Trude Støren

Comparison of speckle reduction using polarization diversity and frequency compounding in optical coherence tomography

We present a novel OCT (Optical Coherence Tomography) instrument which enables us to detect two orthogonal polarization states at two different wavelengths simultaneously. We have used this instrument to demonstrate, study, and compare the properties of speckle averaging using frequency compounding and polarization diversity separately and in combination. Reductions in speckle contrast obtained by measurements are compared to theoretical values and results from computer simulations of OCT signals.

Functional imaging of dye concentration in tissue phantoms by spectroscopic optical coherence tomography

We present functional imaging of the concentration of a photodynamic therapy (PDT)-related dye in scattering tissue phantoms based on spatially resolved measurements of optical properties through spectroscopic optical coherence tomography (OCT). Expressions for the OCT signal are developed, enabling estimation of depth-resolved sample optical properties. Based on these expressions, we discuss speckle statistics and speckle correlations of the OCT signal. Speckle noise reduction is performed by spatial filtering and is used to improve accuracy in the estimated optical properties at the expense of spatial resolution. An analytic expression for the precision in the estimated optical properties is derived. This expression shows that axial filtering, and thereby a reduction of axial resolution, gives a larger improvement in precision compared to the same filtering and reduction in the transversal resolution. It also shows that imaging with a shorter coherence length, or a larger numerical aperture, improves precision when the filter length determines the spatial resolution. Good agreement is obtained between experimentally determined and theoretically predicted variance in the estimated attenuation coefficients and dye concentration. Finally, we present guidelines for spectroscopic OCT systems for concentration imaging and discuss application of the method to more realistic phantoms and tissue.

Quantitative measurements of flow velocity and direction using transversal Doppler optical coherence tomography

We present the first demonstration of measurements of velocity and direction of flow using Transversal Doppler Optical Coherence Tomography. The experiments are carried out using a four-channel quadrant detector at the output of a freespace Michelson interferometer. This allows real three dimensional mapping of both flow and velocity with no limitation on the Doppler angle.

Measurement of dye diffusion in agar gel by use of low-coherence interferometry.

We demonstrate low-coherence interferometry for diffusion measurements. We have measured the diffusion coefficient of a phthalocyanine dye in 1.5% agar gel with a two-wavelength interferometer; one wavelength was matched to the absorption peak of the dye at 675 nm, while the other, 805 nm, was not affected by the dye. The diffusion coefficient of the dye was found by fitting a mathematical model for the interferometer signal to the measured low-coherence interferometry amplitude. A 95% confidence interval for the diffusion coefficient was found to be D = (2.5 +/- 0.2) x 10(-10) m2/s. The influence of speckle averaging and experiment time on the determination of the diffusion coefficient has been studied. The presented technique allows in situ characterization of diffusion in semitransparent media.

Measurement of dye diffusion in scattering tissue phantoms using dual-wavelength low-coherence interferometry.

We demonstrate low-coherence interferometry (LCI) for dye diffusion measurements in scattering tissue phantoms. The diffusion coefficient of a phthalocyanine dye in 1.5% agar gel containing scattering Intralipid was measured using a dual-wavelength interfero-meter. One wavelength was matched to the absorption peak of the dye at 675 nm. The other, 805 nm, was not affected by the dye, and was used to correct for varying sample scattering as a function of depth, assuming a constant ratio between scattering at the two wavelengths. The same wavelength dependence of scattering is assumed for the entire sample, but no a priori knowledge about the amount of scattering is needed. The dye diffusion coefficient was estimated by fitting a mathematical model of the interferometer signal to the measured LCI envelope. We compare results obtained using both a constant-scattering and a depth-resolved-scattering approach to determine the sample scattering. The presented method provides robust estimation of the diffusion coefficient when spatial resolution in determining the depth-resolved scattering is varied. Results indicate that the method is valid for samples having continuous spatial variations in the scattering coefficient over lengths as short as the coherence length of the probing light. The method allows in situ characterization of diffusion in scattering media.

Low-coherence speckle interferometry (LCSI) for detection of interfacial instabilities in adhesive-bonded joints

In this paper the basic principles of Low Coherence Speckle Interferometry (LCSI) are described. Theoretical background and experimental results for the systematic investigation of LCSI are presented. To understand and quantify the measurement results of adhesive bonded joints a modelling of the interference signal is required. For this purpose, a one-dimensional transmission line model is developed, including changes in the refractive index in a stressed adhesive layer and delamination of the glued interface. A new method for the detection of zero path length difference is introduced. Investigations of the probing depth in semi-transparent adhesive and recent experimental results of the characterisation of adhesive-bonded aluminium joints are presented.

Time-resolved spectroscopic optical coherence tomography for diffusion measurements

We demonstrate that spectroscopic optical coherence tomography can be used for measurement of diffusion. We measured the diffusion coefficient of a Phthalocyanine dye in an Agar gel as a first model of dye diffusing into tissue. We used a two-wavelength interferometer, with one of the wavelengths matched to the absorption peak of the dye at 675nm while the other wavelength at 805nm is not affected by the dye. The diffusion constant of the dye in Agar gel is found by fitting the measured OCT amplitude (depth and time dependent) to a mathematical model for the OCT signal. This method may be used as a tool for dosimetry in in Photo Dynamic Therapy (PDT). In PDT the therapeutic light exposure should be applied at a time when the concentration of sensitizer is optimal in the diseased tissue relative to normal tissue. By studying how the OCT signal changes with time and depth at two wavelengths differently affected by the diffusing dye, it should be possible to extract parameters determining diffusion of the sensitizer in live tissue. In comparison with fluorescence-based methods, this OCT approach has the advantage of better depth penetration and being able to account for attenuation effects due to scattering.

Multiple-depth probing by wavelength-multiplexed low-coherence interferometry

We demonstrate a wavelength multiplexed low coherence interferometer that detects and demodulates four subbands of the source spectrum in parallel. By introducing dispersion into one of the interferometer arms we obtain a wavelength dependent measurement depth in the object. We analyze the influence of the dispersion on the signals and demonstrate simultaneous Doppler measurements of dynamic flow at three different positions within a tube. The method can be used to remove false Doppler signals caused by an unsteady object and therefore has potential in blood flow monitoring.

Absorption measurements using low-coherence interferometry

The basic use of low coherence interferometry is measuring the intensity of light reflected from defined depths in a partially transparent object. The light back-scattered from a certain depth in the object carries information about the medium through which it has passed. Absorption and scattering attenuate the intensity of the light, and will thus reduce the interference signal from a given depth. One single interference measurement will not discriminate between attenuation due to absorption and scattering. However, by measuring the interference signal at several different wavelengths, discrimination between the two parameters is possible, if they vary differently with wavelength. Especially when measurements are performed in a wavelength region where the scattering coefficient is constant, measurements at two different wavelengths will give the difference in absorption at the two wavelengths. We present preliminary measurements on scattering and absorbing solutions showing a qualitative difference in absorption measured at 810 and 830 nm. As an absorbing solution we used a commercially available dye with an absorption maximum at 800 nm while intralipid was used to introduce scattering. The aim of the work is to measure the oxygenation saturation of blood through absorption measurements at different wavelengths using low coherence interferometry.

Speckle noise in polarization sensitive optical coherence tomography

We have developed a novel noise model for the analysis of speckle noise and accuracy in birefringence imaging with polarization sensitive optical coherence tomography. Analytical expressions for the signal and noise in Stokes vectors are found, and these are used to investigate the uncertainty in the estimation of amplitude and orientation of birefringence. The important parameter in the model is the correlation between local reflectivity of the two orthogonal polarizations.