Computed tomography (CT) is the technique of choice in the evaluation of craniosynostosis. It has high sensitivity and specificity, especially with the use of 3D reconstructions [5]. CT also provides information regarding brain parenchyma and intracranial abnormalities. Judicious utilization of CT imaging and the use of appropriate low dose techniques protocols is essential to limit radiation risk in children.

Radiography has traditionally been the first-line study when non-syndromic craniosynostosis is suspected. However, it has low sensitivity in the first three months of life, due to poor mineralization.

There are primary and secondary signs on radiography (Fig. 2) [4]:

Primary signs:

• Perisutural sclerosis
• Ridging or beaking
• Bony bridges
• Absence of sutures

Secondary signs:

• Fingerprinting and copper beating, sign of raised intracranial pressure (ICP)

Fig. 2: Scaphocephaly. Radiographic signs of craniosynostosis: perisutural sclerosis (yellow arrow), beaking (blue arrow) and Scaphocephaly morfology (red bracket).

There has been an increasing development of specific magnetic resonance (MR) sequences for the study of the skull, however it is a less available technique and may require sedation of the patient [6]. MR is recommended in syndromic craniosynostoses, for a correct evaluation of the brain parenchyma and hydrocephalus (unusual finding in non-syndromic cases).

Ultrasound plays also a role in craniosynostosis, but it gives limited information about the brain parenchyma.

Non-syndromic synostosis

The closure of each suture determines particular types of morphological alterations (table).

Table 1

Scaphocephaly and dolichocephaly

• Secondary to the premature closure of the sagittal suture (Fig. 3). 
• It is the most common form (50% approximately) [6]. 
• Increase of the anterior-posterior diameter of the skull, with frontal and occipital prominence. Sometimes associated with hypotelorism.

Fig. 3: (Same patient of Fig. 2.). Sagittal synostosis. CT with 3D reconstructions demonstrate fused sagittal suture with ridging (yellow arrow), which results in increased AP dimension and scaphocephaly (elongated skull).

Trigonocephaly

• Secondary to the premature closure of the metopic suture (Fig. 4). 
• It produces a triangular frontal deformity (trigonocephaly), associated with hypotelorism, parieto-occipital prominence, narrow anterior cranial fossa, lateral hypoplasia of the orbits, internal metopic notch and external metopic beak, hypoplastic ethmoidal sinuses and medial inclination of the orbital roofs [7]. 
• One third of the cases are syndromic, with abnormalities of the midline in the brain and palate. 

Fig. 4: Metopic synostosis. A head CT with axial brain window (A), axial bone window (B), Maximun Intensity Projection (C) and 3D reconstruction (D) show complete fusion of the metopic suture (yellow arrow), with a metopic ridge (red arrow), narrowing of the anterior cranial fossa (trigonocephaly) and hypotelorism.

Brachycephaly

• It is the result of the premature bilateral closure of the coronal or lambdoid sutures, producing an increase in the transverse diameter of the skull. [6]
• Coronal synostosis (Fig. 5): when they are unilateral, plagiocephaly occurs, with flattening over the fused suture, ipsilateral temporal expansion and contralateral fronto-parietal expansion. It usually compromises the skull-base, determining a shallow anterior cranial fossa as well as the orbits, also depression of the temporal bone and deformity with harlequin eyes. [7, 8]

Fig. 5: Unilateral coronal synostosis. A head CT with axial bone window and 3D reconstructions of the skull demonstrate fusion of the right coronal suture (yellow arrows) with ipsilateral anterior plagiocephaly, with contralateral frontal and parietal expansion. 3D reconstructions demonstrate convexity of right orbital lateral wall in “Harlequin eye” deformity (blue arrow).

Syndromic synostosis

Usually multiple sutures involved.

The most common entities are Crouzon syndrome, Apert syndrome and Pfeiffer syndrome. [3]

The deformity depends on the combination of affected sutures and there are multi some pattern described: [3, 8] 

• Clover leaf: premature fusion of the sagittal, coronal and lambdoid sutures, there is prominence of the temporal regions.
• Mercedes Benz synostosis: Bilateral lambdoid fusion with sagittal suture synostosis.
• Anteriorly tall cranium: Sagittal, bilateral coronal, and metopic suture synostoses.

Pitfalls

Positional plagiocephaly (Fig. 6)

• Positional plagiocephaly is a non-synostotic skull deformity, characterized by an asymmetric lambdoid flattening. It is produced by prolongated supine positioning of infants during sleep. It manifests in the first months, and stabilizes after the sixth month of life. 
• Clinical observation might be enough for making the diagnosis and differentiate it from a early closure of the lambdoid suture. CT may be helpful in equivocal cases, by demonstrating patency of sutures, ipsilateral frontal bulge and normal position of the temporal bone.

Fig. 6: Positional plagiocephaly. A head CT with axial bone window (A and B), axial brain window (C) and 3D reconstruction (D) demonstrate asymmetric flattening of the left posterior parietooccipital skull (yellow arrow). However, there are patent sutures (red arrow), ipsilateral frontal bulge (blue arrow) and normal position of temporal bones (green arrow), compatible with positional plagiocephaly.

Metopic ridge (Fig. 7):

• It is a normal variant of the metopic closure, seen in asymptomatic children. There are no other findings of trigonocephaly, such as a narrow anterior cranial fossa, hypotelorism or internal metopic notch.

Fig. 7: Metopic ridge. A Head TC with axial brain window (A), bone window (B) and 3D reconstruction (C) demonstrate fused metopic suture, with mild beaking (yellow arrow) but absence of trigonocephaly. There is not metopic notch (blue arrow).

Postsurgical findings:

• The objectives of surgical treatment are to expand intracranial volume to reduce the risk of developing increased intracranial pressure and to allow a normal growth of the brain and the shape of the skull. [3]
• Surgical techniques vary case by case; however, the role of the images is to demonstrate an adequate position of the installed devices (distractors) in the suturectomies (Fig. 8 and 9).

Fig. 8: Top) A head CT with coronal and axial bone window and 3D reconstructions demonstrates fused sagittal suture (yellow arrow) and scaphocephaly. Bottom left) Post-surgical changes of a suturectomy and a distractor, the last at good position. Bottom right) Control 3 years later, there are partially dural calcification and normal parenchymal expansion.

Fig. 9: Top) A head CT with axial brain and bone windows and 3D reconstruction demonstrates fused metopic suture (yellow arrow) and trigonocephaly. Bottom) Post-surgical changes of a suturectomy and a distractor, the last at good position.