الفهرس 
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Abstract
White matter diseases of the brain encompass a wide spectrum of inherited and acqiured neurodegenerative disorders affecting the
integrity of myelin in the brain and peripheral nerves.
Most of these disorders fall into one of four main categories:
demylinating diseases, age-related disorders, metabolic and immunemedited
diseses, each has distinctive clinical, biochemical, pathologic,
and radiologic features.
Magnetic resonance (MR) imaging has become the primary imaging modality in patients with white matter diseases and plays an important
role in the identification, localization, and characterization of underlying white matter abnormalities in affected patients.
MR imaging has also been extensively used to monitor the natural progression of various white matter disorders and the response to
therapy.
Although the MR imaging features are often nonspecific, systematic analysis of the finer details of disease involvement may permit a
narrower differential diagnosis, which the clinician can then further refine with knowledge of patient history, clinical testing, and metabolic
analysis.
This is particularly true for multiple sclerosis (MS), where the sensitivity of T2-weighted MRI in the detection of white matter lesions,
together with the ability of post-contrast T1-weighted images to reflect
the presence of acute inflammatory activity, may allow us to demonstrate the dissemination of MS pathology in space and time earlier than the clinical assessment, thus leading to an earlier and more confident diagnosis.
However, conventional MRI has three major limitations. First, T2-weighted signal abnormalities just reflect the presence of increased water content, which may range from transient edema to irreversible demyelination and axonal loss.
Secondly, the presence of contrast enhancement indicates that blood brain barrier permeability is increased and associated with ongoing
inflammation, but it does not provide any information about the nature and extent of associated tissue damage.
Thirdly, conventional MRI is unable to detect and quantify the presence of damage occurring in the normal appearing CNS tissues,which have been shown to be diffusely and sometimes severely
damaged in many white matter disorders.
Recent modalities of MR imaging have evolved dramatically since their introduction into clinical work. Looking back, there are several
major steps that took MR imaging of the central nervous system to the next level.
One of the first steps was the introduction of the clinical usefulness of contrast agents. Other steps were the development of fat suppression
techniques; fast spin echo imaging, and, more recently, the development of a clinically useful diffusion imaging (DWI), MR spectroscopy and
perfusion imaging techniques.
The basic principle in DWI is that additional attenuation of the signal in proton MRI results from diffusion of water molecules. Different
microscopic structures such as cell membranes play the role of microbarriers that can significantly alter random motion of water molecules in tissue.
Because of these micro-barriers, diffusion of water molecules in tissue is usually restricted to a limited region in contrast to self-diffusion in pure water.
With development of MR magnets, the time it takes to obtain a DW image is so short that in many institutions it is now being used as a
routine part of any MR imaging of the brain.
MR spectroscopy (MRS) is a non-invasive method of brain metabolism assessment. Proton (1Н) МRS is based on a “chemical shift”—the change of proton resonant frequency.
This term was developed by N. Ràmsey in 1951, for defining a distinction between frequencies of separate spectral peaks. The chemical
shift measured unit is in parts per million (ppm).
The main metabolites that are detected in the spectrum of in vivo proton MRS are: N-acetylaspartate (NAA), choline (Cho), creatine (Cr),
myo-inositol (mI), glutamate and glutamine (Glx), lactate (Lac), and a complex of lipids (Lip).
Many of the principles of MRS are the same as those of MRI,although their focus is somewhat different. While theoretically possible
on any MRI system, most MRS studies are performed using magnets of 1.5 T or higher, due to the low intrinsic sensitivity of the technique.
Hydrogen spectroscopic studies can be performed on standard imaging systems with no additional hardware required.
MR perfusion imaging -simply- is the process by which we observe changes in tissue signal due to the blood flow through it .Although angiographic techniques visualize the vascular network within a patient,they do not have sufficient spatial resolution to visualize blood flow through a tissue in bulk.
Proper tissue perfusion is critical to ensure an adequate supply of nutrients to the constituent cells as well as removal of metabolic byproducts. It also aids in maintenance of a stable tissue temperature.
This is particularly important in evaluation of cerebral white matter diseases as many new theories suggested that there is a primary vascular
evidence supported by diminished tissue perfusion and hypoxia-like tissue injuries.
So it has predictive role of lesion reactivity and/or new lesion formation and, therefore, has the potential to predict disease activity,
and monitor disease progression or the effects of therapy.
To conclude, the ultimate aim of newly developed functional MRI techniques is to study crucial aspects of neuro-metabolism in a quantitative way (diffusion weighted MRI and proton MR
spectroscopy), while others provide insight into neurovascular physiology and brain and spinal cord function (perfusion-weighted
MRI).
In this state-of-the-art compendium, interested neuro-radiologists,neuro-clinicians, and neuro-researchers can find practically everything
worthwhile knowing on the in vivo imaging of white matter diseases of the brain.