By Daniel X. Hammer, William Calhoun, Do-Hyun Kim, Robert James, Ilko K. Ilev and Victor Krauthamer
The Food and Drug Administration’s Center for Devices and Radiological Health (FDA/CDRH) regulates medical devices and radiological products in the US. We approve new devices that are deemed to be safe and effective and clear for sale devices which are substantially equivalent to older products. A large and growing number of medical devices include lasers and coherent optical sources that require special consideration in the approval process. Moreover, newly available sources and applications have characteristics that make determination of safety difficult. The Office of Science and Engineering Laboratories (OSEL) is the research arm of CDRH, providing basic and applied scientific expertise and consultative technical review to the FDA. OSEL has expertise in biology, physics, solid and fluid mechanics, chemistry and material science, imaging and applied mathematics, and electrical and software engineering. The Division of Physics (DP) has several active research programs that relate to device laser safety. This article summarizes a few of those projects.
Femtosecond lasers are widely used in LASIK and cataract surgery; emerging applications include dentistry, cancer therapy, drug development, and neurology. Femtosecond lasers are also used in several commercial imaging technologies such as two-photon microscopy. Ultrashort lasers work in a regime of thermal confinement, have lower damage thresholds, precise spatial control, and reduced collateral damage to adjacent tissue, compared to continuous-wave or longer pulsed lasers. DP is studying safety concerns related to nonlinear effects in corneal tissue, including frequency up-conversion (e.g., second and third harmonic generation, SHG and THG), self-focusing, multiphoton absorption, spectral broadening and supercontinuum generation [1]. We have mapped SHG generated throughout the corneal depth while the THG peaks in the epi- and endothelial layers of the cornea (Fig. 1). Another project involves use of low-coherence interferometry to determine the safe distance for optimal delivery of energy below the ablation threshold in optical nerve stimulation therapy [2].
Fig. 1: Second and third harmonic generation in ex vivo corneal tissue.
We have a project to characterize the output of green laser pointers [3]. The advances in laser technology in the last decade have been truly amazing, but they have come with some risks. A diode-pumped, solid state green laser that formerly occupied a table top in a physics lab is now hand-held and cost $25. There has been a rapid proliferation of these products on the market, often from unregulated sources. The pointers also demonstrably exhibit poor manufacturing practices, for example in the omission of the NIR filter that normally would limit the output to visible wavelengths. The Federal Aviation Administration has seen a doubling in the number of incidents reported from 2009 to 2012. We tested 15 laser pointers, and found power exceeding 60 mW in some cases, highly variable temporal output, and NIR emission from the pump source.
We also have a project in the area of safety from laser scanning systems. The problem here is that no standard method of evaluating laser scanning safety exists. The consensus of the laser safety community has been to treat scanning lasers with pulsed source criteria. The project goals are to determine if the assumptions underlying this treatment are valid. So we sought to determine if the photothermal effect of scanning and pulsed lasers are the same for equivalent energy deposition. We also want to define appropriate methodology to evaluate scanning laser systems. We found differences in the thermal processes using a melanin granule lattice model [4]. We also developed a projected scanning pattern method to characterize scanning beams, and developed models to simulate radiant exposure at specific points of the apertured beam, and found significant differences in the radiant energy [5].
DP research is also active in several emerging technologies areas, investigating devices that currently reside in the research realm but will need FDA clearance in the coming years. Examples include adaptive optics retinal imagers and new pediatric devices that use laser light. In both areas, serious laser safety questions exist that need further investigation.
The FDA and CDRH are committed to making science-based regulatory decisions. OSEL research supports those decisions in a multitude of ways throughout the product life cycle for a broad range of applications, including those that involve laser and laser safety.
References
[1] W. Calhoun, D. Kernik, A. Beylin, R. Weiblinger, I. Ilev, “Nonlinear optical frequency conversions of femtosecond laser in corneal tissue”, Proceedings of the SPIE Photonics West-BiOS International Conference, Paper 8579-10, 2013.
[2] K. Zhang, E. Katz, D. Kim, J. Kang, and I. Ilev, “Common-path optical coherence tomography guided fiber probe for spatially precise optical nerve stimulation,” Electronics Letters, 46, 118-120, (2010).
[3] W. Strzelecki, R. James, I. Ilev, “An Eye Hazard Posed by DPSS Green Laser Pointers: Regulatory Assessment,” International Laser Safety Conference (2013).
[4] D. Kim, “Using a Melanin Granule Lattice Model to study the Thermal Effects of Pulsed and Scanning Light Irradiations through a Measurement Aperture,” J. Biomed. Opt., 16, 125002 (2011).
[5] D. Kim, D. Shi, J. Hwang, B. Stuck, and R. J. Landry, “Measurement of Radiant Exposure of Scanning Light Sources in Optical Devices using Pulsed-Source Criteria,” J. Biomed. Opt., submitted.