Multimodal Biomedical Imaging Systems - Biomedical Optical Imaging Technologies: Design and Applications - page 297

Confocal imaging uses point scanning. A scanning system scans a focused spot
across the object of interest to obtain a two-dimensional image. Imaging speed is
limited by the scanning system. Imaging speed is also limited by the numerical
aperture (NA) of the objective lens since only a small fraction of the light from
the focal point is collected by the objective lens. Another limitation of the confocal
imaging technique is its limited imaging depth due to light scattering.

9.1.6 Optical Coherence Tomography

Optical coherence tomography (OCT) is based on a low-coherence interferometric
technique. In OCT, the image contrast originates from scattering, birefringence,
absorption, and difference in refractive index. Backscattered light is measured as
a function of axial range and transverse position through coherence gating. Cross-
sectional, tomographic images are generated by scanning the focused point across
the tissue.
The interference between the sample and reference signals can be measured
using either time-domain or frequency-domain techniques. In time-domain OCT, the
reference mirror is scanned back and forth, and the interference signal is measured
by a photodiode detector. In frequency-domain OCT, the optical path length of
the reference arm is fixed and the interference signal is acquired by encoding the
optical frequency in time with a spectrally scanning source (swept-source OCT)
or is acquired with a dispersive detector (spectral-domain OCT). Frequency-domain
OCT has a much higher sensitivity and imaging speed. In spectral OCT, a diffraction
grating, and a line-scan CCD array are used in the detection arm to record the
interference spectrum in parallel. In swept-source OCT, a wavelength-swept laser
is used as the light source and a photodetector in the detection arm is employed to
record the interference spectrum as the wavelength is swept.
OCT is capable of imaging the layer-to-layer architectonics of healthy tissues
that have different optical characteristics. OCT can also reveal structural changes in
tissues caused by pathologic conditions, such as inflammation and tumors.
While OCT can provide detailed information on underlying tissue structure, it
has some potential drawbacks, such as small FOV, lack of tissue surface detail,
and absence of information on biochemical and molecular compositions. Another
shortcoming of OCT is its relatively slow imaging speed because it usually works
as point scanning system. Some multiple point scanning systems and en face
systems have been developed to address this shortcoming. In evaluating OCT image
results, it may also be difficult to distinguish between different tissues, such as
between normal and pathological tissues, because these tissues may have similar
variations in optical properties, such as scattering, absorption, and refractive index.
Another problem with OCT imaging is that soft tissue is compliant, moving both
involuntarily and voluntarily. When the imaging device contacts the tissue, it can
cause folds, stretching, and other deformations in the tissue that can make the tissue
structure obtained from OCT difficult to interpret.