The confounding effects of skin colour in photoacoustic imaging

Skin colour is known to confound readouts from optical devices that make measurements through the skin, which can adversely impact the care of patients with darker skin. Photoacoustic imaging (PAI) is making its way from the laboratory to the clinic, however, combining optics and ultrasound for deep tissue imaging leads to a complex relationship between photoacoustic-derived imaging biomarkers and skin melanin concentration. Furthermore, no generalisable correction of the confounding effects of skin colour in PAI has been demonstrated. We sought to overcome this limitation by recruiting a healthy volunteer cohort with the most diverse range of skin tones ever assembled in the field, with participants from Fitzpatrick types I to VI and with vitiligo. From this comprehensive dataset, we identified and characterised two physical mechanisms responsible for skin colour-dependent degradation in both image quality and biomarker quantification. Accompanied by detailed theoretical modelling, we demonstrated that strong light absorption by melanin leads to spectral colouring, which dominates in individuals with low skin melanin pigmentation. We further identified the backscattering of ultrasound waves generated in the skin as a major source of image artefacts for individuals with high skin melanin pigmentation. With this improved understanding of the physical basis, we were able to develop a fast and practicable correction method for spectral colouring and adapted a plane-wave ultrasound reconstruction algorithm to reveal the ultrasound scatterer distribution encoded in the photoacoustic timeseries. Our findings highlight the need for more advanced image reconstruction methods to enable equitable clinical application of PAI.