• Normal pressure hydrocephalus
• Meningitis
• Prior subarachnoid hemorrhage
• Venous sinus thrombosis
Obstructive Hydrocephalus
• Aqueductal stenosis
• Colloid cyst of third ventricle
• Tectal glioma
• Intraventricular tumor
Parenchymal Hemorrhage
• Hypertensive hemorrhage
• Amyloid angiopathy
• Hemorrhagic metastasis
• Herpes encephalitis
• Acute leukemia
• Arteriovenous malformation
• Aneurysm
• Contusion (in trauma)
Subarachnoid Hemorrhage
• Ruptured aneurysm
• Benign perimesencephalic hemorrhage
• Traumatic subarachnoid hemorrhage
Global Cerebral Volume Loss
• Chronic alcohol use
• Anticonvulsants (particularly cerebellar)
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pressure leads to transependymal migra-tion of CSF into the periventricular white matter, where it is resorbed via parenchy-mal capillaries.
On CT, cerebral edema is lower in density than surrounding normal parenchyma. On MRI, signal intensity of edema is lower than that of brain parenchyma on T1-weighted images and higher on T2-weighted im-ages. Diusion-weighted image (DWI) and apparent diusion coecient (ADC) map-ping sequences can distinguish between cytotoxic edema (restricted diusion) and vasogenic or interstitial edema (normal or increased diusion). Fluid attenuation inversion recovery (FLAIR) sequences sup-press intraventricular CSF signal and are sensitive for detecting adjacent areas of in-terstitial edema (Fig. 2.3).
◆Cerebral Edema Patterns
Localized cerebral edema may be classified as cytotoxic, vasogenic, or interstitial, al-though overlap is often present. Cytotoxic edema consists of intracellular swelling caused by depletion of adenosine triphos-phate (ATP) and consequent sodium/potas-sium membrane pump failure. Most typical of cerebral infarct and anoxic injury, cyto-toxic edema aects both white and gray matter similarly on most imaging studies.
Vasogenic edema, in contrast, prefer-entially involves white matter and results from conditions that increase intracerebral capillary permeability. Vasogenic edema is often due to brain contusion, severe hy-pertensive disease, primary and metastatic neoplasms, or inflammatory conditions.
Acute hydrocephalus can cause inter-stitial edema, as elevated intraventricular
Fig. 2.3a–f a,b Cytotoxic edema in cerebral infarction. Extensive right frontal and temporal low-density change with loss of dierentiation between gray and white matter, sulcal eacement, and ipsilateral ventricular eace-ment. The middle cerebral artery is dense, indicating thrombosis.
c,d Vasogenic edema due to hemorrhagic brain metastasis from lung carcinoma. Left frontal white mat-ter hemorrhage with surrounding low attenuation edema that spares the cortical gray matter. FLAIR MRI shows a hemorrhagic nodule with extensive surrounding high-signal edema limited to the white matter. e,f Interstitial edema in hydrocephalus due to fourth-ventricle obstruction by meningioma. Acute hydro-cephalus with low-density transependymal CSF resorption, most conspicuous at the frontal, occipital, and temporal horns of the lateral ventricles.
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◆ Anatomic Variants and Incidental Findings
Common incidental findings on head CT include arachnoid cysts, prominent arach-noid granulations, choroid plexus and choroidal fissure cysts, remote lacunar in-farcts, focal encephalomalacia from prior infarct or trauma, and prominent perivas-cular spaces. The ventricles may be slightly asymmetric, and the septum pellucidum may contain a central CSF-filled cavity (cavum septum pellucidum). Unless symp-tomatic, these conditions usually do not require specific follow-up (Fig. 2.4).
Fig. 2.4a–f a,b Arachnoid granulation. Whenvisible, arachnoid granulations, which resorb CSF into the venous sys-tem, appear as lling defects in the opacied venous sinuses. They may also cause smooth erosion of the bone adjacent to the sinus. (a) Round lling defect in opacied left transverse sinus. Round osseous ero-sions near the internal occipital protruberance/torcular Hirophili.
c Choroidal ssure cyst. These small CSF attenuation cysts arise in the choroidal ssure and appear lat-eral to the midbrain or cerebral peduncle on axial images.
d Enlarged perivascular space. 8-mm CSF attenuation space located in the right posterior putamen. e Cavum septum pellucidum. A developmental variant due to failed embryonic fusion of the leaves of the septum pellucidum, it is present in ~15% of individuals. This CT image also shows cortical contusions, traumatic subarachnoid hemorrhage, and subacute bifrontal frontal subdural hygromas.
f Arachnoid cyst. CSF attenuation, extra-axial collection due to duplication in the arachnoid membrane, which compresses the adjacent brain and may smoothly expand the overlying calvarium. These are usu-ally asymptomatic; however, larger cysts may predispose to hemorrhage in minor trauma or can cause symptoms by compression of the brain. Epidermoids can have a similar appearance on CT, but they have characteristically high signal on FLAIR MRI.
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num, mastoid ecchymosis (“Battle