Time Domain OCT axial length is commonly expressed in A-scans; what is the typical value?

• A. 100 A-scans
• B. 400 A-scans
• C. 4000 A-scans
• D. 40 A-scans

Answer: (B)

In time-domain OCT, the depth information collected in one cross-sectional image is built from a set of depth samples called A-scans. A typical TD-OCT system covers about 2 millimeters of axial tissue depth per cross-section, and sampling that range at a useful density results in around 400 A-scans. That gives roughly 5 micrometers per sample, which balances sufficient depth resolution with practical scan speeds. So the common value is about 400 A-scans per cross-section. Fewer samples (like 100 or 40) would be too coarse to resolve retinal layers, while many thousands (like 4000) are more characteristic of newer, higher-density spectral-domain systems rather than classic time-domain setups.