Pump-probe detection of laser-induced microbubble formation in retinal pigment epithelium cells

Abstract

Microsecond laser pulses are currently being investigated in a new ophthalmic procedure for treatment of disorders associated with the retinal pigment epithelium (RPE). The precise mechanism for microsecond laser-induced RPE damage, however, has not been determined. We have previously shown that short pulse laser irradiation in the nanosecond to picosecond time domain causes transient microbubble formation around melanin granules in pigmented cells. Nanosecond time-resolved microscopy was previously used to visualize the intracellular cavitation dynamics. However, this technique is difficult to use with microsecond laser exposures, especially when multiple laser pulses are applied in a rapid sequence as in the clinical setting. Here we describe a simple pump-probe method for detecting transient light scattering signal from individual RPE cells when they are irradiated with nanosecond and microsecond laser pulses. For single 12 ns pulses the threshold for bubble detection was the same as the ED(50) threshold for cell death. For 6 micros pulse duration the threshold for bubble detection was about 10% higher than the threshold for cell death. With repetitive pulse trains at 500 Hz the ED(50) decreased about 25% for 10 and 100 pulses. Cells die when a single bubble was detected in a multiple pulse sequence.

Optical detection of intracellular cavitation during selective laser targeting of the retinal pigment epithelium: dependence of cell death mechanism on pulse duration

“…30 Previous studies have suggested that the transition from cavitation-induced to thermal damage is expected to take place somewhere within the microsecond regime. 31,32 It was shown that RPE cell death coincides with bubble formation for 3-, 5-, and 6-s pulses, 31,32 while RPE cells were damaged without cavitation formation for 50-s pulses. 32 However, these reports did not examine pulse durations between 5 and 50 s, a critical parameter range for scanning selective targeting of the RPE.…”

Trends in melanosome microcavitation thresholds for nanosecond pulse exposures in the near infrared

“…7,11,25 Studies have also shown that the microcavitation thresholds decrease with increasing number of pulses. 26 The RPE is believed to be the site for threshold damage for all nanosecond and picosecond laser exposures in the visible and NIR. In the NIR wavelength region, the ocular media and anterior structures of the eye (cornea and lens) begin to strongly absorb, but still transmit adequate energy to pose a risk to the retina.…”

“…Light melanosomes are predominately spherical, while the heavy melanosomes are predominately elliptical in shape, resulting in slight size differences between the two fractions. 26,29 However, no high-resolution imaging was used, and therefore, no precise knowledge of shape variance within each fraction was obtained. The melanosome fractions were separated in order to determine if differences in microcavitation threshold values occur.…”

Comparison between multi-spot laser and conventional laser in treatment of diabetic clinically signifcant macular edema

“…At subnanosecond exposures, other nonlinear damage mechanisms appear, such as shock-waves and laser-induced breakdown. (9) Multispot laser is also less painful for the patient, as thermal diffusion in the choroid, where pain receptors reside, has been reduced. Multispot laser allows spots to be delivered almost simultaneously and with greater regularity than conventional treatment.…”