Optical-coherence tomography of a dense tissue: statistics of attenuation and backscattering

This paper addresses fundamental issues that underlie the interpretation of images acquired from turbid tissues by optical-coherence tomography (OCT). The attenuation and backscattering properties of freshly excised rat arteries and their dependence on the focusing and collection optics of the OCT system were measured at two wavelengths in the near infrared (830 nm and 1300 nm). Determined from the ratio of the magnitudes of the reflections from glass plates placed on both sides of the arteries, the mean attenuation coefficient of the arterial wall was found to be in the range 14 < microt < 22 mm(-1) at 830 nm and 11 < microt < 20 mm(-1) at 1300 nm. The measured values of microt were lowest for the longer source wavelength and for probe beams with the smallest average diameters. The observed dependence of microt on beam size indicates that relatively large-scale variations in the index of refraction of the tissue contributed to degradation of the tranverse spatial coherence of the beam. We introduce a framework for understanding and quantifying beam-size effects by way of the mutual-coherence function. The fact that spatial variations in backscattering and attenuation (which includes spatial-coherence losses) have similar effects on OCT signals makes the origin of the signals difficult to determine. Evidence is given that suggests that, in spite of this difficulty, certain features of microstructures embedded several hundred micrometres deep in a turbid tissue can still be detected and characterized.