Pediatric patients with hydrocephalus rely on ventriculo-peritoneal shunts to drain cerebrospinal fluid (CSF) from the brain. Quick, safe detection of shunt failure is crucial to preventing neurologic injury. Previously, a non-invasive MRI-based monitoring technique, Shunt Flow Enhancement of Signal Intensity (Shunt-FENSI), was proposed for accurate and quantitative CSF flow measurement in the slow flowing shunt. We are introducing a more advanced version, Phase-Sweep FENSI (PS-FENSI) to deal with magnetic field inhomogeneity which can cause changes to the tag effectiveness. This new method is accompanied by new fitting methods to more reliably measure FENSI signals.