Puncture of the ventricular system is one of the most frequently performed neurosurgical interventions. This procedure is commonly performed in order to treat and/or measure pathologically elevated intracranial pressure.Therefore a safe and fast surgical procedure is needed. Currently the "landmark-based" placement of intraventricular catheters is the gold standard. However it is known that more than 60% of the catheters are not accurately placed in accordance with "landmark-based" procedures. When the catheter is not placed accurately multiple punctures may be required. In this study, the investigators aim to investigate prospectively whether ultrasound guidance leads to a lower number of incorrect catheter placements, and whether this guidance consequently decreases the number of punctures.