Well Circumscribed Solid Orbital Lesions

The most common well-circumscribed orbital tumors are cavernous hemangioma, neurilemoma, neurofi-broma, fibrous histiocytoma, and hemangiopericy-toma.1-4,25 Cavernous hemangioma is one of the most common primary orbital tumors in adults and presents on MRI as a well-defined, oval-to-round intra-conal orbital mass. On T1-weighted images, cavernous

TABLE 10.1. MR Features of the Most Frequent Well-Circumscribed Orbital Lesions on Spin-Echo Sequences."

Lesion appearance and signal with respect to vitreous Tl-Weighted image T2-Weighted image

Degree of lesion enhancement after Gd-DTPA

Cavernous hemangioma Neurilemoma Neurofibroma Fibrous histiocytoma Hemangiopericytoma

Homo

Iso/Hyper

Hetero

Iso/Hyper

Hetero

Iso/Hyper

Hetero

Hyper

Homo

Iso/Hypo

Hetero

Iso/Hypo

Hetero

Iso/Hypo

Hetero

Hypo

Homo

Hetero

Hetero

Hetero

Hetero

++++

^The abbreviations apply to all tables in this chapter: Homo, homogeneous; Hetero, heterogeneous; Iso, isointense; Hyper, hyperintense; Hypo, hypointense.

^The abbreviations apply to all tables in this chapter: Homo, homogeneous; Hetero, heterogeneous; Iso, isointense; Hyper, hyperintense; Hypo, hypointense.

hemangioma appears to have a homogeneous, isointense to slightly hyperintense signal with respect to the vitreous, and a hypointense signal with respect to the orbital fat.4 On T2-weighted images, the tumor has a high signal intensity with respect to the orbital fat (Table 10.1). Heterogeneity in tumor signal may be related to the presence of calcified phleboliths, which produce signal void on T1- and T2-weighted images and may mimic vessels with high blood flow. Other well-circumscribed solid lesions that can conceivably have identical MR characteristics include neurile-moma, neurofibroma, fibrous histiocytoma, and he-mangiopericytoma.4,6,26 However, less common well-circumscribed orbital lesions such as lympho-proliferative disorder, metastasis from skin melanoma or carcinoid tumor, capillary hemangioma, orbital varix, rhabdomyosarcoma, and extraocular extension of intraocular malignancies may also present with the same nonenhancing MR features as cavernous hem-angioma (Table 10.2).4,7,27-30 Gradient echo images are most helpful in differentiating calcification from intrinsic blood vessels.

Well-circumscribed solid orbital lesions usually demonstrate heterogeneous moderate to marked enhancement after gadolinium-DTPA administration (Tables 10.1 and 10.2).4,8,26-29 However, unlike other orbital lesions, cavernous hemangioma shows increas ing homogeneous enhancement on delayed images, owing to the pooling of the contrast medium within the tumor (Figure 10.4). Tumor enhancement and delineation are best evaluated on Gd-DTPA-enhanced Tl-weighted images by means of fat suppression (frequency-selective presaturation) techniques.

Orbital CT will give less information than MRI regarding spatial location or tissue components.3 All solid well-circumscribed lesions demonstrate higher density than those in the vitreous body, as well as variable degree of enhancement after contrast admin-istration.3,8 Calcified phleboliths are occasionally seen as highly dense foci within a cavernous hemangioma.3

The age of the patient, the clinical presentation, and the anatomic location of the lesion will help the clinician or radiologist in arriving at a differential diagnosis.

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