Image Processing Reference
In-Depth Information
this complementary SPAMM (CSPAMM) technique has subsequently proved use-
ful in eliminating artifacts due to T1 relaxation in the HARP and DENSE tech-
niques, as described in the following sections.
10.3
SPAMM IMAGE ANALYSIS
10.3.1
S
T
TRIPE
RACKING
The image stripes visualized in SPAMM images can be tracked in the image plane
and used to calculate 2-D motion and deformation (ignoring for the moment the
effects of through-plane motion). Kraitchman et al. [17] described a semiautomatic,
interactive procedure for tracking tag intersection points. A template matching
procedure was used to filter the tagged images to estimate an image intersection
likelihood function. Adapting the active contour (“snakes”) formulation of Kass
et al. [18], a 2-D active contour mesh model comprising interconnected tag inter-
section points was then used to optimally find the best location of the stripe
intersections, subject to constraints on the relative displacement between neighbor-
ing tag intersection points. This method was extended to track the entire stripe
(including points between stripe intersections) using a 2-D active “carpet” model
of interconnected snakes [19]. Each stripe was modeled as a thin, flexible beam
with resistance to stretching and bending. Stripes were connected at the intersection
points (in the case of grid-tagged images), and the entire mesh deformed to max-
imize the image-derived feature values (Figure 10.6). Similar stripe tracking pro-
cedures have been developed by Guttman et al. [20] and others [9]. User interaction
is essential to these tracking processes in order to correct errors due to image
contamination and resolve correspondence between frames in cases where the
tissue motion was greater than stripe spacing. Image contamination arises due to
respiration and gating artifacts, stripe T1 relaxation, spin dephasing due to sus-
ceptibility, and poor image SNR. It is, therefore, imperative that the user inter-
action is efficient, intuitive, and minimal.
(a)
(b)
(c)
FIGURE 10.6
Grid-tagged image stripes tracked with an active carpet model. (a) end-
diastole, (b) end-systole, and (c) late diastole. Note tracking error (arrow) due to spin
dephasing at myocardium/fat boundary in late diastole.
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