MR, CT, and Plain Film Imaging of the Developing Skull Base in Fetal Specimens

A and B, Lateral skull radiograph of a fetal specimen with a gestational age of 19 weeks 5 days (A) and T2-weighted fast spin-echo MR image (4500/90/4) of a fetal specimen with a gestational age of 19 weeks 3 days (B) show progressive ossification of the skull base from posterior to anterior. The ossification has extended to the postsphenoid (open arrow), which encloses the pituitary to form the sella turcica. The cerebral hemispheres have greatly expanded. Note ossification of presphenoid (curved arrow) on lateral radiograph (arrow, basiocciput; O, orbitosphenoid; ts, tuberculum sellae)

fig 4. T2-weighted fast spin-echo MR image (4500/87/4) of a fetal specimen with a gestational age of 20 weeks 5 days shows progressive skull base ossification that has extended from the basiocciput (straight arrow) to the postsphenoid (open arrow) to the presphenoid (curved arrow) in the area of the chiasmatic sulcus. fig 5. A and B, Lateral skull radiograph of a fetal specimen with a gestational age of 22 weeks 3 days (A) and T2-weighted fast spin-echo MR image (4000/90/3) of a fetal specimen with a gestational age of 24 weeks 4 days (B) show advancing ossification of the basiocciput (arrow), basisphenoid (postsphenoid, open arrow), and presphenoid (curved arrow). Most of the growth of the head is due to the dominant development of the cerebral hemispheres (O, orbitosphenoid)

CT scans at 16 weeks 2 days, 18 weeks 4 days, 18 weeks 5 days, and 24 weeks 4 days show progressive ossification of the postsphenoid. Note that the medial ossification centers (solid arrows) appear first followed by ossification of the lateral centers (open arrows)

fig 7. CT scan of fetal specimen with a gestational age of 23 weeks 3 days shows persistence of the medial ossification centers of the postsphenoid as separate structures (straight solid arrows). Note the bones of Bertin (open arrow) adjacent to the vomer (v) and rostrum of the basisphenoid. This is the site of sphenoid pneumatization. Note also carotid groove in sphenoid (curved arrow). fig 8. CT scan of fetal specimen with a gestational age of 21 weeks 4 days shows early ossification of the main centers (solid arrow) and corporal middle centers (open straight arrow) of the presphenoid. Note small defect that represents the remnant of the obliterated craniopharyngeal canal (Rathke's pouch) in the postsphenoid (curved arrow). Asterisk indicates olivary eminence. fig 9. CT scan of fetal specimen with a gestational age of 24 weeks 4 days shows progressive ossification and fusion of the main (solid arrow) and corporal middle (open arrow) portions of the presphenoid. Note the olivary eminence (asterisk), which is a triangular area filled with unossified cartilage in the presphenoid

fig 10. CT scans show progressive ossification of the orbitosphenoid (arrow) at 16 weeks 2 days, 17 weeks 4 days, 18 weeks 4 days, and 24 weeks 4 days. fig 11. CT scan of fetal specimen with a gestational age of 22 weeks 3 days shows ossification of alisphenoid (A), which forms the greater wing of the sphenoid. Foramen ovale and spinosum are seen as a large defect in the greater wing of the sphenoid (arrow)

Diagram of the skull base ossification centers. Asterisk indicates olivary eminence (anterior foramen)

Diagram of progression of skull base ossification. Arrow indicates normal sequence of ossification of the skull base from posterior to anterior