How To Read a CT Scan of the Brain (Part 2)

B) Examination

Search for radiological findings such as size, shape, symmetry, density, displacement, integrity, and lesions.

Midline structures
Midline structures include the following (Numbered according to the CT Scan below):

1. Anterior falx cerebri
2. Septum pellucidum
3. Third ventricle
4. Pineal
5. Posterior falx cerebri
6. Fourth ventricle (not shown on the scan below)

Midline shifts usually indicate raised intracranial pressure and significant risk for herniation. Less often, they are due to non-threatening atrophy on one side. They can be associated with intra-axial or extra-axial pathology. Different processes can shift more the anterior, middle, or posterior midline structures.

Cerebral Ventricles: Common Abnormalities

Changes in size, shape, and density are frequent abnormalities in the ventricles. They can involve only parts of the ventricular system and may be symmetric or asymmetric. Mass lesions often displace all or part of a ventricle.

• Hydrocephalus = ventricular enlargement = ventriculomegaly can be due to CSF flow problems OR to cerebral atrophy OR a normal variant if mild or moderate

• Intraventricular hemorrhage shown by intraventricular hyperdensity if acute

• Intraventricular mass lesions such as meningioma, colloid cyst, etc. are unusual but important

• Asymmetry in size suggesting mass effect OR atrophy focal or diffuse OR normal variant OR developmental anomalie

• Odd shape due to congenital abnormality such as agenesis of corpus callosum or else acquired area of brain loss as occurs in Huntington disease

Transaxial CT unenhanced brain window below shows shunt in right lateral ventricle, moderate symmetric enlargement of bodies of lateral ventricles, normal CSF density but with periventricular hypodensity and some staples in the scalp from recent shunt insertion.

Cisterns are named parts of the subarachnoid spaces around the brain and brain stem. They can show important abnormalities in density (attenuation), size, sn symmetry.

- Larger arteries and cranial nerves travel in cisterns and so the locations ofcerebral aneurysms and cranial nerve tumours involve cisterns.

- Arachnoid cysts can form if fluid dynamics are abnormal in a cistern.

- Cisterns are important landmarks in surgical anatomy of the brain.

- Subarachnoid hemorrhage usually involves cisterns more than convexity sulci because cerebral arterial aneurysms usually arise within cisterns

- Cisterns can contain pus in cases of meningitis whether bacterial, tuberculous, or other inflammatory conditions such as sarcoid.

- They can fill with tumour in meningeal carcinomatosis and in some meningioma cases

- Air in cisterns due to spontaneous or traumatic or postsurgical CSF leak

This Transaxial CT unenhanced brain window below shows marked hypodensities classic for pneumocephalus (air in the head) in cisterns including quadrigeminal, ambient, superior cerebellar, and Sylvian as well frontal subarachnoid space. Trauma is suggested by swelling in the left posterior temporal scalp.

Transaxial CT unenhanced brain window below shows hyperdensities in all visualized cisterns (Sylvian, ambient, and interpeduncular) as well as enlargement of the temporal horns of the lateral ventricles. This picture is classic for acute subarachnoid hemorrhage (SAH) due to cerebral aneurysm rupture.

Cerebral sulci are the subarachnoid spaces between the gyri. Normally they contain blood vessels and CSF. Sulci are normally much smaller in volume in infants and children and are a little larger in older people. They are subject to changes in size, symmetry, and density.

- Sulci can fill with blood due to subarachnoid hemorrhage whether aneurysmal, traumatic, hypertensive, or of other origin.

- They commonly enlarge in cases of brain injury of any etiology when the pathology has evolved into its chronic state.

- Diffuse enlargement of sulci can be due rarely to communicating hydrocephalus.

The sulci in the scan below are diffusely & symmetrically enlarged to a moderate degree.

Following Transaxial CT shows asymmetry of sulci (and bodies of lateral ventricles) with sulci compressed on the left due to a tumour not shown on this image but obvious on more superior cuts.