Biomedical Engineering Reference
In-Depth Information
3. Motion compensated re-reconstruction
4. Event rebinning
5. Joint reconstruction of image and motion
3.1.1
Optimal Gating
In contrast to cardiac motion induced by myocardial contraction, respiratory motion
has a quite irregular pattern. Most of the time, the thorax is resting at the expiration
phase, whereas the inspiration phase is often relatively short. Recently, a new
amplitude-based gating approach, called
optimal gating
, was introduced by van
Elmpt et al. [
131
] that defines the maximum time phase at end-expiration where
the motion is minimal. Therefore, a histogram is formed of the respiratory signals,
acquired during the list mode PET scan, and converted to a cumulative distribution
function. On this function, a lower and upper level is determined that forces the total
sensitivity to equal to a certain percentage (e.g., 35 %) of the acquired breathing
signals. This percentage is called the
optimal gating yield
.
Although most of the acquired data (e.g., 65 %) is discarded in performing
optimal gating leading to noisier images, this approach provides a robust method
of motion reduction. It could be demonstrated that optimal gating is a user-friendly
respiratory gating method to increase detectability and quantification of upper
abdominal lesions compared to conventional acquisition techniques [
130
].
3.1.2
Averaging of Aligned Images
After gating the acquired PET data, each gate is reconstructed individually and
aligned to one designated reference gate. To overcome the problem of low SNRs,
the aligned images are averaged (summed) afterwards. In the context of respiratory
motion correction, Dawood et al. [
34
] propose an optical flow based approach
and Bai et al. [
6
-
8
] use a regularized B-spline approach with a Markov random
field regularizer. In the context of cardiac motion correction, Klein et al. [
73
,
75
]
propose a technique using 3D optical flow and model the myocardium as an elastic
membrane. In their approach intensity modulations caused by cardiac motion (see
Sect.
2.1.4
) are not addressed. The averaging of aligned images is described in more
detail in Sect.
3.2.1
of this topic.
3.1.3
Motion Compensated Re-reconstruction
Similar to the averaging approach, gating is applied and each individually
reconstructed gate is aligned to one assigned reference gate. The obtained motion
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