Coherent Microscopy and Optical Coherence Tomography for Biomedical Applications - Bioanalysis Advanced Materials Methods and Devices - page 123

Fig. 8.21 Volumetric Doppler OCT imaging of retinal vasculature. ( a and b ) B-scan images close
to the optic disk at 100 kHz and 200 kHz, respectively. ( c and d ) volumetric images of the retinal
arteries and veins branching in the optic disk at 100 and 200 kHz

The PSFs of OCT, CSI and confocal microscopy reduce much more quickly in
the axial direction than those of DHM and consequently many more particles can be
distinguished per unit volume. The number of particles that can be distinguished is
now inversely proportional to the volume of the PSF. From the dimensions of the
OCT PSF shown in Fig. 8.13 we might expect around 10 8 particles per mm 3 while a
lower NA OCT system (NA

0.01) might return 10 4 particles per mm 3 .
As mentioned previously, if the number concentration is increased the images of
particles will overlap and interfere and the image will take on a speckled appearance.
With the tomographic techniques, OCT, CSI, and confocal microscopy, it is possible
to identify local changes in number density as the speckle will become proportion-
ately brighter or darker. This is not true for the case of DHM, however; the slow
decay of the PSF in the axial direction means that any 3D information is lost. For this
reason it is necessary to use a tomographic method to image through tissue. As the
number concentration or equivalently the scattering coefficient increases, however,
the assumption of weak scattering (the Born approximation) breaks down and the
effects of multiple scattering are observed. Typically this increases background noise
in the reconstruction as a function of depth but can sometimes lead to a significant
shadowing effect in low NA tomographic systems [ 18 ].

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