The CT scan of the head is one of the most common imaging studies anyone may face. It is also one of the most frequently requested modalities by the accident & emergency department. This article will cover some of the underlying principles of CT head studies and discuss the basic method for their interpretation.
Mnemonic
“Blood Can Be Very Bad” [B=Blood; C=Cisterns; B=Brain; V=Ventricles; B=Bone].
Blood on CT Scan
Look for any evidence of bleeding throughout all slices of the head CT.
Blood will appear hyperdense (white).
It is typically in the range of 50-100 Houndsfield units.
Basic types of bleed in the brain are epidural, subdural, intraparenchymal/intracerebral, intraventricular, and subarachnoid.
Intraparenchymal hemorrhage (IPH)
The most common cause of intraparenchymal hemorrhage is hypertension.
These bleeds may extend into the ventricles and show as intraventricular hemorrhage.
Primary intraventricular hemorrhages are rare.
They are often released by vascular malformations, Berry aneurysms, and intraventricular tumours.
Subarachnoid hemorrhage (SAH)
Most commonly, they look like, ‘spider (arachnoid) shaped’ white areas around the circle of Willis (~65%), or in the Sylvian fissure (~30%).
The most common cause of SAHs is the rupture of a Berry aneurysm.
Other causes include trauma and tumors.
Cisterns on CT scan of Head
The CSF in the brain collects in sulci, fissures, basal cisterns, and ventricles.
CSF appears darker on CT images because it is of lower density than the grey or white matter of the brain.
Basal cisterns or cerebral cisterns or subarachnoid cisterns are compartments (cisterns=boxes / compartments) within the subarachnoid space where the pia mater and arachnoid membrane are not in close contact and form cavities filled with cerebrospinal fluid.
Cisterns are interconnected, hence their patency is essential for CSF circulation.
Blood vessels and/or cranial nerves pass through cisterns.
The four key cisterns are Circummesencephalic or perimesencephalic, Suprasellar or chiasmatic cistern, Quadrigeminal and Sylvian.
These cisterns lie at the base of the brain and are therefore called basal cisterns.
Circummesencephalic or perimesencephalic basal cistern
On a CT scan, the circummesencephalic basal cistern appears as a ring-like structure surrounding the midbrain.
It should normally appear as a hypodense (dark) area.
The cistern should be well-defined and symmetrical, with no signs of compression or obliteration.
In subarachnoid Hemorrhage, blood in the cistern will appear hyperdense (bright/white) on a CT scan.
In the presence of increased intracranial pressure, the cistern may appear compressed or effaced.
Tumours can displace or distort the cistern.
Suprasellar or chiasmatic cistern
On a CT scan, the suprasellar basal cistern is stellate (star) or pentagon-shaped, depending on the scan’s angulation.
It encloses vessels of the circle of Willis. Posteriorly, the suprasellar cistern fuses with the interpeduncular cistern anteriorly, the ambient cisterns laterally, and the quadrigeminal plate cisterns posteriorly.
Normally it is a clear, hypodense (dark) area surrounding the optic chiasm.
Blood in the cistern will appear hyperdense (bright/white) on a CT scan.
The cistern may appear compressed or obliterated if there is increased Increased Intracranial Pressure and displaced or distorted if there is a tumor nearby.
Quadrigeminal cistern
On a CT scan, the quadrigeminal cistern, also known as the superior cistern or cistern of the great cerebral vein, appears as a W-shaped CSF-filled dark space located posterior to the midbrain.
Normal Appearance: a clear, hypodense (dark) area surrounding the quadrigeminal plate of the midbrain.
In subarachnoid Hemorrhage, blood in the cistern will appear hyperdense (bright) on a CT scan.
Tumors can displace or distort the cistern.
Sylvian Cistern
The Sylvian cistern is the deep part of the Sylvian fissure located between the frontal and the temporal lobes.
It contains the origin of the middle cerebral artery and vein.
Blood in the cistern will appear hyperdense (bright) on a CT scan.
Brain Parenchyma on CT scan of Head
Look for symmetry, grey-white differentiation, midline shift and hyper/hypodensity.
Sulci and gyri should appear the same on both sides.
The earliest sign of a Cerebro Vascular Accident on a CT scan is the loss of the grey-white interface.
Compare both sides.
Normally the fax cerebri should be in the midline.
Infarcted areas in ischaemic strokes look like hypodensity (darker areas) on the CT scan.
Other hypodense things include air and gas, fat, and tumors.
Ventricles on CT scan of Head
The ventricles are fluid-filled spaces within the brain.
They contain cerebrospinal fluid (CSF). The lateral ventricles contain the choroid
The lateral ventricles contain the choroid plexus which produces CSF.
The choroid plexus is almost always calcified in adults.
The main ventricles we typically see are the Lateral Ventricles, Third ventricle, and Fourth Ventricle.
In a normal CT scan, these ventricles appear as hypodense (dark) areas because the CSF is less dense than brain tissue. Abnormalities, such as deviation (shift), dilation, or compression indicate various pathologies.
Look at 3rd, 4th, and lateral ventricles for dilation or compression or shift.
Also, examine for effacement and blood.
Intraventricular hemorrhage (IVH) results in significant morbidity due to the risk of obstructive hydrocephalus.
IVH may be both primary and secondary, primary hemorrhage being far less common than secondary.
In adults, secondary intraventricular hemorrhage is usually a typical bleed or subarachnoid hemorrhage with ventricular reflux.
The main ventricles we typically see in a Head CT are: Lateral, Third, and Fourth.
Lateral Ventricles
These are a pair of C-shaped structures located in each hemisphere of the brain.
They are the largest of the ventricles.
The lateral ventricles look occasionally smaller on one side, usually on the right side.
This asymmetry of the lateral ventricles (ALV) is an anatomic variant in most cases if there is no evidence of space-occupying lesions, cerebral infarction, trauma, or other underlying structural abnormality.
In hydrocephalus, there is an enlargement of lateral ventricles due to an accumulation of cerebrospinal fluid (CSF) due to an obstruction in the flow of CSF, overproduction of CSF, or its poor absorption.
Lateral ventricular effacement refers to the compression or obliteration of the ventricles, often due to mass effect from a tumor, hemorrhage, or edema.
Bleeding within the ventricles can occur due to trauma, aneurysm rupture, or other vascular abnormalities.
Enlargement of the ventricles can also occur due to brain atrophy when the brain tissue shrinks, a condition often seen in neurodegenerative diseases like Alzheimer’s disease.
Third Ventricle
This is a narrow, midline cavity located between the two halves of the thalamus.
It connects to the lateral ventricles via the foramina of Monro.
The ventricle may enlarge due to an excessive build-up of CSF caused by an obstruction within the third ventricle or other parts of the ventricular system leading to hydrocephalus.
Colloid cysts are benign cysts that form in the middle of the brain, usually in the third ventricle.
They are usually asymptomatic.
But rarely can they cause obstructive hydrocephalus if they block the flow of cerebrospinal fluid (CSF).
This leads to symptoms like headaches and, in severe cases, sudden loss of consciousness.
They appear as a rounded hyperdense (white) lesion located near the interventricular foramina of Monro.
Tumours including gliomas and ependymomas can cause mass effect, leading to compression or displacement of the third ventricle.
Bleeding within the third ventricle can result from trauma, aneurysm rupture, or other vascular abnormalities.
This can lead to acute hydrocephalus and increased intracranial pressure.
Fourth Ventricle
In non-communicating hydrocephalus, the lateral and third ventricles dilate, but the fourth ventricle do not.
In communicating hydrocephalus, the lateral ventricles, third ventricle, and fourth ventricle are all dilated.
Tumours such as medulloblastomas or ependymomas around the fourth ventricle can cause mass effect and displacement of the 4th ventricle.
In Dandy-Walker Malformation there is enlargement of the fourth ventricle and cystic expansion of the posterior fossa, often associated with agenesis of the cerebellar vermis.
A large cystic area is seen on the posterior aspect of the posterior fossa communicating directly with the fourth ventricle and there is typically an absence or hypoplasia of the cerebellar vermis.
Bleeding within the 4th ventricle can result from trauma, aneurysmal rupture, or other vascular pathologies.
In Chiari 1, the most common variant of the Chiari malformations, there is caudal descent of the cerebellar tonsils through the foramen magnum which can compress the fourth ventricle and disrupt CSF flow.
Bone on CT scan of Head
Bone has the highest density on CT scan i.e. whitest in appearance.
Common bony abnormalities seen on a CT scan of the head are Fractures, Sclerotic (Osteoblastic) Lesions, Lytic Lesions, Mixed Sclerotic and Lytic Lesions, Congenital Defects, Arachnoid Granulations and Hemangiomas.
Skull fractures can be linear, depressed, or basilar.
They often appear as dark lines or areas of discontinuity in the bone.
Sclerotic or osteoblastic bone lesions look like areas of higher attenuation (brighter) within the skull bone.
These can be seen in conditions like osteomas, fibrous dysplasia, or metastatic disease (e.g., prostate cancer).
Lytic bone lesions look as areas of lower attenuation (darker) within the skull bone often seen in conditions like multiple myeloma, metastases (e.g., from lung or breast cancer), or infections.
Congenital conditions such as craniosynostosis (premature fusion of skull sutures) or calvarial defects (e.g. encephaloceles) can be identified on CT scans.
Arachnoid Granulations are benign protrusions of the arachnoid membrane into the dural venous sinuses, often seen as small, well-defined areas of low density.
Hemangiomas are benign vascular tumours that appear as well-defined, hyperdense lesions within the bone.
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