Early signs of Alzheimer\’s disease may show up in subtle changes in the diffusion of water in key brain structures, as revealed by MRI, researchers here suggested. IRVINE, Calif., Sept. 26 — In a small study, patients with mild cognitive impairment were found to have elevations in apparent diffusion coefficient in brain areas governing memory when compared with control patients, and reported Min-Ying Su, Ph.D., of the University of California at Irvine, and colleagues, in the October issue of Radiology. These elevations correlated with worse memory performance scores, the investigators added.
“Our methods may enable earlier diagnosis of Alzheimer’s disease, allowing earlier intervention to slow down disease progression,” they said.
Apparent diffusion coefficient is a measure of the diffusion of water in the brain, which is normally hindered by local tissues such as cell membranes, intracellular organelles, axons and myelin sheaths. MRI evidence of elevated apparent diffusion coefficient is therefore believed to signal tissue atrophy.
Dr. Su and colleagues conducted a prospective study of alteration in the apparent diffusion coefficient of cortical gray and white matter and subcortical structures known to be involved in mild cognitive impairment.
They performed MRI imaging in nine men and four women, mean age 74 + 6 years, who had been diagnosed with mild cognitive impairment (clinical dementia rating score < 0.05) and in 13 healthy controls (seven men and six women, age 75 + 4 years, clinical dementia rating score=0).
The authors measured the apparent diffusion coefficient on T1-images acquired in the coronal plane using both manual and automated techniques. Investigators experienced with brain imaging used a computer mouse to manually delineate specific regions of interest in the hippocampus, parahippocampal gyrus, amygdala, corpus callosum, and anterior and posterior cingulate gyrus.
They also identified regions of interest in the frontal, parietal, temporal, and occipital lobes with an automated template masking program.
The outlined regions of interest were then superimposed onto apparent diffusion coefficient maps, and a skeleton-based region competition segmentation algorithm, which accurately analyzes digital brain images, was used for segmentation of gray and white matter.
The authors determined group differences in apparent diffusion coefficient values with independent-sample t tests, and used Pearson correlation analysis to examine the relationship between apparent diffusion coefficient values and age and memory test scores.
They saw evidence of significantly higher apparent diffusion coefficients in the hippocampus, temporal lobe gray matter, and corpus callosum of patients with mild cognitive impairment compared with that of control subjects (P<0.05).
When they pooled the results from all participants, they found that elevated hippocampal apparent diffusion coefficient, indicative of hippocampal atrophy, was significantly correlated with worse memory performance scores in both 5-minute and 30-minute delayed word-list recall tasks (P<0.05).
Our finding that hippocampal apparent diffusion coefficient correlated with severity of memory impairment raises the possibility that elevated hippocampal apparent diffusion coefficient might correlate with an increased conversion rate to Alzheimer’s disease,” they wrote.
That hypothesis, they noted, is supported by Mayo Clinic researchers, who reported in Neurology in 2005 that higher baseline hippocampal diffusivity on diffusion-weight MRI was associated with a greater risk of progression to Alzheimer’s disease in patients with amnestic mild cognitive impairment.
“The results have supported our objective to develop a computer-based analysis technique that can analyze different regions in the entire brain, to provide a comprehensive evaluation of cellular changes,” Dr. Su said.
Primary source: Radiology
Source reference: Ray KM et al. “Mild Cognitive Impairment: Apparent Diffusion Coefficient in Regional Gray Matter and White Matter Structures.” Radiology 2006; 241:197-205