Transient-mode photothermal optical coherence tomography (TM-PT-OCT) is introduced as a high-speed and video rate implementation of PT-OCT. Here, the transient thermal response of samples to a low power PT laser pulsed excitation is interrogated through temperature-induced variations in the OCT phase signals. Results suggest that the proposed method enhances the PT-OCT imaging speed by more than two orders of magnitude compared to conventional PT-OCT with lockin detection. This enhancement not only enables video rate visualization of molecules of interest (MOI), but also opens the door for use of multiple PT lasers to perform spectroscopic PT-OCT for detection and differentiation of multiple MOI’s with distinct absorption spectra. To demonstrate feasibility, experimental results on detection and differentiation of lipid and collagen are presented and discussed.
A comprehensive theoretical model for photothermal optical coherence tomography (PT-OCT) has been developed considering opto-thermo-mechanical properties of sample and illumination conditions. Parametric studies show developed model, unlike previous models, can predict characteristic PT-OCT signal behaviours.
Photothermal optical coherence tomography (PT-OCT) is an extension of OCT that forms images based on both scattering and absorption of light. Conventional OCT measures the path length of elastically back-scattered light. Variation of the tissue’s local refractive index due to photothermal modulation results in a modulated OCT phase signal that relates to the absorption of light. Detailed understanding of the PT-OCT signal has the potential to provide insight into the chemical composition of tissue, and may pave a way to detecting and characterizing lipid-rich atherosclerotic plaques with intracoronary imaging in patients. Here, we investigated the dependence of the PT-OCT signal on concentration of a specific tissue component. We modeled the generated thermal wave field and the resulting PT-OCT signal as a function of concentration of this component using the bio-heat equation. We found that the significant parameters in determining the amplitude of the PT-OCT signals are the density, the absorption coefficient and the specific heat of the sample. All these parameters vary as a function of the sample composition, leading to a non-linear relation between PT-OCT signal and the concentration of the component of interest. Only in special cases, e.g. the oxygenation level of blood, when the absorption coefficient is the only varying parameter does a linear dependence arise. PT-OCT experiments on tissue-like samples, prepared by mixing mayonnaise (<80% lipid) and agar gel (<90% water) to mimic lipid-rich atherosclerotic plaques, confirmed the non-linear relation predicted by our model.
Significance: Photothermal optical coherence tomography (PT-OCT) has the promise to offer structural images coregistered with chemical composition information, which can offer a significant impact in early detection of diseases such as atherosclerosis.
Aim: We take the first step in understanding the relation between PT-OCT signals and the endogenous tissue composition by considering the interplay between the opto-thermo-physical properties of tissue as a function of its lipid composition and the ensuing effects on the PT-OCT signals.
Approach: Multiparameter theoretical estimates for PT-OCT signal as a function of composition in a two-component lipid–water model are derived and discussed. Experimental data from various concentrations of lipid in the form of droplets and injections under bovine cardiac muscle align with theoretical predictions.
Results: Theoretical and experimental results suggest that the variations of heat capacity and mass density with tissue composition significantly contribute to the amount of optical path length difference measured by OCT phase.
Conclusion: PT-OCT has the potential to offer key insights into the chemical composition of the subsurface lipid pools in tissue; however, the interpretation of results needs to be carried out by keeping the nonlinear interplay between the tissue of opto-thermo-physical properties and PT-OCT signals in mind.
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