Thermal damage thresholds for multiple-pulse porcine skin laser exposures at 1070 nm

Abstract

As solid-state laser technology continues to mature, high-energy lasers operating in the near-infrared (NIR) band have seen increased utilization in manufacturing, medical, and military applications. Formulations of maximum permissible exposure limits establish guidelines for the safe use of these systems for a given set of laser parameters, based on past experimental and analytical studies of exposure thresholds causing injury to the skin and eyes. The purpose of our study is to characterize the skin response to multiple-pulsed laser exposures at the NIR wavelength of 1070 nm, at a constant beam diameter of 1 cm, using anesthetized Yucatan mini-pig subjects. Our study explores three constant total laser-on times of 0.01, 0.1, and 10 s as single- and multiple-pulse sequences. Exposures consisting of 10, 30, and 100 pulses have identical individual pulse durations but different duty cycles in order to include variable degrees of thermal additivity. A plurality of three observers quantifies skin damage with the minimally visible lesion metric, judged at the 1- and 24-h intervals postexposure. Calculation of the median effective dose (ED₅₀) provides injury thresholds for all exposure conditions, based on varying laser power across subjects. The results of this study will provide a quantitative basis for the incorporation of multiple-pulsed laser exposure into standards and augment data contained in the existing ED₅₀ database.

Keywords: laser damage; minimum visible lesion; multiple pulse; near-infrared lasers; skin injury.

Fig. 1

Experimental setup used for 1070-nm skin exposures.

Fig. 2

Example of a thermal lesion observed on the skin. This lesion was observed 24 h following exposure to 600 W for 0.1 s from a 1070-nm laser with a 1.04-cm diameter, for a radiant exposure over the $1/e^2$ diameter of $71.0{\mathrm{J}/\mathrm{cm}}^2$.

Fig. 3

(a) A thermal frame from an exposure and (b) the corresponding binary image mask of the exposure. The highlighted square indicates the ROI around the center of mass of the binary image: (c) the result of multiplying mask and the thermal frame and (d) hair follicles identified within the ROI.

Fig. 4

Fit of peak temperature data for a single-laser pulse of 10 s, discarding points outside of $\pm 2\mathrm{STD}$ of the difference between the peak temperature data and the fitted temperature data at 10 frames after the laser-off time.

Fig. 5

Skin surface temperature changes at all doses for waxed single-pulse exposures of 0.01, 0.1, and 10 s.

Fig. 6

${\mathrm{ED}}_{50}$ values and fiducial limits for shaved porcine skin exposures with 0.01-s TOT.

Fig. 7

${\mathrm{ED}}_{50}$ values and fiducial limits for shaved porcine skin exposures with 0.1-s TOT.

Fig. 8

${\mathrm{ED}}_{50}$ values and fiducial limits for shaved porcine skin exposures with 10-s TOT.

Fig. 9

24-h ED50 with respect to total sequence duration for 0.01-, 0.1-, and 10-s TOTs. The dashed line indicates a power function fit to all of the data.

Fig. 10

$\mathrm{\Delta }T$ slope at the 24-h ${\mathrm{ED}}_{50}$ with respect to the total sequence duration. The $\mathrm{\Delta }T$ slope is the change in peak skin surface temperature at the 24-h ${\mathrm{ED}}_{50}$ over the 24-h radiant exposure ${\mathrm{ED}}_{50}$.

Fig. 11

24-h ${\mathrm{ED}}_{50}$ per pulse with respect to the number of pulses and the PRF. The perspective of this plot is on the same plane as the fit to the data.

Fig. 12

Comparison of the radiant exposure ${\mathrm{ED}}_{50}$ values of waxed and shaved subjects for single-pulse exposures. The dashed lines represent the boundaries of the upper and lower fiducial limits for each ${\mathrm{ED}}_{50}$.

Fig. 13

Skin lesion focused around a hair follicle due to a single pulse of 0.01 s and a radiant exposure of $17.6{\mathrm{J}/\mathrm{cm}}^2$.