role of CT SCAN in OPHTHALMOLOGY, CT findings in different diseases of orbit and eye, and trauma of orbit and globe.

1. D R . S U N I T A K U M A W A T
D E P T T . O F O P H T H A L M O L O G Y
S . P . M . C . B I K A N E R
COMPUTED TOMOGRAPHY

2. Computer tomography (CT), originally known as
computed axial tomography (CAT or CT scan) and
body section rentenography.
Designed by Godfrey N.
Hounsfield to overcome
the visual representation
challenges in radiography
and conventional
tomography

3. Plain radiography
involves X-rays that pass
through the patient, and
create an image directly
on a photographic film.
Image is basically a
shadow.
Shadows give you an
incomplete picture of an
object's shape

4. Thus a three dimensional
structure is depicted on a
two dimensional plane,
giving rise to disturbing
superimposition.

5. CT completely eliminates the superimposition of
images of structures outside the area of interest.
The word "tomography" is derived from the Greek
tomos (slice) and graphein (to write).
● definition - imaging of an object by analyzing its
slices

6. Cross section slices
Think like looking into a loaf of bread by cutting it
into thin slices and then viewing the slices
individually.

7. Because of inherent high-contrast resolution
of CT, tissues that differ in physical density
by less than 1% can be distinguished.
It provides quicker scans, is able to image
bone directly, shows the presence of
calcification better, and is the modality of
choice in patients with suspected metallic
orbital foreign bodies.

8. CT generally more widely available and cheaper.
Allows us to discern the location, extent and
configuration of the lesion and lesion’s effect on
adjacent structures.
In addition, knowing the precise location of a
lesion, it facilitates the planning of an appropriate
surgical approach to minimise morbidity.

9. The CT machine: Principle
X-rays are attenuated, on their way through
tissues due to absorption of energy.
Different tissues provide different degrees of X-ray
attenuation, and it is this property that forms the
basis of CT imaging technique.
CT machine combines X radiation and radiation
detectors coupled with a computer to create cross
sectional image of any part of the body.

10. The X-ray tube of the CT machine emits a thin
collimated fan-shaped beam of X-rays that are
attenuated as they pass through the tissues.
They are detected by an array of special detectors.

11. Within these detectors, X-ray photons generate
electrical signals, which are converted into images.
High density areas are arbitrarily depicted as white
whereas low density areas appear black.
CT images contain information from thin slices of
tissue only, and are thus devoid of superimposition.

12. Slice thickness:
Spatial resolution of a CT depends on slice
thickness. The thinner the slice, the higher the
resolution.
Vary from 1-10 mm.
Usually, 2mm cuts are optimal for the eye and
orbit.
In special situations (like evaluation of the orbital
apex), thinner slices of 1mm can be more
informative.

13. Thin slices are good for spatial resolution, but
require higher radiation dose, a greater number of
slices, and eventually longer examination time. The
choice of slice thickness therefore is a balance of
these factors.

14. CT terminology
Attenuation
 Hyperattenuating (hyperintense): tissue with
high protien content (lens, clotted blood,
tenacious mucus secretions)
 Hypoattenuating (hypointense): pathologies with
high water content (edema, necrosis)
 Isoattenuating (isodense)

15. Attenuation is measured in Hounsfield units.
2000 HU Scale: (-1000 to 1000)
1. -1000 is air (allows 100% transmission of X-
rays)
2. 0 is water
3. 1000 is cortical bone

16. What we can see
 The brain is grey
White matter is usually dark grey (40)
Grey matter is usually light grey (45)
CSF is black (0)
Things that are bright on CT
Bone or calcification (>300)
Contrast
Hemorrhage (Acute ~ 70)
Hypercellular masses
Metallic foreign bodies

17. Tissue window:
Tissues around the orbit form a spectrum of
composition and density, ranging from air (within
the para-nasal sinuses) to the bony orbit.
Tissue window refers to the selection of a small
range from this variable spectrum to decipher the
finer details of the tissue of interest. Each tissue
window has a specific window width and window
level. Thus we have bone window, soft tissue
window, brain window and so on

18. A thorough evaluation of
any tissue is possible only
when it is scanned under
appropriate window
settings.
Soft-tissue window is best
for evaluating orbital soft
tissue lesions, whereas
fractures and bony details
are better seen with bone
window settings

19. Window width(WW): refers to the span of CT
numbers on the Hounsfield scale that are selected to
display the given image.
Can vary from a few CT numbers to the entire range
available on the system.
Since the Hounsfield scale usually ranges from –
1000 to +1000 HU or above, the maximum WW can
be approximately 2000.

20. Thus at a WW of 2000, air will be black and bone
will be white. The rest of the tissues will be depicted
in shades of gray between these two extremes of the
spectrum.
A wider WW thus depicts a large number of tissues,
and bone details can be better appreciated.

21. Window level (WL):
Midpoint of the selected span
of Hounsfield value,(point
midway between totally black
and totally white).
Eg:WW of 100 with a WL of
+50 displays all tissues with
Hounsfield value ranging from
zero to +100 HU.
Values above +100 HU will be
white, those below zero will be
black.

22. Those between the two will
have all shades of gray. This
window setting is ideal for
soft tissue evaluation.
WW of 2000 with a WL of
+200 displays all tissues
with Hounsfield value
ranging from –800 HU to
+1200 HU.
This setting is ideal for
evaluation of bone.

23. Intraocular structures show very low variations in
tissue consistency and thus need a fairly narrow
window setting,
whereas structures within the orbit show a wide
variation in tissue consistency, and require a wide
window setting.

24. Contrast enhancement:
Contrast study involves imaging the area of
interest after intravenous injection of a radiological
contrast medium.
Most orbital pathologies can be easily visualised
without infusion of a contrast medium as orbital
fat provides intrinsic background contrast.
A contrast-enhancing lesion is one which becomes
bright or more intense after contrast medium
infusion.

25. Cotrast agent Increases tissue’s Hounsfield value
and thus increases its brightness.
Evaluation of optic chiasma, perisellar region and
extraorbital extensions of orbital tumours is best
possible with contrast enhancement.
Contrast enhancement also helps to define
vascular and cystic lesions as well as optic nerve
lesions, particularly meningioma and glioma.

27. Imaging planes
1. Axial
2. Coronal
3. Sagittal
Plane-axial imaging parellel to infra-orbitomeatal line

28. The plane inclined at 30° to the orbito-meatal line
best depicts the optic canal and the entire anterior
visual pathway.

30. Passages of the Bony
Orbit
1.Superior orbital fissure
2.Inferior orbital fissure
3.Optic canal

31. Components of CT scan:
1. Patient data and laterality
2. Slice thickness
3. Type of CT scan:

32. Axial scan

34. Coronal scan

36. Approach to differential diagnosis:
General principle:
Location,
Anatomic structure,
Imaging feature and
Clinical presentation of patient.

37. Using a compartmental approach, a lesion is first
localized to one of four compartments:
1. Globe,
2. Optic nerve sheath complex,
3. Intraconal space,
4. Extraconal space.

38. Once primary location of a lesion determined we
should consider other parameters:
Characterstics of margins of lesion,
Associated bony changes,
Enhancement pattern,
Pathophysiologic basis,
Age of presentation .

39. Imaging in different pathophysiologies

40. Trauma
CT is the procedure of choice in evaluating
patients with orbital trauma. It is a rapid test that
can detect bony and soft tissue injury,
haemorrhage and foreign bodies.
May be blunt, penetrating or involve implantation
of foreign bodies.
The classical injury seen in blunt trauma is the
blowout fracture. This commonly involves the floor
and medial wall of the orbit.

41. Orbital fracture
A “tear drop” shape pointing toward the fracture,
indicating muscle sheath tethering.

43. Orbital fracture
Inferior and medial
blow out fractures:
Coronal CT showing
displacement of bony
fragments of left
orbital floor and
fracture of left lamina
papyracea.

44. Orbital fracture
Partial herniation of medial rectus and orbital fat
into left ethmoid air cells through fractured lamina
papyracea.
Intra orbital emphysema, should raise suspicion of a
blowout fracture.

45. Orbital fracture

46. Orbital fracture

47. Globe rupture
Anterior chamber is
shallow,
Density in AC is
higher,
Area of high density
in vitreous .
Globe is flat on
posterior side.

48. Proptosis
A line connecting the most distal tips of lateral
orbital walls is drawn.
The distance from ant. Margin of globe to this line
should not exceed 21 mm.

49. Intra ocular foreign body
Axial scan of a patient with retained intraocular
foreign body. The radiodense streaks radiating from
the foreign body represent beam hardening artifact
which is typically seen with metallic foreign bodies

50. Intra ocular foreign body

51. Infections
Patient with right
periorbital
inflammation.
Axial CT demonstrates
right lid swelling and
extent of preseptal
inflammatory swelling.

53. Bilateral discrete lacrimal gland enlargement

54. Sub periosteal abscess

55. Fronto ethmoid mucocoele:

56. Thyroid associated ophthalmopathy
Isodense Fusiform
enlargement of extraocular
muscle with sparing
tendinous attatchment.
Additional increased orbital
fat, lacrimal gland
enargement, eyelid edema,
stretching of optic nerve may
be seen.

57. Thyroid ass. ophthalmopathy

58. Orbital pseudotumor
Moderate diffuse enhancement
Presenting as infiltrative soft tissue mass in right orbit
with loss of normal tissue planes.

59. Orbital pseudotumor
Focal &infiltrative,
poorly circumscribed,
thickening on lacrimal
and orbital structures
may be seen.
Thickening of muscles
involves tendinous
insertion.

60. Optic neuritis
CT usualy normal,
May show minimal optic nerve enlargement,
Best imaging is MRI,
Ct showing thickening and straightening of optic nerve

61. Optic nerve glioma
Usually involve optic nerve,
chiasm, and optic tract.
Causes enlargement and
tortuosity these structures.
optic nerve may show kinking &
tortuosity or fusiform
enlargement.
Iso to slightly hypointense.

62. Optic nerve glioma
Enlargement of optic canal
Tubular thickening of the
right intraorbital optic
nerve.(a)
(b, different patient) There
is marked fusiform
enlargement of the left
optic nerve causing
anterior displacement
of the globe.

63. Optic nerve sheath meningioma
Segmental or diffuse
thickening of optic nerve
Fusiform and uniform
thickening of optic nerve
sheath.
Normal optic nerve running
through the tumor have
“tram- track” appearance on
axial and saggital images.

64. Lymphoma
On CT it appears
commonly as
hyperdense contrast
enhancing mass.
Axial, oblique sagittal
and coronal contrast
enhancing CT showing
patient with preseptal
lymphomatous mass
(arrow)

65. Retinoblastoma
Hyperattenuating
mass.
Presence of
calcification
Margins may
smooh or
irregular.
Enhances on
contrast.

68. Metastasis
Aggressive destruction of b/l orbital walls.
Intra orbital and extradural enhancing soft tissue
mass.

69. Fibrous dysplasia
Fibrous dysplasia most
commonly involves the
frontal or sphenoid bone.
It characteristically
produces expansion of the
bone shown as prominent
calcified bone density
material on CT.

70. Dermoid cyst
Well defined non enhancing low density lesions.
Bony erosion may occur related to their slow growth
Dermoid cyst showing low fat density contents, and
remodeling of the adjacent bone.

71. Capillary haemangioma
Extraconal in location and
tend to occur in anterior
part of orbit.
On CT: Well
circumscribed or
infiltrative lesions with
characteristic intense
homogenous
enhancement.
Intracranial extension
through superior orbital
fissure or optic canal can
occur

72. Cavernous haemangioma
Coronal CT scan
showing left
cavernous
haemangioma. mass
is intraconal, well
defined,
homogeneous, with
smooth margins
Hyperdense
Well circumscribed

73. Encapsulated
Can be easily separated from the optic nerve and
extraocular muscles.

74. Orbital lymphangioma
Axial CT scan of A case of orbital lymphangioma
irregular margin, heterogenous consistency, and the
presence of phleboliths;

75. Degenerative
Optic drusen: on CT, disc shows calcification.