a larger imaging voxel. We demonstrate high accuracy in phantom scans, where we match our tagged signal measurement with a simulation which includes tagging, flow, and T1 recovery.