Embryology, Anatomy, Normal Findings, and Imaging Techniques

Chapter 18

Embryology, Anatomy, Normal Findings, and Imaging Techniques*

Anatomy of the Skull

The skull is divided into three interconnected portions: the neurocranium, the facial area, and the base. The neurocranium includes the calvarium, which is composed of the membranous portions of the occipital, parietal, frontal, and temporal bones and is bounded inferiorly by the base of the skull, which is composed of the cartilaginous portions of these bones plus the sphenoid and ethmoid bones. The facial area is the portion of the skull between the forehead and the chin.

Routine views of the skull include the frontal projection (usually posteroanterior), the Towne view of the occipital bone, and the lateral view. These views may be supplemented by submentovertical and Waters and Caldwell (posteroanterior 15 degrees) views for specific indications.

The radiation dose (with the thyroid and lens being the most sensitive structures) varies with the view obtained and the age of the patient. Best practice skin doses for a lateral vew of the skull range from 0.09 mGy in the first year of life to 0.46 mGy in a child who is 10 to 15 years old (see Huda in Suggested Readings).

Indications for plain film skull examination are listed in Box 18-1. Computed tomography (CT) and magnetic resonance imaging (MRI) generally are used for detailed evaluation of facial structures and intracranial contents.

Neonatal and Infant Skull

Size and Shape

During infancy the neurocranium is larger relative to the face than at any other time during normal growth. Ratios of the respective areas of the neurocranium in lateral projection are roughly 3 : 1 to 4 : 1 at birth, and they decrease to 2 : 1 to 2.5 : 1 by age 6 years. The bones of the calvarium lie in their incompletely mineralized membranous capsule; they are separated by broad strips of connective tissue that form the sutures and by patches of connective tissue, the fontanelles. The six constant, or major, fontanelles are located at the four corners of the parietal bones—two in the midline of the skull and two pairs on each side (Fig. 18-1). Accessory fontanelles may occur in several parts of the cranium but usually are located in the sagittal suture. The sutures and the synchondroses in the base are prominent in newborns but diminish in width during the first 2 to 3 months. Obliteration of the sutures does not begin until the second to third decades. Figures 18-1 through 18-4 and e-Figure 18-5 illustrate sutures, fontanelles, and synchondroses.

The sphenoid bone at birth consists of a single central mass composed of the body and the lesser wings and two symmetric lateral osseous masses, each of which is made up of a greater wing and a pterygoid process. The pituitary fossa in the body of the sphenoid bone tends to be round with smooth margins; the dorsum sella is short and blunt, and the clinoid processes are rudimentary. The angle between the body of the neonatal mandible and the ascending ramus in lateral projection is about 160 degrees; the relatively large bodies are separated in the midline by a prominent cartilaginous symphysis mentalis (see Fig. 18-3). Early calcification of teeth is seen in the fifth fetal month.1

Components of the individual bones that are not united in infancy may lead to confusion unless they are correctly recognized. The frontal bone is divided in half laterally by the metopic suture (see Figs. 18-1 and 18-3). Apparent discontinuity of the sphenoid bone with the frontal bone superiorly and the occipital bone posteriorly indicates the sites of the sphenoid bone’s synchondroses with these two bones (see Fig. 18-2). The four major components of the occipital bone (e-Fig. 18-5 and Fig. 18-2) likewise may simulate discontinuities of structure.

Growth and Development

Most of the postnatal growth and differentiation of the skull occurs during the first 2 years of life, and thus after 24 months, most of the features of the adult skull are present. During childhood, growth continues at a greatly reduced velocity but shows a slight postpubertal spurt. The thickness of the bones increases. The inner and outer tables, diploic space, vascular markings, and grooves for the dural sinuses on the internal surface of the calvarium all make their appearance by the end of the second year.

With increasing age, the fontanelles and sutures become smaller and narrower. The anterior fontanelle usually is reduced to fingertip size during the first half of the second year; the posterior fontanelle may be closed at birth (range of closure: birth to several months). Closure of the fontanelles occurs clinically before it is seen radiographically. The metopic suture is quite variable and may be obliterated at birth, but it usually is closed during the third year; however, it persists throughout life in about 10% of cases. In the occipital bone, the mendosal suture (see Fig. 18-4 and e-Fig. 18-5) usually disappears during the first 2 years, but it too can persist; the synchondrosis between the supraoccipital and exoccipital (supracondylar) portions usually disappears during the second or third year. The spheno-occipital synchondrosis begins to close near the time of puberty but may persist until the twentieth year. This variation and irregularity make suture lines unreliable criteria for estimation of the developmental age of the skull. At about the twentieth year, the skull attains its definitive size.

Normal Variations

Intrasutural, or wormian, bones occur most frequently along the lambdoid sutures (Fig. 18-6 and e-Fig. 18-7; Box 18-2). They occur much less frequently in the fontanelles (see e-Fig. 18-7). The interparietal or Inca bone (Fig. 18-8) results from division of the supraoccipital portion of the occipital bone into two parts by the mendosal suture, with the superior part arising from membranous bone and the inferior part arising from cartilage continuous with that of the supracondylar portions and the basiocciput. A rare synchondrosis or suture line runs vertically through the squamous portion of the occipital bone (Fig. 18-9); persisting superior and inferior portions of the line are known as the superior longitudinal fissure or bi-interparietal suture and the cerebellar synchondrosis or median cerebellar suture. Where the supraoccipital portion of the occipital bone forms the posterior border of the foramen magnum, accessory supraoccipital bones occasionally are found (e-Fig. 18-10). The configuration caused by an outward bulge of the occipital squamosa just above the torcular Herophili in a newborn (Fig. 18-11) is called bathrocephaly. Rarely, a horizontal interparietal suture divides the parietal bones into superior and inferior moieties (e-Fig. 18-12).

Compression of the fetal skull and its molding during passage through the maternal pelvis produce significant radiographic findings that persist after birth (Fig. 18-13).2 During the first weeks and months of life, widths of sutures vary so much that caution is required in their evaluation for the diagnosis of increased intracranial pressure, particularly because positioning is difficult and partial superimposition of bilateral sutures can produce spurious widening (Fig. 18-14).

In children older than 2 years, the sutures extend through both tables and the diploic space. The outer table portion of the suture may be deeply serrated when the inner table portion is practically a straight line (e-Fig. 18-15) and may be interpreted erroneously as a “fracture through a suture.” Persistence of the metopic suture may simulate a vertical fracture in the occipital bone in anteroposterior, caudally angulated exposures if extension of the superimposed radiolucent line into the area of the foramen magnum is invisible or if the inferior portion of the suture has been obliterated. The frontal crest on the internal surface of the frontal squamosa in the midsagittal plane may be sufficiently prominent to simulate calcification of the falx cerebri that attaches to it (e-Fig. 18-16).

Dec 20, 2015 | Posted by in PEDIATRIC IMAGING | Comments Off on Embryology, Anatomy, Normal Findings, and Imaging Techniques
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