Fig. 4
Modified from Deck et al. [42]
The Three Air Displacement Phases within the Ventricular System. The patient is in the supine position when circa 80 mL CSF is drained via a lumbar puncture followed by its replacement with air. Sequential roentgenographic imaging of the ventricles, the aqueduct of the third ventricle, and the fourth ventricle are acquired. Phase 1: the fluid level rises as the newly formed CSF accumulates in the lateral ventricles. The air is displaced into the third ventricles. Phase 2: the CSF flows into the aqueduct and the fourth ventricle when the lateral ventricles are filled with CSF to the level of the foramen of Monro. Phase 3: the CSF level elevates when the fluid level reaches the posterior margin of the foramen of Monro as the air is re-absorbed.