Role of Multi Detector Computed Tomography (MDCT) in diagnosis of PROPTOSIS

Abstract

Pathologic lesions of the orbit continue to be a great challenge to the diagnostic radiologist. The complex anatomy of the orbit on the one hand and the multitude of disease entities that may affect the orbit on the other hand demand a simple, well-structured approach to diagnostic imaging. Subdividing the orbit into four (or five) distinct spaces, i.e., the eyeball, the intraconal space, the optic nerve, and the extraconal space (with some authors adding the conal space as a separate compartment), facilitates both the localization and characterization of orbital lesions and helps the ophthalmic surgeon to select the best approach to a lesion. Multidetector CT (MDCT), due to its thin collimation and resulting multiplanar image reformatting capabilities, has greatly improved the precision of CT imaging of orbital pathology. MDCT allows for multiplanar views of the bony orbital walls and their apertures, i.e., the optic foramen, the superior and inferior orbital fissures, and their respective affection by trauma, tumor, or inflammation. Inclusions of gas or air within the orbit indicate complications of facial trauma, inflammation of the paranasal sinuses, or head and neck tumors. However, due to the separation of the various soft tissue contents of the orbit by orbital fat tissue, MDCT also lends itselfto the assessment of primary intra-orbital lesions or secondary affection of the orbit by lesions extending from the face, the neurocranium, the skull base, or distant primary malignancies. Proptosis can be the result of a myriad of disease processes including infections, inflammations, tumours, trauma, metastases, endocrine lesions, vascular diseases & extra orbital lesions.The evaluation of proptosis include a detailed clinical history , ocular examination, laboratory investigations & imaging studies Understanding orbital anatomy and vascular blood flow patterns and optimal use of imaging techniques facilitate confident evaluation of the fine structures of the orbit and improve diagnostic accuracy. Orbital imaging has been through many transitions in the past several years, though magnetic resonance imaging (MRI) studies have improved soft tissue contrast, complementary studies including computed tomography (CT) scans, plain films, conventional angiography and ultrasonography all have their place in diagnosis of orbital pathology. The orbit is one of the areas of the body where MRI has not supplanted CT as the clear choice for diagnostic imaging evaluation. Firstly, because of the presence of superb natural contrast provided by retro-bulbar fat, bony orbit and sinus air and any lesion that may be present can be seen on CT. Secondly, the inherent sensitivity of MRI to globe and eyelid motion. Since, the advent of MDCT (multi-detector CT) particularly with 16 and 64 slice CT scanners, CT has become isotopic imaging, that means multi-axial imaging is now possible like MRI without changing the patient’s position CT is useful to characterize: the precise location of the lesion - the intraconal space (including muscles & Optic nerve), the extraconal space (associated or not to an extra orbital lesion), or the eyeball; the features of the lesion (density, calcification, enhancement.). These findings are helpful to generate a differential diagnosis. CT is also useful to demonstrate the precise extension of the orbital lesion, the involvement of adjacent paranasal sinuses & nasal cavity, the evidence of bone erosion and intracranial extension which helps in pre treatment evaluation & post treatment follow up. To conclude CT scan can be considered as a cost effective, non invasive, reliable diagnostic tool for evaluation of proptosis. Multi–detector row CT allows substantial reduction in examination time for standard protocols, coverage of extended anatomic volumes, and, most important, substantially increased longitudinal resolution by means of reduced section width. Near-isotropic spatial resolution in routine examinations, which has been achieved with 16-section CT systems, enables 3D renderings of diagnostic quality and oblique MPRs and maximum intensity projections with resolution similar to that of the transverse images. Scanning at narrow collimation does not markedly increase the radiation dose to the patient, as long as the effective milliampere-seconds level is kept constant..