Brain waste clearance is emerging as one of the most consequential mechanisms in Alzheimer's disease prevention, and a non-invasive MRI technique may now offer a practical window into that process. Understanding whether the brain effectively flushes amyloid-β and tau proteins during sleep and rest could reshape how clinicians assess neurodegeneration risk years before symptoms manifest — and this systematic review offers the most structured evaluation yet of that imaging tool's clinical validity.
The review analyzed ten human studies investigating the DTI-ALPS index — a diffusion tensor imaging metric designed to indirectly capture fluid dynamics within perivascular spaces, the microscopic channels surrounding cerebral blood vessels through which the glymphatic system operates. Across these studies, lower ALPS index values were consistently associated with greater cognitive impairment, higher amyloid burden, and more advanced disease severity, especially in Alzheimer's disease. Several studies strengthened biological plausibility by pairing ALPS measurements with amyloid PET imaging and cerebrospinal fluid tau and amyloid markers, allowing researchers to cross-validate the MRI signal against established pathological benchmarks.
The glymphatic system — a glia-dependent lymphatic analog in the brain — has attracted intense research interest since its formal characterization roughly a decade ago, largely because its impairment appears to accelerate toxic protein aggregation. DTI-ALPS is conceptually appealing because it requires no contrast agents, no lumbar puncture, and no radioactive tracers, potentially lowering the barrier for population-scale screening. However, the review surfaces a critical limitation: substantial methodological heterogeneity across studies — differing region-of-interest placement, variable MRI acquisition parameters, and inconsistent preprocessing pipelines — makes cross-study comparison unreliable and normative thresholds impossible to establish. This variability is not merely technical noise; it threatens reproducibility at the foundational level. The finding base, while directionally consistent, rests on small cohorts and heterogeneous protocols. Before DTI-ALPS enters clinical use as a glymphatic biomarker, standardized acquisition and analysis protocols are an essential prerequisite. Incremental but directionally important.