Biomedical Engineering Reference
In-Depth Information
D
(
k-n
)
D
1.0
1.0
1
2
1
2
3
0.8
0.8
3
1.0
1.0
0.4
0.4
0.2
0.2
0.004
0
0.008
0.012
0
10
20
30
40
(
k-n
)
T,
s
Fig. 7.6
Decorrelation function of the intensity variations in biospeckle patterns formed in laser
light scattered from an apple tissue (
1
), human tissue in vivo (
2
), and a fixed ground glass (
3
). On
the
x
-axis—numbers of the chosen frames with time interval between subsequent frames equal to
40 ms (
left
). At the
right
—decorrelation functions for three different parts of the human tissue
in vivo.
x
-axis shows real time in seconds
To evaluate this function two sequential image maps,
Ik
and
In
, are subsampled.
The size of the interrogation window is chosen to be equal to 32
62
pixels, since such windows contain a sufficient amount (10-100) of speckles for
statistical averaging. This gives the possibility of using statistical averaging and to
obtain 18
32 or 62
9 interrogation windows. The total illuminated area is about
2-5 mm in diameter, resulting in the size of interrogation window ranging from
100
18 or 9
0.5 mm. The values obtained can be coded either as gray or
color variations and displayed at a monitor. Examples of such decorrelation
functions obtained in our research are shown in Fig.
7.6
.
Two different tissues have been analyzed using this approach. First, Fig.
7.6
shows the decorrelation function variations in biospeckle patterns obtained under
illumination of an apple surface by a He-Ne laser.
The speckle movement in such patterns is due to two types of intra-tissue
dynamics. There is not only the movement of the chloroplasts and amyloplasts
(small particles with a mean size of 1
100
m
mto0.5
m) in the cytoplasm medium of the apple
tissue but also the movement of small mineral particles inside the apple cell
vacuoles [
46
]. As the speed of all of these scatterers is less than 1 mm/min
1
,
there is a clear linear dependence of the evaluated decorrelation function on the
time interval between frames. The decorrelation technique was also used in in vivo
studies. The speckle pattern variation in this case is due to erythrocyte movement in
a living human tissue. The typical speckle patterns for such are shown in Fig.
7.6
.
Erythrocyte movement can be two orders of magnitude faster than that in an apple
tissue, so no linear increase of the structural function with the time interval between
successive exposures was evident. Instead, random fluctuations in blood flow
velocity were recorded. The values of the decorrelation function obtained are
m
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