Biomedical Engineering Reference
In-Depth Information
Optimal spline control point spacing/number of spline control points (cf.
Sect.
2.1.3
), and the
Influence of additional spline control points outside the image domain.
4.1.1
Gating
The gating schemes for V
AMPIRE
used in [
51
] and MPOF in [
37
] use an equidistant
time-based gating for the cardiac cycle. A more sophisticated cardiac gating with
time varying cardiac gates [
127
] or with grouping of cardiac phases [
74
] could
better resolve the cardiac phases (in particular regarding the systole) and should
be considered in future work.
For respiratory motion, the proposed amplitude-based gating has the advantage
of (almost) equal statistics in each gate, which manifests itself in a comparable
noise level in the reconstructed images. This allows us to choose a constant set
of regularization parameters for all motion estimation tasks. The downside of this
gating scheme is the varying motion blur in different gates. Infrequent motion states
feature an increased blurring induced by motion. A phase-based gating scheme
would allow to create gates with equal motion blur in each gate, however, with
the effect of different SNRs in the gates. The optimal trade-off between remaining
motion and statistics per gate should thus be explored. This is directly related to the
analysis of the optimal number of dual gates, which is not addressed in this work
but interesting to analyze, cf. [
36
].
4.1.2
Spatial-Temporal Approach
Respiratory and cardiac motion is cyclic and smooth over time. It should thus be
taken advantage of this information. The approaches described in Chap.
2
estimate
the motion between each motion phase and the reference phase independently. Mod-
ifications could be an estimation of motion between each phase and its subsequent
phase to keep the expected motion vectors small [
73
]. This is particularly interesting
for optical flow methods due to the Taylor linearization. Another extension is to
explore the cyclic nature of motion [
83
]. As hyperelastic regularization allows
for large motion, this is not essential in the case of V
AMPIRE
. Still, a global
smoothness constraint which accounts for smooth transitions and cyclicality should
be investigated to further improve the robustness of motion estimation towards
noise.
Klein et al. [
73
] also propose a tissue-dependent regularization in their work by
modeling the incompressibility of myocardial tissue. If different tissue compartment
are given, e.g., by applying a prior segmentation, non-uniform regularization could
be applied to incorporate further a priori knowledge about tissue-dependent motion
behavior.
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