Ultrafast Imaging of Physiological Brain Pulsations With Magnetic Resonance Encephalography-From Noise to Predictive Clinical Biomarker.

Over the past decade, novel in vivo imaging techniques have revealed that physiological pulsations drive the transport of brain solutes and that impairment of fluid flow precedes certain neuropathologies. Although the pioneering investigations on brain solute transport mechanisms mainly employed imaging of exogenous tracers, novel advanced ultrafast functional MRI sequences enable critical sampling of propagating physiological pulsations driving the brain fluids devoid of aliased mixing of signals. In this review, we summarize the emerging magnetic resonance encephalography (MREG) technique, beginning with a historical perspective and physiological background of the phenomena of brain pulsatility as measured in the parenchyma and cerebrospinal fluid (CSF). We give a detailed account of how functional contrast mechanisms evident in the T2(*)-weighted MREG signal enable the simultaneous mapping of three distinct physiological signals. Our narrative review continues with an account of signal analysis and methodological considerations arising from 12 years of experience in ultrafast brain scanning. Our review concludes with a presentation of how sleep-related physiological changes in the driving pulsations influence solute transport in a healthy brain and our perspective on the potential of these pulsations as emerging biomarkers for predictive, diagnostic, and treatment monitoring in the context of Alzheimer's disease and other central nervous system (CNS) conditions.