Noninvasive fluorescence imaging through strongly scattering layers

Summary form only given. Non-invasive imaging requires the ability to form sharp pictures even when an opaque material act as a screen between the object and the detector. Light scattering scrambles the spatial information of the object, thereby blurring the picture and making imaging impossible. Gated imaging methods [1,2] such as optical coherence tomography [3] can separate the small amount of ballistic light that did not change direction from the scattered background, and diffuse tomography methods [4] offer high-depth imaging at low resolution even if no ballistic light is present at all. It has been theoretically suggested that a complete knowledge of the scattering screen will allow one to image objects hidden behind it [5]. Major steps in this direction were achieved using ultrasound and electromagnetic waves in both the microwave and in the optical regime [6-12]. Yet to obtain the required knowledge of the scattering screen, it is necessary to access its back, thus severely limiting the usefulness of these approach. We have recently demonstrated a reference-free imaging method that can obtain an image of a fluorescent object behind a thin layer that scatters all incident light [13]. The incident laser light is scrambled by a diffuser which transmits negligible ballistic light. The speckles that hit the object excite fluorescence, that appear as a diffused blob on the front side of the diffuser. However the optical memory effect allows deterministic scanning of this overlap when the scattering layer has a physical thickness that is small compared to the distance to the object. By varying the incident angle, an angle-dependent intensity is measured from which the autocorrelation of the object can be extracted exploiting the speckle correlation properties. Subsequently the shape of the object can be retrieved from the autocorrelation using a Gerchberg-Saxton-type iterative algorithm [14-16]. We demonstrated imaging through a thin, strongly scattering screen b- sed on fluorescence contrast. Other contrast methods such as nonlinear conversion or photoacoustic imaging may be possible. In addition, our method may be adapted for a reflection geometry to allow around-the-corner imaging [12]. Three-dimensional imaging will be possible by varying the curvature of the incident wavefronts in addition to the angles of incidence [17].