Biomedical Engineering Reference
In-Depth Information
such that the curvature is not limited by the right-hand term of inequality (5.19).
Remember that the curvature was also bounded by
1
x
, so the resolution of the
image grid provides another constraint to the algorithm in the capture of narrow
structures.
Traditionally, two functions have been used as contour detectors
1
(1
+|∇
σ
I
(
x
)
|
)
p
,
p
=
1
,
2
g
(
x
)
=
(5.20)
g
(
x
)
=
e
−
|∇
σ
I
(
x
)
|
,>
0
(5.21)
where
∇
σ
I
=∇
(
I
∗
G
σ
) and
G
σ
is a Gaussian kernel. In this work, the Leclerc
function (Eq. (5.21)) was used with
σ
=
0
.
8 mm, as edge detector function. For
the selection of
, we noted that in places where
k
out
≈
k
in
, Inequality (5.19) leads
to
|
κ
|≤
||∇
σ
I
||
∞
.Soif
1
<
2
, the steady-state model associated with
2
will
be able to capture narrower structures than the steady-state model associated
to
1
. Therefore, in all the experiments, the Leclerc contour detector function
g
was selected with
=
10. Less than 20 iterations were then enough to assure
convergence of the algorithm.
5.3
Evaluation: Materials and Methods
5.3.1
Clinical Protocol
The evaluation study was performed on a data base of 39 brain aneurysms
located mostly at the Circle of Willis; more specifically, at the Anterior Commu-
nicating (16), Posterior Communicating (10) and Middle Cerebral (10) Arteries.
There were also three aneurysms located at the Pericallosal (1) and Internal
Carotid Arteries (2). Aneurysm sizes (dome depth size) varied from 2
.
9to16
.
3
mm. Table 5.1 shows the distribution of sizes, manually measured, according to
the location of the aneurysm.
Image acquisition was performed using an Helical Elscint CT Twin scanner
(Marconi; Haifa, Israel) with 120 kV/300 mA for the amplifier tube, 1.2-mm col-
limation with an helical pitch of 1 and slice spacing of 0.65 mm. Images were
reconstructed on a 512
×
512 matrix with a square Field Of View (FOV) of 20.8
cm yielding an in-plane resolution of 0.4 mm. A total of 140 ml of non-ionic
contrast fluid was intravenously administrated (Omnitrast 300 mg; Schering,
