Label-free bond-selective imaging of atherosclerosis

Atherosclerosis, the major cause of cardiovascular diseases, has been a leading contributor to morbidity and mortality in the United States. The statistics that account for the rise in incidence consequently call for new imaging techniques to advance the research and diagnosis of atherosclerosis. My thesis work focused on developing novel imaging tools for diagnosis of early lesions and plaque vulnerability. In particular, my study involved the development of label-free chemically selective imaging tools based on signals from inherent molecular vibration. The first part of my study involves the development of multimodal coherent anti-Stokes Raman scattering (CARS) microscopy for label-free imaging of multiple components in an arterial wall. My studies have demonstrated CARS-based nonlinear optical imaging for identifying arterial cells and stratifications ex vivo [Opt. Comm. 281(7), 2008], verifying different atherosclerotic lesion types quantitatively [ATVB 29(9), 2009] and inspecting stentation effects on stenosis and collagen expression in pig coronaries [JBO 16(2), 2011]. Supported by an AHA Predoctoral Fellowship, my study on ApoE deficient mice further revealed an inflammation milieu of populated foam cells in adventitia as a surrogate marker of plaque vulnerability. My research work also suggested the applicability of the vibrational signal in monitoring dynamic cell growth [Organogenesis 2009] and cell counting [Opt. Express 2008; 16]. Together these studies herald the usefulness of vibrational imaging in providing mechanism insights of diseases. The second part of my thesis work aimed to overcome the limited penetration depth (∼100 micron) of nonlinear optical imaging modalities. Via team work, I developed a vibrational photoacoustic (VPA) microscope for deep tissue imaging by using optical excitation of molecular overtone vibration and acoustic detection of the resultant pressure transients [Phys. Rev. Lett. 106(23), 2011]. This approach eliminates the tissue scattering problem encountered in near-infrared spectroscopy and enables depth-resolved signal collection. The overtone vibration at near-infrared range, where blood interference is minimal, is excited. 3-D VPA imaging of lipid-rich atherosclerotic plaques in intact arteries, and lipid storage in live Drosophila larvae, is demonstrated with millimeter-scale penetration depth. This method opens a new avenue toward intravital diagnosis of, but not limited to, lipid-related disorders.