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lacrimal gland lies in the lacrimal fossa and is adjacent to the tendinous insertion of the superior and lateral rectus muscles.
The optic nerve, which is 2–3 cm in length and 3–4 mm in width, has imaging characteristics similar to the white matter in the brain [2]. The nerve leaves the posterior aspect of the globe, and there is a slight inferolateral bowing of the midportion of the nerve. The nerve courses posteriorly, medially, and superiorly to exit the orbit through the optic canal. The optic nerves join in the suprasellar cistern to form the optic chiasm.
The ophthalmic division of the trigeminal nerve (V1) provides sensory innervation to the eye, lacrimal gland, conjunctiva, upper eyelids, forehead, and scalp. The largest branch, the frontal nerve, enters the orbit through the superior orbital fissure (Fig. 12.8). Within the orbit, the nerve divides into the supratrochlear and supraorbital nerves, which run along the orbital roof. These nerves are difficult to see on imaging when not involved with tumor (Fig. 12.17).
Branches of the maxillary division of the trigeminal nerve (V2) provide sensory innervation to the skin of the nose, midface, and cheek. This nerve extends through the foramen rotundum to the pterygopalatine fossa (Fig. 12.18b, c) and enters the orbit through the inferior orbit fissure. The nerve then courses through the infraorbital canal, where it is named the infraorbital nerve (Fig. 12.18a), and extends through the infraorbital foramen.
The zygomatic nerve is a branch of V2 that arises in the pterygopalatine fossa. The nerve enters the orbit through the inferior orbital fissure, traverses the lateral wall, and exits the orbit along the lateral orbital wall. Branches of the nerve supply the skin of the temporal region and lateral cheek (zygomaticotemporal and zygomaticofacial nerves).
The intraocular structures can be appreciated to some extent on imaging studies. The lens is hyperdense on CT and the vitreous is hypodense. The sclera, choroid, and retina form a well-defined, enhancing line on CT. On MRI, the vitreous has high water content and thus is hyperintense on T2-weighted images and does not enhance. The lens has low signal intensity on T1 and T2 images. The sclera, choroid, and retina are hypointense.
12.3 Intraocular Lesions
12.3.1 Retinoblastoma
Retinoblastoma is a type of primitive neuroectodermal tumor caused by a mutation in the tumor suppressor oncogene RB1, located on chromosome 13q14 [3, 4]. Approximately 30% of retinoblastomas are hereditary and 85% of these are bilateral [5]. The remainder arises from spontaneous mutations and occurs unilaterally. It is important to recognize retinoblastoma in the early stages [5–8], as patients with tumors confined to the globe have a 5-year survival rate of greater than 90% whereas patients with tumors extending outside the globe have a 5-year survival rate of less
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than 10% [9]. Retinoblastoma and its management are discussed in great detail in Chapter 14.
As most cases of retinoblastoma are initially diagnosed by the patient’s primary ophthalmologist, the role of the radiologist is to use imaging to confirm the diagnosis, evaluate for metastasis along the optic pathway, and detect other intracranial masses [10]. MRI is primarily used to evaluate the globe and brain. CT may also be used in questionable cases to identify calcification, which occurs in 90% of cases and is highly characteristic, especially in children under 3 years [11].
On CT, the tumor is smoothly marginated and hyperdense with punctate or nodular calcification (Fig. 12.1a). Associated subretinal and vitreous hemorrhage appears as nonenhancing areas of hyperdensity. Lesions in the vitreous are intermediate to slightly hyperintense on T1 sequences and hypointense on T2 sequences
Fig. 12.1 Retinoblastoma, right globe. (a) Axial precontrast-enhanced CT demonstrates a calcified mass in the right globe (arrow). (b) Axial T2 image shows a T2 hypointense mass extending into the vitreous (arrow). (c) Axial T1 postcontrast image with heterogeneous enhancement of the mass (arrow)
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(Fig. 12.1b). In children, the hypointensity on T2 sequences is relatively specific for retinoblastoma. The lesions usually have moderate to marked enhancement (Fig. 12.1c).
With optic nerve involvement, the nerve appears thickened and enhancing. Seeding of the cerebrospinal fluid may present as diffuse leptomeningeal enhancement. Bilateral retinoblastoma may be accompanied by an intracranial primitive neuroectodermal tumor along the midline of the brain; this tumor occurs most commonly in the pineal gland (pineoblastoma), suprasellar region, or parasellar region [12, 13]. These lesions are hypointense on T1 sequences and intermediate to hyperintense on T2 images, and there may be associated hydrocephalus. Response to treatment appears on MRI as a reduction in the volume of the lesion, loss of vascularity, and replacement of the tumor with calcification. Secondary tumors, such as osteosarcoma and chondrosarcoma, may develop in the radiation field.
12.3.2 Uveal Melanoma
Uveal melanoma is the most common intraocular tumor in adults. Lesions usually occur unilaterally and may be associated with retinal detachment and vitreous hemorrhage. The role of imaging studies is complimentary to echography; imaging studies can help determine the size and extent of the mass, including extraocular extension, and detect associated retinal detachment. On CT, uveal melanoma presents as a hyperdense thickening of the wall of the globe, which is enhancing. Because of the paramagnetic properties of melanin, lesions are often hyperintense on T1 images, are often hypointense on T2 images, and demonstrate moderate enhancement (Fig. 12.2). The lesions are often well defined and may extend into
Fig. 12.2 Uveal melanoma, left globe. (a) Axial T2 image demonstrates a hypointense uveal lesion (arrow). (b) Coronal T1 postcontrast image demonstrates the homogeneously enhancing uveal mass (arrow)