Hydrocephalus, as the name implies, refers to the accumulation of water in the cavities of brain tissue. The main causes are as follows: 1. Enlargement of the ventricles and/or widening of the subarachnoid space due to imbalance in the production and absorption of cerebrospinal fluid. 2. Obstruction of all ventricular outlets and enlargement of the ventricles. 3.Decrease in brain parenchyma, widening of the subarachnoid space, widening of the sulcus and fissure depth and/or enlargement of the ventricles (cerebral hypoplasia or farcical atrophy). CT and MRI manifestations: the common manifestation of traffic hydrocephalus is the general enlargement of the ventricular system and the disappearance or normalization of the sulcus and fissure. 2, obstructive hydrocephalus: also known as non-traffic hydrocephalus, refers to hydrocephalus caused by obstruction in any part of the ventricular system. It is the most common type of hydrocephalus, and hemorrhage and infection are the most common causes of obstructive hydrocephalus. The paraventricular interstitial edema of obstructive hydrocephalus is more obvious and extensive, but it is mostly reversible and can be relieved after drainage. External hydrocephalus: also known as extracerebral hydrocephalus or extracerebral fluid accumulation, is a group of diseases with widening of the subarachnoid space in the anterior part of the mediastinal fissure and the top of the frontal area, widening of the basal pool (mainly the widening of the suprasellar pool), with or without ventricular enlargement as the main manifestation on CT and MRI. The widened subarachnoid space can increase in a short period of time as the disease evolves, and the open cranial suture is the condition for this hydrocephalus, which occurs in infants with unclosed fontanelles, but also in children with closed fontanelles. 4.Normal pressure hydrocephalus: It is named after the determination of the normal pressure of cerebrospinal fluid, which is actually a relative normal of the normal absolute value of cerebrospinal fluid pressure. It is seen in the following two cases: First, the cerebrospinal fluid pressure of the patient was low in the past, and after the disease, the cerebrospinal fluid pressure rose and produced clinical symptoms, while the absolute value of the cerebrospinal fluid pressure was normal when the patient’s cerebrospinal fluid pressure was measured. Second, the cerebrospinal fluid pressure fluctuates widely, sometimes low and sometimes high, when hydrocephalus signs are obvious and the cerebrospinal fluid pressure fluctuates above and below normal, and such fluctuations can cause neurological damage. Therefore, normal pressure hydrocephalus is actually the result of traffic hydrocephalus with partially intact cerebrospinal fluid circulation function as a substitute. 5, compensatory hydrocephalus: actually “cerebral atrophy” synonymous with it. There are no clinical symptoms and manifestations of hydrocephalus, and it shows limited or diffuse cerebral atrophy on CT and MRI. The reduced brain volume is partially filled by cerebrospinal fluid, which maintains the normal intracranial pressure, so it is called compensatory hydrocephalus. Compared with other causes of ventricular enlargement such as cerebral atrophy, traffic hydrocephalus has its regularity and characteristics: (1) The sequence of ventricular enlargement: early enlargement of the temporal horn of the lateral ventricle, then frontal horn enlargement, followed by enlargement of the three ventricles and the body of the lateral ventricle, and finally enlargement of the fourth ventricle. (2) The incidence of interstitial edema in the paraventricular white matter of traffic hydrocephalus is 40%, but this sign is often absent in chronic traffic hydrocephalus because intraventricular hypertension causes damage to the ventricular canal and prevents extravasation of cerebrospinal fluid due to the formation of ventricular canal scar by gliosis. (3) The enlargement of the fourth ventricle, although late, is the most significant and establishes the diagnosis once it is present. (4) The temporal horn enlargement is relatively late and the third ventricle enlargement is mild in cerebral atrophy.