Feature Of The Week 08/06/2017: Depth-resolved multimodal imaging: Wavelength modulated spatially offset Raman spectroscopy with OCT
There is now a major drive to obtain complementary information using photonics approaches for biomedical studies. Such multimodality can improve diagnosis and increase the accuracy of determining the sample under investigation. A key advance to address this goal would be to obtain molecular as well as morphological information in a given 3D volume. Optical coherence tomography (OCT) is very powerful and can deliver morphological information at depths of up to millimeters. Raman is an ideal companion as it may deliver molecular information but is typically hampered by its poor penetration into a sample, of only 1/10 of a millimeter.
Our latest advance develops a new route using modulation of the input Raman illumination light as well as imposing an offset between the excitation and collection routes for the Raman signal in conjunction with OCT imaging. In turn this allows us to extract Raman and OCT signals at depth in tandem, thus giving multimodal information over a 3D volume.
We implement spatially offset Raman spectroscopy (SORS) using wavelength modulation. The laser spot and the collection point are imaged on the slit of the spectrometer, which is then relayed onto the CCD camera to get the spectral information. Therefore, the rows of binned pixels on CCD contain SORS information when they are not exactly on the rows of pixels, which correspond to the image of laser spot. In this way, we can acquire fluorescence-free Raman spectra using different spatial offsets simultaneously with one single snapshot. Due to the geometry we use, this is achieved in the absence of any moving parts. We call this original combination of wavelength modulated Raman spectroscopy (WMRS) and the technique of SORS, as WMSORS.
Our multimodal WM-SORS/OCT system can penetrate deep into highly scattering media to acquire Raman signals from the hidden targets. The system may achieve an enhancement factor as high as 14 using an optimal offset. We demonstrate our approach on samples including pharmaceutical tablets and brain tissue to show the ability of our system to reveal the hidden targets at depth (> 0.5 mm). We envisage our work will enable multimodal imaging studies for neuroscience.
For more information see recent Article. Courtesy Mingzhou Chen and Kishan Dholakia from University of St. Andrews.