Brief Review Of Basic Aspects Of Nonconventional Mr Techniques

Proven MS Treatment By Dr Gary Levin

Multiple Sclerosis Causes and Treatments

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MT MRI is based on the interactions between protons in a relatively free environment and those where motion is restricted. Off-resonance irradiation is applied, which saturates the magnetization of the less mobile protons, but this is transferred to the mobile protons, thus reducing the signal intensity from the observable magnetization. Thus, a low MT ratio (MTR) indicates a reduced capacity of the macromol-ecules in the CNS to exchange magnetization with the surrounding water molecules, reflecting damage to myelin or to the axonal membrane (Fig. 3). The most compelling evidence indicating that markedly decreased MTR values correspond to areas where severe and irreversible tissue loss has occurred comes from a postmortem

Magnetisation Transfer Weighted Images

Figure 3 Axial magnetic resonance images from a patient with multiple sclerosis. The proton density weighted scan (A) shows multiple lesions. On the scalp-stripped magnetization transfer ratio map (B), lesions appear as hypointense areas. The degree of hypointensity is related to decrease in magnetization transfer ratio and indicates damage to myelin and to axonal membranes.

Figure 3 Axial magnetic resonance images from a patient with multiple sclerosis. The proton density weighted scan (A) shows multiple lesions. On the scalp-stripped magnetization transfer ratio map (B), lesions appear as hypointense areas. The degree of hypointensity is related to decrease in magnetization transfer ratio and indicates damage to myelin and to axonal membranes.

study showing a strong correlation of MTR values from MS lesions and NAWM with the percentage of residual axons and the degree of demyelination (24). Another postmortem study has also shown the potential of this technique to monitor the extent of remyelination in MS lesions (25).

Diffusion is the microscopic random translational motion of molecules in a fluid system. In the CNS, diffusion is influenced by the microstructural components of tissue, including cell membranes and organelles. The diffusion coefficient of biological tissues (which can be measured in vivo by MRI) is, therefore, lower than the diffusion coefficient in free water and for this reason it is named as apparent diffusion coefficient (ADC) (26). Pathological processes that modify tissue integrity, thus resulting in a loss or increased permeability of "restricting" barriers, can determine an increase of the ADC. Since some cellular structures are aligned on the scale of an image pixel, the measurement of diffusion is also dependent on the direction in which diffusion is measured. As a consequence, diffusion measurements can give information about the size, shape, integrity, and orientation of the tissues (27). A measure of diffusion, which is independent of the orientation of structures, is provided by the mean diffusivity (MD), the average of the ADCs measured in three orthogonal directions. A full characterization of diffusion can be obtained in terms of a tensor (28), a 3 x 3 matrix which accounts for the correlation existing between molecular displacement along orthogonal directions. From the tensor, it is possible to derive MD, equal to the one-third of its trace, and some other dimensionless indices of anisotropy. One of the most used of these indices is named fractional ani-sotropy (FA) (29). The pathological elements of MS have the potential to alter the permeability or geometry of structural barriers to water molecular diffusion in the brain (Fig. 4). The application of DW MRI technology to MS is therefore appealing to provide quantitative estimates of the degree of tissue damage and, as a consequence, to improve the understanding of the mechanisms leading to irreversible disability.

Water-suppressed proton MR spectra of normal human brain at long echo times reveal four major resonances: one at 3.2 ppm from tetramethylamines [mainly from choline-containing phospholipids (Cho)], one at 3.0 ppm from creatine (Cr) and phosphocreatine, one at 2.0 ppm from N-acetyl groups (mainly NAA), and one at 1.3 ppm from the methyl resonance of lactate (Lac). NAA is a marker of axonal integrity, while Cho and Lac are considered as chemical correlates of acute inflammatory/demyelinating changes (30). An immunopathologic study of MS (31) has indeed shown that a decrease in NAA levels is correlated with axonal loss, and an increase in Cho correlates with the presence of active demyelination and gliosis. 1H-MRS studies with shorter echo times can detect additional metabolites, such as lipids and myoinositol (mI), which are also regarded as markers of ongoing myelin damage. Therefore, 1H-MRS can complement conventional MRI in the assessment of MS patients by defining simultaneously several chemical correlates of the pathological changes occurring within and outside T2-visible lesions (Fig. 5).

fMRI aids in the mapping of regions of brain activation during motor, sensitive, and cognitive tasks and can define changes in brain activation associated with disease. fMRI quantitates the blood oxygenation level dependent (BOLD) effect and detects areas of brain that have greater local blood flow, reflecting increased neuronal activity during task performance compared with rest (32). As a consequence, fMRI work has the potential to detect adaptive cortical reorganization with the potential to limit the clinical consequences of irreversible MS-related tissue injury.

Figure 4 Axial magnetic resonance images from a patient with multiple sclerosis. (A) Proton density-weighted image. On the scalp-stripped mean diffusivity map (B), some of the lesions appear as hyperintense areas. The degree of hyperintensity is related to mean diffusivity increase and indicates a loss of structural barriers to water molecular motion. On the fractional anisotropy map (C), white matter pixels are bright because of the directionality of the white matter fiber tracts. Dark areas corresponding to some of the macroscopic lesions indicate a loss of fractional anisotropy and suggest the presence of structural disorganization.

Figure 4 Axial magnetic resonance images from a patient with multiple sclerosis. (A) Proton density-weighted image. On the scalp-stripped mean diffusivity map (B), some of the lesions appear as hyperintense areas. The degree of hyperintensity is related to mean diffusivity increase and indicates a loss of structural barriers to water molecular motion. On the fractional anisotropy map (C), white matter pixels are bright because of the directionality of the white matter fiber tracts. Dark areas corresponding to some of the macroscopic lesions indicate a loss of fractional anisotropy and suggest the presence of structural disorganization.

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