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Extracellular methemoglobin Cerebrospinal fluid Edema Neoplasms Abscess Demyelination Dysmyelination Acute to subacute infarcts

Bone

Ligaments Air

Calcifications Deoxyhemoglobin Intracellular methemoglobin Hemosiderin Flow void in vessels Fat

* In relation to gray matter.

TABLE 23-5 -- MR SIGNAL CHANGE OF HEMORRHAGE

T1WI

Acute (6 hr-3 days)

Early Subacute (day 3 to day 7)

Late Subacute (1 wk to 1 mo)

Chronic (months to years)

T2WI

Pathophysiology

Deoxy-Hb

Intracellular Met-Hb

Extracellular Met-Hb

Extracellular Met-Hb and hemosiderin ring*

*After slow reabsorption of the extracellular methemoglobin only decreased signal hemosiderin persists.

*After slow reabsorption of the extracellular methemoglobin only decreased signal hemosiderin persists.

foreign bodies, pacemakers, or non-MRI compatible artificial heart valves, vascular clips, or ventilators. When approaching a lesion that is seen on an MR image of the brain, the specific location of this lesion must be identified. If one begins superficially and moves inward toward the deeper spaces of the brain, the first important space would be the extracalvarial soft tissues, consisting of skin, muscle, and facial structures. The bony calvarium is next composed of the outer table, bone marrow within the diploic space, and inner table. The next space to consider is the extra-axial structures, which include the epidural, subdural, and subarachnoid spaces. The layer beneath the subarachnoid space is the pia mater, which is closely adherent to the gray matter cortical ribbon. Deep to the pial layer lies the intra-axial structures. These structures include the cortical gray matter, the underlying white matter, the ventricular system, the deeper basal ganglionic structures, followed by the brain stem. On routine MRI of the brain there are additional structures to examine, including the orbits, sinuses, temporal bone, and skull base.

CALVARIAL RELATED LESIONS

Benign growths or neoplastic processes include neurofibroma in a patient with neurofibromatosis (type I), soft tissue fibromas, and dermoid or epidermoid tumors. Bone abnormalities include secondary metastatic lesions and rare primary osteogenic sarcomas or giant cell tumors. Secondary neoplastic processes that involve the diploic (marrow) space are leukemia and lymphoma.

Inflammatory lesions within the extracalvarial soft tissues typically include benign processes such as sebaceous cysts or cellulitis. Primary calvarial inflammatory lesions include osteomyelitis or an infected postoperative bone flap.

Bony lesions and congenital fibro-osseous lesions such as fibrous dysplasia or Paget's disease are common calvarial abnormalities. A common marrow disorder is conversion from fatty marrow to hematopoietic marrow in patients with anemia. Primary loss of normal hematopoietic elements with conversion to dense fibrous matrix will be seen in myelofibrosis.

CRANIAL DISEASES

Extra-axial Space Lesions. Congenital tumors of epidermal and mesodermal origin that arise from the skull base and cerebellopontine angle are typically epidermoid tumors. These tumors are usually eccentric and are locally invasive. Congenital tumors that contain all three embryonic layers are dermoid tumors. These tumors tend to occur in the midline at the skull base and within the suprasellar cistern. More aggressive neoplasms to consider are ependymomas, which arise from ependymal cells most commonly within the fourth ventricle or cerebellopontine angle. Secondary neoplastic involvement of the pachymeninges include lymphoma, metastasis from lung or breast, or possibly an adenocarcinoma from an unknown primary tumor. These lesions may represent a single metastatic foci of diffuse carcinomatosis.

T1Wl

Intracellular methemoglobin Extracellular methemoglobin Complex proteins Complex calcifications Entry slice Flow in vessels Slow flow in vessels Melanin

Contrast medium enhancement

Cerebrospinal fluid

Bone

Ligaments Air

Calcifications

Deoxyhemoglobin

Hemosiderin

Flow void in vessels

Acute to subacute infarcts

The most common tumor is a meningioma, which represents a primary neoplasm of the dura (Fig 23-9.). Inflammatory processes such as epidural empyema from

Figure 23-9 Olfactory groove meningiomas, Axial T2:Wdightdd image shows a mass attached to the anterior falx (dural extension). Note relative low signal on T2WI and surrounding signal ecB, Coronal ^-weighted image after contrast shows homogeneous enhancement of the dural based extra-axial mass.

an adjacent sinus infection may occur. Subdural effusions have been described in children with systemic Haemophilus influenzae infections. The most common inflammatory process to consider is a meningitis, which may be of viral, fungal, or bacterial origin.

An important pattern of abnormality is diffuse pachymeningeal thickening with homogeneous enhancement following an intravenous injection of contrast agent (gadolinium). The differential diagnosis includes carcinomatosis, sarcoidosis, meningitis, or a more recently described entity referred to as central CSF hypotensive pachymeningitis.'^ The exact cause of this last entity is not well understood, but it is believed to be related to either a CSF leak or an abnormal CSF metabolic process.^ One of the more common extra-axial lesions is a post-traumatic epidural or subdural hematoma. Lesions within the subarachnoid space that follow CSF signal on all pulse sequences and demonstrate mass effect represent congenital or post-traumatic (acquired) arachnoid cysts.

Intra-axial Space Lesions. Small focal lesions at the gray matter/white matter junction, which demonstrate mass effect, surrounding edema, and contrast medium enhancement most commonly represent metastatic lesions. Large enhancing masslike lesions that involve both the gray matter and white matter and that do not follow a wedgelike distribution typically represent primary brain gliomas. Low-grade (I, II) gliomas typically do not enhance, whereas higher-grade gliomas (Ill, IV) usually do. A low-grade neoplastic process must also be considered if there is a focal, nonenhancing enlargement of the gray matter cortex that continues to have mass effect over a 3- to 4- week interval. This lack of change will help differentiate a low-grade glioma from a cortical infarct.

Peripheral cortical inflammatory processes secondary to meningitis may lead to a cerebritis that may subsequently form a focal abscess. Viral inflammatory processes of the cortex and subcortical white matter may occur and are most commonly secondary to a herpesvirus infection. Multifocal enhancing lesions in the gray matter/white matter junction, brain stem, and cerebellum may represent multifocal abscesses in the appropriate clinical setting ( ...Fig..23:10 ).

A wedge-shaped lesion that involves both white matter and gray matter and follows a vascular distribution usually represents an infarction from cerebrovascular disease. Acute infarctions may become evident on routine T2WI spin-echo images within 12 to 24 hours of onset. The most sensitive pulse sequence is a water diffusion coefficient weighted technique. '171 This pulse sequence can detect an infarct within the first few hours of onset. Peripheral vascular venous occlusions of the superior sagittal sinus or adjacent cortical draining veins may lead to a cortical or a cortical/subcortical venous infarction.

Lesions that primarily involve the white matter are most commonly secondary to small vessel occlusive disease or a demyelinating process. Small rounded infarctions less than 1 cm within the deep white matter, basal ganglion, cerebellum, or brain stem in an adult older than 50 years of age most commonly represent lacunar infarctions. Also in this population, tiny lesions within the periventricular white matter are usually secondary to small vessel ischemic disease. Demyelinating lesions secondary to multiple sclerosis may mimic this appearance but typically are present in younger adults. Lesions of multiple sclerosis may present as early as the second decade as rounded or ovoid lesions in the periventricular deep white matter. Ovoid lesions that are perpendicular to the long axis of the body of the lateral ventricles represent demyelination within perivascular spaces termed Dawson's fingers. Additional lesions are commonly present in the corpus callosum and brain stem. In children younger than the age of 10, periventricular white matter lesions and basal ganglionic, brain stem, or cerebellar lesions may represent an acute disseminated encephalomyelitis, which is a postviral or postvaccine autoimmune process (...Fig 23-11 ). Multiple sclerosis and acute disseminated encephalomyelitis are the most common acquired leukoencephalopathies. Another leukoencephalopathy to consider is human immunodeficiency virus (HIV) encephalopathy, which is a diffuse process. Multifocal lesions with minimal mass effect and no abnormal contrast medium enhancement in an immunocompromised patient may represent progressive multifocal leukoencephalopathy. Postradiation and/or postchemotherapeutic leukoencephalopathy

Figure 23-10 Cysticercosis.A, Axial T2:Wdightdd image shows two parietal lobe lesions with up signB, Coronal ^-weighted image after contrast shows small ring-enhancing lesions.

Figure 23-11 Acute disseminated encephalomyelitis. Axial T2:Wdightdd image shows multiple white matter lesions.

should be considered. There are a wide variety of congenital dysmyelinating disorders, the most common of which are metachromatic leukodystrophy, adrenoleukodystrophy, and Canavan's disease. All of these dysmyelinating diseases represent congenital enzymatic defects.

Within the deeper basal ganglionic structures, brain stem, or cerebellum, the most common lesions are focal infarctions and demyelination. Much less common lesions include focal encephalitis or gliomas. Focal mass lesions within the brain stem or cerebellum may occur within an older adult and most commonly represent metastasis. A primarily cystic lesion with an enhancing mural nodule within the cerebellum in a child represents a pilocystic astrocytoma. In a young adult, a similar-appearing lesion most likely represents a hemangioblastoma, which may be part of von Hippel-Lindau disease. A focal expansile lesion within the brain stem most commonly in the pons in a young child typically represents a brain stem glioma (Fig. 23-12) . In a child, the primary differential diagnosis for a posterior fossa mass includes in decreasing frequency: a primitive neuroectodermal tumor (medulloblastoma), brain stem glioma, cerebellar pilocystic astrocytoma, and ependymoma.

Vascular malformations are most commonly found within the intra-axial space, although they may be present extra-axially. These lesions represent a spectrum from benign cavernous angiomas, which have a low bleeding potential, to arterial venous malformations that carry a bleeding potential of 2 to 4 percent per year. y Cavernous angiomas lesions have characteristic appearances of lobulated blood-filled channels with a rim of hemosiderin. Arterial venous malformations have serpiginous tangles of flow voids with adjacent feeding arteries and draining veins.

Primary neoplasms, although uncommon, do occur. These tumors include choroid plexus papillo or carcinoma, central neurocytomas, which are characteristically attached to the septum pellucidum (.Fig, 23-13 ), interventricular meningiomas, and ependymomas. Colloid cysts typically originate within the region of the roof of the anterior portion of the third ventricle and typically are usually benign. Occasionally, these cysts may cause acute hydrocephalus and rarely death in a young adult from brain herniation.

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