Using magnetic resonance imaging and diffusion tensor imaging to assess brain damage in alcoholics

Abstract

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.

Figures

Figure 1

Isotropic and anisotropic diffusion. (A) Water molecules in the brain are constantly moving (i.e., in Brownian motion). When motion is unconstrained, as in the large fluid-filled spaces deep in the brain (i.e., the ventricles, as illustrated in the MR image on the left), diffusion is isotropic, which means that motion occurs equally and randomly in all directions. (B) When motion is constrained, as in white-matter tracts (illustrated on the right), diffusion is anisotropic, meaning that motion is oriented more in one direction than another (e.g., along the y axis rather than along the x axis).

Figure 2

Images of the same axial slice of the brain acquired with varying diffusion gradients. The strength of the diffusion gradient is indicated by b. The image at the top left was acquired without diffusion gradients (b = 0). The remaining six images were acquired when applying diffusion gradients (b = 860s/mm2) in six of the many possible combinations of directions (i.e., x = left to right, −x = right to left, y = front to back, z = top to bottom, and −z = bottom to top). For example, in the bottom right picture, diffusion gradients were applied from right to left and from front to back. When no gradients were applied, regions such as the ventricles and sulci (spaces between folds of brain tissue), where there is free movement of molecules, appear bright. When gradients were applied, these spaces appear dark.

Figure 3

Two types of diffusion tensor imaging. (A) The trace image reflects the total amount of diffusion occurring in each region and highlights the ventricles, with little difference between white and gray matter. (B) The fractional anisotropy (FA) image highlights regions where diffusion is oriented in a single direction. The ventricles and gray matter are dark, whereas the white matter tracts are bright.

Figure 4

Images displayed in the coronal orientation from MRI and DTI studies of a 61-year-old healthy man (upper images) and a 60-year-old alcoholic man (lower images). The high-resolution MRI slices are at the same locations as the fractional anisotropy images of the DTI panels. Note on the MRI the thinner corpus callosum displaced upward by enlarged ventricles and, on the DTI, less well delineated white matter tracts in the alcoholic man compared with the healthy man.

Figure 5

The colored bars represent the means, and the represents the standard errors of fractional anisotropy (FA) in three white-matter brain regions in 15 alcoholic men, 12 alcoholic women, and 49 healthy control men and women. As indicated by the stars, the alcoholic men and women had lower regional FA in the genu of the corpus callosum and the centrum semiovale (the mass of white matter composing the interior of the cerbral hemispheres). Only the alcoholic men had lower FA than control subjects in the splenium, as noted by the cross. SOURCES: Pfefferbaum et al. 2000a; Pfefferbaum and Sullivan 2002.