Image Processing Reference
In-Depth Information
9.3.2
M
OTION
AND
D
EFORMATION
A
NALYSIS
9.3.2.1
Motion Analysis
A number of techniques have been used in order to describe and quantify the
motion of the heart. They can be divided into three main categories according to
the method used [65]: (i) detecting endocardial motion by observing image
intensity changes, (ii) determining the boundary wall of the ventricle and subse-
quently tracking it, and (iii) attempting to track anatomical [66-69], implanted
[70-76], or induced [53,54,77-83] myocardial landmarks. There are a few prob-
lems with each of these techniques. Assumptions must be made about the motion
(motion model) in the first two groups in order to obtain a unique pointwise
correspondence between frames. To this end, optic flow methods [84-89]
*
and
phase-contrast MR [91-94] have been applied for (i), and curvature-based match-
ing [95-99] has been used to find point correspondences in (ii). Landmark-based
methods [53,54,66-83] provide information on material point correspondences.
However, this information is mostly sparse and, again, assumptions on the type
of motion have to be made in order to regularize the problem of finding a dense
displacement field. The use of implanted markers adds the extra complication of
being invasive, which precludes routine use of this technique in humans. Although
implanted markers are usually regarded as the gold standard, there are some
concerns in the literature about their influence on image quality and about their
modification of the motion patterns.
9.3.2.2
Wall Thickening
Azhari et al. [100] have compared wall thickening (WT) and wall motion in the
detection of dysfunctional myocardium. From their study, it was concluded that
WT is a more sensitive indicator of dysfunctional contraction [100]. This finding
has prompted several researchers to define methods to quantify wall thickness.
Azhari et al. [100], and Taratorin and Sideman [101] carried out a regional
analysis of wall thickness by dividing the myocardium into small cuboid ele-
ments. The local wall thickness is then defined as the ratio between the volume
of the particular element and the average area of its endocardial and epicardial
surfaces [50].
The most widely employed method for WT computation, however, is the cen-
terline method [102] and several improvements thereof [47,103-105]. Starting with
the endo- and epicardial contours at each slice, the centerline method, in its original
formulation, measures WT in chords drawn perpendicular to a line that is equidistant
to both contours (the centerline). Although more accurate than methods relying on
a fixed coordinate system, this method still assumes that the contours are perpen-
dicular to the long axis of the LV. If this is not the case, the myocardial wall thickness
*
At this point it is worth mentioning an excellent online bibliographic database maintained by the Special
Interest Group on Cardiac Motion Analysis (SigCMA) that can be accessed at
http://www.creatis.insa-
lyon.fr/sigcma
. It also provides general bibliographic information on model-based cardiac image analysis.
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