2.1 Imaging Findings of Various Calvarial Bone Lesions
CT and MRI have been incorporated into the basic imaging tools for evaluating calvarial lesions with their increasing frequency. Therefore, it is useful to categorize diagnostic features of images taken by CT and MRI and not the conventional radiography. It is possible to categorize different types of calvarial lesions based on four key features: thickening or thinning of calvarial bone, sclerosis or lysis of bone, focal or generalized lesions, and singularity or multiplicity of lesions. On CT and MRI, sclerotic lesions appear as thickening of calvarial bone (either the inner/outer tables, or bone marrow, or both). They can be further subdivided into focal (solitary) or diffuse lesions. The lytic category can be subdivided into either single (solitary) or multiple lesions. Furthermore, we can expect to get additional information about matrix characteristics with contrast enhancement and diverse advanced MR sequences such as diffusion-weighted imaging, perfusion-weighted imaging, and MR spectroscopy.
CT scan is considered to be the best examination to characterize bone alterations whereas MRI depicts bone marrow abnormalities and invasion of adjacent tissues.
2.1.1 Clinical features for evaluation of a calvarial lesion
Patient age, symptoms, clinical history, and laboratory findings are important clinical factors in making radiological diagnosis. The imaging appearance of calvarial lesions can indicate the lesion growth rate and sometimes suggest a specific diagnosis or limit the differential diagnosis. Systematic analysis of certain radiological features can be used for imaging evaluation of a calvarial lesion.
2.1.2 Radiological features for evaluation of a calvarial lesion
Size and number of lesions
The number of lesions, either single or multiple, can suggest a specific diagnosis. For example, in patients over 50 years of age, multiple osteolytic lesions with variable size are highly suggestive of multiple metastases.
Pattern of bone destruction and margins (transitional zone)
Benign tumors usually exhibit geographic/sharp bone destruction and a clear narrow transitional zone between normal and abnormal bone. Sclerotic margin may be present. Aggressive tumors often have poorly defined, moth-eaten, or permeated bone destruction and a wide zone of transition.
Periosteal reaction
A unilamellate uninterrupted pattern is usually associated with slow-growing lesions, and an interrupted periosteal reaction signifies an aggressive lesion.
Soft-tissue component and local extension
Malignant tumors often have a soft-tissue component that can extend to the scalp or to extra-axial spaces and/or the cerebral cortex.
Type of matrix and internal characteristics
The type of tumor matrix or other internal characteristics can suggest a specific diagnosis. Chondroid- and osteoid-producing tumors are usually easily detected by CT. Bone-forming tumors most often display amorphous or cloud-like mineralization, but the amount and degree of matrix mineralization are widely variable. Cartilage-forming tumors typically exhibit punctuate, comma-like, ring, or popcorn-like mineralization. Nonmineralized chondroid matrix, vascular tissue, fibrous matrix, and fatty, cystic, hemorrhagic contents within the lesions are more easily detected by MRI.
2.2 Solitary Radiolucent Skull Lesion without Sclerotic Margins in Adults
Foramina, canals, and unfused sutures
Vascular markings and emissary channels
Arachnoidal granulations (near midline or superior sagittal sinus)
Parietal thinning: Involves only the outer table of elderly individuals.
Sinus pericrania: Anomalous venous diploic channel between the extra cranial and intracranial venous system most commonly seen in the frontal bones; clinically it appears as a soft mass under the scalp that changes in size with alteration in the intracranial blood volume.
Congenital and developmental defects
Encephaloceles: Extracranial protrusions of brain and/or meninges through skull defects; occipital in 70% and frontal in 15%. Congenital midline masses include encephaloceles, nasal gliomas, and dermoid and epidermoid cysts. These lesions occur in the nasofrontal region, the occiput, or the cranial vertex. Transsphenoidal encephaloceles occur at the skull base and are not visible at clinical examination but may be seen as nasopharyngeal masses and are part of the spectrum of encephaloceles seen in children. (▶Fig. 2.1)
Dermoid cyst: Midline orbital in 80% lesion originating from e ctodermal inclusions. (▶Fig. 2.2)
Neurofibroma: May cause a lucent defect in the occipital bone usually adjacent to the left lambdoidal suture.
Intradiploic arachnoid cyst: Expansion of diploic space and thinning of the outer table.
Traumatic and iatrogenic defects
Fig. 2.1 (a–c) Chiari III (Chiari II malformation with an associated cervical-occipital encephalocele). Multiplanar T2W images demonstrate an intracranial Chiari II hindbrain malformation with a high cervical/occipital encephalocele. The arrows(a, c) indicate the site of the occipital calvarial defect resulting in herniation of disorganized cerebellar tissue. (Reproduced from Case 116. In: Tsiouris A, Sanelli P, Comunale J, ed. Case-Based Brain Imaging. 2nd edition. Thieme; 2013.)
Fig. 2.2 (a, b) Occipital dermoid cyst. Sagittal T2-weighted (a) and axial contrast-enhanced T1-weighted (b) surface coil MR images show a midline suboccipital cystic lesion (white arrow), with a sinus tract in the occipital bone (black arrow), and an infratorcular intracranial connection. The enhancement is due to infection of the cyst.
2.3 Solitary Radiolucent Skull Lesion without Sclerotic Margins in Children
Venous lakes and emissary channels
Arachnoidal granulations (near midline or superior sagittal sinus)
Leptomeningeal cyst or “growing fracture”: Under a skull fracture if the dura is torn, the arachnoid membrane can prolapse, and the cerebrospinal fluid (CSF) pulsations can cause over several weeks’ time a progressive widening and scalloping of the fracture line.
Congenital and developmental defects
Cranium bifidum, meningocele, encephalocele, dermal sinus
Epidermoid or dermoid cyst (▶Fig. 2.3): Midline orbital in 80% lesion originating from ectodermal inclusions.
Intradiploic arachnoid cyst: Expansion of diploic space and thinning of the outer table
Neurofibromatosis (▶Fig. 2.4)
Metastasis: commonly from a neuroblastoma and leukemia
Langerhans cell histiocytosis (▶Fig. 2.5)
i. Eosinophilic granuloma: A solitary lesion which causes only local pain. Only has sclerotic margins if it is in the healing process.
ii. Hand–Schüller–Christian disease: “Geographic” as well as multiple lytic lesions are common, associated with systemic symptoms such as exophthalmos, diabetes insipidus, chronic otitis media, “honeycomb lung.” There are solitary or multiple areas of bone destruction. The edges of the individual lesions are sharp, having a slightly cupped shape or irregular, but do not have a transitional zone sclerosis. Typically, the lesion originates in the diploe and involves one or both of the bone plates, causing a well-defined radiolucent defect and slightly irregular “punched-out” lesion with beveled edge, which is caused by asymptomatic destruction of the inner and outer cortices of the skull bone.
In the Hand–Schuller–Christian disease, the lesions can coalesce, become similar to a very large map, a finding referred to as a “geographic skull,” whereas eosinophilic granuloma lesions tend to be smaller, of the order of 1–2 cm.
Skull lesions can be asymptomatic but may manifest with focal pain and soft-tissue swelling in the scalp. At MRI, the soft-tissue component is hyperintense on T2-weighted images (T2WI) and isointense on T1-weighted images (T1WI), with enhancement after gadolinium-based contrast agent administration.
Sarcoma (i.e., Ewing’s brown tumor, osteosarcoma)
Fig. 2.3 (a–c) An epidermoid in 1-year-old infant. (a) Arrows point to a small oval defect in the parietal bone with a sharply defined sclerotic border. (b) An epidermoid in another child shows that these lesions may not be as well demarcated by sclerotic edges. (c) A computed tomography scan reveals the lesion and soft-tissue swelling.
Fig. 2.4 (a–d) Skull changes of neurofibromatosis (NF) type 1. A frontal view of the orbits reveals an elevated sphenoid wing (bilateral) and hypoplasia of the left sphenoid bone (a). A 5-year-old boy with NF has a defect in the left lambdoid suture seen on lateral (b) and oblique (c) views. A lateral view of another child with NF who has a large skull defect seen in the left lambdoid suture (d). (These images are provided courtesy of Peter Strouse, MD, Ann Arbor, MI.)
Fig. 2.5 Langerhans cell histiocytosis. (a) Axial bone algorithm computed tomographic image shows a lytic lesion within the right parietal bone, that has extended through the inner table of the skull, with a sharp “beveled” margin (red arrows) and with early extension through the outer table (red arrowhead). (b) Sagittal thick-section reformatted image from a computed tomographic scan, simulating a lateral radiograph of the skull, shows a lytic lesion with circumscribed margins within the right parietal bone (red arrowheads). (c) Coronal T1 W plus contrast image shows an enhancing soft tissue component causing the lytic lesion (red arrow), and reactive dural thickening in the region (red arrowhead). (Reproduced from Calvarial Defects. In: Choudhri A, ed. Pediatric Neuroradiology. Clinical Practice Essentials. 1st edition. Thieme; 2016.)
Fig. 2.6 Aneurysmal bone cyst. This 12-year-old girl presented with a nontender firm mass in the right temple. (a, b) Computed tomography shows calcification surrounding the lesion; bone windows show a thin shell of intact bone externally. (c, d) Axial and coronal T1-weighted images with contrast reveal an enhancing soft tissue portion of the tumor. (e) Axial T2-weighted image shows typical fluid–fluid levels of nonclotted blood. (Reproduced from Surgical Concepts. In: Albright A, Pollack I, Adelson P, ed. Principles and Practice of Pediatric Neurosurgery. 3rd edition. Thieme; 2014.)
Fig. 2.7 Intraosseous hemangioma. (a) Serial reformatted sagittal CT scans, showing a frontal bone hemangioma encroaching on the superior orbit. This may be mistaken for a benign fibro-osseous lesion such as fibrous dysplasia. (b) Coronal CT scan in another patient, showing a histologically proven hemangioma of the right frontal bone (arrows). (Reproduced from Mafee M. Pathology. In: Valvassori G, Mafee M, Becker M, ed. Imaging of the Head and Neck. 2nd edition. Stuttgart: Thieme; 2004.)
2.4 Solitary Radiolucent Skull Lesion with Sclerotic Margins
Epidermoid (▶Fig. 2.8): Arises from the diploic region and so it can expand both the inner and the outer tables. Most common location is the squamous portion of the occipital bone, less common are the frontal and temporal portions. It is the most common erosive lesion of the cranial vault.
Meningocele: Midline skull defect with a smooth sclerotic margin and an overlying soft-tissue mass. In 70% of the cases, it appears in the occipital bone; in 15%, occurs in the frontal and less commonly in the basal or parietal bones.
Langerhans cell histiocytosis (▶Fig. 2.9): Only has a sclerotic margin if it is in the healing process.
Hemangioma: Originates in the diploic area and rarely has a sclerotic margin.
Frontal sinus mucocele (secondary to chronic sinusitis)
Chronic osteomyelitis: Most commonly pyogenic but may be fungal, syphilitic, or tubercular. Reactive sclerosis dominates particularly with fungal infections, i.e., actinomycosis, with only a few lytic areas.
Fig. 2.8 (a) Axial DWI and (b) T2W FLAIR images reveal restricted diffusion in a heterogeneously hyperintense intradiploic calvarial lesion. (c) Sagittal postcontrast T1W image shows both inner and outer skull table involvement of this intradiploic epidermoid cyst. (Reproduced from Case 37. In: Tsiouris A, Sanelli P, Comunale J, ed. Case-Based Brain Imaging. 2nd edition. Thieme; 2013.)
Fig. 2.9 Eight-year-old girl with Langerhans cell histiocytosis. (a) Radiograph shows lesion without sclerotic solitary osteolytic border. (b) Axial CT image with bone window reveals small lesion with well-defined contours; inner and outer tables are unequally affected, giving lesion its characteristic beveled edges on CT.
2.5 Multiple Radiolucent Skull Lesions
Fissures, parietal foramina, and channels
Pacchionian depressions from arachnoidal granulations (near midline or superior sagittal sinus)
Hyperparathyroidism: The multiple punctate lytic changes in the cranium cause the so-called pepperpot appearance. The focal lucencies are made of fibrous tissue and giant cells known as brown tumors, indicated by the old term osteitis fibrosa cystica.
Renal osteodystrophy: Excessive excretion or loss of calcium due to kidney disease results in calcium mobilization and in a skull appearance identical to primary hyperthyroidism.
Osteoporosis: Loss of protein matrix results in lytic areas in the diploic and inner table of the skull in elderly and in patients with endocrine diseases such as Cushing’s disease.
Metastatic tumors: The most frequent neoplastic involvement of the skull is by hematogenous metastases from breast, lung, prostate, kidney, and thyroid or by invasion from adjacent primary neoplasms with osteolytic metastases such as the medulloblastoma.
Multiple myeloma: Produces small, discrete round holes of variable size, also referred as punched-out lesions. (▶Fig. 2.10)
Leukemia and lymphoma: Produce small, not well-defined, or separated multiple lesions which tend to coalesce.
Neuroblastoma: In infants, it is the most common metastatic tumor of the skull.
Radiation necrosis: Focal irradiation results in multiple small areas of bone destruction localized in the area treated.
Avascular necrosis: A few months following local ischemia from trauma destructive changes occur in the outer and diploic region of the cranium.
Hand–Schüller–Christian disease: Multiple large areas of bone destruction with irregular edges and without marginal sclerosis; the latter feature differentiates this form of histiocytosis X from eosinophilic granuloma which is believed to be the more benign form of the two.
Osteoporosis circumscripta: Represents the first stage of an idiopathic decalcification/ossification condition which results in areas of lucency sharply separated from normal bone. The second stage is characterized by an abnormal recalcification and ossification, suggesting an initial insult followed by disordered repair. The coexistence of these two stages of bone destruction and sclerosis characterize the pathological changes seen in Paget’s disease.
Fig. 2.10 (a) Multiple myeloma. Lateral x-ray demonstrates multiple lytic lesions. (Reproduced from XI. Differential Diagnosis by Location. In: Citow J, Macdonald R, Refai D, ed. Comprehensive Neurosurgery Board Review. 2nd edition. Thieme; 2009.) (b) Multiple osteolytic lesions from multiple myeloma are seen in the skull on axial CT. (c) The lesions show gadolinium contrast enhancement on axial fat-suppressed T1-weighted imaging. (Reproduced from Table 1.3 Multiple lesions involving the skull. In: Meyers S, ed. Differential Diagnosis in Neuroimaging: Head and Neck. 1st edition. Thieme; 2016.)
2.6 Localized Increased Density or Hyperostosis of the Skull Vault
Depressed skull fracture: Occurs due to overlapping bone fragments.
Cephalohematoma: Old calcified hematoma under elevated periosteum. It is commonly found in the parietal area; may be bilateral.
Paget’s disease: It involves all skull layers and is characterized with the appearance of both lytic, osteogenesis circuscripta, and sclerotic phases. (▶Fig. 2.11)
Fibrous dysplasia: Affects the craniofacial bones in approximately 20% cases and it may be monostotic or polyostotic and diffuse. It consists of abundant myofibromatous tissue woven with dysplastic, nonmaturing, or atypical bone. CT imaging shows thickened, sclerotic bone with a “ground glass” appearance (70–130 HU), with cystic components found in the early stages of the disease. On MRI, the expanded, thickened bone is typically of low to intermediate signal intensity on both T1WI and T2WI, although scattered hyperintensity areas may be present. Following gadolinium injection, a variable enhancement occurs. (▶Fig. 2.12)
Hyperostosis frontalis interna: This idiopathic condition refers to the thickening of the inner table. It is commonly found in the frontal bone of sexually active females indicating a true endocrine relationship.
Osteoblastic metastases: Metastatic prostatic carcinoma is most frequently osteoblastic and is the most common cause of osteoblastic metastasis in the males. Medulloblastoma is a rare example of blastic metastasis.
Osteoid osteoma: When arising from the dura, it stimulates a calvarial lesion. In order to be disclosed, the neurosurgeon needs to open the dura.
ii. Malignant skull tumors: Chondrosarcoma, osteosarcoma, fibrosarcoma, and angiosarcoma.
The distinct radiological aspects of conventional osteosarcoma are bone marrow lesions, cortical bone destruction, an aggressive periosteal reaction, a soft-tissue mass, and a tumor matrix in the destructive lesion, as well as within the soft-tissue mass. Although the tumors can present as purely sclerotic or purely osteolytic, most are a combination of the two. The borders are generally indistinct, with a broad zone of transition. The bone destruction is infiltrative, with a “moth-eaten” appearance, and only rarely geographic. The most common forms of periosteal reaction seen in osteosarcomas are the speculated (sunburst) type and Codman’s triangle with the laminated (onion-skin) type being less common.
Meningioma: Focal hyperostosis and enlargement of meningeal arterial grooves are the classic findings in a plain skull x-ray.
Fig. 2.11 Skull base lesions. (a) Fibrous dysplasia. Axial proton density MRI with thickening of the right sphenoid bone and reduction of the size of the orbit and associated exophthalmos. (b) Meningioma of the right cavernous sinus. Coronal T1 WI shows expansion of the right cavernous sinus and a very high signal intensity following contrast enhancement. (c) Metastasis. Axial CT demonstrating an osteolytic lesion of the sphenoid tip of the petrous bone. (d) Chordoma. Axial CT with a high-density space-occupying lesion of the left temporal fossa and the parasellar region. The mass is eroding the apex of the petrous bone and is extending to the cerebellopontine angle of the same side. (e) Paraganglioma or glomus jugulare. Axial CT shows a space-occupying lesion of the right CP angle that occupies the right jugular foramen and demonstrates intense, heterogeneous postcontrast enhancement. (f) Paget′s disease. Axial CT shows a marked thickening of all bones of the skull base with reduction of the size of the posterior fossa.
2.7 Diseases Affecting the Temporal Bone
2.7.1 Destructive (lucencies with irregular margins)
Inflammatory: Acute petrositis is a nondestructive inflammatory condition (rare complication of otomastoiditis) affecting only 30–50% of the cases with the aerated petrous apex and is characterized by irregular spotty opacifications scattered through the petrous pyramid. Spread of the inflammation may lead to osteomyelitis and abscess formation in the petrous pyramid. The involvement of the surrounding tissues causes the irritation of cranial nerve Vth with periorbital pain, VIth nerve palsy causing diplopia and ipsilateral ottorhea which is referred to as Gradenigo’s syndrome, although it may not be present in every patient. The facial pain is due to focal meningitis over the petrous apex with irritation of the Gasserian ganglion in the Meckel cavity. Abducens nerve involvement occurs at its course through the Dorello canal. Imaging studies demonstrate erosive changes of the petrous apex with abnormal enhancement of the adjacent meninges. Differential diagnosis should be performed with neoplastic disease (rhabdomyosarcoma, metastasis) and epidermoid tumors. On MRI, it presents typically of low signal intensity on T1WI and high intensity on T2WI. In chronic petrositis, the high content of protein and viscosity causes high signal on T1WI and/or lower signal intensity on T2WI. (▶Fig. 2.13)
i. Nasopharyngeal carcinoma: Usually large area of destruction in the floor of the middle cranial fossa are also seen.
ii. Metastatic tumors: Any site of the petrous pyramid, particularly lung, breast, and kidney carcinoma (▶Fig. 2.14)
iv. Chordoma: Arises from a notochordal remnant, usually in the midline at the spheno-occipital synchondrosis. The origin is 35% from the clivus, 50% sacrococcygeal, and 15% spinal. The presence of dense retrosellar calcification with bone destruction of the clivus, dorsum sellae, and petrous bones is characteristic of clivus chordoma. Frequent tumor calcification shows lytic destruction of bone, and has mild enhancement. On T1WI, the lesions are usually isointense (75%) or hypointense (25%) but nearly all are hyperintense on T2WI.
The classic appearance of intracranial chordoma at high-resolution CT is that of a centrally located, well-circumscribed, expansile soft-tissue mass that arises from the clivus with associated extensive lytic bone destruction. The bulk of the tumor is usually hyperattenuating relative to the adjacent neural axis. Intratumoral calcifications appear irregular at CT and are usually thought to represent sequestra from bone destruction rather than dystrophic calcifications in the tumor itself. (▶Fig. 2.15)
MRI is the single best modality for radiologic evaluation of intracranial chordomas. It is similar to CT in detecting intracranial chordomas. However, it is considerably superior to CT in the delineation of lesion extent because it provides excellent tissue contrast and exquisite anatomic detail. The multiplanar capability of MRI is also helpful in this regard. Sagittal images are generally most valuable in defining the posterior margin of the tumor, showing the relation between the tumor and brainstem, and depicting nasopharyngeal extension of the tumor. Sagittal imaging is also useful in disclosing transdural transgression by a tumor, an important factor in surgical planning. Coronal images, on the other hand, are helpful in detecting tumor extension into the cavernous sinus and depicting the position of the optic chiasm and tract.
Angiographic evaluation of intracranial chordomas is nonspecific. Abnormal tumor vascularity or staining is rare. Angiographic evaluation is reserved for cases in which there is significant displacement, encasement, or narrowing of the internal carotid or vertebral artery at MR angiography. Cerebral angiography can better demonstrate the degree of luminal narrowing or occlusion and the extent of collateral circulation.
i. Glomus jugulare or ganglioglioma or chemodectoma: Arise from chemoreceptor organs on the promontory in the jugular fossa in the superior portion of the jugular bulb. Usually, these tumors spread superiorly and laterally through the inferior surface of the petrous pyramids. At this stage, they show an irregular enlargement of the jugular foramen and an irregular destruction of the inferior aspect of the petrous pyramid. As the tumor grows, it causes further destruction involving the ossicular system, the internal jugular vein, the posterior margin of the carotid canal, and the posterior fossa. On CT scan, it seems this mass erodes the jugular foramen of the temporal bone. The mass may grow inferiorly into the jugular vein or may grow from the jugular bulb region into the sigmoid and transverse sinuses or the vein. Mass within the vessel plexus may be distinguished from thrombosis by the presence of enhancement in the former. On MRI, the glomus jugulare has a typical “salt and pepper” appearance. Characteristically, they reveal undulating channel-like voids, especially on T2WI. Following gadolinium injection, there is moderate enhancement. Angiography was needed in the definitive diagnosis of these lesions previously but now the location of the lesion at or extending into the jugular bulb plus the vascularity and the “salt and pepper” appearance of MR images makes this an easy diagnosis.
i. Acute or chronic bacterial: There are four main mechanisms of extension of the infection in acute mastoiditis: preformed pathways, osseous erosion, thrombophlebitis, and hematogenous seeding. When mastoid region inflammation cannot be arrested, the suppuration under pressure causes local acidosis and osseous decalcification, ischemia, and osteoclastic dissolution of the pneumatic cell walls. The pneumatic cells can coalesce into larger cavities filled with purulent exudates and granulations, resulting in empyema and the stage of coalescent mastoiditis. This osteoclastic osseous resorption proceeds in all directions, and intratemporal or intracranial complications threaten to occur before spontaneous resolution. Spread of the inflammatory debris anteriorly to the middle ear via the aditus ad antrum can result in spontaneous resolution if the tympanic membrane was previously perforated. The infection may also spread laterally and produce a subperiosteal abscess or spread medially to the petrous air cells, causing petrositis. Coalescent mastoiditis is diagnosed when temporal bone CT scan demonstrates erosion of the mastoid septa or mastoid walls. This complication can follow a more acute and aggressive course (coalescent acute mastoiditis) or a more subclinical progression (latent or “masked” mastoiditis). (▶Fig. 2.16) Chronic mastoiditis was commonly associated with benign intracranial hypertension due to the contiguous extension of the inflammation to the neighboring sigmoid and lateral sinuses.
ii. Tuberculosis: Very rare; causes bone destruction without sclerosis.
i. Squamous-cell carcinoma: The most common malignant tumor of the middle ear. Irregularly marginated lucent defects, without any evidence of sclerosis.
ii. Adenocarcinoma: Less common than squamous-cell carcinomas.
i. Glomus hypotympanicum tumor—Chemodectoma: Most common benign tumor of the middle ear. Arises from the receptor organs on the promontory in the hypotympanum. These are locally invasive and extremely vascular tumors.
i. Langerhans granulomatosis: This disease has a propensity for the mastoid portion of the temporal bone of children and young adults. It presents as a lytic process clinically resulting in loss of hearing without pain or tenderness. The patients are afebrile and otherwise healthy children. The lesion is intense on T1WI and hyperintense and enhanced on T2WI.
Fig. 2.13 Petrous apicitis. (a) Axial and (b) coronal computed tomography images demonstrate diffuse opacification localized to the petrous apex with loss of septations (arrows), indicating coalescent disease. (c) Axial precontrast T1-weighted magnetic resonance image (T1WI) reveals petrous apex disease (arrow). (d) Axial post-contrast T1WI reveals intensely enhancing debris. Leptomeningeal disease is manifest most notably by enhancement of the facial and superior vestibular nerves within the internal auditory canal (arrow). (Reproduced from Pathology and Treatment. In: Swartz J, Loevner L, ed. Imaging of the Temporal Bone. 4th edition. Thieme; 2008.)
Fig. 2.14 A 68-year-old woman with metastatic breast carcinoma who has multiple lesions in skull marrow associated with bone destruction and extraosseous tumor extension. (a) The tumors have intermediate to slightly high signal on axial T2-weighted imaging and (b) show gadolinium contrast enhancement on axial T1-weighted imaging. Thickened contrast enhancing dura is also seen from neoplastic invasion. (Reproduced from Table 1.3 Multiple lesions involving the skull. In: Meyers S, ed. Differential Diagnosis in Neuroimaging: Head and Neck. 1st edition. Thieme; 2016.)
Fig. 2.15 T2 axial MRI of a C3 Di3 jugular paraganglioma that has caused significant compression of the brainstem with associated edema. The patient required a ventriculoperitoneal shunt. (Reproduced from 39.1 Introduction. In: Sekhar L, Fessler R, ed. Atlas of Neurosurgical Techniques: Brain, Volume 2. 2nd edition. Thieme; 2015.)
Fig. 2.16 Chronic otitis media in a 12-year-old female. Axial CT shows opacification of the right middle ear and mastoid air cells. The mastoid is small, with zones of lysis of mastoid bone septa related to an intramastoid empyema (referred to as coalescent mastoiditis) (arrow). (Reproduced from Table 1.8 Lesions involving the middle ear. In: Meyers S, ed. Differential Diagnosis in Neuro imaging: Head and Neck. 1st edition. Thieme; 2016.)
2.7.2 Erosive (lucencies with well-defined margins, with or without sclerosis)
Petrous pyramid or apex (▶Fig. 2.17)
Bone neoplasm, benign, or malignant: Such as hemangioma, osteoblastoma, chordoma, chondroma, metastasis.
Epidermoid: In the cerebellopontine angle (CPA) cistern.
Aneurysm of the intracavernous or intrapetrous internal carotid artery
Neurinoma of V, IX, or X nerve
Acoustic neurinoma: It represents 8% of all intracranial tumors. It arises from the Schwann cells which invest the eighth nerve as it enters the internal auditory canal (IAC). Ninety-five percent (95%) of these tumors originate within the auditory canal and the other 5% arise from the nerve at its CPA course, proximal to the canal. Often bilateral in neurofibromatosis. Most acoustic neuromas arise from the superior vestibular branch of the eighth cranial nerve. As vestibular schwannomas enlarge, they may expand medially into the CPA and laterally toward the fundus and/or into the cochlear aperture. The most noticeable X-ray change caused by these tumors is erosion of the superior and posterior lips of the porus acusticus. On CT images, most vestibular schwannomas are isoattenuating with the cerebellum and are difficult to delineate without contrast material enhancement. However, if the tumor is large and causes expansion of the porus acousticus, this may be readily seen on CT bone window images, with the porus acousticus asymmetrically wider on the affected side. Calcification and hemorrhage are rare unless the tumor has not been treated. (▶Fig. 2.18)
On MR images, schwannomas are usually isointense to mildly hypointense to brain parenchyma and hyperintense to CSF on T1WI, whereas on T2WI, these are mildly hyperintense to brain parenchyma and isointense to hypointense to CSF and enhance avidly. Large tumors may be heterogeneous, with intra- or extramural cystic components, and may deform and displace the brainstem, causing parenchymal edema, and compress the fourth ventricle. Heavily T2-weighted sequences are helpful for outlining the tumor, as most structures except CSF appear quite dark. CSF, thus, provides natural contrast around the dark tumor mass. Heavily T2-weighted sequences may also reveal decreased signal intensity of the labyrinthine fluid ipsilateral to the tumor, thought to be related to higher protein content in the fluid; this may also be seen as increased signal intensity on fluid-attenuated inversion recovery images. Enhancement is always evident and homogeneous in approximately 70% of patients. Peritumoral edema may be seen in 30–35% of cases with larger lesions and less frequent calcification, cystic change, and hemorrhage. (▶Fig. 2.19 and ▶Fig. 2.20)
Facial nerve neuroma: Very rare tumors but may cause similar roentgenographic changes to that of an acoustic neuroma.
Meningioma of the Gasserian cavity: Meningiomas of the auditory canal may cause erosion of the canal and usually extend to involve the posterior surface of the petrous apex. In contrast to vestibular schwannomas, meningiomas are often eccentric to the porus acousticus, centered at the CPA; when they do extend into the IAC, they seldom expand the porus or the IAC. Meningiomas may extend into the middle cranial fossa by means of herniation, growth through the tentorium, or growth through the temporal bone. They may also extend into the middle ear and cavernous sinus. As mentioned above, meningiomas tend to be broad-based along the tposterior petrous wall, forming an obtuse angle at the bone–tumor interface, appearing either hemispherical or plaquelike. Dural enhancement extending outward from the margins of the tumor is often seen. (▶Fig. 2.21)
i. Aneurysm of the intracavernous or intrapetrous carotid artery
ii. Arteriovenous malformation or occlusive disease of the anterior inferior cerebellar artery may cause erosion of the IAC causing it to have a funnel-shaped appearance.
iii. Aneurysm at the origin of the internal auditory artery may cause erosion of the canal.
i. Acute or chronic bacterial: Chronic mastoiditis was commonly associated with benign intracranial hypertension due to the contiguous extension of the inflammation to the neighboring sigmoid and lateral sinuses.
ii. Tuberculosis: Very rare; causes bone destruction without sclerosis.
Trauma (postoperative changes)
Cholesteatoma: Primary cholesteatomas are developmental in origin and less common than the secondary ones that result from inflammatory ear disease; although their x-ray findings are identical. The earliest x-ray sign is partial to complete destruction of the bony ridge or drum spur of the innermost portion of the roof of the external auditory canal in 80% of the cases. More than 95% of cholesteatomas are visible on otoscopic examination. The mastoid antrum is enlarged and may often be sclerotic due to the associated chronic infection. A soft-tissue mass within the tympanic cavity with destruction or demineralization of the ossicular chain may also be seen. These latter x-ray changes may also be seen after involvement of the tympanic cavity by granulation tissue due to chronic inflammation, in which case are indistinguishable roentgenographically. On CT, cholesteatomas appear as noninvasive, erosive, and well-circumscribed lesions in the temporal bone with scalloped margins. On MR imaging, they usually hypointense on T1WI and hyperintense on T2WI. (▶Fig. 2.22)
i. Metastases: Hematogenous from breast, lung, prostate, kidney, other primary neoplasms with osteolytic metastases
ii. Carcinoma of the middle ear: It is associated with chronic otitis media in 30% of the cases; pain and bleeding appear late. Bone destruction is seen in 12% cases, particularly in the temporal fossa of the temporomandibular joint.
iii. Glomus jugulare tumor: The jugular foramen is enlarged and destroyed. It is a very vascular lesion.
iv. Nasopharyngeal tumor invasion.
v. Rhabdomyosarcoma: This is a tumor of children and young adults with a predilection for the nasopharynx. It may be very vascular and may displace the posterior antral wall forward, thus stimulating angiofibroma. Imaging studies show a bulky soft-tissue mass with areas of bone destruction. Signal intensity is similar to muscle on T1WI but becomes hyperintense on T2WI. Some contrast enhancement is usual.
Cholesterol granuloma: In the setting of eustachian tube dysfunction, there may be build-up of negative pressure or vacuum phenomenon in the middle ear cavity, leading to mucosal edema and rupture of blood vessels. The breakdown of erythrocytes and tissue elements release cholesterol, which incites a foreign body giant cell reaction leading to formation of a chronic granuloma termed cholesterol granuloma. This lesion is also referred to as a cholesterol cyst, chocolate cyst, or blue-domed cyst. Common locations for this entity include the middle ear and the petrous apex; it can also rarely occur in a mastoidectomy cavity. In the middle ear, patients will present with a blue tympanic membrane, hemotympanum, or conductive hearing loss.
In the petrous apex, an expansile lesion with imperceptible bone margins may be seen on CT scans. On MR images, the characteristic finding is the presence of intrinsic T1 shortening (hyperintensity) due to the presence of blood products. On T2WI, signal intensity is usually heterogeneously hyperintense. A hypointense rim on T2WI may be present, which is believed to represent hemosiderin or a preserved rim of bone. Cholesterol granulomas do not enhance. An important differential diagnosis for this entity is entrapped simple fluid or a petrous apex effusion. However, although an effusion may mimic a cholesterol granuloma by way of MR signal characteristics, it does not cause expansion or destruction of the petrous apex air cells, a distinction that is best evaluated with CT.
Tuberculosis: Rare and may be present without evidence of TB elsewhere. Lytic lesions with no sclerotic margins.
Fig. 2.17 Chondrosarcoma. (a) Axial T2-weighted magnetic resonance image (MRI) shows an irregular mass (arrow) that is hyperintense but located laterally at the petroclival synchondrosis and invading the right petrous apex. (b) The mass is minimally hypointense compared with brain on sagittal T1-weighted MRI, with a focal area of T1 hyperintensity (curved arrow) that may represent hemorrhage or calcification. (c) Axial gadolinium-enhanced T1-weighted image shows uniform intense contrast enhancement. (Reproduced from Pathology. In: Swartz J, Loevner L, ed. Imaging of the Temporal Bone. 4th edition. Thieme; 2008.)
Fig. 2.18 Axial contrast-enhanced T1-weighted MR image of vestibular schwannoma. This middle-aged man presented with sudden asymmetric sensorineural hearing loss on the right. Image demonstrates an avidly enhancing tumor in the internal auditory canal extending through an expanded porus acousticus (between arrows) into the cerebellopontine angle, where it forms an acute angle with the posterior petrous ridge.
Fig. 2.19 Facial schwannoma on a heavily T2-weighted sequence (T2 DRIVE) (a) and corresponding postgadolinium T1 section (b). The enhancing “ice-cream cone”–like cisternal and IAC component of the mass is similar to a vestibular schwannoma. The distinguishing feature is the involvement of the labyrinthine segment of the facial nerve (thin arrow) and the geniculate ganglion (thick arrow). (Reproduced from Role of Neuroimaging in Skull Base Surgery. In: Di Ieva A, Lee J, Cusimano M, ed. Handbook of Skull Base Surgery. 1st edition. Thieme; 2016.)
Fig. 2.20 Acoustic schwannoma (large with cystic change). (a) Axial T1 C+ fat sat. Left cerebellopontine angle extra-axial mass with cystic component. It shows typical extension through the internal auditory meatus and displays avid enhancement in post contrast study. (b) Axial T2 of facial nerve neuroma. Very rare tumors but may cause similar roentgenographic changes to that of an acoustic neuroma.
Fig. 2.21 Axial contrast-enhanced T1-weighted MR images of meningioma. (a) Lesion having a broad-based component against the posterior petrous surface (arrowhead) and an en plaque component extending into the internal auditory canal (IAC) and along the posterior surface of the mastoid (arrows). The IAC is not expanded. There is extension to the middle cranial fossa (*). (b) Extension into the middle cranial fossa is well seen in this image from the same sequence at a more cranial level (*), along with invasion of the Meckel cave (arrow) and cavernous sinus (white arrowhead). Tumor surrounds the internal carotid artery, causing mild narrowing. Note the obtuse angles between the tumor and the bone surfaces (black arrowhead).
Fig. 2.22 A 76-year-old woman with an epidermoid/cholesteatoma originating in the middle ear and extending into bone at the upper medial external auditory canal (EAC), resulting in an automastoidectomy, which caused extrusion of eroded ossicles out of the external auditory canal. (a) Coronal and (b) axial CT images show absence of the ossicular chain in the middle ear, residual cholesteatoma deep to the tympanic membrane, and bony erosion at the epitympanum and upper medial wall of the right EAC. (Reproduced from Table 1.7 Acquired lesions involving the external auditory canal (EAC). In: Meyers S, ed. Differential Diagnosis in Neuroimaging: Head and Neck. 1st edition. Thieme; 2016.) (c) Cholesteatoma, congenital, petrous pyramid. The cholesteatoma appears in the T2W image as a high signal intensity mass involving the anterior aspect of the petrous pyramid and middle ear cavity. In the sagittal spin-density-weighted sections, the signal intensity is less than in T2W. (Reproduced from Valvassori G. Cholesteatoma of the Middle Ear. In: Valvassori G, Mafee M, Becker M, ed. Imaging of the Head and Neck. 2nd edition. Stuttgart: Thieme; 2004.)
2.8 Abnormalities of the Craniovertebral Junction
These abnormalities may involve either the bones and joints, or the meninges and the nervous system, or all of the above.
2.8.1 Congenital anomalies and malformations
Malformations of the occipital bone
Manifestations of occipital vertebrae: These are ridges and outgrowths around the bony margins of the foramen magnum. Even though the bony anomaly occurs extracranially at the anterior margin, it is often associated with an abnormal angulation of the craniovertebral junction resulting in a ventral compression of the cervicomedullary junction. This particular anomaly is frequently associated with primary syringomyelia and Chiari malformation.
Basilar invagination: The term basilar invagination applies to the primary form of invagination of the margins of the foramen magnum upward into the skull. The roentgenographic diagnosis is based on pathologic features seen on plain x-rays, CT, and MRI. Basilar invagination is often associated with anomalies of the notochord of the cervical spine, i.e., atlanto-occipital fusion, stenosis of foramen magnum, Klippel–Feil syndrome, and maldevelopments of the syringomyelia. The term basilar impression applies to the secondary acquired form of basilar invagination that is due to softening of the bone secondary to diseases such as Paget’s disease, osteomalacia, hyperparathyroidism, osteogenesis imperfecta, renal rickets, and achondroplasia.
The term platybasia applies to a condition in which the basal angle formed by joining the planes of the clivus and of the anterior cranial fossa is greater than 140°. By itself it causes no symptoms or sings but if associated with basilar invagination then obstructive hydrocephalus may occur. (▶Fig. 2.23)
Condylar hypoplasia: The elevated position of the atlas and axis may lead to vertebral artery compression, compensatory scoliotic changes, and lateral medullary compression.
Assimilation or occipitalization of atlas: Occurs in 0.25% of the population and in only 1/4 or 1/3 of the affected population does it cause neurologic symptoms and signs.
Atlanto-axial fusion: Very rare except when associated with Klippel–Feil syndrome.
Irregular atlanto-axial segmentation
i. Ossiculum terminale: Results from the persistence of the summit ossification center; seldom appears before the age of 5 years.
ii. Os odontoideum: Results from the nonfusion of the epiphyseal plate and separation of the deformed odontoid process from the axial centrum. Increased incidence in patients with Down’s syndrome, spondyloepiphyseal dysplasia, and Morquio syndrome. (▶Fig. 2.24)
