DEVELOPMENT OF THE SKULL
The skull is divided into two distinct portions: the neurocranium,
which surrounds the brain and special sense organs,
and the viscerocranium,
which forms the lower face and jaws.
The neurocranium is further divided into the membranous neurocranium,
which forms the cranial vault,
and the cartilaginous neurocranium,
which forms the skull base.
In the newborn,
the membranous bones of the vault are separated by the intervening sutures.
Where the sutures intersect,
they widen and assume the shape of fontanelles.
The larger anterior fontanelle lies at the intersection of the sagittal,
coronal,
and metopic sutures and closes by the end of the second year.
The posterior fontanelle lies at the intersection of the sagittal and lambdoid sutures and closes before the third month.
The most significant growth of the skull occurs along the sagittal and coronal sutures .
At birth,
the volume of the neurocranium is eight to nine times greater than that of the face.
This ratio is 5:1 by 2 years,
3:1 at 6 years,
and 2:1 in the adult.
A lateral skull radiograph with the jaws closed reveals the relative areas in the midsagittal plane of the cranium.
The ratio of cranium to facial bones is 4–4.5 at birth and decreases with age: 3–3.5 at 2 years,
2.5 at 6 years,
and 1.5–2 in the adult.
THE SKULL XRAY
Skull radiographs of infants are obtained in the supine position.
Radiographic views include the following:
(a) A straight anteroposterior view is obtained to demonstrate the calvaria.
(b) One or both lateral views are obtained to demonstrate the calvaria and skull base in the lateral projection; both lateral views are indicated in trauma and focal lesion evaluation.
(c) An antero-posterior view with 30° caudal tilt (Towne view) is obtained to demonstrate the occipital bone and foramen magnum.
SKULL ANTERO-POSTERIOR AND TOWNE’S PROJECTION
Demonstrates: Occipital bone,
petrous pyramids,
posterior foramen magnum,
dorsum sellae,
posterior clinoids,
zygomatic arches,
and mandibular condyle.
Measure: Through the CR.
kVp: 85 (80 to 90).
Film Size: 10 × 12 inches (24 × 30 cm),
vertical orientation.
Grid: Yes.
TFD: 40 inches (102 cm); must correct TFD to 35 inches (89 cm) for tube tilt.
Tube Tilt: 35° caudad (for Townes view).
Patient Position: Supine or upright.
Part Position: Centered,
with removal of lateral head tilt and rotation.
Infra-orbital meatal line is perpendicular to the cassette.
CR: Passes through the midline at the external auditory meatus.
Collimation: To skull size.
Side Marker: In an open space at a corner of the film.
Breathing Instructions: Suspended expiration.
Common Pitfalls:
a)Collimation: Care must be taken not to crop off the vertex.
b)Underexposure: The film is commonly too light,
as this view requires the greatest exposure of any skull view.
c)Head rotation: Will displace the pineal gland and unequally project the petrous pyramids.
Clinicoradiologic Correlations:
a)Alignment: The dorsum sellae and posterior clinoid processes should project into the anterior portions of the foramen magnum.
b)Bone: The occipital bone is best demonstrated as are the petrous ridges,
auditory meatus,
zygoma,
and mandibular condyle.
Fractures through the occipital bone,
zygomatic arches,
and the foramen magnum as well as bone disease of these structures are shown to advantage in this view.
c)Cartilage: The temporomandibular joint is poorly demonstrated.
The lambdoidal suture can usually be seen at the periphery of the occipital convexity.
d)Soft tissue: The pineal gland should lie in the midline.
SKULL LATERAL PROJECTION
Demonstrates: Lateral cranial structures closest to the bucky (temporal,
parietal),
sella turcica,
sphenoid sinus,
occipito-cervical junction,
and calvarium.
Measure: At the CR.
kVp: 85 (80 to 90).
Film Size: 10 × 12 inches (24 × 30 cm),
horizontal orientation.
Grid: Yes.
TFD: 40 inches (102 cm).
Tube Tilt: None.
Patient Position: Semi prone.
Part Position: Head is in true lateral position against the bucky.
The infra-orbital meatal line is parallel with the long edge of the cassette,
and the inter-pupillary line is perpendicular.
CR: Passes ¾ inch superior and ¾ inch anterior to the external auditory meatus.
Collimation: To skull size.
Side Marker: Side closest to the film,
in a corner.
Breathing Instructions: Suspended expiration.
Common Pitfalls:
a)Head rotation: Must be parallel to the film for proper demonstration.
b)Artifacts: Removal of head/hair jewelry wherever possible.
Tight hair braids and tie bands also can produce confusing artifacts.
Clinico-radiologic Correlations: Both right and left laterals should be performed routinely.
The most common clinical indications for skull radiography are trauma,
bone malignancy,
and metabolic bone disease.
a)Alignment: A well-positioned lateral should show superimposition of the mandibular rami,
orbital roofs,
and sella turcica.
The tip of the dens should lie not more than 8 mm above the plane from the hard palate to the occipital convexity (McGregor’s line).
b)Bone: The vascular markings off the middle meningeal artery should not be confused with a fracture line.
Fractures are better seen on plain film rather than CT studies.
The sclerotic density of the skull base may mimic bone pathology.
Normal bone thinning of the squamous portion of the temporal bone and occipital bone create a normal decrease in radiographic density of these regions and should not be confused with bone destruction.
c)Cartilage: The lambdoidal and coronal sutures can occasionally be recognized by their characteristic zigzag pattern.
The atlantodental interspace should be inspected.
d)Soft tissue: The palate and retropharyngeal tissues should be perused for evidence of swelling or abnormal density.
A fluid level in the sphenoid sinus when performed upright is an indicator of a skull base fracture.
ABNORMALITIES OF HEAD SIZE
Microcephaly
The term microcephaly signifies a head circumference that is more than 2 standard deviations below the mean for age or is below the third percentile.
Primary microcephaly includes familial and autosomal dominant microcephaly and several chromosomal syndromes.
The small cranial size typically reflects underlying damage to the developing brain (radial microbrain).
Infants with trisomy 21 are typically microcephalic.
In Rubinstein-Taybi syndrome,
microcephaly is associated with severe developmental delay and broad distal phalanges of the thumbs and first toes.
This syndrome arises from microdeletions of the CREB-binding protein gene.
CREB-binding protein is an essential transcriptional coactivator.
In trisomy 13,
the microcephalic neonate also has mandibular hypoplasia,
a dysgenetic corpus callosum,
and ventriculomegaly.
Infrequently,
malformation that leads to reduced brain growth can result from acquired disease such as congenital TORCH infections (toxoplasmosis,
rubella,
cytomegalovirus,
herpes simplex),
maternal diabetes,
and alcohol or hydantoin embryopathy.
Lack of normal postnatal brain growth is associated with secondary pancraniosynostosis,
resulting in a small head of normal shape.
Macrocephaly
The term macrocephaly signifies a head circumference that is more than 2 standard deviations above the mean or exceeds the 97th percentile by at least 0.5 cm.
Macrocephaly may result from a variety of genetic defects or acquired causes of increased intracranial volume,
such as hydrocephalus or subdural collections or any cause of raised intracranial volume.
Genetic causes include neurofibromatosis,
achondroplasia,
cerebral giantism (Sotos syndrome),
and campomelic dysplasia.
Cerebral giantism is characterized by a large dolichocephalic skull,
large hands and feet,
weight above the 90th percentile,
and profound developmental delay.
Pathognomonic signs of campomelic dysplasia are hypoplastic scapulae,
short limbs with anterior angulation in the femora and tibiae,
and hypoplastic fibulae.
Macrocephaly is encountered in later infancy in patients with mucopolysaccharidoses,
mucolipidoses,
and other metabolic storage diseases,
where abnormal metabolites deposit in the meninges,
thicken them,
and interfere with cerebrospinal fluid absorption.
Large head size is also seen in two dysmyelinating diseases: Alexander disease and Canavan disease.
ABNORMALITIES OF HEAD SHAPE
The skull shape can be altered by deformations or true sutural synostosis.
In utero,
deformation of the skull may result from faulty fetal packing or the syndrome of amniotic bands.
Amniotic bands may cross the calvaria at any level to produce variable skull deformities or may cross the face,
causing hypoplasias and clefting.
Postnatally,
head deformity commonly results from preferential sleep position (positional plagiocephaly) or premature sutural synostosis.
Faulty Fetal Packing
The term faulty fetal packing signifies concave depressions in the neonatal skull that are caused by prolonged extrinsic pressure from a malpositioned limb in utero.
The skull deformity is not permanent and will resolve with time.
Parietal bone compression has also been shown to occur from extrinsic pressure caused by uterine leiomyomas.
A differential possibility for localized skull depression is compression applied during forceps delivery.
Positional Plagiocephaly
Calvarial deformation results from external pressure after birth when an infant is consistently placed in the same position for rest and sleep.
This can be marked in very premature infants,
whose heads become flattened and scaphocephalic when they are positioned on their side for mechanical ventilation.
The long hours spent supine and the infants’ inability to change position caused persistent pressure against one portion of the skull (preferred sleeping position) and occipital flattening.
The deformity may be bilateral and symmetric or one side may be substantially more flattened than the other,
leading to secondary changes in the position and appearance of the ear.
Posterior positional plagiocephaly has become increasingly common.
Extreme cases come to radiologic evaluation to exclude possible unilateral or bilateral lambdoid synostosis.
Normal outward pressure from brain growth,
most marked in the first 2 years of life,
provides sufficient stimulus for remodeling once the child is old enough to roll from side to side or sit up,
even with support.
Craniosynostosis
Primary synostosis is subdivided into syndromic (familial or hereditary) and the more common nonsyndromic (isolated and sporadic).
Premature fusion of the sutures may be isolated or form part of a syndrome,
including Crouzon syndrome (premature synostosis,
maxillary hypoplasia,
shallow orbits),
Apert syndrome (craniosynostosis with syndactyly of fingers and toes),
and Pfeiffer syndrome (premature synostosis,
broad thumbs and great toes,
and mild soft-tissue syndactyly).
The latter three syndromes are associated with fibroblast growth factor receptor 2 (FGFR2) mutations.
FGFR2 mutations lead to increased numbers of precursor cells that are involved in the osteogenic pathway.
Carpenter syndrome (premature synostosis,
severe developmental delay,
brachydactyly,
syndactyly,
thumb duplication) has been linked to the ATR-X gene.
Cloverleaf skull (kleeblattschaedel) develops when there is premature synostosis of all except the squamosal suture.
This configuration may be seen with severe Apert or Crouzon syndrome or in association with thanatophoric dysplasia.
Thanatophoric dysplasia is a sporadic lethal dysplasia caused by mutations in fibroblast growth factor receptor 3 (FGFR3),
resulting in chronic FGFR3 hyperactivation and inhibition of bone growth.
Additional diagnostic findings are short ribs,
short curved long bones,
and platyspondyly.
Normal skull growth occurs in a direction perpendicular to the axis of the sutures.
The term premature cranial suture synostosis signifies premature closure of one or more of the cranial sutures.
Secondary synostoses are encountered in a variety of unrelated conditions,
including metabolic derangements such as hypophosphatasia and rickets,
bone dysplasias such as mucopolysaccharidoses and thanatophoric dysplasia,
and effects of fetal teratogens such as hydantoin.
Secondary sutural closure occurs after ventriculoperitoneal shunting,
when reduction in ventricular size reduces the expansile forces on the calvaria.
When sutures fuse prematurely,
head growth occurs along the axis of the fused suture.
The altered skull shape is diagnostic.
Because closure of the suture does not always occur along its entire length and may not involve the entire depth of the suture,
the entire length of each suture must be evaluated.
Radiographs obtained tangential to a palpable ridge or sutural bump may prove to be diagnostic in situations where standard radiographs are equivocal or show only an abnormal head shape but not suture closure.
Synostosis may occur in utero and manifest at birth,
but diagnosis is usually delayed until misdirected growth manifests as calvarial asymmetry.
Scaphocephaly and Dolichocephaly
These terms denote calvarial elongation in the anteroposterior diameter.
This condition results from premature sagittal synostosis This is the most common type of synostosis,
accounting for up to 50% of cases,
and is more common in males.
Sagittal synostosis is frequently inherited as an autosomal dominant trait.
Brachycephaly
The term signifies abnormal calvarial widening in the transverse diameter.
It typically arises when coronal or lambdoid synostosis limits anteroposterior growth.
There is a slightly higher incidence of bilateral coronal synostosis in females.
The ipsilateral frontal bone is flattened and the orbit is deformed with elevation of its superior lateral angle,
resulting in the so-called harlequin eye .
The midline of the face is skewed with respect to the midline of the skull base.
The incidence of associated anomalies is higher with bilateral coronal synostosis than with sagittal synostosis.
Plagiocephaly
Plagiocephaly,
or asymmetry,
usually occurs with unilateral coronal synostosis or asynchronous synostoses of multiple sutures bilaterally.
Asymmetric growth results in displacement of the sagittal suture,
nasal septum,
and skull base to the affected side.