Introduction toBrain CT : Part 2

RAH Radiology

Introduction to Brain Imaging : CT scans

How to interpret CT brain:

• Film quality / technical factors

• Important anatomic structures

• Basic patterns of disease

Warning:This is a big topic. Important phrases and concepts are bolded.

Anatomy

Introduction to Brain Imaging : CT scans

• Important anatomic structures• Describe cortical features• Deep brain structures• CSF spaces• Vessels

There are many ways to describe the organisation of the cerebrum (upper brain).

The most common method is to describe the cerebrum anatomically; naming areas by location. The major divisions are the lobes.

These divisions are not particularly useful for diagnosis.

Frontal lobe Parietal lobe

Temporal lobe

Occipital lobe

Introduction to Brain Imaging : CT scans

• Important anatomic structures• Describe cortical features• Deep brain structures• CSF spaces• Vessels

The cerebrum can also be described functionally. There are many methods to do this, but the important radiological elements are fairly macroscopic:

Inputs / sensorium - blue

Output / motor - red

Complex functions - yellow

Introduction to Brain Imaging : CT scans

• Important anatomic structures• Describe cortical features• Deep brain structures• CSF spaces• Vessels

Several specific areas of interest are shown on the diagram. These regions help us identify areas to focus on given the clinical symptoms.

Note the proximity of Broca’s area and the “mouth area” of the motor cortex, and Wernicke’s area and the auditory cortex. Areas with similar functions are often co-located.

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3

4

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1. Broca’s area – expressive dysphasia2. Lower motor cortex (mouth/tongue) – dysarthria 3. Upper motor cortex (limbs) – hemi/monoparesis4. Wernickie’s area – receptive dysphasia5. Auditory cortex – cortical deafness6. Visual cortex – homonymous hemianopia

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Introduction to Brain Imaging : CT scans

• Important anatomic structures• Describe cortical features• Deep brain structures• CSF spaces• Vessels

The cerebrum can also be described in terms of the blood supply:

Anterior cerebral artery - red

Middle cerebral artery - green

Posterior cerebral artery - purple

These divisions can help differentiate diagnosis, for example embolic CVA vs “watershed” CVA (global hypoxia).

Introduction to Brain Imaging : CT scans

• Important anatomic structures• Describe cortical features• Deep brain structures• CSF spaces• Vessels

Another useful way to describe the blood supply is:

Anterior circulation - orange

Posterior circulation - blue

The anterior circulation is supplied via the carotids, the posterior circulation via the vertebral arteries. This helps us identify a likely source of embolus, e.g. cardiac vs ICA origin.

Introduction to Brain Imaging : CT scans

From the skull base on the left to vertex on the right identify:• Anterior and posterior circulation• ACA, MCA and PCA territories

Click forward to highlight these distributions.

Introduction to Brain Imaging : CT scans

• Important anatomic structures• CSF spaces• Vessels• Deep brain structures

One of the most useful diagnostic features to assess an intracranial abnormality is whether the pathology is within the brain tissue, or outside it.

These locations are called intra-axial and extra-axial respectively.

The extra-axial spaces are the meningeal spaces. These either contain CSF or exist as potential spaces.

Occipital lobe

Introduction to Brain Imaging : CT scans

• Important anatomic structures• CSF spaces• Vessels• Deep brain structures

The other major CSF spaces are the ventricles.

These are technically in continuity with the subarachnoid space, but they are best thought of as somewhere in between intra-axial and extra-axial because pathology from both locations can extend into the ventricles.

Occipital lobe

Introduction to Brain Imaging : CT scans

Occipital lobe

The ventricles are somewhat difficult to understand anatomically, because the lateral ventricles are oriented obliquely in the coronal plane. This is an important shape to understand in relation to the deep brain structures.

The easiest way to think of the orientation of the lateral ventricles is as reversed “C “ shapes that are more lateral at inferiorly when seen from the front. This follows the shape of the brain; the temporal lobes are more lateral than the frontal or parietal lobes.

Introduction to Brain Imaging : CT scans

• Important anatomic structures• CSF spaces• Vessels• Deep brain structures

The lateral ventricles are also called the 1st and 2nd ventricles (although which is which is not important).

The 3rd and 4th ventricles are unpaired CSF spaces, located in the midline.

The 3rd is just below the lateral ventricles, the 4th is at the level of the medulla.

Occipital lobe

Introduction to Brain Imaging : CT scans

• Important anatomic structures• CSF spaces• Vessels• Deep brain structures

CSF flows between the ventricles via small channels which can be easily obstructed.

The communication between the lateral ventricles and the 3rd ventricle is via the paired foramina of Monroe. These are at the front of the third ventricle.

Occipital lobe

Lateral ventricles

Third ventricle

Foramen of Munroe

Introduction to Brain Imaging : CT scans

• Important anatomic structures• CSF spaces• Vessels• Deep brain structures

CSF flows between the ventricles via small channels which can be easily obstructed.

The communication between the 3rd ventricle and 4th ventricle is via the cerebral aqueduct, at the level of the midbrain.

The fourth ventricle drains into the subarachnoid space at the craniocervical junction (blue circle)

Fourth ventricleThird ventricle

Cerebral aqueduct

Introduction to Brain Imaging : CT scans

• Important anatomic structures• CSF spaces

The extra-axial spaces are defined by the meninges; connective tissue layers between the brain and the skull.

Dura - runs along the inner surface of the skull. The inner layers run along the falx and tentorium.

Arachnoid - runs loosely around the outside of the brain.

Pia - is closely adherent to the brain, following the gyri.

Occipital lobe

Introduction to Brain Imaging : CT scans

• Important anatomic structures• CSF spaces

The extra-axial spaces are described by their relationship to the meninges.

Extradural space - a potential space between the bone and dura. The outer part of the dura is bound to the skull sutures.

Subdural space - a potential between the dura and arachnoid.

Subarachnoid space - between the arachnoid and pia. Contains CSF.

Subpial space – a potential space between the pia and brain, of little diagnostic significance.

Occipital lobe

Introduction to Brain Imaging : CT scans

• Important anatomic structures• CSF spaces

Of these meningeal spaces, only the subarachnoid is normally appreciated; it contains CSF.

This includes the CSF over the cerebral convexities, in the sulci and in the basal cisterns.

The most important basal cistern is the suprasellar cistern. This is low in the brain (just above the pituitary fossa or sella) and looks like a star.

Occipital lobe

Introduction to Brain Imaging : CT scans

• Important anatomic structures• CSF spaces

The CSF spaces (i.e. the subarachnoid space) contains most of the vessels that supply and drain the brain.

The Circle of Willis matches the “star” shape of the suprasellar cistern.

The limbs of the star contain the cerebral arteries.

Introduction to Brain Imaging : CT scans

• Important anatomic structures• CSF spaces• Vessels

Reviewing the arterial vascular territories, you can appreciate how the path of the vessel determines the pattern of blood supply.

Click to show the territories in relation to the vessels.

• ACA, MCA and PCA territories

Occipital lobe

Introduction to Brain Imaging : CT scans

• Important anatomic structures• CSF spaces• Vessels• Deep brain structures

There are important structures in the brain other than the cerebral cortex. These include aggregations of neurons, called nuclei, and communication pathways.

Nuclei, being the locations of neurons, are part of the grey matter. What central grey matter can you see on this image?

The basal ganglia are partially visible here. Notice the density is the same as the cerebral cortex.

Introduction to Brain Imaging : CT scans

• Important anatomic structures• CSF spaces• Vessels• Deep brain structures

The basal ganglia are a group of deep brain nuclei clustered in the lower cerebral hemispheres.

The parts of the basal ganglia seen readily on CT imaging are the lentiform nucleus and the head of the caudate nucleus.

The lentiform nucleus is ‘lens-shaped’ and is appreciated alongside the third ventricle.

The caudate head sits against the anterior lateral ventricle.

Introduction to Brain Imaging : CT scans

• Important anatomic structures• CSF spaces• Vessels• Deep brain structures

The other major grey matter structure associated with the basal ganglia are the paired thalami, which look like eggs along the third ventricle.

The deep brain nuclei are similar, in that they are multipurpose / not specialised.

They are often thought of as “integrating” other functions, with major roles in sensation, motor control and learning.

Introduction to Brain Imaging : CT scans

• Important anatomic structures• CSF spaces• Vessels• Deep brain structures

There are also several major white matter pathways in the brain, communicating between brain areas or to / from the spinal cord.

At the level of the basal ganglia, the most important is the internal capsule. This contains the axons of the sensory and motor neurons. A single insult here, like a stroke, can cause severe disability.

Much like the cortex itself, the anterior half carries motor fibres and the posterior half carries sensory fibres.

Introduction to Brain Imaging : CT scans

• Important anatomic structures• CSF spaces• Vessels• Deep brain structures

There are also several major white matter pathways in the brain, communicating between brain areas or to / from the spinal cord.

The major communication between hemispheres is the corpus callosum. This is best seen on the sagittal view in the midline.

The corpus callosum is only rarely involved with pathology.

Introduction to Brain Imaging : CT scans

• Important anatomic structures• CSF spaces• Vessels• Deep brain structures

The corpus callosum can also be seen on axial views. It curls around the ventricles, so the superior portion is hard to see, but the anterior and posterior elements are easily identified.

Pathology

Introduction to Brain Imaging : CT scans

The most important high density pathology you will see is acute haemorrhage.

There are many different types of haemorrhage. Location is usually the best clue to identify the cause (aside from clinical history).

Extra-axial pathology pushes the brain away. Intra-axial pathology expands the brain.

This is an acute intra-axial haemorrhage.

• Basic patterns of disease• Approach to CT scans• Low density pathology• High density pathology

Introduction to Brain Imaging : CT scans

Intra-axial bleeds are inside the brain, and are usually caused by hypertension (deep brain regions), underlying disease (e.g. amyloidosis) or trauma (coup / contrecoup injury, shown here).

Extra-axial bleeds within the meningeal spaces, and are usually traumatic or aneurysmal.

• Basic patterns of disease• Approach to CT scans• Low density pathology• High density pathology

Introduction to Brain Imaging : CT scans

The shape of extra-axial bleeding helps differentiate the source.

The outer layer of the dura is strongly bound to the skull sutures, so extradural blood cannot pass.

This creates the classic biconvex or “lens shaped” appearance, as the haematoma is pinned down at either end.

This is an acute extradural haemorrhage.

• Basic patterns of disease• Approach to CT scans• Low density pathology• High density pathology

Introduction to Brain Imaging : CT scans

Subdural haematomas are not bound by the skull sutures, so can flow around the brain. This creates the typical semilunar or “crescent moon” appearance.

Blood outside vessels becomes less dense over time, and can be similar to brain tissue or even fluid.

This is an image of bifrontal chronic subdural haematomas. Notice that you can see the displaced arachnoid mater on the left.

• Basic patterns of disease• Approach to CT scans• Low density pathology• High density pathology

Introduction to Brain Imaging : CT scans

Subarachnoid haematomas follow the shape of the brain.

This is an acute subarachnoid haemorrhage.

In extra-axial bleeds, a central location (the basal cisterns) is suspicious for aneurysm rupture (shown here).

A peripheral location is often related to trauma.

• Basic patterns of disease• Approach to CT scans• Low density pathology• High density pathology

Introduction to Brain Imaging : CT scans

The only other common high density change you will see in the brain is with calcification.

Throughout the body, calcification is usually benign. In the brain, the arteries, pineal gland, choroid plexus and basal ganglia (shown here) often calcify with age.

Some tumours calcify, such as meningiomas. Again, calcified tumour are usually benign.

• Basic patterns of disease• Approach to CT scans• Low density pathology• High density pathology

Introduction to Brain Imaging : CT scans

CSF flows through the ventricular system and extra-axial spaces, constantly produced in the ventricles and resorbed in the peripheral subarachnoid spaces.

If the flow of CSF is obstructed, the ventricles can dilate, this is called hydrocephalus.

The most important causes are tumours and subarachnoid blood (which can block the resorption of the CSF).

• Basic patterns of disease• Approach to CT scans• Low density pathology• High density pathology

Introduction to Brain Imaging : CT scans

The obstruction can occur at any level. Most common locations are: • mass at the foramen of Munroe (unilateral lateral ventricle hydrocephalus)• stenosis at the cerebral aqueduct (lateral and 3rd ventricle dilatation)• subarachnoid blood obstructing CSF resorption (global ventricular dilatation).

This case shows stenosis of the cerebral aqueduct.

Third ventricleFourth ventricle

Introduction to Brain Imaging : CT scans

As we have seen, expansion of CSV spaces can also be due to volume loss.

In this case the ventricles of this elderly patient are larger than expected, but the sulci are also prominent. This suggest generalised atrophy / volume loss.

Always look for enlargement of the ventricles out of keeping with the sulcal size.

• Basic patterns of disease• Approach to CT scans• Low density pathology• High density pathology