Biomedical Engineering Reference
In-Depth Information
As a final point we note again that when a particle moves, its reconstructed
image will move in sympathy. Because the backscatter PSF's have a cyclic phase
variation in the axial direction, the phase of the reconstruction at a given point in
space will change according to the movement of the surrounding particles in this
direction. By measuring the rate of change of phase, or Doppler frequency, the axial
velocity of the particles can be deduced. This is the basis of laser Doppler
velocimetry that typically exploits a confocal configuration [ 38 ] and more recently
has been exploited in Doppler OCT [ 5 ]. A good example of Doppler OCT showing
retinal vasculature by Baumann et al. [ 37 ] is presented in Fig. 8.21 .
The arterial and venous vessels in the papilla can be clearly distinguished. It is
noted, however, that in Fig. 8.21a there appears to be bidirectional flow within the
vessels. This is because the individual depth scans are obtained at a rate of 100 kHz
which is insufficient to resolve the phase change unambiguously. By increasing the
sample rate to 200 kHz the problem is resolved.
8.7 Conclusion and Discussion
This chapter has compared the 3D imaging methods DHM, OCT, CSI, and confocal
microscopy with reference to their point spread and transfer characteristics. There is
a major distinction between coherent microscopy, that records a single coherent
image at a single wavelength, and the other, tomographic, techniques that construct
an image from a range of wavelengths and illumination conditions. The tomo-
graphic techniques provide superior performance when compared to coherent
microscopy but require additional images to be taken with different illumination
conditions. This additional information comes at the expense of time and frequently
it is necessary to compromise performance.
Although coherent microscopy can only be used to study relatively sparsely
seeded flows, it is currently the only flow mapping technique capable of simulta-
neous whole-field flow measurement. Line transfer CCD cameras and double
pulsed laser sources, that are often used in particle image velocimetry, make
it possible to record pairs of holograms separated by 1-2 m s at megapixel resolution
and at more than 1,000 frame pairs per second. For micro-flow measurement this
means a maximum flow velocity of approximately 1-10 m/s depending on the
configuration.
To image through scattering media or increase the data density by introducing
more seeding particles it is necessary to use a tomographic technique. Doppler OCT
is now used routinely, both to image and measure blood flow within the retina. In its
original form OCT used a configuration similar to low NA CSI and used
a broadband incoherent source. This configuration is referred to as space domain
OCT and required slow mechanical scanning. Commercial swept-source OCT
systems now exploit fast scanning laser sources to measure the backscattered
light scattering along the path of a weakly focused beam. 2D scanning mirrors
are required to produce a complete 3D image. Typically the depth scan rate
is around 100-200 kHz, and this limits the maximum velocity to around
Search WWH ::




Custom Search