Evaluation of the Effect of Extracerebral Layers in Cerebral Photoplethysmography (PPG) Using a Head Phantom

Optical brain monitoring (NIRS / cerebral PPG) mixes cerebral and extracerebral circulation. This study used a multilayer head phantom with independent scalp and brain perfusion, three conditions (both layers perfused; scalp occlusion; brain occlusion), and a custom forehead probe at 770 / 810 / 880 nm with proximal (10 mm) and distal (30 mm) source–detector separations.

• Form factor: forehead

Optical brain monitoring techniques such as near-infrared spectroscopy (NIRS) and cerebral photoplethysmography (PPG) are promising non-invasive approaches for cerebral haemodynamic assessment; however, the measured signal can reflect contributions from both cerebral and extracerebral circulation. This study investigated the effect of extracerebral layers on forehead PPG signals using a multilayer head phantom with independent control of scalp and cerebral perfusion. Three perfusion conditions were performed: both layers perfused, scalp occlusion, and brain occlusion. PPG signals were acquired at 770, 810, and 880 nm using a custom optical probe with proximal (10 mm) and distal (30 mm) source–detector separations. Signal features were extracted and compared across conditions. Scalp occlusion produced a strong reduction in pulsatile signal, with complete signal collapse at the proximal detector. A small pulsatile signal (~5% of baseline amplitude) was detected at the distal detector, indicating that longer source–detector separations (30 mm) can detect signals originating from the brain phantom. Brain occlusion resulted in a partial amplitude reduction (~45%) with preserved waveform morphology across all wavelengths. While this demonstrates that cerebral perfusion changes are detectable in the presence of intact extracerebral circulation, it also highlights a potential masking effect as scalp circulation alone can produce a good-quality PPG signal despite complete cerebral hypoperfusion. These findings have clinical implications and highlight the need for mitigation strategies that account for extracerebral contributions in brain optical monitoring techniques.

Verbatim from the publisher's DOI record (opens in a new tab) · Publisher licence: https://creativecommons.org/licenses/by/4.0/

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Wavelengths

770nm, 810nm, 880nm