External ventricular drainage (EVD) is a high-risk medical procedure that involves inserting a catheter inside a patient’s skull, through the brain and into a ventricle, to drain cerebrospinal fluid and thus relieve elevated intracranial pressure. Once the catheter has entered the skull, its tip cannot be seen. The neurosurgeon has to imagine its location inside the cranium and direct it toward the ventricle using only anatomic landmarks. The EVD catheter is thin and thus hard to track using infra-red depth sensors. Traditional optical tracking using fiducial or other markers inevitably changes the shape or weight of the medical instrument. We present an augmented reality system that depicts the catheter for EVD and a new technique to precisely track the catheter inside the skull. Our technique uses a new linear marker detection method that requires minimal changes to the catheter and is well suited for tracking other thin medical devices that require high-precision tracking.