Examination of the oxygenation state of hemoglobin in a phantom and in-vivo tissue applying absorption balancing with two and three laser wavelengths

Two-dimensional absorption measurements of hemoglobin in (i) a phantom, imitating the optical properties of tissue, and (ii) in-vivo breast tissue are reported. The method is based on quasi-simultaneous balanced absorption measurements of hemoglobin and oxygenated hemoglobin at selected wavelengths in order to minimize baseline fluctuations during spatial scanning of the sample. The method allows the detection of 0.8 cm-diameter inclusions of oxygenated hemoglobin differing by about 30% in its oxygenation state from the surrounding hemoglobin if two-wavelengths balancing is applied. However, small wavelength detunings from the optimum wavelength positions cause considerable fluctuations of the signal baseline and deteriorate the detection sensitivity, if there is an inhomogeneous distribution of another weak wavelength dependent absorber in the sample. Although the optical absorption of water is relatively small for the wavelengths applied, a considerable influence of the water on the signal baseline was observed in in-vivo measurements of the hemoglobin status in healthy breast tissue. For better compensation of interfering water absorption, a third laser wavelength was applied in order to balance the water absorption. The prospects of this new optical technique for screening of breast tumors are discussed.