Biophotonics_Concepts_to_Applications

9.5 Diffuse Correlation Spectroscopy

Diffuse correlation spectroscopy(DCS) is a continuous noninvasive technique that
is used for probing theflow of blood in thick blood vessels or in vessels located in
deep tissue, such as brain, muscle, and breast tissues [ 32 – 35 ]. DCS is based on
using a time-averaged intensity autocorrelation function of afluctuating diffusely
reflected light signal. This technique also is known asdiffusing-wave spectroscopy.
DCS procedures are carried out in the 600-to-900-nm spectral range where the
relatively low absorption enables deep penetration of light into tissue (see Fig.6.8).
The blood-flow measurements are made by monitoring the specklefluctuations of
photons that are diffusely scattered by blood cells as they move in the tissue (see
Sect.6.6).
The typical DCS setup shown in Fig.9.13uses a laser that has a long coherence
length, a photodetetctor that can count single photons (such as an avalanche pho-
todiode or a PMT), and a hardware autocorrelator. The purpose of theautocorre-
latoris to compute the degree of similarity of a signal with itself at varying time
intervals. If the measurement times of the signal intensities are widely separated,
then for a randomlyfluctuating signal the intensities are not going to be related in
any way. If the intensities are compared within a very short time intervalδt, there
will be a strong relationship or correlation between the intensities of two signals.
To measure the bloodflow, near-infrared light from a laser that has a long
coherence length is launched into the tissue through a MMF that has its probing end
placed on the tissue surface [ 2 ]. A fraction of the light that is scattered by blood
cells in the tissue is collected by a SMF or a few-modefiber, which is placed a few
millimeters or centimeters away from the illuminatingfiber. The temporalfluctu-
ation of the light intensity in a single speckle area is related to the movements of the
red blood cells in microvasculature (the smallest blood vessels located throughout
the tissue). The bloodflow can be quantified by calculating the decay of the light
intensity, which is derived from the autocorrelation function results.

Single-mode

collection

fiber

Multimode

illumination

fiber

Scattering events

in the tissue

Long-coherence-

length laser

Photon-counting

photodetector

Hardware

autocorrelator

Computer for

autocorrelation

function

calculation

mm to cm

separation

Fig. 9.13 Schematic of a representative experimental DCS setup

9.5 Diffuse Correlation Spectroscopy 275