Hi, Thanks for writing in. An arachnoid cyst is a balloon like fluid filled structure and it is completely harmless. The sub cortical T2 FLAIR hyperintense area is a non specific finding (cannot be pin pointed to any one particular condition). It is seen in brain aging, with migraine, ischaemic changes, vasculitis, hypoxic-ischaemic changes, haemorrhagic foci, inflammatory changes to name some. You must discuss in details with your neurologist. Bilateral maxillary sinusitis may need to be controlled with medications when symptomatic. It is most likely due to allergy or upper airway infection.