title: Monte Carlo simulation of laser beams interaction with the human eye using Geant4.
abstract: BACKGROUND
  Due to the unique characteristics of the eye, ophthalmologic diagnostic techniques often rely on the photons interaction with the retina to infer its internal structure. Although these techniques are widely used, the interpretation of the generated images is not always fully understood, as in scanning laser ophthalmoscopy dark field imaging. This limits the exploitation of its full potential as a diagnostic tool for deep abnormalities in the retina, as in the situation of drusen.
  
  
  METHODS
  With the aim of better understanding the retinal diagnostic images, we have carried out computer simulations of incident laser beams interacting with different structures of the human eye, including a retina with and without drusen. We have used the Geant4 simulation toolkit, applying the optical package of the electromagnetic (EM) physics working group, to simulate the physical processes of reflection, refraction, absorption, and scattering of low energy photons (2 eV) in biological tissues. For each simulation it was used a single beam of orange light, with a Gaussian profile, that travels through all optical elements of the eye. The reflected beam characteristics were analyzed by virtual detectors in different locations, which collected information about the number and position of photons. The geometry and optical properties of all components of the eye were considered according to the published data.
  
  
  RESULTS
  Simulation results put in evidence that the presence of drusen influences the profile of the reflected beams. It changes the mean free path of the photons, modifying its reflection pattern, which depends on the area illuminated by the incident beam. This result is also visible when the reflected beam is analyzed outside of the eye, when the profile has no longer a symmetrical Gaussian distribution. These results will support the retinal diagnostic images that will be obtained in a near future with a new developed ophthalmic apparatus.
  
  
  CONCLUSIONS
  The shape analysis of the reflected beams in retinal laser scanning techniques could increase its potential as a diagnostic examination tool for the deeper structures of the retina.
authors.forename: Diogo
authors.initials: D
authors.lastname: Tendeiro
authors.forename: Gonçalo
authors.initials: G
authors.lastname: Lopes
authors.affiliation: Centro de Física Atómica, CFA, Departamento de Física, Faculdade de Ciências e Tecnologia, FCT, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal. pmv@fct.unl.pt.
authors.forename: Pedro
authors.initials: P
authors.lastname: Vieira
authors.forename: José Paulo
authors.initials: JP
authors.lastname: Santos
mesh_terms: D005123:Eye / Q000528:radiation effects*; D006801:Humans; D007834:Lasers; D009010:Monte Carlo Method; D012160:Retina / Q000528:radiation effects; D012542:Scattering, Radiation