Methods and Principles of Peripheral Retinal OCT-Scanning



Fig. 1.1
(a) Lattice degeneration at the superior retina equator (right eye); (b) Scanning direction



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Fig. 1.2
Scanning of the retinal periphery depending on the degeneration’s location; (a) Schematic location of the degeneration; (b) Patient positioning during superior peripheral retina scanning


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Fig. 1.3
OCT image of lattice degeneration (according to OCT-scanning direction indicated by line in Fig. 1.1b) in Line protocol. Marked vitreoretinal tractions on the lesion margins, atrophic holes, marked thickening (hyperreflectivity area), and a shallow detachment of the neurosensory retina


While performing the OCT scan, the patient’s gaze was oriented in the desired direction with the head slightly rotated towards the area of degeneration (Figs. 1.2 and 1.4). If required, the OCT was calibrated according to the patient’s refraction prior to the procedure. We made several scans and selected images with the highest detail. The scanning direction and the axial scan depth were adapted to the location of the retinal lesion. We aimed to standardize the procedure by making the scan in the optimal signal area between the horizontal reference arms and aligning the B-scan edges at the same height to ensure equal image intensification. We used the following scanning protocols: Line (axial length 6–12 mm), Enhanced HD Line (axial length 12 mm), 3D Macular (scan area 6 × 6 mm), and 3D Retina (scan area 7 × 7 mm). The maximal scan size was limited by the optimal signal area (see above) and depended on the location of the fundus scanning area and the degree of pupil dilation. The automatic dioptric adjustment (both unifocal and bifocal) and axial adjustment is very important since these parameters change while switching from the central retina to the periphery (Fig. 1.4). The superior and inferior fundus regions are preferably investigated in the horizontal direction, while the temporal and nasal regions are usually investigated in the vertical direction. We shifted the scanning pattern in the viewfinder to ensure a more precise positioning of the line scanner and a successful imaging of the far-peripheral lesions. This also allowed to perform the scanning of any part of the lesion with no change in the patient’s gaze direction and head positioning.

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Fig. 1.4
Gaze direction and head positioning for the OCT-scanning (right eye): (a) Central retina; (b) Inferior retina; (c) Superior retina; (d) Medial (nasal) retina; (e) Lateral (temporal) peripheral retina

The following parameters were used in the linear OCT-scanning:



  • RTVue-100 – axial resolution: 5 μm; scanning speed: 26,000 A-scans/s; 1,024 pixels per A-scan; linear scanning speed: 0.038 s; averaging no. of scans: 32


  • RTVue XR Avanti – axial resolution: 5 μm; scanning speed: 70,000 A-scans/s; 1,024 pixels per A-scan; linear scanning speed: 0.014 s; average no. of scans: 120

To analyze OCT scans, the following morphometric parameters were measured: thickness of the unaltered retina, depth and length of the lesion, size of the vitreoretinal traction at the site of vitreoretinal adhesion.

It is important to mention several factors that may influence the quality of retinal photography and OCT scanning, such as: media transparency, degree of pupil dilation, deep/shallow setting of the eyes, nystagmus, head tremor, patient compliance and operator’s experience.



OCT-Scanning Principles


The basic principle of SD OCT technology is to investigate the target tissue by comparing spectral characteristics of the back-reflected light with those of the reference beam (A-scan).

The coherent light beam (laser beam) passes through the ocular structures and is reflected, scattered and/or absorbed by different tissues depending on their characteristics and depth, which results in the interaction between the back-reflected and scattered light and changes in the spectral characteristics of the back-reflected light. The reflected light with multiple “optical echoes” is detected by the multichannel spectrometer, and the resulting interference patterns are compared to those of the reference beam using Fourier analysis. The spectral differences are used to reconstruct an A-scan, which shows the optical characteristics of the target tissues and their location. A compilation of A-scans represents a B-scan line. Three-dimensional scans are obtained from the array of cross-sectional B-scans; 3D scans have different densities of A-scans compared to B-scans.

The majority of the OCT scans in this Atlas were obtained using the RTVue-100 scan (Optovue, USA). The RTVue-100 scan uses a low-coherent infrared (IR) laser centered at a wavelength of approximately 840 nm with a bandwidth of 50 nm. The axial resolution of the retinal scanning is 5 μm, and the scanning speed is 26,000 A-scans/s. The peripheral retina was usually scanned in the linear and 3D modes. Table 1.1 summarizes the characteristics of the linear and 3D scanning by RTVue-100.


Table 1.1
RTVue-100 scanning protocols
































Scan protocol

Scan range (mm)

Number of A-scans in a B-scan

Maximal number of averaged scans

Maximal number of lines in scan

Line

2 × 12

1,024

32

1

Line HD

2 × 12

4,096

1

1

Cross line

2 × 12

2 × 1,024

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Jun 27, 2017 | Posted by in OPHTHALMOLOGY | Comments Off on Methods and Principles of Peripheral Retinal OCT-Scanning
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