Diagnostic and surgical techniquesAnatomy of the Orbital Apex and Cavernous Sinus on High-Resolution Magnetic Resonance Images
Section snippets
Examination Protocols
Magnetic resonance imaging examinations were performed in eight normal volunteers on a 1.5-T MRI unit (Gyroscan ACS-NT; Philips Medical Systems, Best, the Netherlands). A circular polarized head coil was used for the cavernous sinus and brain stem, and a surface coil with a diameter of 17 cm was used for the orbital apex. Scans 1.2 to 3 mm thick with −0.6- to 0.3-mm interslice gap were obtained in axial, oblique-axial (along the neuro-ophthalmic plane82, 83), coronal, oblique-coronal (along the
Bony anatomy
Cortical bone does not produce a perceptible signal and is seen only indirectly by contrast demarcation to adjacent signal-generating tissue (e.g., brain, cerebrospinal fluid [CSF], muscle, fat, sinus mucosa, etc.). Cancellous bone is visualized indirectly by its fatty tissue content.
The orbital apex is defined as the region between the posterior ethmoidal foramen and the openings of the optic canal and the superior orbital fissure. The posterior ethmoidal foramen is visible on axial and
Sequences and coils
Magnetic resonance imaging demonstrates the anatomy of the orbit and retro-orbital region with superb detail. The best resolution of orbital structures is presently obtained by using standard T1w SE2, 3 or T2w TSE pulse sequences. Conventional T2w and proton density images need too long an acquisition time leading to motion artifacts. For the orbits, local surface coils2, 3 and, whenever possible, phased assay coils should be used. However, since the signal decay in the orbital apex depends on
Anatomic Features Demonstrated by MRI
All major anatomic structures, including the origin of the extraocular muscles27 and the apical orbital connective tissue system, can be demonstrated by MRI.
In the selected pulse sequences, blood vessels usually appear dark (“signal void”), as discussed earlier.52 All important arterial and venous vessels of the orbit can be identified without contrast enhancement.26
It is also possible to delineate the intraorbital and intracranial course of sensory and motor cranial nerves of the orbit on MRI.
Clinical Implications
High-resolution MRI enables exact delineation of space occupying orbital processes in relation to surrounding anatomical structures, thus facilitating planning of surgical procedures. This feature will be essential for computer-assisted surgery using neuronavigation.59
MRI reveals information on blood flow and may differentiate between flowing and stagnant blood in orbital vascular lesions. This is extremely important for treatment planning. Magnetic resonance angiography may provide further
Method of Literature Search
An on-line search of the international literature was conducted by MEDLINE, covering all years from 1966 to 1999, and using the following search words: magnetic resonance imaging (MRI), cavernous sinus, orbital apex. Other sources included original and review articles from our own files, articles cited in the reference lists of other articles, and standard textbooks on orbital imaging and anatomy. An important inclusion criterion was the coverage of anatomic and radiologic rather than purely
Outline
I. Examination protocols
II. Imaging
A. Bony anatomy
B. Extraocular muscle and connective tissue
system anatomy
C. Cranial nerve anatomy
1. Optic nerve and chiasm
2. Motor nerves
a. Oculomotor nerve
b. Trochlear nerve
c. Abducens nerve
3. Sensory nerves
a. Ophthalmic nerve (V.1)
b. Maxillary nerve (V.2)
D. Vascular anatomy
1. Arterial system
a. Internal carotid artery
b. Ophthalmic artery
2. Venous system
a. Veins
b. Parasellar venous plexus (cavernous sinus)
III. Discussion of techniques
A. Sequences and coils
B.
Key to Abbreviations
ACA anterior cerebral artery
ACP anterior clinoid process
AICA anterior inferior cerebellar artery
BA basilar artery
C clivus
CC chiasmatic cistern
CLT cisterna laminae tecti
COI colliculus inferior laminae tecti
COS colliculus superior laminae tecti
CP cerebral peduncle
CRA central retinal artery
CSF cerebrospinal fluid
DS diaphragm sellae
ES ethmoidal sinus
FLB frontal lobe of brain
FN frontal nerve
FV fourth ventricle
G globe
GWS greater wing of sphenoid bone
H hypophysis
ICA internal carotid artery
IH
Acknowledgements
We thank our radiologic technicians for collecting the imaging data, Erika Just for her secretarial assistance, and Peter Mentil and Anton Jaeger for preparing the figures.
The authors have no proprietary or commercial interest in any products or concepts discussed in this article.
This study was supported by the Ludwig Boltzmann-Institute for Interventional Magnetic Resonance, St. Pölten, Austria.
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