Abstract
A novel method (Sophia) is presented to track oxygen saturation changes in a controlled environment using an RGB camera placed approximately 1.5 m away from the subject. The method is evaluated on five healthy volunteers (Fitzpatrick skin phenotypes II, III, and IV) whose oxygen saturations were varied between 80% and 100% in a purpose-built chamber over 40 minutes each. The method carefully selects regions of interest (ROI) in the camera image by calculating signal-to-noise ratios for each ROI. This allows it to track changes in oxygen saturation accurately with respect to a conventional pulse oximeter (median coefficient of determination, 0.85).
Highlights
Peripheral blood oxygen saturation (SpO2) is a measure of the relative concentration of oxygenated haemoglobin molecules in the arterial blood with respect to the total amount of haemoglobin
Pulse oximetry is based on the technique of photoplethysmography, and takes advantage of the fact that oxyhaemoglobin (HbO2) and reduced haemoglobin (Hb) absorb light differently at different wavelengths; that arterial blood is mostly pulsatile in nature; and that an optical window exists in the far visible and short-wave infrared for water which allows radiation in this wavelength range to probe the vasculature of the dermis [6]
The resulting metric is robust to changes in the position of the observed subject. These conclusions are taken into account in Skin-Oxygen Photoplethysmographic Image Analysis (Sophia), which is based on the heart rate maps introduced in [15] and further explored in [19]
Summary
Peripheral blood oxygen saturation (SpO2) is a measure of the relative concentration of oxygenated haemoglobin molecules in the arterial blood with respect to the total amount of haemoglobin. The invasive technique of blood gas analysis remains the clinical gold standard for the assessment of arterial blood oxygenation and is still widely used in many clinical contexts [1,3], it was only with the development of the current standard for SpO2 measurement in the early 1980s [4] that SpO2 could be measured both continuously and non-invasively This new technique, known as pulse oximetry, was a revolution for in-hospital monitoring [5]. Pulse oximetry is based on the technique of photoplethysmography, and takes advantage of the fact that oxyhaemoglobin (HbO2) and reduced haemoglobin (Hb) absorb light differently at different wavelengths; that arterial blood is mostly pulsatile in nature; and that an optical window exists in the far visible and short-wave infrared for water which allows radiation in this wavelength range to probe the vasculature of the dermis [6]. Recent work in the field of non-contact imaging using cameras has concentrated mostly on methods for the determination of heart rate, with little or no effort dedicated to deriving oxygen saturation [9,10,11,12]
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