Applied Radiology

This work presents an overview of the radiologic appearance in bone window setting of different primary and secondary skull tumors, bone dysplasia, congenital and inflammatory diseases affecting the skull, and extrinsic lesions exhibiting secondary effects on the skull. The differential diagnosis of lesions affecting the inner and/or outer table, or the diploic space also is presented. Recommendations for routine use of bone window setting in brain CT scanning includes 1) abnormal skull films; 2) suspected congenital anomalies; 3) presence of enhancing lesions in close proximity to skull bone; and 4) suspected metastatic disease.

Dr. Snow and Dr. Brogdon are in the Department of Radiology at University of South Alabama Medical Center in Mobile, AL. Dr. Williams was in the Department of Radiology at University of South Alabama Medical Center in Mobile, AL. Dr. Georgy is in the Department of Radiology at the University of California in San Diego, CA.

Bone window images that routinely are acquired in CT examinations of the head for trauma or paranasal sinus disease can be useful in the characterization of different skull lesions. These images are excellent for demonstration of skull tables and the diploic space. Bone algorithm and filters also enhance the ability to visualize fine bony detail. Bone window images obtained in conjunction with routine brain scans may lead to fortuitous discovery or superior characterization of skull pathology.

CT window widths of 600 to 2,000 Hounsfield units (HU) and window levels of 160 HU to 500 HU accurately delineate tissues of a large difference in CT attenuation, such as bone and air. Narrower window widths (80 HU to 150 HU) and lower center levels (40 HU to 50 HU) are employed to show small differences in attenuation of soft tissues such as brain, but are not as accurate as bone window settings in depicting contour, edge interface, and size of bone structures.

Features of the calvaria

The calvaria is well differentiated into three layers: inner, middle, and outer tables. The inner and the outer tables are composed of compact bone; the middle table is composed of cancellous bone. The inner and outer tables vary little in thickness, except in places where they are eroded by vascular structures and gyral impressions. The diploic space is composed of an irregular network of bony trabeculae and vascular spaces. Blood within this space may act as a fluid cushion to absorb traumatic forces.1

Normal variants

Common benign lesions may represent normal variants or developmental abnormalities in the skull. Such lesions usually have characteristic radiologic findings and should not be confused with other pathological entities. Pacchionian granulations or venous lakes (figure 1) are arachnoid extensions projecting into the lumen of the main venous sinuses. The edges are further apart at the inner table level than at the outer table, indicating a benign process originating inside the skull.1 Another benign finding is the presence of circular lucent defects of the calvaria, known as doughnut lesions.

These usually do not exceed 2 cm in diameter and may represent smaller versions of abnormal lesions such as fibrous dysplasia, eosinophilic granuloma, epidermoid inclusion cyst, or osteoid osteoma.3 Some cases of doughnut lesions may be familial in nature.4

Parietal thinning (figure 2) is characterized by the generally bilateral and symmetrical thinning of the parietal bones with partial or complete absence of the diploe and the outer table of the skull.5 Parietal foramina are holes in the skull that represent a benign variant of incomplete ossification of the bones in the region of the obelion on both sides of the sagittal suture, and may be associated with severe pain and headache with gentle pressure.6 Parietal foramina also may be familial and is possibly associated with faulty ossification of the clavicles.7 Another abnormality that may be seen is hyperostosis frontalis interna,8 which is characterized by thickening of the inner table that may extend to the diploe of the frontal bone; this lesion is commonly found in middle-aged women (figure 3). It is important to differentiate this normal variant from hyperostosis that may occur secondary to meningiomas.

Hereditary and developmental abnormalities

A long list of developmental and hereditary diseases can affect the calvarial bones. Hodges3 divided the hereditary abnormalities of the calvaria into five groups: suture abnormalities, abnormalities in shape and size, increased thickness and density, generalized thinning, and skull defects or holes. Table 1 summarizes the radiological findings of the common developmental and hereditary diseases that can affect the skull calvaria (figures 4-6). Bone window setting also is extremely valuable in the evaluation of nasal bone and temporal bone developmental anomalies (figure 7).

Inflammatory lesions

Infections of the skull are rare because of the relative resistance of the calvarial bones to infection. When infection does occur, it usually is by direct extension from the paranasal sinuses and mastoid air cells or from trauma.18 Subperiosteal abscess formation secondary to osteomyelitis extension from acute suppurative frontal sinusitis results in the well known Pott's puffy tumor (figure 8).19 Common predisposing factors of Pott's puffy tumor are diabetes and immunosuppression.

If a Pseudomonas infection extends through the cartilage of the external auditory canal to the skull base in cases of malignant otitis externa it may result in osteomyelitis.14 Additionally, aspergillosis and mucormycosis can invade the calvaria (figure 9).20 Osteitis is seen as a focal or diffuse wavy thickening of periosteum with sclerosis of bony margins; this periosteal thickening may enhance following intravenous injection of contrast material.

When an infection extends into the bone marrow (osteomyelitis),14 lytic expansion and destruction are noted and usually associated with a soft-tissue mass. Granulomatous infection is seen as a lytic area with or without sclerotic margins and localized soft-tissue swelling.18 Mucoceles of the paranasal sinuses often show smooth defects in relation to the sinus walls and can be seen as extending into the orbit or the intracranial cavity.21

CT is not specific in differentiating tumor from infection; however, it can help to define the extent of the lesion and is useful in following up the efficacy of treatment.14

Neoplastic diseases

The most frequent neoplastic involvement of the skull occurs by metastatic disease or invasion from adjacent neoplasms. Table 2 shows the radiologic findings in bone window setting of the most common primary osseous and cartilaginous tumors arising from the skull bones. Tumors of neuronal origin also can cause secondary effects on the skull bones (table 3).

Some tumors are particularly common in the skull base; these include chordoma, chondroma, chondrosarcoma, giant cell tumors, dermoid and epidermoid tumors, sellar tumors, chemodectomas, and neuromas that arise from cranial nerves traversing skull base foramina. Coronal and sagittal images are very helpful in evaluation of the presence of such lesions. Scanning at a gantry angle of + 30 degrees to the canthomeatal line is of particular use in evaluation of the jugular foramina.28

Metastatic disease, multiple myeloma, and lymphomas are the most common malignant tumors affecting the calvaria, presenting chiefly as multiple lytic lesions.3 Kido et al29 found that CT scans using bone window settings are more sensitive than skull radiographs in detecting calvarial lesions.

Lytic metastases are more frequent than blastic or mixed, and usually erode either the inner or the outer table of the calvaria. Lesions of this type usually are sharply circumscribed and concave toward the eroded surface (figures 15 and 16).29 Blastic metastases typically are caused by carcinoma of the breast, prostate, colon, or bone. Medulloblastoma also has been reported to cause calvarial blastic metastasis.30 Metastasis to the endocranial epidural space is prevalent in patients with prostate cancer, and may be associated with a soft-tissue mass that simulates meningioma.3

When a calvarial metastatic deposit is detected, the presence of other skeletal deposits should be suspected.29 Leukemia and lymphoma deposits usually are detected as less well-defined multiple lesions that tend to coalesce and sometimes resemble a coarse version of normal diploic bone;3 a soft-tissue component may be associated.14 Multiple myeloma (figure 17) usually presents with multiple discrete, rounded holes of variable sizes, referred to as punched-out lesions.3 Dural based lymphoma and metastatic disease can cause erosion and destruction of the inner table and can be confused with other non-neoplastic dural lesions, such as those found in chronic infections and sarcoidosis.

Endocrine, metabolic, and idiopathic lesions

In hyperparathyroidism, bone window images on CT scans may show a granular or mottled appearance known as the "salt and pepper" appearance.3,31,32 Brown tumors show a low CT density similar to that of epidermoids and neurofibromatosis skull defects.14 An accentuated temporal line due to subligamentous bone resorption under the temporalis muscle has been described recently as a radiologic sign of hyperparathyroidism.33

In acromegaly, hypertrophic thickening of all the skull tables and expansion of the paranasal sinuses and sella turcica can be noted. Diffuse osteoporosis may be seen in Cushing's syndrome.3,34 In hypothyroidism (myxedema), retardation of cranial and facial development is manifested by an absence or underdevelopment of paranasal sinuses and poor pneumatization of the mastoid air cells. Additionally, there is a delay in closure of the sutures, with poorly differentiated inner and outer tables and poorly developed diploic space.3,34 In hypervitaminosis A, generalized osteoporosis with a thin, poorly mineralized skull is usually seen, associated with relatively dense suture margins and hydrocephalus.35

Generalized increased bone density and thickening with metastatic calcification in the falx, tentorium, and dura are seen in cases of hypervitaminosis D.35 Sclerosis and coarse trabeculation are associated with fluorine ingestion.36

In fibrous dysplasia, the radiologic appearance will depend on the proportion of fibrous tissue to bone (figure 18).31,37 Three categories of radiologic appearances of fibrous dysplasia have been described: pagetoid type, which involves expansion of the bone with areas of sclerosis and luciences (mixed type); sclerotic type; and cystic type, which may be complicated by spontaneous hemorrhage.31,38,39 The outer table has a thin and delicate appearance, with diffuse widening of the diploic space. Areas of poorly-defined dense nodules14 or the characteristic homogeneous ground-glass appearance also are commonly seen.3 Extensive sclerosis may affect the base of the skull, sphenoid bones, or temporal bones.14 Arterial grooves generally are enlarged, indicating hypervascularity of the tumor.3 The lesions may cause facial deformities, and may encroach upon the cranial nerves.14

Bone window images of a patient with Paget's disease can show a continuous spectrum of the disease, from the early active lytic phase (osteoporosis circumscripta) to the dense sclerosis of the healing phase (cotton-wool appearance) (figure 19). Sutures and vascular grooves do not restrict the progress of this disease.14 Softening of the skull base occurs with basilar invagination and compression on the basal foramina. Sarcomatous degeneration may occur and is characterized by bone destruction or adjacent bulky soft-tissue masses.40

Eosinophilic granuloma are usually well-defined lytic lesions that rarely have sclerotic margins. Two characteristic radiologic findings have been described in relation to eosinophilic granuloma:3 beveled edges due to greater involvement of the outer table than the inner table (figure 20) and button sequestrum41 due to a bone island or density within the lesion. However, these findings, besides being rare, also have been described with other lesions.3,42 Eosinophilic granuloma can heal spontaneously or with radiation therapy, especially in children.43

Conclusion

Judicious use of bone window settings in conjunction with routine brain CT scanning allows better characterization of associated bony lesions. The ability to define the origin of the lesion may be helpful in reaching the final diagnosis (table 4). In a study of 333 patients with calvarial translucencies, Thomas and Baker44 suggested some guidelines to reach a final diagnosis based on plain film findings (table 5). The following indications are suggested for routine use of bone window settings in brain CT examinations: 1) abnormal skull film; 2) suspected congenital abnormalities; 3) presence of an enhan-

cing lesion in relation to the skull bones; and 4) suspected metastatic disease. AR

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