Molecular Imaging

Spectroscopic OCT Spectroscopic OCT is an extension of OCT technology where not only the structural information, but also the spectroscopic information is retrieved. It is based on the principle that the bandwidth of a light source used in OCT is broad, therefore by using appropriate time-frequency analysis, a depth-resolved spectroscopy study can be performed. Spectroscopic OCT has at least two imaging targets: imaging spectral absorption and spectral scattering. These can be used to detect either endogenous molecules or exogenous agents.

Wavelength-dependent spectral scattering The spectral-scattering information contained within the SOCT signal can be used diagnostically to assess scatterer size and spatial distribution in cells and tissues. Dominant scatterers include nuclei and mitochondria. This technique is similar to Light Scattering Spectroscopy, except SOCT can perform analysis in depth, and in three-dimensions.

Coherent CARS beam is generated in both arms and interfered to generate a molecule-specific image.

Nonlinear Interferometric Vibrational Imaging (NIVI) NIVI draws from two known imaging techniques: coherent anti-Stokes Raman scattering (CARS) to present molecular contrast and OCT to present structural contrast. CARS is used to determine whether a specific molecular species is present and OCT is used to determine where in the sample the molecular species is located. CARS signals are generated in both interferometer arms and the interference signal between these two arms is detected. Therefore, an image can be generated that maps out the location of the known species in the sample. In the future, we hope NIVI will be used as a novel diagnostic technique for early detection of cancer through molecular contrast.

NIVI, which utilizes interferometric detection and three-dimensional spatial localization of the coherent CARS signals, offers numerous advantages over standard photon-counting methods typically used in CARS microscopy. Interferometric spectral detection enables increased sensitivity and phase determination of the nonlinear susceptibility. This can be used to separate the resonant molecular signals from the non-resonant background noise and improve detection.

Experimental set-up and high-resolution CARS spectral data obtained using interferometric detection of chirped broadband pulses.

Contrast Agents Novel molecularly-targeted contrast agents are being developed which enhance the diagnostic imaging capabilities of OCT and other non-fluorescent or non-bioluminescent imaging techniques. These include scattering microspheres as contrast and drug-delivery agents, magnetomotive nanoparticles, plasmon-resonant nanoparticles, and absorbing near-infrared dyes. In addition, there are a large number of optical properties and characteristics from contrast agents that can be leveraged to enable optical molecular imaging of cells and tissues.

Magnetomotive OCT of single cells dispersed in a 3-D gel. Cells containing magnetite are clearly identified.

Ultrastructural Imaging Polarization-sensitive OCT (PS-OCT) is being used to reveal alterations in the ultrastructure of tissue. Specifically, skeletal muscle is highly ordered and birefringent. Genetic mutations which alter the structural proteins in muscle result in more disorganization and subsequently, less birefringence. PS-OCT may prove useful for non-invasively or minimally-invasively asessing tissue health, injury, and disease by tracking changes in the ultrastructure.