Biomedical Engineering Reference
In-Depth Information
30
25
20
15
10
5
1500 nm
1300 nm
800 nm
0
0
50
100 150 200
Source Spectral Bandwidth (nm)
250
300
350
Fig. 8.4.
Dependence of coherence length (axial resolution) on optical source band-
width. Curves are plotted for 800, 1,300, and 1,500 nm, three common wavelengths
used in OCT. High axial imaging resolution is achieved with broad spectral band-
widths and shorter wavelengths. Shorter wavelengths, however, are more highly ab-
sorbed in biological tissue, decreasing imaging penetration depth
The transverse resolution is also related to the depth of focus or the confocal
parameter 2z
R
(two times the Raleigh range).
2
z
R
=
π
∆
x
2
2
λ
.
(8.3)
Increasing the transverse resolution (decreasing the spot size at the focus)
subsequently results in a reduced depth of field. For OCT imaging, the con-
focal parameter or depth of focus is typically chosen to match the desired
depth of imaging. High transverse resolutions are often required and may be
utilized in OCT. However, the short depth of field requires additional optical
or mechanical techniques to spatially track the focus in depth along with the
axial OCT scanning.
Relatively small incident powers of 1-10 mW are commonly required for
OCT imaging. Typically, the real-time (
∼
30 frames per second) acquisition
can be achieved at a signal-to-noise ratio of
100 dB with 5-10 mW of incident
optical power. Recent advances in spectral-domain OCT (SD-OCT) [39, 40]
and swept-source OCT (SS-OCT) [41, 42] have enabled extremely fast axial
scanning and real-time volumetric imaging at micron-scale resolution, while
maintaining su
ciently high SNR for imaging larger volumes of tissue.
∼
