Panayiotis A. Kyriacou

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.

Generalised oedema is characterised by tissue swelling due to excess water accumulation. It is a frequent complication encountered in infants admitted to paediatric and neonatal intensive care. In clinical practice, oedema assessment depends on a combination of weight monitoring, fluid balance calculations and direct visual inspection of the skin. However, all of these lack accuracy and sensitivity, especially in critically ill infants. We report the use of near infrared (NIR) spectroscopy using a portable spectrometer and an optical fibre with a custom attachment allowing continuous recordings for 30 min. Data were collected from 20 neonates and infants admitted to the intensive care unit, with or without oedema, across three consecutive days. Statistically significant differences were found in the 1450 nm features, amplitude (p = 0.009), ratio (p < 0.001) and band area (p < 0.001), and the 970 nm ratio followed the same direction (p = 0.005), all with higher values in the oedema group. A late fusion model, combining spectral data and clinical features, effectively differentiated between oedematous and non-oedematous infants, achieving AUC 0.909 ± 0.016, p = 0.004. This proof-of-concept study suggests that near-infrared spectroscopy can detect oedema in neonates and infants, as assessed against clinical evaluation, laying the groundwork for its potential application as a non-invasive clinical detection and monitoring tool.

Objective . Vascular ageing (VA) is an age-related process characterised by vessel stiffening and altered endothelial function. Recognised as a major predictor of cardiovascular events, VA remains under-assessed in clinical practice due to limitations of current measurement systems. Photoplethysmography (PPG) emerges as a promising non-invasive, cost-effective tool, but its application to VA is hindered by signal complexity and lack of standardisation. This study investigated the relationship between VA and PPG waveform morphology through combined in vitro and in vivo analyses. Approach . In vitro , PPG signals were acquired from vascular phantoms with 3 stiffness levels. in vivo , PPG signals from 31 healthy subjects in two age groups ( < 30 years and > 50 years) were analysed, assuming increased stiffness with age. A novel skew-Gaussian decomposition model was developed to reconstruct pulses and extract morphological features. In vitro feature differences were assessed using the Kruskal–Wallis test; in vivo data were analysed with an XGBoost classifier to discriminate age groups. Main results . The skew-Gaussian model exhibited excellent waveform reconstruction ( R mean 2 > 0.98 ). In vitro , 23 of 60 features differed significantly across stiffness levels, particularly between the stiffest phantom and the others. Amplitude and energy parameters correlated negatively with stiffness, indicating reduced vascular compliance. Temporal features related to wave reflection correlated positively, suggesting altered signal dynamics. in vivo , the XGBoost classifier achieved 87% mean accuracy, with 83% sensitivity and 92% specificity in differentiating age groups. Significance . Feature relevance analysis confirmed the physiological significance of the extracted parameters, supporting the potential of PPG as a non-invasive and scalable technology for monitoring VA.

SignificanceOptical techniques are promising tools for non-invasive cerebral monitoring; however, optical measurements acquired from the forehead can be influenced by extracranial tissues and skin pigmentation, potentially complicating interpretation of cerebral signals. Tissue-mimicking phantoms provide controlled platforms for studying these effects; however, existing head phantoms employ simplified geometries and lack the features required for cerebral photoplethysmography (PPG) investigations.AimWe present the development and characterization of a multilayer head phantom reproducing key anatomical, optical, and hemodynamic features relevant to optical brain monitoring while enabling future investigation of confounding factors.ApproachThe phantom incorporates a custom-made silicone brain with embedded vessels, a three-dimensional-printed skull, a vascularized scalp, and interchangeable skin layers representing different pigmentation levels, integrated with an in vitro cardiovascular system enabling selective perfusion of cerebral and extracranial compartments. The optical and colorimetric properties of the tissue-mimicking layers were assessed.ResultsThe scalp and skin layers exhibited absorption and reduced scattering coefficients within ranges reported for human tissues. Colorimetric assessment confirmed physiologically relevant pigmentation characteristics across the skin layers. Using the pale skin, the phantom generated measurable PPG signals across multiple wavelengths and source–detector separations.ConclusionsThe multilayer head phantom provides an anatomically and optically representative platform for optical brain monitoring research. Its interchangeable skin layers and independently perfusable cerebral and extracranial compartments enable future controlled investigations of skin pigmentation and extracranial hemodynamics as confounding factors in cerebral optical measurements.