Basic Scan Patterns and OCT Output

1.2


Basic Scan Patterns and OCT Output


Each commercially available OCT device has unique scan patterns that are programmed into the machine. There is considerable overlap between devices, however, with several general scan patterns available across all devices. The scan patterns for the major commercially available machines are summarized in Table 1.2.1. The two most commonly used scans in evaluating retinal disease are:




Depending on the particular machine, scan patterns may be programmable with respect to functions such as pixel density, B-scan density, speed, ability to oversample, and length of scanned image.



Macular Cube Scan


Cube scans are ‘volume’ or ‘3D’ scans analogous to computed tomography or magnetic resonance scans that acquire volumetric cubes of data. SD-OCT machines acquire a rapid series of line scans (B-scans), generally in a 6 mm × 6 mm square area centered on the fovea. The scans are generally at relatively lower resolution, in order to minimize the time of scanning. As a result, when examining individual line scans from a cube scan, some detail is lost. As a default the cube scan is centered at the fovea, but other areas of interest can be captured by manually centering the scan elsewhere in the retina. Optic nerve topographic scans are cube scans centered on the nerve.


In the Zeiss Cirrus SD-OCT, there are two macular cube scans available, with no ability to customize. Both scans capture a 6 mm × 6 mm area centered at the macula. There is a faster 200 × 200 cube (200 B-scans each comprised of 200 A-scans) or the slightly slower 512 × 128 cube (128 B-scans each comprised of 512 A-scans) that has higher quality horizontal scans. The ‘volume scan’ on the Heidelberg Spectralis uses a similar raster scanning protocol with a ‘fast’ 25 B-scans each consisting of 512 sample points or A-scans, or with a ‘dense’ 1024 × 49 default scanning protocol. The Topcon 3D OCT uses a 256 × 256 or a 512 × 128 scanning protocol. The RT-Vue ‘3D macular scan’ consists of a 4 mm × 4 mm macular cube scan with 101 B-scans consisting of 512 A-scans each, and the MM5 protocol uses a mix of vertical and horizontal B-scans to create a grid-like (not true raster) scanning pattern.


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Jun 19, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on Basic Scan Patterns and OCT Output

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