Damage threshold from large retinal spot size repetitive-pulse laser exposures

Nanosecond multipulse retinal damage thresholds of elongated irradiance profiles in explant measurements and simulations

Significance

The database for multipulse retinal damage thresholds for the laser safety standard (IEC 60825-1:2014) is confined, especially for elongated irradiation profiles. To ensure eye safety, retinal damage thresholds (ED 50 values) need to be determined.

Aim

This study aims to examine nanosecond multipulse scenarios.

Approach

To determine ED 50 values in ex vivo measurements, an optical laser setup is presented. Porcine explant tissue is irradiated with rectangular top-hat profiles. Thermal simulations are carried out on a validated computer model and retinal injury thresholds are obtained.

Results

The measurements resulted in ED 50 values from 8.46 to 42.72    μ J with a slope from 1.15 to 1.4. A thermal damage in the measurements can be excluded due to the level value in combination with a different type of declining behavior for increasing pulses compared to the simulations. A dependence with increasing elongation or area of the retinal image emerges in the simulations but could not be confirmed in the measurements due to the influencing factors (biological variability, focusing, and measuring procedure).

Conclusions

Using slit apertures for beam shaping, variable rectangular spot geometries are realized without changing elements in the setup. For further evaluation of the behavior of elongated irradiation profiles, additional measurements to improve the measurement accuracy are necessary.

Safety Evaluation of Photoacoustic Tomography System for Intraocular Tumors

Purpose

Photoacoustic tomography (PAT) has demonstrated the ability to characterize molecular components and architectural heterogeneities of intraocular tumors in enucleated human globes and in animals in vivo. Although laser safety levels have been established for illumination through the cornea, the safety limit for PAT illumination through the sclera has not been investigated. The purpose of this study is to examine if the energy level used in intraocular PAT results in ocular damage.

Methods

Rabbit eyes were exposed to pulsed laser illumination at 20 mJ/cm² at the scleral surface. Eyes were examined at 1, 7, and 28 days after the laser exposure. Examination procedures included white light and fluorescence fundus imaging, optical coherence tomography (OCT), electroretinography (ERG), and histology with hematoxylin and eosin (H&E) staining as well as terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling (TUNEL staining).

Results

Fundus imaging and OCT of rabbit eyes at 1, 7, and 28 days following exposure of the laser illumination of the PAT system did not reveal any damage to the retinal structures. ERG showed no significant difference between the experimental and control eyes. Similarly, H&E histology did not show abnormalities in either the scleral tissue where the laser illumination was delivered or in the retinal layers. No sign of apoptosis in the layers of the retina, choroid, or optic nerve was found on TUNEL staining.

Conclusions

Similar to the application of PAT to other organs, the proposed laser illumination energy level at 20 mJ/cm² does not impose detectable harm to the ocular tissue.

Translational Relevance

This study addresses illumination safety issues for PAT.

Characteristics of Non-Ablative Resurfacing of Soft Tissues by Repetitive Er:YAG Laser Pulse Irradiation

Background and Objectives

Recently, several minimally invasive gynecological, ENT and esthetic procedures have been introduced that are based on delivering “smooth” sequences of Er:YAG laser pulses to cutaneous or mucosal tissue at moderate cumulative fluences that are not only below the ablation threshold but typically also do not require local anesthesia.  To explain the observed clinical results using “smooth‐resurfacing,” it has been suggested that in addition to the direct heat injury to deeper‐lying connective tissues, there is an additional mechanism based on indirect triggering of tissue regeneration through short‐exposure, intense heat shocking of epithelia. The goal of this study is to improve understanding of the complex dynamics of the exposure of tissues to a series of short Er:YAG laser pulses, during which the thermal exposure times transition from extremely short to long durations.

Study Design/Materials and Methods

A physical model of laser‐tissue interaction was used to calculate the temperature evolution at the irradiated surface and deeper within the tissue, in combination with a chemical model of tissue response based on the recently introduced variable heat shock (VHS) model, which assumes that the tissue damage represents a combined effect of two limiting Arrhenius′ processes, defining cell viability at extremely long and short exposure times. Superficial tissue temperature evolution was measured during smooth‐resurfacing of cutaneous and mucosal tissue, and compared with the model. Two modalities of non‐ablative resurfacing were explored: a standard “sub‐resurfacing” modality with cumulative fluences near the ablation threshold, and the “smooth‐resurfacing” modality with fluences below the patient′s pain threshold. An exemplary skin tightening clinical situation was explored by measuring pain tolerance threshold fluences for treatments on abdominal skin with and without topical anesthesia. The obtained temperature data and pain thresholds were then used to study the influence of Er:YAG laser sequence parameters on the superficial (triggering) and deep (coagulative) tissue response.

Results

The simulations show that for the sub‐resurfacing modality, the parameter range where no excessive damage to the tissue will occur is very narrow. On the other hand, using pain tolerance as an indicator, the smooth‐resurfacing treatments can be performed more safely and without sacrificing the treatment efficacy. Two preferred smooth‐resurfacing treatment modalities were identified. One involves using optimally long pulse sequence durations (≈1–3 seconds) with an optimal number of pulses (N ≈ 10–30), resulting in a maximal short‐exposure superficial tissue response and moderate coagulation depths. And for deeper coagulation, without significant superficial heat shocking, very long pulse sequences (>5 seconds) with a large number of delivered pulses are to be used in combination with topical anesthesia.

Conclusions

A comparison of the simulations with the established smooth‐resurfacing clinical protocols in gynecology, ENT, and esthetics suggests that, through clinical experience, the clinical protocols have been optimized for the maximal superficial heat shock triggering effect. Further research is needed to gain a better understanding of the proposed role of heat shock triggering in the clinically observed regeneration of cutaneous, vaginal, and oral tissues following Er:YAG laser smooth‐resurfacing. Lasers Surg. Med. © 2021 The Authors. Lasers in Surgery and Medicine published by Wiley Periodicals LLC.