Measurements of laser-stimulated action potentials in the sciatic nerve of leopard frogs (Rana pipiens) are made using two infrared lasers. The dorsal sides of the frog's hind limbs are exposed to short-pulsed 1540- and 1064-nm wavelengths at three separate spot sizes: 2, 3, and 4 mm. Energy density thresholds are determined for eliciting an action potential at each experimental condition. Results from these exposures show similar evoked potential thresholds for both wavelengths. The 2-mm-diam spot sizes yield action potentials at radiant exposure levels almost double that seen with larger beam sizes.
The Yucatan mini-pig <i>(Sus scrofa)</i> is one of the most widely used animal models for skin damage studies because it
shares many of the same physical properties as human skin. While the Yucatan is ideal for laser exposure studies using a
large spot size, its size and cost are excessive for projects using smaller beams. This experiment performed histological
analysis of skin biopsies from pigmented Hairless Guinea Pigs <i>(Cavia porcellus)</i> for epidermal thickness and melanin
concentration. That data was then compared to similar information on the Yucatan.
Historically, safety analyses for radio frequency emission and optical laser exposures have been designed to define the threshold level for tissue damage. To date, no experimental studies have documented damage thresholds to living tissues in the terahertz (THz) range of electromagnetic frequencies (0.1 - 10 THz). Exposure limits exist as extrapolated estimates at the extreme bounds of current occupational safety standards for lasers and radio frequency sources. Therefore, due to the lack of published data on the safety of terahertz emissions, an understanding of the bioeffects of tissue exposures to terahertz beams is necessary. The terahertz frequency band represents an intermediate range in which both optical and radiofrequency methods of theory and experimentation can be selectively employed and compared for consistency. We report on work recently completed to reconcile the theoretical methods of optical and radio-frequency radiative transport modeling, while additionally discussing preliminary theoretical estimates of damage thresholds to skin tissue from terahertz energy and work planned to validate these findings experimentally.
The reflectance and absorption of the skin plays a vital role in determining how much radiation will be absorbed by human tissue. Any substance covering the skin would change the way radiation is reflected and absorbed and thus the extent of thermal injury. Hairless guinea pigs (<i>cavia porcellus</i>) in vivo were used to evaluate how the minimum visible lesion threshold for single-pulse laser exposure is changed with a topical agent applied to the skin. The ED<sub>50</sub> for visible lesions due to an Er: glass laser at 1540-nm with a pulse width of 50-ns was determined, and the results were compared with model predictions using a skin thermal model. The ED50 is compared with the damage threshold of skin coated with a highly absorbing topical cream at 1540 nm to determine its effect on damage pathology and threshold. The ED<sub>50</sub> for the guinea pig was then compared to similar studies using Yucatan minipigs and Yorkshire pigs at 1540-nm and nanosecond pulse duration.<sup>1,2</sup> The damage threshold at 24-hours of a Yorkshire pig for a 2.5-3.5-mm diameter beam for 100 ns was 3.2 Jcm<sup>-2</sup>; very similar to our ED<sub>50</sub> of 3.00 Jcm<sup>-2</sup> for the hairless guinea pigs.
An optical phantom was designed to physically and optically resemble human tissue, in an effort to provide an alternative for detecting visual damage resulting from inadvertent exposure to infrared lasers. The phantom was exposed to a 1540-nm, Erbium:Glass, Q-switched laser with a beam diameter of 5 mm for 30 ns at varying power levels. Various materials were tested for use in the phantom; including agar, ballistic media, and silicone rubber. The samples were analyzed for damage lesions immediately after exposure and the Minimum Visible Lesion - Estimated Dose 50% (MVL-ED<sub>50</sub> ) thresholds were determined from the data. In addition, any visible damage was evaluated for similarity to human tissue damage to determine if the phantom tissue would be a suitable substitute for in vivo exposures.