Placement of a CSF drainage catheter (external ventricular drainage (EVD) or lumbar drainage (LD)) is a frequent neurosurgical emergency intervention. It is used in case of obstructive hydrocephalus as it can occur after e.g. intracerebral hemorrhage with intraventricular involvement, subarachnoid hemorrhage or traumatic brain injury but also in situations with reduced CSF resorption or increased CSF production (e.g. meningitis, ventriculitis). Furthermore, this technique can be used to monitor intracranial pressure. In all these situations, the EVD/LD can be used to reduce the elevated intracranial pressure during the acute phase. However, because of impairment of the Paccioni granulations by blood and its breakdown products, a continuing hydrocephalus may develop, leading to permanent necessity of CSF drainage. To evaluate the need for a permanent (implanted) drainage system (e.g. ventriculoperitoneal or ventriculoatrial shunt) attempts to clamp the EVD/LD are carried out during the subacute phase. Successful clamping is verified by absent increase of intracranial pressure, absent clinical deterioration and lack of ventricular enlargement on serial CT scans. The latter means that a first CT scan has to be performed before clamping of the drainage catheter (baseline examination) and again 24 hours after clamping of the drainage catheter. This leads to the need for repeated transportations of critically ill patients with possible harm of the patient, high costs, need for more staff and repeated exposure of the patient to radiation. Hence, a simple, repeatable, non-invasive, bedside tool for diagnosis of ventricular enlargement due to clamping of EVD/LD is strongly needed.