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1. Introduction to
Brain Imaging : CT Scans
RAH Radiology

2. 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.

3. How to interpret CT brain:
• Film quality / technical factors
• Window levels
• Movement artefact
• Beam hardening artefact
• Important anatomic structures
• Basic patterns of disease
Introduction to Brain Imaging : CT scans

4. 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.
Introduction to Brain Imaging : CT scans
Increasing window width
Increasingwindowheight

5. Film quality / technical factors:
• Window levels
• Movement artefact
• Beam hardening artefact
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.
Introduction to Brain Imaging : CT scans
Increasing window width
Increasingwindowheight

6. Film quality / technical factors:
• Window levels
• Movement artefact
• Beam hardening artefact
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.
Introduction to Brain Imaging : CT scans
Study courtesy of Dr David Cuete at radiopaedia.org

7. Film quality / technical factors:
• Window levels
• Movement artefact
• Beam hardening artefact
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.
Introduction to Brain Imaging : CT scans

8. How to interpret CT brain:
• Film quality / technical factors
• Important anatomic structures
• Describe cortical features
• Deep brain structures
• CSF spaces
• Vessels
• Basic patterns of disease
Introduction to Brain Imaging : CT scans

9. 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

10. 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

11. 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.
1
3
4
6
1. Broca’s area – expressive dysphasia
2. Lower motor cortex (mouth/tongue) – dysarthria
3. Upper motor cortex (limbs) – hemi/monoparesis
4. Wernickie’s area – receptive dysphasia
5. Auditory cortex – cortical deafness
6. Visual cortex – homonymous hemianopia
5
2

12. 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).

13. 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.

14. 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.

15. 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).
Introduction to Brain Imaging : CT scans

16. How to interpret CT brain:
• Film quality / technical factors
• Important anatomic structures
• Basic patterns of disease
• Approach to CT scans
• Low density pathology
• High density pathology
Introduction to Brain Imaging : CT scans

17. • 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.”
Introduction to Brain Imaging : CT scans

18. • Basic patterns of disease
• Approach to CT scans
• Low density pathology
• High density pathology
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).
Introduction to Brain Imaging : CT scans

19. • 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).
Introduction to Brain Imaging : CT scans

20. • Low density pathology
• Water
• Air
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.
Introduction to Brain Imaging : CT scans

21. 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.”
Introduction to Brain Imaging : CT scans

22. 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.
Introduction to Brain Imaging : CT scans
• Low density pathology
• Water
• Oedema
• Air
Image: © Lucien Monfils found at Wikimedia commons

23. 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.
Introduction to Brain Imaging : CT scans
• Low density pathology
• Water
• Oedema
• Air
Image: © Lucien Monfils found at Wikimedia commons

24. “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”.
Introduction to Brain Imaging : CT scans
How would you describe this film?
• Asymmetry
• Density
• Oedema vs
encephalomalacia
• Cytotoxic vs vasogenic
oedema
Image: © Lucien Monfils found at Wikimedia commons

25. “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”.
Introduction to Brain Imaging : CT scans
How would you describe this film?
• Asymmetry
• Density
• Oedema vs
encephalomalacia
• Cytotoxic vs vasogenic
oedema

26. Introduction to Brain Imaging : CT scans
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?
• Low density pathology
• Water
• Oedema
• Air

27. Introduction to Brain Imaging : CT scans
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?
• Low density pathology
• Water
• Oedema
• Air

28. Introduction to Brain Imaging : CT scans
“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.
• Low density pathology
• Water
• Oedema
• Air

29. Introduction to Brain Imaging : CT scans
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.
• Low density pathology
• Water
• Air

30. Introduction to Brain Imaging : CT scans
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.
• Basic patterns of disease
• Approach to CT scans
• Low density pathology
• High density pathology

31. Introduction to Brain Imaging : CT scans
Subacute to old blood becomes less
dense, and can be similar to brain
tissue or even fluid.
This image show bifrontal chronic
subdural haematomas.
• Basic patterns of disease
• Approach to CT scans
• Low density pathology
• High density pathology

32. Introduction to Brain Imaging : CT scans
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.
• Basic patterns of disease
• Approach to CT scans
• Low density pathology
• High density pathology

33. Introduction to Brain Imaging : CT scans
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

34. 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