Development and application of a non-contact, depth-resolved optical skin oximeter

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Oxygen is vital for life. Every cell in the body requires a continuous supply of oxygen or it will die. The delivery of oxygen from the lungs to cells is the most important function of the microcirculation. The skin represents the largest organ' of the body and plays a vital role in protecting the body from the outside environment. Its health is thus of critical importance to our general well-being. The skin is comprised of distinct layers. the uppermost layer is the stratum corneum and consists of dead cells. Below this lies the epidermis, about 0.1mm thick and below this is the dermis. 1-2 mm thick. The epidermis is unique in that it contains living cells but no blood vessels: to remain alive it relies totally on the passive diffusion of oxygen from elsewhere. If the supply of oxygen to the epidermis is interrupted for any reason then a serious medical condition is likely to result. Painful skin conditions such as venous ulcers. pressure ulcers (e.g. 'bed sores') or diabetic ulcers (i.e. the condition known as 'diabetic foot ) are potentially caused by disturbed oxygen transport to the epidermis. There is thus a need to measure oxygen delivery to the skin via the microcirculation.Over 50 years ago an approach was suggested that has remained of value today. The American scientist Millikan demonstrated that optical spectroscopy can measure the amount of oxygen bound to haemoglobin in blood via changes in the colour of haemoglobin. Oxygenated haemoglobin. which predominates in arterial blood, appears bright crimson whereas deoxygenated haemoglobin. more prevalent in venous blood. appears reddish-brown. Millikan used this effect to allow American bomber pilots to check that they were breathing in sufficient oxygen whilst flying at altitudes of 5-6 miles. All current instruments are of the same basic conceptual design as Millikan's instrument. Consequently they have limitations that become very restrictive when studying the skin. The microcirculation of the skin is highly structured on a depth-scale of 1 mm. but current instruments provide no direct information about the distribution of oxygenated haemoglobin with depth. The devices make physical contact with the skin. which can easily produce mechanical pressures sufficient to close off the smallest blood vessels and thus distort the measurements. Our project proposal is to use modern technology. developed over the last few years. to address these shortcomings and build a better optical skin oximeter.Our approach will apply an optical analogue of ultrasound imaging called optical coherence tomography (OCT), which has emerged over the last 12 years as the most promising new optical technique for skin imaging. Spectroscopic OCT can yield depth-resoived information about the distribution of absorbing compounds by performing OCT at a number of different wavelengths. To date, this technique has mainly been used to measure the water content of tissues, as water shows a strong absorption band between 1.3 pm and 1.55 pm. OCT light sources are commonly available at these wavelengths because they are also the wavelengths used for optical fibre communications. Spectroscopic OCT has not been attempted over the wavelength range 450 to 600 nmi ideal for measuring haemoglobin oxygenation, due to the unavailability of suitable light sources. Spectroscopic OCT at these wavelengths can form the basis of the next generation of skin oximeters, effectively overcoming all the limitations of current devices. Recently the chief technical obstacle, the lack of blue-green OCT sources, has started to be overcome via the invention of supercontinuum' light sources.Our project will explore the potential that blue-green spectrosopic OCT. using this new type of light source, has for improving the diagnosis and management of skin conditions that involve disturbed oxygen delivery by the microcirculation.