Alexander Knuettel

Analytical modeling of spatial resolution curves in turbid media acquired with optical coherence tomography (OCT)

Optical coherence tomography (OCT) is an emerging alternative imaging tool to confocal microscopy. In layers beyond about 200 - 300 micrometers depth, an increasing fraction of multiple scattered photons begins to deteriorate diffraction limited axial and lateral resolution curves, which otherwise can be obtained only in very superficial layers (single-scatter regime). At greater depths, the contrast and resolution from OCT (and confocal microscopy) are determined by the parameters of the turbid medium rather than by the focusing optics. We have developed an analytical model to describe spatial resolution curves in homogeneous turbid media employing the heterodyne interferometric principle. Analogous to basic ideas from theoretical work done in the atmospheric LIDAR (Light Detecting and Ranging) community we derived the heterodyne detector signal from a mutual coherent function (MCF). The MCF is a function of the parameters of the focusing optics, the object position and the degree of coherence (lateral coherence length), which in turn characterizes the turbid medium. Axial resolution curves were acquired with our interferometer in reflection mode to characterize various turbid media by fitting the experimental data to simulation curves. Particularly when light propagates through suspensions of large particles before impinging on the object, a considerably loss in contrast (and even resolution) of the curves is noticeable. We studied the effects of a mirror and a diffuse plate serving as reflecting targets. In ex vivo tissue, we obtained a lateral coherence length on the order of 1 micrometers under the assumption of the validity of the model used.

Tissue characterization with optical coherence tomography (OCT)

Optical coherence tomography (OCT) is an emerging alterative imaging tool to confocal microcopy for diagnosing turbid tissue. In layers beyond about 200 to 300 micrometer depth, an increasing fraction of multiple scattered photons begins to deteriorate diffraction limited axial and lateral resolution curves, which otherwise can only be obtained in very superficial layers where the single scattering regime prevails. At greater depths, OCT images suffer contrast (and resolution) degradation due to multiple scattering. Recently, we have developed an analytical model to describe spatial point-spread function (PSF) curves in homogeneous turbid media base on the interferometric principle. It is shown that the parameter mean scattering angle can be derived with reasonable accuracy under the small-angle approximation (SAA) at a given (average) scattering coefficient. Axial PSF curves were acquired with our OCT interferometer in reflection mode to characterize skin tissue in vivo by fitting simulated curves to the experimental data. Mechanical through-focus translation of the focusing objective (around particular penetration depth) generated a single contrast arising from the single and multiple scattered photons. We made two assumptions: (1) the tissue is homogeneous on average and (2) this particular contrast is independent of the type of backscattering (on average). The latter assumption was approximately validated by simulations. Skin tissue probed at 300 and 400 micrometer penetration depth yielded a mean scattering angle (theta) RMS approximately equals 4 degrees at an average scattering coefficient of (mu) s approximately equals 11 mm-1. The small angle value indicates strong forward scattering from large particles.