Most shunt systems consist of a ventricular catheter, a flush pump, a unidirectional flow valve, and a distal catheter. All are radiopaque for easy visualization after placement, and all are tested for accuracy before insertion. A reservoir is frequently added to allow direct access to the ventricular system for administration of medications and removal of fluid. In all models the valves are designed to open at a predetermined intraventricular pressure and close when the pressure falls below that level, thus preventing backflow of fluid. Most shunts now in use have differential pressure and adjustable valves with capability for changing the pressures with an external magnet, thus avoiding additional surgery (Kestle, 2003).