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Molecular Imaging True Color Spectroscopic (METRiCS) OCT Francisco E. Robles 1,2, Christy Wilson 3, Gerald Grant 3 and Adam Wax 1,2 Nature Photonics DOI: 10.1038/NPHOTON.2011.257 Department of Biomedical Engineering 1, Medical Physics Program 2, and Pediatric Neurosurgery 3 Duke University, Durham, North Carolina 27708, USA
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Introduction OCT provides high resolution cross- sectional imaging A novel form of spectroscopic OCT (SOCT) is introduced to provide molecular imaging with high spatial and spectral resolution Spectroscopy provides insight into the molecular composition of samples A dual window processing methods is used to achieve high spatial and spectral resolution A laser light source with a bandwidth (BW) that spans the visible region of the spectrum is used Quantitative molecular imaging in true color is achieved
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Dual Window (DW) Method Typical processing methods for SOCT, including short time Fourier transforms and wavelet transforms, suffer from an inherent trade- off between the spatial and spectral resolution The DW method avoids this resolution trade-off by using two orthogonal windows that independently tune the resolution in each dimension * *Robles et al. Opt. Express 17, 6799-6812 (2009) High spatial and spectral resolution from the DW method allows for a quantitative treatment of the depth resolved spectra
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Parallel Frequency Domain OCT System The light source consists of a super continuum laser – Detection centered in the visible spectrum (450-700 nm) – Large BW yields an experimental axial resolution of 1.2 µm A cylindrical lens is used to deliver a line of illumination onto the sample Lateral resolution is 6 µm Detection is achieved by using an imaging spectrograph which detects 400 interferograms simultaneously Sample is translated along the y- dimension to acquire the three- dimensional (3D) data set Robles et al. Nature Photonics DOI: 10.1038/NPHOTON.2011.257
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Processing for METRiCS OCT Each interferogram is processed using the DW method The spectra at all points in the sample is divided into red, greed, and blue channels to provide a hue map with the sample’s true colors Robles et al. Nature Photonics DOI: 10.1038/NPHOTON.2011.257 – This provides an intuitive form of display of the spectral data The spectra may also be analyzed quantitatively to obtain parameters of interest such as hemoglobin oxygen levels (SO 2 )
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Animal model An in-vivo CD1 nu/nu normal mouse dorsal skinfold window chamber model was used * Mice were anaesthetized and the window chambers were removed before imaging Endogenous (Hb) and exogenous (sodium fluorescein; NaFS) molecular contrast were utilized – NaFS has an extinction maxima at ~494 nm, thus transmitted light appears red (bottom left). It fluoresces with a peak wavelength of ~521 nm thus appearing green at low concentrations (bottom right). FDA approved sodium fluorescein *Huang, Q. et al. Nature Biotechnol. 17, (1999).
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METRiCS OCT using Endogenous Contrast Conventional OCT imaging revealed tissue structures -E.g., muscle layer layer at the surface, lumen of blood vessels and the subcutaneous layer METRiCS OCT reveals the same structures with the addition of true-color molecular contrast -Muscle layer appears relatively colorless due to low Hb concentrations -Once light traverses through the vasculature network, a red shift is clearly observed due to the higher concentrations of Hb -Highly attenuating regions (e.g., vessels >100 µm in diameter) produce ‘shadow’ effect Robles et al. Nature Photonics DOI: 10.1038/NPHOTON.2011.257 Conventional OCT image METRiCS OCT image x-z scale bars are 100 µm
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Robles et al. Nature Photonics DOI: 10.1038/NPHOTON.2011.257
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METRiCS OCT using Endogenous Contrast Robles et al. Nature Photonics DOI: 10.1038/NPHOTON.2011.257 An en-face view of the volumetric data provides a global prospective of the vasculature network -The major vessel on the left is an artery, while the one of the right is a vain An important capability of METRiCS OCT is the ability to provide a quantitative analysis from the spatially- resolved spectra -Spectra from points (b)- (e) are measured and plotted on the right -The figure also shows the computed Hb SO 2 levels x-y scale bars are 100 µm
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METRiCS OCT using Exogenous Contrast Conventional OCT image METRiCS OCT image Robles et al. Nature Photonics DOI: 10.1038/NPHOTON.2011.257 x-z scale bars are 100 µm To date, conventional OCT has shown limited success in using exogenous contrast agents METRiCS OCT provides clear molecular contrast form the exogenous agent -The presence of NaFS is evident by a severe red shift in hue -NaFS also shows an increase in scattering from within the vessels
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Robles et al. Nature Photonics DOI: 10.1038/NPHOTON.2011.257
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METRiCS OCT using Exogenous Contrast Robles et al. Nature Photonics DOI: 10.1038/NPHOTON.2011.257 x-y scale bars are 100 µm Vessels in the en-face METRiCS OCT image are now characterized by the red hue of NaFS Large vessels still exhibit a ‘shadow’ Spectra of four points are quantitatively analyzed -The spectra now exhibits contributions form three absorbing species: oxy-Hb, deoxy-Hb and NaFS -NaFS only absorbes in the lower wavelength regions, thus SO2 levels may still be computed -ε = NaFS max abs /Hb max abs
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Conclusions METRiCS OCT provides molecular contrast using endogenous and/or exogenous agents The DW method allows for a thorough quantitative analysis Use of the large visible spectral bandwidth provides 1.2 µm resolution and allows for a true color representation of samples Introduces possibility of using readily available contrast agents for molecular contrast (e.g., using FDA approved NaFS) Robles et al. Nature Photonics DOI: 10.1038/NPHOTON.2011.257
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Acknowledgements Duke University BIOS Lab: Francisco E Robles, PhD Adam Wax, PhD Pediatric Neurosurgery: Christy Wilson, PhD Gerald Grant, MD A.W. is the founder and chairman of Oncoscope, which licenses the rights to intellectual property underlying this work. Funding NIH (NCI R01 CA 138594-01) Robles et al. Nature Photonics DOI: 10.1038/NPHOTON.2011.257
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