Other patients with hydrocephalus have a very narrow therapeutic window between symptomatic overdrainage and symptomatic underdrainage. Patients with NPH often require multiple adjustments of opening pressure to optimize cognitive function and gait stability, while avoiding overdrainage and the secondary subdural effusion. Although most valves placed in the United States are still fixed-pressure valves, the programmable valve has become an important tool in hydrocephalus treatment, particularly in the NPH population and in pediatric patients with complex hydrocephalus. It provides the option of changing the opening pressure of the valve transcutaneously, most commonly with a device using a coded magnetic field. The programmable valve is an important advancement in shunt technology. This required the surgeon to select a specific valve for implantation, and if overdrainage or underdrainage resulted, a second operation was required to change the valve. Historically, shunt valves permitted a fixed amount of CSF drainage. To counter a siphoning effect associated with upright posture, many shunt systems also include an antisiphon device this reduces overdrainage when the patient is standing. The ventricular catheter is connected to a valve that regulates flow. The ventricular catheter sits within 1 of the ventricular spaces in the brain, most commonly the right lateral ventricle. They are frequently impregnated with radiopaque material to aid in their radiographic visualization. The catheter components of a shunt are made from Silastic (Dow Corning, Midland, Michigan), a form of rubber tubing resistant to breakdown in the body. A shunt is a completely internalized system, as opposed to an external ventricular drain in which a ventricular catheter drains to a collection system at the bedside. Most CSF shunts consist of 3 components: a ventricular catheter, a valve, and a distal catheter. 3 These evaluations include not only cross-sectional imaging with CT and MR imaging but also plain radiographs of the shunt system. 2 Because one-third of all shunts fail within 1 year of placement and the manifestations of shunt failure are protean, patients with shunts frequently undergo radiographic evaluation. 1 It accounts for 70,000 hospital admissions annually and the placement of between 18,000 and 33,000 CSF shunts in the United States each year. Hydrocephalus affects between 1% and 2% of the population. Abbreviations H high L low M medium NPH normal pressure hydrocephalus P/L performance levels

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