Skull X-Ray

Skull radiography images the cranial vault, base, sutures, sinuses, pituitary fossa and facial bones in projection. The usual series includes a lateral view, an anteroposterior or occipitofrontal (Caldwell) view and a Towne (fronto occipital, angled 30 degrees caudally) view, with a basal (submentovertical) view in selected cases.

Bone detail is reasonable, but the brain, extradural and subdural blood, and soft tissue cannot be assessed, and overlapping structures limit interpretation. The effective dose is about 0.03 to 0.1 mSv for the series, but the main reason to avoid it is poor diagnostic value.

Skull radiography has largely been replaced by CT, which shows fractures and intracranial injury together. In adults with head injury, CT is chosen by clinical decision rules (Glasgow Coma Scale under 15 at 2 hours, suspected open or depressed fracture, signs of basal skull fracture, post-traumatic seizure, focal deficit, repeated vomiting, anticoagulation, or age 65 years or over).

A skull film does not exclude intracranial injury, and many patients with intracranial haemorrhage have no fracture, whereas the presence of a fracture raises the chance of haemorrhage many times over.

Remaining uses are skeletal surveys in suspected non-accidental injury in infants (where skull films are included with CT as needed), penetrating injury and foreign bodies, shunt series, myeloma and metabolic bone disease screening in some centres, and craniosynostosis and calvarial lesions in children.

Indications. Suspected non-accidental injury in a child under 2 years (as a part of the skeletal survey). Penetrating injury or suspected foreign body where CT is unavailable. Depressed skull fracture suspected on palpation. Head injury in settings without CT, with strict clinical criteria and transfer plans. Ventriculoperitoneal shunt series to check continuity.

Multiple myeloma and bone metastases (if whole body imaging is not available). Paget disease of the skull. Suspected craniosynostosis or abnormal head shape in infants (before CT). Skull lesions found clinically (lump, pain). Not for routine investigation of headache, sinusitis or minor head injury.

Contraindications and precautions. No absolute contraindication. Pregnancy: low foetal dose with shielding; image when necessary. In head injury, do not delay CT or transfer to obtain skull films, and the cervical spine must be protected (immobilised) until cleared. Infants need immobilisation for symmetrical views. Remove hair clips, dentures, earrings and hair braids.

Preparation and technique. Erect or supine positioning with the head in the neutral anatomical position, the orbitomeatal line perpendicular to the film for the Caldwell view, and horizontal beam lateral view for suspected fluid levels in the sinuses or air fluid in the cranium. In a trauma patient, use a cross-table lateral film.

Normal appearances. Smooth inner and outer tables with diploe between them; thin vascular grooves for middle meningeal vessels (branching, 1 to 2 mm wide, with sclerotic margins), venous lakes and diploic veins; sutures (coronal, sagittal, lambdoid and squamosal) as interdigitating lines that close from 20 to 40 years of age; the anterior fontanelle closes by 18 months;

pineal gland calcification in adults (central, within 2 mm of the midline); choroid plexus and falx calcification; pituitary fossa up to 12 mm deep and 16 mm wide; and pneumatised sinuses.

Interpretation.

• Linear fracture: sharp, straight, lucent line, wider than vascular grooves, crossing the vault without branching and without sclerotic margins, often with soft tissue swelling. Fractures crossing the middle meningeal groove or the dural venous sinuses carry a risk of extradural haematoma. Sutures and grooves are the main mimics.
• Depressed fracture: a fragment displaced inwards past the inner table, shown as a double density; tangential views are needed. Compound depressed fractures need surgical review.
• Basal skull fracture: air fluid level in the sphenoid sinus, intracranial air (pneumocephalus), opacified mastoid or sinus, and clinical signs (haemotympanum, cerebrospinal fluid leak, periorbital and mastoid bruising).
• Sutural diastasis: separation of sutures in raised intracranial pressure in children (more than 2 mm) or after trauma; "copper beaten" appearance indicates chronic raised pressure.
• Craniosynostosis: early suture closure with abnormal skull shape (scaphocephaly, brachycephaly, trigonocephaly).
• Lytic lesions: sharply punched out (multiple myeloma, "raindrop" or "pepper pot" skull in hyperparathyroidism), single (metastasis, epidermoid, eosinophilic granuloma, haemangioma), or geographic bone destruction (osteomyelitis, tumours). Sclerotic lesions (osteoma, meningioma hyperostosis, osteoblastic metastasis).
• Paget disease: thickened vault with mixed lucency and sclerosis ("cotton wool"), osteoporosis circumscripta.
• Pituitary fossa: enlarged, eroded or double floor in pituitary tumours (now assessed by MRI). Calcification of the sellar and suprasellar region in craniopharyngioma, aneurysm or meningioma. Shift of calcified pineal gland over 3 mm indicates mass effect (obsolete).
• Shunt series: continuity of the tubing from the reservoir to the peritoneum, kinks, disconnections and fractures.
• Foreign bodies: bullets, pellets, shrapnel and metallic objects; radiolucent wood and plastic are invisible.

Critical values and findings needing immediate action.

• Depressed skull fracture, compound fracture or penetrating injury: urgent CT and neurosurgical review.
• Fracture crossing a vascular groove or venous sinus: urgent CT for extradural haematoma, even if neurologically intact.
• Intracranial air (pneumocephalus) or air fluid level in the sphenoid sinus: basal fracture and risk of meningitis, so CT and neurosurgical or ENT review.
• Multiple fractures or fractures in an infant, especially complex or bilateral: suspected non-accidental injury, with skeletal survey, safeguarding referral and CT.
• Shunt disconnection or fracture with symptoms of raised intracranial pressure: urgent neurosurgical review.
• Widely separated sutures with vomiting or papilloedema: raised intracranial pressure and urgent CT or MRI.

Limitations and artefacts.

• Plain skull films cannot show intracranial haemorrhage, contusion or oedema, and a normal film does not exclude serious intracranial injury, which makes them unsuitable as a screening test for head injury.
• Mimics of fractures include sutures, vascular channels, venous lakes, accessory sutures, Pacchionian granulations, hair braids and scalp folds; comparing with the opposite side and the clinical examination helps.
• Fractures parallel to the beam can be missed; a depressed fracture may be seen only on tangential views.
• Radiolucent foreign bodies are not shown.
• Superimposition of the skull base, face, cervical spine and petrous bones limits assessment of the base.
• Children under 2 years have wide sutures, fontanelles, and venous markings that vary; interpretation requires paediatric radiology expertise.
• Radiation dose is low but the diagnostic yield is limited; do not repeat the study.
• Many calcifications (pineal, choroid plexus, falx) are normal; abnormal calcification is better shown on CT.

Special groups.

• Infants and children: skull radiographs have a role in non-accidental injury, with a skeletal survey; CT or MRI is used for head injury; accidental falls from low heights rarely cause fractures.
• Elderly patients on anticoagulants: low threshold for CT regardless of fracture status.
• Pregnancy: avoid unless essential; CT head is justified for serious injury.
• Patients with shunts: shunt series with CT or rapid MRI for suspected malfunction.
• Regions: in hospitals without CT, skull films with clinical decision rules direct transfer in head injury, while transfer should not be delayed by imaging; trepanation and firearms injuries are seen with metallic foreign bodies.

Follow up tests. CT head without contrast for any suspected intracranial injury, CT with bone windows or three dimensional reconstruction for complex fractures and craniosynostosis, MRI for brain, pituitary and posterior fossa, cervical spine imaging in trauma, skeletal survey in suspected non-accidental injury, serum calcium,

alkaline phosphatase and protein electrophoresis for lytic lesions, bone scan or whole body imaging for metastases and myeloma, ophthalmic examination for papilloedema, and neurosurgical review.