Ultrahigh-resolution microstructural diffusion tensor imaging reveals perforant path degradation in aged humans in vivo.

The perforant path (PP) undergoes synaptic changes in the course of aging and dementia. Previous studies attempting to assess the integrity of the PP in humans using diffusion tensor imaging (DTI) were limited by low resolution and the inability to identify PP fibers specifically. Here we present an application of DTI at ultrahigh submillimeter resolution that has allowed us to successfully identify diffusion signals unique to the PP and compare the intensity of these signals in a sample of young adults and older adults. We report direct evidence of age-related PP degradation in humans in vivo. We find no evidence of such loss in a control pathway, the alveus, suggesting that these findings are not evidence for a global decline. We also find no evidence for specific entorhinal gray matter atrophy. The extent of PP degradation correlated with performance on a word-list learning task sensitive to hippocampal deficits. We also show evidence for gray matter diffusion signals consistent with pyramidal dendrite orientation in the hippocampus and cerebral cortex. Ultrahigh-resolution microstructural DTI is a unique biomarker that can be used in combination with traditional structural and functional neuroimaging methods to enhance detection of Alzheimer disease in its earliest stages, test the effectiveness of new therapies, and monitor disease progression.