Optical Taxonomic Signal and the Diagnosis of Alzheimer's Disease.

We previously demonstrated that near-infrared spectroscopy in vivo presents spectral features at 895 and 861 nm that accurately classify Alzheimer's disease, mild cognitive impairment, and age-matched control subjects. Our purpose here is to associate the 895 nm signal with [Formula: see text]-amyloid. We applied our feature selection technique to subjects with and without leptomeningeal amyloid. We developed a novel concept, optical taxonomic signal, to determine the dependence of signal on source-detector distance. Features at 891 and 768 nm discriminate between subjects with and without leptomeningeal [Formula: see text]-amyloid. The variation of optical taxonomic signal with source-detector distance indicates that both signals come from the leptomeninges and not cerebral cortex. The two features are highly correlated and likely result from the same cellular material. The discovery of an 891 nm feature that clearly depends upon the presence of [Formula: see text]-amyloid supports our hypothesis that the 895 nm feature previously discovered also reports [Formula: see text]-amyloid.