Interpret / Orbital imaging
Orbital imaging
CT for bone and the acute orbit; MRI for the nerve and the brain behind it.
Ophthalmology reaches for imaging when the problem is behind the globe: an orbital fracture, cellulitis, thyroid eye disease, a mass, a compressed optic nerve, or a chiasm under a pituitary tumour. CT and MRI answer different questions, and RadSpace is where the reading is taught in depth.
CT
Fast, available, and the study for bone and the acute orbit: a blow-out fracture of the floor or medial wall with the inferior rectus caught in it, an orbital abscess and the sinusitis it came from, a metallic foreign body (which forbids MRI), and the enlarged muscle bellies with spared tendons of thyroid eye disease. Contrast for cellulitis and masses. The radiation dose to the lens is a reason not to repeat it casually in children.
| Question | Study |
|---|---|
| Fracture after trauma, foreign body | CT orbits, thin cuts, coronal reformats |
| Orbital cellulitis, abscess | CT orbits and sinuses with contrast, same day |
| Thyroid eye disease, compressive optic neuropathy | CT (muscles and apex) or MRI |
| Optic neuritis, demyelination | MRI brain and orbits with contrast and fat suppression |
| Bitemporal hemianopia, pituitary | MRI pituitary with contrast |
| Third nerve palsy with a dilated pupil | CT angiography or MR angiography, same day, for an aneurysm |
| Horner syndrome with pain | CT or MR angiography of the carotid for dissection |
MRI
The study for soft tissue: the optic nerve (enhancing and swollen in optic neuritis, with the demyelinating lesions of multiple sclerosis in the brain), the chiasm and the pituitary, orbital tumours and inflammation, the cavernous sinus, and the cortex behind a homonymous hemianopia. Fat-suppressed sequences with gadolinium make the nerve visible against the orbital fat. Not with a metallic foreign body, and slower, which matters in the acute orbit.
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