Neuroimaging

Brain

“Text Syllabus”

Brain
Basic Principles of MR Imaging
Congenital Disorders of the Brain
Infectious and Inflammatory Disorders
Supratentorial Brain Tumors
Brain Stem and Posterior Fossa
Sella and Parasellar Region
Head Trauma
Degenerative & Metabolic Disorders
Stroke and Cerebral Ischemia
Aneurysms and Subarachnoid Hemorrhage
Intracranial Hemorrhage and AV Malformations
White Matter Diseases
The Temporal Lobe and Limbic System
MR Angiography: CNS Applications
Proton MR Spectroscopy
Non-Traumatic CNS Emergencies
Orbit and ENT
Imaging the Eye and Orbit
The Temporal Bone: Anatomy & Pathology
Deep Spaces, Paranasal Sinuses, & Nasopharynx
Infections of the Sinuses and Face
Lower Face and Salivary Glands
Imaging Cervical Lymph Nodes
Neck, Larynx, and Thyroid
Spine
Pediatric Spinal Anomalies
Spinal Cord and Intradural Disease
Degenerative Spine Disease
Vertebral and Paravertebral Abnormalities
High-Field Spine MRI

INFECTIOUS AND INFLAMMATORY DISORDERS

John R. Hesselink, MD, FACR

Inflammatory diseases of the brain include abscess, meningitis, encephalitis and vasculitis. The brain is protected from invading infectious agents by the calvarium, dura and blood- brain barrier. Moreover, the cerebral tissue itself is relatively resistant to infection. Most pyogenic infections are hematogenous and related to septicemia and endocarditis. Direct extension from an infected paranasal sinus or middle ear/mastoid is less common than in the pre-antibiotic era. Fungal infections are less common than bacterial infections, but are taking on more importance in AIDS patients and those immunocompromised by way of chemotherapy, neoplasia, or immunosuppressive therapy for organ transplantation. The most important viral infections of the central nervous system from an imaging point of view are aseptic meningitis, encephalitis, and progressive multifocal leukoencephalopathy (PML). Herpes simplex is responsible for a fulminant viral encephalitis, and both the human immunodeficiency virus (HIV) and cytomegalovirus (CMV) produce a white matter encephalitis associated with the AIDS epidemic. ^,

ABSCESS

Bacterial

Brain abscesses may be related to infections of the paranasal sinuses, mastoids, middle ears as well as hematogenous seeding, but in 20% of cases a source is not discovered. Very rarely an abscess is secondary to meningitis. In children, more than 60% of cerebral abscesses are associated with congenital heart disease and right to left shunts. Presenting symptoms of a cerebral abscess include headache, drowsiness, confusion, seizures and focal neurologic deficits. Fever and leukocytosis are common during the invasive phase of a cerebral abscess but may resolve as the abscess becomes encapsulated. Organisms most frequently cultured from brain abscesses in otherwise immunocompetent individuals are staphylococcus and streptococcus.

When the brain is inoculated with a pathogen, a local cerebritis develops. Pathologically, an area of cerebritis consists of vascular congestion, petechial hemorrhage and brain edema. The infection goes through a stage of cerebral softening, followed by liquefaction and central cavitation. With time, the central necrotic areas become confluent and are encapsulated after one to two weeks. Edema, a prominent feature of cerebral abscess, may actually subside after the capsule forms.

In the cerebritis stage, MR reveals high signal intensity on T2-weighted images, both centrally from inflammation and peripherally from edema. Areas of low signal are variably imaged on T1-weighted scans. As the progression to abscess ensues there is further prolongation of T1 and T2 centrally. The capsule becomes highlighted as a relatively isointense structure containing and surrounded by low signal on T1- weighted images, and high signal on T2-weighted images. Mottled areas of enhancement are seen with gadolinium-enhanced MR during the cerebritis stage, with an enhancing rim developing as the abscess matures. The enhancing rim may appear late in the cerebritis stage, prior to actual central necrosis. In some instances, the central area of necrosis has also enhanced on delayed scans, but not as commonly as is seen in necrotic tumors. ^,

Advanced neuroimaging techniques, such as MR spectroscopy (MRS) and diffusion-weighted imaging (DWI) , have markedly improved the specificity of MRI for distinguishing bacteria abscess from other infections and from cystic and necrotic tumors. MRS reveals metabolites of bacterial origin, including acetate, lactate, succinate, cytosolic acid, and amino acids (alanine, valine, leucine). The spectral pattern of cystic or necrotic brain tumors is quite different and normally contains elevated choline and decreased N-acetyl-aspartate (NAA), with variable amounts of lactate and lipids. DWI shows restricted diffusion and high signal intensity in bacterial abscesses. The presence of pus within the abscess cavity, which consists of numerous leukocytes and proteinaceous fluid with high viscosity, accounts for the restricted diffusion and high signal intensity on DWI and low ADC values. In contrast, the cystic or necrotic portions of brain tumors typically are less cellular and have less viscous fluid consistency. As a result, tumors show low signal intensity on DWI and higher ADC values.

Fungal

Fungal organisms can start as a meningitis or cerebral abscess, or can invade directly from an extracranial compartment. As mentioned above, fungal infections are primarily found in immuno compromised hosts. In immunocompetent patients, fungal abscesses tend to evolve more slowly than bacterial abscesses, but that is not the case in patients with deficient immunity. In general, there are no specific MR imaging features to distinguish the infecting agent.

Coccidioidomycosis is endemic to the central valley regions of California and desert areas of the southwestern United States. Infection occurs by inhalation of dust from soil usually heavily infected with arthrospores. Primary coccidioidomycosis, a pulmonary infection, is followed by dissemination in only about 0.2% of immunocompetent patients. Central nervous system involvement most often represents a meningitis, but cerebral abscess and granuloma formation can also occur.

Mucormycosis is seen most often in patients with poorly controlled diabetes. It starts as a necrotizing vasculitis of the nose and sinuses, and spreads by direct invasion of adjacent facial compartments. Extension into the intracranial cavity occurs through the cribriform plate, superior orbital fissure, and basal foramina or indirectly via involvement of vascular structures. Once within the intracranial cavity, it produces a purulent meningitis, cerebral infarction from arterial occlusion, and acute cerebritis due to direct invasion of the olfactory tracts and inferior frontal and temporal lobes. Cranial nerve involvement and cavernous sinus thrombosis are common. The MR findings reflect the observed pathologic changes. Regions of meningeal and cerebral inflammation are hyperintense on T2 and proton density weighted MR images. Infarction and edema account for additional high signal parenchymal abnormalities. Gadolinium-enhanced T1-weighted scans show enhancement of the basal meningeal inflammation, as well as the adjacent cerebral involvement. Coronal scans are especially helpful to display the relationships of the meningeal process to the brain and adjacent extracranial compartments.

Aspergillosis is an aggressive opportunistic fungal infection. The organism gains entrance with inhalation of infected grains or dusts and results in primarily a pulmonary infection. Pathologic changes include a combination of suppuration and granulomas. Dissemination to the CNS may start as a basal meningitis, but the organism readily invades vascular structures and extends into the brain parenchyma.

Cysticercosis

Neurocysticercosis is the most frequently encountered parasitic infestation of the CNS. Originally endemic in underdeveloped countries, predominantly Latin America, Africa, Asia and some portions of eastern Europe, it is becoming increasingly frequent in North America in immigrant populations. Humans become accidental hosts for the larval stage of Taenia Solium, the pork tapeworm, by ingesting contaminated material. The eggs hatch in the stomach and larvae burrow through the gut wall and become distributed by the circulatory system. There is a predilection for involvement of the brain. Patients most often present with seizures, elevated intracranial pressure, focal neurologic abnormalities and altered mental status. Asymptomatic infections are common.

Four forms of neurocysticercosis are described: meningeal, parenchymal, ventricular and mixed. In all locations, death of the larva provokes a more intense inflammatory response, and in the case of an intraventricular lesion may lead to ependymitis. Parenchymal lesions consist of small cysts, large cysts and calcified lesions. Small (approximately 1.5 cm. in diameter) cysts may have a central area of relatively shorter T1 (isointense or hyperintense to cortex) and are uniformly hyperintense on T2-weighted images. Large (4-7 cm) cysts are usually multiloculated, adjacent to the subarachnoid space and may contain a mural nodule. The presence of a mural nodule or a T2-hypointense rim in encapsulated lesions may correlate with larval death. Visualization of calcified lesions has been variable with MR; overall there is an advantage for CT in this regard. Sometimes, calcified lesions are surrounded by edema, making them more conspicuous on MR. Basal cistern lesions can be difficult to identify but have been visualized as areas of intermediate signal intensity on T1-weighted images. Intraventricular cysticercosis results in deformable and mobile cysts that may cause intermittent hydrocephalus.

MENINGITIS

Bacterial

Bacterial meningitis is an infection of the pia and arachnoid and adjacent cerebrospinal fluid. The outer arachnoid serves as a barrier to the spread of infection, but involvement of the subdural space can occur, resulting in a subdural empyema. This complication is more common in children than adults. The most common organisms involved are Hemophilus influenza, Neisseria meningitides (Meningococcus) and Streptococcus pneumoniae. Patients present with fever, headache, seizures, altered consciousness and neck stiffness. The overall mortality rate ranges from 5 to 15% for H. influenza and meningococcal meningitis to as high as 30% with streptococcal meningitis. In addition, persistent neurologic deficits are found in 10% of children after H. influenza meningitis and in 30% of patients with streptococcal meningitis.

The ability of nonenhanced MR to image meningitis is extremely limited, and the majority of cases are normal or have mild hydrocephalus. In severe cases, the basal cisterns may be completely obliterated, with high signal intensity replacing the normal CSF signal on proton density images. Intermediate signal intensity may be seen in the basal cisterns on T1-weighted images in these cases. Meningeal enhancement often is not present, unless a chronic infection develops. Infection within the ventricles, either from direct extension from a shunt or abscess or progression of meningitis, may lead to ependymitis, resulting in hyperintensity outlining the ventricles on T2- weighted images and enhancement of the ependyma on T1-weighted images with gadolinium. Subdural empyemas are better seen with MR than with CT, and the signal characteristics of the exudate in subdural empyema (higher signal than CSF) helps to differentiate it from benign extra-axial collections.

Tuberculosis

Tuberculous meningitis remains an important disease, becoming more common as an infectious agent in AIDS patients. As a rule, the evolution is less rapid than in pyogenic infections. Vasculitis and cerebral infarction, caused by inflammatory changes in the basal cisterns, are more prevalent. The MR features of tuberculous meningitis are similar to the bacterial agents, but the chronic inflammation induces thick granulation tissue that produces a more striking enhancement pattern. Actual intracranial tuberculomas are rare in the United States. Mature tuberculomas are T2 hypointense. Central necrosis in some lesions results in a T2 bright core with a low signal intensity rim.

Sarcoidosis

Sarcoidosis is a granulomatous disease of unknown etiology. In approximately 5% of cases, the CNS is involved as a granulomatous infiltration of the meninges and underlying parenchyma, most notably at the base of the brain. It may also affect cranial or peripheral nerves as isolated disease. Cranial nerve palsies, chronic meningitis and hypothalamic- pituitary dysfunction are frequent manifestations.

MR is well suited to imaging the focal pituitary/hypothalamic lesions and white matter lesions that have been noted in these patients. The basal cisterns may enhance in patients with meningeal sarcoidosis, but a nodular pattern usually distinguishes it from the infectious varieties. A particularly interesting form of meningeal sarcoid results in thick meningeal plaques, often over the convexities. These may mimic meningiomas in that they remain isointense or hypointense relative to cortex on T2- as well as T1-weighted images. ^,

ENCEPHALITIS

Encephalitis refers to a diffuse parenchymal inflammation of the brain. Acute encephalitis of the non-herpetic type presents with signs and symptoms similar to meningitis but with the added features of any combination of convulsions, delirium, altered consciousness, aphasia, hemiparesis, ataxia, ocular palsies and facial weakness. The major causative agents are arthropod-borne arboviruses (Eastern and Western equine encephalitis, St. Louis encephalitis, California virus encephalitis). Eastern equine encephalitis is the most serious but fortunately also the least frequent of the arbovirus infections. The enteroviruses, such as coxsackie-virus and echoviruses, can produce a meningoencephalitis, but a more benign aseptic meningitis is more common with these organisms. MR reveals hyperintensity on T2-weighted scans within the cortical areas of involvement, associated with subcortical edema and mass effect.

Herpes Simplex

Herpes simplex is the commonest and gravest form of acute encephalitis with a 30-70% fatality rate and an equally high morbidity rate. It is almost always caused by Type 1 virus except in neonates where Type 2 predominates. Symptoms may reflect the propensity to involve the inferomedial frontal and temporal lobes- hallucinations, seizures, personality changes and aphasia. MR has demonstrated positive findings in viral encephalitis as soon as 2 days after symptoms, more quickly and definitively than CT. Early involvement of the limbic system and temporal lobes is characteristic of herpes simplex encephalitis. The cortical abnormalities are first noted as ill-defined areas of high signal on T2-weighted scans, usually beginning unilaterally but progressing to become bilateral. Edema, mass effect and gyral enhancement may also be present. Since MR is more sensitive than CT for detecting these early changes of encephalitis, hopefully it will improve the prognosis of this devastating disease.

Lyme Disease

Borrelia burgdorferi, the cause of Lyme disease, is a spirochete associated with chronic meningitis or meningoencephalitis, and cranial or peripheral neuropathy. It is transmitted by ticks and is prevalent in the eastern United States. Meningitis occurs in the earlier stages of disease from direct spirochetal invasion of the CSF. Multifocal white matter lesions from the encephalitis can mimic multiple sclerosis. Definitive diagnosis requires serologic studies.

Post-Infectious Encephalitis

Post-infectious encephalitis, also known as acute disseminated encephalomyelitis (ADEM), is an acute demyelinating disease thought to represent an immune-mediated complication of infection, rather than a direct viral infection of the CNS. The clinical presentation is one of confusion, seizures, headaches and fevers. Ataxia may occur. Spinal cord involvement may lead to paraplegia or quadriplegia. The most common viruses implicated are measles and chicken pox. It occasionally is seen after vaccination for rabies or smallpox or following nondescript respiratory infections. MR demonstrates lesions in the white matter of the cerebrum, cerebellum and brainstem, often while CT is normal or non-diagnostic. The lesions may be patchy and involve the deep and subcortical white matter. Involvement of the deep gray matter has also been reported.

CONGENITAL INFECTIONS

Congenital infections refer to maternally transmitted infections, which are most frequently caused by the group of TORCH pathogens, which include Toxoplasma, Others (Listeria, Treponema), Rubella, Cytomegalovirus, and Herpes simplex type 2. Nowadays, maybe another “H” should be added to emphasize the common occurrence of HIV in this subgroup of CNS infections. Congenital infections of the brain may produce diffuse, parenchymal inflammation with some unique characteristics, such as microcephaly, brain atrophy, hydrocephalus, neuronal migrational anomalies and cerebral calcifications. The degree of the destructive brain process and the resultant developmental abnormalities depend on the timing of the infection. The earlier in gestation the CNS involvement occurs, the more profound the brain destruction will be. In cases of congenital infections, where the prerequisite is involvement of the mother, even in a subclinical form, the causative agents may reach the fetus, either during the gestation via a hematogenous – transplacental route, or during the birth as the fetus passes through the infected birth canal. ^,

Toxoplasmosis

Toxoplasmosis is caused by the parasite Toxoplasma gondii, which is typically passed hematogenously through the placenta to the fetus. There is a large percentage of the population, approaching 50%, which has been infected by the parasite sometime in their life, but congenital toxoplasmosis occurs only when the mother becomes infected during pregnancy. Infected fetuses have a high incidence (almost 50%) of CNS involvement. Early infection before 20 weeks of pregnancy is associated with severe, persistent neurologic abnormalities, whereas late infection after 30 weeks is rarely associated with deficits. Neuroimaging of congenital toxoplasmosis may reveal a whole spectrum of findings such as intracranial calcifications, hydrocephalus, brain atrophy, microcephaly and neuronal migrational anomalies.

Cytomegalovirus

Cytomegalovirus (CMV) is a member of the herpesvirus family, which subclinically infects nearly all the population at some time in their life and is the most frequent cause of a congenital viral infection. Congenital infection occurs after primary or secondary (reactivation) maternal infection, and the virus reaches the fetus via the transplacental route. CNS involvement is a very important manifestation of the disease, and as with toxoplasmosis, earlier infection results in poorer outcome with more severe and persistent neurologic sequelae.

CMV produces a diffuse encephalitic infectious process, which results in multifocal destructive changes in the brain that lead to calcifications and microcephaly. The immature cells in the germinal matrix region are the first involved areas in the brain. Necrosis and calcifications of those areas explain the predilection for thick or nodular calcifications in the periventricular area. Intracranial calcifications may also be found in the cortical and subcortical region, as well as in the basal ganglia, so differentiation between congenital infection from CMV or toxoplasmosis is not certain based on imaging criteria alone.

Rubella

Due to maternal screening methods and systematic immunization, congenital rubella has become quite rare in the developed countries. Rubella virus is transmitted transplacentally during the primary maternal infection, and as in the other congenital infections, the earlier in gestation the infection occurs, the more severe and extensive is the brain damage. Clinically, the infected neonates have multi-system involvement with various abnormalities of the heart, eyes, and CNS. Brain involvement typically consists of a meningo-encephalitis that may cause brain atrophy, intracranial calcifications (not as common as in the other congenital infections), microcephaly, delayed myelination, and vasculitis.

SUPRATENTORIAL BRAIN TUMORS

John R. Hesselink, MD, FACR and Richard J. Hicks, MD

In the diagnostic work-up of intracranial tumors, the primary goals of the imaging studies are to detect the abnormality, localize and determine its extent, characterize the lesion, and provide a list of differential diagnoses or, if possible, the specific diagnosis. Correlative studies have proved that MR is more sensitive than CT for detecting intracranial masses. Moreover, the multiplanar capability of MR is very helpful to determine the anatomic site of origin of lesions and to demarcate extension into adjacent compartments and brain structures. The superior contrast resolution of MR displays the different components of lesions more clearly. MR can assess the vascularity of lesions without contrast infusion. On the other hand, CT detects calcification far better than MR, a useful finding for differential diagnosis. Gradient-echo techniques improve MR detection of calcification by accentuating the diamagnetic susceptibility properties of calcium salts, but the observed low signal on T2-weighted images is nonspecific, in that any accompanying paramagnetic ions would produce the same effect.

Contrast enhancement with gadolinium increases both the sensitivity and specificity of MR. Gadolinium is a blood-brain barrier (BBB) contrast agent like iodinated agents for CT. It does not cross the intact BBB, but when the BBB is absent or deficient, gadolinium enters the interstitial space to produce enhancement (increased signal) on T1-weighted images. All the collective knowledge learned from contrast-enhanced CT can be applied directly to the gadolinium-enhanced MR images. Although the enhancement patterns are not tumor specific, the additional information is often helpful for diagnosis. Lesions can be classified as homogeneous or heterogeneous, and necrotic and cystic components are seen more clearly. The margins of enhancement provide a gross measure of tumor extension. Contrast MR is particularly valuable for extra-axial tumors because they tend to be isointense to the brain on plain scan.

CEREBRAL GLIOMAS

Gliomas are malignant tumors of the glial cells of the brain and account for 30-40% of all primary intracranial tumors. They occur predominantly in the cerebral hemispheres, but the brain stem and cerebellum are frequent locations in children, and they are also found in the spinal cord. The peak incidence is during middle adult life, when patients present with seizures or symptoms related to the location of the gliomas and the brain structures involved.

Astrocytomas are graded according to their histologic appearance. Grade 1 astrocytomas have well-differentiated astrocytes and well-defined margins. The clinical course often proceeds over many years and complete cures are possible. The pilocytic variant is a low-grade tumor with a distinct capsule that is commonly found in children. The giant cell astrocytoma is a specialized tumor that develops from pre-existing hamartomas in patients with tuberous sclerosis. Grade 2 astrocytomas are well-differentiated but diffusely infiltrating tumors. The fibrillary type is most common, and although initially benign, they may evolve into a higher grade tumor over time. This changing character of gliomas makes histological classification difficult from sample biopsies, because different parts of the tumor often exhibit varying degrees of malignancy. The higher grade astrocytomas are very cellular and pleomorphic. Anaplastic astrocytomas (Grade 3) are very aggressive tumors, readily infiltrate adjacent brain structures, and have a uniformly poor prognosis. Glioblastoma multiforme (Grade 4) has the added histologic features of endothelial proliferation and necrosis. Multicentric foci of tumor may be seen in 4 to 6% of glioblastomas. Gliomatosis cerebri is an unusual condition with diffuse contiguous involvement of multiple lobes of the brain.

Oligodendrogliomas are the most benign of the gliomas. Calcification is common, and they occur predominantly in the frontal lobes. The mixed neuronal and glial tumors are found mostly in children and young adults. They are slow-growing and are found predominantly in the temporal lobes and around the third ventricle. Intratumoral cysts and calcification are common.

The common signal characteristics of intra-axial tumors include high signal intensity on T2-weighted images and low signal on T1-weighted images, unless fat or hemorrhage is present. Fat and subacute hemorrhage (methemoglobin) exhibit high signal on T1-weighted images, and acute hemorrhage (deoxyhemoglobin) and chronic hemorrhage (hemosiderin/ferritin) show low signal intensity on T2-weighted scans. Gliomas have poorly defined margins on plain MR. They infiltrate along white matter fiber tracts, and the deeper lesions have a propensity to extend across the corpus callosum into the opposite hemisphere. They are often quite large by the time of clinical presentation.

The higher grade gliomas, particularly glioblastomas, appear heterogeneous due to central necrosis with cellular debris, fluid, and hemorrhage. Peritumoral edema and mass effect are common features. Following injection of gadolinium, T1-weighted images show irregular ring enhancement, with nodularity and nonenhancing necrotic foci. As mentioned above, gliomas are infiltrative lesions, and microscopic fingers of tumor usually extend beyond the margin of enhancement. Enhanced scans are particularly helpful to outline subependymal spread of tumor along a ventricular surface, as well as leptomeningeal involvement. Although highly malignant, anaplastic astrocytomas may or may not exhibit breakdown of the blood-brain barrier. In general, the presence or lack of enhancement alone is not helpful in grading astrocytomas.

The lower grade astrocytomas tend to be more homogeneous without central necrosis. Large cystic components may be present. The cysts have smooth walls, and the fluid is of uniform signal, to distinguish them from necrosis. Enhancement is variable, depending on the integrity of the blood-brain barrier.

Perfusion imaging has shown promise as a technique for determining the grade of intracranial mass lesions. Perfusion imaging relies on a first-pass susceptibility-related signal loss on T2*-weighted images, from which relative cerebral blood flow and volume can be calculated. Several studies have shown a correlation between relative cerebral blood volume and tumor grade, likely due to the relationship of blood volume to vascular proliferation in high-grade gliomas.

MR Spectroscopy

MR spectroscopy provides a measure of brain chemistry and can help characterize tumors and and grade the degree of malignancy. As a general rule, as malignancy increases, NAA and creatine decrease, and choline, lactate, and lipids increase. NAA decreases as tumor growth displaces or destroys neurons. Very malignant tumors have high metabolic activity and deplete the energy stores, resulting in reduced creatine. Very hypercellular tumors with rapid growth elevate choline. Lipids are found in necrotic portions of tumors, and lactate appears when tumors outgrow their blood supply and start utilizing anaerobic glycolysis. To get an accurate assessment of the tumor chemistry, the spectroscopic voxel should be placed over an enhancing region of the tumor, avoiding areas of necrosis, hemorrhage, calcification, or cysts.

Multi-voxel spectroscopy is best to detect infiltration of malignant cells beyond the enhancing margins of tumors. Particularly in the case of cerebral glioma, elevated choline levels are frequently detected in edematous regions of the brain outside the enhancing mass. Finally, MRS can direct the surgeon to the most metabolically active part of the tumor for biopsy to obtain accurate grading of the malignancy.

A common clinical problem is distinguishing tumor recurrence from radiation effects several months following surgery and radiation therapy. Elevated choline is a marker for recurrent tumor. Radiation change generally exhibits low NAA, creatine, and choline on spectroscopy. If radiation necrosis is present, the spectrum may reveal elevated lipids and lactate.

MRS cannot always distinguish primary and secondary tumors of the brain from one another. As mentioned above, one key feature of gliomas is elevated choline beyond the margin of enhancement due to infiltration of tumor into the adjacent brain tissue. Most non-glial tumors have little or no NAA. Elevated alanine at 1.48 ppm is a signature of meningiomas. They also have no NAA, very low creatine, and elevated glutamates.

LYMPHOMA

Primary malignant lymphoma is a non-Hodgkin’s lymphoma that occurs in the brain in the absence of systemic involvement. These tumors are highly cellular and grow rapidly. Favorite sites include the deeper parts of the frontal and parietal lobes, basal ganglia, and hypothalamus. Most occur in patients who are immunocompromised secondary to chemotherapy or acquired immunodeficiency syndrome (AIDS) or in organ transplant recipients who are on immunosuppressant drugs. Cerebral lymphomas are very radiosensitive and respond dramatically to steroid therapy.

Lymphomas typically appear as homogeneous, slightly high signal to isointense masses deep within the brain on T2-weighted images. The observed mild T2 prolongation is probably related to dense cell packing within these tumors, leaving relatively little interstitial space for accumulation of water. They are frequently found in close proximity to the corpus callosum and have a propensity to extend across the corpus callosum into the opposite hemisphere, a feature that mimics glioblastoma. Multiple lesions are present in as many as 50%. Despite their rapid growth, central necrosis is uncommon. They are associated with only a mild or moderate amount of peritumoral edema. By time of presentation they can be quite large and yet produce relatively little mass effect, a feature that sets lymphoma apart from glioblastoma and metastases. Intratumoral cysts and hemorrhage are unusual. Most lymphomas show bright homogeneous contrast enhancement.

The pattern is modified somewhat in AIDS patients. Multiplicity seems to be more common. Moreover, lymphomas exhibit more aggressive behavior and readily outgrow their blood supply. As a result, central necrosis and ring enhancement are often seen in lymphomatous masses in AIDS patients. On MR spectroscopy, lymphomas exhibit elevated choline little or no NAA.

METASTATIC DISEASE

Metastases to the brain occur by hematogenous spread, and multiple lesions are found in 70% of cases. The most common primaries are lung, breast, and melanoma, in that order of frequency. Other potential sources include the gastrointestinal tract, kidney, and thyroid. Metastases from other locations are uncommon. Clinical symptoms are nonspecific and no different from primary brain tumors. If a parenchymal lesion breaks through the cortex, tumor can extend and seed along the leptomeninges.

Metastatic lesions can be found anywhere in the brain but a favorite site is near the brain surface at the corticomedullary junction of both the cerebrum and cerebellum. They are hyperintense on plain T2-weighted images. Areas of necrosis are prevalent in the larger lesions, accounting for their heterogeneous internal texture. Peritumoral edema is a prominent feature, but multiplicity is the most helpful sign to suggest metastatic disease as the likely diagnosis. Correlative studies have shown MR to be more sensitive than CT for detecting metastases, particularly lesions near the base of the brain and in the posterior fossa. One limitation of plain MR is the frequency of periventricular white matter hyperintensities found in the same older age group at risk for metastatic disease.

Gadolinium enhanced MR has resulted in improved delineation of metastatic disease compared with nonenhanced scans. Moderate to marked enhancement is the rule, nodular for the smaller lesions and ringlike with central nonenhancing areas for the larger ones. Controlled clinical trials have also shown that contrast-enhanced MR is more sensitive than both plain MR and contrast-enhanced CT for detecting cerebral metastases. In patients with a known primary, T1-weighted enhanced MR is probably sufficient to screen the brain for metastatic disease.

Hemorrhage is present in 3 to 14% of brain metastases, mainly in melanoma, choriocarcinoma, renal cell carcinoma, bronchogenic carcinoma, and thyroid carcinoma. The presence of nonhemorrhagic tissue and pronounced surrounding vasogenic edema are clues to the underlying neoplasm.

Metastatic melanoma has been a topic of special interest in the MR literature because of the presence of paramagnetic, stable free radicals within melanin. The MR appearance is variable depending on the histology of the melanoma and the components of hemoglobin. Most are hyperintense to white matter on T1-weighted scans and hypointense on T2-weighted scans. Atlas and coworkers observed three distinct signal intensity patterns. Nonhemorrhagic melanotic melanoma was markedly hyperintense on T1-weighted images and isointense or mildly hypointense on T2-weighted images. Nonhemorrhagic amelanotic melanoma appeared isointense or slightly hypointense on T1-weighted scans and isointense or slightly hyperintense on T2-weighted scans. The signal pattern for hemorrhagic melanoma was variable depending on the components of hemoglobin. Some uncertainty remains as to whether the predominant effect on signal intensity within melanomas is due to stable free radicals, chelated metal ions, or hemoglobin.

INTRAVENTRICULAR TUMORS

The intraventricular location is unique in that many of the tumor types are more commonly associated with extra-axial locations. Patients often present with obstructive hydrocephalus. Most intraventricular tumors are relatively benign and have well-defined margins. As they grow, the tumors expand the ventricle of origin. With malignant degeneration, extension into the brain parenchymal occurs. The primary blood supply to intraventricular lesions is derived from the choroidal arteries.

MENINGIOMA

Meningiomas account for 15% of all intracranial tumors and are the most common extra-axial tumor. They originate from the dura or arachnoid and occur in middle-aged adults. Women are affected twice as often as men. Meningiomas are well-differentiated, benign, and encapsulated lesions that indent the brain as they enlarge. They grow slowly and may be present for many years before producing symptoms. The histologic picture shows cells of uniform size that tend to form whorls or psammoma bodies.

The parasagittal region is the most frequent site for meningiomas, followed by the sphenoid wings, parasellar region, olfactory groove, cerebello-pontine angle, and rarely the intraventricular region. Meningiomas often induce an osteoblastic reaction in the adjacent bone, resulting in a characteristic focal hyperostosis. They are also hypervascular, receiving their blood supply predominantly from dural vessels.

Most meningiomas are isointense with cortex on T1- and T2-weighted images. A heterogeneous internal texture is found in all but the smallest meningiomas. The mottled pattern is likely due to a combination of flow void from vascularity, focal calcification, small cystic foci, and entrapped CSF spaces. Hemorrhage is not a common feature. An interface between the brain and lesion is often present, representing a CSF cleft, a vascular rim, or a dural margin. MR has special advantages over CT in assessing venous sinus involvement and arterial encasement. Occasionally, a densely calcified meningioma is encountered that is distinctly hypointense on all pulse sequences.

Meningiomas show intense enhancement with gadolinium and are sharply circumscribed. They have a characteristic broad base of attachment against a dural surface. Associated hyperostosis may result in thickening of low signal bone as well as diminished signal from the diploic spaces. Although meningiomas are not invasive, vasogenic edema is present in the adjacent brain in 30% of cases. Contrast scans are especially helpful for imaging the en plaque meningiomas that occur at the skull base. MR spectroscopy shows elevated alanine and glutamates, no NAA, and markedly decreased creatine.

PINEAL REGION TUMORS

Tumors in the pineal region can be classified into three major groups based on their origin: germ cell, pineal parenchyma, and parapineal. Germinoma is the least differentiated of the germ cell group. It occurs in children and young adults and accounts for more than 50% of all pineal region tumors. The other germ cell tumors include embryonal carcinoma, yolk-sac tumor, and choriocarcinoma. Differentiation along three germ layers results in a teratoma. The true pinealomas consist of pineoblastoma and pineocytoma. Pineoblastoma is an embryonal tumor of neuroectoderm, related to neuroblastoma and medulloblastoma, and is found primarily in young children. Pineocytomas are less cellular and exhibit benign behavior. The parapineal lesions include gliomasof the tectum and posterior third ventricle, meningiomas arising within the quadrigeminal cistern, and developmental cysts (epidermoid, dermoid, arachnoid cyst).

The clinical expression of these tumors is usually related to mass effect upon adjacent brain structures. Hydrocephalus secondary to aqueductal obstruction is a common presentation. Compression of the tectum of the midbrain can produce paralysis of upward gaze, the classic Parinaud’s syndrome. Germinomas and gliomas have a propensity to grow into the third ventricle and compress the hypothalamus, resulting in endocrine dysfunction. Dissemination through the CSF pathways is a known complication of pineoblastoma and germinoma.

Pineal germinomas and primary pineal tumors are most often isointense with the brain on T1- and T2-weighted images. A few lesions exhibit long T1 and T2, which may correlate with embryonal cell elements. Despite this relative lack of contrast, with multiplanar imaging plain MR delineates pineal region masses better than CT, showing the relationships of the tumor to the posterior third ventricle, vein of Galen, and aqueduct. These tumors are well defined and enhance to a moderate degree, usually without central necrosis, cystic change, or hemorrhage. Enhanced scans are essential to assess CSF spread of tumor. In young patients with germinoma, the difficulty of visualizing calcium is a disadvantage of MR, as this may be the only evidence of tumor.

Meningiomas can appear very similar on plain scan, but their intense enhancement may set them apart from other lesions. Gliomas infiltrate the tectum and posterior walls of the third ventricle. They tend to be poorly circumscribed and produce symptoms earlier. Edema is not a consistent finding, and enhancement is variable. Larger gliomas in the splenium of the corpus callosum may present as pineal region masses.

Teratomas are of mixed signal intensity, frequently with calcification. They may also have cystic components and fat. Arachnoid cysts, epidermoid and dermoid tumors can usually be distinguished from other pineal region tumors by their increased signal on T2-weighted images.

Pineal cysts were visualized in 4.3% of normal patients in one MR study. These apparently benign lesions are seen best as areas of high signal on intermediate T2-weighted images. They are not associated with hydrocephalus or a pineal mass and are not clinically significant.

BENIGN CYSTIC MASSES

Cystic lesions occur most often in the basal cisterns, a midline location or within the ventricular system. They include arachnoid cyst, dermoid, epidermoid, and neuroepithelial cysts, including colloid cyst. These lesions are interesting in that their MR appearance is quite distinct from solid masses. Their signal characteristics depend to a large extent on the cyst contents, but associated solid components may also have specific features.

Arachnoid Cyst

Arachnoid cysts are CSF-containing cysts that are found in the middle fossa, posterior fossa, suprasellar cistern, or near the vertex. They are benign but slowly grow as they accumulate fluid, compressing normal brain structures. Remodeling of the adjacent skull is an important clue for a benign expansile process.

Arachnoid cysts are smoothly marginated and homogeneous. They are not calcified and do not enhance. The multiplanar capability of MR is particularly helpful in establishing the exact location, and the diagnosis is supported by the cyst fluid being isointense with CSF on all pulse sequences. The cysts may appear higher signal than CSF on intermediate T2-weighted images. The exact reason for this is uncertain, although it may reflect dampening of the CSF pulsations that normally results in signal loss in the ventricles and cisterns. This effect will be less apparent with pulse sequences that incorporate flow compensation techniques.

Epidermoid Cyst

Epidermoid cysts are referred to as “pearly tumors” because of their glistening white appearance at surgery. They arise from epithelial cell rests in the basal cisterns. They are benign and grow slowly along the subarachnoid spaces and into the various crevices found at the base of the brain. Intradural epidermoids are usually quite large with lobulated outer margins and an insinuating pattern of growth. They have a heterogeneous texture and variable signal intensity on MR. Most are slightly higher signal than CSF on both T1 and T2-weighted images. An occasional epidermoid has a very short T1 and appears bright on T1-weighted images. The heterogeneous signal pattern is likely related to varying concentrations of keratin, cholesterol, and water within the cyst, as well as the proportion of cholesterol and keratin in crystalline form. Calcification is sometimes present. Epidermoid tumors do not enhance with contrast.

Dermoid Cyst

Dermoid cysts have both dermal and epidermal derivatives, accounting for their more varied histologic and MR appearance. They are primarily midline lesions, occurring in the pineal and suprasellar regions. Dermoids have some distinctive features on MR. They tend to be heterogeneous owing to the multiple cell types within them. Fatty components are common, producing high signal on T1-weighted images. On axial and sagittal scans, a fat-fluid level may be seen, or a level between fat and matted hair within the cyst. Rupture of a dermoid and leakage of cyst contents into a ventricle or subarachnoid space may produce an ependymitis or meningitis, respectively.

Lipomas are also midline lesions and are often associated with partial or complete agenesis of the corpus callosum. Occasionally, an incidental lipoma will be found in the region of the quadrigeminal plate or cerebellopontine angle.

Colloid Cyst

Colloid cysts originate from primitive neuroepithelium within the roof of the anterior third ventricle. They are positioned just posterior to the foramina of Monro between the columns of the fornix. Histologically, they consist of a thin, fibrous capsule with an epithelial lining. The cysts contain a mucinous fluid with variable amounts of proteinaceous debris, blood components, and desquamated cells.

Colloid cysts are smoothly marginated spherical lesions without surrounding brain reaction. Two signal patterns have been reported on MR scans and correlated with their CT features. Those that are low density on CT are isointense on T1-weighted images and hyperintense on T2-weighted images, probably indicating a fluid composition similar to CSF. Most colloid cysts are isodense or slightly hyperdense on CT. The MR counterpart is a high signal capsule and a hypointense center on T2-weighted images. The signal characteristics of the fluid depend on the protein content of the cyst fluid and is similar to that observed in sinonasal secretions.

Dilatation of the lateral ventricles is a common finding, and the enlargement may be unequal owing to asymmetric positioning of the cyst at the foramina of Monro. The expanding cyst also enlarges the anterior third ventricle, but the posterior third, aqueduct, and fourth ventricle should be normal. Following contrast infusion, colloid cysts may show ring enhancement, due to either enhancement of the cyst wall or choroid plexus draped around the cyst.

Herpes Simplex Virus

Herpes simplex virus (HSV) is a DNA virus and a member of the herpesvirus family, which has two different serotypes, herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2). They produce the most important acute viral encephalitis in the neonate. In over 80% of cases of herpes simplex encephalitis, HSV-2 is the causative agent. The infection is most commonly acquired during delivery through an infected birth canal, although hematogenous transmission through the placenta does occur. An explanation for the observed rarity of early transplacental infection is that it causes severe destruction in the fetus, resulting in spontaneous abortions rather than maldevelopment of the CNS. However, if infants survive the early hematogenous infection, the devastating effect of the panencephalitis results in findings similar to those of other placentally transmitted infections, such as microcephaly, cerebral atrophy and necrosis, and intracranial calcifications, but to a greater degree and with more severe neurological sequelae. An important and unique imaging finding in HSV-2 encephalitis is a linear, gyriform cortical pattern of increased attenuation on CT and hyperintensity on T₁-weighted images, overlying abnormal edematous and/or necrotic white matter. The cortical imaging features have been attributed to the presence of microcalcifications or to changes in local vascularity.

Human Immunodeficiency Virus (HIV)

The majority of cases are transmitted hematogenously through the placenta. Infants infected with HIV are asymptomatic at birth, presenting in time with developmental delay and recurrent infections. Later on at about 2-3 years of age, a progressive clinical syndrome evolves, manifested by seizures, motor deficits, acquired microcephaly, and behavioral and cognitive decline. Secondary opportunistic infections or CNS tumors, common in adults infected with HIV, are relatively uncommon in the pediatric age group. Neuroimaging displays cerebral atrophy, primarily central, as well as intracranial calcification in the basal ganglia, evaluated better on CT. MRI demonstrates the white matter abnormalities, such as white matter hypoplasia and delayed myelination, better than CT, as well as the presence of the secondary CNS complications of HIV infection.

AIDS RELATED INFECTIONS

Acquired immunodeficiency syndrome (AIDS) results in neurologic symptoms in approximately 1/3 of patients. The most commonly reported CNS complications include opportunistic fungal, viral and protozoan infections and lymphoma. Direct infection of the CNS with human immunodeficiency virus (HIV) also occurs. More Recently, highly active antiretroviral treatment (HAART) has resulted in changes of the neuropathologic abnormalities, with an overall decreased incidence of opportunistic cerebral infections, while lymphomas and progressive multifocal leukoencephalopathy remain unchanged.

Toxoplasmosis

Toxoplasma gondii, a protozoan, is the most common opportunistic infection in AIDS patients, accounting for between 13.4% and 33% of all CNS complications. The characteristic MR appearance is multiple ring-enhancing lesions located at the cortical-medullary junction, but the basal ganglia and white matter are also frequently involved. The amount of peripheral edema is variable. Earlier in the evolution of the abscesses, the nodule may exhibit more homogeneous enhancement with little mass effect or edema. In general, toxo and fungal abscesses evolve more slowly than bacterial ones, but in immunocompromised individuals they can be quite aggressive. Dual infections are common is AIDS patients. In such cases, invariably one of the pathogens is toxoplasma gondii. Toxoplasmosis may also co-exist with lymphoma. Another confounding fact is that the inflammatory reaction to toxoplasmosis may mimic lymphoma on biopsy or CSF cytology.

Cryptococcosis

Cryptococcosis is the most common CNS fungal infection in AIDS, occurring in 8.7-13% of patients. Cryptococcus neoformans has a peculiar propensity to affect individuals with cell mediated immunity, and it usually produces a meningitis. CSF antibody titers are not always reliable for diagnosis because the immune response in AIDS patients is so variable. Imaging studies may be negative or show only mild ventricular dilatation. Since cerebral atrophy is common in AIDS patients, distinguishing central atrophy from hydrocephalus is not always easy, and sometimes followup studies or correlation with the clinical picture is necessary. The cryptococcal organisms may enter the brain via the VR spaces at the base of the brain. Proliferation of the organisms within the VR spaces produces gelatinous pseudocysts of variable size to give a mottled appearance on imaging studies. Meningeal enhancement is not often present unless a chronic inflammation has developed. A chronic relapsing infection can result in cryptococcal brain abscesses.

Progressive Multifocal Leukoencephalopathy (PML)

PML is a disorder characterized by widespread foci of demyelination caused by reactivation of a latent papovavirus. Most cases occur in the setting of an immunocompromised host secondary to neoplasia, chemotherapy and increasingly, AIDS. The lesions are initially round or oval, becoming larger and more confluent with time. Subcortical white matter may be the first area of involvement with later spread to the deeper white matter. The pattern is often asymmetric. The MR appearance reflects the long T1 and long T2 of the lesions, typically without mass effect. The high T2 signal is related to both demyelination and edema. These lesions do not usually enhance on CT, and experience with MR shows that most do not enhance with gadolinium.

HIV/CMV Encephalitis

Research interests have centered on a subacute encephalitis involving the white matter because of its association with HIV infection of the brain and correlation with a primary AIDS dementia complex. Subacute encephalitis is seen in up to 30% of patients with AIDS, and cytomegalovirus (CMV) may be the causal agent in some cases. The two viruses often coexist in brain specimens taken from AIDS patients. In studies of patients with HIV encephalitis, both CT and MR have been found to be relatively insensitive to the early stages of involvement, manifested pathologically by widespread microscopic microglial nodules with multinucleated giant cells. Later, bilateral patchy to confluent lesions in the white matter develop that are readily visible on MR.

BRAINSTEM AND POSTERIOR FOSSA

John R. Hesselink, MD, FACR and John F. Healy, MD, FACR

The posterior fossa houses the brainstem and cerebellum. The brainstem contains all the cranial nerve nuclei and many efferent and afferent fiber tracts that connect the brain with the rest of the body. The cerebellum is the major organ of coordination for all motor functions, as well as mental activities of the brain.

Many disease processes can occur in the posterior fossa. Tumors can arise from the brain tissue itself, the cranial nerves, the meninges, or the skull. All these structures are also susceptible to infection and inflammation. Vascular disease can lead to infarction or hemorrhage. Finally, the posterior fossa can be invaded by neoplasms and infections from the head and neck regions.

Magnetic resonance (MR) is ideally suited for imaging the posterior fossa. The high contrast resolution and multiplanar capabilities of MR make possible accurate correlation of pathology with the complex anatomy of this region. Gadolinium enhancement is often helpful to identify and characterize lesions.

CRANIAL NERVES

The anatomy of the brainstem and cerebellum is very intricate. Some of the larger nuclei and fiber tracts are visible on MR images. Although others may not be visible, a general knowledge of the locations of major structures is essential to correlate the imaging findings with the clinical picture.

Anatomy

The cranial nerve nuclei are located in the tegmentum of the brainstem, just ventral to the cerebral aqueduct and 4th ventricle. The 3rd nerves (oculomotor) pick up parasympathetic fibers from the Edinger-Westfall nucleus and course ventrally through the substance of the midbrain to exit in the interpeduncular cistern. The cisternal segments continue ventrally between the posterior cerebral and superior cerebellar arteries and enter the cavernous sinuses. The 4th nerves (trochlear) are the only cranial nerves to cross the midline. They course dorsally and cross behind the aqueduct, exit the dorsal midbrain, and travel forward in the ambient cisterns to reach the cavernous sinuses. Other major structures within the midbrain include the pyramidal (corticospinal and corticobulbar) tracts within the cerebral peduncles, the substantia nigra, the red nuclei, the decussation of the superior cerebellar peduncles, and the superior and inferior colliculi of the quadrigeminal plate.

The pons contains the nuclei for the 5th (trigeminal), 6th (abducens), 7th (facial), and the 8th (acoustic) cranial nerves. The 5th nerve enters the mid-portion of the pons ventrolaterally. The spinal tract and nucleus of the 5th nerve extends from the upper pons all the way down into the upper spinal cord. The 6th exists ventrally at the pontomedullary junction. Both the 5th and 6th nerves course through the cavernous sinus. The 7th nerve loops posteriorly around the 6th nerve nucleus and indents the floor of the 4th ventricle (facial colliculus). The 7th and 8th nerves exist the inferior pons inferiolaterally, traverse the cerebellopontine cistern and enter the internal auditory canal. The anterior pons (basis pontis) contains a large number of transverse fibers from the middle cerebellar peduncles and longitudinal, dispersed bundles of the pyramidal tracts.

The medulla contains the remaining cranial nerves. Nerves 9 (glossopharyngeal), 10 (vagus), and 11 (spinal accessory) exist laterally just posterior to the olivary nucleus and course toward the jugular foramen. The 12th cranial nerve (hypoglossal) exists the medulla ventral to the olive and courses ventrally to the hypoglossal canal. The medulla also contains the decussation of the pyramids (corticospinal tracts) ventrally and the inferior cerebellar peduncles posteriorly.

Two other important fiber tracts are the medial longitudinal fasciculus (MLF) and the medial lemniscus. The MLF, which connects the 3rd, 4th, and 6th cranial nerve nuclei, lies in a paramedian position just ventral to the aqueduct and 4th ventricle. The medial lemniscus, the major sensory tract, ascends through the brainstem just ventral to the MLF.

Pathology

Nerve sheath tumors

Tumors of schwann cell origin include schwannoma and neurofibroma. Schwannomas are more common and most arise from the 8th cranial nerve. Neurofibromas are usually associated with neurofibromatosis. Acoustic neuromas originate on the vestibular division of the eighth cranial nerve just within the internal auditory canal. Bilateral lesions are common with NF 2. They usually present in middle-aged adults with a sensorineural hearing loss, but other symptoms include headache, vertigo, tinnitus, unsteady gait, and facial weakness. Large tumors may fill the cerebellopontine angle cistern and compress adjacent brain structures, producing additional symptoms.

Most schwannomas are isointense to the brain on MR images, but some are distinctly hyperintense with T2-weighted sequences. Occasionally, a schwannoma will be hyperintense on T1-weighted images owing to foci of hemorrhage. They may be heterogeneous on T2-weighted images as well, particularly the larger ones, due to necrosis, hemorrhagic components, and occasional calcification. With small intracanalicular tumors, partial voluming effects may result in uneven signal intensity.

Gadolinium causes approximately 50% shortening of the T1 relaxation time of schwannomas, making them appear very bright on T1-weighted images. Those lesions that are heterogeneous on plain scan will likely exhibit heterogeneous enhancement as well.

Meningioma

Meningiomas originate from the dura or arachnoid and occur in middle-aged adults. In the posterior fossa, most meningiomas are found in the cerebellopontine angle. Women are affected twice as often as men. Meningiomas are well-differentiated, benign, and encapsulated lesions that indent the brain as they enlarge. They grow slowly and may be present for many years before producing symptoms. The histologic picture shows cells of uniform size that tend to form whorls or psammoma bodies. They are hypervascular, receiving their blood supply predominantly from dural vessels.

Most meningiomas are isointense with cortex on T1- and T2-weighted images. A heterogeneous internal texture is found in all but the smallest meningiomas. The mottled pattern is likely due to a combination of flow void from vascularity, focal calcification, small cystic foci, and entrapped CSF spaces. Hemorrhage is not a common feature. An interface between the brain and the lesion is often present, representing a CSF cleft, a vascular rim, or a dural margin. MR has special advantages over CT in assessing venous sinus involvement and arterial encasement. Occasionally, a densely calcified meningioma is encountered that is distinctly hypointense on all pulse sequences.

Meningiomas show intense enhancement with gadolinium and are sharply circumscribed. They have a characteristic broad base of attachment against a dural surface. Contrast scans are especially helpful for imaging the en plaque meningiomas that occur at the skull base.

Glomus Jugulare

These tumors arise from paraganglia along the auricular branch of the vagus nerve (nerve of Arnold) in the jugular fossa. They are benign but locally invasive lesions that slowly erode into adjacent areas of the skull base and posterior fossa. Most patients present with pulsatile tinnitus or neurological deficits secondary to involvement of cranial nerves 9, 10 and 11. Multiple tumors occur in about 10% of cases.

Paragangliomas have a unique MR appearance related to their extreme hypervascularity. Serpiginous areas of signal void from blood flow are interspersed among areas of flow-related enhancement and more isointense tumor cells. On T2-weighted images, they are of variable signal intensity but usually hyperintense to the surrounding muscles. MR angiography is helpful in determining involvement of the carotid artery and jugular vein. CT is better for showing subtle bone erosions.

Epidermoid Tumors

Epidermoids are referred to as “pearly tumors” because of their glistening white appearance at surgery. They arise from epithelial cell rests in the basal cisterns. They are benign and grow slowly along the subarachnoid spaces and into the various crevices found at the base of the brain.

Intradural epidermoids are usually quite large with lobulated outer margins and an insinuating pattern of growth. They have a heterogeneous texture and variable signal intensity on MR. Most are slightly higher signal than CSF on both T1 and T2-weighted images. An occasional epidermoid has a very short T1 and appears bright on T1-weighted images. The heterogeneous signal pattern is likely related to varying concentrations of keratin, cholesterol, and water within the cyst, as well as the proportion of cholesterol and keratin in crystalline form. Calcification is sometimes present. Epidermoid tumors do not enhance with contrast. The cystic contents of epidermoids often exhibit restricted diffusion on diffusion-weighted imaging.

Arachnoid Cyst

Arachnoid cysts are benign but slowly grow as they accumulate fluid, compressing normal brain structures. Most are smoothly marginated and homogeneous. They are not calcified and do not enhance. The cyst fluid is usually isointense with CSF on all pulse sequences. The cysts may appear higher signal than CSF on intermediate T2-weighted images due to dampening of the CSF pulsations that normally results in signal loss in the ventricles and cisterns. This effect will be less apparent with pulse sequences that incorporate flow compensation techniques.

Fatty Lesions

Dermoid cysts and lipomas are unusual in the posterior fossa. Dermoids are primarily midline lesions, occurring near the torcula or the quadrigeminal plate. Lipomas are sometimes found in the cerebellopontine angle. Both are characterized by their fatty components on MR imaging.

Leptomeningeal Diseases

Neoplastic and inflammatory diseases can involve the leptomeninges. Leptomeningeal tumor can develop from seeding of a malignant intracranial tumor or via a hematogenous route from an extracranial site. Pediatric tumors that commonly seed the CSF include germinoma, medulloblastoma, and pineoblastoma. In the adult, leptomeningeal carcinomatosis is usually secondary to breast or lung carcinoma. Highly malignant gliomas can also seed the CSF.

Most bacterial meningitis is treated before changes become evident on imaging studies. The most common finding is a communicating hydrocephalus due to obstruction of the CSF pathways in the basal cisterns. The infectious agents that are more likely to show meningeal enhancement are tuberculosis and fungal infections, such as coccidioidomycosis. These organisms induce a vascular granulation tissue that readily enhances with gadolinium or iodinated contrast agents. A secondary vasculitis can also develop, leading to vessel occlusion and brain infarction.

Another inflammatory disease that produces a granulomatous reaction within the basal meninges is sarcoidosis. CNS involvement occurs in about 5% of patients with sarcoidosis. The pattern of meningeal involvement is usually more focal and patchy than with the infectious agents. Vasculitis is less common, but the inflammation can extend along the Virchow-Robin spaces to enter the brain substance.

INTRAAXIAL TUMORS

Except for hemangioblastoma and metastatic disease, the majority of intra-axial posterior fossa tumors occur in children. Cerebellar astrocytoma accounts for 33% of these childhood tumors, medulloblastoma 26%, brain stem glioma 21%, ependymoma 14% and choroid plexus papilloma, only 2%.

Brain Stem Glioma

Most brain stem gliomas are relatively benign initially but frequently evolve to a higher grade. They usually present with a cranial nerve palsy, most often involving the 6th or 7th nerves. The pons is the common location, but they also occur in the medulla and midbrain. These tumors infiltrate the brain stem and induce surrounding vasogenic edema in the brain parenchyma. Since both the tumor and edema are hyperintense on T2-weighted images, tumor margins tend to be indistinct and poorly defined.

Brain stem gliomas are relatively homogeneous masses without much cystic change, necrosis, vascularity or calcification. About 50% of cases will show mild enhancement. As the gliomas grow, they enlarge the brain stem, producing effacement of the basal cisterns, anterior displacement of the basilar artery against the clivus, and compression and posterior bowing of the fourth ventricle. Hydrocephalus is often present. Exophytic growth is a well-known feature of these tumors.

Cerebellar Astrocytoma

Cerebellar astrocytoma is the most common CNS tumor in children. They tend to be lower grade than the supratentorial variety found in adults and are often quite large by time of presentation. The majority are hemispheric in location, a helpful but not absolute criterion to distinguish them from medulloblastoma.

More than 50% of cerebellar astrocytomas are cystic, and the cyst contents often have elevated protein, making them slightly higher signal than CSF but lower signal than brain on T1-weighted images. The solid components are hyperintense to brain on proton density-weighted images. Both solid tumor and cyst are bright on T2-weighted scans. Calcification is occasionally present. Peritumoral edema is not pronounced, and in general, their margins are defined better than in supratentorial gliomas. Cerebellar astrocytomas exhibit nodular or ringlike enhancement. Since these tumors are frequently large, mass effect is a prominent feature. Anterior and lateral displacement of the fourth ventricle is common. Upward herniation of the superior vermis and downward herniation of the cerebellar tonsils can also occur.

Medulloblastoma (and PNET)

The majority of medulloblastomas occur in children between four and eight years old, and males outnumber females three to one. Primitive neuro-ectodermal tumors (PNET) may present at birth or early infancy. Medulloblastomas and PNETS arise from remnants of primitive neuro-ectoderm in the roof of the fourth ventricle. These tumors are very malignant and exhibit an aggressive biologic behavior, commonly invading the adjacent brain stem and leptomeninges. Widespread dissemination through the ventricular system and distant seeding to other areas of the neuraxis occurs in as high as 30%.

Medulloblastomas are primarily midline vermian lesions, but hemispheric locations are also possible. Since they arise close to the fourth ventricle, growth predominantly into the ventricle may make them simulate an intraventricular mass. Necrosis, hemorrhage and cavitation are common features, giving these tumors a heterogeneous appearance on MR, but not to the same degree as seen with ependymomas. Calcification is rare in medulloblastomas. ^, They are hypervascular lesions and show moderate contrast enhancement.

Ependymoma

About 70% of ependymomas are found in the fourth ventricle. The atria of the lateral ventricles are another common site. Males are affected twice as often as females. They originate from the ependyma of the ventricles but may grow either into the ventricle or into the brain substance. Ependymomas are slow-growing, but malignant, tumors and grow by expansion and infiltration. Ventricular and subarachnoid seeding are not infrequent.

Most ependymomas arise in the floor of the fourth ventricle. They have a propensity to extend through the foramina of Luschka and Magendie into the basal cisterns. They tend to be well defined, particularly if they are marginated by CSF within a ventricle or cistern. Calcification is present in 50%, cysts and necrotic areas are common, and most are moderately vascular. These properties account for their heterogeneous internal texture on both plain and contrast scans.

Choroid Plexus Papilloma

Choroid plexus papillomas are rare tumors that arise from cells of the choroid plexus. Their histologic picture is strikingly similar to normal choroid plexus. These lesions are benign and expand within a ventricle, eventually leading to obstruction of CSF pathways. Moreover, they may be associated with increased CSF production, another factor contributing to hydrocephalus.

Choroid plexus papillomas usually have well-defined margins, with parts of the tumor outlined by CSF. On T2-weighted scans they are only mildly hyperintense because their T2 relaxation times are not as prolonged as in parenchymal tumors. They are relatively homogeneous, but hypervascularity can result in areas of flow void or flow enhancement. Choroid plexus papillomas demonstrate intense homogeneous contrast enhancement.

Hemangioblastoma

Hemangioblastoma is a benign tumor of middle age. In fact, it is the most common primary intra-axial tumor of the posterior fossa in adults. About 20% are associated with Hippel-Lindau disease, and hereditary factors have been implicated in another 20%. The cerebellum and vermis are the common sites, but hemangioblastomas can also be found in the medulla and spinal cord. Multiplicity is a well-known feature but is present in only about 10% of cases. Histologic examination reveals a meshwork of capillaries and small vessels.

The classic MR appearance of hemangioblastoma is a cystic mass with a brightly enhancing nodule. About 60% are cystic, so solid lesions are not uncommon. Calcification is rare. Hemangioblastomas are sharply marginated and induce minimal surrounding parenchymal reaction. The tumor nodules are hypervascular and the vascular pedicle often produces a characteristic flow void on MR.

Lymphoma

Primary malignant lymphoma is a non-Hodgkin’s lymphoma that occurs in the brain in the absence of systemic involvement. Most occur in the setting of immune suppression related to HIV infection, chemotherapy, or immunosuppressive therapy for organ transplantation. These tumors are highly cellular and grow rapidly. Cerebral lymphomas are very radiosensitive and respond dramatically to steroid therapy.

Lymphomas typically appear as homogeneous, slightly high signal to isointense masses on T2-weighted images. The observed mild T2 prolongation is probably related to dense cell-packing within these tumors, leaving relatively little interstitial space for accumulation of water. Multiple lesions are present in as many as 50%. Despite their rapid growth, central necrosis is uncommon. They are associated with only a mild or moderate amount of peritumoral edema. By time of presentation they can be quite large and yet produce relatively little mass effect, a feature that sets lymphoma apart from glioblastoma and metastases. Intratumoral cysts and hemorrhage are unusual. Most lymphomas show bright homogeneous contrast enhancement.

Metastatic disease

Metastatic lesions can be found anywhere in the brain but a favorite site is near the brain surface at the corticomedullary junction of both the cerebrum and cerebellum. They are hyperintense on plain T2-weighted images. Areas of necrosis are prevalent in the larger lesions, accounting for their heterogeneous internal texture. Peritumoral edema is a prominent feature, but multiplicity is the most helpful sign to suggest metastatic disease as the likely diagnosis. Hemorrhage is present in 3 to 14% of brain metastases, mainly in melanoma, choriocarcinoma, renal cell carcinoma, bronchogenic carcinoma, and thyroid carcinoma. The presence of nonhemorrhagic tissue and pronounced surrounding vasogenic edema are clues to the underlying neoplasm.

Gadolinium enhanced MR results in improved delineation of metastatic disease compared with nonenhanced scans. Moderate to marked enhancement is the rule, nodular for the smaller lesions and ringlike with central nonenhancing areas for the larger ones. Correlative studies have shown MR to be more sensitive than CT for detecting metastases, particularly lesions near the base of the brain and in the posterior fossa.

Neuroimaging

Brain

“Text Syllabus”

Brain

Orbit and ENT

Spine

INFECTIOUS AND INFLAMMATORY DISORDERS

SUPRATENTORIAL BRAIN TUMORS

BRAINSTEM AND POSTERIOR FOSSA

Head and Neck

Spine

Neuroanatomy modules