Choice of MRI or CT in orbital lesions

Proptosis: For most of the non-vascular lesions, pseudotumor, orbital cellulitis, thyroid eye diseases orbital cysticercosis, MRI is modality of choice. However, certain facts must be kept in mind if we suspect calcification in lesion, bony erosion or hyperostosis then CT is a better option, but MRI will be a better option if we suspect extension of lesion in orbital apex, optic canal or intracranial extension. For vascular lesions such as arterio- venous malformations, orbital varix and carotico- cavernous fistulas, MRI and MRI with contrast are better options.

Retinoblastoma: MRI is the better choice. However, it will not pick up calcification.

Papilledema and optic neuritis: MRI is the better option.

Isolated 3 rd nerve plasy: MRI with MRA to detect posterior communicating artery aneurysm, but sometimes MRA can miss aneurysm less than 5 mm in size.

Multiple cranial nerve palsy: MRI is better choice. Sometimes T2* weighted images are required to study the intracranial course of the nerves.

Unexplained vision loss: MRI.

Optic atrophy: MRI.

Trauma: Bony fracture can be better depicted by CT but soft tissue damage can be better assessed by MRI. CT is indicated for intraocular and intra orbital foreign body.

Ferromagnetic foreign body is absolute contraindication for MRI; however, MRI is better for wooden foreign body.

Orbital structures are studied with the slice thickness of 3 or 4 mm and the inter slice distance of 0.5 to 1.0 mm. Sometimes we may have to go for thinner slices depending upon the type of the lesions to be studied. Please remember by reducing the slice thickness the test duration as well as the price goes up. Usually, intracranial cuts are 3 to 5 mm with inter slice distance of 0.5 to 2.5 mm.