The nomenclature for the middle two bands has much less supportive evidence. This band is typically thinner and fainter than the others. 29,30,36 The innermost band has been attributed to the external limiting membrane (ELM). There is discordance between different authors regarding which anatomic structure correlates with each band. Outside the central fovea, commercially available SD-OCT instruments typically resolve four bands in the outer retina. 35 The retinal vessels may sometimes be seen on OCT images as circular hyperreflective structures located in the inner retina, with a vertical shadow or reduced reflectivity extending into deeper layers. A recent study demonstrated that the incidence of the light beam could affect the appearance of Henle fiber layer by OCT, resulting in a thin hyperreflective layer corresponding to the photoreceptor synapses or a thicker hyperreflective layer corresponding to photoreceptor axonal extensions enveloped by the outer cytoplasm of Müller cells ( Fig. Within the retina, the retinal nerve fiber layer and the plexiform layers (both inner and outer) are seen as hyperreflective while the ganglion cell layer and the nuclear layers (both inner and outer) are relatively hyporeflective. In some patients, the posterior hyaloid can be seen above the ILM as a hyperreflective layer. The first detected layer in most OCT scans is the ILM that appears as a hyperreflective layer at the vitreoretinal interface. 2,30Īlthough the interpretation of features of the retina, which can be defined for our purpose to span from the ILM to the outer segments of the photoreceptors appears to correlate well with histology, the OCT features of the outer retina are less well understood and remain a topic of discussion ( Fig. Therefore, one-to-one correspondence of histology with OCT images cannot be expected. The angle of incidence of the light, motion artifacts, speckled noise, and image contrast can affect the axial resolution of the retinal imaging. 30 When light travels through the retinal tissue it can be reflected, scattered, or absorbed, and this creates the multilayered pattern of the retina. 29 However, care must be taken when making assumptions about these correlations because histologic sections require fixation and exogenous staining to produce contrast within tissue, and this can introduce artifacts, while OCT relies on intrinsic differences in tissue optical properties to produce image contrast.
![coherence explained coherence explained](https://i.ytimg.com/vi/FaUwN_uR0us/maxresdefault.jpg)
With the increase in the axial resolution of the new SD-OCT instruments (5–8 µm) and the ultrahigh-resolution OCT (2 µm), it has become possible to correlate OCT images accurately with histologic features of the retina. The OCT image closely approximates the histologic appearance of the macula and, for this reason, it has been referred to as an in vivo optical biopsy. Schachat MD, in Ryan's Retina, 2018 Normal Macular Anatomy This can be understood by expanding the Coherence equation to show the averaging process.Andrew P. The coherence computation for a single set of input and output “snapshots” will always evaluate to 1.0 at every frequency. Note that ensemble averaging must be used for the Coherence to have meaning. Experienced analysts always use the Coherence measurement to quantify the quality of an FRF measurement at every frequency. Departures from unity can indicate input noise, output noise, a non-linear process or any combination of these things. Values in-between state the fraction of measured output power explained by the measured input power and a linear process. A coherence of 0 means the output and input are totally unrelated.
![coherence explained coherence explained](https://cdn.slidesharecdn.com/ss_thumbnails/coherence-1225479750435189-8-thumbnail-4.jpg)
the system is linear and the Hxy(f) measurement is perfect). A coherence of 1.0 means the output is perfectly explained by the input (i.e.
![coherence explained coherence explained](https://signalhorizon.com/wp-content/uploads/2020/08/Coherence-Em-1536x864.jpg)
Thus, the coherence magnitude at every frequency is always between 0 and 1. If the system is non-linear or if extraneous noise has been interjected at the input or output, the round-trip will be less efficient and the Coherence will be less than one (but never more). That is, the Coherence will be exactly 1.0. If the system is linear and none of our measurements are contaminated by noise, the round-trip is perfect and we get back everything we put in. In a perfect world, this would result in exactly Gxx as the output of Hyx. Then we conjugate Gxy (equivalent to flipping or reflecting x(t) in time) and pass it back through Hyx. We apply Gxx to Hxy and get Gxy at the output.
![coherence explained coherence explained](https://i.ytimg.com/vi/fglRfaD9Tdk/maxresdefault.jpg)
This product definition indicates the coherence characterizes an “energy round trip” or a reflection through the process.