A sinus within a sinus.
Belcher
H E
HE
Zammit-Maempel
I
I
Rose
J D
JD
D005260:Female; D006801:Humans; D008443:Maxillary Sinus / Q000002:abnormalities* / Q000000981:diagnostic imaging*; D008875:Middle Aged; D014057:Tomography, X-Ray Computed