City Research Online

Abstract

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.

Publication Type:	Article
Additional Information:	© 2026 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license.
Publisher Keywords:	cerebral photoplethysmography; extracranial circulation; optical brain monitoring; multilayer head phantom
Subjects:	R Medicine > R Medicine (General) T Technology > TA Engineering (General). Civil engineering (General) T Technology > TK Electrical engineering. Electronics Nuclear engineering
Departments:	School of Science & Technology School of Science & Technology > Department of Engineering
SWORD Depositor:	Symplectic Administrator

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