NORMAL CRANIAL OSSIFICATION

AND NEAR FIELD REVERBERATION

INTRAMEMBRANOUS OSSIFICATION

Mesenchymal cells lay down osteoid which forms primary centers for ossification.

• Frontal bone, parietal bone, portions of the sphenoid, temporal and occipital bone.
• Normal ossification depends on the presence of brain tissue.
• Size and shape of cranium depends on internal pressures exerted on the inner table of neurocranial bones.
• Sutural bone growth is stimulated by the tensile force that the developing and expanding brain exerts on the sutures.
• Head circumference, because of its relationship to intracranial volume, is a good indicator of brain growth.

No brain (anencephaly)	Þ	no cranium
Small brain	Þ	microcephaly
Hydrocephalus or mass	Þ	macrocephaly

ENDOCHONDRAL OR INTRACARTILAGINOUS OSSIFICATION

• Ossification of a pre-existing cartilaginous model.
• Ossification centers in portions of the sphenoid, temporal and occipital bone.
• Shape of cranial base is probably caused by rapid brain growth during the 1st and early 2nd trimester (chondrocranium retains its fetal flexure in anencephaly).
• Cranial base is uneven in keeping with the highly irregular shape it develops to accommodate the undulating ventral surface of the brain.

NORMAL CRANIAL OSSIFICATION CENTERS

Normal ossification centers appear as areas of increased echogenicity of the bones and may be seen endovaginally from 9 weeks onwards. Osteogenesis starts centrally and spreads peripherally until the whole skeletal element is ossified.

• Frontal bones Þ Single primary ossification center at 7 weeks. Three pairs of secondary centers appear later in the nasal spine, zygomatic process and trochlear fossa.
• Parietal bones Þ Two primary ossification centers for each bone at week 8.
• Supranuchal squamous and Occipital bone Þ Two primary ossification centers at week 8. Rest of occipital bone ossifies endochondrally.
• Squamous temporal bone, Occipital bone Þ Single primary center at week 8.

Primary cranial ossification complete by week 12.
Always visualized by week 14.

* - Occipital ossification. ** - Frontal ossification.

Other Ossification Centers
8 weeks MA	Þ	Clavicle.
9 weeks MA	Þ	Mandible and palate. Primary centers in vertebral bodies and neural arches.
10-11 weeks MA	Þ	Frontal and occipital bones. Maxilla. Humerus. Radius and ulna. Femur. Tibia and fibula.
11-12 weeks MA	Þ	Skull base. Inferior portion of skull. Ilium. Scapula. Ribs.
12-13 weeks MA	Þ	Vertebra. Metacarpals and phalanges. Metatarsals slightly later than metacarpals.
18-22 weeks MA	Þ	Calcaneous.
25 weeks MA	Þ	Pubic bone.
32 weeks MA	Þ	Distal femoral epiphysis.
After birth	Þ	Carpal bones.

Non-visualization of near field lateral ventricle due to reverberation artifact from a normally ossified cranial vault. Near field ventricle is visualized by scanning through the fontanelles and sutures.
Normal choroid plexus and ventricles scanned through the anterior fontanelle.

REFERENCES

1. van Zalen-Sprock PM, Brons JTJ, van Vugt JMG et.al. Ultrasonographic and radiologic visualization of the developing embryonic skeleton. Ultrasound Obstet Gynecol 1997;9:392-397.