Next Wave of Optical Imaging—Clinical Applications of Laser Speckle

Abstract

President Obama’s recent pronouncement on the under-utilization of medical imaging 1 may serve to increase the use of imaging technologies in healthcare. Currently a minor player, optical imaging modalities such as optical coherence tomography (OCT), orthogonal polarization spectroscopy (OPS), laser Doppler imaging (LDI), and laser speckle imaging (LSI) could fulfill several niche application areas, such as wound healing and perfusion. OCT, an optical interferometric technique, provides high quality 3D images from within optical scattering media. OPS utilizes the depolarization of photons due to multiple scattering effects in tissues to provide high contrast video images of microcirculation. LDI and LSI are both based on the optical coherent effect of photon interactions with moving particles, i.e., red blood cells (RBCs) in vessels. LDI employs a focused laser beam to measure the frequency shifts of the radiation scattered from a point of interest. It requires a scanning mechanism for imaging applications. Its measurement accuracy is strongly affected by motion artifacts. Laser speckle (LS) had been considered a nuisance until Fercher and Briers 2 demonstrated that with a proper exposure, flow velocity information can be extracted from the blurring of the LS patterns generated from interference of the scattered laser beam. The development of LSI devices has been advanced significantly by technological progress and continued price reduction of charged-coupled devices (CCD) and optical fibers. Clinical Applications LSI is currently a research tool that shows great promise in a wide range of clinical applications including retinal perfusion, wounds, cerebral blood flow, sublingual microcirculation, and others. Advantages of LSI include fast image acquisition (in milliseconds), full-field imaging, and relatively low-cost.