The maxillary sinus is the cavity behind your cheeks, very close to your nose. When a CT scan is taken of the head, the sinuses should show up black since they are cavities. When the area shows up white or gray, it is called opaque or opacification of the sinus.
There are several possible causes for a CT scan to reveal the maxillary sinus as opaque. Polyps can appear opaque, as can tumors. Inflamed tissue, which is common with chronic sinus infections, and thick mucus also can cause opaqueness.
Polyps can form in the sinus cavities, which can cause poor drainage. This in turn can lead to chronic sinusitis (infection). These combined factors can cause total opacification.
Since tumors in the maxillary sinus are often large and appear opaque, they can cause total opacification of the sinus.
If your scan was done during a sinus infection or if you have chronic sinus infections, your maxillary sinus was likely inflamed. This can cause the area to be opaque on a CT scan.
If you had sinusitis, you might have had thick mucus lining or even filling the sinus, which will make it appear opaque on a CT scan.