Preseptal cellulitis appears as an area of increased density on CT scan, with swelling of the anterior orbital tissues and obliteration of the fat planes. The fibrous orbital septum offers some resistance to extension of the infection into the posterior compartment of the orbit. Often, the first sign of involvement of the orbit is a slight increase in density of the orbital fat. This is followed by development of more discrete densities as the infectious process progresses. In most cases, the cellulitis is confined to the extraconal space but, if left untreated, it can enter the muscle cone and intraconal space. The cellulitis, in turn, can lead to orbital abscess formation. On MR, orbital cellulitis is hypointense on T1- and hyperintense on T2-weighted images. Fat-suppression MR imaging in conjunction with gadolinium enhancement may be the most sensitive imaging method for detecting early inflammatory disease of the orbit. Endnote

      Extension of an ethmoiditis into the orbit usually begins as a subperiosteal abscess. This can be identified on CT scan as a thin layer of high density immediately lateral to the lamina papyracea.  

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