MRI diffusion changes may signal cognitive decline

Ischemic small-vessel disease (SVD) wreaks havoc on the brain’s executive function and processing speed and is thought to be caused by gradual disconnects in the white matter tract and other cortical tissues leading to long-term cognitive impairment and disability. However, diffusion changes in apparently healthy tissues can reveal tell-tale signs of impending cognitive decline, according to a study published February 19 in the Annals of Neurology.

Hanna Jokinen, PhD, from the department of neurology of Helsinki University Central Hospital and department of neurological sciences, University of Helsinki, Finland, and colleagues performed a  study of MRI to find subtle diffusion-weighted imaging (DWI) abnormalities in neurological tissue and in white matter hyperintensities indicating early warnings of disease.

“It has been suggested that white matter signal abnormalities visible on conventional T2-weighted MRI scans only represent the tip of an iceberg, when in fact SVD and ischemia cause subtle structural changes in the entire brain,” wrote Jokinen et al. “Quantitative data on the white matter microstructure have been obtained by [DWI], which provides a more direct measurement of brain tissue integrity than standard MRI sequences. The average apparent diffusion coefficient (ADC), or mean diffusivity, is a quantitative measure of the extent of tissue water mobility and a sensitive marker of early morphologic abnormalities such as myelin loss and axonal damage in vivo. In SVD, diffusion changes have been observed not only within visible lesions but also in the normal-appearing brain tissue (NABT).”

This is a sub study of the multi-site LADIS (Leukoaraiosis and Disability Study). A total of 340 subjects from this cohort aged 65 to 84 years underwent MRI to calculate changes in diffusion-weighted imaging (DWI) and white matter hyperintensities. Results showed that the mean global apparent diffusion coefficient (ADC) and the relative peak height and peak position of patient ADC histograms in apparently healthy tissue predicted a faster rate of dysfunction when analyzed in a composite score evaluating motor control and processing speed. If mean ADC was higher, lower peak height was seen and attributed to degeneration of working and other memory functions and executive function. Peak height was additionally considered predictive of relatively quick decline in cognition and a higher death rate, while mean ADC in white matter hyperintensities did not have the same significant effect on brain function and cognition. 

The most marked MRI findings were correlated with high mean and peak ADC scores and low ADC height in apparently healthy tissues and high mean ADC score in regions of white matter hyperintensity.  Multiple measures of DWI were used and correlated strongly with white matter hyperintensity volume: Pearson correlation,  brain atrophy score and number of lacunes.

“DWI microstructural changes in NABT predict faster decline in psychomotor speed, executive functions, and working memory regardless of conventional MRI findings,” wrote the authors. “Moreover, these changes are related to functional disability and higher mortality.”