Medical fibers:Imaging of the Irradiation of Skin With a Clinical CO2 Laser System

Keywords:medical fibers, surgical fibers,  Time:24-02-2016

The thermal response of subsequent pulses will depend on any residual temperature increase from the preceding pulse, the physical condition and integrity of the surface, and the local hydration level of the exposed tissue. Quantitative and objective data can provide a better understanding of the thermodynamic mechanisms underlying skin resurfacing. This study examines the thermal response of skin to CO2 laser irradiation in terms of an in vivo study performed on a rat model. The implications of the thermographic and video images that were obtained during pulsed CO2 irradiation are addressed and discussed. The effects of pulse stacking are examined both quantitatively and qualitatively.

MATERIALS AND METHODS Animal Model

In vivo experiments were performed on ‘‘Fuzzy’’ rats (Sprague Dawley strain), which are immunologically competent rats that do not produce coarse hair but are covered with fine undercoat hair. They were anesthetized with a mixture of Ketamine and Rompun (4:3 ratio, 0.1 mL/100 g body weight). The hair on the backs and flanks of the rats was removed by applying a depilatory cream (Nair) with cotton-tipped applicators and then by wiping with dry gauze. During the experiments, the rats were kept on a heating pad in order to counteract the hypothermic effects of the anesthesia and of the absence of body hair. The depth of anesthesia was monitored constantly by checking heart rate, breathing rate, and the toe-pinch response. After the experiments were completed, the rats were euthanized in a carbon dioxide chamber according to the required procedure of The University of Texas at Austin Institutional Animal Care and Use Committee.

Laser System

The laser system used for the experiments was the TruPulse CO2 laser fibers (Tissue Technologies, Albuquerque, NM). It is characterized by: (1) the emission of relatively short pulses (tp 4 65–125 ms) and (2) a 3 mm × 3 mm square spot size at the focus with a uniform irradiance profile over the entire spot (called the ‘‘Mesa Mode’’). An articulated arm served as the delivery system for the TruPulse laser medical fibers, and a 3.5-inch focal length lens focused the beam to a spot size of 3 mm × 3 mm at the treatment focal plane. For ease of use, the tip of the laser handpiece was in the treatment focal plane. Theenergyoutputofthelaserwasmeasured with a joulemeter (Labmaster, Coherent Laser Group, Palo Alto, CA) placed at the focus of the laser handpiece. The radiant exposure was calculated by dividing these energy measurements by the spot area (9 mm2). A pulse duration of 100 ms was used for all of the experiments.

Thermal Camera

Surface temperatures were measured with a 3–5 mm band-limited thermal camera (Model 600L, Inframetrics, Billerica, MA) composed of a HgCdTe detector and two oscillating mirrors that scanned the camera’s field of view (FOV) horizontally and vertically. To reduce the effects of thermal noise, the detector was cooled with liquid nitrogen to −196°C. The radiant energy that the camera detects was converted to a voltage; this voltage was displayed as a grayscale or false color image on the video display of the camera. The thermal camera imaging mode was the fast line scan. In this setting, the vertical oscillating mirror was frozen into place, and the horizontalscanningmirrorscannedrepeatedlyacrossthe same line in the center of the camera’s FOV. The timeforeachscanacrossthislinewas125 ms,and 256 samples were taken across this line. Using this mode, a time-temperature history of a single line was obtained. To reduce the FOV of the thermal camera, a 3× telescope and a 9.5-inch focal distance close-up lens were attached to the camera. With the camera placed at a distance of 9.5 inches from the treatment plane, the camera’s FOV was ∼3 cm × 3 cm. Internal calibration of the thermal camera compensated for the presence of the external optics.Forourexperiments,anemissivityof1.0was assumed [10].

Basic Experimental Setup

A diagram of the setup used in these experiments is shown in Figure 1. A heating pad was placed on a lab jack, and an anesthetized ‘‘Fuzzy’’ rat was placed on top of the pad. The thermal camera was placed 9.59 from the rat. The handpiece of the laser was measured and removed. A spacer of the same length was attached to the articulated arm; the tip of the applicator was placed in the same plane as the treatment focal plane. The handpiece was removed because its relatively large size blocked a significant portion of the camera’s FOV during irradiation; the spacer was much smaller in size. The thermal images were recorded with a Super VHS recorder (Diamond Pro, Mitsubishi, Japan) and were digitized and processed on a PC equipped with a frame grabber. Microsoft Excel 97 and Kaleidagraph Version 3.08 software packages were used to convert the grayscale values into temperatures. To aid in the analysis of the videotapes, a frame counter was used to label each individual frame. A fan was used to blow the ablation plume away from the scene during irradiation. The fan speed and the distance between the rat and fan were constant. The fan ensured that the measuredpeaktemperatureswerenotduetothehigh temperatures of the ablation ejecta [11].

Temperature Response to a Single Pulse Asinglepulsewasappliedtoinvivoratskin. Pulseenergiesof215mJand350mJ(corresponding to radiant exposures (Ho) of 2.4 J/cm2 and 3.9 J/cm2, respectively) were used. The surface temperatures were measured with the thermal camera, and the temperature decay as a function of time was obtained. Prior to irradiation of the rat skin, burn paper was irradiated, and the camera was moved on its swivel mount so that its FOV was centered on the location of pulse impact. This ensured that the measured temperatures were obtained from the center of the laser spot [12].

Temperature Response to Multiple Pulses To determine the peak temperatures due to pulse stacking on a single spot, 15 pulses were applied to a single location on the rat skin.