Estimation of the scattering coefficients of turbid media using angle-resolved optical frequency-domain imaging

Abstract

Noninvasive measurements of the scattering coefficients of optically turbid media using angleresolved optical frequency-domain imaging (OFDI) are demonstrated. It is shown that, by incoherently averaging OFDI reflectance signals acquired at different backscattering angles, speckle noise is reduced, allowing scattering coefficients to be extracted from a single A-line with much higher accuracy than with measurements from conventional OFDI and optical coherence tomography systems. Modeling speckle as a random phasor sum, the relationship between the measurement accuracy and the number of compounded angles is derived. The sensitivity analysis is validated with measurements from a tissue phantom.Knowledge of the scattering coefficients of biological tissues is important for the development and application of noninvasive optical diagnostic and therapeutic methods. Many different methods have been documented for quantifying the scattering (μ s ), absorption (μ a ), total scattering coefficients (μ t =μ a + μ s ), and reduced scattering coefficients noninvasively, using approximations and inversion models of varying complexity. These methods include spatially resolved steady-state diffuse reflectance [1], time-domain diffuse reflectance [2], frequency-domain diffuse reflectance [3], confocal microscopy [4], modulated imaging [5], and optical coherence tomography (OCT) [6]. OCT and its Fourier-domain counterpart, optical frequency-domain imaging (OFDI) [7], are particularly attractive methods, as they allow measurements of heterogeneous structures in vivo. With the single-backscattering model, measurements of μ s or μ t can be made, depending on whether the effects of absorption can be neglected. Models that account for multiple scattering have been shown to allow measurements of μ s and g, the anisotropy parameter [8]. The accuracy of scattering coefficient measurements with conventional OCT and OFDI is limited by speckle, however, which greatly reduces the signal-to-noise ratio. Although averaging over many spatial locations can reduce speckle and permit measurements of μ s with high accuracy [9], this type of averaging reduces the resolution with which differences in scattering coefficients can be resolved. Angle-resolved OFDI was recently demonstrated as a novel method of acquiring OFDI images that allows substantial speckle reduction by averaging the magnitudes of multiple interferometric scans (A-lines) obtained simultaneously from a broad range of backscattering angles [10].Angle-resolved OFDI signals are acquired in the Fourier domain as a function of the laser source wavelength, which is swept in time. Reconstruction of depth-resolved reflectance profiles is performed separately for signals that correspond to different backscattering angles. As such, the reconstructed signals I(z m , θ n ) are functions of the discrete depth variable z m and the discrete angular variable θ n ; the latter is measured relative to the incident beam. In the case that only one polarization component of the backscattered fiel...

Frequency domain multiplexing for speckle reduction in optical coherence tomography

“…Compounding of amplitudes (rather than magnitudes; k • k 2 ) to facilitate comparison with earlier studies on OCT speckle compounding. 9,[12][13][14][15][16] We show results in this study for 2 < w < 7, using a Hamming window for apodization. We quantify the effect of the filter by two measures:…”

“…It requires, however, that multiple realizations of the same scene or object can be sampled while one parameter (angle, wavelength, position) is varied in a controlled manner. [12][13][14][15][16] Compounding strategies generally achieve a CNR improvement proportional to the square root of the number of realizations. Digital filtering aims to reduce speckle by post-processing, while preserving image resolution, contrast, and edge fidelity.…”

Tissue characterization with ballistic photons: counting scattering and/or absorption centres

“…Biomedical diagnosis is often based on the characterization of tissue samples using the optical tech− niques, such as the response to the coherent or non-coherent illumination, including reflection (optical coherence tomog− raphy), transmission [4,5], absorption, and scattering [6][7][8][9], in either time−integrated or time−resolved applications [10][11][12][13][14][15].…”

Efficient reduction of speckle noise in Optical Coherence Tomography

“…Another application of speckle pattern analysis is optical elastography, in which position of single speckles in the image is tracked as a function of pressure applied to the sample [21,22]. There were also attempts to measure mechanic, scattering and absorptive properties of samples using speckle pattern analysis [23][24][25][26][27][28].…”

“…Retinal layer visibility and segmentation can be improved by the speckle reduction [30,31]. Additionally, the reduced speckle pattern caused by multiple scattering can improve the reliability of detecting scattering coefficient, as well as the spectroscopic analysis [24][25][26]32,33]. Because of that, several research groups and companies attempt to create techniques to decrease speckle noise (in other words: to decrease speckle pattern contrast) using either software or hardware solutions.…”