Basic Scan Patterns and OCT Output

Published on 09/05/2015 by admin

Last modified 22/04/2025

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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:

Table 1.2.1

Scan patterns in commonly used OCT devices

▶ Macular cube scan

▶ Line scan(s)

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.

▶ Raster Scans: raster scanning is one method used to obtain cube scans of the macula. This involves a systematic pattern of image capture over a rectangular area using closely spaced parallel lines. It leads to a uniform sampling density over the entire area being scanned with the OCT.

▶ Radial Scans: these consist of six to 12 high resolution line scans taken at radial orientations, all passing through the fovea. The RT-Vue’s MM6 is a radial line scanning pattern with 12 lines radially oriented to the fovea, each 6 mm long. The macular radial scanning pattern of the Spectralis and the 6-line radial scan of the Topcon 3D OCT 100 are similar. A disadvantage of the radial line scans is that the machine interpolates between the scans when generating macular thickness maps. This is reasonable for the fovea where the lines are close to each other, but can miss lesions further out in the macula where the lines are spaced further apart.

▶ Mesh Scans: Some machines include a mesh scanning pattern that acquire vertical and horizontal B-scans over the area of interest. The MM5 protocol of the RT-Vue uses a less dense outer and a more dense inner grid. The outer grid has horizontal and vertical B-scans 0.5 mm apart and the inner grid has horizontal and vertical B-scans each 0.1 mm apart.

Line, Cross-Line and Raster Scans

SD-OCT line scans are a single B-scan composed of generally a higher number of A-scans than the cube scans. This higher sampling density allows higher resolution scans of the retinal tissue to be acquired. In addition, oversampling can be performed to increase signal-noise ratio (Fig. 1.2.1). The Cirrus 5-line raster consists of five horizontal 6 mm lines each scanned four times and averaged. The five lines in the raster can be collapsed to obtain a single line scan that consists of 20 averaged B-scans. The ‘cross-line’ scan of the RT-Vue consists of a horizontal and vertical line scan while the 7-line raster of the Heidelberg also spans a 6 mm × 6 mm area of the macula. Heidelberg can be programmed to oversample a line scan up to 100 times at each point.