CT of the orbit Some essential CT technical points

1. CT TEACHING
LECTURES

2. CHALLENGE
‫لكفى‬ ‫الفراغ‬ ‫أوقات‬ ‫شغل‬ ‫غير‬ ‫فائدة‬ ‫من‬ ‫وتحصيله‬ ‫العلم‬ ‫طلب‬ ‫في‬ ‫يكـن‬ ‫لم‬ ‫لو‬
‫ت‬ ‫الحنبلي‬ ‫عقيل‬ ‫ابن‬ ‫الوفاء‬ ‫أبو‬:495‫هـ‬

3. CURRICULUM:
Lecture Parts Parts Additional
Lecture1 Anatomy, vascular anatomy 26-32 Radiant Dicom viewer
Lecture2 Variants, artifacts, trauma,
Hemorrhage, tumors, inflammatory
52-60 PACS
Lecture3 Orbit 33-40 CT technique: image
acquisition, WW, WL
Lecture4 PNS-Petrous 41-49, 60-
63
MIP, VR, inverted MIP
Lecture5 Neck, thyroid, LN 64-73 word skills

4. THE ORBITAL CONTENTS
• The orbit contains the globe, the extraocular
muscles, the lacrimal gland, the optic nerve
and the ophthalmic vessels.
• The whole is embedded in fat
• The orbit is limited anteriorly by the orbital
septum = thin layer of fascia that extends
from the orbital rim to the superior and
inferior tarsal plates, separating the orbital
contents from the eyelids

5. 6 Extrinsic ocular muscles: insert into the sclera
• 4 recti = superior, inferior, medial and
lateral recti,
• origin from a common tendinous ring =
annulus of Zinn : attached to the superior
orbital fissure. Insertion = the
corresponding aspects of the globe,
anterior to its equator.
• 2 obliques = superior, inferior

6. BLOOD SUPPLY
• The arterial supply of the orbit is
from the ophthalmic artery
• The venous drainage of the orbit is
through the superior and inferior
ophthalmic veins into the
cavernous sinus

7. THE SKULL BASE
The inner aspect of the skull base is made up of the
following bones from ant. to post:
1. The orbital plates of the frontal bone, with the
cribriform plate of the ethmoid bone and crista galli
in the midline
2. The sphenoid bone with its lesser wings anteriorly,
the greater wings posteriorly, and body with the
elevated sella turcica in the midline;
3. Part of the squamous temporal bone and the
petrous temporal bone; and
4. The occipital bone

8. THE SPHENOID BONE
consists of a body and greater and lesser wings.
• It houses the sphenoid sinuses -cavernous
sinus and carotid artery run.
• A deep fossa superiorly known as the sella
turcica
• Anteriorly; tuberculum sellae;
• anterior to it = optic chiasm
• Two bony projections
• anterior clinoid processes
• The posterior part of the sella = dorsum sellae
• posterior clinoid processes

9. THE TEMPORAL BONE consists of four parts:
1. flat squamous part, which forms part of the
vault and part of the skull base
2. pyramidal petrous part, which houses the
middle and inner ears and forms part of the
skull base
3. aerated mastoid part
4. inferior projection known as the styloid
process

10. COMPUTED TOMOGRAPHY
• CT is an excellent modality for demonstrating the extraocular contents of the orbit
The lacrimal gland, extraocular muscles, globe, optic nerve and superior
ophthalmic vein are routinely seen The lens has a low water content and is dense
on CT
• The bony walls of the orbit are demonstrated, and the foramina of the orbit and
related anatomy are readily assessed
• Coronal images are best for assessment of the orbital floor, especially in trauma

11. MAGNETIC RESONANCE IMAGING
• MRI demonstrates the soft tissues of the orbit It may be performed in any plane It is of
particular value in demonstrating the optic nerve, allowing excellent visualization of
the entire nerve, including the intracanalicular segment on vertical oblique images
along the nerve’s long axis
• On coronal images the 3rd, 4th and 6th nerves and the first division of the 5th can be
seen just below the anterior clinoid process
• Images of the intraorbital part of the optic nerve are performed with fat saturation
pulses to help distinguish the optic nerve and its sleeve of dura and cerebrospinal
fluid (CSF) from the surrounding high-signal fat
• The lacrimal gland lies lateral to the levator palpebrae

12. CT IMAGE ACQUISITION
• Acquire = Get & hold

13. WINDOW VERSUS RECONTRUCTION

14. WINDOW/LEVEL : WW-WL
Window = how many HU within the
256 shades of gray.
• Wider the window (larger #)
• More densities seen
• Less contrast
• Narrow window (smaller #)
• Less densities seen
• More contrast
Level = where is this window
centered
• More dense tissue  higher level
• Less dense tissues  Lower level

15. Window Name Window Level Window Width
Lung = Air -400 1500
Chest = Mediastinal
Window
40 400
Abdomen 60 400
Brain 40 80
Angio 300 600
Bone 300 1500

16. RECONSTRUCTION
• Multiplanar reconstruction
• MIP - MinIP – VR
• Pixel/ Voxel
• Slice/ Slab

17. MAXIMUM INTENSITY PROJECTION (MIP)
• Maximum Intensity Projection
(MIP) consists of projecting the voxel
with the highest attenuation value on
every view throughout the volume
onto a 2D image

18. MIP
• This method tends to display bone and contrast material–filled structures
preferentially, and other lower-attenuation structures are not well visualized.
• The primary clinical application of MIP is :
• improve the detection of pulmonary nodules and assess their profusion. MIP also
helps characterize the distribution of small nodules.
• In addition, MIP sections of variable thickness are excellent for assessing the size and
location of vessels, including the pulmonary arteries and veins.