Orbit Overview

General Approach

Imaging of the orbit encompasses 2 clinically distinct areas of ophthalmology: (1) The eye or globe and (2) the bony orbit, soft tissues, and periorbita.

Lesions in these 2 areas result in specific clinical profiles that affect different patient groups. When a patient is referred for imaging, it is usually clear to the clinician whether the problem involves the eye proper versus some other structure of the orbit.

The term “orbital” refers to those bony structures and soft tissues that are extrinsic to the eye, as opposed to the term “ocular,” which refers to the globe itself. Most imaging referrals come from oculoplastic surgeons, neurologists, neuroophthalmologists, neurosurgeons, and otolaryngologists who need help characterizing orbital processes extrinsic to the globe. However, in some instances imaging of the globe provides complementary information to the physical and ophthalmoscopic examination.

Ultrasound

Ultrasound of the eye is a readily available complement to funduscopic examination and is traditionally performed in the ophthalmology clinic. In addition to providing imaging of the globe, transocular ultrasound provides a limited, high-resolution assessment of other soft tissue orbital structures.

Because of the availability of ultrasound, many simple diagnoses can be made without the need of further imaging. In this regard, imaging of the orbit is a function that is shared between the ophthalmologist and the neuroradiologist.

CT

Because of its superior bony characterization, CT has advantages over MR for orbital lesions that arise from or directly affect the bones, such as epithelial inclusions, osteocartilaginous tumors with matrix, osteodystrophic processes, benign masses that cause bony scalloping, and aggressive malignancies that cause bony destruction.

The presence of calcification is a specific differentiating feature in some lesions, and, therefore, CT can provide essential diagnostic information, even after an MR has been obtained. Examples include retinoblastoma, perioptic meningioma, and end-stage ocular disease (phthisis bulbi). It is worth noting that orbital cavernous hemangioma, a common intraconal orbital mass in the adult, rarely shows phleboliths or calcification.

In many instances, CT can provide enough information to allow for a definitive diagnosis and guide therapy without the need for MR. Examples include thyroid ophthalmopathy, clinically benign lacrimal mass, and orbital cavernous hemangioma.

When the diagnosis is clinically apparent, CT is often adequate to identify associated findings or complications that directly impact treatment decisions. For example, in a patient with orbital cellulitis, enhanced CT can identify the presence of sinus disease &/or abscess, thus guiding surgical therapy.

In children, CT has the particular advantage of rapid acquisition that obviates the need for sedation. However, the risk of radiation exposure is a mitigating factor that must also be considered.

MR

For evaluating complex orbital disease, MR is the preferred modality. Superior soft tissue differentiation and enhancement make MR ideal for characterizing the extent of complicated lesions, including extraocular tumors, vascular malformations, and complex inflammatory processes.

In particular, MR is the optimal modality for delineating the extent of malignant orbital disease. Important features visible on MR include perineural tumor spread, optic nerve invasion, hematogenous or cerebrospinal fluid (CSF) disseminated metastases, and intracranial extension.

Although ultrasound is usually the 1st line for imaging the globe, MR can provide a more accurate visualization of retrobulbar extension of intraocular malignancy, including retinoblastoma, ocular melanoma, and ocular metastases. Additionally, MR provides exquisite characterization of the globe itself, which is particularly useful in circumstances in which funduscopic evaluation is obscured, such as swollen or injured eye, retinal detachment, large intraocular mass, vitreal hemorrhage, or opaque media from any cause.