RAH Med 4 MHU - Brain CT 1

Introduction toBrain Imaging : CT Scans

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.


• Film quality / technical factors• Window levels• Movement artefact• Beam hardening artefact




Film quality / technical factors:

• Window levels• Movement artefact• Beam hardening artefact

Window levels alter how the image is displayed. These are called window height and window width.

Window height:• The density value the displayed image

in centred on.

Window width:• The range of density values displayed

around the centre point.

You can think of these like TV brightness and contrast.


Increasing window width

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You aren’t expected to understand window levels. All modern CT viewers offer preset values that are optimised for certain tasks.

Common presets include:• Brain windows (central image)• Bone windows (top right)• Lung windows (bottom right)

You can see that the details of certain structures can be completely obscured by the choice of window levels.


Increasing window width

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CT scans take several seconds to acquire, as the patient moves through the machine. This is less of an issue with modern scanners.

If the patient moves during the scan it can ruin the images (see left).

Unfortunately patients who need CT brain scans are often confused and can’t stay still. Sedation can be used if we expect movement to be a problem.


Study courtesy of Dr David Cuete at radiopaedia.org



Very dense materials like metal and bone block too much of the x-ray beam, so too few x-rays reach the detector. This causes beam hardening or streak artefact.

This is often a problem in the posterior fossa (the brainstem and cerebellum), as the dense bone of the skull base impairs assessment for subtle changes, like early ischaemia.

Here you can see the anterior pons appears hypodense.




• 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



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



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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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).



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.


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.

So how would you describe the location of this old infarct?• Anatomic location• Functional region• Vascular supply• Significance of this description

There is an old infarct in the left frontal lobe.

It involves the inferior motor cortex, possibly affecting Broca’s area. This patient may have expressive dysphasia.

The infarct is in the left MCA vascular territory, which is part of the anterior circulation. This may suggest a left ICA origin embolism (among other differentials).





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



Asymmetry:Very useful in CT head, because the brain structure is nearly perfectly symmetrical (with mild variation).

Density:Once you see asymmetry, the density is a useful discriminator. Describe relative to normal tissue.

“There is a left frontal hypodensity.”

“There is a left frontal CSF density abnormality.”



Hypodense (compared to brain parenchyma) materials include water, air and fat.

Water is the most important diagnostically, it is a sign of cell injury and cell death.

Air is always abnormal, and must come from outside the skull vault.

Fat is rare inside the skull, and is seen in some tumours (outside of the scope of this tute).


• Low density pathology• Water• Air

Water in the brain is a sign of cell injury or cell death.

When cells are injured, inflammation make the local capillaries “leaky”, and fluid leaks out into the intercellular space. This appears low density because the region is a mixture of soft tissue and fluid.

After cells die and are removed, fluid fills the space left behind. If no cells are left, the area is the same density as pure water (i.e. CSF).



Acute, inflammatory fluid is called oedema.

Loss of brain cells is called encephalomalacia.

These can be distinguished by the presence of mass effect or volume loss respectively.

Oedema is a mix of fluid and normal tissue so the volume expands, pushing on surrounding structures.

Encephalomalacia is the loss of cells, so the volume shrinks, making more space for surrounding structures.


How would you describe this film?• Asymmetry• Density• Oedema vs

encephalomalacia

“There is a left frontal CSF density with prominence of the adjacent lateral ventricle and sulci, suggesting volume loss. Findings are consistent with encephalomalacia related to an old infarction.”


Almost all acute pathology causes oedema, and it is readily identified on CT and MRI imaging. Looking for oedema is the easiest way to identify pathology.

In the brain, we differentiate between two “types” of oedema –cytotoxic vs vasogenic.

Cytotoxic oedema is caused by acute cell death (recent infarction).

Vasogenic oedema is caused by other inflammatory processes.


• Low density pathology• Water

• Oedema• Air

Image: © Lucien Monfils found at Wikimedia commons

So cytotoxic oedema is specific to acute stroke. We identify this by the loss of grey-white differentiation.

Grey matter is usually more dense than white matter (brighter on CT). When the cells die, both tissues become necrotic debris. The density becomes the same.

Note that you cannot identify cortex peripherally in the low density region of this image.



• Oedema• Air


“There is a large region of hypodensity in the right cerebral hemisphere, with associated mass effect and midline shift. Findings suggest oedema.

I note loss of grey-white differentiation in the right MCA territory, the appearance is consistent with an acute CVA”.


How would you describe this film?• Asymmetry • Density• Oedema vs

encephalomalacia• Cytotoxic vs vasogenic

oedema


“There is hypodensity in the left frontal lobe, with associated mass effect and midline shift. Findings suggest oedema.

I note preservation of grey-white differentiation, the appearance is not consistent with an acute CVA”.


How would you describe this film?• Asymmetry • Density• Oedema vs

encephalomalacia• Cytotoxic vs vasogenic

oedema


Vasogenic oedema is caused by all other inflammatory pathologies:

• Tumour• Infection• Trauma• Vasculitis• Etc.

The differential list is wide. Generally, clinical history and the distribution of the findings are the most useful features.

If you still can’t identify a specific cause, what else can you do?


• Oedema• Air


Post-contrast imaging can be useful to differentiate between causes of vasogenic oedema.

Contrast agents in CT scanning are dense liquids injected into the bloodstream. They circulate and accumulate in areas with increased blood flow. These areas appear more dense with contrast.

Inflammation generally increases blood flow.

How would you describe this?


• Oedema• Air


“There is vasogenic oedema in the left frontal lobe, surrounding a ring-enhancing lesion with central hypodensity.

Findings are concerning for malignancy or cerebral abscess.”

There are many patterns of disease in CNS imaging, but enhancing lesions can usually be discriminated by clinical history.


• Oedema• Air


Pneumocephaly (air in the skull vault) is always abnormal.

The most likely sources are:• Surgery• Penetrating trauma• Skull base fracture

A skull base fracture must extend into an air filled cavity (e.g. mastoids, paranasal sinuses) to cause pneumocephaly. Ectopic gas in the skull vault is a sensitive sign for subtle skull base fractures.



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

Blood outside of vessels changes density over time.

Just remember that acute blood is hyperdense (as in this image) and slowly becomes less dense as it ages.



Subacute to old blood becomes less dense, and can be similar to brain tissue or even fluid.

This image show bifrontal chronic subdural haematomas.



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

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 are within the meningeal spaces, and are usually traumatic or aneurysmal.



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.



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.


RAH Med 4 MHU - Brain CT 1

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