Biomedical Engineering Reference
In-Depth Information
then is swept to a different wavelength. Wide tuning ranges make high imaging
resolution possible and at the rate of hundreds of thousands of axial scans per
second, fast volumetric imaging can be performed. The development of fast
swept sources has utilized innovative methods, including a rotating polygonal
mirror array [41] or Fourier-domain mode-locking [42].
High-speed SD-OCT has been demonstrated in the living human eye and
in other real-time applications. Because the acquisition rate is no longer de-
pendent on a mechanically scanned reference arm, significantly faster rates are
achievable, up to typical rates of 30,000 axial scans per second, depending on
the read-out rates of the linear CCD arrays in the spectrometer. The computa-
tional complexity has been increased for both SD-OCT and SS-OCT because
the inverse Fourier transform of the acquired data is required for each axial
scan line. Because dedicated digital-signal-processing (DSP) chips are readily
available, it appears that the computational power necessary for this appli-
cation is feasible and available to the research investigator. Spectral-domain
OCT also has the advantages of improved phase stability, because no mechan-
ical scanning is performed, and higher signal-to-noise ratio, because individual
wavelengths are now detected by separate elements in a linear detector array.
Linear detector arrays, especially indium-gallium-arsenide arrays used for de-
tecting the commonly used 1,300 nm OCT wavelength, are expensive, shifting
the costs of rapid scanning optical delay lines for time-domain systems to the
detector array for SD-OCT.
8.5 Beam Delivery Instruments for Optical Coherence
Tomography
The OCT imaging technology is modular in design. This is most evident
in the optical instruments through which the OCT beam can be delivered
to the tissue. Because OCT is fiber-optic based, single optical fibers can be
used to deliver the OCT beam and collect the reflected light, thereby mak-
ing the beam delivery system potentially very small, on the order of the
size of an optical fiber (125
m diameter) itself. The OCT technology can
also be readily integrated into existing optical instruments such as research
and surgical microscopes [51, 52], ophthalmic slit-lamp biomicroscopes [3],
catheters [26], endoscopes [29], laparoscopes [27], needles [28], and hand-held
imaging probes [27] (Fig. 8.6). Imaging penetration is determined by the opti-
cal absorption and scattering properties of the tissue or specimen. The imaging
penetration for OCT ranges from tens of millimeters for transparent tissues
such as the eye to less than 3 mm in highly scattering tissues such as skin.
To image highly scattering tissues deep within the body, novel beam-delivery
instruments have been developed to relay the OCT beam to the site of the
tissue to be imaged. An OCT catheter has been developed for insertion into
biological lumens such as the gastrointestinal tract [29]. Used in conjunction
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