Design of an optoelectronic system for noninvasively mapping of oxygen saturation in skin tissue to detect various pathologies

The optical analysis of human skin is an important issue for the diagnosis of many diseases and disor-ders like tumors, ischemia, skin cancer and other skin tissue disorders. The most often used approach for diagnosis of skin diseases such as pigmented lesions is visual inspection for the skin tissue. Careful observation and visual assessment of the suspected area is always the first and most important step. This is usually followed by a punch biopsy, in which a tissue sample is removed from skin for micro-scopic analysis. Through punch biopsy subject is exposed to scars and pain during excision. Optical techniques on the other hand, are usually non-invasive and results are often satisfactory without pain accompanied while screening the skin tissue under examination. Mapping of the oxygen saturation distribution in human skin tissues using the optical techniques avoids the biopsy and the associated pain. These maps provide information about the health of the skin tissues, which is useful in medical diagnosis and in planning medical intervention for skin lesions. Many researches have been conducted in this filed. However, most of the methods which are used in research have drawbacks, for instance using chemical agent in order to find the oxygen saturation distribution map. But rare methods are reported used the oximetry principle. Finding a modality for mapping the distribution of oxygen saturation in skin tissues noninvasively depending on the pulse oximetry principle is a new idea in this discipline. A linear photodiode array is used to scan the skin tissue of human body, so that an image can be constructed from the reflected (scattered) light intensity of internal tissues. The photoplethysmogram (PPG) signals are obtained after illuminating the tissues by light emitting diodes (LEDs) that oscillate in two wavelengths; red (660nm) and infrared (880nm). The absorption in these wavelengths are mainly due to oxy-hemoglobin (HbO2) and deoxy-hemoglobin (Hb) while other blood contents and tissue contents fairly have low influence on the ac-quired signals. The acquired signals are processed and manipulated by computer simulation pro grams (Matlab and LabVIEW). The next step is applying the light model which represents light inter-action with skin tissues. The modified Beer Lambert law (MBLL) is used to model the light scattered from the skin tissues. The distribution of the oxygen saturation map is extracted. The final map is used to differentiate between different skin tissue state and for diagnosing many skin disorders.