A method for determination of the absorption and scattering properties interstitially in turbid media

Abstract

We have developed a method to quickly determine tissue optical properties (absorption coefficient μa and transport scattering coefficient μ′s) by measuring the ratio of light fluence rate to source power along a linear channel at a fixed distance (5 mm) from an isotropic point source. Diffuse light is collected by an isotropic detector whose position is determined by a computer-controlled step motor, with a positioning accuracy of better than 0.1 mm. The system automatically records and plots the light fluence rate per unit source power as a function of position. The result is fitted with a diffusion equation to determine μa and μ′s. We use an integrating sphere to calibrate each source–detector pair, thus reducing uncertainty of individual calibrations. To test the ability of this algorithm to accurately recover the optical properties of the tissue, we made measurements in tissue simulating phantoms consisting of Liposyn at concentrations of 0.23, 0.53 and 1.14% (μ′s = 1.7–9.1 cm−1) in the presence of Higgins black India ink at concentrations of 0.002, 0.012 and 0.023% (μa = 0.1–1 cm−1). For comparison, the optical properties of each phantom are determined independently using broad-beam illumination. We find that μa and μ′s can be determined by this method with a standard (maximum) deviation of 8% (15%) and 18% (32%) for μa and μ′s, respectively. The current method is effective for samples whose optical properties satisfy the requirement of the diffusion approximation. The error caused by the air cavity introduced by the catheter is small, except when μa is large (μa > 1 cm−1). We presented in vivo data measured in human prostate using this method.