Mucoceles result from obstruction of a sinus ostium. Because of continued secretion of mucus and the resulting increased pressure in the sinus cavity, the walls of the sinus eventually are displaced and encroach upon adjacent facial compartments. Mucoceles are most often associated with chronic infection and allergic sinusitis but may also occur as the result of trauma, previous surgery, or a bone dysplasia. Proptosis is the most common presenting sign of a mucocele. Other clinical features include a mass in the upper medial quadrant of the orbit, pain, vertical diplopia, limited upward gaze, bifrontal headache, and increased tearing. Mucoceles are most commonly found in the frontal and ethmoid sinuses, are infrequent in the sphenoid sinus, and occur rarely in the maxillary sinuses. If a mucocele becomes infected, it is called a mucopyocele.

Mucoceles are recognized on plain films by the presence of an opacified sinus with expansion and thinning of its bony margins. On CT the majority of mucopyoceles are of average density and are homogeneous. Because they are slow growing, these lesions displace and thin the bone around a sinus cavity, but they usually do not cause bone destruction. Occasionally, an aggressive mucocele will cause bone destruction, especially of the orbital wall, and may then simulate a malignant neoplasm. In these cases, CT is particularly helpful to distinguish benign from malignant disease, because an expanding mucocele often preserves a thin fat plane between the margins of the mucocele and the muscle cone.

Mucoceles of the frontal and ethmoid sinuses commonly involve the orbit. In particular, the thin lamina papyracea forming the medial orbital wall offers little resistance to an expanding mucocele. Progressive compression of the orbital contents may lead to visual impairment and, if of sufficient degree, to optic atrophy because of excessive stretching of the optic nerve. As they expand, frontoethmoid mucoceles can produce dehiscence of the posterior wall of the frontal sinus or roof of the ethmoid sinuses and expose the adjacent dura. In these cases, contrast-enhanced CT scans are helpful in defining the position of the dura. Not infrequently, ethmoid mucoceles extend medially into the superior nasal cavity and erode the nasal septum or cribriform plate. Coronal imaging is essential for precise evaluation of these structures. It is also useful for optimal visualization of the roof of the ethmoid sinuses and the ethmoidomaxillary plate and for judging the degree of orbital extension of frontal mucoceles.

The MR signal intensity of the mucous contents of an obstructed sinus or a mucocele depends on the protein content. Typically, fluid is hypointense on T1 and hyperintense on T2-weighted images. As the protein content increases, the T1 relaxation time decreases, increasing the signal intensity on T1-weighted images. When the protein content approaches 25%, T2 shortening results in progressively lower signal intensity on T2-weighted images.