Diagnostic Accuracy of ShuntCheck Compared to Radionuclide Shunt: Patency in Patients With Normal Pressure Hydrocephalus

Inclusion Criteria: * Men or women 40 yo or older and have a VP shunt placed for NPH and are suspected of shunt obstruction. * Capable of providing valid signed informed consent, or has a legal guardian, health care agent, or surrogate decision maker (according to local statutes, and collectively referred to as "surrogates" in this protocol) capable of providing valid, signed informed consent Exclusion Criteria: * Presence of multiple shunts, or presence of more than one distal shunt catheter (regardless of function) crossing the clavicle ipsilateral to the shunt with suspected obstruction; * ShuntCheck test would interfere with standard patient care, or emergency shunt surgery that cannot be delayed is indicated; * Presence of an interfering open wound or edema over any portion of the VP shunt.