Wavelength Selection Method with Standard Deviation: Application to Pulse Oximetry CAMILLE VAZQUEZ-JACCAUD,GONZALO PAEZ, and MARIJA STROJNIK Infrared Physics Group, Centro de Investigaciones en Optica, Apartado Postal 1-948, 37150 Leon, Gto., Mexico (Received 14 December 2010; accepted 23 March 2011; published online 2 April 2011) Associate Editor Miklos Gratzl oversaw the review of this article. Abstract—Near-infrared spectroscopy provides useful bio- logical information after the radiation has penetrated through the tissue, within the therapeutic window. One of the significant shortcomings of the current applications of spectroscopic techniques to a live subject is that the subject may be uncooperative and the sample undergoes significant temporal variations, due to his health status that, from radiometric point of view, introduce measurement noise. We describe a novel wavelength selection method for monitoring, based on a standard deviation map, that allows low-noise sensitivity. It may be used with spectral transillumination, transmission, or reflection signals, including those corrupted by noise and unavoidable temporal effects. We apply it to the selection of two wavelengths for the case of pulse oximetry. Using spectroscopic data, we generate a map of standard deviation that we propose as a figure-of-merit in the presence of the noise introduced by the living subject. Even in the presence of diverse sources of noise, we identify four wavelength domains with standard deviation, minimally sensitive to temporal noise, and two wavelengths domains with low sensitivity to temporal noise. Keywords—Standard deviation, Pulse oximetry, Spectros- copy, Medical optics, Human functions as temporal noise, Near-IR. INTRODUCTION The relatively low radiation attenuation within the biological tissues in the spectral interval [650– 1050 nm], also known as the therapeutic window, makes it of great current interest for patient monitor- ing. 6,8–10,12,13,18,35,40 The great advantage of the optical techniques is that they are noninvasive and nonde- structive, producing no damage to the subject under test. Possibly, their greatest disadvantage is that the diagnostic work on a live subject introduces many adverse conditions, as compared to those in vitro. As an example, we may mention the introduction of miscellaneous temporal noises due to functioning of live organs, producing uncontrollable temporal effects, some of which may be quite irregular and unpredict- able. They include unexpected (muscle) spasms or involuntary contractions, due to the patient’s health status. 26,30,32,41 We refer to these effects as wiggly human, similar to a small child who wiggles, refusing to be immunized, until he/she is immobilized. Spec- troscopic data on a living specimen are obscured by appreciably more noise than those on material with comparable composition, such as phantoms, or tissue in vitro. This is probably one of the principal reasons that the optical techniques have been slow in their incorporation into patient diagnosis, despite strong funding for over the last 30 years. The (pulse) oximetry to monitor the degree of oxy- gen saturation is a technology that has been success- fully transferred from the conceptual stage to clinical applications. An important fact of such monitors is their capability of early determination of physiologic condition. 1,42 However, under less than favorable conditions, including excessive noise, the potential benefits may be limited. Hence, it is decisive to achieve higher performance and robustness. Next, we briefly digress and review the current state of (pulse) oximetry as it leads to the main objective of the current research: finding the probe wavelengths using a scientific opti- mization method that produce reliable results despite presence of known and random sources of noise. Oximetry is a noninvasive technique to monitor oxygen saturation in tissue for medical diagnosis: in patients suffering respiratory or chronic obstructive pulmonary diseases, or acute asthma, and their effects. 15,33–35 It has been incorporated as a standard monitoring device in surgery, intensive care, and Address correspondence to Gonzalo Paez, Infrared Physics Group, Centro de Investigaciones en Optica, Apartado Postal 1-948, 37150 Leon, Gto., Mexico. Electronic mail: jaccaud1@cio.mx, gpaez@cio.mx Annals of Biomedical Engineering, Vol. 39, No. 7, July 2011 (Ó 2011) pp. 1994–2009 DOI: 10.1007/s10439-011-0304-7 0090-6964/11/0700-1994/0 Ó 2011 Biomedical Engineering Society 1994