A reflectance model for non-contact mapping of venous oxygen saturation using a CCD camera Jun Li a , Barbrina Dunmire a , Kirk W. Beach b , Daniel F. Leotta a,n a Applied Physics Laboratory, University of Washington, Seattle, WA 98195, USA b Department of Surgery, University of Washington, Seattle, WA 98195, USA article info Article history: Received 12 March 2013 Received in revised form 10 June 2013 Accepted 17 June 2013 Available online 8 July 2013 Keywords: Tissue optics Tissue diagnostics Blood gas monitoring Oxygen saturation Venous blood Venous occlusion abstract A method of non-contact mapping of venous oxygen saturation (SvO 2 ) is presented. A CCD camera is used to image skin tissue illuminated alternately by a red (660 nm) and an infrared (800 nm) LED light source. Low cuff pressures of 30–40 mmHg are applied to induce a venous blood volume change with negligible change in the arterial blood volume. A hybrid model combining the Beer–Lambert law and the light diffusion model is developed and used to convert the change in the light intensity to the change in skin tissue absorption coefficient. A simulation study incorporating the full light diffusion model is used to verify the hybrid model and to correct a calculation bias. SvO 2 in the fingers, palm, and forearm for five volunteers are presented and compared with results in the published literature. Two-dimensional maps of venous oxygen saturation are given for the three anatomical regions. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Pulse oximetry measures blood oxygen saturation by monitor- ing the pulsatile change in light transmission (absorbance) due to changes in arterial blood volume. Conventional pulse oximetry devices measure arteriolar oxygen saturation (SaO 2 ) at the finger- tip or earlobe. The concentrations of oxyhemoglobin (HbO) and deoxyhemoglobin (Hb) are estimated from the change in light transmission associated with the cardiac cycle for two wave- lengths, typically one in the red and the other in the infrared range. It is a low-cost, non-invasive, simple technique that pro- vides continuous real time monitoring of blood oxygen supply. Research to improve pulse oximetry and expand its application has led to devices incorporating more than two transducers [1,2], reflectance mode operation [3–8], and venous oximetry [9–15]. The technique for measuring venous oxygen saturation (SvO 2 ) is consistent with traditional pulse oximeters, except the measure- ments are based on the venous blood volume change. This can be induced naturally with respiration [10,13], or through artificial means such as mechanical ventilation [14,15] or a blood pressure cuff [9,11,12]. In contrast to arterial oxygenation monitoring, which represents lung function, venous oxygenation provides a measure of hemoglobin saturation after the tissue is supplied, that is, after tissue oxygen consumption. In addition, when used in reflectance mode, the probe can positioned anywhere on the body. Therefore, reflectance venous oximetry can provide an assessment of local tissue health at sites (such as the limbs) that are not well suited for through-transmission measurements. Most recently, two dimensional (2D) camera-based non-contact oximetry systems have been developed [5,7,8,16,17]. This approach is especially suitable in cases where tissue contact discouraged, such as with ulcers and burns. Moreover, camera devices simplify the image analysis compared with using an array of fiber probes. Wierenga et al. [17] demonstrated that 2D photoplethysmographic (PPG) waveforms containing both the cardiac and venous components could be obtained at multiple wavelengths, but no measurements of tissue oxygenation were made. In the device developed by Hum- phreys and Ward [16], SaO 2 values were consistent with commercial pulse oximeter measurements taken from the finger. In recent years multispectral imaging techniques have also been developed for oxygenation measurements [5,7,8,18]. Multi- spectral imaging techniques provide a 2D map of tissue oxygena- tion (StO 2 ) through spectral analysis of a reflected white light source. StO 2 is a weighted average of SaO 2 and SvO 2 based on the volume ratio of the arterial blood to the venous blood. Oxygena- tion is extracted by fitting the results across multiple wavelengths to the known chromophore concentration absorption curves. Blood oxygenation measurement accuracy is improved with more wavelengths, but this method generally requires more specialized and expensive equipment. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/optcom Optics Communications 0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.06.041 n Corresponding author. Tel.: +1 206 598 9761; fax: +1 206 221 6578. E-mail address: leotta@u.washington.edu (D.F. Leotta). Optics Communications 308 (2013) 78–84