Rationale and Objectives
The stenosis degree of carotid artery (CA) can be a critical factor for treatment of cerebrovascular disease and for determining candidate of carotid endarterectomy. Currently, three different measuring methods are applied only on projectional cervical images. These measurement methods introduce several demerits such as a thromboembolic event, and three reference positions provide the different measurement results even on same subject. In addition, projection image could not provide the most severe stenosis position by nature; and the manual measurements also provide the inter-observer and intra-observer variability. Therefore, a computerized measuring scheme is necessary to overcome these drawbacks.
Materials and Methods
By applying local adaptive thresholding technique on cervical magnetic resonance angiogram image sequence, CA objects are initially identified. These are used to determine the three-dimensional central axis of CA by using circumscribed quadrangle. The oblique slices are reformatted into two-dimensional image planes, which are perpendicular to the central axis of CA, to provide the circular shape of blood vessel provided that the artery runs horizontally across the scanning axis. After that, region growing technique is applied on obliquely reformatted image sequence followed by geometrically restoration of segmented CA objects.
Results
The percentage of stenosis can be defined by the area ratio of segmented CA to restored CA object. The stenosis grading of is [(A−B)/A]×100% [
(
A
−
B
)
/
A
]
×
100
% , where A represents area measure of restored object, B represents area measure of segmented CA object. Experiments have been conducted on both phantom that simulated the mild (30%), moderate (50%), and severe (70%) stenosis degree for validation of proposed measurement approach and 86 carotid arteries from 43 clinical data sets (including 5 occlusion cases).
Conclusions
The automated approach is recommended to measure the carotid stenosis by using axial image sequence. This technique is not only accurate as possible but also robust, simple to handle, and less time consuming as compared to manual measurements. In addition, a computerized carotid stenosis measuring method is necessary to overcome the drawbacks introduced by using the projectional image and measurement variability of inter-observer, intra-observer.
Stroke is the third leading cause of death and a principal cause of severe, long-term disability in much of the industrialized world . A stroke occurs when a blood vessel that brings oxygen and nutrients to the brain bursts or is clogged by a blood clot or some other mass. Because of this rupture or blockage, part of the brain does not get the blood and oxygen it needs. The carotid arteries, which mainly carry blood to the brain, are located on the left and right sides of the front neck. The stenosis occurs when the carotid artery (CA) becomes clogged with fatty deposits. Fatty deposits can narrow the artery so severely that not enough blood can get through to the brain. In addition, pieces of the atherosclerosis may break off and travel with the flowing blood to the brain. These may block small blood vessels and cause the brain infarction. Either way, a stroke could occur. For the purpose of stroke treatment caused by carotid stenosis, carotid angioplasty involves using a balloon catheter to flatten plaque blockage against the artery wall, whereas endarterectomy is the surgical procedure to remove fatty plaque from cervical arteries. Carotid endarterectomy surgery is a treatment that has been proven safe and effective in providing long-term benefits to patients.
The North American Symptomatic Carotid Endarterectomy Trial (NASCET) provided a method to measure the carotid stenosis using the distal internal CA (ICA) diameter as the reference diameter, and recommended that carotid endarterectomy be performed in symptomatic patients with stenosis of 70% or greater . The European Carotid Surgery Trial (ECST) provided a method to measure the carotid stenosis using the approximate diameter of the carotid bulb as the reference diameter and recommended that carotid endarterectomy should be performed in symptomatic patients with stenosis of 80% or greater . Another method was developed to measure the carotid stenosis based on the measurement of the common carotid (CC) lumen diameter as the reference diameter .
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Materials and methods
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Image Segmentation
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Determine Central Axis of Carotid Artery
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Oblique Slice Reformation
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Ax+By+Cz+D=0 A
x
+
B
y
+
C
z
+
D
=
0
If A = B = 0 , the plane is parallel to the xy -plane as axial slice image. If B = C = 0 , the plane is parallel to the yz-plane as sagittal slice image. If A = C = 0 , the plane is parallel to the xz-plane as coronal slice image. However, oblique slice images are defined as planes that are perpendicular to the arbitrary defined lines connecting the two coordinates in image volume space by applying the equation (1) . Suppose points Pn(xn,yn,zn) P
n
(
x
n
,
y
n
,
z
n
) and Pn+1(xn+1,yn+1,zn+1) P
n
+
1
(
x
n
+
1
,
y
n
+
1
,
z
n
+
1
) are two consecutive central coordinates based on circumscribed quadrangle of segmented CA, and then L is a line in a image volume space that passes through two given points Pn P
n and Pn+1 P
n
+
1 ; and which is one of the line segment from central axis of CA object. Then the oblique slice image can be obtained by locating the tangent plane with normal line L in the image volume space. Figure 3 shows the oblique slice images on the central axis of CA object.
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Region Growing on Obliquely Reformatted Image Sequence
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CAs⊂fsandCAu⊂fu C
A
s
⊂
f
s
and
C
A
u
⊂
f
u
RGSc=CAsICAu={(x,y)|(x,y)∈CAsand(x,y)∈CAu} R
G
S
c
=
C
A
s
I
C
A
u
=
{
(
x
,
y
)
|
(
x
,
y
)
∈
C
A
s
and
(
x
,
y
)
∈
C
A
u
}
where RGSc R
G
S
c represents the set of candidate pixel for region growing seed; CAs(x,y) C
A
s
(
x
,
y
) represents the segmented CA area that is the proper subset of fs(x,y) f
s
(
x
,
y
) ; and CAu(x,y) C
A
u
(
x
,
y
) represents the unsegmented CA area that is the proper subset of fu(x,y) f
u
(
x
,
y
) .
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Geometric Restoration
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Results
Stenosis Measurement
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[(A−B)/A]×100% [
(
A
−
B
)
/
A
]
×
100
%
where A represents area measure of restored CA object and B represents area measure of segmented CA object. This formula is applied to all axial images from the MRA cervical image sequence; and the stenosis grade is provided as a percentage for each axial image. The percentage of maximum ICA stenosis degree can be determined from each individual stenosis grade of axial image sequence; these can provide the specific location of maximum ICA stenosis.
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Experiment with Simulated Stenosis on Phantom
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Clinical Data Sets
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Discussion
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