Biomedical Engineering Reference
In-Depth Information
Fig. 8.7.
Spectroscopic OCT imaging. Spatially resolved spectroscopic information
can be extracted from the detected OCT signal. By digitizing the interferogram
fringes and transforming the data using a Fourier-transform, the spectrum of the
returned light can be determined. When compared to the original laser spectrum,
spectral differences can be identified, which are related to the absorption and scat-
tering of the incident light. Spectroscopic OCT images of control and experimental
botanical specimens where a highly absorbing and fluorescent chemical dye has
accumulated in the vascular system. Corresponding fluorescence and bright-field
microscopy images are shown. Figure modified with permission from [58]
can be obtained by digitizing the full interference signal and applying digital
signal processing algorithms to transform the data from the time (spatial) do-
main to the frequency (spectral) domain. Transformation algorithms include
the Morlet wavelet and the short-time Fourier transform with attention to
reduction of windowing artifacts. While spectroscopic data can be extracted
from each point within the specimen, there is an inherent trade-off between
high spatial resolution and high spectral resolution and novel algorithms are
being investigated to optimize this data. Once the spectral data is obtained
at a point in the tissue, the spectral center of mass can be calculated and
compared with the original spectrum from the laser source. Spectral shifts to
longer or shorter wavelengths, from the original center of mass, are displayed
on a 2D image using a multidimensional hue-saturation-luminance (HSL) color
space. At localized regions within the tissue, a color is assigned with a hue
that varies according to the direction of the spectral shift (longer or shorter
wavelength) and a saturation that corresponds to the magnitude of that shift.
The luminance is held constant.
Longer wavelengths of light are scattered less in turbid media. In a homo-
geneously scattering sample, one can observe using spectroscopic OCT that
shorter wavelengths are scattered near the surface and a smooth color-shift
occurs with increasing depth, as longer wavelengths are scattered. In more
heterogeneous samples, such as tissue, scattering objects such as cells and
sub-cellular organelles produce variations in the spectroscopic OCT data.
