A wide range of processes can produce space-occupying lesions inside the orbit.
Most of them arise from orbital contents.
Cross-sectional imaging has been traditionally used for detection and mapping.
Nonetheless,
latest techniques allow differentiation between aggressive and non-aggressive tumors and might play a valuable role in characterization of these lesions.
ANATOMIC CONSIDERATIONS
A general approach to orbital tumours should consider the anatomic compartment of involvement as a first step.
The globe is located centrally in orbit. The eyeball will not be discussed in this review.
The remaining orbit is divided into two main compartments: the preseptal and the postseptal space.
These two compartments are separated by the orbital septum.
This septum is a thin membrane of fibrous tissue extending from the orbital rims to the eyelids.
It defines an anterior compartment,
anteseptal or preseptal,
and a posterior compartment,
postseptal.
The anteseptal space basically contains fat from eyelid and the orbicularis muscle.
The postseptal space can be further divided into an extraconal compartment and an intraconal compartment.
The ocular muscles originate posteriorly in the apex of the orbit (annulus of Zinn) and diversely extent towards the globe.
These muscles and their surrounding fascia form a muscle-cone,
which divides the postseptal compartment into the (intra)conal space,
internally,
and the extraconal space,
externally.
The conal or intraconal compartment is located behind the globe and peripherally limited by the myofascial-cone.
It contains: nerves,
the ophthalmic artery and fat.
The extraconal compartment lies outside the myofascial-cone and contains: nerves,
arteries,
veins,
fat and the lacrimal gland.
IMAGINING TECHNIQUES
Regarding orbital imaging the two main techniques are CT & MRI,
which are complementary.
MRI is the best initial exam in setting of suspected orbital malignancy but both are indicated for the evaluation of complex lesions.
MRI provides great soft tissue contrast for globe,
optic nerve,
orbital structures and cranial nerves.
The protocol usually includes T1 and T2 sequences,
in axial and coronal plane,
with and without fat suppression.
Gadolinium administration helps in the characterization.
CT is optimal for the evaluation of the orbit,
aided by the natural contrast between fat,
bone,
air and soft tissues.
It may provide information about invasion of adjacent bone.
One of the strongest points is the detection of calcifications.
Contrast usually is not needed if MRI is available.