Mr. Tomaž Cvetko Profile

Tomaž Cvetko

at Univ of Ljubljana

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Proceedings Article | 13 March 2024 Presentation + Paper

Photothermal tomographic imaging of optically absorbing inclusions in human skin in vivo

Katja Arh, Sara Klopčič, Tomaž Cvetko, Boris Majaron

Proceedings Volume 12816, 1281606 (2024) https://doi.org/10.1117/12.3000045

KEYWORDS: Skin, Tomography, Mid infrared, In vivo imaging, Tissue optics, Biological samples, Medical image reconstruction, Photothermal effect

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The concept of photothermal tomography (PTT) involves spatially and temporally resolved detection of blackbody emission from the sample surface after irradiation with a short light pulse. In principle, this allows reconstruction of the light-induced temperature field inside the sample, thus enabling three-dimensional imaging of absorbing structures in strongly scattering biological tissues and organs. However, development of accurate and robust PTT methodology has proven difficult due to the large size and severe ill-posedness of the underlying inverse problem, aggravated by the inherently low signal-to-noise ratios of active infrared radiometry. We discuss here our recently developed PTT system and its first application to human skin in vivo. The experimental setup involves a medical-grade laser emitting milisecond pulses at 532 nm, and a fast mid-infrared camera equipped with a microscope objective. A custom code written in Python is used to reconstruct three-dimensional images of the absorbing structures by performing iterative multidimensional minimization of the difference between the analytically predicted and experimental radiometric record, using a projected v-method algorithm. The described approach produces a rather sharp and high-contrast tomographic image of a tattoo layer in a human volunteer’s skin with the onset depth of ~0.15 mm. No evident artifacts or even noise appear elsewhere in the reconstructed volume. Applying quadratic binning of the radiometric record significantly reduced the computational load, enabling us to reconstruct a volume of 5.6 x 2.8 x 0.9 mm³ with nominal resolution of 25 x 25 x 15 μm³ using a personal computer in less than one minute.

Proceedings Article | 11 August 2023 Paper

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

Katja Arh, Tomaž Cvetko, Boris Majaron

Proceedings Volume 12629, 126290I (2023) https://doi.org/10.1117/12.2670924

KEYWORDS: Tissues, Mid-IR, 3D image reconstruction, Infrared cameras, Tomography, Thermography

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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.

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