Basic concepts of computed tomography scan of the brain 1.ppt

gg

1. Basic concepts of
computed tomography scan of the brain
Part 1
By
Amir AbdelGhaffar
Lecturer of neurology
Al-Azhar faculty of medicine

2. “Advances in clinical imaging, particularly
in neuroradiology, are some of the most
exciting recent advances in medicine.”
CT became mandatory initial investigating tool
in all cases of acute CNS dysfunction.
2CT of CNS disorders by Amir
AbdelGhaffar

3. X ray of skull
•Necessitates large dose “ hard beam” to pass through bone of
both sides.
•X ray image is taken to the whole skull tissues can not be
visualized except if calcified. Also, image is not several
cross-sectional planes but one whole image.
•X ray is a shadow while CT is the result of reconstitution of
the several shadows.
3CT of CNS disorders by Amir
AbdelGhaffar

4. Major landmarks used for skull
radiography
1. Vertex.
2. External occipital protuberance.
3. External auditory meatus.
4. Outer canthus of eye.
5. Infra-orbital point.
6. Nasion.
7. Glabella.
X ray is a powerful form of electromagnetic energy

5. Other uses of skull
X ray
•Localisation of calcifications.
•Skull fractures.
•Certain diseases.
5
Myeloma skull
CT of CNS disorders by Amir
AbdelGhaffar

6. Introduction
6

7. Computerized tomography (CT)
Tissue visualization based on image reconstruction from sets
of quantitative X-ray measurements through the body.
CT is the name given to an imaging technique in which an X-
ray tube and series of X ray detectors lie opposite one
another in a fixed geometry. The resultant image is
represented in a computer to create cross sectional image.
7
CT of CNS disorders by Amir
AbdelGhaffar

8. 8
CT of CNS disorders by Amir
AbdelGhaffar

9. Important definitions
•Scout: plane of CT view.
•Voxel: the X-ray attenuation through the section plane. A
voxel is volume element (similar to picture element ‘pixel’) with
the added dimension of the section thickness to create an
image volume component.
•Hounsfeld unit: attenuation values in each pixel are
reconstructed by mathematical means. These attenuation
values are displayed as Hounsfield units. The typical range
of Hounsfield units is from -1000 to +1000. Water measures
by definition as zero Hounsfield units, air as -1000.
•Photon: unit of light beam.
•Phantom: the space around the imaged structure.
9

10. Gantry: to which an X-ray source and detector are fixed.
10CT of CNS disorders by Amir
AbdelGhaffar

11. 11CT of CNS disorders by Amir
AbdelGhaffar

12. CT data acquisition
•In order to perform a head CT, the patient is placed on the
CT table in a supine position and the tube rotates around the
patient in the gantry.
•In order to prevent unnecessary irradiation of the orbits
(especially the lenses), Head CTs are performed at an angle
parallel to the base of the skull. So, Orbito-auricular (meatal)
line is the horizontal plane of CT cuts.
•Slice thickness may vary, but in general, it is between 5 and
10 mm for a routine Head CT.
•IV contrast is not routinely used, but may be useful for
evaluation of:
–Tumors,
–CNS infections, and
–Vascular malformations & occasionally stroke.
12

13. Computerized Tomography (CT)
13
CT of CNS disorders by Amir
AbdelGhaffar

14. Advantages of CT
•More accessible, less expensive.
•Rapid & simple. This reduces patient discomfort &
motion artifact.
•CT is superior for evaluation of:
–Acute hemorrhage.
–Bones (skull & vertebrae).
•Less incidence of claustrophobia.
•No contradiction with:
–Artificial pace maker or other metallic implants.
–Mechanical ventilator work.
14
CT of CNS disorders by Amir
AbdelGhaffar

15. Disadvantages of CT
•Artifacts when imaging the posterior fossa or near
bony structures.
•The iodinated contrast used in CECT is potentially
toxic.
15
CT of CNS disorders by Amir
AbdelGhaffar

16. Different techniques of CT
•Contrast enhanced CT (CECT); for detection of BBB
breakdown.
•Spiral (helical) CT; increases scanning speed & image
acquisition (<1 sec/section). Single or multi-slice spiral CT is
one of its applications. In this type of system, the patient
table moves while the x-ray tube makes a rotation to acquire
3D volumetric data.
•CT angiography (CTA); in which iodinated IV contrast
is injected & visualized by high speed CT apparatus.
•CT perfusion (CTP); high speed CT apparatus capable
of detecting cerebral blood flow & changes in tissue
perfusion.
•Three dimensional CT.
Portable CT
16

17. Single-section helical
CT
Multi-slice
(quadsection) CT
17
CT of CNS disorders by Amir
AbdelGhaffar

18. Bone & calcified structures are dense and appear white in CT scans.
Less dense materials such as air appear black.
Clear fluid such as CSF is black or dark gray.
Adipose tissue is hypodense because fat is less dense than water.
Muscle has similar density to brain tissue.
White matter is darker than gray matter due to lipid content of myelin.
Isodense = intermediate density,
similar to brain tissue
Hyperdense = increased tissue
density, much lighter than brain tissue
Hypodense = decreased tissue
density, much darker than brain tissue
18

19. Different densities of hemorrhage
Fresh blood coagulates quickly and is hyperdense.
As clot breaks down, after ~ 1 week it is isodense.
After 2-3 weeks the clot is dissolved and becomes hypodense.
Hyperdense (acute) Isodense (1 week) Hypodense (2-3 weeks)
19CT of CNS disorders by Amir
AbdelGhaffar

20. From here we can start
•Cranial cross-sectional anatomy is very important to
know prior to analyze a head CT. Once the normal
structures are identified, abnormalities can be
detected and a diagnosis may be possible.

•Symmetry is an important concept in anatomy and is
almost always present in a normal head CT unless
the patient is incorrectly positioned with the head
tilted at an angle.
20CT of CNS disorders by Amir
AbdelGhaffar

21. 21
CT of CNS disorders by Amir
AbdelGhaffar

22. Normal anatomy
Base of the brain
A. Orbit
B. Sphenoid Sinus
C. Temporal Lobe
D. External Auditory Canal
E. Mastoid Air Cells
F. Cerebellar Hemisphere
22

23. A. Frontal Lobe
B. Frontal Bone
(superior surface of orbit)
C. Dorsum Sellae
D. Basilar Artery
E. Temporal Lobe
F. Mastoid Air Cells
G. Cerebellar Hemisphere
Normal anatomy
Base of the brain, one plane above
23

24. A. Frontal Lobe
B. Sylvian Fissure
C. Temporal Lobe
D. Suprasellar Cistern
E. Midbrain (lower part)
F. Fourth Ventricle
G. Cerebellar Hemisphere
Normal anatomy
Base of the brain, two planes above
24

25. Normal anatomy
Base of the brain, three planes above
25CT of CNS disorders by Amir
AbdelGhaffar

26. A. Anterior horn of the lateral ventricle
B. Caudate nucleus
C. Anterior limb of the internal capsule
D. Putamen & globus pallidus
E. Posterior limb of the internal capsule
F. Third ventricle
G. Quadrigeminal plate cistern
H. Cerebellar vermis
I. Occipital lobe
Normal anatomy
Base of the brain, four planes above
26

27. A. Genu of the corpus callosum
B. Anterior horn of the lat vent
C. Internal capsule
D. Thalamus
E. Pineal gland
F. Choroid plexus
G. Straight sinus
Normal anatomy
Base of the brain, five planes above
27

28. A. Falx cerebri
B. Frontal lobe
C. Body of the lateral ventricle
D. Splenium (corpus callosum)
E. Parietal lobe
F. Occipital lobe
G. Superior sagittal sinus
Normal anatomy
Base of the brain, six planes above
28

29. A. Falx Cerebri
B. Sulcus
C. Gyrus
D. Superior Sagittal
Sinus
Normal anatomy
Base of the brain, seven planes above
29CT of CNS disorders by Amir
AbdelGhaffar

30. Dural Venous Sinuses (DVS)
•DVS are venous channels lying between layers of dura mater in
the brain. They receive blood from cerebral veins (& some
external veins) & receive CSF from the subarachnoid space, and
ultimately empty into the internal jugular vein.
•The walls of DVS are composed of dura mater lined with
endothelium. They differ from other blood vessels in that they
lack a full set of vessel layers (e.g. tunica media) characteristic of
arteries and veins. It also lacks valves as seen in veins.
30CT of CNS disorders by Amir
AbdelGhaffar

31. Cerebral venous sinuses in relation
to falx and tentorium
Diagram of major DVS (transverse
sinus, sigmoid sinus and internal
jugular vein are bilateral)
The anterior part of SSS is narrow or sometimes absent, replaced by two
superior cerebral veins that join behind the coronal suture. This fact should
be borne in mind while evaluating for cerebral venous thrombosis (CVT).
31

32. Base of skull showing location of DVS
32

33. Dural Venous Sinuses:
A. Superior Sagittal Sinus
B. Great Cerebral Vein of Galen
C. Ophthalmic Veins
D. Facial Vein
E. Cavernous Sinus
F. Inferior Petrosal Sinus
G. Jugular Vein
H. Sigmoid Sinus
I. Superior Petrosal Sinus
J. Transverse Sinus
K. Straight Sinus
L. Inferior Sagittal Sinus
33CT of CNS disorders by Amir
AbdelGhaffar

34. Role of Specific Anatomic Features of Cerebral Venous
System in Pathophysiology of Cerebral Venous Thrombosis
•DVS lack valves & so blood flows in different directions. Moreover,
the cortical veins are linked by numerous anastamoses explaining the
good prognosis of cerebral venous thromboses (CVT).
•In addition to draining most of the cerebral hemisphere, SSS also
receives blood from diploic and emissary veins. Same is the case with
other DVS. This explains the frequent occurrence of CVT as a
complication of infective pathologies in the catchments areas e.g.
cavernous sinus thrombosis in facial infections, lateral sinus thrombosis
in chronic otitis media and SSS thrombosis in scalp infections.
•DVS especially the SSS contain most of the arachnoid granulations, in
which absorption of CSF takes place. So CVT leads to intracranial
hypertension and papilloedema.

35. Noncontrast CT shows a hyperattenuating (thrombosed) SSS (arrow A)
and slightly hyperattenuating transverse sinuses (arrows B).
Sinus thrombosis in CT
35

36. 36CT of CNS disorders by Amir
AbdelGhaffar

37. Skull fractures
•The brain is surrounded by CSF, enclosed in
meningeal covering, and protected inside the skull.
Furthermore, the fascia and muscles of the scalp
provide additional cushioning to the brain.
•Although these layers play a protective role,
meningeal attachments to the interior of the skull
may limit the movement of the brain, transmitting
shearing forces on the brain.
37CT of CNS disorders by Amir
AbdelGhaffar

38. Anatomy of the fractures
•The causative forces and fracture pattern, extent and position are
important in assessing the sustained injury.
•The skull is thickened at:
–Glabella.
–External occipital protuberance.
–Mastoid process.
•The skull vault is composed of cancellous bone (diploë) sandwiched
between 2 tables, the lamina externa (1.5 mm) and the lamina interna
(0.5 mm). The diploë does not form where the skull is covered with
muscles, leaving the vault thin and prone to fracture.
•The skull is prone to fracture at certain anatomic sites that include:
–The thin squamous temporal & parietal bones.
–The foramen magnum.
–The petrous temporal ridge, and the inner parts of the sphenoid wings at the skull
base in the middle cranial fossa
–The cribriform plate and the roof of orbits in the anterior cranial fossa.
–The areas between the mastoid and dural sinuses in the posterior cranial fossa.38

39. Skull fractures
•Skull fractures are categorized as linear or depressed.
Depressed fractures are characterized by inward (below the
skull surface) displacement of fracture fragments.
Linear fractures are more common. It results from low-
energy blunt trauma over a wide surface area of the skull. It
runs through the entire thickness of the bone.
•By itself, it is of little significance except:
–Vascular channel → Epidural hematoma.
–Venous sinus groove → Venous sinus thrombosis.
–Suture → Sutural diastasis.
39CT of CNS disorders by Amir
AbdelGhaffar

40. Skull fractures
Linear skull fracture of the right parietal
bone (arrows)
Depressed skull fracture
40
CT of CNS disorders by Amir
AbdelGhaffar

41. Basal Skull Fracture
Clinical features indicate the presence of a basal skull fracture which
may not be evident on routine CT or even on specific views of the skull
base. However, Base of skull fracture may be suggested by air-fluid
(blood) level in the sphenoid sinus
These signs if present, a potential route of infection exists with the
concomitant risk of CNS infection.
CSF leak:
• CSF Rhinorrhoea (Ant fossa fracture); if the nasal discharge contains
glucose, then the fluid is CSF rather than mucin.
• CSF otorrhoea (Middle fossa fracture) ; if combined with Bl must be
differentiated from a laceration of the ext auditory meatus.
41
CT of CNS disorders by Amir
AbdelGhaffar

42. 42

43. 43CT of CNS disorders by Amir
AbdelGhaffar

44. Delayed signs after
fracture base of the skull
•Battle's sign is bruising over
the mastoid sinus (just behind
the auricle) after 24-48 hours.
•Hemotympanum blood
behind the ear drum.
•Raccoon eyes periorbital
bruising.
•Subconjunctival hge;
bruising extending to the
posterior limit of sclera
indicated Bl tracking from
orbital cavity.
44
CT of CNS disorders by Amir
AbdelGhaffar

45. Subconjunctival haemorrhage due
to fracture base of skull
Subconjunctival haemorrhage
due to ocular trauma or
haemorrhagic blood disease
45

46. Traumatic brain injury (TBI)
TBI may lead to:
•Hemorrhages:
•Subdural hemorrhage (SDH).
•Epidural (extradural) hemorrhage (EDH).
•Intraventricular hemorrhage (IVH).
•Parenchymal injury:
•Difuse axonal injury (DAI).
•Cerebral contusion (CC).
Brain hemorrhages following trauma often evolve
over the first 24-72 hours.
46

47. Acute subdural hemorrhage (SDH)
•Deceleration and acceleration or rotational forces that tear
bridging veins can cause SDH.
• The blood collects in the space above the arachnoid matter
and below the dura matter.
•The hematoma on CT has the following characteristics:
- Crescent (concave) shaped.
- Hyperdense, but may contain hypodense foci due to:
–Serum.
–CSF.
–Active bleeding!
- Does not cross dural (pia matter) reflections at sulci.
47
CT of CNS disorders by Amir
AbdelGhaffar

48. 48
CT of CNS disorders by Amir
AbdelGhaffar

49. 49CT of CNS disorders by Amir
AbdelGhaffar

50. 50

51. The hypodense region (arrow) within the
high density hematoma (arrowheads)
51

52. Subacute Subdural Hematoma (SDH)
•Subacute SDH may be difficult to be visualized by CT. As
the hematoma is being reabsorbed it becomes isodense.
Subacute SDH should be suspected when you identify shift of
midline structures without an obvious mass.

•IV contrast leads to enhancement of the dura & adjacent
vascular structures. So, the interface between the hematoma
and the adjacent brain becomes more obvious.
•Some of the notable characteristics of subacute SDH are:
- Compressed lateral ventricle.
- Effaced sulci.
- White matter "buckling“.
- Thick cortical "mantle“. 52
CT of CNS disorders by Amir
AbdelGhaffar

53. Subacute SDH (arrowheads). Note the
compression of gray & white matter in
the left hemisphere due to the mass effect
This haematoma is almost isodense.
It is probably about one to two weeks old.
53

54. Chronic Subdural Hematoma (SDH)
•Chronic SDH becomes low density as the
hemorrhage is further reabsorbed.
•It is usually uniformly low density but may be
loculated.
•Rebleeding often occurs and causes mixed density
and fluid levels.
54CT of CNS disorders by Amir
AbdelGhaffar

55. Chronic SDH (arrowheads) shows the
separations and loculations that often
occur over time
Crescent shaped chronic SDH
(arrowheads). Notice the low attenuation
due to reabsorbtion of the hge over time
55

56. Epidural hematoma (EDH)
•EDH is usually associated with a skull fracture. It often
occurs when an impact fractures the calvarium (skull cap).
•The fractured bone lacerates a dural artery or a venous sinus.
The blood from the ruptured vessel collects above the dura &
below the inner table of the skull.
•On CT, EDH forms a hyperdense biconvex (lenticular) mass
with sharp margins. It is usually uniformly high density but
may contain hypodense foci due to active bleeding.
•Since EDH is extradural it can cross the dural reflections
unlike SDH. However EDH does not cross suture lines where
the dura tightly adheres to the inner table of the skull.
56

57. EDH vs SDH
In EDH, lateral transtentorial herniation occurs and there is a lucid interval
between consciousness and coma.
There is no focal signs.
In SDH, There are no lucid periods and focal signs usually appear later
57

58. EDH commonly occurs in association with temporal-parietal
bone fractures because the middle meningeal artery is
lacerated.
58

59. Biconvex EDH (arrowheads), deep
the parietal skull fracture (arrow)
High density, crescent shaped hematoma
(arrowheads) overlying the Rt cerebral
hemisphere. Note the shift of the ‘midline’
septum pellucidum due to mass effect (arrow)
59

60. Intraventricular Hemorrhage (IVH)
•Traumatic IVH is associated with diffuse axonal
injury (DAI), deep gray matter injury and brainstem
contusion.
•Potential causes for IVH are:
–Extension from an intraparenchymal hemorrhage.
–Rupture of subependymal veins.
–Aneurysm or AVM rupture.
–Vertebral artery dissection!.
•IVH from a ruptured aneurysm has a poor prognosis
(mortality 64%).
60CT of CNS disorders by Amir
AbdelGhaffar

61. IVH
Traumatic IVH (arrow).
Note the SAH in the sulci (arrowheads)
Isolated IVH
61

62. Diffuse Axonal Injury (DAI)
•DAI is a "shear injury". It is the most common cause of significant
morbidity in head trauma.

•Fifty percent (50%) of all primary intra-axial injuries are DAI.
•Acceleration, deceleration and rotational forces cause portions of the
brain with different densities to move resulting in tearing of axons.

Immediate loss of consciousness is typical of these injuries
•CT in DAI may be normal despite the patient's profound neurological
deficit. However, it may show ill-defined areas of hemorrhage in
characteristic locations based on the pattern of trauma.
•The following list of frequent DAI locations:
- Subcortical white matter
- Posterior limb of internal capsule
- Corpus callosum
- Dorsolateral midbrain
62

63. 63

64. DAI-hge in the corpus callosum (arrow)
DAI-hge of the posterior limb of the
internal capsule (arrow) and hge of the
thalamus (arrowhead)
64

65. Cerebral Contusion (CC)
•CC is the most common intra-axial injury. It often occurs
when the brain impacts an osseous ridge or a dural fold.
•On CT, CC appears as an ill-defined hypodense areas mixed
with foci of hemorrhages along gyral crests. Adjacent SAH
is common.
•After 24-48 hours, hemorrhagic transformation or
coalescence of petechial hemorrhages into a rounded
hematoma is common.
•The following are common locations of CC:
- Temporal lobe; anterior tip, inferior surface, sylvian region.
- Frontal lobe; anterior pole, inferior surface.
- Midbrain; dorsolateral parts.
- Cerebellum; inferior parts.
65
CT of CNS disorders by Amir
AbdelGhaffar

66. 66

67. Multiple foci of hyperdensity corresponding to hge (arrows) in an
area of low density (arrowheads) in the left frontal lobe
67

68. 68CT of CNS disorders by Amir
AbdelGhaffar