Interferometric non-contact on-line dosimetry control during selective retina treatment (SRT)

In selective retina treatment (SRT) spatial confined tissue damage in the absorbing retinal pigment epithelium (RPE) is obtained by applying microsecond laser pulses. The damage in the RPE is caused by transient microbubbles forming around laser heated melanin granules inside the cells. For treatment of RPE related diseases, SRT is thought to share the therapeutic benefits of conventional photocoagulation but without affecting the photoreceptors. A drawback for effective clinical SRT is that the laser-induced lesions are ophthalmoscopically invisible. Therefore, a real-time feedback system for dosimetry is demanded in order to avoid undertreatment or unwanted collateral damage to the adjacent tissue, which is observed at about twofold threshold radiant exposure for RPE damage. We demonstrate that interferometry is capable of detecting microbubbles at threshold irradiation for RPE damage. A porcine ex-vivo RPE model, which was irradiated by a Nd:YLF laser (350ns/1700ns@527nm), was employed. The irradiated area was simultaneously probed by a Michelson interferometer. Cell viability assays were performed after irradiation. Cell damage was compared to the interferometric data. At threshold for cell damage a change in the interferometric transients was observed due to microbubble formation. Thus, interferometry could serve as a non-contact real-time dosimetry control during selective targeting of the RPE in vivo. From the time resolved motion of the bubble interface, which is determined by interferometry, the maximum bubble size is obtained. This gives an estimation for the damage range in the tissue. Knowledge of the damage range as a function of the irradiation parameters is helpful in optimizing SRT.