الفهرس 
Anatomy of the white matterOnly 14 pages are availabe for public view
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Abstract
The white course and connections, into three distinct systems:
1. Projection fibers,
2. Transverse or Commissural fibers, and
3. Association fibers.
The white matter of the brain is located in the central and subcortical regions of the cerebral and cerebellar hemispheres and accounts for about 60% of the total brain volume.
Myelin sheath is a modified plasma membrane that is formed by the spiral wrapping of oligodendrocyte and Schwann cell plasma membrane extensions around axons of the central nervous system (CNS) and the peripheral nervous system (PNS), respectively. Myelin is not only essential for the fast conduction of the action potential but also to maintain axonal integrity.
Demyelinating and dysmyelinating white matter diseases are important components of neurological problems.
With the ever rising burden of the white matter disease, it is imperative to better understand pathophysiological, clinical and imaging findings for proper management of patients. The advent of Magnetic Resonance Imaging (MRI) has revolutionized the concept of diagnosing and understanding of white matter diseases.
Many of the white matter diseases, if detected early, can be cured and MRI plays a vital role in its early diagnosis. MRI is considered far superior to Computed Tomography (CT) and the imaging modality of choice in white matter diseases.
Multiple Sclerosis (MS) is the commonest of all the white matter diseases. Is an autoimmune condition in which the immune system attacks the central nervous system (CNS), leading to demyelination.
MS affects the white matter areas of the brain and spinal cord. More specifically, MS destroys oligodendrocytes which are the cells responsible for creating and maintaining a fatty layer, known as the myelin sheath, which helps the neurons carry electrical signals.
Although certain clinical features are characteristic of MS, investigative studies are often needed to confirm the clinical suspension and exclude other possibilities.
A diagnosis of MS is based on showing disease dissemination in space and time and excluding other neurological disorders that can clinically and radiologically mimic MS.
The 2005 modified McDonald criteria have a simpler approach to dissemination in time, and a further simplification has been proposed in terms of dissemination in time and space.
Variants of MS include:
• Charcot variant
• Marburg variant
• Balo concentric sclerosis
• Schilder’s disease
• Neuromyelitis optica (Devic’s disease).
MRI is the most important paraclinical test for the diagnosis of MS. The lesions can be readily seen with conventional MRI techniques early in the disease course, even before the diagnosis is established clinically. MRI is the most sensitive neuroimaging method for detecting these lesions for dissemination in both space and time.
MS plaques are most commonly found in the periventricular region, corpus callosum, centrum semiovale, optic nerves, and, more rarely, deep white matter structures and basal ganglia.
These new MRI techniques allow a more precise assessment of the pathological mechanisms involved in MS; they provide a better means of establishing the correlation between clinical impact and the destructive nature of the MS lesion.
Inflammatory Demyelinating Pseudotumor is a rare form of demyelinating disorders included in the group of primary demyelinating disorders, as neuroimaging reveals a solitary large demyelinating lesion.
Acute Disseminated Encephalomyelitis (ADEM) is an immune mediate demyelination which is typically seen five days to two weeks following a viral illness or immunization. On MR, lesions are located in the subcortical white mater with asymmetric involvement of both hemispheres with or without brainstem involvement. Though it predominantly involves the white matter, it can involve the gray matter as well.
Progressive multifocal leukoencephalopathy (PML (is caused by reactivation of a latent infection by a papovavirus (JC virus). Before the acquired immunodeficiency syndrome (AIDS) epidemic, PML was a rare disorder occurring most often in association with leukemia or lymphoma. Most cases of PML occur as a complication of AIDS, and estimates indicate that 5% of all patients with AIDS ultimately develop PML. MRI shows multifocal and asymmetric lesions in the cerebral white matter that typically start in a juxtacortical location and progressively enlarge over weeks. These lesions never develop in the optic nerves, very rarely in the spinal cord, and rarely display mass effect.
Subacute sclerosing panencephalitis (SSPE) is a progressive neurologic disorder that is considered to be caused by persistent measles virus. MR imaging studies on SSPE have reported typical bilateral asymmetric hyperintense lesions in the parietal and temporal lobes in the acute stage. In time, lesions become more prominent, and periventricular white matter, corpus callosum, and basal ganglia can be involved. Later, encephalomalacia and atrophy develops.
HIV encephalopathy (HIVE) is the infection of the CNS caused by HIV. If untreated, some 15-20 % of patients will eventually develop the disease. Neurologic manifestations are the initial manifestations of AIDS in 7% to 20% of patients. The most common finding on imaging is atrophy of the brain. In early stages of the disease, small areas of high signal intensity are seen on the T2-weighted sequences in the periventricular white matter that lack mass effect or edema.
There are other diseases that have a relation to myelin dysfunction and different MRI techniques play a vital role in differential diagnosis between them in addition to the clinical picture and the investigation of each one like, Leukodystrophies.
matter consists of medullated fibers, varying in size, and arranged in bundles consist mostly of axons with their envelope of myelin, along with two types of neuroglia which are oligodendrocytes and astrocytes. They may be divided, according to their