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
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
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
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
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
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
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
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
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
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
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
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
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
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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