EXPERIMENTAL STUDY OF CORNEAL THERMAL DAMAGE TO 2000nm LASER RADIATION

Bo Chen,MS,1 Jeffery Oliver,PhD,2 Soumak Dutta,BS,2 Sharon L. Thomsen,MD,3 H Grady Rylander III,PhD,1 Ashley J. Welch,PhD1

1 Biomedical Engineering Laser Laboratory, University of Texas at Austin, Austin, Texas
2 Air Force AFRL/HEDO, Brooks City-Base, Texas
3 Pathology Consultant to Engineers and Physicists, 182 Chickdee Lane, Sequim, WA 98382.

Abstract

To support refinement of the Maximum Permissible Exposure (MPE) safety limits, a series of experiments were conducted in vivo on Dutch Belted rabbit corneas to determine corneal minimum visible lesion thresholds for 2.0 micrometer continuous-wave laser irradiation.

Single pulse radiant exposures were made at specified pulse durations of 0.1, 0.25, 0.5, 1.0, 2.0 and 4.0 seconds for spot 1/e2 diameters of 1.17 mm and 4.02 mm. Lesions were placed in rows without overlap on rabbit cornea. The effect of each irradiation was evaluated within one minute post exposure and the final determination of lesion formation was made using a slit lamp one hour post exposure. Threshold lesions were defined as the presence of a superficial surface whitening one hour after irradiation. Probit analysis was conducted to estimate the dose for 50% probability (ED50) of laser-induced damage. Approximately 20 different radiant exposures were made for each exposure duration-spot size combination. The associated transient temperature during laser irradiation was recorded using an IR thermal camera.

Introduction

The eye and skin are the most susceptible parts of the body to accidental laser irradiation and due to the importance of vision to the quality of life, eye hazards are by far the more important consideration for safety. Wavelengths greater than 1.4 mm are primarily absorbed in the cornea and aqueous humor with insignificant energy reaching the retina. Since the absorption coefficient of cornea at 2.0 micrometer has been reported to be 45.9 cm-1 [1], approximately 90% of the irradiation delivered to the anterior surface of the human cornea is absorbed within average central thickness of 520 mm.

Early safety studies for wavelengths beyond 1.4 micrometer investigated CO2 laser radiation of the cornea at 10.6 mm, where the 1/e penetration depth was approximately 10 mm [1]. Bargeron et al. found that most of the CO2 radiation was absorbed with in the 50 mm thick human corneal epithelium [2]. In 1992, McCally et al. reported corneal damage thresholds for Tm:YAG laser radiation (2.02 mm) on New Zealand white rabbits [3]. The laser spot diameter was approximately 1 mm and exposure durations were 0.082 sec, 0.235 sec and 4.28 seconds. Based on the very little experimental data and mainly on the extrapolation of CO2 threshold data, the American National Standard for Safe Use of Lasers (ANSI Z136.1-2000 [4]) defined the Maximum Permissible Exposure (MPE) for the eye at wavelengths between 1.8 mm and 2.6 mm and laser exposures from 1.0 ms to 10.0 s:

Table 1. Maximum Permissible Exposure (MPE) for Corneal Exposure to a Laser Beam (From ANSI Z136.1-2000).

Wavelength (μm) Exposure Duration,
t (s)		 MPE
(J cm-2)		 Limit Aperture Diameter (mm)
1.800 to 2.600 10-3 to 0.3 0.56 t0.25 1.0
0.3 to 10 0.56 t0.25 1.5 t0.375

A recent threshold damage study of skin to 2.0 mm laser irradiation suggested that the current laser safety standard may need to be adjusted for spot diameters larger than 3.5 mm [5]. Therefore, this study was conducted to provide large spot size thresholds for the cornea to 2.0 micrometer wavelength laser irradiation.

In this paper, we report the corneal minimal visible damage thresholds for two spot diameters and exposure duration from 0.1 sec to 4 seconds and describe the thermal response of the cornea to laser irradiation.

Material and Methods

Experimental Setup
A rack mountable thulium fiber optic CW laser (IPG Photonics Corporation; Oxford, MA; Model: TLR-20-2000-LP) with a maximum 20 W output at a wavelength 2.0 micrometer provided the source of CW irradiation. A small fraction of the incident laser power was reflected onto a powermeter (Molectron Detector, Inc.; Portland, OR; Model: EPM2000 with an air-cooled powermeter probe PM30) using a beam splitter. Telescopes were employed to generate collimated laser beams with desired spot diameters. A low power alignment beam (fiber optic stable source 600nm-700nm; OZ optics, Ltd; Canada) was injected into the 2.0 micron beam path using a beam combining cube. Co-alignment of the two laser beams was accomplished at the cornea plane and at an intermediate aperture by adjustment of the position of the alignment laser fiber mount with respect to the combining cube. An iris shutter system (Uniblitz, Inc.; Rochester, NY; Model VMM-T1) was used to control exposure duration. Laser power was controlled by adjusting the current setting on the control panel of the laser. After energizing the laser, a settling time prior to corneal exposure was allowed until a stable power meter reading was obtained. A pulse generator (Stanford Research Systems. Inc; Sunnyvale, CA; Model DG535) was used to trigger the iris shutter system as well as a function generator (Hewlett-Packard, Ltd; Model HP 33120A), which controlled the imaging rate of an IR array detector thermal camera (PhoenixTM DAS camera system, Indigo, CA). The imaging rate of IR camera was set at 100 Hz. The measurement system was arranged as depicted in Figure 1. To facilitate this alignment, fine positioning of the cornea was accomplished with the use of a goniometric animal stage which had 5-1/2 axes of adjustment.

Figure 1
Figure 1. Experimental configuration for corneal damage study.

Damage Determination
At lease two examiners evaluated all exposure locations acutely and approximately one-hour post irradiation. In addition, twenty-four hour examinations were conducted for selected laser conditions. A grade of “yes” or “no” was recorded acutely and a numerical grade as described below was assigned at the one and twenty-four hour examinations.
Grade 0, no visible damage;
Grade 1, superficial damage minimally visible without magnification;
Grade 2, readily apparent lesion on surface with some circular symmetry;
Grade 3, severe lesion, circular symmetric opacity with shrinking of epithelium at the center.

Results and Discussions

Most of the one hour threshold lesions were still visible 24 hours after exposure. However, some barely visible superficial lesions disappeared. No new lesions were observed at 24 hours. All one hour threshold lesions were included in our evaluation of threshold. Damage threshold evaluated at one hour may give slightly more conservative threshold values than at 24 hours. As laser power was increased beyond threshold, readily apparent surface lesions with somewhat circular symmetry were seen. More severe lesions with circular symmetric opacity and shrinking of epithelium at the center were observed at powers around 1.5 times threshold. In this case, dehydration and coagulation of corneal epithelium as well as denaturization of corneal stroma occurred.

Figure 2
Figure 2. Slit lamp images of thermal damage at 1 hour post exposure. a) spot diameter 4.02 mm, exposure duration 0.5 s, power 789.7 mw (approximately equals threshold power). Grade 1 was assigned as the severity of the lesion. This superficial surface whitening was defined as threshold lesion. b) spot diameter 1.17 mm, exposure duration 0.25 s, power 181 mw (approximately 1.2 times threshold power). Grade 2 was assigned. c) spot diameter 1.17 mm, exposure duration 0.25 s, power 231.2 mw (approximately 1.5 times threshold power). Grade 3 was assigned.

Generally, the small standard deviations of threshold powers, closeness of lower and upper fiducial limits and sharp slopes of the probit curves indicated that the damage thresholds were well defined with a little overlap between exposures that produce damage and those that do not. The larger spot size (4.02 mm) had more uncertainty in damage thresholds than 1.17 mm spot at most of the exposure durations.

A range of peak temperatures associated with thermal damage was found. A threshold range for peak temperature associated with damage was determined from the maximum temperature at which no damage was observed compared to the minimum temperature at which damage was consistently observed.

Table 2. Threshold Peak Temperature: (°C)

T (s) 4 2 1 .5 .25 .1
D (mm)
1.17 54.2-57.0* 62.7-63.5 63.5-70.0 65.0-69.2 65.4-71.7 69.0-74.1
4.02 60.5-61.0 62.5-66.2 66.0-72.4 67.1-69.5 64.6-68.9 71.4-72.4

*: Threshold peak temperature range is defined as: Maximum peak temperature with consistent absence of damage - Minimum peak temperature with consistent presence of damage.

The exposure duration dependence of threshold average radiant exposure was described by an empirical power law equation: Threshold radiant exposure[J/cm²] = ax exposure duration [s] ^b,

Figure 3
Figure 3. Comparison of threshold average radiant exposures along with the power law fitting curves for 1.17 mm and 4.02 mm spot sizes.

Table 3. The Results of Power-Law Fitting of Threshold Radiant Exposure.

Spot size [mm] a [J/cm2] b R (correlation coefficient)
1.17 9.79 0.669 0.98
4.02 4.57 0.456 0.97
MPE* 0.56 0.25 N/A

Based on the experimental data and the empirical power law, the safety factors which were defined as threshold radiant exposure divided by MPE values were predicted for the 2.0 mm wavelength at various exposure durations and spot diameters. The minimum limit of the safety factor was approximately a factor of four for both 4.02 mm and 1.17 mm spot diameters. Due to the very sharp boundary and small uncertainties of damage threshold determination, it is suggesting that a factor of 4 “padding” is adequate and safety standard may not need to be changed.

Table 4. Experimental Threshold Average Radiant Exposures [J/c²] and ANSI MPE Values along with Their Safety Factors.

T (s) 4 2 1 .5 .25 .1
D (mm)
1.17 27.2 16.3 10.7 5.5 3.58 2.19
4.02 9.6 6.3 4.2 3.1 2.38 1.79
MPE 0.79 0.67 0.56 0.47 0.40 0.31
Safety factor, k 34.4(12.2)* 24.3(9.4) 19.1(7.5) 11.7(6.6) 9.0(6.0) 7.1(5.8)

*: safety factor calculated based on: 1.17 mm data (4.02 mm data)

Reference:
  • Maher EJ. Transmission and absorption coefficients for ocular media of the rhesus monkey. San Antonio, Texas: Brooks Air Force Base; 1978.
  • Bargeron CB, et al. Health Phys 1989; 56(1):85-95.
  • McCally RL, et al. Lasers Surg Med 1992; 12(6):598-603.
  • ANSI Z136.1-2000, Laser Institute of America, Orlando, FL American National Standards Institute; 2000.
  • Chen B, et al. Lasers Surg Med 2005; 37(5):373-381.