Figure 1 demonstrates the scanning preferences for conventional OCT to produce fast tomograms versus the TS OCT configuration for fast en face or C-scan imaging. The resulting en face images are similar to fundus photographs, that ophthalmologists are used to, but with the additional benefit of exhibiting also information about the layered retinal structure. In order to produce a depth scan, the reference arm delay is changed slowly or in a stepwise manner. This allows recording of a temporally gated OCT signal from a given sample depth as defined by the reference arm length. This is accomplished by introducing a defined temporal modulation to the OCT signal and using a lock-in detection scheme (see section 6.2). However, in this case, it is necessary to distinguish the actual coherently gated interference or OCT signal at a defined sample depth from the axially non-localized backscattered sample light intensity. Equipped with resonant scanners, en face OCT was thereby capable to record already densely sampled en face images with rates of several tens of Hz. In en face TS OCT the preferential scanning direction is not in depth, but laterally. This gap was for the first time closed by en face transverse scanning (TS) time domain (TD) OCT. To keep the measurement times low, only a few tomograms were recorded for covering a volume, with increased risk of missing important pathological details. For this task it was necessary to record full tissue volumes. However, for disease evaluation and treatment monitoring, it was often important to evaluate lesion extensions such as in geographic atrophy, or lesion volumes of edema or drusen. Faster depth scanning critically reduced the detection sensitivity. In the early days of OCT, the recording speed of tomograms was still slow, due to the in effect low detection efficiency of by that time established time domain OCT. OCT has already become the golden standard for the treatment planning and monitoring of AMD as well as of many other retinal diseases, such as diabetic retinopathy. Also, the technology development coincided with perfect timing with the market release of antiVEGF drugs for treatment of wet AMD. Together with the easy operation of OCT systems this formed the basis for the huge success of this relatively young medical imaging technology. Abnormal morphological changes can immediately be spotted as disruption of the layered structure even by non-specialists. The retina with its preferentially layered structure has been the ideal target for OCT, since this structure is well appreciated in recorded tomograms. Arranging successively recorded A-scans along one scanning direction in a 2D array forms a tomogram or brightness scan (B-scan). A depth scan is called amplitude scan (A-scan), analogous to ultrasound imaging. Conventional OCT scans an illumination spot across a tissue surface and records at each spot the depth structure. There are numerous excellent reviews and book sections highlighting the many applications of OCT in medical imaging today, although its most successful application is still retinal imaging, in ophthalmology. Optical coherence tomography (OCT) provides image slices of tissue with high optical resolution in a non- or minimally invasive manner.
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