ESTIMATING HEMODYNAMIC METRICS FROM PPG SIGNALS ACQUIRED FROM THE TEMPLE REGION AND BENCHMARKING THOSE WITH TCD, FNIRS, AND MRI - A PILOT

The human brain comprises only ~2% of body weight, yet it consumes ~20% of total blood supply (Elia, 1992). Given its high metabolic demand and limited energy reserves, uninterrupted and precise cerebral blood flow (CBF) is essential for cognitive and neurological health. Subtle fluctuations may signal early physiological dysfunction, highlighting the need to monitor cerebral hemodynamics beyond clinical settings. Additionally, CBF declines with age (Sabayan et al., 2013) and is strongly associated with cognitive decline (van Dinther et al., 2023), suggesting that tracking these changes may help preserve cognitive function and support healthy aging. This study aims to benchmark a non-invasive wearable that uses photoplethysmography (PPG) signals acquired at the temple to estimate real-time hemodynamic changes relevant to cerebral and cephalic circulation. The output may serve as a relative proxy measure, rather than a direct estimate of absolute cerebral blood flow (CBF). We benchmarked the wearable’s PPG-derived changes against established modalities sensitive to cerebral and cephalic hemodynamics, including transcranial Doppler (TCD)-derived middle cerebral artery velocity (MCAv), functional near-infrared spectroscopy (fNIRS)-derived hemoglobin signals, and MRI-based perfusion measures. Concurrent recordings were acquired during physiological challenges designed to elicit reproducible hemodynamic changes, including orthostatic transitions and seated cycling. Across protocols, temple PPG-derived changes showed moderate-to-strong correlations with concurrent changes in MCAv and fNIRS-derived hemoglobin signals. We also observed patterns consistent with age-associated differences reported in prior literature. These findings support larger studies to test reliability and sensitivity to change in broader, real-world settings.