Biomedical Engineering Reference
In-Depth Information
proportional to the blood flow velocity, but due to the complex processes of light
scattering in a living tissue direct experimental calibration is necessary to evaluate
absolute levels of velocity.
7.7.2 Contrast Evaluation
The second method is autocorrelation analysis. In this case speckle contrast is
evaluated. Each image is interrogated with a window of varying size from 8
8
to 50
N
and the extent of
overlapping were optimized for each particular optical scheme of the experiment.
Contrast is calculated in each window
k
according to the equation:
50 and the windows can overlap. The window size
M
q
I
hi
I
k
2
s
I
k
C
k
¼
Ihi
¼
:
Ihi
s
2
MN
P
P
MN
P
P
M
N
M
N
1
1
I
k
I
k
m¼
1
n¼
1
m¼
1
n¼
1
¼
:
(7.7)
MN
P
P
M
N
1
I
k
m¼
1
n¼
1
Color coding of the contrast is used for visualizing the results, this contrast field
being displayed on the monitor. An example of such a field is shown in Fig.
7.5
(right). In this case, the speckle field is obtained by laser illumination of the rotating
ground glass. When the image corresponds to the part of the ground glass moving
with higher velocity, the image is blurred and the contrast is lower. When the
contrast of the speckle field is integrated over time it can characterize the mean
speckle displacement during the exposure time. As shown by Briers [
10
] this
contrast is inversely proportional to the velocity of scatterers in the interrogation
area. The relation between speckle field contrast and relative speckle enlargement
(exposure time) was analyzed with the help of a phantom speckle pattern. This
phantom was an ideal speckle pattern with a contrast equal to 1. Then mathematical
blurring of the speckle pattern was carried out so that the entire windowwas covered,
this giving a contrast value of 0. Contrast was calculated for different relative
speckle enlargements and the results of the modeling show nearly linear dependence
of the contrast on speckle displacement. Thus, at 50% blurring the contrast was 0.55.
7.7.3 Cross-correlation Analysis
The most general approach in image analysis is cross-correlation analysis. The
image is interrogated with small windows as in the approaches above, allowing the
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