Abstract

Pulsed photothermal radiometry involves measurements of transient changes in blackbody emission from a sample surface after irradiation with a short light pulse. From such a radiometric record, light-induced temperature field inside the sample can be reconstructed by solving the inverse problem of heat diffusion and radiation. In principle, this enables three - dimensional visualization of selectively absorbing structures inside strongly scattering biological tissues and organs, a.k.a. photothermal tomography (PTT).

We present an up-to-date realization and testing of PTT in an agarose tissue phantom with a suspended human hair, imitating a subsurface blood vessel. After irradiating the phantom with a milisecond laser pulse at 532 nm, its surface was imaged with a fast mid-infrared (IR) camera equiped with a microscope objective. A custom code was used to reconstruct the laser-induced temperature field in three dimensions by running multidimensional optimization based on analytically formulated forward problem of heat transport and IR emission, using the projected v-method algorithm. We demonstrate that quadratic binning of the radiometric record enables a 10-fold reduction of the computational time without adversely affecting the results. In the presented example, a sharp image of a hair at a subsurface depth of >200 μm with no significant noise or artifacts elsewhere in the imaged volume of 3 x 3 x 0.6 mm³ was obtained in only 45 seconds.

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