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

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 ED50 threshold for cell death. For 6 μs 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 ED₅₀decreased about 25% for 10 and 100 pulses. Cells die when a single bubble was detected in a multiple pulse sequence.