Development of Molecular Contrast in Coherence Domain Optical Imaging

Optical imaging has been developed quickly in the past decades because it has become an important research tool in biology, biochemistry, and biomedical sciences. Coherence domain optical imaging is one of the well developed optical imaging modalities, as it provides high resolution and long penetration depth. In this dissertation, we will report our work on development of molecular contrast in coherence domain optical imaging.

In order to image important molecules which are poor fluorophores, we developed a high resolution molecular imaging technique, pump-probe optical coherence microscopy (PPOCM), which does not rely on fluorescent tags. PPOCM is the fusion of Pump-Probe spectroscopy and optical coherence microscopy (OCM). We have demonstrated the prototype system on a fixed human skin sample containing a nodular melanoma. The results indicate that PPOCM can clearly provide strong contrast between the melanotic and amelanotic regions. This technique can be applied to early diagnosis of melanoma and the mapping of tumor margins during excision. It also can be extended to any biological chromophore with a known absorption spectrum and sufficient concentration. In order to differentiate further multiple chromophores, we developed a spectrally resolved two color pump-probe Optical Coherence Microscopy (SRPPOCM). We showed the prototype system on a red hair and a black hair. Our preliminary results show that the SRPPOCM technique could provide a contrast between pheomelanin and eumelanin. This technique could be used potentially as a clinical tool for diagnosing different progression stages of melanoma. This technique could also be applied to differentiate other mixed chromophores.

Second harmonic optical coherence tomography (SHOCT) is non-linear high resolution optical molecular imaging modality which is widely used in non-centrosymmetric material. However, depth ambiguity is associated with SHOCT in tissue sample because forward generated second harmonic signal does not correctly report where the second harmonic signal is generated. We studied the feasibility of collecting the backward generated second harmonic signal from nanocrystals through a Second Harmonic Optical Coherence Tomography in Fourier domain. The preliminary result shows that we can collect backward generated second harmonic signal from nanocrystals which indicates that this technique could suppress the depth ambiguity.