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.he 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.