Adult Infratentorial By Larissa
T. Bilaniuk
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NFRATENTORIAL tumors in adults are less frequent and have a different distribution than tumors in supratentorial locations. In contradistinction to the supratentorial tumors, where the intraaxial tumors are more frequent, the extraaxial tumors represent the most common group of tumors that occur in the posterior fossa of adults. Most of the tumors are slow growing neoplasms; thus, they may manifest themselves insidiously. It becomes important to detect these tumors as early as possible because surgical resection is easier and there is less morbidity. Also, there is a greater chance to preserve function. The smaller volume of the posterior fossa as compared to the supratentorial space and the presence of vital structures contribute to a critical situation if there is rapid increase in the volume of a tumor, such as may occur with hemorrhage. Emergency surgical decompression may be life saving and, therefore, it is important to recognize these lesions and delineate them. The differentiation of intraaxial from extraaxial tumors is important because it not only aids in correct prediction of histology, but influences surgical approach and planning. Identification and characterization of posterior fossa lesions has presented a particular challenge to the neuroradiologist. Although CT has made a tremendous impact on the evaluation of intracranial lesions, it has had several drawbacks as far as the posterior fossa is concerned. The major one has been the presence of beam-hardening artifacts that can obscure the detail of the posterior-fossa structures. Thus, magnetic resonance imaging (MRI), with its multiplanar capability, superb soft tissue discrimination, and lack of major artifacts, has become the procedure of choice for the evaluation of the posterior fossa.“’ Availability of MR contrast agents has further expanded the use of MR. At present, CT plays a complementary role in cases where there is a question of bony changes or calcification. Angiography is used when detailed vascular anatomy is required, such as in cases of hemangioblastomas, paragangliomas, or some meningiomas.
Seminars in Roentgenology, Vol XXV, No 2 (April), i990:
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Tumors
INTRAAXIAL
TUMORS
Metastases
Metastatic lesion is the most common posterior fossa intraaxial tumor in the adult. The source is usually a tumor of the lung, breast, colon and kidney, and melanoma.3 The cerebellum is more often involved than the brainstem. Overall the supratentorial space is a more common site for metastases.4 However, there are some specific exceptions. One study, dealing with distribution of brain metastases as shown by CT in 288 patients, reports that when there is a solitary metastasis (49% in that series) and when the primary tumor is pelvic (prostate or uterus) or gastrointestinal then the posterior fossa is involved in 50% of cases, but only 10% if the source is from other tumors, such as lung and breast carcinomas.5 Gadolinium (Gd)-DTPA enhanced MR is the most sensitive technique for the evaluation of posterior fossa for metastases. Small metastases located in the inferior portion of the cerebellum or brainstem, that may be missed by contrastenhanced CT due to the beam-hardening artifacts, are well shown by MR. Therefore, as information from larger series of patients examined using contrast-enhanced MR is accumulated, more accurate statistics will become available on the distribution and number of metastases. On MR and CT, most metastases contrast enhance, are surrounded by edema, and produce mass effect (Fig 1). Because of the tight fitting of structures in the posterior fossa and the smaller volume of brain relative to the metastatic lesion, peritumoral edema may be difficult to detect and delineate, particularly using CT. Metastatic lesions generally are hypodense on plain CT and hypointense on Tl-weighted MR images. Excep-
From the Department of Radiology. Hospital of the University of Pennsylvania, Philadelphia, PA. Address reprint requests to Larissa T. Bilaniuk, MD, Department of Radiology, Hospital of the University of Pennsylvania, 3400 Spruce St, Philadelphia. PA 19104. 0 1990 by W.B. Saunders Company. 0037-198X/90/2502-0004$5.00/0
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and some gastrointestinal mucin producing tumors). On proton density and TZ-weighted images a metastasis may be difficult to delineate from edema, because either its intensity is similar to edema or it is isointense to the normal brain and, therefore, is obscured by the edema. Some metastases are not associated with edema and may be difficult to detect due to the similarity of their intensity to that of the brain (Fig 2A). Thus, contrast-enhanced MR is necessary for the identification and delineation of metastatic lesions (Fig 2). Some metastatic lesions that are not associated with edema may mimic ischemic lesions. On the other hand, a single, large metastasis may be difficult to differentiate from other primary posterior fossa tumors or an abscess. Some metastases, particularly in the posterior fossa, may mimic cysts. Correlation with the clinical picture and idemification of metastases at other sites are helpful in making the diagnosis of posterior fossa metastasis. In some cases the metastases may be both intraaxial and extraaxial (Fig 2).
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lung carcinoma. (A) Contrast-enFig 1. Metsstatic hanced CT shows a peripherally enhancing mass with central necrosis. There is prominent edema around the mass. The fourth ventricle is compressed and displaced slightly to the right. (6) Axial MR scan at 2500/70 (TR/TE). The mass is of more homogeneous and slightly greater intensitv than the surrounding edema.
tions to this are melanotic melanomas,6 hemorrhagic tumors7 (such as melanoma, choriocarcinoma, lung carcinoma, and renal carcinoma), and calcified lesions (such as osteogenic sarcoma
Hemangioblastomas Hemangioblastoma, a benign tumor of vascular origin, is considered to be the most common primary intraaxial tumor in the posterior fossa in adults. It accounts for 1.1% to 2.4% of all intracranial tumors and 7.3% of all primary posterior fossa tumors.’ The tumors most often occur in young to middle-aged adults and show male preponderance. Hemangioblastoma is most commonly located in the cerebellum and usually lies at the periphery of the brain near the pial surface (Figs 3 through 5).’ Most typically, the lesion consists of a cyst with a vascular mural nodule. However, the tumors may also be completely solid or may be solid with a central cyst. When the tumor is multiple, it is frequently associated with Hippel-Lindau disease. In cases of hemangioblastomas, MR is superior to CT, not only because it can detect and define smaller posterior fossa lesions,” but it also characterizes them better by showing the prominent tumor vasculature (Figs 3B and 4), tumor hemorrhage, and also demonstrates associated spinal lesions. On CT, the solid portion of hemangioblastomas is isodense to the brain and shows marked enhancement (Fig 3A). The cyst content is isodense or slightly increased in density to the
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Fig 2. Metastatic adenocarcinoma both with intraaxial and extraaxial lesions. (A) Axial MR scan (3WD/sO). Low-intensity mass (arrowhead) expands the left cavernous sinus and projects posteriorly into the prepontine cistern. (BJ Axial contrast-enhanced MR scan fBClD/2Oj. There is enhancement of the mass and encasement of the carotid artery, which is narrower than the normal right intracavernous carotid artery. (Cj Axial MR scan (3ooO/BDj. Low intensity metastasis (arrowheads) is identified by its mess effect, but could be overlooked because it has similar intensity to that of the cerebellum. (D) Contrast-enhanced coronal MR scan. Multiple contrast enhancing nodules (arrows) are shown on a coronal MR scan.
cerebrospinal fluid (CSF). The rim of the cyst, which does not contain tumor and is composed of fibrillary neuroglia,8 may show slight contrast enhancement. On Tl-weighted MR images, the solid tumor or nodules are of similar or slightly lower intensity than the gray matter and show increase in intensity on proton density (Fig 3B) and on T2-weighted sequences. Curvilinear or punctate regions of signal void that represent enlarged tumor vessels are demonstrated within and on periphery of the tumor (Figs 3B and 4).
The vascularity can also be demonstrated using MR angiography (compressed images from threedimensional acquired gradient-echo scans). However, the best anatomic detail of vasculature is still provided by conventional angiography. Just as on CT (Fig 3A), the solid components of the tumor enhance on MR with Gd-DTPA (Fig 4B). The cysts on MR images are well defined and are hypointense on Tl weighting and hyperintense on proton density and T2-weighted images (Fig 5). This is most likely related to increased protein
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should be achieved with high resolution multiplanar Gd-enhanced MR images. Other Primary Posterior Fossa Tumors Primary posterior fossa tumors, such as cerebellar astrocytoma, ependymoma, and primitive neuroectodermal tumor that are common in the pediatric age group, occur infrequently in adults, but when they occur it is in the young adult. Brainstem gliomas represent approximately 2.5% of adult gliomas.” EXTRAAXIAL
TUMORS
Acoustic Schwannomas
Solid hemangioblastoma. (A) Contrast-enhanced Fig 3. scan shows markedly enhancing tumor on the enterosuperior aspect of the left cerebellar hemisphere. The tumor compresses posterior aspect of the midbrain. There is hydrocephalus. (B) Proton density axial MR scan (2500/30). The mass shows prominent hyperintensity and contains large vessels within it as well as on the periphery.
content of the fluid within the cysts. Some cysts may contain blood products.’ In some cases the mural nodule may be so small that it is difficult to demonstrate. Detection of such small lesions
Acoustic schwannoma is the most common tumor of the cerebellopontine angle region. It represents about 80% of the cerebellopontine angle tumors and approximately 10% of all intracranial neoplasms. It is bilateral in about 5% of cases and this is generally associated with neurofibromatosis type 2.i2 Since the original description of the acoustic tumor, numerous names, more than 25 (among them neuroma, neurinoma, neurilemmoma, neurofibroma), have been assigned to it.13 This confusion in nomenclature arose from a controversy concerning the origin of the tumor. The advent of electron microscopy has resolved most of the disagreements. At present the most widely accepted view is that the acoustic tumor originates in the Schwann cell and, therefore, the name schwannoma is most appropriate.* The axons of peripheral nerves are surrounded by Schwann cells, endoneural fibroblasts, and perineural cells. Proliferation of Schwann cells gives rise to schwannomas, whereas neurofibromas consist of Schwann cells and endoneural fibroblasts. Neurofibromas contain more connective tissue fibers and more nerve fibers than schwannomas.’ Another distinguishing feature between the two tumors is that schwannomas are encapsulated and neurofibromas lack a capsule. Even when associated with neurofibromatosis, the acoustic tumors are schwannomas. The eighth cranial nerve, which consists of an auditory and a vestibular component, courses from the cochlea and vestibular apparatus, through the internal auditory canal and the subarachnoid space of the cerebellopontine angle to enter the uppermost medulla. Throughout its course it is accompanied by the seventh cranial nerve, the facial nerve. The eighth cranial nerve
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Fig 4. Small solid hemangioblastoma. (Aj A sagittal MR scan (SOO/2Oj shows a small, slightly hypointense mass on the periphery of the cerebellar hemisphere. The mass has a prominent vessel on its periphery (arrow) as well as within it. (B) There is marked enhancement of the tumor following intravenous injection of Gd-DTPA.
Fig 6. Cystic hemengioblastoma. An axial MR scan (25iOO/BOj shows a well-defined cystic lesion located on the periphery of the left cerebellar hemisphere. No tumor nodule is identified. The use of contrast enhancement would have proven helpful in identifying a nodule.
consists of two parts, the proximal part where the sheath surrounding the axons is formed by Schwann cells, and the distal part where the sheath consists of oligodendroglial cells. The junction of the two parts is usually located near or at the opening of the internal auditory canal (porus acusticus). Acoustic tumors originate only from the distal portion and usually from the vestibular component. The overproduction of Schwann cells by the vestibular nerve and ganglion is thought to be the reason for the predominant origin of the acoustic tumors from the vestibular component.14 The acoustic schwannomas occur from the second to the seventh decade; however, they most often manifest themselves during the fifth decade and more frequently in women. These tumors grow slowly and may remain asymptomatic for a long period of time. Because of the slowness of the process, the cranial nerves can tolerate a marked degree of stretching and compression prior to the appearance of deficits. Tinnitus and sensorineural hearing loss are the earliest symp-
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Fig 6. Small left intracanalicular acoustic schwannoma. (A) Axial MR scan (600/20). There is a question of slightly more prominent soft tissue (white arrow) in the lateral aspect of the left internal auditory canal. Nerves oen be seen leaving the brainstem. crossing the subarachnoid space in the cerebelopontine angle region. and entering the internal auditory canals. (B) There is prominent enhancement of the left intracanalicular acoustfc schwannoma (arrow) after injection of Gd-DTPA.
Fig 7. Acoustic schwannoma. Axial MR scan (600/25). A small round mass (arrow) expands the right porus acusticus and projects into the right carebellopontine angle cistern. The cistern is widened compared to the normal left side. The tumor does not fill the canal. Cranial nerves can be seen coursing from the brainstem toward the mass.
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Fig 8. Left acoustic schwannoma expands the cerebellopontine angle cistern and the internal auditory canal. (A) Axial MR scan (3DDD/3D). On this proton density image it is difficult to delineate the mass (arrow) from the adjacent CSF. There is irregular expansion of the internal auditory canal. (B) Axial MR scan K38W/88). The tumor is of heterogeneous hypointensity and is well delineated from the high-intensity CSF. (Cl Axial MR scan I600/20). The tumor has similar intensity to that of the brain. There is a small cleft of CSF between the posterior margin of the mass and the cerebellum. IDI Enhancement with Gd-DTPA provides best delineation of the tumor.
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toms, in spite of the fact that the tumor originates in the vestibular portion. Vestibular symptoms (ie, unsteadiness) appear late. Other symptoms depend on the extent and size of the tumor and are related to compression of the cranial nerves, cerebellum, and the brainstem. Hydrocephalus may develop secondary to compression of the fourth ventricle. MR with the use of Gd-DTPA has superseded CT and become the imaging diagnostic procedure of choice for the evaluation of patients who are suspected of having acoustic schwannomas. It is of particular value in the identification of small intracanalicular tumors (Fig 6) and also in the detection of residual or recurrent tumors.15 On unenhanced CT the schwannoma is isodense or slightly hypodense to the adjacent brain. On MR, lesions show a variable intensity pattern On Tl-weighted MR images, the tumors are of slightly lower or of similar intensity (Figs 7 and 8C) to that of the brain and range from isointense to slightly increased intensity to prominently increased intensity on the proton density
Fig 9. Large left acoustic schwannoma. Contrastenhanced axial MR scan (600/20. Gd-DTPAI. There is markedly heterogeneous enhancement of a large tumor that has produced marked compression of the left brainstem and left middle cerebellar peduncle. The fourth ventricle is asymmetric and displaced to the right.
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Fig 10. Acoustic schwannoma with arachnoid cysts. Axial MR scan (3ClOO/SO). Low-intensity heterogeneous tumor in the right cerebellopontine angle region has highintensity arachnoid cysts on its periphery. The internal auditory canal is expanded by the tumor.
and T2-weighted images (Fig 8A and 8B). The tumors enhance prominently, both on CT and MR (Figs 6B, 8D, and 9). The large tumors may be heterogeneous due to cystic degeneration (Fig 9) and presence of large vessels. They may also be associated with peripheral cysts due to trapped CSF (Fig 10). To detect small intracanalicular lesions it is essential to use a contrast agent and to obtain thin MR sections. However, identification of an intracanalicular mass and/or contrast enhancement does not necessarily indicate the presence of a schwannoma. Clinical findings must be taken into consideration in order to arrive at a correct diagnosis. Metastatic (Fig 11) and inflammatory processes may involve the contents of the internal auditory canal. Findings of both the rapid onset of hearing loss or paralysis of the seventh cranial nerve at presentation are unusual with acoustic schwannomas. When there is enhancement of the intracanalicular seventh cranial nerve related to Bell’s palsy, there generally is contrast enhancement of other portions of the seventh cranial nerve and this is well shown on
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Fig 11. Lyrnphomatous (B) On contrast-enhanced arrowhead).
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meningitis. (A) Axial MR scan i600/20). The internal auditory canals appear within axial MR scan there is abnormal enhancement within the right internal auditory
normal canal
limits. (white
MR.16 Intracanalicular schwannomas and meningiomas may have the same clinical and radiologic picture. MR plays an important role in the evaluation of patients who have sensorineural hearing loss because it can demonstrate a spectrum of abnormalities, all of which can result in hearing loss. Brainstem lesions, abnormal vascular loops, and siderosis” can be detected in addition to extraaxial neoplasms. Meningiomas
Fig 12. Left cerebellopontine angle meningioma. Contrast-enhanced axial CT scan shows a large well-defined left cerebellopontine angle mass that has invaginated itself into the brainstem and has a flat base with the petrous bone. The meningioma extends into the posterior cavernous sinus (arrow).
Meningioma is the second most frequent tumor of the cerebellopontine region. Meningiomas are more common in women and occur most frequently during middle age. Meningiomas usually arise from meningothelial cells of the arachnoid; however, they may also originate from the dural or pial cells.’ In addition to the cerebellopontine angle region, the meningiomas may arise anywhere along the walls of the posterior fossa. The more frequent sites are the free edge of the tentorium cerebelli and the foramen magnum.
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Fig 13. Left cerebellopontine angle meningioma with intracanalicular component. (A) Axial contrast-enhanced MR scan @00/20). A hyperintense mass with a flat base against the left petrous bone is present on the left. There is an intracanelicular component that is unusual for a meningioma. (B) Coronal contrast-enhanced MR scan EOCJ/20). The meningioma extends superiorly end inferiorly along the brainstem. (C) An axial contrast-enhanced MR scan at a higher level than A shows a tail of meningioma (arrowhead) squeezing itself between the brainstem medially and dura laterally.
When the cerebellopontine angle meningioma is small or intracanalicular, it is not distinguishable from an acoustic schwannoma on the basis of its appearance on images. Larger meningiomas have a more distinctive morphology. They have a flat base with the dura, thus having a configuration of a “half-ball” (Fig 12). In the cerebellopontine angle they generally lack an
intracanalicular component that schwannomas usually have. However, some meningiomas may extend into the internal auditory canal (Fig 13) and then the diagnosis has to be based on the other morphologic features. On nonenhanced CT, most meningiomas are hyperdense to adjacent brain; however, some are isodense and some may contain calcifications. Generally they en-
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hance homogeneously and prominently (Fig 12). On Tl-weighted images meningioma is isointense or hypointense to the brain. On T2weighted images, meningioma intensity ranges from low, isointense to hyperintense in reference to the brain. Elster and coinvestigators” found a correlation between the intensity pattern and histology of the meningioma.” Because of the variability of the intensity, some meningiomas may not be distinguishable from the brain unless contrast material is used. Vessels and a cleft may be noted on the periphery of the meningioma where it invaginates into the brain.” Curvilinear and punctate regions of signal void may be seen within the tumor due to vascularity and/or calcifications. Contrast enhancement with GdDTPA is prominent and homogeneous (Fig 13). Due to necrosis and hemorrhages, schwannomas tend to be more heterogeneous than meningiomas. The meningiomas typically extend along the dura and, therefore, a “tail” of meningioma may be seen extending anteriorly from the cerebellopontine angle mass (Fig 13C). In addition, meningiomas can grow through bone and thus a cerebellopontine angle meningioma may extend through the petrous apex (Fig 12) or around the tentorial edge. In such a circumstance, it is important to differentiate the meningioma from a fifth cranial nerve schwannoma or neurofibroma. The latter tumors tend to scallop or amputate the apex (Fig 14), whereas meningiomas most often grow through bone, producing sclerosis with the structure of the bone being preserved. However, it should be noted that in the region of the cerebellopontine angle, the sclerosis, due to the meningioma, may be fairly subtle in contrast to marked sclerosis and bone expansion produced by sphenoid wing meningiomas. Cranial Nerve Tumors at Other Sites in the Posterior Fossa The trigeminal nerve is the second in frequency to be involved by schwannoma. It may have a primarily cerebellopontine angle location and may be difficult to distinguish from the acoustic tumor. However, it is generally more anteriorly placed, being closely related to the petrous apex, which it may stradle and erode (Fig 14). Thus these tumors may have a dumb-
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Right fifth cranial nerve schwannoma. Axial Fig 14. contrast-enhanced MR scan (600/20. Gd-DTPA). A lobulated markedly enhancing mass has amputated the right petrous apex and eroded the lateral aspact of the clivus. The mass projects posteriorly into the posterior fossa and anteriorly into the cavernous sinus. lntracavernous carotid (arrowhead) is displaced anteromedially.
bell configuration, having one component in the posterior fossa and another in the middle cranial fossa. The seventh cranial nerve and the cranial nerves of the jugular fossa (9th, lOth, and 1 lth cranial nerves) may also be involved by schwannoma or neurofibroma. If there is a solitary tumor of one of these nerves, then diagnostic imaging cannot differentiate between the two types of tumors. However, if there are multiple cranial nerves involved by tumors then it is more likely that the tumors are neurofibromas. Epidermoid
Cysts
Epidermoid cysts represent the third most common tumor of the cerebellopontine angle. Epidermoid, as well as dermoid cysts, are lesions of maldevelopmental origin. They arise from the
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Fig 15. Large left epidermoid cyst. (A) Contrast-enhanced CT scan. A large low density lobulated mass compresses and displaces the brainstem to the right. Contrast-enhanced basilar artery (arrow) can be identified. A lobule of the tumor has extended into the middle cranial fossa (arrowhead). (B) Axial MR scan (600/20). A heterogeneous low-intensity mass is identified compressing and displacing the brainstem and extending over the left margin of the tentorium. Multiple serpiginous regions of slightly greater intensity are identified within the msss. These most likely reflect the gross structure of the tumor that consists of concentric lamellae of desquamated material. (C) A proton density MR scsn (2500/40) shows the tumor to be of heterogeneous but increased intensity. The multilobulated nature of the tumor is well demonstrated here. Displaced vessels can be identified on periphery due to the signal void within them.
inclusion of ectodermal elements that occurs during embryogenesis. In cases of dermoid cysts, which are generally in the midline, there may be an associated dermal sinus. The inclusions that take place earlier in the embryogenesis tend to produce midline lesions that mostly are dermoid cysts. Inclusions that occur later are responsible
for the more laterally located lesions, most of which are epidermoid cysts.” In addition to the developmental origin, it is believed that both types of cysts may arise secondary to previous procedures, infection, or trauma, which may have resulted in the implantation of skin fragments in deeper tissues. Cere-
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Epidermoid cyst Fig 16. within the fourth ventricle. (A) A sagittal MR scan (800/20) shows a low-intensity slightly heterogeneous mass that produces marked and irregular expansion of the fourth ventricle. The mass extends anteriorly to deform the medulla. (B) A series of CT scans after intrathecal instillation of metrizamide shows a multilobulated mass within the fourth ventricle and extending anteriorly around the medulla.
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Fig 17. Epidermoid cyst. A sagittal MR scan (800/25) shows an epidermoid cyst lying along the clivus and scalloping and displacing the brainstem. The mass is of slightly greater intensity than CSF and shows fine heterogeneitywithin it. The basilar artery, shown in part as regions of signal void, is encased by the epidermoid cyst.
bellopontine angle epidermoid cysts are of a developmental origin, whereas middle ear cholesteatomas are usually of the acquired type. Epidermoid and dermoid cysts consist of breakdown material of desquamated keratin from the epithelial lining of the cyst. The material is rich in cholesterol. The presence of skin appendages, such as hairs and sebaceous glands, differentiates the dermoid cysts from epidermoid cysts. The epidermoid cysts are also referred to as congenital cholesteatomas, epidermoids, epidermoidomas, and epidermoid tumors. In the head they may be located in the subarachnoid space or may be intradiploic. Because of their slow growth, epidermoid cysts become symptomatic insidiously and when detected may have achieved a large size. They are more common in males and generally are discovered during adulthaod. Symptoms are related to stretching of cranial nerves and displacement and compression of the cerebellum and brainstem. The subarachnoid epidermoid cysts show propensity for insinuating themselves between structures and also for encircling nerves and vessels. For this reason complete resection is frequently not possible. When large, the epidermoid cysts are multilobulated and have an amoeboid configuration. These lesions scallop and deform the intracranial soft tissues, but do
not produce bone destruction, as is the case with the intradiploic variety. On CT, the density of epidermoid cysts is most typically similar to that of CSF and usually they do not enhance (Fig 15A). In the past, it was at times necessary to use intrathecal contrast material in order to identify and delineate them (Fig 16B).” Some of the cysts may have calcifications in their wall, some may become diffusely dense due to saponification, others may enhance on periphery.2’ The epidermoid cysts have similar intensity to that of CSF, both on Tl- (Figs 15B,C, 16, and 17) and TZ-weighted images. For this reason small lesions could be overlooked, particularly if the images are photographed with narrow windows. Careful viewing of the images on a screen or wide window photography will often show internal heterogeneity (Figs 15B, 16A, and 17) and a difference in the intensity between the tumors and CSF. Particularly on Tl-weighted and proton density images, the lesions tend to be of slightly greater intensity than CSF. Attention should be given to proton density images, which at times may provide more specific information. The tumors may have a mixed signal, both hypointense and isointense or even hyperintense (Fig 15C), showing multilobular morphology of
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Fig 18. Cerebellopontine angle lipomas in two different patients. iA) Contrast-enhanced axial CT scan shows a lesion of fatty density in the right cerebellopontine angle. The lipoma is traversed by nerves and vessels. (B) Axial MR scan @000/ 30) shows a left cerebellopontine angle high-intensity lesion that has a hypointense margin on its posterior s&ace. This represents a chemical shift artifact, thus confirming the ‘fatty composition of the lesion. (A Tl-weighted MR scan, not shown here, showed increased intensity in the lesion.) (Cl Axial MR seen (3000/80) on the seme patient, as shown in B, shows low intensity in the lipoma and a chemical shii artifact consisting of a bright stripe on its anterior surface and a black stripe on its posterior surface.
the lesion. Also, some lesions may be surrounded by a hyperintense rim that is interpreted to represent trapped CSF between the lesion and the brain.22 The use of multiple pulse sequences and multiple planes makes MR the procedure of
choice for the evaluation of epidermoid cysts. It provides the best delineation of the lesions and also shows best their effect on blood vessels, nerves, and brain. Only in rare circumstances, when the lesion is small and there is no expansion
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Fig 19. Clival chordoma. (A) Sagittal MR scan (800/20). Low-intensity multilobulated mass deforms and displaces the brainstam, destroys the clivus, and extends into the sella turcica (arrowhead) and nasopharynx (two arrowheads). (9) Axial MR scan (2900136). Hyperintense mass with peripheral vessels inveginates itself into the brainstem and also occupies the region of the sella turcica and left cavernous sinus.
Fat has lower density than CSF and with proper photography can be identified on CT (Fig 18A). Generally, identification of fatty lesions on MR is not a problem because they have a characteristic intensity pattern, high intensity on Tl weighting and proton density and low intensity on T2 weighting (Fig 18B and C). In the posterior fossa, lipomas occur more commonly around the cerebellar vermis and quadrigeminal plate cistern than at the cerebellopontine angle region. Their shape may be fairly irregular due to the fact that they infiltrate around the structures, including vessels and nerves. The symptoms are related to the size of the lipoma and which structures are involved. POSTERIOR FOSSA WALL LESIONS
of the subarachnoid space and MR is inconclusive, it may still be necessary to perform CT after intrathecal installation of a contrast agent.
Bony walls of the posterior fossa may be involved in such lesions as chordomas, intradiploic epidermoid cysts, jugular paragangliomas, schwannomas, as well as metastases. These lesions may extend into the confines of the posterior fossa cavity.
Lipomas
Chordomas
Lipoma is a lesion of maldevelopmental origin that rarely occurs in the cerebellopontine angle.
Chordoma, a rare tumor that arises in remnants of the notochord, frequently involves the
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clivus. It may also arise off the midline in the petrous apex. 23 Chordomas grow slowly and, therefore, when discovered are large and multilobmated. They destroy the bony cranial base and extend into various directions-posterior fossa, middle cranial fossa, and anteriorly into the nasopharynx. With all the lobules they can become ameboid in configuration. On CT, most chordomas are of increased density and contrast enhance. On MR they are hypointense to the brain on Tl-weighted images and become hyperintense on proton density and T2-weighted images24 (Fig 19). They show enhancement with Gd-DTPA. Some contain irregular regions of signal void that may represent calcifications and/or enlarged vessels (Fig 19B). Jugular Paraganglioma Tumor)
(Glomus Jugulare
Jugular paragangliomas originate from the paraganglion (a group of granule storing chief cells and schwann-like satellite cells) located in the adventitia of the jugular bulb.*’ Jugular paragangliomas occur most often in women who are middle aged and present with pulsatile tinnitus and conductive hearing loss. Although most tumors are benign and grow slowly, they can produce extensive bony destruction. The bony destruction is typically permeative resulting in a lacelike erosive pattern throughout the bone. The tumors can extend up into the ear and medially into the cerebellopontine angle region. Thus a patient can be noted as having a vascular mass behind the tympanic membrane, conductive hearing loss, and facial nerve palsy. The tumor can also extend downward to the jugular foramen to involve the 9th, lOth, and 11th cranial nerves, and into the hypoglossal canal to involve the 12th cranial nerve. On CT, paragangliomas appear as dense irregular masses that enhance markedly (Fig 20A). They show a permeative pattern of bony destruction. On MR, these tumors are isointense to the brain on Tl -weighted images and can be noted to contain multiple signal voids representing enlarged vessels (Fig 20B). On T2-weighted images the tumors remain heterogeneous due to vascularity, but become hyperintense. They show marked contrast enhancement with Gd-DTPA.26 Thus,
Jugular paragangliomas in two different paFig 20. tients. (A) Contrast-enhanced CT scan shows an enhanced lesion (arrowheads) that has produced irregular bone destruction in the left temporal bone. (B) Coronal contrastenhanced MR scan (TR, 600; TE, 20) shows a heterogeneous hypointense mass (arrowheads) that destroys the inferomedial portion of the right temporal bone and extends inferiorly into the neck.
on the basis of the morphology and intensity pattern, a paraganglioma can be diagnosed. Angiography is performed for the identification of vascular supply and also for the purpose of presurgical embolization. The differential diagno-
Abbreviations:
ternHoer
HYPE HYPE (* calcification)
Iso or sl. hypo Hyper (+ calcification)
angle region; hypo, hypointense;
Irregular
CPA, cerebellopontine
Jugular-infralabyrinthine poral bone
Multilobulated Multilobulated
CPA Clivus-petrous
CT Density
Tumors-Typical
and
iso, isointense;
Hypo kaxcept melanoma hemorrhagic tumors) Cyst: hypo Solid: iso
lnfratentorial
Round/oval Semilunar flat-based
Round/oval
Round
Configuration
1. Adult
CPA CPA
apex
Periphery of cerebellum
Cerebellum
CE, contrast enhancement:
cyst
Paraganglioma
Epidermoid Chordoma
Extraaxial Acoustic schwannoma Meningioma
Hemangioblastoma
lntraaxial Metastasis
Lesion
Frequent Location
Table
Hoer
Ww
SI. hyper
Variable Variable
Hyper
Iso or hyper
hyper, hyperintense.
Is0
HYPE HYPE
HYPE Iso or hypo
Iso or sl. hypo
Hypo
Proton Density T2
Hwr
Hoer
HvPer
Variable Variable
Hoer
Iso or hyper
Intensity Compared With Brain
Features
Tl-W
Imaging
Yes
No Yes
Yes Yes
Yes
Yes
CE. CT, or MR
No
No No
Some Rare
Yes
Yes
Necrotic or Cystic
Yes
No Some
Some Yes
Yes
Some
Vascularity
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sis includes vascular metastases, such as those due to renal cell carcinoma. SUMMARY
In adults, the combination of clinical presentation and imaging results (Table 1) permit predic-
tion of the histology of a posterior fossa tumor. MR imaging, which uses various pulse sequences and contrast material is the most sensitive and precise technique for the evaluation of patients who have symptoms referrable to the posterior fossa.
REFERENCES 1. Gentry LR, Jacoby CG, Turski PA, et al: Cerebellopontine angle-petromastoid mass lesions: Comparative study of diagnosis with MR imaging and CT. Radiology 162513-520, 1987 2. Press GA, Hesselink JR: MR imaging of cerebellopontine angle and internal auditory canal lesions at 1.5 T. AJNR 9:241-251, 1988 3. Potts DG, Abbott GF, von Sneidern JV: National Cancer Institute study: Evaluation of computed tomography in the diagnosis of intracranial neoplasms. III. Metastatic tumors. Radiology 136:657-664, 1980 4. Davis JM, Zimmerman RA, Bilaniuk LT: Metastases to the central nervous system. Radio1 Clin North Am 20:417-435, 1982 5. Delattre JY, Krol G, Thaler HT, et al: Distribution of brain metastases. Arch Neural 45741-744, 1988 6. Atlas SW, Grossman RI, Gomori JM, et al: MR imaging of intracranial metastatic melanoma. J Comput Assist Tomogr 11577-582, 1987 7. Zimmerman RA, Bilaniuk LT: Computed tomography of acute intratumoral hemorrhage. Radiology 135:355-359, 1980 8. Russel DS, Rubinstein LJ: Pathology of Tumors of the Nervous System (ed 5). Baltimore, MD, Williams & Wilkins, 1989 9. Lee SR, Sanches J, Mark AS, et al: Posterior fossa hemangioblastomas: MR imaging. Radiology 171:463-468, 1989 10. Elster AD, Arthur DW: Intracranial hemangioblastomas: CT and MR findings. J Comput Assist Tomogr 12:736739,1988 11. Bilaniuk LT, Zimmerman RA, Littman P, et al: Computed tomography of brainstem gliomas in children. Radiology 134:89-95, 1980 12. National Institutes of Health Consensus Development: Neurofibromatosis. Arch Neural 45:575-578, 1988 13. Gruskin P, Carberry JN: Pathology of acoustic tumors, in House WF, Leutje CM (eds): Acoustic Tumors. Baltimore, MD University Park, 1979
14. Bebin J: Pathophysiology of acoustic tumors, in House WF, Leutje CM (eds): Acoustic Tumors. Baltimore, MD, University Park, 1979 15. Haughton VM, Rimm AA, Czervionke LF, et al: Sensitivity of Gd-DTPA-enhanced MR imaging of benign extraaxial tumors. Radiology 166:829-833, 1988 16. Daniels DL, Czervionke LF, Millen SJ, et al: MR imaging of facial nerve enhancement in Bell palsy or after temporal bone surgery. Radiology 171:807-809, 1989 17. Gomori JM, Grossman RI, Bilaniuk LT, et al: Highfield MR imaging of superficial siderosis of the central nervous system. J Comput Assist Tomogr 9:972-975, 1985 18. Elster AD, Challa VR, Gilbert TH, et al: Meningiomas: MR and histopathologic features. Radiology 170:857-862, 1989 19. Spagnoli MV, Goldberg HI, Grossman RI, et al: Intracranial meningiomas: High-field MR imaging. Radiology 161:369-376,198O 20. Latack JT, Kartush JM, Kemink JL, et al: Epidermoidomas of the cerebellopontine angle and temporal bone: CT and MR aspects. Radiology 157:361-366, 1985 21. Zimmerman RA, Bilaniuk LT: Cranial CT of epidermoid and congenital fatty tumors of maldevelopmental origin. J Comput Tomogr 3:40-50, 1979 22. Tampieri D, Melanson D, Ethier R: MR imaging of epidermoid cysts. AJNR 10:351-356, 1989 23. Brown RV, Sage MR, Brophy BP: CT and MR findings in patients with chordomas of the petrous apex. AJNR 11:121-124, 1990 24. Oot RF, Melville GE, New PFJ, et al: The role of MR and CT in evaluating clival chordomas and chondrosarcomas. AJNR9:715-723, 1988 25. Batsakis JG: Tumors of the Head and Neck (ed 2). Baltimore, MD, Williams & Wilkins, 1982 26. Vogl T, Bruning R, Schedel H, et al: Paragangliomas of the jugular bulb and carotid body: MR imaging with short sequences and Gd-DTPA enhancement. AJNR 10:823-827, 1989
T. Bilaniuk
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NFRATENTORIAL tumors in adults are less frequent and have a different distribution than tumors in supratentorial locations. In contradistinction to the supratentorial tumors, where the intraaxial tumors are more frequent, the extraaxial tumors represent the most common group of tumors that occur in the posterior fossa of adults. Most of the tumors are slow growing neoplasms; thus, they may manifest themselves insidiously. It becomes important to detect these tumors as early as possible because surgical resection is easier and there is less morbidity. Also, there is a greater chance to preserve function. The smaller volume of the posterior fossa as compared to the supratentorial space and the presence of vital structures contribute to a critical situation if there is rapid increase in the volume of a tumor, such as may occur with hemorrhage. Emergency surgical decompression may be life saving and, therefore, it is important to recognize these lesions and delineate them. The differentiation of intraaxial from extraaxial tumors is important because it not only aids in correct prediction of histology, but influences surgical approach and planning. Identification and characterization of posterior fossa lesions has presented a particular challenge to the neuroradiologist. Although CT has made a tremendous impact on the evaluation of intracranial lesions, it has had several drawbacks as far as the posterior fossa is concerned. The major one has been the presence of beam-hardening artifacts that can obscure the detail of the posterior-fossa structures. Thus, magnetic resonance imaging (MRI), with its multiplanar capability, superb soft tissue discrimination, and lack of major artifacts, has become the procedure of choice for the evaluation of the posterior fossa.“’ Availability of MR contrast agents has further expanded the use of MR. At present, CT plays a complementary role in cases where there is a question of bony changes or calcification. Angiography is used when detailed vascular anatomy is required, such as in cases of hemangioblastomas, paragangliomas, or some meningiomas.
Seminars in Roentgenology, Vol XXV, No 2 (April), i990:
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Tumors
INTRAAXIAL
TUMORS
Metastases
Metastatic lesion is the most common posterior fossa intraaxial tumor in the adult. The source is usually a tumor of the lung, breast, colon and kidney, and melanoma.3 The cerebellum is more often involved than the brainstem. Overall the supratentorial space is a more common site for metastases.4 However, there are some specific exceptions. One study, dealing with distribution of brain metastases as shown by CT in 288 patients, reports that when there is a solitary metastasis (49% in that series) and when the primary tumor is pelvic (prostate or uterus) or gastrointestinal then the posterior fossa is involved in 50% of cases, but only 10% if the source is from other tumors, such as lung and breast carcinomas.5 Gadolinium (Gd)-DTPA enhanced MR is the most sensitive technique for the evaluation of posterior fossa for metastases. Small metastases located in the inferior portion of the cerebellum or brainstem, that may be missed by contrastenhanced CT due to the beam-hardening artifacts, are well shown by MR. Therefore, as information from larger series of patients examined using contrast-enhanced MR is accumulated, more accurate statistics will become available on the distribution and number of metastases. On MR and CT, most metastases contrast enhance, are surrounded by edema, and produce mass effect (Fig 1). Because of the tight fitting of structures in the posterior fossa and the smaller volume of brain relative to the metastatic lesion, peritumoral edema may be difficult to detect and delineate, particularly using CT. Metastatic lesions generally are hypodense on plain CT and hypointense on Tl-weighted MR images. Excep-
From the Department of Radiology. Hospital of the University of Pennsylvania, Philadelphia, PA. Address reprint requests to Larissa T. Bilaniuk, MD, Department of Radiology, Hospital of the University of Pennsylvania, 3400 Spruce St, Philadelphia. PA 19104. 0 1990 by W.B. Saunders Company. 0037-198X/90/2502-0004$5.00/0
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and some gastrointestinal mucin producing tumors). On proton density and TZ-weighted images a metastasis may be difficult to delineate from edema, because either its intensity is similar to edema or it is isointense to the normal brain and, therefore, is obscured by the edema. Some metastases are not associated with edema and may be difficult to detect due to the similarity of their intensity to that of the brain (Fig 2A). Thus, contrast-enhanced MR is necessary for the identification and delineation of metastatic lesions (Fig 2). Some metastatic lesions that are not associated with edema may mimic ischemic lesions. On the other hand, a single, large metastasis may be difficult to differentiate from other primary posterior fossa tumors or an abscess. Some metastases, particularly in the posterior fossa, may mimic cysts. Correlation with the clinical picture and idemification of metastases at other sites are helpful in making the diagnosis of posterior fossa metastasis. In some cases the metastases may be both intraaxial and extraaxial (Fig 2).
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lung carcinoma. (A) Contrast-enFig 1. Metsstatic hanced CT shows a peripherally enhancing mass with central necrosis. There is prominent edema around the mass. The fourth ventricle is compressed and displaced slightly to the right. (6) Axial MR scan at 2500/70 (TR/TE). The mass is of more homogeneous and slightly greater intensitv than the surrounding edema.
tions to this are melanotic melanomas,6 hemorrhagic tumors7 (such as melanoma, choriocarcinoma, lung carcinoma, and renal carcinoma), and calcified lesions (such as osteogenic sarcoma
Hemangioblastomas Hemangioblastoma, a benign tumor of vascular origin, is considered to be the most common primary intraaxial tumor in the posterior fossa in adults. It accounts for 1.1% to 2.4% of all intracranial tumors and 7.3% of all primary posterior fossa tumors.’ The tumors most often occur in young to middle-aged adults and show male preponderance. Hemangioblastoma is most commonly located in the cerebellum and usually lies at the periphery of the brain near the pial surface (Figs 3 through 5).’ Most typically, the lesion consists of a cyst with a vascular mural nodule. However, the tumors may also be completely solid or may be solid with a central cyst. When the tumor is multiple, it is frequently associated with Hippel-Lindau disease. In cases of hemangioblastomas, MR is superior to CT, not only because it can detect and define smaller posterior fossa lesions,” but it also characterizes them better by showing the prominent tumor vasculature (Figs 3B and 4), tumor hemorrhage, and also demonstrates associated spinal lesions. On CT, the solid portion of hemangioblastomas is isodense to the brain and shows marked enhancement (Fig 3A). The cyst content is isodense or slightly increased in density to the
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Fig 2. Metastatic adenocarcinoma both with intraaxial and extraaxial lesions. (A) Axial MR scan (3WD/sO). Low-intensity mass (arrowhead) expands the left cavernous sinus and projects posteriorly into the prepontine cistern. (BJ Axial contrast-enhanced MR scan fBClD/2Oj. There is enhancement of the mass and encasement of the carotid artery, which is narrower than the normal right intracavernous carotid artery. (Cj Axial MR scan (3ooO/BDj. Low intensity metastasis (arrowheads) is identified by its mess effect, but could be overlooked because it has similar intensity to that of the cerebellum. (D) Contrast-enhanced coronal MR scan. Multiple contrast enhancing nodules (arrows) are shown on a coronal MR scan.
cerebrospinal fluid (CSF). The rim of the cyst, which does not contain tumor and is composed of fibrillary neuroglia,8 may show slight contrast enhancement. On Tl-weighted MR images, the solid tumor or nodules are of similar or slightly lower intensity than the gray matter and show increase in intensity on proton density (Fig 3B) and on T2-weighted sequences. Curvilinear or punctate regions of signal void that represent enlarged tumor vessels are demonstrated within and on periphery of the tumor (Figs 3B and 4).
The vascularity can also be demonstrated using MR angiography (compressed images from threedimensional acquired gradient-echo scans). However, the best anatomic detail of vasculature is still provided by conventional angiography. Just as on CT (Fig 3A), the solid components of the tumor enhance on MR with Gd-DTPA (Fig 4B). The cysts on MR images are well defined and are hypointense on Tl weighting and hyperintense on proton density and T2-weighted images (Fig 5). This is most likely related to increased protein
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should be achieved with high resolution multiplanar Gd-enhanced MR images. Other Primary Posterior Fossa Tumors Primary posterior fossa tumors, such as cerebellar astrocytoma, ependymoma, and primitive neuroectodermal tumor that are common in the pediatric age group, occur infrequently in adults, but when they occur it is in the young adult. Brainstem gliomas represent approximately 2.5% of adult gliomas.” EXTRAAXIAL
TUMORS
Acoustic Schwannomas
Solid hemangioblastoma. (A) Contrast-enhanced Fig 3. scan shows markedly enhancing tumor on the enterosuperior aspect of the left cerebellar hemisphere. The tumor compresses posterior aspect of the midbrain. There is hydrocephalus. (B) Proton density axial MR scan (2500/30). The mass shows prominent hyperintensity and contains large vessels within it as well as on the periphery.
content of the fluid within the cysts. Some cysts may contain blood products.’ In some cases the mural nodule may be so small that it is difficult to demonstrate. Detection of such small lesions
Acoustic schwannoma is the most common tumor of the cerebellopontine angle region. It represents about 80% of the cerebellopontine angle tumors and approximately 10% of all intracranial neoplasms. It is bilateral in about 5% of cases and this is generally associated with neurofibromatosis type 2.i2 Since the original description of the acoustic tumor, numerous names, more than 25 (among them neuroma, neurinoma, neurilemmoma, neurofibroma), have been assigned to it.13 This confusion in nomenclature arose from a controversy concerning the origin of the tumor. The advent of electron microscopy has resolved most of the disagreements. At present the most widely accepted view is that the acoustic tumor originates in the Schwann cell and, therefore, the name schwannoma is most appropriate.* The axons of peripheral nerves are surrounded by Schwann cells, endoneural fibroblasts, and perineural cells. Proliferation of Schwann cells gives rise to schwannomas, whereas neurofibromas consist of Schwann cells and endoneural fibroblasts. Neurofibromas contain more connective tissue fibers and more nerve fibers than schwannomas.’ Another distinguishing feature between the two tumors is that schwannomas are encapsulated and neurofibromas lack a capsule. Even when associated with neurofibromatosis, the acoustic tumors are schwannomas. The eighth cranial nerve, which consists of an auditory and a vestibular component, courses from the cochlea and vestibular apparatus, through the internal auditory canal and the subarachnoid space of the cerebellopontine angle to enter the uppermost medulla. Throughout its course it is accompanied by the seventh cranial nerve, the facial nerve. The eighth cranial nerve
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Fig 4. Small solid hemangioblastoma. (Aj A sagittal MR scan (SOO/2Oj shows a small, slightly hypointense mass on the periphery of the cerebellar hemisphere. The mass has a prominent vessel on its periphery (arrow) as well as within it. (B) There is marked enhancement of the tumor following intravenous injection of Gd-DTPA.
Fig 6. Cystic hemengioblastoma. An axial MR scan (25iOO/BOj shows a well-defined cystic lesion located on the periphery of the left cerebellar hemisphere. No tumor nodule is identified. The use of contrast enhancement would have proven helpful in identifying a nodule.
consists of two parts, the proximal part where the sheath surrounding the axons is formed by Schwann cells, and the distal part where the sheath consists of oligodendroglial cells. The junction of the two parts is usually located near or at the opening of the internal auditory canal (porus acusticus). Acoustic tumors originate only from the distal portion and usually from the vestibular component. The overproduction of Schwann cells by the vestibular nerve and ganglion is thought to be the reason for the predominant origin of the acoustic tumors from the vestibular component.14 The acoustic schwannomas occur from the second to the seventh decade; however, they most often manifest themselves during the fifth decade and more frequently in women. These tumors grow slowly and may remain asymptomatic for a long period of time. Because of the slowness of the process, the cranial nerves can tolerate a marked degree of stretching and compression prior to the appearance of deficits. Tinnitus and sensorineural hearing loss are the earliest symp-
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Fig 6. Small left intracanalicular acoustic schwannoma. (A) Axial MR scan (600/20). There is a question of slightly more prominent soft tissue (white arrow) in the lateral aspect of the left internal auditory canal. Nerves oen be seen leaving the brainstem. crossing the subarachnoid space in the cerebelopontine angle region. and entering the internal auditory canals. (B) There is prominent enhancement of the left intracanalicular acoustfc schwannoma (arrow) after injection of Gd-DTPA.
Fig 7. Acoustic schwannoma. Axial MR scan (600/25). A small round mass (arrow) expands the right porus acusticus and projects into the right carebellopontine angle cistern. The cistern is widened compared to the normal left side. The tumor does not fill the canal. Cranial nerves can be seen coursing from the brainstem toward the mass.
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Fig 8. Left acoustic schwannoma expands the cerebellopontine angle cistern and the internal auditory canal. (A) Axial MR scan (3DDD/3D). On this proton density image it is difficult to delineate the mass (arrow) from the adjacent CSF. There is irregular expansion of the internal auditory canal. (B) Axial MR scan K38W/88). The tumor is of heterogeneous hypointensity and is well delineated from the high-intensity CSF. (Cl Axial MR scan I600/20). The tumor has similar intensity to that of the brain. There is a small cleft of CSF between the posterior margin of the mass and the cerebellum. IDI Enhancement with Gd-DTPA provides best delineation of the tumor.
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toms, in spite of the fact that the tumor originates in the vestibular portion. Vestibular symptoms (ie, unsteadiness) appear late. Other symptoms depend on the extent and size of the tumor and are related to compression of the cranial nerves, cerebellum, and the brainstem. Hydrocephalus may develop secondary to compression of the fourth ventricle. MR with the use of Gd-DTPA has superseded CT and become the imaging diagnostic procedure of choice for the evaluation of patients who are suspected of having acoustic schwannomas. It is of particular value in the identification of small intracanalicular tumors (Fig 6) and also in the detection of residual or recurrent tumors.15 On unenhanced CT the schwannoma is isodense or slightly hypodense to the adjacent brain. On MR, lesions show a variable intensity pattern On Tl-weighted MR images, the tumors are of slightly lower or of similar intensity (Figs 7 and 8C) to that of the brain and range from isointense to slightly increased intensity to prominently increased intensity on the proton density
Fig 9. Large left acoustic schwannoma. Contrastenhanced axial MR scan (600/20. Gd-DTPAI. There is markedly heterogeneous enhancement of a large tumor that has produced marked compression of the left brainstem and left middle cerebellar peduncle. The fourth ventricle is asymmetric and displaced to the right.
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Fig 10. Acoustic schwannoma with arachnoid cysts. Axial MR scan (3ClOO/SO). Low-intensity heterogeneous tumor in the right cerebellopontine angle region has highintensity arachnoid cysts on its periphery. The internal auditory canal is expanded by the tumor.
and T2-weighted images (Fig 8A and 8B). The tumors enhance prominently, both on CT and MR (Figs 6B, 8D, and 9). The large tumors may be heterogeneous due to cystic degeneration (Fig 9) and presence of large vessels. They may also be associated with peripheral cysts due to trapped CSF (Fig 10). To detect small intracanalicular lesions it is essential to use a contrast agent and to obtain thin MR sections. However, identification of an intracanalicular mass and/or contrast enhancement does not necessarily indicate the presence of a schwannoma. Clinical findings must be taken into consideration in order to arrive at a correct diagnosis. Metastatic (Fig 11) and inflammatory processes may involve the contents of the internal auditory canal. Findings of both the rapid onset of hearing loss or paralysis of the seventh cranial nerve at presentation are unusual with acoustic schwannomas. When there is enhancement of the intracanalicular seventh cranial nerve related to Bell’s palsy, there generally is contrast enhancement of other portions of the seventh cranial nerve and this is well shown on
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Fig 11. Lyrnphomatous (B) On contrast-enhanced arrowhead).
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meningitis. (A) Axial MR scan i600/20). The internal auditory canals appear within axial MR scan there is abnormal enhancement within the right internal auditory
normal canal
limits. (white
MR.16 Intracanalicular schwannomas and meningiomas may have the same clinical and radiologic picture. MR plays an important role in the evaluation of patients who have sensorineural hearing loss because it can demonstrate a spectrum of abnormalities, all of which can result in hearing loss. Brainstem lesions, abnormal vascular loops, and siderosis” can be detected in addition to extraaxial neoplasms. Meningiomas
Fig 12. Left cerebellopontine angle meningioma. Contrast-enhanced axial CT scan shows a large well-defined left cerebellopontine angle mass that has invaginated itself into the brainstem and has a flat base with the petrous bone. The meningioma extends into the posterior cavernous sinus (arrow).
Meningioma is the second most frequent tumor of the cerebellopontine region. Meningiomas are more common in women and occur most frequently during middle age. Meningiomas usually arise from meningothelial cells of the arachnoid; however, they may also originate from the dural or pial cells.’ In addition to the cerebellopontine angle region, the meningiomas may arise anywhere along the walls of the posterior fossa. The more frequent sites are the free edge of the tentorium cerebelli and the foramen magnum.
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Fig 13. Left cerebellopontine angle meningioma with intracanalicular component. (A) Axial contrast-enhanced MR scan @00/20). A hyperintense mass with a flat base against the left petrous bone is present on the left. There is an intracanelicular component that is unusual for a meningioma. (B) Coronal contrast-enhanced MR scan EOCJ/20). The meningioma extends superiorly end inferiorly along the brainstem. (C) An axial contrast-enhanced MR scan at a higher level than A shows a tail of meningioma (arrowhead) squeezing itself between the brainstem medially and dura laterally.
When the cerebellopontine angle meningioma is small or intracanalicular, it is not distinguishable from an acoustic schwannoma on the basis of its appearance on images. Larger meningiomas have a more distinctive morphology. They have a flat base with the dura, thus having a configuration of a “half-ball” (Fig 12). In the cerebellopontine angle they generally lack an
intracanalicular component that schwannomas usually have. However, some meningiomas may extend into the internal auditory canal (Fig 13) and then the diagnosis has to be based on the other morphologic features. On nonenhanced CT, most meningiomas are hyperdense to adjacent brain; however, some are isodense and some may contain calcifications. Generally they en-
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hance homogeneously and prominently (Fig 12). On Tl-weighted images meningioma is isointense or hypointense to the brain. On T2weighted images, meningioma intensity ranges from low, isointense to hyperintense in reference to the brain. Elster and coinvestigators” found a correlation between the intensity pattern and histology of the meningioma.” Because of the variability of the intensity, some meningiomas may not be distinguishable from the brain unless contrast material is used. Vessels and a cleft may be noted on the periphery of the meningioma where it invaginates into the brain.” Curvilinear and punctate regions of signal void may be seen within the tumor due to vascularity and/or calcifications. Contrast enhancement with GdDTPA is prominent and homogeneous (Fig 13). Due to necrosis and hemorrhages, schwannomas tend to be more heterogeneous than meningiomas. The meningiomas typically extend along the dura and, therefore, a “tail” of meningioma may be seen extending anteriorly from the cerebellopontine angle mass (Fig 13C). In addition, meningiomas can grow through bone and thus a cerebellopontine angle meningioma may extend through the petrous apex (Fig 12) or around the tentorial edge. In such a circumstance, it is important to differentiate the meningioma from a fifth cranial nerve schwannoma or neurofibroma. The latter tumors tend to scallop or amputate the apex (Fig 14), whereas meningiomas most often grow through bone, producing sclerosis with the structure of the bone being preserved. However, it should be noted that in the region of the cerebellopontine angle, the sclerosis, due to the meningioma, may be fairly subtle in contrast to marked sclerosis and bone expansion produced by sphenoid wing meningiomas. Cranial Nerve Tumors at Other Sites in the Posterior Fossa The trigeminal nerve is the second in frequency to be involved by schwannoma. It may have a primarily cerebellopontine angle location and may be difficult to distinguish from the acoustic tumor. However, it is generally more anteriorly placed, being closely related to the petrous apex, which it may stradle and erode (Fig 14). Thus these tumors may have a dumb-
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Right fifth cranial nerve schwannoma. Axial Fig 14. contrast-enhanced MR scan (600/20. Gd-DTPA). A lobulated markedly enhancing mass has amputated the right petrous apex and eroded the lateral aspact of the clivus. The mass projects posteriorly into the posterior fossa and anteriorly into the cavernous sinus. lntracavernous carotid (arrowhead) is displaced anteromedially.
bell configuration, having one component in the posterior fossa and another in the middle cranial fossa. The seventh cranial nerve and the cranial nerves of the jugular fossa (9th, lOth, and 1 lth cranial nerves) may also be involved by schwannoma or neurofibroma. If there is a solitary tumor of one of these nerves, then diagnostic imaging cannot differentiate between the two types of tumors. However, if there are multiple cranial nerves involved by tumors then it is more likely that the tumors are neurofibromas. Epidermoid
Cysts
Epidermoid cysts represent the third most common tumor of the cerebellopontine angle. Epidermoid, as well as dermoid cysts, are lesions of maldevelopmental origin. They arise from the
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Fig 15. Large left epidermoid cyst. (A) Contrast-enhanced CT scan. A large low density lobulated mass compresses and displaces the brainstem to the right. Contrast-enhanced basilar artery (arrow) can be identified. A lobule of the tumor has extended into the middle cranial fossa (arrowhead). (B) Axial MR scan (600/20). A heterogeneous low-intensity mass is identified compressing and displacing the brainstem and extending over the left margin of the tentorium. Multiple serpiginous regions of slightly greater intensity are identified within the msss. These most likely reflect the gross structure of the tumor that consists of concentric lamellae of desquamated material. (C) A proton density MR scsn (2500/40) shows the tumor to be of heterogeneous but increased intensity. The multilobulated nature of the tumor is well demonstrated here. Displaced vessels can be identified on periphery due to the signal void within them.
inclusion of ectodermal elements that occurs during embryogenesis. In cases of dermoid cysts, which are generally in the midline, there may be an associated dermal sinus. The inclusions that take place earlier in the embryogenesis tend to produce midline lesions that mostly are dermoid cysts. Inclusions that occur later are responsible
for the more laterally located lesions, most of which are epidermoid cysts.” In addition to the developmental origin, it is believed that both types of cysts may arise secondary to previous procedures, infection, or trauma, which may have resulted in the implantation of skin fragments in deeper tissues. Cere-
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Epidermoid cyst Fig 16. within the fourth ventricle. (A) A sagittal MR scan (800/20) shows a low-intensity slightly heterogeneous mass that produces marked and irregular expansion of the fourth ventricle. The mass extends anteriorly to deform the medulla. (B) A series of CT scans after intrathecal instillation of metrizamide shows a multilobulated mass within the fourth ventricle and extending anteriorly around the medulla.
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Fig 17. Epidermoid cyst. A sagittal MR scan (800/25) shows an epidermoid cyst lying along the clivus and scalloping and displacing the brainstem. The mass is of slightly greater intensity than CSF and shows fine heterogeneitywithin it. The basilar artery, shown in part as regions of signal void, is encased by the epidermoid cyst.
bellopontine angle epidermoid cysts are of a developmental origin, whereas middle ear cholesteatomas are usually of the acquired type. Epidermoid and dermoid cysts consist of breakdown material of desquamated keratin from the epithelial lining of the cyst. The material is rich in cholesterol. The presence of skin appendages, such as hairs and sebaceous glands, differentiates the dermoid cysts from epidermoid cysts. The epidermoid cysts are also referred to as congenital cholesteatomas, epidermoids, epidermoidomas, and epidermoid tumors. In the head they may be located in the subarachnoid space or may be intradiploic. Because of their slow growth, epidermoid cysts become symptomatic insidiously and when detected may have achieved a large size. They are more common in males and generally are discovered during adulthaod. Symptoms are related to stretching of cranial nerves and displacement and compression of the cerebellum and brainstem. The subarachnoid epidermoid cysts show propensity for insinuating themselves between structures and also for encircling nerves and vessels. For this reason complete resection is frequently not possible. When large, the epidermoid cysts are multilobulated and have an amoeboid configuration. These lesions scallop and deform the intracranial soft tissues, but do
not produce bone destruction, as is the case with the intradiploic variety. On CT, the density of epidermoid cysts is most typically similar to that of CSF and usually they do not enhance (Fig 15A). In the past, it was at times necessary to use intrathecal contrast material in order to identify and delineate them (Fig 16B).” Some of the cysts may have calcifications in their wall, some may become diffusely dense due to saponification, others may enhance on periphery.2’ The epidermoid cysts have similar intensity to that of CSF, both on Tl- (Figs 15B,C, 16, and 17) and TZ-weighted images. For this reason small lesions could be overlooked, particularly if the images are photographed with narrow windows. Careful viewing of the images on a screen or wide window photography will often show internal heterogeneity (Figs 15B, 16A, and 17) and a difference in the intensity between the tumors and CSF. Particularly on Tl-weighted and proton density images, the lesions tend to be of slightly greater intensity than CSF. Attention should be given to proton density images, which at times may provide more specific information. The tumors may have a mixed signal, both hypointense and isointense or even hyperintense (Fig 15C), showing multilobular morphology of
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Fig 18. Cerebellopontine angle lipomas in two different patients. iA) Contrast-enhanced axial CT scan shows a lesion of fatty density in the right cerebellopontine angle. The lipoma is traversed by nerves and vessels. (B) Axial MR scan @000/ 30) shows a left cerebellopontine angle high-intensity lesion that has a hypointense margin on its posterior s&ace. This represents a chemical shift artifact, thus confirming the ‘fatty composition of the lesion. (A Tl-weighted MR scan, not shown here, showed increased intensity in the lesion.) (Cl Axial MR seen (3000/80) on the seme patient, as shown in B, shows low intensity in the lipoma and a chemical shii artifact consisting of a bright stripe on its anterior surface and a black stripe on its posterior surface.
the lesion. Also, some lesions may be surrounded by a hyperintense rim that is interpreted to represent trapped CSF between the lesion and the brain.22 The use of multiple pulse sequences and multiple planes makes MR the procedure of
choice for the evaluation of epidermoid cysts. It provides the best delineation of the lesions and also shows best their effect on blood vessels, nerves, and brain. Only in rare circumstances, when the lesion is small and there is no expansion
LARISSA
170
T. BILANIUK
Fig 19. Clival chordoma. (A) Sagittal MR scan (800/20). Low-intensity multilobulated mass deforms and displaces the brainstam, destroys the clivus, and extends into the sella turcica (arrowhead) and nasopharynx (two arrowheads). (9) Axial MR scan (2900136). Hyperintense mass with peripheral vessels inveginates itself into the brainstem and also occupies the region of the sella turcica and left cavernous sinus.
Fat has lower density than CSF and with proper photography can be identified on CT (Fig 18A). Generally, identification of fatty lesions on MR is not a problem because they have a characteristic intensity pattern, high intensity on Tl weighting and proton density and low intensity on T2 weighting (Fig 18B and C). In the posterior fossa, lipomas occur more commonly around the cerebellar vermis and quadrigeminal plate cistern than at the cerebellopontine angle region. Their shape may be fairly irregular due to the fact that they infiltrate around the structures, including vessels and nerves. The symptoms are related to the size of the lipoma and which structures are involved. POSTERIOR FOSSA WALL LESIONS
of the subarachnoid space and MR is inconclusive, it may still be necessary to perform CT after intrathecal installation of a contrast agent.
Bony walls of the posterior fossa may be involved in such lesions as chordomas, intradiploic epidermoid cysts, jugular paragangliomas, schwannomas, as well as metastases. These lesions may extend into the confines of the posterior fossa cavity.
Lipomas
Chordomas
Lipoma is a lesion of maldevelopmental origin that rarely occurs in the cerebellopontine angle.
Chordoma, a rare tumor that arises in remnants of the notochord, frequently involves the
ADULT
INFRATENTORIAL
TUMORS
clivus. It may also arise off the midline in the petrous apex. 23 Chordomas grow slowly and, therefore, when discovered are large and multilobmated. They destroy the bony cranial base and extend into various directions-posterior fossa, middle cranial fossa, and anteriorly into the nasopharynx. With all the lobules they can become ameboid in configuration. On CT, most chordomas are of increased density and contrast enhance. On MR they are hypointense to the brain on Tl-weighted images and become hyperintense on proton density and T2-weighted images24 (Fig 19). They show enhancement with Gd-DTPA. Some contain irregular regions of signal void that may represent calcifications and/or enlarged vessels (Fig 19B). Jugular Paraganglioma Tumor)
(Glomus Jugulare
Jugular paragangliomas originate from the paraganglion (a group of granule storing chief cells and schwann-like satellite cells) located in the adventitia of the jugular bulb.*’ Jugular paragangliomas occur most often in women who are middle aged and present with pulsatile tinnitus and conductive hearing loss. Although most tumors are benign and grow slowly, they can produce extensive bony destruction. The bony destruction is typically permeative resulting in a lacelike erosive pattern throughout the bone. The tumors can extend up into the ear and medially into the cerebellopontine angle region. Thus a patient can be noted as having a vascular mass behind the tympanic membrane, conductive hearing loss, and facial nerve palsy. The tumor can also extend downward to the jugular foramen to involve the 9th, lOth, and 11th cranial nerves, and into the hypoglossal canal to involve the 12th cranial nerve. On CT, paragangliomas appear as dense irregular masses that enhance markedly (Fig 20A). They show a permeative pattern of bony destruction. On MR, these tumors are isointense to the brain on Tl -weighted images and can be noted to contain multiple signal voids representing enlarged vessels (Fig 20B). On T2-weighted images the tumors remain heterogeneous due to vascularity, but become hyperintense. They show marked contrast enhancement with Gd-DTPA.26 Thus,
Jugular paragangliomas in two different paFig 20. tients. (A) Contrast-enhanced CT scan shows an enhanced lesion (arrowheads) that has produced irregular bone destruction in the left temporal bone. (B) Coronal contrastenhanced MR scan (TR, 600; TE, 20) shows a heterogeneous hypointense mass (arrowheads) that destroys the inferomedial portion of the right temporal bone and extends inferiorly into the neck.
on the basis of the morphology and intensity pattern, a paraganglioma can be diagnosed. Angiography is performed for the identification of vascular supply and also for the purpose of presurgical embolization. The differential diagno-
Abbreviations:
ternHoer
HYPE HYPE (* calcification)
Iso or sl. hypo Hyper (+ calcification)
angle region; hypo, hypointense;
Irregular
CPA, cerebellopontine
Jugular-infralabyrinthine poral bone
Multilobulated Multilobulated
CPA Clivus-petrous
CT Density
Tumors-Typical
and
iso, isointense;
Hypo kaxcept melanoma hemorrhagic tumors) Cyst: hypo Solid: iso
lnfratentorial
Round/oval Semilunar flat-based
Round/oval
Round
Configuration
1. Adult
CPA CPA
apex
Periphery of cerebellum
Cerebellum
CE, contrast enhancement:
cyst
Paraganglioma
Epidermoid Chordoma
Extraaxial Acoustic schwannoma Meningioma
Hemangioblastoma
lntraaxial Metastasis
Lesion
Frequent Location
Table
Hoer
Ww
SI. hyper
Variable Variable
Hyper
Iso or hyper
hyper, hyperintense.
Is0
HYPE HYPE
HYPE Iso or hypo
Iso or sl. hypo
Hypo
Proton Density T2
Hwr
Hoer
HvPer
Variable Variable
Hoer
Iso or hyper
Intensity Compared With Brain
Features
Tl-W
Imaging
Yes
No Yes
Yes Yes
Yes
Yes
CE. CT, or MR
No
No No
Some Rare
Yes
Yes
Necrotic or Cystic
Yes
No Some
Some Yes
Yes
Some
Vascularity
ADULT
INFRATENTORIAL
173
TUMORS
sis includes vascular metastases, such as those due to renal cell carcinoma. SUMMARY
In adults, the combination of clinical presentation and imaging results (Table 1) permit predic-
tion of the histology of a posterior fossa tumor. MR imaging, which uses various pulse sequences and contrast material is the most sensitive and precise technique for the evaluation of patients who have symptoms referrable to the posterior fossa.
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14. Bebin J: Pathophysiology of acoustic tumors, in House WF, Leutje CM (eds): Acoustic Tumors. Baltimore, MD, University Park, 1979 15. Haughton VM, Rimm AA, Czervionke LF, et al: Sensitivity of Gd-DTPA-enhanced MR imaging of benign extraaxial tumors. Radiology 166:829-833, 1988 16. Daniels DL, Czervionke LF, Millen SJ, et al: MR imaging of facial nerve enhancement in Bell palsy or after temporal bone surgery. Radiology 171:807-809, 1989 17. Gomori JM, Grossman RI, Bilaniuk LT, et al: Highfield MR imaging of superficial siderosis of the central nervous system. J Comput Assist Tomogr 9:972-975, 1985 18. Elster AD, Challa VR, Gilbert TH, et al: Meningiomas: MR and histopathologic features. Radiology 170:857-862, 1989 19. Spagnoli MV, Goldberg HI, Grossman RI, et al: Intracranial meningiomas: High-field MR imaging. Radiology 161:369-376,198O 20. Latack JT, Kartush JM, Kemink JL, et al: Epidermoidomas of the cerebellopontine angle and temporal bone: CT and MR aspects. Radiology 157:361-366, 1985 21. Zimmerman RA, Bilaniuk LT: Cranial CT of epidermoid and congenital fatty tumors of maldevelopmental origin. J Comput Tomogr 3:40-50, 1979 22. Tampieri D, Melanson D, Ethier R: MR imaging of epidermoid cysts. AJNR 10:351-356, 1989 23. Brown RV, Sage MR, Brophy BP: CT and MR findings in patients with chordomas of the petrous apex. AJNR 11:121-124, 1990 24. Oot RF, Melville GE, New PFJ, et al: The role of MR and CT in evaluating clival chordomas and chondrosarcomas. AJNR9:715-723, 1988 25. Batsakis JG: Tumors of the Head and Neck (ed 2). Baltimore, MD, Williams & Wilkins, 1982 26. Vogl T, Bruning R, Schedel H, et al: Paragangliomas of the jugular bulb and carotid body: MR imaging with short sequences and Gd-DTPA enhancement. AJNR 10:823-827, 1989
