Ultrafast-laser-radiation transfer in heterogeneous tissues with the discrete-ordinates method

Abstract

Here light propagation and radiation transfer of ultrafast laser pulses in heterogeneous biological tissues are simulated by use of the discrete-ordinates method (DOM). Formulations for solving the time-dependent radiation-transfer equation are deduced for three-dimensional geometries incorporating the Fresnel specularly reflecting boundary condition and characteristics of ultrafast laser pulses. The present method can treat both the incident laser intensity and the scattered radiation intensity from the walls of the targeted tissue as two components, i.e., a diffuse part and a specular part. Reflectivity at the tissue-air interface is calculated by use of Snell's law and the Fresnel equation. The high-order S10 DOM method is found to be adequate for describing the propagation and transfer of ultrafast laser radiation in heterogeneous tissues. The time-dependent radiation field in the tissue as well as the temporal radiation intensity profiles at the boundaries can be obtained simultaneously. The absolute values of the logarithmic slope of the temporal reflectance and transmittance at various detector positions are found to converge to a constant value in a homogeneous tissue model. With the inclusion of a small inhomogeneity, such a value will change in line with the property of the embedded inhomogeneity. The orientation of heterogeneity of the tissues also substantially affects the radiation intensity at the boundaries. The effect of the Fresnel boundary in the modeling is pronounced. The simulated transmitted signals are broadened and amplified under specularly reflecting boundary condition as compared with those under diffusely reflecting boundary conditions.

Maximum time-resolved hemispherical reflectance of absorbing and isotropically scattering media

“…Other studies have shown that the diffusion approximation fails to predict the transmittance at early times for all optical thicknesses and also at long times for optically thin slabs [6]. In addition, Guo et al [7] showed that the diffusion approximation fails for both collimated radiation and strong anisotropically scattering media.…”

Numerical feasibility analysis of an epidermal glucose sensor based on time-resolved fluorescence

“…In the case of optically thick slabs, the diffusion approximation fails for short times. This issue has also been discussed by Guo and Kim [20] in the transport of ultra-fast laser pulses in biological tissues. In addition, it is difficult to describe collimated sources using the diffusion approximation without carefully choosing the source terms [21].…”

Laser-assisted vessel welding: state of the art and future outlook

“…To limit thermal damage, external irradiation should be performed with a continuous-wave laser with cooling intervals (i.e., intermittent irradiation) [86] or with ultrafast irradiation with a pulsed laser (e.g., picosecond laser pulses) [109]. The combination of irradiation-and cooling intervals facilitates heat distribution to the solder-tissue interface and prevents excessive heat buildup in the underlying tissue.…”

“…In contrast to continuous-wave lasers that distribute heat to the deeper layer via conduction, the pulsed laser enables sufficient heat generation at the solder-tissue interface without over-coagulating the solder superficial layer. The pulsed laser produced stable welds with minimal thermal damage [109].…”