Thresholds for visible lesions in the primate eye produced by ultrashort near-infrared laser pulses.

PURPOSE

To evaluate the effects of near-infrared (near-IR) ultrashort laser pulses on the retinas of rhesus monkey eyes and to perform threshold measurements for minimum visible lesions (MVLs) at pulse widths ranging from nanoseconds to femtoseconds.

METHODS

Near-infrared single laser pulses were placed within the macular area of live rhesus monkey eyes for five different pulse widths (7 nsec; 80, 20, and 1 psec; and 150 fsec). One visible wavelength of 530 nm at 100 fsec was also included in the study. Visible lesion thresholds (MVL-ED50) were determined 1 hour and 24 hours after exposure. Fluorescein angiography thresholds (FAVL-ED50) were also determined using a probit analysis of the dosage. Thresholds were calculated as that dosage causing a 50% probability for damage, and the fiducial limits were calculated at the 95% confidence level.

RESULTS

For all pulse widths, the 24-hour MVL-ED50 was lower than the 1-hour MVL-ED50, and they both decreased with decreasing pulse width. Thresholds at the 1-hour reading decreased from 28.7 microJ at 7 nsec to 1.8 microJ at 150 fsec, whereas thresholds at 24 hours decreased from 19.1 microJ at 7 nsec to 1.0 microJ at 150 fsec. The doubled 1060-nm wavelength of the 530-nm threshold decreased from 0.36 to 0.16 microJ after 24 hours. FAVL-ED50s were much higher than MVL-ED50s, showing that FA was not as sensitive in determining damage levels.

CONCLUSIONS

Laser pulse widths less than 1 nsec in the near-IR are capable of producing visible lesions in rhesus monkey eyes with pulse energies between 5 and 1 microJ. Also, the near-IR thresholds for these pulse widths are much higher than for the visible wavelengths. As with visible wavelengths, FA is not as sensitive in determining threshold levels as is visually observing the retina through a fundus camera.