What does a normal computed tomography (CT) scan of the brain look like in a healthy adult, from the bones to the tissue of the brain?

Normal Brain CT Appearance: Bones to Tissue

Skull and Bone Structures

On a normal brain CT, the skull appears as a bright white (hyperdense) ring surrounding the brain, with uniform thickness and smooth inner and outer tables. 1 The bone should show:

• Symmetric calvarial thickness without focal thinning, thickening, or fractures 2
• Mean skull density (Hounsfield units) of approximately 935-987 HU in normal adults 2
• No evidence of depressed or displaced fractures, which would appear as discontinuities or step-offs in the bright white bone 3
• Intact skull base with normal foramina and no erosions 1

Cerebrospinal Fluid (CSF) Spaces

CSF appears dark (hypodense) on CT, with attenuation values near water density. 4 Normal CSF spaces include:

• Ventricular system: The lateral ventricles, third ventricle, and fourth ventricle should be symmetric and appropriately sized for age 5
• Subarachnoid spaces: The sulci and cisterns should be visible but not enlarged 6
• CSF volume: Typically comprises 1.4-4.7% of total brain segment volume in young adults 4

Gray Matter Structures

Gray matter appears lighter (higher density) than white matter on CT, with mean attenuation of approximately 35 Hounsfield units. 7 Key gray matter structures include:

• Cerebral cortex: Should appear as a ribbon of higher density along the brain surface, clearly differentiated from underlying white matter 7
• Basal ganglia: The caudate, putamen, and globus pallidus should be symmetric and well-defined 2
• Thalami: Should be symmetric bilaterally with clear borders 5
• Deep gray nuclei: All should demonstrate symmetric appearance without mass effect 5

White Matter

White matter appears darker (lower density) than gray matter, with mean attenuation of approximately 29 Hounsfield units. 7 Normal characteristics include:

• Clear gray-white differentiation: The boundary between cortical gray matter and subcortical white matter should be distinct 7, 2
• Symmetric appearance: White matter should be symmetric bilaterally without focal hypodensities 5
• Homogeneous density: No areas of abnormal lucency (leuko-araiosis) in younger adults, though some periventricular hypodensity may be normal after age 60 6
• White matter volume: Typically comprises 37.5-48.2% of brain segment volume 4

Critical Symmetry Principle

The most important feature of a normal brain CT is bilateral symmetry—any asymmetry should raise suspicion for pathology. 5 However, be aware that:

• Symmetrical abnormalities are the most difficult to detect and can be easily missed 5
• Compare corresponding structures side-to-side systematically to identify subtle differences 5

Age-Related Considerations

Normal aging produces predictable CT changes that should not be mistaken for pathology:

• Progressive cortical gray matter density decline (polio-araiosis) after age 60 6
• Mild ventricular enlargement and sulcal prominence with advancing age 6
• Some periventricular white matter hypodensity (leuko-araiosis) correlating with cortical atrophy in elderly patients 6

Technical Factors Affecting Image Quality

Skull thickness and density significantly affect gray-white matter differentiation. 2 Important considerations:

• Patients with thick skulls (41% more bone volume than normal) show decreased Hounsfield unit differences between gray and white matter 2
• Increased skull density makes detecting subtle brain abnormalities more difficult, including early infarction 2
• Contrast enhancement improves gray-white differentiation when administered 7