Evaluation of Plain X-ray Skull—A Systematic Approach1

In the past skull radiographs were considered an essential step in the investigative protocol of a patient suspected to have neurological disease. With the availability of CT and MRI there has been a dramatic decline in the use of plain films and the indications for skull radiographs have been redefined.^1,2 Patients presenting with stroke, epilepsy, dementia or in postoperative cases, skull X-rays generally provide no useful information and CT is the investigation of choice.³ In patients of trauma, CT should be the first line investigation except in patients who suffer from facial and orbital fractures where plain films are helpful in orientation and in medicolegal cases.^4–6 Occasionally skull X-rays may reveal linear fractures with more certainty than CT scan.⁴ The skull radiographs play a major role in the diagnosis of dysplasias, infections and tumours affecting the skull bones, metabolic conditions such as leukaemia and multiple myeloma. In patients suspected to have intracerebral tumours, PA and lateral view of skull may provide additional information like detection of hyperostosis in case of meningiomas, presence of lytic and sclerotic metastasis in neuroblastomas and tram track calcification in Sturge-Weber syndrome which may compliment the diagnosis on CT. The present chapter will describe the normal roentgen anatomy as seen in the basic views of skull followed by systematic approach to the analysis of the abnormal skull X-rays.^7–9

A single lateral view of the skull is the most common radiographic X-ray examination performed. A systematic approach to the examination consists of evaluation of the size and shape of the cranium, the thickness and density of the bones, the sutures, the vascular markings, the base of skull and the cranial cavity.

Width of the sutures can be best assessed at the top of the coronal suture in the lateral view. To see the sagittal and the lambdoid sutures, PA and Towne's views are performed. Vascular markings are seen along the coronal suture due to middle meningeal vessels. Arterial grooves become narrower as they go distally. They may be confused with fracture lines but the latter are more radiolucent whereas vascular marking have a halo of increased density around them. Posterior branch of middle meningeal artery as it ascends upwards and posteriorly sometimes causes a shadow over the temporal bone that should not be mistaken for a fracture. Enlargement of the arterial grooves may occur in meningioma and arteriovenous malformations (Fig. 1.1B).

Posteroanterior (PA) projection with 15–20° craniocaudal angulation is preferred to straight PA projection as the petrous pyramids are projected below the orbits and the superior orbital fissure as well as greater and lesser wings of the sphenoid are clearly visualised. PA view is also examined for shape of the skull, with special attention to the symmetry of the two sides. The bony landmarks which require to be carefully examined for any erosions, sclerosis or lack of continuity include crista galli in the midline, planum sphenoidale, floor of the sella, lesser and greater wings of the sphenoid and the three lines of the orbit formed by the palpable superior border of orbit, highest point of roof of the orbit and the sphenoid ridge which represents the floor of the anterior cranial fossa. The floor of the posterior cranial fossa can also be seen inferiorly. Pacchionian depressions due to arachnoid granulations can be seen in both PA and lateral views as tiny radiolucent areas usually within 2.5 to 3 cm from the midline. Their margins are well defined superiorly whereas inferiorly the margins fade away-a feature helpful in distinguishing these from destructive lesions.

Towne's view is performed by angling the tube 35° caudally from the orbitomeatal line. It is generally performed when pathology is suspected in the petrous pyramids.

This projection also shows the occipital bone, foramen magnum, dorsum sellae, the internal acoustic canals, mastoids and the condyles of mandible.

Basal view of the skull or the submentovertical view is an infrequent examination and is generally performed in specific situations such as looking for the skull base lesions, middle ear or inner ear lesions, nasopharyngeal masses or oropharyngeal lesions and sinus pathologies. The bony landmarks that should always be identified and carefully examined include three lines, constituted by the lateral wall of the maxillary antrum (S-shaped), the posterolateral wall of the orbit, and the anterior wall of the middle cranial fossa which is arched with concavity pointing posteriorly. The lesser wing of the sphenoid is seen just behind the anterior wall of middle cranial fossa. A transverse dense line in the centre 5represents the anterior margin of sella. The medial and lateral pterygoid processes are projected over the sphenoid ridge. Sphenoid sinuses should be carefully seen as early bone destruction in patients of nasopharyngeal carcinoma or sphenoid sinus carcinoma is well demonstrated in this view.

It is one of the standard views to study the maxillary and anterior ethmoidal sinuses. Waters view is generally performed with the patient in sitting position to facilitate demonstration of any fluid level in the sinuses. Patient is instructed to keep the mouth open with nose and chin touching the cassette in order to visualise the sphenoid sinuses. It is performed by placing the orbitomeatal line at an angle of 35 degrees with the plane of film by either raising the chin or by tilting the tube. It also gives clear picture of the roof of the orbits, destruction of which may be seen in mucocele of frontal sinus and in carcinoma of lacrimal gland.

Caldwell's view is best projection for examining the frontal and ethmoid sinuses. Patient is positioned directly facing the cassette in either sitting or prone position with midsagittal plane and orbitomeatal line perpendicular to the film with nose and forehead touching the cassette. Central ray is directed 15 degrees caudally to the nasion.

Normal contour of the skull is maintained by sutures, the intracranial contents and normal bone formation. Abnormality in any of these may result in abnormal contour of the skull.

Size of the calvarium is dependent on the size of the intracranial contents. The most accurate way to determine abnormal cranial volume is to measure the skull directly and compare the measurements to standard for age and body size. A simple method of assessing the size of the skull is to compare the skull vault to the size of the face.

At birth, the volume of skull is approximately four times that of face. This ratio decreases to 3:1 by age 2 and 1.5:1 by adulthood.⁸

Enlarged head, size may result from hydrocephalus, macrocephaly, hydranencephaly and in pituitary dwarfism. The most common cause of hydrocephalus in children is congenital obstruction of the ventricular system and is associated with raised intracranial tension. Sutures become wide due to expansion of the intracranial contents.

Small skull but otherwise normal contour is characteristically seen in microcephaly associated with mental retardation. Cranial sutures fuse early but this is a result of microcephaly and not the cause. The sinuses are large and the digital or convolutional markings are absent or decreased. It is important to differentiate premature closure of all the sutures from microcephaly with fused sutures. When multiple sutures fuse prematurely, the fusion does not occur simultaneously and the result is an irregular skull due to expansion of the skull in unusual directions to accommodate the brain. Clinically signs of raised intracranial tension are present. Convolutional markings are exaggerated.

Increased thickness of the skull may result due to early cessation of brain growth or due to cerebral atrophy. Increased width of diploic space due to increased haematopoiesis is seen in haemolytic anaemias. Progressive hydrocephalus leads to large bony calvarium and a decreased diploic space. However, if a ventricular shunt is performed and abnormal expansion ceases resulting in arrested hydrocephalus, the cranial sutures close and the inner table of bones of the skull become thicker and the diploic space becomes larger. A history of hydrocephalus and the presence of a ventricular shunt facilitates the diagnosis

When there is a single lucent lesion, the important considerations that help in deciding its nature are its location, associated soft tissue swelling, table of the bone involved and margins of the lytic lesion, whether sharp, ill-defined or sclerotic. Radiolucent defects in the skull bone may be due to variety of causes which may be congenital or acquired. Congenital causes may be parietal foramina, anomalous apertures, meningoencephalocele or dermal sinus. The acquired causes include trauma, infections, tumours and histiocytosis.

Fractures generally occur at the site of injury and may be associated with soft tissue swelling. Linear non-depressed fractures may be seen as radiolucent lines and should not be confused with sutures or vascular grooves.

Fracture lines are non-tapering, non-branching and have sharp borders whereas vascular grooves have ill-defined borders and an undulating course. Sutures are seen in known anatomical positions and have saw tooth edges. Depressed fractures generally occur after severe trauma and are considered more serious than linear fractures. Radiologically the depressed fragment presents as area of increased radiodensity surrounded by a radiolucent zone (Fig. 1.10). In children, when the dura beneath the suture is torn, the arachnoid membrane herniates through the dura into the bony defect. The pulsations of the brain lead to progressive enlargement of the arachnoid collection resulting in expansion of the fracture line termed as growing skull fracture. The bulging membranes result in the formation of leptomeningeal cyst (Fig. 1.11)

Infections of the skull are uncommon and generally follow trauma or arise secondary to infection elsewhere in the body. The radiographic appearance consists of mottled irregular lucencies which have ill-defined borders and are associated with soft tissue swelling of the scalp.

Epidermoid tumours develop from a congenital inclusion of epithelial cells within the calvarium.

Radiologically these lesions present as well-defined lytic lesions which have sclerotic border and are not necessarily located in the midline. Intracranial epidermoids may also produce a radiolucent shadow which may mimick a lytic lesion (Figs 1.12A and B).

Malignant lesions such as primary osteosarcoma or metastasis can also produce lytic defects. Osteosarcoma causes gross destruction of the bone with ill-defined margins and soft tissue swelling (Figs 1.13A and B). Neurofibromatosis is a rare cause of a lytic defect seen along the suture. This defect is not due to the presence of neurofibroma but is a manifestation of mesenchymal defect. Intracranial mass lesions can also rarely present as lytic areas of the skull.

Eosinophilic granulomas, Hand-Schüller Christian disease and Letterer-Siwe disease all form part of a complex comprising, histiocytosis. The severity ranges from mild in eosinophilic granuloma to very malignant course in Letterer-Siwe disease. A single lytic lesion having sharp non-sclerotic border and bevelled edges is characteristic of eosinophilic granulomas. Occasionally a small bone is seen in the centre representing button sequestrum. Lytic lesions in the other two variants are larger, multiple and punched out (Fig. 1.14).

Multiple radiolucent defects in the skull in children may be due to craniolacunia, presence of wormian bones, increased convolutional makings due to raised intracranial tension, histiocytosis and metastasis from neuroblastoma (Figs 1.15A and B), leukaemia or Ewing's sarcoma. In adults multiple myeloma (Fig. 1.16), metastasis and hyperparathyroidism (Fig. 1.17) are the usual causes.

Craniolacunia is due to a defect in ossification of the bones which develop from membranous tissue. There are multiple radiolucent defects seen all over the cranial vault interspersed with strips of normal bone which appear dense. Craniolacunia (Lacunar skull) by itself is not of much significance but it is generally associated with myelomeningocele or encephalocele (Fig. 1.9).

Appearance must not be confused with increased convolutional markings that result from raised intracranial tension and are seen as multiple radiolucent areas not exceeding the diameter of a finger. Convolutional marking may also be seen in normal children in the frontal and occipital region. Presence of increased convolutional markings in the parietal region should generally be considered abnormal. Wormian bones are seen along the sutures and results due to defective mineralisation. Multiple wormian bones are seen in cleidocranial dysostosis, osteogenesis imperfecta, hypothyroidism and pyknodysostosis.

Areas of increased density in the skull may be seen in both normal as well as pathological conditions. Osteopetrosis is a rare condition which is characterized by diffuse thickening of the skull and face (Fig. 1.18). Fibrous dysplasia may involve the vault or base of skull. There may be a single lesion or it may be part of syndrome (McCune-Albright syndrome) seen in females when it affects multiple bones and is associated with precocious puberty. The lesions are sclerotic with loss of normal trabecular pattern. Mixed type of lesions with sclerotic and lytic areas are also known to occur (Figs 1.19A and B). Multiple hyperostotic lesions affecting the calvarium measuring 5–10 mm in size may be seen in tuberous sclerosis in association with calcified lesion in periventricular region. Thickening of the frontal and parietal bones may occur in rickets due to presence of poorly mineralised bone which on healing becomes dense.

An osteoma affecting the skull bones is a benign tumour which appears as a dense lesion projecting extracranially from the outer table of skull. Osteoma is also the commonest benign tumour affecting the sinuses (Fig. 1.20). Focal areas of hyperostosis are characteristic of meningioma (Figs 1.21A and B). When the hyperostosis affects the frontal bone, in a case of convexity meningioma, it must be differentiated from hyperostosis frontalis interna (Fig. 1.22), the later is generally seen in elderly females and affects the inner table with sparing of diploic space and does not cross the midline.

Presence of calcification can provide important clue to the diagnosis in several conditions. Although causes are numerous (Table 1.1) some of these conditions have specific appearance which can be diagnostic.

It lies above and anterior to pineal gland. Choroid plexus calcification is generally bilateral and may be unequal on the two sides. Other normal sites of calcification are the falx and anterior petroclinoid ligaments above the sella.

Tuberous sclerosis is a syndrome comprised of epilepsy, mental retardation and adenoma sebaceum. Multiple hamartomas occur in the brain as well as at other sites such as kidneys. Tumours consist of glial tissue and ganglion cells. In the brain they are usually multiple and are seen in the subcortical, subependymal and basal ganglia regions. Calcification is seen in 50 per cent of the lesions.

Sturge-Weber syndrome is another important cause of intracranial calcification. Patients present with epilepsy and mental retardation and often have cutaneous haemangioma in the distribution of trigeminal nerve on the same side as calcification. Calcification has a typical tram track appearance and is seen in the cerebral cortex (Figs 1.23A and B).

Basal ganglia calcification is an important feature of hypoparathyroidism and pseudohypoparathyroidism. Wide spread irregular and punctate areas of calcification which are diffusely scattered are characteristic of Fahr's disease. In this condition patients present with severe growth and mental retardation. The disease is hereditary and is characterised by microscopic deposits of iron and calcium in the basal ganglia, cerebellum and subcortical region. Infections due to toxoplasma and cytomegalovirus are important causes of intracranial calcification in the newborn. Calcifications are multiple and diffusely scattered in the brain parenchyma or paraventricular region. Bacterial infections may progress to cerebral abscess which may get calcified.

A large arc like calcification seen in the region of pineal gland in a newborn presenting with congestive heart failure and hydrocephalus is characteristic of vein of Galen aneurysm. Intracerebral or chronic subdural haematomas may reveal curvilinear calcification. A variety of tumours may show calcification. In children, suprasellar craniopharyngioma is the most common tumour which reveals calcification whereas in adults oligodendrogliomas and meningiomas are the common tumours to calcify.