Using Magnetic Resonance Imaging and Diffusion Tensor Imaging to Assess Brain Damage in Alcoholics

Article excerpt

Brain imaging using conventional magnetic resonance imaging (MRI) has revealed that several brain structures in people with a history of chronic alcohol dependence are smaller in volume than the same brain structures in nonalcoholic control subjects. Areas that are particularly affected are the frontal lobes, which are involved in reasoning, judgment, and problem solving. Older people are especially vulnerable to the damaging effects of alcohol. It is unclear whether women show consistently more vulnerability to these changes in the brain than men do. In general, alcoholics evaluated before and after a period of abstinence show some recovery of tissue volume, whereas alcoholics evaluated again after continued drinking show further reductions in brain tissue volume. A new MR technique called diffusion tensor imaging (DTI) can aid in detecting the degradation of fibers (i.e., white matter) that carry information between brain cells (i.e., gray matter). With DTI, researchers studying alcoholics have been able to detect abnormalities in white matter not visible with conventional MRI. Ultimately DTI may be useful in elucidating the mechanisms that underlie macrostructural and functional brain changes seen with abstinence and relapse.

KEY WORDS: AODR (alcohol and other drug related) structural brain damage; magnetic resonance imaging; diffusion tensor imaging; brain imaging; patient assessment; neural tissue; gender differences; chronic AODE (alcohol and other drug effects), AOD dependent

Excessive chronic alcohol consumption is associated with significant shrinkage of brain tissue, degradation of fibers (i.e., white matter) that cany information between brain cells (i.e., gray matter), reduced viability of these brain cells, and impairment of associated cognitive and motor functions (for reviews, see Oscar-Berman 2000; Sullivan 2000). Some alcoholism-related tissue damage is partially reversible with abstinence, although residual tissue volume deficits persist even in long-abstinent alcoholics (Pfefferbaum et al. 1998).

Abnormalities are found in both the gray matter and the white matter of the brain. The brain's gray matter consists of nerve cells (i.e., neurons), which account for its grayish color, and the surrounding glia cells, which provide mechanical support, guidance, nutrients, and other substances to the neurons. White matter is made up of long, thin extensions of the neurons, called axons, which carry information between neurons. White matter is paler in color than gray matter because the axons are surrounded by myelin-a fatty substance that protects the nerve fibers. The axons form fiber tracts linking nearby and distant neurons across different brain regions (i.e., white-matter tracts).

Imaging in living patients (i.e., in vivo) can be used to detect and quantify gray-and white-matter abnormalities on both macrostructural and microstructural levels. Conventional structural magnetic resonance imaging (MRI) reveals the size, shape, and tissue composition (gray vs. white matter) of the brain and its constituent parts. Diffusion tensor imaging (DTI) reveals the integrity of white-matter tracts that link regions of the brain to each other.

MRI is based on the observation that the protons of hydrogen atoms, when placed in a strong magnetic field, can be detected by manipulating the magnetic field. Because the human body is composed primarily of fat and water, it is made up mostly of hydrogen atoms. Variations in behavior of hydrogen atoms in different brain tissue types and structures show up as intensity differences that clinical structural MRI can detect and map to visualize and measure gross brain neuroanatomy.

Diffusion tensor imaging makes use of the fact that water molecules in the brain are always moving-that is, they are in Brownian motion. DTI detects the diffusion, or Brownian movement, of water protons within and between individual cells and yields measures of the magnitude and predominant orientation of this movement. …

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