Holographic Motility Contrast Imaging of Live Tissues - Biomedical Optical Phase Microscopy and Nanoscopy - page 216

) of the exponential is the product of the
single-scattering rate multiplied by the average number of scattering events along the path
( s /

the incident light [19] . The argument 2( τ / τ 0 )( s/ '

). Longer paths lead to faster decorrelation because more scattering events add to
scramble the phase.

'

The multiple scattering analysis is important for fluctuation spectroscopy of dense tissue.
The characteristic frequencies that appear in Table 11.1 are for quasi-elastic light scattering
(QELS), but in multiple scattering, these frequencies must be multiplied by ( s / ' ). For

coherence gating to a depth of 200 8. Therefore, the

characteristic frequencies in MCI can be up to an order of magnitude higher than those
obtained from single-scattering studies.

400 μ m, this is a factor of 4

11.5 Tissue Dynamics Spectroscopy

MCI, described in Section 11.3 , uses statistical properties to generate motility metrics and
motility maps of live tissue samples. While these motility metrics, such as the NSD, capture
the total motion, they do not provide information on the many different types of motion that
occur inside the tissues. To access this more specific information, we perform fluctuation
analysis of the fluctuating speckle originating from a coherence-gated depth inside the live
tissue. Our emphasis for tissue dynamics spectroscopy (TDS) is on the spectral power
density [20] .

An example of the spectral power density from the proliferating shell of a tumor that is
500 m in diameter is shown in Figure 11.8 at two temperatures. The spectrum spans three

decades in frequency and in dynamic range. There is more spectral weight at higher
frequencies at 37 C than at 24 C, and the knee frequency has shifted to higher rates
as well.

μ

To capture subtle effects of environmental or drug-induced changes in the power spectra,
we define a normalized spectral difference as

S 2 ðωÞ 2 S 1 ðωÞ
S 1 ðωÞ

D ðωÞ 5 (11.21)

where S 1 ( ) is the altered spectrum.

An example for different osmolarities applied to four tumors is shown in Figure 11.9A .
Hypotonic conditions enhance low frequencies and suppress high frequencies relative to
hypertonic conditions. In the hypotonic case, the cells swell significantly to the bursting
point and beyond. The enhanced low frequencies are likely to be related to gross shape
changes of the membranes as they deform and form blebs. In the hypertonic case, the
cells shrink and activate vesicle transport to attempt to compensate for the loss of
water from the cytoplasm. The enhanced high frequencies may be related to enhanced

ω ) is the baseline spectral power density, and S 2 ( ω