CT Brain Bone window shows right side deep sigmoid sulcus.
The inner surface of the mastoid part of temporal bone shows a deep, curved groove on either side known as sigmoid sulcus, which lodges sigmoid portion of lateral sinus.
The sulcus is separated from the innermost of the mastoid air cells by a very thin lamina of bone and even this may be partly deficient. Deep sigmoid sulcus noted an incidental finding and is of no clinical significance. But needs to be mentioned, as it is important to note during Mastoidectomy.