Imaging of Tumors of the Eye and Orbit
Diagnosis of diseases affecting the eye and orbit with modern techniques has various meanings. Aspects of differential diagnosis are important in tumors or diseases of the retrobulbar space. but less important in tumors of the eyeball, because these tumor
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LEMKE
and N.
HOSTEN
CONTENTS 22.1 22.2 22.3 22.3.1 22.3.2 22.3.3 22.3.4 22.3.5 22.3.6 22.3.7 22.3.8
Introduction 213 Compartment Classification 213 Pathologies of the Orbit 214 Intraconal Compartment: Optic Nerve Intraconal Compartment: Other 214 External Eye Muscles 216 Extraconal Compartment 218 Subperiosteal Compartment 219 Lacrimal Gland 219 Globe 220 Preseptal Compartment 222 References 224
214
22.1
Introduction
Diagnosis of diseases affecting the eye and orbit with modern techniques has various meanings. Aspects of differential diagnosis are important in tumors or diseases of the retrobulbar space. but less important in tumors of the eyeball, because these tumors are usually well diagnosed by ophthalmoscopy and ultrasound. Determination of tumor staging may be performed in all pathologies of the eye and orbit using such imaging techniques as computed tomography (CT) and magnetic resonance imaging (MRI). Controllable tissue contrasts mean that the extent of the tumor and the infiltration of surrounding tissue can be evaluated exactly. Improved visualization techniques increasingly allow an integration of CT and MRI images into radiation planning systems. A precondition for this integration is the performance of images with isotropic voxels, i.e., the edges of the A.-J. LEMKE Universitatsklinikum Charite, Medizinische Fakultat der Humboldt-Universitat zu Berlin, Campus Virchow-Klinikum, Klinik fUr Strahlenheilkunde, Augustenburger Platz 1, 13353 Berlin, Germany N. HOSTEN
Zentrum fUr Radiologie, Ernst-Moritz-Arndt-Universitat, Greifswald, Germany
voxels are equal in all three dimensions and they are cube-shaped. This requirement is met by using modern scanners and slice thicknesses between 0.3 and 0.5 mm. With a slice thickness of 0.3 mm, isotropic voxels are reached with a resolution of 512 to 512 pixels and a field of view of 150 mm or a resolution of 256 to 256 pixels and a field of view of 75 mm, respectively. The cross-sectional imaging modalities, CT and MRI, have been improved drastically by the developments in scanner technique. The improvements of spatial resolution and soft tissue contrast in particular are very important for image quality in small fields of view imaging. Whether CT or MRI is used depends on the clinical question; the two methods have different indications. The key to the correct differential diagnosis of orbital pathologies is the division of the orbit into several compartments. The assignment of orbital pathologies to a compartment reduces the number of differential diagnoses, because the frequency of a tumor entity differs rather with the compartment (ZONNEVELD et al. 1987; HOSTEN et al. 1992a).
22.2
Compartment Classification The number of different pathologies found in each orbital compartment is lower than the total number of orbital diseases. Some changes seem to have an affinity for certain compartments. The first step in orbital diagnosis is the assignment of the detected pathology to the correct compartment (ZONNEVELD et al. 1987; HOSTEN et al. 1992a). The divisio
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