Prof. Dr. Boris Majaron (IJS, FMF): Photothermal tomography: an alternative approach to imaging of subsurface structures in strongly scattering biological tissues

Pulsed photothermal radiometry explores transient changes in Planck's emission from an object's surface after irradiation with a short light pulse. From the radiometric record acquired with a fast mid-IR camera, light-induced temperature field inside the sample can be reconstructed by solving the inverse problem of heat diffusion and blackbody emission. This principle could enable tomographic imaging of selectively absorbing structures in strongly scattering biological tissues without application of ionizing radiation, strong magnetic fields, or problematic contrast agents. However, the task is very challenging due to severe ill-posedness of the underlying inverse problem confounded with inherently low signal-to-noise ratios of mid-IR radiometry.
I will present our recent realization of such a photothermal tomography (PTT) system and discuss the steps introduced to overcome various adverse issues. These range from the huge size and poor convergence of the optimization problem to non-uniform emissivity of the sample surface and unavoidable limitations of the mid-IR imaging system. In physical models made from strongly scattering collagen gels, we obtain quite sharp and high-contrast tomographic images of hairs or threads with diameters of 40-160 mm at depths over 1 mm. Imaging with a lateral resolution exceeding that of the imaging system was demonstrated with no concerning artifacts or noise in the reconstructed volume. I will also present one example of PTT imaging in human skin in vivo and discuss the possibility of using the described system for accurate assessment of thermal diffusivity in multicomposite media, which thus far eludes theoretical derivation.
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