Worldwide, over 65 million people suffer visual impairment from glaucoma. Because the vast majority of patients rarely experience symptoms of glaucoma until relatively late in the disease, early diagnosis and treatment is essential in preserving vision. Currently, the diagnosis of glaucoma is based on a combination of structural optic disc changes and functional visual field changes. Unfortunately, both of these tests are relatively subjective and definitive diagnosis of glaucoma can be delayed or missed as a result. However, it is well accepted that defects in the retinal nerve fiber layer (RNFL;i.e. structural changes) precede the onset of visual field loss by as much as six years. A number of imaging methods including confocal scanning laser ophthalmoscopy (cSLO), optical coherence tomography (OCT) and scanning laser polarimetry (SLP) have been developed to allow early detection of these subtle structural defects. Of these, SLP provides promise in achieving the earliest detection of glaucoma-induced retinal changes. However, since SLP is not a complete polarimeter, its measurements are prone to error due to the RNFL's diattenuation and depolarization. Thus, accuracy of the SLP's retardance distribution is influenced by the unknown retinal diattenuation and depolarization, decreasing the device's sensitivity and specificity. Furthermore, commercial SLPs only image small sections of the retina in the neighborhood of the optic nerve head, suffer from motion artifacts, and are expensive (estimated cost of $17k using commercial off-the-shelf (COTS) parts and mechanical scanners). We have overcome these limitations with a less expensive (estimated $10k using COTS parts), complete Mueller matrix (MM) polarimeter with no moving parts: the Snapshot Retinal Imaging Mueller Matrix (SRIMM) polarimeter. This device avoids temporal scanning and allows a real-time measurement of the retina's complete polarization state over a large field of view. This device has several advantages. First, the device's cost reduction may make it a more practical and cost-effective instrument for screening patients. Second, since the design has no temporal scanning, large retinal areas can be observed, in a single fundus snapshot, without the need for complex image registration algorithms. Third, the ability to acquire a complete MM data set with high temporal resolution will improve the clinical sensitivity and specificity of this device by removing measurement errors related to depolarization and diattenuation. This is a collaborative effort between North Carolina State University, USC Eye Institute, and the University of Arizona.
The specific aims are 1. Design, calibrate, validate and test a fundus camera prototype of a Snapshot Retinal Imaging Mueller Matrix (SRIMM) polarimeter. 2. Design and build a model eye to characterize and validate the performance of the polarimeter. 3. Validate the SRIMM polarimeter's performance in a rabbit model and in a limited number of human patients via comparative measurements taken with a high-resolution SD-OCT and a Zeiss GDx.
An inexpensive high resolution snapshot system for capturing the complete polarization content (i.e., retardance, depolarization, and diattenuation) of the retina would present a benefit to public health by providing clinics with a potentially more accurate and affordable means to diagnose glaucoma. By allowing surgeons improved visualization and treatment options, earlier diagnosis and intervention may be possible, allowing a preservation or prolongation of good vision.