 Diabetic retinopathy DR is caused by changes in the blood vessels of the retina associated with long-term Type 1 or Type 2 diabetes mellitus. DR is a leading cause of blindness in the United States. Standard optical coherence tomography OCT cannot directly detect vascular changes, which may occur early affecting the passage of blood through the tiny capillaries reduced capillary flow or cause the greatest damage through formation of abnormal blood vessel growth neovascularization. Currently, fluorescein angiography FA is the gold standard for detecting these changes, but FA requires an injection of a dye into the vein of the arm of the patient. This dye can cause undesirable side effects. Recently, OCT has been used to make functional measurements such as total retinal blood flow among others and to perform angiography. Thus, functional OCT may provide a useful, alternate way to evaluate diabetic retinopathy. 