Rodney E. Amnotte

Ultrashort laser pulse bioeffects and safety

Recent studies of retinal damage due to ultrashort laser pulses have shown that less energy is required for retinal damage for pulses shorter than 1 ns than that for longer pulses. It has also been shown that more energy is required for near-infrared (NIR) wavelengths than in the visible because the light focuses behind the retina, requiring more energy to produce a damaging fluence on the retina. We review the progress made in determining the trends in retinal damage from laser pulses of 1 ns to 100 fs in the visible and NIR wavelength regimes. We have determined the most likely damage mechanism(s) operative in this pulse width regime.

Visible lesion thresholds from near-infrared pico- and nanosecond laser pulses in the primate eye

Minimum visible lesions (MVL) are reported for picosecond and nanosecond laser pulses at near-IR wavelengths in the primate eye, Macaca Mulatta. The 50 percent probability for damage (ED50) dosages are reported for the 24 hour for both MVL and fluorescein angiography visible lesion thresholds at the 95 percent confidence level. The thresholds decreased by as much as 48 percent between the 1- hour reading and were lower in all cases at 24 hours. MVL- (ED50) threshold doses were 19.1 uJ at 7 ns and 4.2 uJ and 4.6 uJ at 80 ps and 20 ps respectively. Our thresholds measured for the near-IR laser pulses were lower by a factor of 4 to 8 lower than previously reported values but almost an order in magnitude higher than visible MVL thresholds for similar pulsewidth in the visible wavelengths.

In vivo laser-induced breakdown in the rabbit eye

Threshold measurements for femtosecond laser pulsewidths have been made for retinal minimum visible lesions (MVLs) in Dutch Belted rabbit and rhesus monkey eyes. Laser-induced breakdown (LIB) thresholds in biological materials including vitreous, normal saline, tap water, and ultrapure water have been measured and reported using an artificial eye. We have recorded on video the first LIB causing bubble formation in any eye in vivo using albino rabbit eyes (New Zealand white) with 120- femtosecond (fs) pulses and pulse energies as low as 5 microjoules ((mu) J). These bubbles were clearly formed anterior to the retina within the vitreous humor and, with 60 (mu) J of energy, they lasted for several seconds before disappearing and leaving no apparent damage to the retina. We believe this to be true LIB because of the lack of pigmentation or melanin granules within the albino rabbit eye (thus no absorptive elements) and because of the extremely high peak powers within the 5-(mu) J, 120-fs laser pulse. These high peak powers produce self-focusing of the pulse within the vitreous. The bubble formation at the breakdown site acts as a limiting mechanism for energy transmission and may explain why high-energy femotsecond pulses at energies up to 100 (mu) J sometimes do not cause severe damage in the pigmented rabbit eye. This fact may also explain why it is so difficult to produce hemorrhagic lesions in either the rabbit or primate eye with 100-fs laser pulses.

Histopathology of ultrashort-laser-pulse retinal damage

Recent studies of retinal damage due to ultrashort laser pulses have shown interesting behavior. Laser induced retinal damage for ultrashort (i.e. less than 1 ns) laser pulses is produced at lower energies than in the nanosecond to microsecond laser pulse regime and the energy required for hemorrhagic lesions is much greater times greater for the nanosecond regime. We investigated the tissue effects exhibited in histopathology of retinal tissues exposed to ultrashort laser pulses.

New noninvasive imaging technique for cataract evaluation in the rhesus monkey

We present the first in vivo study using Optical Coherence Tomography (OCT) as the imaging device for lenticular cataracts in the geriatric rhesus monkey. OCT is a non-invasive imaging technique that produces a 2D cross sectional image of intraocular tissue similar to ultrasound B scan. In OCT the images are formed by measuring optical reflections from the tissue. Eighteen geriatric subjects with documented lenticular opacities and one control subject were imaged. The OCT images produced are compared to current and previous clinical cataract grading exams and slit-lamp photography. Histopathology was collected on one subject and is compared to the OCT image. OCT provides information on nuclear, cortical and subcapsular opacities. The image formation is presented based on a color coded computer generated log reflective scale. The log reflective scale is converted to a qualitative grading system. Although movement and shadow artifact can occur, these are readily identifiable and can be differentiated from underlying lenticular abnormalities. OCT has great potential to assist in further characterization of cataracts.

Ultrashort-laser-pulse retinal damage

Recent studies of retinal damage due to ultrashort laser pulses have shown that less energy is required for retinal damage for pulses shorter than one nanosecond. Laser minimum visible lesion thresholds for retinal damage from ultrashort laser pulses are produced at lower energies than in the nanosecond to microsecond laser pulse regime. We review the progress made in determining the trends in retinal damage from laser pulses of one nanosecond to one hundred femtoseconds in the visible and near-infrared wavelength regimes. We have determined the most likely damage mechanism operative in this pulse width regime and discuss implications on laser safety standards.

In-vivo laser-induced bubbles in the primate eye with femtosecond pulses

Threshold measurements for laser-induced breakdown (LIB) and bubble generation for femtosecond laser pulsewidths have been made in vivo for rhesus monkey eyes. These LIB thresholds are compared with model-predicted thresholds for water and minimum visible lesion thresholds in Dutch Belted rabbit and rhesus monkey eyes. LIB thresholds in biological materials including vitreous, normal saline, tap water, and ultrapure water have been measured and reported using an artificial eye. We have recorded on video the first LIB causing bubble formation in any eye in vivo using albino rabbit eyes, pigmented rabbit eyes, and rhesus monkey eyes. External optics were used to focus the image within the vitreous and the bubbles generated were clearly formed anterior to the retina within the vitreous humor. The length of time that the bubbles are visible depends on the pulse energy delivered and may last for several seconds. However, for pulse energies near thresholds, the bubbles have a very short lifetime and may be seen on the video for only one frame. The plasma formation at the breakdown site acts as a limiting mechanism for energy transmission and may explain why high-energy femtosecond pulses at energies up to 100 microjoules sometimes do not cause severe damage to the retina. This fact may also explain why it is so difficult to product hemmorrhagic lesions in either the rabbit or primate eye with 100-femtosecond laser pulses.

Subnanosecond single laser pulse minimum-visible-lesion studies in the near infrared

Lacking established national laser safety standards for sub-nanosecond, single pulse laser systems operating in the visible to near infrared spectral regions has resulted in research efforts designed to define the risks associated with human ocular exposure. The bulk of this work has been focused on visible wavelength laser pulses and resulting retinal threshold damage. We report threshold measurements for Minimum Visible Lesions (MVL) at the retina for picosecond (ps) laser pulses in Macaca Mulatta eyes using near infrared wavelengths (80 ps and 1064 nm). The 50% probability for damage (ED50) dosages are calculated for 1 hour and 24 hour post exposures using the SAS probit analysis. The MVL ED50 threshold and the fiducial limits at the 95% confidence level were found to be 4.16 (3.00 - 5.77) microjoules ((mu) J). Fluorescein angiography (FA) was accomplished at both 1 hour and 24 hour post exposure, however the analysis for FA is currently underway and results will not be reported here.

Retinal damage mechanisms from ultrashort laser exposure

Extensive research of ultrashort ocular damage mechanisms has shown that less energy is required for retinal damage for pulses shorter than one nanosecond. Laser minimum visible lesion thresholds for retinal damage from ultrashort (i.e. < 1 ns) laser pulses occur at lower energies than in the nanosecond to microsecond laser pulse regime. WE review the progress made in determining the trends in retinal damage from laser pulses of one nanosecond to one hundred femtoseconds in the visible and near-infrared wavelength regimes. We discuss the most likely damage mechanism(s) operative in this pulse width regime and discuss implications on laser safety standards.