Spectroscopic visible light optical coherence tomography

In this project the possibility of using a supercontinuum in the visible wavelength range for spectroscopic analysis in combination with imaging, so called spectroscopic optical coherence tomography is investigated. Spectroscopic optical coherence tomography combines imaging of optical coherence tomography with localized absorption, which is used as functional information. These functional information are asked for from several medical doctors to improve diagnostics and screening.

By applying a different processing of the spectrometer data compared to conventional optical coherence tomography (OCT) spectral and structural information can be linked. Measurements show that by using several samples, such as blood or laser dyes their absorption spectrum can be reconstructed and that it is influenced by the used concentration. These data are measured in combination with high resolved structural information due to the wide bandwidth of the supercontinuum source and the broadband spectrometer. The measurements are done to possibly investigate the absorption of oxygen by comparing oxygenated and deoxygenated haemoglobin. This can be used in applications, such as retinal venous and arteriolar occlusions, diabetic retinopathy, glaucoma or assessment of central venous and arteriolar oxygen saturation.

As the laser dye rhodamine B has a similar absorption peak as oxygenated haemoglobin, as can be seen in Figure 1, it is used to evaluate our set-up and its performance.

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Figure 1
: Absorption spectra of haemoglobin and rhodamine B. Rhodamine B and haemoglobin have a similar absorption peak around 542 nm.

In a first measurement rhodamine B is filled into a 10 mm quartz cuvette and inserted into one interferometer arm and the absorption is measured. By varying the dye concentration (1:1000; 1:2000; 1:4000; 1:5000 and 1:1000000 [dye in ml: water in ml]) the influence of concentration variation on the spectroscopic OCT signal is investigated, which can be seen in Figure 2. Plotting the transmission as function of concentration, the data can be fitted by an exponential function, which describes the absorption as function of concentration (comes from the Beer-Lambert law).

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Figure 2: With spectroscopic OCT reconstructed absorption spectrum from the laser dye rhodamine B and their fitted absorption as a function of the concentration. 

In a next experiment the spectral information are combined with structural information. For this a small glass capillary is filled with rhodamine B and the localized absorption between the two glass-dye surfaces is reconstructed. A total of eight spectral bands are used to reconstruct the absorption spectrum. Each band has a different central wavelength, but a constant spectral width in wavenumber, which they cover.

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Figure 3: Depth information with marked dye position and reconstructed absorption spectrum of three different dye concentrations.

In Figure 3 the structural information and the reconstructed absorption spectra of different dye concentrations (not diluted; 1:10; 1:50 [dye in ml: ethanol in ml]) of rhodamine B in between the two glass-dye surfaces are shown.

Conclusion

We have shown that SOCT in the visible spectral range is suitable for measuring absorption of rhodamine B, which has similar absorption properties as proteins in the globin group. These measurements are verified in cuvette measurements and in localised reflection samples. Both samples are based on reflection, but we expect to be able to reconstruct similar results for a scattering sample and detect different absorption amplitude due to concentration changes. A phantom which combines absorption with scattering that imitates biological samples better than the cuvette and capillary measurements is considered for further optimisation.

Written by: Felix FLEISCHHAUER

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