Ultrasound of the orbit is best performed with a high-frequency linear transducer, ideally 10 MHz or higher. US is used to evaluate the eye and superficial structures and provides limited evaluation of deeper intraorbital structures. It is particularly useful when ophthalmoscopic examination is limited due to the presence of opaque ocular media such as by cataract or vitreous hemorrhage. US is commonly performed by the ophthalmologist as an office procedure for evaluating a mass seen during ophthalmoscopy, to look for a mass underlying a retinal detachment, to evaluate ocular trauma, and for biometry. Color Doppler can be used to evaluate lesional vascularity.

CT is the modality of choice in suspected orbital trauma, in orbital infection in the setting of sinusitis, and in the initial evaluation of an orbital mass. In patients with trauma, an unenhanced scan is sufficient. CT demonstrates bony changes and calcifications better than MRI. However, it must be employed judiciously in children and young patients where radiation exposure to the lens is a legitimate concern. CT scan and MRI can both provide complimentary information in the evaluation of orbital masses. A standard orbit MR study comprises of axial and coronal thin section unenhanced non-fat-suppressed T1-weighted images (the hyperintense fat provides excellent background contrast), a coronal fat-suppressed T2W sequence (for evaluating the optic nerve), and post-contrast fat-suppressed axial and coronal T1W images. CTA and MRA are useful when a vascular process such as a carotid-cavernous fistula or AV malformation is suspected.

The primary role of the radiologist is to precisely localize the lesion, estimate its site of origin and describe its extent. An attempt should be made to describe the lesion as focal or multispatial (Box 7.2), whether it involves intra or extraconal compartments, and its origin from the globe, optic nerve, lacrimal gland, vascular structures, extraocular muscles or the bony orbit.

A coloboma is a focal discontinuity in the structure of the globe resulting from the failure of closure of the embryonic choroid fissure between the 5th and 7th weeks of life. Posterior colobomas, which include choroidoretinal, and optic disc colobomas are the more common type. They can occur as isolated lesions or as a part of a more widespread syndromic process. Isolated colobomas are usually unilateral, while syndromic forms tend to be bilateral. They appear as focal protrusions isodense to the vitreous on non-enhanced CT (NECT) scan and isointense to the vitreous of MRI, unless complicated by retinal detachment and hemorrhage (Fig. 7.4). A large coloboma may also present as an intraorbital cyst associated with a small globe (microphthalmia).

Persistent hyperplastic primary vitreous results from failure of regression of embryonic ocular blood supply. There are three types—posterior, anterior, and mixed. In posterior PHPV, there is an enhancing retrolental soft tissue with a stalk, representing the remnant of the primitive hyaloid artery, extending from the posterior surface of the lens to the head of the optic nerve (Fig. 7.5). Retinal detachment is frequently present and the globe is small. In anterior PHPV, the anterior chamber is shallow, the lens is thin and dysplastic, and an abnormal enhancing tissue is present in the region of the ciliary body and lens. The mixed type demonstrates features of posterior and anterior PHPV.

Coats’ disease, seen in children, usually boys, under the age of ten, is caused by retinal telangiectasias and a leaky blood retinal barrier, which results in subretinal exudate accumulation. Coats’ disease is usually unilateral and results in retinal detachment. The exudate appears hyperdense compared to vitreous on CT. The exudate, comprised mainly of cholesterol, is hyperintense on T1W MR images. Patients usually present with leukocoria and vision loss. The most important differential is retinoblastoma. On CT, the lack of calcification of the exudate of Coats enables differentiation from retinoblastoma. Also, post-contrast studies show no enhancement of the subretinal collection. Linear enhancement of the retinal detachment may however be present.

A dermoid is a congenital inclusion cyst occurring near suture lines, adjacent to the periosteum. The most common site is the superolateral portion of the orbit. Dermoid cysts commonly result in bony remodeling. They may be exophytic or endophytic. Exophytic cysts grow externally and are discovered early in childhood, while endophytic cysts grow internally and are encountered later in youth or early adulthood. Exophytic dermoids are usually small as they are identified earlier. Endophytic cysts are larger and may be associated with extensive bony changes. Other cystic lesions are summarized in Box 7.3.

They appear on unenhanced CT as well-defined fat density lesions (Fig. 7.6). The fat density is by no means universal, and it is not uncommon for them to present as fluid-containing lesions with no fat density. On MRI, the high T1 signal on fat, when present, is characteristic. Occasionally a heterogeneous appearance due to intracystic debris may be encountered (Fig. 7.6). Sometimes, thin marginal enhancement on CT and MRI may be present. Irregular margins and perilesional inflammatory changes indicate rupture.

Epidermoid cysts demonstrate fluid signal/density on MRI and CT. A characteristic finding on MRI is restricted diffusion.

The ocular and orbital manifestations of NF1 include sphenoid wing dysplasia, plexiform neurofibroma, optic nerve glioma, buphthalmos, and optic nerve sheath ectasia. Sphenoid dysplasia, thinning, remodeling, or deficiency of the greater sphenoid wing, can result in the “bare orbit sign” on a frontal radiograph of the skull. Plexiform neurofibromas appear as variably enhancing infiltrative soft tissue masses on cross-sectional imaging. The target sign produced by foci of central T2 hypointensity is a typical finding. Perioptic dural sheath ectasia is an uncommon benign finding in patients with NF1. The distended CSF-filled sheaths are easily recognized on T2W MRI. Optic nerve gliomas are characterized by expansion of the nerve itself. They may be limited to the nerve or extend substantially along the visual pathway to involve the chiasm and hypothalamus (Fig. 7.7). They are low-grade neoplasms (pilocytic or fibrillary astrocytomas) and are frequently bilateral. No calcification is present on CT. They are of intermediate signal intensity and enhance variably on MRI.

Venous-lymphatic malformations are poorly marginated frequently trans-spatial multicystic lesions comprised of variable amounts of dysplastic lymphatic and venous elements. These lesions are best characterized on MRI. Cystic areas may show fluid-fluid levels and can contain blood products (Fig. 7.8). Enhancement is variable and is a function of the proportion of the venous component. Phleboliths, which appear on CT, as well-defined rounded calcific foci are a useful clue when present.

An orbital venous varix is a distensible low-flow venous malformation connected with the systemic circulation. When large enough, it can result in intermittent reversible proptosis. Varices increase in size when systemic venous pressure increases, such as with coughing, straining, or compression of the jugular vein. Occasionally thrombosis, calcification, or hemorrhage can occur. Uncomplicated lesions appear as tubular soft tissue densities that enhance intensely with contrast. Patent varices can be made more conspicuous with the Valsalva maneuver. On MRI complex signal intensities due to slow flow and thrombosis may be present (Fig. 7.9).

Cavernous hemangiomas, the most commonly encountered orbital masses in adults, are vascular malformations comprised of endothelial-lined cavernous spaces with a pseudocapsule of compressed normal orbital tissue. They are often incidentally detected on imaging but can present with slowly progressive painless proptosis. They are typically located in the intraconal compartment. They appear as well-defined homogeneous, noncalcified round or oval masses that exhibit progressive enhancement on delayed scans (Fig. 7.10).

Idiopathic orbital inflammatory disease (orbital pseudotumor) is a poorly understood non-granulomatous steroid responsive disorder that can afflict any structure in the orbit. IOID occurs in five forms—anterior (globe and retrobulbar soft tissues), lacrimal, myositic, apical (involvement of the orbital apex and cavernous sinus, Tolosa-Hunt syndrome), and diffuse and sclerosing (bilateral affliction of the orbits and paranasal sinuses) (Figs. 7.11 and 7.12). Painful unilateral ophthalmoplegia is the most common presenting feature. The myositic form, which on imaging is characterized by uniform enlargement of one or more extraocular muscles and their tendon sheaths, is the most frequently encountered variety of IOID. Involvement of the tendon sheaths is a typical finding in IOP and enables differentiation from thyroid ophthalmopathy, where despite smooth enlargement of the muscle belly, the tendon is unaffected. IOID may also occur as a diffusely infiltrative relatively T2 hypointense enhancing process. When fibrosis is marked, the lesion may be profoundly T2 hypointense. Orbital lymphoma, sarcoidosis, and Wegener’s granulomatosis can have very similar imaging appearances (Box 7.6), and one may have to rely on clinical data to arrive at the correct diagnosis. IgG4-related sclerosing disease is a type of orbital pseudotumor characterized by lymphoplasmacytic tissue infiltration with a predominance of IgG4-positive plasma cells, usually accompanied by fibrosis, and elevated serum levels of IgG4. IgG4-related sclerosing disease radiologically is indistinguishable from IOID.