Molecular imaging of endogenous and exogenous chromophores using ground state recovery pump-probe optical coherence tomography.

We present a novel molecular imaging technique which combines the 3-D tomographic imaging capability of optical coherence tomography with the molecular sensitivity of pump-probe spectroscopy. This technique, based on transient absorption, is sensitive to any molecular chromophore. It is particularly promising for the many important biomarkers, such as hemoglobin, which are poor fluorophores and therefore difficult to image with current optical techniques without chemical labeling. Previous implementations of pump-probe optical coherence tomography have suffered from inefficient pump-probe schemes which hurt the sensitivity and applicability of the technique. Here we optimize the efficiency of the pump-probe approach by avoiding the steady-state kinetics and spontaneous processes exploited in the past in favor of measuring the transient absorption of fully allowed electronic transitions on very short time scales before a steady-state is achieved. In this article, we detail the optimization and characterization of the prototype system, comparing experimental results for the system sensitivity to theoretical predictions. We demonstrate in situ imaging of tissue samples with two different chromophores; the transfectable protein dsRed and the protein hemoglobin. We also demonstrate, with a simple sample vessel and a mixture of human whole blood and rhodamine 6G, the potential to use ground state recovery time to separate the contributions of multiple chromophores to the ground state recovery signal.