CT Head

• Blood—Acute hemorrhage appears hyperdense (bright white) on CT. This is due to the fact that the
globin molecule is relatively dense and hence effectively absorbs x-ray beams. As the blood becomes older
and the globin breaks down, it loses this hyperdense appearance, beginning at the periphery. The precise
localization of the blood is as important as identifying its presence.

• Cisterns—Cerebrospinal fl uid collections jacketing the brain; the following four key cisterns must be
examined for blood, asymmetry, and effacement (representing increased intracranial pressure):

• Circummesencephalic—Cerebrospinal fl uid ring around the midbrain; fi rst to be effaced with increased
intracranial pressure
• Suprasellar (star-shaped)—Location of the circle of Willis; frequent site of aneurysmal subarachnoid
• Quadrigeminal—W-shaped cistern at top of midbrain; effaced early by rostrocaudal herniation
• Sylvian—Between temporal and frontal lobes; site of traumatic and distal mid-cerebral aneurysm and
subarachnoid hemorrhage

• Brain—Examine for:
• Symmetry—Sulcal pattern (gyri) well differentiated in adults and symmetric side-to-side.
• Gray-white differentiation—Earliest sign of cerebrovascular aneurysm is loss of gray-white differentiation;
metastatic lesions often found at gray-white border
• Shift—Falx should be midline, with ventricles evenly spaced to the sides; can also have rostrocaudal
shift, evidenced by loss of cisternal space; unilateral effacement of sulci signals increased pressure in
one compartment; bilateral effacement signals global increased pressure
• Hyper-/hypodensity—Increased density with blood, calcifi cation, intravenous contrast media; decreased
density with air/gas (pneumocephalus), fat, ischemia (cerebrovascular aneurysm), tumor
• Ventricles—Pathologic processes cause dilation (hydrocephalus) or compression/shift; hydrocephalus
usually fi rst evident in dilation of the temporal horns (normally small and slit-like); examiner must take
in the “whole picture” to determine if the ventricles are enlarged due to lack of brain tissue or to
increased cerebrospinal fl uid pressure

• Bone—Highest density on CT scan; diagnosis of skull fracture can be confusing due to the presence of
sutures in the skull; compare other side of skull for symmetry (suture) versus asymmetry (fracture); basilar

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