الفهرس 
COVEROnly 14 pages are availabe for public view
 |  | from | 109 |  |  |
| | | from | 109 | | |
Abstract
Due to the complex anatomy of the human face and the close proximity of its structures to one another and to the neurocranium, the human orbit presents a great challenge to radiologists. The radiologist’s task may be described as visualizing and seeing what ophthalmologists do not see with their own eyes and optical tools.
Ophthalmic ultrasonography is the main diagnostic imaging modality of the eye. It is a safe, noninvasive diagnostic tool that provides instant feedback for the evaluation of various ophthalmic disorders.
Computed tomography is the modality of choice for showing bony detail and for detecting calcifications and foreign bodies. Irradiation to the ocular and orbital structures is a disadvantage, however. MR imaging, on the other hand, has no known biologic side effects and is superior to CT when evaluating soft tissue detail in the globe, orbit, visual pathways, and intracranial structures.
Most neoplasms of the ocular globe and optic nerve are of neuroepithelial origin. Retinoblastoma is an embryonal tumor arising from the retina in infants Medulloepithelioma is a very rare embryonal tumor arising from the medullary epithelium of the ciliary body, which, like retinoblastoma, arises from and is composed of primitive neuroepithelial tumor cells.
Retinoblastoma is the most common intraocular tumor of childhood, but it remains a rare disease the most frequent presenting sign of retinoblastoma is leukocoria. There are both heritable and nonheritable forms of retinoblastoma . The average age when children are found to have retinoblastoma is 18 months. Most often, it is found before age 3. It is rare to find this disease in a child past age 5.
The diagnosis of RB is made primarily by indirect ophthalmoscopic examination with ultrasonography used as a confirmatory procedure. Computed tomography delineates extraocular extension and can detect an associated pinealoblastoma Magnetic resonance imaging is specifically indicated if optic nerve invasion or intracranial extension is suspected.
Diffuse infiltrating retinoblastoma is a rare histologic form of retinoblastoma characterized by diffuse infiltration of the retina. The association of bilateral retinoblastoma with a midline intracranial neoplasm was first reported by Jakobiec et al. in 1977, and the term “trilateral retinoblastoma” was introduced a few years later to refer to this association.
Children with retinoblastoma are at risk for three important, life-threatening problems including metastasis from retinoblastoma, intracranial neuroblastic malignancy (trilateral retinoblastoma/pinealoblastoma), and second primary cancers.
The presence of calcifications distinguishes retinoblastoma from other intraocular lesions so the presence of intraocular calcium in children under three years of age is highly suggestive of retinoblastoma.
Intraocular Medulloepithelioma is a rare primary intraocular neoplasm derived from neuroectoderm. This type of tumor characteristically arises from the nonpigmented epithelium of the ciliary body. On rare occasions, medulloepithelioma also arises from the iris, retina, or optic disc. The tumors range from benign proliferations to malignant neoplasms with unequivocal invasive capacity but limited metastatic potential.
Clinical eye examination is still the most important step in the evaluation of Medulloepithelioma; imaging studies like ultrasonography and magnetic resonance imaging (MRI) can help in the diagnosis. Medulloepithelioma rarely metastasizes; however, they can recur locally. The outcome for localized disease after treatment is good but the prognosis is worse when extraocular extension is present.
Optic nerve Glioma is a neoplasm of astrocytes that affects primarily children (mean age, 8 years). It comprises about 1% of all intracranial tumors. They are almost always unilateral and occur more frequently in females than in males. Rarely, tumors will extend beyond the lateral geniculate bodies into the optic radiations. Clinical studies indicate a poorer prognosis for tumors that involve the optic chiasm or the optic tracts as compared to those restricted to the optic nerves.
Optic nerve or optic pathway tumors (typically low-grade astrocytomas) are the most common central nervous system (CNS) neoplasms associated with NF1. By MRI criteria, 15 to 20% of children with NF1 have these tumors; the natural history of optic nerve Glioma is almost always benign. Most grow slowly in a self-limited manner and some spontaneously regress.
There is little literature regarding the imaging features of optic pathway Glioma at US because the tumor is largely inaccessible to that modality. CT allows good evaluation of the optic nerve due to the intrinsic contrast between the nerve and the conal fat, but is less sensitive than MR imaging for intracranial extension In general.
Magnetic resonance is the preferred imaging modality for evaluating the entire central nervous system in NF1. The size and course of the optic nerve are best evaluated on T1-weighted images without fat saturation. The normal maximum diameter is 5 mm or less in children.