![]() Recent histological studies have shown that the second hyper-reflective outer retinal OCT band, currently attributed to IS/OS junction, is likely due to mitochondria packing geometry and the organelle’s high refractive index (1.4). Even though it is barely visible on histological sections, PIL is highly reflective with SD-OCT imaging. PIL represents the junction between the inner (IS) and outer (OS) segments of the photoreceptors ( Figure 1). 1 One of the most important new SD-OCT developments secondary to the higher resolution is the recognition and clinical importance of the photoreceptor integrity line (PIL), 10 also referred to as the IS/OS line. 3 The interface between different retinal layers is determined by changes in reflective properties between the tissues. SD-OCT is often referred to in literature as “optical biopsy method” due to its extraordinary ability to provide information on ocular subsurface structures with retinal layer-by-layer analysis, otherwise obtainable only by histology. 1, 7 The high sensitivity of this instrument provides entirely new imaging capabilities and, therefore, has significant impact on diagnosis and decision making. Clinically detectable drusen by ophthalmoscopy, for example, is 30 µm 8 and resolution of fundus photography is 25 µm, 9 whereas axial resolution of SD-OCT has been reported to be 3.5–6 µm. 1, 2 The unique capability of SD-OCT technology is to provide a layer-by-layer analysis of the retina with a resolution highly superior to contact lens fundus ophthalmoscopy and/or fundus photography. 2 Thus, SD-OCT imaging results in considerable improvement in image acquisition speed and resolution, minimizing the well-known limitations of TD-OCT, such as limited retinal nerve fiber layer (RNFL) resolution, operator error, and increased motion artifacts. 3 – 6 SD-OCT is capable of obtaining 27,000–40,000 A-scans/second with an axial resolution of 3.5–6 µm, 1, 7 as opposed to an acquisition speed of 400 A-scans/second, as in TD-OCT. 1, 2 Recent work in the field of ophthalmic OCT has demonstrated that spectral or Fourier detection methods (SD-OCT) enable OCT imaging with significantly improved speed, resolution, and sensitivity, as opposed to time domain OCT (TD-OCT). It is a noninvasive imaging procedure of ocular structures that employs nonionizing optical radiation. Optical coherence tomography (OCT) has revolutionized our approach to retinal disease identification and diagnosis.
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