Cross-sectional image of the SSS in a 27-week gestation preterm infant at the level of the anterior fontanel. Although ultrasound can penetrate bone, good images require the fontanel as an acoustic window. The black layer between the inner surface of the skull/fontanel and the surface of the brain is unusually wide in this infant, about 7 mm, and is constituted by cerebrospinal fluid in the subarachnoidal space. The subarachnoidal space separates the two brain hemispheres in a deep V-shape. The thin line in the middle represents the falc cerebri. On top of this, the triangular SSS is seen, measuring 2 to 3 mm on each side. It is black on the ultrasound image, since flowing blood, such as cerebrospinal fluid, gives few echoes to ultrasound.