Quantitative Diffuse Reflectance Spectroscopy

In the field of biomedical optics, diffuse reflectance spectroscopy (DRS) is a frequently used technique for obtaining information about the optical properties of the medium under investigation. The method utilizes spectral difference between incident and backscattered light intensity for quantifying the underlying absorption and scattering processes that affects the light-medium interaction. In this thesis, diffuse reflectance spectroscopy (DRS) measurements have been combined with an empirical photon migration model in order to quantify myocardial tissue chromophore content and status. The term qDRS (quantitative DRS) is introduced in the thesis to emphasize the ability of absolute quantification of tissue chromophore content. To enable this, the photon migration models have been calibrated using liquid optical phantoms. Methods for phantom characterization in terms of scattering coefficient, absorption coefficient, and phase function determination are also presented and evaluated. In-vivo qDRS measurements were performed on both human subjects undergoing routine coronary artery bypass grafting (CABG), and on bovine heart during open-chest surgery involving hemodynamic and respiratory provocations. The application of a hand-held fiber-optic surface probe (human subjects) proved the clinical applicability of the technique as the results were in agreement with other studies. However, problems with non-physiological variations in detected intensity due to intermittent probetissue discontact were observed. Also, systematic deviations between modeled and measured spectra were found. By model inclusion of additional chromophores revealing the mitochondrial oxygen uptake ability, an improved model fit to measured data was achieved. Measurements performed with an intramuscular probe (animal subjects) diminished the influence of probe-tissue discontact on the detected intensity. It was demonstrated that qDRS could quantify variations in myocardial oxygenation induced by physiological provocations, and that absolute quantification of tissue chromophore content could be obtained. The suggested qDRS method has the potential of becoming a valuable tool in clinical practice, as it has the unique ability of monitoring both the coronary vessel oxygen delivery and the myocardial mitochondrial oxygen uptake ability. This makes qDRS suitable for directly measuring the result of different therapies, which can lead to a paradigm shift in the monitoring during cardiac anesthesia.