QUANTITATIVE BROAD-BAND NEAR-INFRARED SPECTROSCOPY OF TISSUE-SIMULATING PHANTOMS CONTAINING ERYTHROCYTES

We report the use of steady-state diffuse reflectance spectroscopy (SS DRS) to measure the near-infrared absorption spectrum of liquid phanto ms containing human erythrocytes in aqueous suspensions of polystyrene spheres which simulate the scattering properties of tissue. The absor ption spectra obtained from these SSDRS measurements of intact red cel ls under oxygenated and deoxygenated conditions are compared with seve ral published spectra of 'stripped' haemoglobin prepared from lysed ce lls. Two fitting algorithms (nonlinear least squares and singular valu e decomposition) which exploit the broad spectral range provided by th ese measurements (170 data points spanning 164 nm in a single acquisit ion) are used to determine haemoglobin oxygen saturation (SO2) from SS DR spectra collected over a wide range of measured oxygen partial pres sures. The validity of these algorithms is assessed by comparing liter ature values of p(50) (the oxygen tension at which haemoglobin is 50% saturated) and the Hill coefficient to values of these parameters dete rmined from the SO2 estimates. The singular value decomposition algori thm can also be used to reconstruct the non-haemoglobin background abs orption spectrum without a priori assumptions regarding its constituen t chromophores or their concentrations. Using this technique, the abso rption spectrum of a small amount of India ink (maximum absorption coe fficient (mu(a max)) approximate to 0.0006 mm(-1)) added to a phantom containing red cells (mu(a max) approximate to 0.026 mm(-1)) was recon structed over a full range of oxygen saturations. The implications of these measurements for detection of weakly absorbing chromophores (suc h as cytochrome aa(3)) in the presence of haemoglobin are discussed.