Plain film

Plain film radiography is no longer used routinely for the evaluation of orbital pathology, but familiarity with normal anatomy remains important when reviewing emergency department trauma radiographs (Fig. 2.1).

Cross-sectional anatomy

The primary imaging modalities used to examine the orbit and visual pathways in clinical practice are CT and MRI. The divergent, conical anatomy of the orbits and their contents means that a combination of axial, coronal and parasagittal scan planes may be required to delineate anatomical structures optimally.

CT demonstrates orbital anatomy well due to the substantial differences in attenuation of bone, air in adjacent paranasal sinuses, orbital fat and sot tissues. In particular, helical CT with multiplanar reconstructions provides excellent bony anatomical detail. Coronal reformatted images are important for the bony anatomy at the orbital apex, the orbital floor and roof.

MRI is valuable for evaluation of intra-orbital soft-tissue anatomy and is unhindered by artefacts from surrounding bone. Imaging protocols usually include axial and coronal sequences, including thin-section coronal T2-weighted scans with fat suppression. Intravenous gadolinium-enhanced T1-weighted imaging is also combined with fat suppression so that enhancing structures are not obscured by the intrinsic high-T1 signal of normal orbital fat. Acquisition times should be short to minimize the ef ects of eye movement.

MRI is the preferred technique for demonstration of the intracranial optic nerves, optic chiasm and tracts.