Three-dimensional reconstruction of subsurface absorbing structures in tissue phantoms from photothermal radiometric records

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 threedimensional 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 -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 × 3 × 0.6 mm3 was obtained in only 45 seconds.