SuperEllipseTrace3D
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− | == Local vascular modelling using | + | == Local vascular modelling using Superellipsoid == |
The superellipsoid provides the basis for a simple shape model that describes localized vessel segments. It has an explicit low-order parameterization relating directly to the local pose of a vessel segment. Using this geometric information, it is straightforward to traverse entire vascular networks.Also, since the superellipsoid model furnishes joint estimates of the boundary and centerpoint of a vessel, the resulting traversal approximates the medial axis of each vessel in the vasculature. Another key | The superellipsoid provides the basis for a simple shape model that describes localized vessel segments. It has an explicit low-order parameterization relating directly to the local pose of a vessel segment. Using this geometric information, it is straightforward to traverse entire vascular networks.Also, since the superellipsoid model furnishes joint estimates of the boundary and centerpoint of a vessel, the resulting traversal approximates the medial axis of each vessel in the vasculature. Another key | ||
advantage when using the superellipsoid model is that its local nature allows us to adapt vessel intensity estimates across different regions of the vasculature. | advantage when using the superellipsoid model is that its local nature allows us to adapt vessel intensity estimates across different regions of the vasculature. |
Revision as of 12:58, 11 May 2009
This page presents automated methods for robust modeling and analysis of 3-D vascular images using a \textit{cylindroidal superellipsoid} to model localized segments of vasculature. The proposed vessel model has an explicit, low-order parameterization, allowing for joint estimation of boundary and centerline information, thereby approximating the medial axis of tubular structures. Further, this explicit parameterization provides a geometric framework for traversing vessels in a directed manner. Topological information like branch point location and connectivity can also be detected as a side effect. An M-estimators provide robust region-based statistics that are used to drive the superellipsoid toward a vessel boundary. The proposed methodology behaves quite well across scale-space, shows a high degree of insensitivity to adjacent structures and implicitly handles branching.
Contents |
Local vascular modelling using Superellipsoid
The superellipsoid provides the basis for a simple shape model that describes localized vessel segments. It has an explicit low-order parameterization relating directly to the local pose of a vessel segment. Using this geometric information, it is straightforward to traverse entire vascular networks.Also, since the superellipsoid model furnishes joint estimates of the boundary and centerpoint of a vessel, the resulting traversal approximates the medial axis of each vessel in the vasculature. Another key advantage when using the superellipsoid model is that its local nature allows us to adapt vessel intensity estimates across different regions of the vasculature.
Also, since the superellipsoid model furnishes joint estimates of the boundary and centerpoint of a vessel, the resulting traversal approximates the medial axis of each vessel in the vasculature. Another key advantage when using the superellipsoid model is that its local nature allows us to adapt vessel intensity estimates across different regions of the vasculature.
Superellipsoids are geometric modeling primitives first introduced
in early computer vision literature [2]. Superellipsoids are expressed implicitly as:
where the parameters control the shape of the superellipsoid. To understand the role of
the shape parameters, first note that with ε1 = ε2 = 1 equation 1 is simply an ellipsoid. With
and ε2 = 1 the superellipsoid becomes a cylindroid, i.e., the implicit surface bounds a convex region, where for constant z, the 2-D level curves are
elliptical. Setting ε1,ε2 to other values produces a variety of shapes including cuboids and pinched cuboids [2]. Since the goal in this work is to find an approximation to an elliptical cylinder, we will restrict our focus to cylindroidal forms. This means that ε2 in equation is set to unity.
Starting with a set of points in the set
(ε
is fixed), a transformation given by
can be performed. This transformation preserves orthogonality of
the local coordinate frame while allowing for anisotropic scaling.
The action of
is to first scale
by
, which is a
matrix with
scale parameters
on the
diagonal. Then, a rotation is performed via the
matrix
with three degrees of freedom that is
parameterized by φ. Finally, a shift via
is performed.
This transformation leads to a new superellipsoid defined by the
set having 10 degrees of freedom in 3D,
,
where,
Vessel Model Formulation
When estimating the superellipsoid model parameters we will assume that a vessel segment consists of two piece-wise constant regions with intensities denoted IF for foreground and IB for background and that the vessel boundary separating these regions is well-modeled by a superellipsoid. The local vessel intensities are modeled using a homogeneous grayscale appearance model as
where
is the full parameter set of the superellipsoid vessel model. This
includes the intensity parameters
that
describe the foreground and background intensities respectively,
as well as the underlying parameters of the superellipsoid shape
model, i.e.,
.
The notation
denotes an indicator
function that evaluates to one on the set specified in the
subscript and is zero elsewhere.
The strategy for maximizing equation is to alternate between estimating the intensity parameters and then updating the
boundary using a gradient search. The following two subsections will detail each step.