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

Abstract

Selective laser targeting of the retinal pigment epithelium (RPE) is an attractive method for treating RPE-associated disorders. We are developing a method for optically detecting intracellular microcavitation that can potentially serve as an immediate feedback of the treatment outcome. Thermal denaturation or intracellular cavitation can kill RPE cells during selective targeting. We examined the cell damage mechanism for laser pulse durations from 1 to 40 micros ex vivo. Intracellular cavitation was detected as a transient increase in the backscattered treatment beam. Cavitation and cell death were correlated for individual cells after single-pulse irradiation. The threshold radiant exposures for cell death (ED(50,d)) and cavitation (ED(50,c)) increased with pulse duration and were approximately equal for pulses of up to 10 micros. For 20 micros, the ED(50,d) was about 10% lower than the ED(50,c); the difference increased with 40-micros pulses. Cells were killed predominantly by cavitation (up to 10-micros pulses); probability of thermally induced cell death without cavitation gradually increases with pulse duration. Threshold measurements are discussed by modeling the temperature distribution around laser-heated melanosomes and the scattering function from the resulting cavitation. Detection of intracellular cavitation is a highly sensitive method that can potentially provide real-time assessment of RPE damage during selective laser targeting.

Selective Retina Therapy in Acute and Chronic-Recurrent Central Serous Chorioretinopathy

“…RPE cells are damaged thermomechanically by the formation of microbubbles by nanosecond and short microsecond pulses, while thermal damage, that is, coagulation, is observed with pulses of a few milliseconds and longer. The transition of photodisruption to photothermal denaturation occurs at exposure durations between 20 and 50 µs [41,42]. With SRT, axial and lateral heating of the tissue is confined to the absorbing RPE cells, whereas conventional photocoagulation is associated with strong heat diffusion into the neural retina and choroid with subsequent scotoma and scar formation.…”

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.…”

“…11 The RPE is a monolayer of cells located within the retina. Melanin-containing organelles known as melanosomes, found in RPE cells, readily absorb ∼50 to 60% of the incident visible light as well as NIR radiation.…”

“…Melanin-containing organelles known as melanosomes, found in RPE cells, readily absorb ∼50 to 60% of the incident visible light as well as NIR radiation. 5,11,12 For visible laser exposures, injury first occurs in the RPE because it contains the highest energy density of absorbed photons. As wavelength increases from the visible to the NIR, laser light is absorbed less in the retina and more in the cornea and anterior segments of the eye.…”

Response to Stanga et al.: Structural Changes of the Retina after Laser Photocoagulation in Spectral Domain Optical Coherence Tomography

“…Histology of such lesions clearly demonstrates damage to the photoreceptors. While 10 ms pulses might have been "shown to achieve effective and safe laser treatments," a selective effect in terms of targeting only the RPE can only be achieved with thermo-mechanical effects following pulse durations < 50 µs (Schuele et al 6 and Lee et al 7 ). When using longer pulse durations, purely selective thermal effects can hardly be achieved owing to the nature of heat diffusion, except when neighboring cells have different resistance to heat, which to the best of our knowledge is not the case between RPE and photoreceptor cells.…”