Biomedical Engineering Reference
In-Depth Information
light retardation. From them, the sinograms R n P of slice n at pressure
P were built for each pressure and slice. For each image, the region
of interest (ROI) is limited to the borders of the model from rows
350 to 449. In the same way, two additional scans were done with
a guide wire inserted into the phantom and inner pressure of 80
mmHg. From them the sinograms A n 80GW representing the OPL
and the R n 80GW representing retardation were obtained (Fig. 4.20).
Optical path length and retardation images were captured for the
slices within the region of interest at 128 different directions for
pressures of 80, 120 and 160 mmHg. When the model had an inner
pressure of 80 mmHg, an additional set of images was captured
when a guide wire was inserted. The resulting sinograms of slice 400
are shown for the four samples in Fig. 4.21. For reconstructing the
stress distribution from A n P and R n P , ML-EM method was used twice.
First, the slices of the blood vessel thickness were reconstructed
by using A n P images as source and using a white image with the
same dimensions of A n P as initial guess of the cross section B 0 P . The
result images are named B n,0 P ; best results were obtained after 20
iterations B n,20 P . B n,20 P are grayscale images where the thickness is
represented by the color value. A threshold level was applied to set
to zero the pixels with a value below 128, and set to 255 those with
a value above. The resulting image B' n, 20 P is used as the irst stress
cross section approximation for using the ML-EM method a second
time to reconstruct the stress distribution (Fig. 4.22).
P=120mmHg
P=160mmHg
;M m]
;M m]
P=80mmHg
P=80mmHg+Guid wire
4000
4000
3000
3000
2000
2000
1000
1000
A 160
400
A 120
400
400
A 80GW
400
A 80
0
0
; nm]
; nm]
250
250
167
167
125
125
62
62
R 160
400
R 80
400
R 400
80GW
R 120
400
b)
a)
0
0
Figure 4.21 (a) Sinograms of optical path length and retardation with
80 mmHg of pressure inside the model and with guide wire
inserted. (b) Sinograms of optical path length and retardation
with pressures of 120 mmHg and 160 mmHg. See also Color
Insert.
 
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