Holographic Interferometry for Monitoring of PDT Treatment and Micro-Surface Displacement

There is a growing demand for non-destructive diagnostic modalities in clinical applications for 3D imaging or monitoring of optically transparent tissues such as tumors in the eye. Holographic interferometry can be used in determining changes in refractive index and surface displacement. Photodynamic therapy (PDT) is a rapidly growing method for treating tumors and lesions. Currently, there is no method of monitoring the PDT reaction in optically transparent tissue such as the eye. During dental implant surgeries, there is also a need for monitoring the amount of pressure applied to surrounding teeth. Both clinical applications have the goal of monitoring their given treatment for increased precision and control. A hologram is a record of a light field’s amplitude and phase. Phase is encoded into the hologram as an interference pattern. Optical path length is dependent on the refractive index and physical path; therefore, altering either will result in an interference pattern coded with information about the given alteration. Holographic interferometry (HI) can be described as a 2D-interferogram between two holograms: a record prior to the alteration and an overlaying "live" hologram. Two experimental setups, a transmission and reflectance mode, have been explored. Reflectance mode monitors changes in physical path and has its object beam reflected directly from the object to the plate in the holographic recorder. Transmission mode monitors changes in refractive index and has its object beam transmitted through the object. A relationship between the phase encoded in the hologram and change of the optical path length (either the refractive index or physical path) has been developed. Applying this relationship to the collected data showed a linear relationship between forces applied to an object and its maximum displacement, and a refractive index change during and after a PDT reaction.