MDCT Scanning Technique

All CT examinations were performed by a 64-slice CT scanner (Aquillon 64, Toshiba Medical Systems, Tochigi, Japan). The scanning parameters included 120 kV, 100-120 mA, section thickness of 0.5 mm and reconstruction interval of 0.5 mm. The scan revolution time was 0.5 seconds. Three-dimensional reconstructions were generated on the CT scanner console and send to picture archiving and communication system (PACS). For patients that need detailed evaulation, 3D volume rendered (VR) and 3D MIP images were evaluated in a workstation (Aquarius workstation, TeraRecon, San Mateo, California, USA).

Embryology and anatomy

Fig. 1: Development of the skull

The development of the skull outlined in Figure 1. Calvaria is a Latin term and it means upper part of the head that surrounds brain and special sense organs. It is formed by pressure of the growing cerebral and cerebellar hemispheres. Dura has a regulatory role in this process (1).

Membranous bones of the vault are seperated by sutures that help vaginal passage and allow uniform growth of the calvarium by its fibrous connective tissue content. The growth of the skull is parallel to a fused suture (Virchow’s law). If there is a premature fusion of a suture, the calvaria show no growth perpendicular to the affected suture (1).

Anterior fontanelle is a space in the conjunction of sagittal, coronal and metopic sutures and closes typically by 12 months of age. Posterior fontanelle is in the conjunction of sagittal and lambdoid sutures and closes by about three months of age (1-3). The closure of the sutures and fontanels outlined in Table. Figure 2-4 showed normal calvarial 3D anatomy.

Fig. 2: Normal 3D calvarial anatomy. (F: frontal bone, O: Occipital bone, P: parietal bone, Sp: sphenoid bone, Ts: temporal bone squamous portion).

Fig. 3: Normal 3D calvarial anatomy. (F: frontal bone, O: Occipital bone, P: parietal bone, Sp: sphenoid bone, Ts: temporal bone squamous portion).

Fig. 4: Normal 3D calvarial anatomy. (F: frontal bone, O: Occipital bone, P: parietal bone, Sp: sphenoid bone, Ts: temporal bone squamous portion). There is a linear fracture in the right parietal bone (double arrows).

Wormian bone (intrasutural bones)

Wormian bones are accessory bones that occur within cranial suture and fontanels, most commonly within the posterior sutures (Figure 5).

Usually they are normal variant but sometimes associated with cleidocranial dysplasia, pyknodysostosis, osteogenesis imperfecta, hypothyroidism, hypophosphatasia, acro-osteolysis and Down Syndrome (4).

Fig. 5: Wormian bones. There are a lot of are accessory bones within lamdoid suture and posterior fontanel (arrows).

A larger, single, centrally located intrasutural bone at the junction of the lambdoid suture and sagittal suture is called os incae (interparietal bone) (Figure 6). It is formed in a persistent mendosal suture (5).

Fig. 6: Os incae (interparietal bone). A large, single intrasutural bone at the junction of the lambdoid suture and sagittal suture (arrows).

Lacunar skull, increased convolutional markings, copper beaten skull

Lacunar skull, increased convolutional markings and copper beaten skull are confusing terms.

Lacunar skull is a dysplasia of the membranous bone. The well-defined lucent areas in calvarium are represent nonossified fibrous bone and they are bounded by normally ossified bone (2). They usually present at birth and most prominent in the parietal and occipital bones. The inner table is more affected than the outer. The lacunae resolves spontaneously by age 6 months and is not related to the degree of hydrocephalus. Lacunar skull is usually associated with Chiari II malformation, and less commonly with encephalocele (2).

Convolutional markings are inner table indentations that is caused by the cerebral surface of the growing brain in infants (Figure 6c). They occur later than lacunar skull, especially during periods of rapid brain growth, between age 2 – 3 years and 5 - 7 years. They become less prominent after approximately 8 years of age. Convolutional markings are now considered to be a reflection of normal brain growth. If they become prominent and seen throughout the skull rather tahan posterior parts, it reflects a pathologic condition so called copper beaten skull (6).

Copper beaten skull is an indicator of chronic elevated intracranial pressure resulting from  craniosynostosis, hydrocephalus and intracranial masses (Figure 8). Macrocrania, splitting of the sutures, skull demineralisation and erosion or enlargement of the sella turcica may be seen due to increased in intracranial pressure (6, 7).

Abnormalities in head size (macrocephaly and microcephaly)

Macrocephaly is a large head which is larger than two standard deviations from the normal distribution. There are three major causes of macrocephaly: Hydrocephalus (increased CSF fluid) (Figure 22), megalencephaly (enlargement of the brain due to neurocutenous syndromes or metabolic diseases) or thickening of the skull (anemia, rickets, hyperphosphataemia, osteopetrosis, osteogenesis imperfecta, cleidocranial dysostosis). It may be contitutional or due to benign causes such as benign enlargement of the subarachnoid space. CT is superior to skull radiograph because it can differantiate these major categories (8).

Microcephaly is small head which is less than two standard deviations from the normal distribution. Head size is smaller in some ethnic groups. It can also be familial. But it is important to diagnose microcephaly and identify the cause. There are two major causes of microcephaly: Primary ( chromosomal disorders, neurolation defects such as anencephaly and encephalocele, prosencephalisation defect such as agenesis corpus callosum and holoprosencephaly, migration defect) and secondary (intrauterine infection, toxins and vascular occlusions, severe hypoxic-ischemic injury  and postnatal systemic diseases) (2,8). Due to the lack of brain growth, the force keeping the cranial bones separated does not exist and there may be early closure of the sutures or even overlapping of the skull bones (Figure 7).

Fig. 7: Microcephaly. Sutures are closed and overlapping (arrows) in 1 year-old patient due to severe hypoxic-ischemic injury.

Abnormalities in head shape

There may be abnormalities in the shape of the neonatal calvaria due to pressure on the head during childbirth. This is called fetal or newborn molding and usually disappears after a few days. Faulty fetal packing means concave depressions in the calvaria due to extrinsic pressure of the limb or uterine leiomyoma (2).

Plagiocephaly without craniosynostosis (posterior deformational, positional plagiocephaly) is associated with sleeping position (sleeping on back), congenital torticolis, abnormal vertebra and neurologic deficits. There is ipsilateral frontal and contralateral occipital bossing (parallelogram shape) and anterior displacement of ipsilateral ear (9) .There is no significant distortion of the anterior-posterior axis of the skull base (10) (Figure 8-10).

Craniosynostosis

Premature fusion of the sutures are commonly isolated and sporadic (non-syndromic). Craniosynostosis may be associated with some syndromes, including Crouzon , Apert, Pfeiffer and Carpenter syndrome (1,11).

Plagiocephaly, means skewed or oblique head (1). Unilateral coronal synostosis (anterior plagiocephaly) (Figure 11), unilateral synostosis of the lambdoid suture (posterior plagiocephaly) or asyncronous synostoses of multipl sutures (Figure 12-13).

Fig. 12: Plagiocephaly and copper beaten skull. Premature fusion of the sagittal suture (white arrow) and left coronal suture (double white arrows). There are multipl lucencies in the parietal and occipital bones due to increased intracranial pressure, copper beaten skull.

Fig. 13: Plagiocephaly and copper beaten skull (the same patient as in Fig. 12). Premature fusion of the sagittal suture (white arrow) and left coronal suture (double white arrows). There are multipl lucencies in the parietal and occipital bones due to increased intracranial pressure, copper beaten skull.

A radiologist should distinguish posterior plagiocephaly, which requires surgey, from positional plagiocephaly, which can be treated conservatively (10,11). There are some important clues:

1) Lamdoid suture synostosis; 3D VR images are very useful for quick assesment of the premature fusion. 3D MIP images, that can be easily done in workstations within seconds, can be added for further detailed evaluation (12).

2) Contralateral frontal and parietal bossing (trapezoidal shape) and posterior displacement of ipsilateral ear (9). In positional plagiocephaly there is ipsilateral frontal and contralateral occipital bossing with a parallelogram shape and anterior displacement of ipsilateral ear (Figure 8, 9).

3) In skull base view, posterior fossa axis line (central line from the basion to opisthion) will be away from the anterior fossa axis line (central line bisecting the cribriform plate) toward the site of the lambdoid fusion (9, 10). In positional plagiocephaly the lines are continuous with each other or have minimal deviation (2.3˚±1.3˚) (10) (Figure 9)

Fig. 8: Positional plagiocephaly. Parallelogram shape of the posterior calvaria with ipsilateral frontal bossing (white arrow) and contralateral occipital bossing (double white arrows) on vertex view.

Fig. 9: Positional plagiocephaly. Skull base view in another patient shows minimum shift in midline lines (white; posterior fossa axis line, black; anterior fossa axis line). Long axis of the left temporal bone (black lines) is anterior than right, which clinically reflects anterior displacement of the left ear.

Fig. 10: Positional plagiocephaly(The same patient as in Fig. 9). There is flattening of the posterior calvaria (white line), ipsilateral frontal bossing (white arrow) and contralateral occipital bossing (double white arrows) and parietal bone fracture paralel to the plane of the imaging (black arrows). Convolutional markings are seen as lucent areas in 3D VR in parietal and occipital bones.

Scaphocephaly (or dolichocephaly) results from premature sagittal synostosis. There is an increased growth following the direction of the sagittal suture (Virchow’s law). This is the most common form of the isolated synostosis (11). (Figure 14-16)

Fig. 14: Scaphocephaly. Vertex view shows premature fusion of the anterior part of the sagittal suture (arrow). There are lucencies in the parietal and occipital bones due to increased intracranial pressure.

Fig. 15: Scaphocephaly. (The same patient as in Fig. 14) Increased AP diameter of the skull. There are lucencies in the parietal and occipital bones due to increased intracranial pressure.

Fig. 16: Scaphocephaly. Increased AP diameter of the skull in another patient. Note the bony ridge (arrows).

 Trigonocephaly is a bulging of the forehead due to fusion of the metopic suture before 6 months of age (1). Metopic suture fuses from glabella to the anterior fontanel. Anterior fontanel ossification, hypotelorism, narrowing of anterior cranial fossa and compensatory increase of middle cranial fossa are seen (11). (Figure 17,18)

Fig. 17: Trigonocephaly. 3D images of premature fusion of the metopic suture.

Fig. 18: Trigonocephaly. (The same patient as in Fig. 17) Axial MPR images of premature fusion of the metopic suture with hypotelorism, narrowing of anterior cranial fossa and compensatory increase of middle cranial fossa.

Oxycephaly, brachycephaly results from bilateral premature fusion of the coronal or lambdoid sutures. There is a flat and high forehead due to growth following the direction of the coronal suture. Transverse diameter of the skull is widened. Superior displacement of the lesser wing of the sphenoid caused the characteristic “harlequin eye” (1,11). Brachycephaly is frequent in syndromic synostosis (e.g. Apert, Crouzon, Pfeiffer, craniofrontonasal syndrome etc.) (13). (Figure 19,20)

Fig. 19: Craniofrontonasal syndrome, brachycephaly. Bilateral coronal synostosis with bony ridge (double arrows). Central defects between frontal bones (arrows). The calvarium is broadened in the transverse plane.

Fig. 20: Craniofrontonasal syndrome, brachycephaly.(The same patient as in Fig. 19)the calvarium is shortened in the sagittal and broadened in the transverse plane.

Congenital calvarial defects

Parietal foramina are paired parasagittal defects that result from delayed or incomplete ossification of the parietal bone (2). They are generally isolated but may be part of a syndrome. They are usually considered benign. Parietal foramina associated with an atretic cephalocele and symmetrical parietal meningoceles with abnormal venous anatomy have been described (14). (Figure 21).

Fig. 21: Parietal foramina seen as paired parasagittal defects in parietal bones.

Open sutures and anterior fontanel can be due to elevated intracranial pressure (Figure 22) or hypothyroidism and skeletal dysplasia, e.g. cleidocranial dysplasias, pycnodysostosis and osteogenesis imperfecta.

Fig. 22: Large sutures and anterior fontanel due to hydrocephalus.

Large anterior fontanel can be associated with achondroplasia, congenital hypothyroidism, Down syndrome, rickets and increased intracranial pressure. Anterior fontanel size is the average of the anteroposterior and transverse diameters. The average size of the anterior fontanel is 2.1 cm, and the median time of closure is 13.8 months. (15). (Figure 23)

Fig. 23: Large anterior fontanel in a 2 year-old boy. Measurement of the fontanel size (a+b/2).

Calvarial bone fractures

Cranial fractures that are parallel or nearly parallel to the section orientation may be missed at interpretation of CT. Pediatric calvarium with multipl sutures and fontanel make diagnosis more diffucult. 3D VR and 3D MIP images are very useful in these patients and fractures and their extension can be assessed easily (12). (Figure 10, Figure 24,25).

Fig. 24: Bilateral parietal bone fractures (arrows), posterior view.

Cephalohematoma is subperiosteal hematoma of the calvaria (2). They do not cross the midline. They generally resolves spontaneously and may calcify periferally (Figure 26,27). If they are not absorbed they can ossify over the surface. Ossified cephalohematoma is a rare entity and it needs surgical management (16). It can mimic osteoma in 3D images.

Fig. 26: Cephalohematoma. 3D image show periferally calcified subperiostal hematoma.

Fig. 27: Cephalohematoma. Axial MPR image show periferally calcified subperiostal hematoma (the same patient as in Fig. 26).

Calvarial bone tumors

3D CT may help to evaluate litic and sclerotic bone tumors. 3D CT is useful for preoperative and postoperative assessment of these patients.

Osteomas are the most common primary benign tumors of the calvaria, They are solid nodular sclerotic lesions and they usually arise from the outer table (17). (Figure 28,29)

Fig. 28: Osteoma. 3D image show left parietal osteoma (arrows).

Fig. 29: Osteoma. Axial MPR image (the same patient as in Fig. 28) show left parietal osteoma.

Langerhans’ cell histiocytosis, epidermoid and dermoid cyst, meningioma, hemangioma, fibrous dysplasia and metastases are other most common lesions of the calvaria.

Postoperative cranium

3D CT is valuable in postoperative evaluation of surgery for craniosynostosis (Figure 30,31). Burr holes, cranioectomy defects and bone grafts may be evaluated with 3D VR images .

Fig. 31: Preoperative images of the patient in Fig. 29. Anterior plagiocephaly due to fusion of the right coronal suture (arrows). Preoperatively there is increased convolutional markings in parietal and occipital bones result from increased cranial pressure.

Fig. 30: Ppostoperative images (1 year after) of the anterior plagiocephaly due to fusion of the right coronal suture.