20 Sinus Disease Incidental sinus disease is a common finding on MRI, with active sinusitis identified much less often. Most sinus infections are viral, but may result in the obstruction of outflow tracts predisposing the sinus to bacterial infection, frequently by S. pneumonia, H. influenza, or M. catarrhalis. MRI findings of retention cysts (with a CSF-like SI) and mucosal thickening are often seen in asymptomatic patients and are not specific for active sinusitis. True active disease is typified by the presence of both mucosal thickening and an air–fluid level, with sinusoidal fluid undergoing a characteristic evolution in appearance on MRI. Initially, the sinusoidal fluid appears as CSF-like SI replacing the normal low SI of the air-filled sinus. Eventually, as demonstrated in Fig. 20.1A, accumulation of protein decreases the SI of the fluid relative to that of CSF on T2WI. Higher protein content also hastens T1 relaxation, leading to a high SI on T1WI. Chronic desiccation of sinusoidal contents results in even higher protein concentrations, leading to low SI on T1 and T2WI that may be confused for the SI of a normal, patent sinus. In these cases, the characteristic peripheral enhancement of sinusitis may provide the only evidence of disease. The major role of MRI in the evaluation of acute sinusitis is in delineating the spread of infection to surrounding structures. Such extension is demonstrated in Fig. 20.1A as an area of isointensity compressing the parenchyma adjacent to the frontal sinus on the right. This area is lined posteriorly by low SI dura and demonstrates restricted diffusion (Fig. 20.1B) consistent with the diagnosis of an epidural abscess. Contrast administration confirms the confinement of this abscess to the epidural space (Fig. 20.1C, white arrow
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