Optical Coherence Tomography: Technical Aspects - Biomedical Optical Imaging Technologies: Design and Applications - page 181

Fig. 5.10 Different scanning modalities in flying-spot optical coherence tomography. ( a ) A-scan.
( b ) B-scan. ( c ) C-scan

5.4.6 Scanning Modalities in OCT

In conventional point-scan OCT (flying-spot OCT), various types of scanning
modalities have been implemented for representing the cross-sectional information
about the sample. Depending upon the priority of longitudinal (depth) and transverse
direction, the scanning modalities can be mainly classified into three: A-scan,
B-scan, and C-scan [ 12 ]. As in ultrasound instrument, an A-scan (axial scan)
typically records the two fundamental parameters of an echo: how large it is
(amplitude) and where it occurs in time with respect to zero time delay (pulse transit
time). As explained in Sect. 5.3 , in a TD-OCT system, an A-scan is implemented by
scanning the reference mirror throughout the entire depth to obtain a time-domain
interference pattern, as shown in Fig. 5.10 a; however, in an FD-OCT system, the
total distribution of an A-scan is measured at one time without any reference scan.
Similarly, a B-scan can be generated by acquiring several A-scans for different
adjacent transverse positions, as shown in Fig. 5.10 b. In case of C-scan, the sample
is scanned along many successive x and y directions at a fixed depth, as shown in
Fig. 5.10 c, which can provide different transverse cross-sectional ( en face )images
of the sample at different depth.

5.5 Different OCT Schemes

In OCT, the coherence-gated information about the elementary volume of the
scatters within the obscuring scattering specimen can be obtained from either
the

time-domain measurement principle (TD-OCT) or the frequency-domain

(FD-OCT) measurement principle.