6. Principle of CT
• Computed tomography scanning an
x-ray source and detector, situated
180o across from each other, move
360o around the patient,
continuously sending and detecting
information on the attenuation of x-
rays as they pass through the body.
Finally, a computer manipulates and
integrates the acquired data and
assigns numerical values based on
the subtle differences in x-ray
attenuation
A CT scan is essentially a computerized assembly of several x-ray
images taken from a series of different angles
7. Technique of CT
- Head CTs are performed at an angle parallel to the base of the skull,
Orbito-meatal line (Reid’s line).
- Slice thickness is generally between 5 and 10 mm.
- The patient is placed in a supine position on the table.
- Evaluation of tumors, infections and some stroke cases may benefit
from the use of contrast, but contrast is not used in the routine head CT.
8. CT Terminology
Attenuation
Hyperattenuating (hyperdense)
Hypoattenuating (hypodense)
Isoattenuating (isodense)
Attenuation is measured in Hounsfield units
Scale -1000 to +1000 H.U
-1000 is air, Fat is -100 HU.
0 is water , Fluids is 20:40 H.U
+1000 is cortical bone & calcification
Hg. 60:70 HU.
10. Sulci and Gyri
• Sulcus
– Fissure in the brain tissue.
– Interhemispheric fissure –
divides the brain into left
and right hemispheres.
• Gyrus
– Elevated “hill” areas
between sulci.
Gyrus
Sulcus
Atamai
11. Meninges
Outer dura mater (Latin for “tough
mother”), beneath which is the subdural
space;
Arachnoid (Latin for “spider,” because it
resembles a cobweb), beneath which is the
subarachnoid space, which is accessed
during a spinal tap
Pia mater (“soft mother”), which is
attached to the brain and dips down into
sulci
A subarachnoid hemorrhage will extend down into
cerebral sulci, while a subdural hematoma will
not.
Pia and dura are vascular while arachnoid is
avascular
12. Ventricular system & CSF
Lateral ventricles – cerebral
hemispheres
Third ventricle, aqueduct and 4th
ventricle are in midline and in
continuity with the central canal
All are symmetrical and are lined
by ependyma
150 ml produced daily by choroid
plexus( mostly in lateral
ventricles)
Flows cephalad from basal
cisterns
19. What items should be included?
• Site(intracerebral-subarachnoid-dubdural-
extradural)
• Distribution
• Stage
• Association (Mass effect)
20. 20
B is for Blood
• Blood becomes
hypodense at
approximately 2 weeks.
• Blood becomes isodense
at approximately 1 week.
• Acute blood is bright white
on CT (once it clots).
21.
22. Well defined hyperdense intracerebral hematoma seen
in the Rt basal ganglia>>>> Acute Rt basal ganglia
intracerebral hematomabasal
23. 23
Subdural Hematoma
• Typically falx or sickle-
shaped.
• Crosses sutures, but does
not cross midline.
• Acute subdural is a marker
for severe head injury.
(Mortality approaches 80%)
• Chronic subdural usually
slow venous bleed and well
tolerated.
Axial CT scan of the brain,, demonstrates hyperdense subdural hematoma
with concave inner margin and mass effect >>>acute sub dural Lt. parietal
hematoma
24. Axial CT scan of the brain,,
demonstrates isodense
subdural hematoma in the LT
frontal region with midline shift
>>>Subacute subdural LT frontal
hematoma
Axial CT scan of the brain,,
demonstrates right fronto-parieto-
occipital subdural with layering
>>>acute on top of chronic
hematoma
25. Axial CT scan of the brain,,
demonstrates hyperdense epidural
hematoma with convex inner margin
and mass effect in the LT temporo-
occipital region >>>acute epidural LT
temporo-occipital hematoma
Extradural hematoma
27. 27
Subarachnoid Hemorrhage
• Blood in the
cisterns/cortical
gyral surface
– Aneurysms
responsible for 75-
80% of SAH
– AVM’s responsible
for 4-5%
– Vasculitis accounts
for small proportion
(<1%)
– No cause is found in
10-15%
28. Axial CT scan of the brain,, demonstrates hyperdense
hematoma in the basal cisterns and both Sylvian fissures
>>>acute subarachnoid hemorrhage
29. Axial CT scan of the brain,, demonstrates
hyperdense hematoma in the basal
cisterns,falx and both Sylvian fissures as
well as intraventricular
extension>>>acute subarachnoid
hemorrhage
34. What items should be included?
• Site and Distribution ((cortical-subcortical-
basal ganglia-lobe-lacunar))
• Stage(Recent –old-hemorrhagic)
• Mass effect or evacudilatation
38. Acute infarction
Axial CT of the brain
shows hypoattenuated
area in the cortical and
subcortical Rt temporal
region …..Rt temporal
recent infarction
43. Old infarction
Axial CT of the brain shows hypodense(CSF like) area in the cortical and subcortical
Rt Fronto-temporal region with evacudilatation>>>Cortical and subcortical Rt
fronto-temporal old infarction
