Design and Optimize a Two Color Fourier Domain Pump Probe Optical Coherence Tomography System

Molecular imaging using fluorescence spectroscopy-based techniques is generally inefficient due to the low quantum yield of most naturally occurring biomolecules. Current fluorescence imaging techniques tag these biomolecules chemically or through genetic manipulation, increasing the complexity of the system. A technique capable of imaging these biomolecules without modifying the chromophore and/or its environment could provide vital biometric parameters and unique insights into various biological processes at a molecular level. Pump probe spectroscopy has been used extensively to study the molecular properties of poorly fluorescing biomolecules, because it utilizes the known absorption spectrum of these chromophores. Optical Coherence Tomography (OCT) is an optical imaging modality that harnesses the power of low coherence interferometry to measure the 3-D spatially resolved reflectivity of a tissue sample. We plan to develop a new molecular imaging modality that combines these techniques to provide 3-D, highresolution molecular images of various important biomolecules. The system uses a Fourier domain OCT setup with a modified sample arm that combines the "pump" and "probe" beams. The pump beam drives the molecules from the ground state to excited state and the probe interrogates the population change due to the pump and is detected interferometrically. The pump and the probe beam wavelengths are optimized to maximize absorption at the pump wavelength and maximize the penetration depth at the probe wavelength. The pump-probe delay can be varied to measure the rate at which the excited state repopulates the ground state, i.e., the ground state recovery time. The ground state recovery time varies for different chromophores and can potentially be used to identify different biomolecules. The system was designed and optimized to increase the SNR of the PPOCT signals. It was tested by imaging hemoglobin and melanin samples and yielded encouraging results. Potential applications of imaging hemoglobin using this technique include the mapping of tissue microvasculature and measuring blood-oxygen saturation levels. These applications could be used to identify hypoxic areas in tissue. Melanin imaging can provide means of demarcation of melanoma in various organs such as skin, eye and intestines.